Synthetic, mechanistic, and biological interrogation of Ginkgo biloba chemical space en route to (-)-bilobalide

Article abstract of DOI:10.1021/jacs.0c08231

Here we interrogate the structurally dense (1.64 mcbits/?3) GABAA receptor antagonist bilobalide, intermediates en route to its synthesis, and related mechanistic questions. 13C isotope labeling identifies an unexpected bromine migration en route to an α-selective, catalytic asymmetric Reformatsky reaction, ruling out an asymmetric allylation pathway. Experiment and computation converge on the driving forces behind two surprising observations. First, an oxetane acetal persists in concentrated mineral acid (1.5 M DCl in THF-d8/D2O); its longevity is correlated to destabilizing steric clash between substituents upon ring-opening. Second, a regioselective oxidation of des-hydroxybilobalide is found to rely on lactone acidification through lonepair delocalization, which leads to extremely rapid intermolecular enolate equilibration. We also establish equivalent effects of (-)-bilobalide and the nonconvulsive sesquiterpene (-)-jiadifenolide on action potential-independent inhibitory currents at GABAergic synapses, using (+)-bilobalide as a negative control. The high information density of bilobalide distinguishes it from other scaffolds and may characterize natural product (NP) space more generally. Therefore, we also include a Python script to quickly (ca. 132 000 molecules/min) calculate information content (B?ttcher scores), which may prove helpful to identify important features of NP space.

Full text of DOI:10.1021/jacs.0c08231

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Article
Synthetic, Mechanistic and Biological Interrogation of
Ginkgo biloba Chemical Space en route to (–)-Bilobalide
Robert M. Demoret, Meghan A. Baker, Masaki Ohtawa, Shuming Chen, Ching-Ching
Lam, Sophia Khom, Marisa Roberto, Stefano Forli, K. N. Houk, and Ryan A. Shenvi
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c08231 • Publication Date (Web): 29 Sep 2020
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Synthetic, Mechanistic and Biological Interrogation of Ginkgo biloba
Chemical Space en route to (–)-Bilobalide
Robert M. Demoret,1,‡ Meghan A. Baker,1,‡ Masaki Ohtawa,1,† Shuming Chen,2 Ching Ching Lam,2 Sophia
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Khom,4 Marisa Roberto,4 Stefano Forli,3 Kendall N. Houk,2 Ryan A. Shenvi1*
1Department of Chemistry, Scripps Research, La Jolla, California 92037, United States
2Department of Chemistry and Biochemistry, University of California, Los Angeles, CA, USA
3DISCoBio, Scripps Research, La Jolla, California 92037, United States
4Departments of Molecular Medicine and Neuroscience, La Jolla, California 92037, United States
ABSTRACT: Here we interrogate the structurally dense (1.63 mcbits/Å3) GABAA receptor antagonist bilobalide, intermediates en
route to its synthesis and related mechanistic questions. 13C isotope labeling identifies an unexpected bromine migration en route to
an α-selective, catalytic asymmetric Reformatsky reaction, ruling out an asymmetric allylation pathway. Experiment and computa-
tion converge on the driving forces behind two surprising observations. First, an oxetane acetal persists in concentrated mineral acid
(1.5 M DCl in THF-d8/D2O); its longevity is correlated to destabilizing steric clash between substituents upon ring-opening. Sec-
ond, a regioselective oxidation of des-hydroxybilobalide is found to rely on lactone acidification through lone-pair delocalization,
which leads to extremely rapid intermolecular enolate equilibration. We also establish equivalent effects of (–)-bilobalide and the
nonconvulsive sesquiterpene (–)-jiadifenolide on action potential-independent inhibitory currents at GABAergic synapses, using
(+)-bilobalide as a negative control. The high information density of bilobalide distinguishes it from other scaffolds, and may char-
acterize natural product (NP) space more generally. Therefore, we also include a Python script to quickly (ca. 132,000 molecules/
minute) calculate information content (Böttcher scores), which may prove helpful to identify important features of NP space.
a.bilobalide (BB, 1)
ginkgolide B
O
picrotoxinin (PXN)
O
O
dieldrin
Introduction. Herbal supplements and remedies constitute a
large and controversial market.1 Not regulated as conventional
foods or drugs by the FDA,2 plant extracts can nevertheless
contain orally bioavailable small molecules with profound
effects on human biology. Successful sales, however, do not
rely on the same demonstrations of efficacy and safety as ap-
proved pharmaceuticals. For instance, leaf extracts of Ginkgo
biloba are widely marketed today as nootropic supplements,
but the ability of these extracts to improve cognition is ques-
tionable.3 Despite the ambiguity, approximately 1.6 million
adults reported using G. biloba extracts in the US in a 2012
survey.4,5 Even with clear target annotation for several constit-
uents, as well as proven brain penetration,6 any benefits of
Ginkgo extracts remain largely unsubstantiated. Additionally,
a two-year study by the National Toxicology Program (NTP)
concluded that one commercially available G. Biloba extract
increased rates of liver and thyroid cancers in mice and rats.7
Adverse events have been long attributed to ingestion of
Gingko extracts, especially increased bleed time (see below).
Even so, individual components of the crude extract may
find useful application if isolated from the off-target activity
of congeners. For example, pure bilobalide (–)-1 exhibited
normalization of cognitive impairment in models of Down
syndrome, Ts65Dn mice, which exhibit excess inhibitory sig-
naling. 8 Four-week administration of bilobalide led to im-
proved cognitive performance, which correlated to enhanced
long-term potentiation (LTP, strengthening of synapses) by a
related compound (pentyltetrazole, PTZ) even months after
drug regimen had ended. Bilobalide also displayed protection
against NMDA-induced excitotoxicity in rat hippocampus.9
O
H
H
O
O
Cl
Cl
O
O
Me
O
H
H
OH
Me
HO
OH
Cl
Cl
O
O
H
O
HO
O
H
OH
H
O
HO
H
O
t-Bu
O
t-Bu
Me
IC50 = 2.4 uM
LD50 = 15 mg/kg
(p.o. mouse)
H
O
O
O
IC50 = 4.6 uM
LD50 = 300 mg/kg
(p.o. mouse)
LDlow = no reports
(human)
IC50 = 92 uM
LD50 = 38 mg/kg
(p.o. mouse)
IC50 = 7.22 uM
PAF IC50 = 8.2 uM
(increased
LDlow = 0.357 mg/kg LD50 = 65 mg/kg
(human) (human)
bleed time)
11-O-debenzoyl
tashironin
b. α1β3γ2L GABAAR • PXN c. views of TMD pore
HO
Me
H
OH
O
H
O
Me
HO
Me
out
in
out
jiadifenolide (J)
OH
O
Me
O
TMD
PXN
H
O
OH
in
O
Me
ICD
O
Figure 1. a. Hyperexcitatory compounds that antagonize
GABAA receptors; b. cryogenic electron microscopy structure
of the human α1β3γ2L GABAA receptors; c. channel binding
site of picrotoxinin, which exhibits allosteric coupling to the
extracellular region (b/c, Ref. 10).
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In addition to human applications,11 ginkgolides and bilob-
Here we describe our efforts to devise a concise synthesis of
1
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alide exhibit insect deterrent or insecticidal activity against the
fall webworm (H. Cunea, 0.31–1.25 mg/mL),12a codling moth
(C. pomonella, 0.1 mg/mL),12b brown rice planthopper (N.
lugens, 0.17–0.26 ng/female) and others.12 Ginkgo leaves have
long been used as bookmarks to protect against page-eating
insects like silverfish and booklice.16b The insecticidal activity
of Ginkgo biloba leaves, roots and stems may have provided
advantage against predation to preserve the lineage of the ear-
ly Paleocene species G. adiantoides.13 Benign effects of bilob-
alide in humans—in contrast to ginkgolides14—commend it as
a potentially safe insecticide. The seemingly disparate applica-
tions of bilobalide in mammals and in insects derive from ac-
tivity at homologous ion channels: RDL (resistance to diel-
drin) receptors in insects and GABAA (gamma-amino butyric
acid A) receptors in mammals (Figure 1b).
bilobalide (1), a challenge due to its two hallmark features: 1)
congestion, both steric and stereogenic, and 2) polyoxygena-
tion. Together, these features give 1 a very high information
density (Figure 2, 1.63 mcbits/ Å3), a measure we introduce
here that accounts for the volume occupied by chemical in-
formation present in a molecule. In contrast to information
content (mcbits),25 which mostly reflects heavy atom count
and stereocenters, information density (mcbits/ Å3) reflects
how that information is packaged. When high levels of stereo-
chemical information are incorporated into small molecular
volumes (i.e. contiguous stereocenters), new interactions and
modes of reactivity result. In other words, molecules with high
information density can have unexpected emergent properties
that alter their stability and reactivity.26 The efficient conver-
sion of low information-density building blocks into high in-
formation-density targets is a fundamental aim of total synthe-
sis. This exercise is particularly important in channel-binding
ligands of ionotropic receptors, especially terpenoids typified
by 1, where atoms must be restricted to a small volume: 1 is
only 260 Å3. In comparison, the steroid-derived anxiolytic
minaxolone,27 which binds the extracellular region of GABAA
receptors occupies 498 Å3 and the insecticidal macrolide
ivermectin, which binds the outer transmembrane region, oc-
cupies 835 Å3. Compared to similarly-sized natural products
pleuromutilin, himbacine and artemisinin, bilobalide possesses
the highest information density, primarily due to its oxygen
content, stereocenters and small volume. Stereocenters play a
major role in the information density of 1: an achiral constitu-
tional isomer contains only half the information density (0.94
vs. 1.63 mcbits/ Å3).
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Bilobalide exhibits insecticidal activity through antagonism
of RDL receptors: heteropentameric ion channels gated by
GABA. These ionotropic receptors mediate fast synaptic
transmission in insect nervous systems. As in mature mamma-
lian systems, the current tends to be inhibitory.15 Antagonism
of these inhibitory channels, therefore, causes increase in
spontaneous firing and can prove lethal. As a consequence,
RDL receptors are common targets of insect-control agents,
such as their namesake, dieldrin, a member of the cyclodiene
class. An Ala→Ser mutation within the channel16 reduces sen-
sitivity to GABA receptor antagonists, including picrotoxinin
(PXN, the more active component of the mixed substance
picrotoxin, PTX), a plant metabolite of similar formula and
topography to bilobalide.17a
Dieldrin, PXN and bilobalide also antagonize homologous
human GABA-gated ion channels or GABAA receptors
(GABAARs). Point mutation of channel residues17b identified a
likely binding site for bilobalide near the same location as
dieldrin and PXN18: a wide cavity between 2’ and 6’ residues
on the M2 helix in the transmembrane domain (Figure 1c).19 (–
)-1 and PXN also display similar potency against a major sub-
type of GABA receptor in the human brain (IC50 = 1.2 μM vs.
4.9 μM, α1β2γ2L @ EC50 GABA), although they respond dif-
ferently to diverse positive allosteric modulators.20
O
H
H
O
HO
Me
H
O
O
MeO2C
O
O
CO2Me
OMe
O
O
O
Me
H
O
OH
H
Me
Me
H
Me
H
O
H
O
Me
Me
O
O
HO
O
H
H
HO
t-Bu
H
Me
t-Bu
O
Me
H
O
N
Me
O
O
pleuromutilin
356 Å3
1.14 mcbit/ A3
himbacine
artemisinin
bilobalide (1) bilobalide isomer
260Å3 270Å3
333 Å3
239 Å3
1.19 mcbit/ A3
1.5 mcbit/ A3 1.63 mcbit/ A3 0.94 mcbit/ A3
Figure 2. Representative secondary metabolites, volumes and
information densities.
Both PXN and dieldrin, however, exhibit marked neurotoxi-
city in mammals. The latter compound, a polychlorinated pen-
tacycle, persists in the environment, transits along the food-
chain and causes multiple disorders in humans.21 In contrast,
bilobalide is well-tolerated in mammals at low to moderate
doses and breaks down easily near neutral pH (see below). As
a result, there is significant interest in the development of bi-
lobalide as an insecticide.12 Crude or semi-purified G. biloba
extracts unfortunately contain ginkgolides A-J, which inhibit
RDL receptors but also antagonize human platelet aggregating
factor (PAF) and are stable to water.
Two overarching strategies differentiate this work from the
,
prior28 29 syntheses: incorporation of targeted carbon oxidation
states in starting materials and formation of hindered C–C
bonds between these building blocks. The result, realized ret-
rospectively, is a generally positive increase in average infor-
mation content;30 each step productively approaches the target
complexity.
The execution of the synthesis relied on recursive feedback
between several discoveries that we analyze here in detail.
First, we provide a full account of the development of a cata-
lytic, enantioselective Zn+2-Reformatsky aldol that overcomes
the thermodynamic instability of the first key building block.
Second, experimental observations and mechanistic hypothe-
ses rationalize the development of a stereoselective Mukaiya-
ma hydration reaction, a considerable challenge associated
with selective diradical (C• + O2) collapse. Third, computa-
tional analysis describes the driving forces behind the unex-
pected formation of an oxetane acetal. Fourth, details behind a
We became interested in 1 due to its similarity to the Illici-
um sesquiterpenes,22 which hyperexcite mammalian neurons22b
but display potentially therapeutic phenotypes in mice, such as
normalization of amphetamine-induced hyperlocomotion, an
antipsychotic effect.23 We also recognized the opportunity to
investigate broader strategies in polyoxygenated carbocycle
synthesis gleaned from work on jiadifenolide22a and O-
debenzoyltashironin,22b namely the merger of highly oxidized
but sterically congested fragments to accelerate synthesis.24
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final, deep oxidation of C10 are provided, including analysis set. The pseudosymmetry of the bilobalide bis-lactone sug-
1
2
3
4
5
6
7
8
gested a symmetric 1,5-diester starting material that could
incorporate many oxidation states and might undergo radical
annulation by a 3-carbon fragment with a pendant tert-butyl
group (see Figure 3 and 4). The C10 hydroxyl of BB, howev-
er, proved hard to incorporate into starting material or early
intermediates (Figure 4). Therefore, our strategy also hinged
on installation of the single hydroxyl late in the synthesis. This
late stage oxidation, while simplifying overall, presented an
inherent risk due to the topolography of the BB ring system.
O
of selectivity using computed pKa.
Results and Discussion. Foundational work by Corey28 and
Crimmins29 established successful strategies to complete bi-
lobalide and informed our own orthogonal approach.31 These
prior routes established the contiguous, fully substituted cen-
ters of bilobalide early in the synthesis via open-shell interme-
diates. For example, Corey’s synthesis installed every carbon
atom of bilobalide in the very first step using a base-mediated
annulation (Figure 3), proposed to proceed by single electron
transfer followed by diradical collapse. Six of the resulting
carbon atoms underwent redox manipulation during elabora-
tion to BB. Of the 21 steps required to elaborate this interme-
diate to the target, 11 were redox manipulations. Similarly,
Crimmins’ synthesis established all four contiguous, fully
substituted carbons early, at step 8 of 17, via a photochemical
enone-olefin [2+2] cycloaddition. Bond formation via the
enone triplet excited state (π→π*), radical cyclization and
subsequent 1,4-diradical32 collapse reflects some mechanistic
similarity to ynoate annulation. The resulting cyclobutane
contains 5 carbons at the incorrect oxidation state, requiring a
total of 8 redox manipulations to reach BB.
9
O
O
10
11
12
13
14
15
16
17
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O
O
O
O
H
SmI2
[O]
CO2R
CH(OR)2
OR
t-Bu
CO2R
CO2R
HO
t-Bu
CH(OR)2
CO2R
HO
t-Bu
CH(OR)2
CO2R
HO
OSmI2
alkylation
and
Michael/ Giese
addition
deprotection
O
O
O
OH
O
O
Me
O
H
O
X
Me
t-Bu
O
Me
O
O
O
H
H
HO
O
RO2C
CO2R
OR
H
CO2Ph
MeO2C
HO
H
OH
H
CO2Me
t-Bu
O
Corey
1987
1988
LDA, HMPA
(72%)
O
O
RO
+
O
CO2Me
each prepared in one step
bilobalide 4 steps LLS (theoretical, not realized)
t-Bu
MeO2C
t-Bu
Figure 4. An early approach using a pseudosymmetric build-
ing block and radical cyclization.
22 steps (racemic)
24 steps (asymmetric)
SET
Michael / Claisen
contains all
carbons
The concave core of BB prevents easy access to either
hydrogen of lactone D (the ‘inner’ lactone) in the presence of
lactone A (the ‘outer’ lactone) (Figure 5). Instead, Hk is buried
deep within the ‘bowl’ shape of bilobalide and Hi is occluded
by the bulky t-Bu group. While retrosynthetically simplifying,
this ‘deep’ oxidation strategy pushed a tremendous amount of
risk to the endgame. If the route, however, were accomplished
in the projected eight step sequence, a robust supply of des-
hydroxybilobalide (2) would be available for experimentation,
mitigating some risk.
TMSO
O
O
OPiv
O
hν (350 nm)
Crimmins
1992
1993
OPiv
H
(50%)
t-Bu
O
t-Bu
HO
TMSO
HO
17 steps (racemic)
photochemical
[2+2]
contains all
quaternary carbons
oxidation state change required
correct oxidation state
O
O
OH
CO2H
CO2H
O
H
O
O
O
H
H
'outer lactone'
H
O
des-hydroxy-bilobalide (2)
bilobalide (BB, 1)
O
O
CHO
HO
- 2 H2O
C10
[O]
HO
HO
t-Bu
OH
OH
HO
t-Bu
O
O
H
H
10
H
Me
O
Me
O
O
O
H
A
t-Bu
C10
[O]
O
Me
Hi
Me
Hi
O
CO2H
dihydrate 3
O
C
B
O
O
Me
Me
O
O
O
O
HO
10
Hk
1
H
10
OH
1
des-hydroxybilobalide (2):
thermodynamic sink?
versus
C1
[O]
H
H
1: 7 oxidized carbons
3 oxidation states
C–O
C–C
D
O
t-Bu
H
H
O
O
O
O
O
O
O
X
CO2H
CO2H
'inner lactone'
buried
surface
buried hydroxyl
HO
O
CO2Bn
O
Figure 5. Structural analysis of BB and identification of the
topological problem with late stage oxidation.
H
t-Bu
O
CO2Bn
CHO
H
t-Bu
O
t-Bu
O
We also hypothesized that des-hydroxybilobalide (2)
might form spontaneously from its dihydrate—an aldehyde
triacid—and could be a useful thermodynamic sink to target
(Figure 3).85a Dihydrate 3 could then arise from hydration,
alkylation and subsequent global condensation of cyclopen-
tene 4. The hindered tertiary alcohol of 3, flanked by two qua-
ternary carbons, would be generated from the corresponding
alkene using a Mukaiyama hydration. We previously pro-
CO2H
conceptual intermediate 4
OMe
3-carbon
symmetric
dilactone
actual intermediate 5
unit
Figure 3. Previous syntheses of 1 relied on early carbon
framework synthesis, followed by oxidation state changes.
The work here uses mostly preoxidized building blocks.
In both cases, successful construction of the congested
BB core can be attributed to the early transition states of radi-
cal reactions that minimize steric repulsion. A more concise
synthesis might be achieved if radical reactivity were retained
but oxidation patterns of BB were incorporated from the out-
,
posed33 34 that this base-metal catalyzed radical hydration pro-
ceeds via an outer-sphere, metal-hydride atom transfer
(MHAT) elementary step, and reasoned that this mechanism
3
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OH
would tolerate the extreme steric hindrance of 4 (Figure 3),
irreversible
addition?
Br
CO2Bn
CO2Bn
CHO
Br
1
2
3
4
5
6
7
8
whereas an inner-sphere coordinative mechanism would fail.
Simplified cyclopentene 4 turned out to be merely conceptual;
the actual intermediate (5) ended up very different.
+
CH(OMe)2
t-Bu
CH(OMe)2
t-Bu
Br
retro-aldol
not possible
CO2Bn
CO2Bn
iso-8
Initial attempts to forge the key C–C bond between symmet-
ric anion of diester 6 and 7 via aldol reaction were met with
failure (Table 1). No product was observed upon addition of 7
to alkali enolates of 6 at cryogenic temperatures. Trace prod-
uct was observed under equilibrating, basic conditions (entry
3)35, but attempts to trap the product alcohol through alkyla-
tion, acylation or silylation were unsuccessful. These data
indicated that the reaction equilibrium lay in favor of 6, an
observation that was subsequently corroborated by the facile
retro-aldol reaction of 8 under basic conditions to give 6 and 7
(entry 5).
7
iso-9
actual
CO2Bn
expected
Br
Br
CO2Bn
Wittig
olefination
O
Br
CO2Bn
+
CH(OMe)2
CH(OMe)2
9
CH(OMe)2
CO2Bn
and
isomerization?
PPh3
CO2Bn
9
CO2Bn
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
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10
11
iso-9
proton migration or bromide migration?
=
13C
Br
CO2Bn
H
CO2Bn
Br
CO2Bn
O
via:
Br
O
PPh3
PPh3
(60%)
CO2Bn
Br
OH
CH(OMe)2
CH(OMe)2
CH(OMe)2
CH(OMe)2
CHO
H
aldol
retro-aldol
CO2Bn
CO2Bn
CO2Bn
CO2Bn
13C-9
E/Z mixture
+
t-Bu
CH(OMe)2
6
13C-11
13C-12
bromide migration
t-Bu
Br
CO2Bn
CO2Bn
Figure 6. 13C isotope labeling identifies bromide migration as
operating in preference to proton migration.
7
8 (not observed)
result
entry
conditions
Initial attempts at a Reformatsky reaction with Zn0 or In0
led only to the recovery of starting material or proto-
dehalogenation. After extensive screening, SmI2 proved to be
the ideal reductant,36,37 affording aldol product 8 in a 15:1 ratio
over diastereomer 13 (Figure 7, entry 1 and top HPLC trace).
The reaction was reliably scaled to 51 mmol in a single pass
using 1 L of a 0.1 M THF solution of SmI2 (MilliporeSigma).
This SmI2-mediated Reformatsky reaction was utilized for
early scale-up campaigns and route scouting, however an
asymmetric synthesis of (–)-BB necessitated the development
of an enantioselective Reformatsky reaction.
6 recovered
6 recovered
6 recovered
trace 8
1
2
3
LDA, THF -78 °C
Li, Na, or KHMDS, THF -78 °C
Cs2CO3
MgCl2, NEt3, TBSCl
treatment of 8 with Et3N
4
5
6
Table 1. Reversible and irreversible enolization conditions
favor starting materials over product.
Instead, we anticipated that vinyl anion addition to the
aldehyde would be irreversible (Figure 6) from adduct iso-8.
To test this hypothesis we sought vinyl bromide iso-9. Surpris-
ingly, Wittig reaction between bromo ylide 10 and β-ketoester
11 resulted in formation of allylic bromide 9, through what
first seemed to be olefin isomerization (Figure 6). The sym-
metry of the diester, however, prevented differentiation of H
versus Br migration, so a 13C-labelled analogue of 11 (13C-11)
was synthesized. Following olefination, the 13C labelled posi-
tion bore the bromide (13C-9), indicating migration of the
bromide from the ylide to the β-ketoester (13C-11→13C-12),
followed by olefination. The unexpected a-bromoester might
be used in a cryogenic Reformatsky reaction that could pro-
mote stabilization of the aldol intermediate through metal che-
lation between a strongly-bound Zn+2 or Sm+3 alkoxide and the
Early trials focused on induction of asymmetry through
the use of chiral ligands with SmI2. Although the work of
Proctor38 led us to test chiral diamines, amino alcohols and
cinchona alkaloids, addition of these ligands shut down SmI2
reactivity or led to unproductive protodehalogenation of 9.
Chiral auxiliaries were unsuccessful due to poor stability of
intermediates.
,
neighboring benzyl ester.36 37 Low temperature acidic quench
could then prevent retro-aldol.
4
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Page 5 of 21
Journal of the American Chemical Society
,
use of chiral ligands39 40 with Zn0 and Zn+2 provided a founda-
reactive at
, Reformatsky-like
OH CH(OMe)2
=
13C
Br
,
,
tion41 for experimentation,42 43 44 however an extensive litera-
ture search indicated a paucity of cases in which Reformatsky
reactions with aldehydes are rendered both enantio- and dia-
stereoselective45 using Et2Zn46 Most relevant methods involved
1
2
3
4
5
6
Br
CO2Bn
H
CO2Bn
t-Bu
BnO2C
CH(OMe)2
CO2Bn
13C-8
Et2Zn (3.0 equiv)
13C-9
,
primary a-haloesters,47 48 resulting in the formation of single
(–)-A (0.1 equiv)
THF, -78 °C
Br
OH
H
CH(OMe)2
Br
CHO
7
,
stereocenters. 49 50 This methodological gap provided an oppor-
CO2Bn
t-Bu
tunity to discover new reactivity.
t-Bu
7
8
9
CO2Bn
13C-13
To this end, a number of easily accessible chiral ligands
were screened in combination with Et2Zn in search of a reac-
tion manifold that would allow for construction of two stereo-
centers in a single step, and in an enantio- and diastereoselec-
tive manner. Optimization data is provided in Figure 7. Unfor-
tunately, many X-type ligands that are commonly employed in
Et2Zn-mediated Reformatsky reactions were unsuccessful in
our system, giving either protodehalogenated starting material
or complex product mixtures. The first positive results came
from the use of L-type ligands including nicotine, and sparte-
ine, and some cinchona alkaloids which generated product,
albeit in low dr and ee (entries 8-10). Fortunately, combination
of Et2Zn with BOX or PyBOX ligands led to product cleanly
and in high conversion. Each ligand resulted in varying dia-
stereomeric ratios, but often in moderate to excellent enanti-
oselectivities. Notably, temperature significantly influenced
background, racemic reaction rate: at -78 °C in the absence of
ligand, reaction mediated by Et2Zn alone was sluggish. The
racemic reaction did not compete with the ligand-catalyzed
pathway until temperatures reached -40 °C. Use of indabox
ligand (–)-A at -78 °C gave the best results with a dr of 2.3:1
in favor of 8, and an excellent ee of 94% in favor of the enan-
tiomer corresponding to nat-1. Other ligands favored the un-
desired diastereomer 13 but with comparable enantioselectivi-
ty. The reaction utilizing indabox ligand (–)-A could be ren-
dered catalytic (10 mol%) and was scalable to 51 mmol with
careful control of temperature.
From the pseudosymmetric enolate of 9, two modes of
reactivity could feasibly produce alcohols 8 and 13: 1) for-
mation of an allyl zinc reagent followed by aldehyde addition
via allyl-metal reactivity, or 2) via the anticipated Refor-
matsky-type zinc-enolate addition. 13C-labelled bromide 13C-9
differentiated these pathways in its reaction with diethylzinc
and aldehyde 7, leading to formation of secondary alcohol 13C-
8 and 13C-13 (2.3:1.0 dr) as single regioisomers, a result of
enolate—not allyl zinc—addition. Despite the low dr, success-
ful formation of this key carbon-carbon bond provided evi-
dence that such an enantioselective, diastereoselective Refor-
matsky aldol could be rendered catalytic in ligand.
Reformatsky or allyl zinc reactivity?
Entry
dr (8 : 13)
Deviation
% ee of 8
1
2
3
4
5
6
7
8
SmI2, no ligand
SmI2, sparteine
none
no ligand
-40 °C
15.0 : 1.0
15.0 : 1.0
2.3 : 1.0
no reaction
1.5 : 1.0
1.0 : 1.0
2.5 : 1.0
1.1 : 1.0
1.0 : 1.2
1.0 : 1.0
no reaction
1.0 : 2.5
2.0 : 1.0
1.0 : 7.0
1.0 : 3.0
1.3 : 1.0
1.0 : 1.0
1.3 : 1.0
1.3 : 1.0
2.3 : 1.0
2.3 : 1.0
—
0
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
94
—
70
—
85
<10
13
<5
—
53
54
58
13
70
83
82
77
94
94
-40 °C, no ligand
-100 °C
quinine, not (–)-A
sparteine, not (–)-A
nicotine, not (–)-A
proline, not (–)-A
C, not (–)-A
B: R = H , R1 = Bn
D: R = Ph, R1 = Ph
D: R = H, R1 = Bn
Et2O
toluene
MTBE
DCM
1.0 equiv (–)-A
(+)-A
9
10
11
12
13
14
15
16
17
18
19
20
21*
* opposite enantiomer of ligand gives opposite enantiomer of products
Ligands:
Me
Me
O
O
N
H
N
HO
N
N
N
N
Me
N
MeO
N
(–)-A
sparteine
quinine
nicotine
F3C
Me
N
Me
N
O
O
O
O
O
R1
R1
N
D
N
R1
R1
N
N
N
R2
R2
R2
R2
Bn
8
B
C
AU
ent-8
entry 1: SmI2, THF, -78°C
15:1 dr, racemic
ent-13
ent-13
13
13
8
entry 3: Et2Zn, 0.1 equiv. (–)-A
THF, -78°C
2.3:1 dr, 97:3 er
ent-8
Further studies are underway to understand if a vinylogous
diester is necessary for diastereoselectivity and high levels of
ee or if simpler secondary α-bromo esters can be used in this
general reaction manifold. To the best of our knowledge, this
represents the first example of a diastereo- and enantioselec-
tive Reformatsky reaction between a zinc enolate and an alde-
hyde.
Access to 8 set the stage for a 5-exo-trig Giese addition via
an sp2-hybridized radical, which we anticipated to be kinetical-
ly more reactive than the lower-energy ketyl radical (see Fig-
ure 4). Whereas ambient temperature cyclizations proved un-
successful, elevated temperatures employing standard condi-
tions (1.5 equiv Bu3SnH, 0.1 equiv AIBN, 85 °C) afforded the
cyclopentene as a single diastereomer. The reaction likely
0 min
1
2
3
Figure 7. Development of a catalytic asymmetric Reformatsky
reaction. 13C isotope labeling identifies alpha-attack operating
in preference to gamma-attack.
Use of alternative metals to effect the Reformatsky reac-
tion initially fell flat; Zn0 and In0 failed to generate product,
whereas Cr+3 or Cr+2/Ni+2 salts led to messy reactions that were
poorly diastereoselective. Diethylzinc (Et2Zn), however, re-
sulted in clean reactions when run at -40 °C, yielding an
equimolar mixture of syn- and anti-aldol diastereomers, 8 and
13 (entry 5). The undesired anti diastereomer could not be
carried forward to BB due to formation of unproductive dia-
stereomers in subsequent steps. Precedent for the successful
5
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Journal of the American Chemical Society
Page 6 of 21
proceeds through the transition state of intermediate 15 (Fig- from either Wieland (radical) or Criegee (cationic) fragmenta-
ure 8) to minimize A1,3 strain of the β,β-disubstituted enoate
and alleviate A1,2 strain (see 16) between the dimethyl acetal
and the benzyl ester. Attempts to install C10 oxidation at this
stage were also unsuccessful;51 instead, radical chain propaga-
tion by Sn-H atom transfer proved efficient. As an aid to bulk
preparation, the desired product could be crystallized from the
crude reaction mixture with hot pentanes, obviating the need
for large-scale chromatography. One representative run pro-
duced 8.1 grams of high purity 14 (>99% ee according to chi-
ral SFC analysis).
tion.57 Either way, we expected that a decrease in the concen-
tration of intermediate peroxide would minimize 19. A semi-
nal mechanistic study by Nishinaga et al. on the mechanism of
Drago’s cobalt salen-catalyzed hydration reported rate accel-
eration by the addition of triphenylphosphine (10-fold relative
to cobalt), which was interpreted to result from the reduction
of an intermediate cobalt-peroxide.58 The hydration of 14 cata-
lyzed by Co(acac)2, however, did not benefit from the addition
of triphenylphosphine (PPh3). Instead, no hydration occurred
until all PPh3 had been oxidized to triphenylphosphine oxide,
at which point the same poor reaction mixture resulted! As a
result, a series of cobalt, manganese and iron catalysts—
shown by Mukaiyama to catalyze
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
OH
H
HO
14 x-ray
CO2Bn
CO2Bn
H
see
t-Bu
Br
CH(OMe)2
Table
CH(OMe)2
t-Bu
CO2Bn
8
This work
CO2Bn
14
M
H
O2
entry
conditions
result
H
H
metal-mediated
hydrogen atom
transfer (MHAT)
Markovnikov
selectivity
1
2
3
4
5
6
7
(Bu3Sn)2, hν
t-Bu group loss
HO
(Me3Sn)2, AIBN, Δ
(Me3Sn)2, Et3B, O2
retro-aldol (6 + 7)
t-Bu
t-Bu
t-Bu
retro-aldol (6 + 7)
decomposition
enoate reduction
no reaction
Bu3SnH
AIBN
(38%)
Bu3SnH, AIBN, O2
≥1 equiv.
OsO4, Py
Et2O
MeO
O
O
SmI2, THF, -78 °C to rt
Bu3SnH, AIBN, PhMe, 65 °C
Bu3SnH, AIBN, PhMe, 85 °C
3 steps)
23 °C
MeO2CCOCl
i-Pr2NEt
HO
HO
t-Bu
O
60% (>20:1 dr)
HO
HO
t-Bu
Corey
HO
t-Bu
MeO
OMe
BnO2C
CO2Bn
H
HO
CO2Bn
t-Bu
BnO2C
H
HO
H
HO
vs.
CO2Bn
CO2Bn
10 mol% Mn(dpm)3, O2,
Ph(i-PrO)SiH2, PPh3,
H
H
H
CH(OMe)2
CO2Bn
OMe
H
H
MeO
t-Bu
CO2Bn
9
14
H
8
17
methylcyclohexane
50 °C
HO
CH(OMe)2
CH(OMe)2
15
16
t-Bu
t-Bu
CO2Bn
CO2Bn
Deviation
combination of Giese with
Reformatsky not successful
C10 oxygen could
not be installed here
[O]
H–SnBu3
Entry
18 : 19
Figure 8. Diastereoselective Giese addition.
none
t-BuOMe
THF
1
2
75 : 25
35 : 65
40 : 60
Formation of the triad of adjacent fully substituted carbon
atoms at C3 (t-butyl), C8 and C9 entailed Markovnikov hydra-
tion of 14 (Figure 9). In a prior synthesis of BB, this transfor-
mation proved challenging and required a multi-step sequence
including olefin dihydroxylation (OsO4, pyridine) and selec-
tive mono-deoxygenation (1. methyl oxalyl chloride, i-Pr2NEt;
2. AIBN, n-Bu3SnH). Notably, osmium tetroxide dihydroxyla-
tion tolerates extreme steric hindrance 52 due to its ‘outer-
sphere’ [3+2] cycloaddition mechanism,53 whereas coordina-
tive ‘inner-sphere’ reactions of hindered alkenes can suffer
very slow rates.54 Inspired by the steric tolerance of outer-
sphere elementary steps, we explored hydration under Drago–
3
4
CO2Bn
CH(OMe)2
HO
H
1,2-DCE
CH2Cl2
52 : 48
53 : 47
54 : 46
57 : 43
60 : 40
5
18
6
EtOH
EtOAc
PhMe
t-Bu
O
7
O
8
9
cyclohexane
PhCF3
66 : 34
67 : 33
69 : 31
50 : 50
NR
10
11
12
hexanes
HO
H
CO2Bn
i-PrOH/PhSiH3
19
13
PhSiH3 not PhSi(Oi-Pr)H2
Co(acac)2 not Mn(dpm)3
Fe(acac)3 no Mn(dpm)3
Mukaiyama conditions.34, These base-metal catalyzed reac-
CH(OMe)2
55
25 : 25 : 50
NR
14
15
O
tions have garnered significant attention due to their ‘mild’
conditions, Markovnikov selectivity and ability to tolerate
sterically demanding systems. Their extensive use across
unique, challenging chemotypes embodies what we call high
‘chemofidelity’: reliable reactivity, independent of molecular
context.56 Success here relies on the ‘outer-sphere’ nature of
the MHAT mechanism, since the most hindered carbon center
does not require a bonding interaction in the transition state,
but instead is converted directly to a carbon centered radical.
Unfortunately, the use of standard Mukaiyama hydration
conditions produced a near equal mixture of diastereomers and
unexpected byproduct 19, which lacked the tert-butyl group
(Figure 9). The low diastereoselectivity reflected capture of
CO2Bn
<15% conv
16
-20 °C
HO
H
HO
H
CO2Bn
CO2Bn
– Co+4
O
– t-Bu
19
O
CH(OMe)2
CO2Bn
O
CH(OMe)2
CO2Bn
LnCo
O
t-Bu
t-Bu
Figure 9. Effect of solvent, metal center and phosphine on
Mukaiyama hydration probed using Ph(i-PrO)SiH2.
the hydrofunctionalization of alkenes—were screened for per-
formance in the presence of PPh3. Mn(dpm)3 proved optimal
but sensitive: PPh3 allowed catalysis and completely sup-
pressed the formation of 19, yet its omission led to 19 exclu-
3
the open-shell carbon with O2, and the byproduct resulted
6
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Page 7 of 21
Journal of the American Chemical Society
sively. The equivalent reaction rate 14 in the presence of ab- Mukaiyama hydration, made possible by iso-propoxy-
1
2
3
4
5
6
7
8
sence of PPh3 suggested that it did not react with any kinetical-
ly relevant species in the Mn-catalyzed reaction. In contrast,
PPh3 shut down the productive reaction pathway of cobalt.
Additionally, a variety of Mn, Fe and Co β-diketonate com-
plexes showed no variation in diastereoselectivity, reflecting
the outer-sphere MHAT mechanism and capture of dioxygen
by a carbon radical.
This discouraging selectivity exposed the inherent limitation
of MHAT reactions: substrate dependent stereoselectivity in
almost all cases.59 Solvent screens were possible using cobalt
catalysis,60 but Mn and Fe catalysts typically required an alco-
holic solvent to function. Our lab found that these catalysts
required alcohol because the standard reductant, PhSiH3, per-
formed poorly compared to its mono-alcoholysis product, iso-
propoxy(phenyl)silane, a highly efficient reductant that func-
tions in numerous solvents.61 This reductant allowed an exten-
sive solvent and temperature screen, which to date, had not
been reported. It was not clear, however, that solvent choice
would increase the stereoselectivity of carbon radical capture
by O2.
(phenyl)silane.
Only a single substitution of the cyclopentane now remained
to reach subtarget 3 (Figure 3): stereoselective formation of
the C5-C1 bond to form the butyrolactone. Although acylation
of the C6 secondary alcohol was achieved with EDCI/ bromo-
acetic acid, cyclization of 22 to 24 under basic conditions was
completely outcompeted by E1cB elimination to 23. Attempts
to choreograph lactone formation earlier, prior to Mukaiyama
hydration, also proved unsuccessful and 25 yielded elimina-
tion product 26, instead of 27. A number of attempts were
made at direct alkylation of 14 by a two-carbon unit via Fra-
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
,
ter–Seebach alkylation63 64 with methyl bromoacetate. Howev-
er, treatment of the alkoxy ester 14 with two equivalents of
strong base still led to elimination, leaving the question of
stereoselectivity open-ended.
Actual
Expected
O
Br
O
CO2Bn
O
O
1
DBU
CO2Bn
H
H
CO2Bn
5
HO
t-Bu
CH(OMe)2
CO2Bn
THF, 22 °C
(84%)
HO
HO
CH(OMe)2
CH(OMe)2
To our surprise, solvent played a significant role in stereose-
lectivity and correlated roughly to Lewis basicity. Ethereal
solvent favored the undesired diastereomer 18, which lac-
tonized due to proximity of the southern benzyl ester. As sol-
vent polarity decreased, the reaction increasingly favored dia-
stereomer 17. Methylcyclohexane proved to be the optimal
solvent and afforded 17 as the major diastereomer, isolable in
67% yield. These data contrast a recent paper from the Woer-
pel group that demonstrated small effects of solvent on dr
(85:15→97:3), the benefits of which were mitigated by de-
creases in yield (60→29%). In this case, stereoselectivity was
explained by a chair versus twist boat, Fürst–Plattner-like
model and required a six-membered ring (Figure 10). Solvent-
based stereochemical reversal of five-membered ring hydra-
tion (14→17), however, is hard to explain with this model.
Instead, stereoselectivity might be explained by an internal
hydrogen bond between the C6 secondary alcohol and the C5
ester (Figure 10) that is strengthened in conformation 20. Non-
polar solvents like methylcyclohexane could increasingly fa-
vor this interaction, whereas hydrogen bond acceptors like
ethers could disrupt it.62
t-Bu
t-Bu
CO2Bn
CO2Bn
22
23
24
O
LHMDS
BrCH2CO2Me
THF, -78 °C
(81%)
RO
CO2Bn
CO2Bn
O
H
H
CO2Bn
CH(OMe)2
CH(OMe)2
CO2Bn
CH(OMe)2
CO2Bn
or
t-Bu
DBU, THF,
22 °C
(84%)
t-Bu
t-Bu
CO2Bn
27
26
14: R = H
25: R = C(O)CH2Br
only E1cB elimination observed
Br
O
O
Bu3SnH
AIBN
Mn(OAc)3
Cu(OAc)2
CO2Bn
R
O
CO2Bn
CO2Bn
HO
t-Bu
CH(OMe)2
OEt
HO
t-Bu
CH(OMe)2
HO
t-Bu
CH(OMe)2
CO2Bn
29
CO2Bn
28
CO2Bn
not observed
30
TMS-EBX,
TBAF
THF
-78 to 0 °C
NO2
O2N
CO2H
O2N
32
O
Woerpel [2018]
NO2
1. NaBH4, EtOH
3O2
O
CO2Bn
CO2Bn
O
O
O
twist
boat
Me
O
31
33
O
O
O
O
O
i-Bu
i-Bu
Me
chair
O
2. EDC, DMAP, 32
59% (2 steps)
HO
CH(OMe)2
O
O
HO
t-Bu
i-Bu
CH(OMe)2
Me
t-Bu
Me
Me
H
O
H
Me
CO2Bn
Me
O
Me
CO2Bn
Me
3O2
Figure 11. Attempts to install final two-carbon unit at C5.
To prevent E1cB elimination and probe substrate control
over the stereoselectivity of C–C bond formation, alcohol 14
was oxidized to β-ketoester 28 (IBX in DMSO). A variety of
electrophiles proved incapable of reacting with the extremely
hindered carbon C5. O-acylation occurred to deliver enol
bromoacetate 29, but radical cyclization to 30 did not. As a
result of the congested steric environment, we explored sp-
hybridized electrophiles, including Waser’s reagent (TMS-
EBX). 65 This isolable alkyne electrophile desilylates with
TBAF to deliver the highly reactive parent compound ethynyl
benziodoxolone (EBX), which has been isolated recently as its
acetonitrile complex.66 Together, its electrophilicity and small
size allowed bond formation at C5 where many other reagents
This work
20
H
21
H
MeO
MeO
MeO
t-Bu
H
H
H
H
MeO
O
BnO2C
BnO
t-Bu
17
18
6
6
5
5
O
OH
CO2Bn
BnO
O
H
Figure 10. Stereoselectivity via proposed chair versus twist-
boat transition state conformations, compared with intercon-
verting envelopes.
These effects highlight the inherent substrate dependent dia-
stereoselectivity in the Mukaiyama hydration, and emphasize
how solvent effects can perturb the conformational equilibria
of pyramidalized radicals. This represents the first example of
a solvent dependence dictating the stereoselectivity of the
7
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Journal of the American Chemical Society
Page 8 of 21
failed. To our frustration, however, the (R)-configuration 31
pears to possess a facile path to decomposition via oxetane
ring-opening.
predominated from cyclopentanone 28.67
1
2
3
4
5
6
7
8
9
To reverse this stereoselectivity, we pursued formation of
the BB bis-lactone earlier than planned (17→34→35, Figure
12), which we thought might direct the delivery of EBX to the
correct Re-face, at that point a convex surface, of the BB par-
tial ring system. To our surprise, attempted global deprotection
of the bis-ester acetal 17 with para-toluenesulfonic acid deliv-
ered an extremely unreactive oxetane acetal as a 1:3 mixture
of diastereomers 36 and 37, both resistant to epimerization.
In contrast, oxetanes 36 and 37, we thought, could easily
open via formation of a methyl oxocarbenium ion, which
could then either hydrolyze or demethylate. Its stability, how-
ever, might reflect a general feature of the bilobalide scaffold.
A similar, albeit internal, acetal was reported recently as an
intermediate in an approach towards the synthesis of bilobal-
ide and survived a strongly acidic, oxidizing step (CrO3,
H2SO4, acetone).73 Our attempts to deprotect or epimerize the
oxetane acetals 36 and 37 led to recovery of starting material,
and caused us to investigate their unusual persistence.
O
HO
HO
H
CO2Bn
CO2H
CO2H
-2 [Me]
-2 [Bn]
H
H
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
[O]
Steric congestion imparted by the multiple quaternary car-
bons and tert-butyl substituent could be considered the defin-
ing idiosyncrasy of the bilobalide ring system, and one which
might reasonably play a role in oxetane persistence. Such an
effect is well precedented in the synthesis and study of tetra-
hedranes, including tetra-tert-butyl tetrahedrane (t-Bu4T),
which despite its strain energy (Estrain ~ 130 kcal/mol) melts at
135 °C.74 This requirement of steric congestion was attributed
to the ‘corset effect’ whereby bond scission increases repul-
sive steric interaction in the transition state, leading to a high
energy barrier. It was not clear whether the unusual stability of
36 and 37, however, derived from substrate stabilization, tran-
sition state destabilization or product destabilization.
HO
t-Bu
HO
t-Bu
HO
CH(OMe)2
CHO
CHO
t-Bu
CO2Bn
17
CO2H
CO2H
34
PTSA
THF/H2O
1:3 36 : 37
dehydration
–2 H2O
HO
OH
CO2Bn
CO2Bn
O
Me
O
Me
O
Me
O
R1
t-Bu
O
H
R2
H
36: R1 = OMe, R2 = H
37: R1 = H, R2 = OMe
‘exo-oxetane’
37 (x-ray)
O
E+
35
HO
t-Bu
HO
BnO
OBn
t-Bu
t-Bu
CO2Me
t-Bu
CO2Me
ΔG
decay
O
+
H2O
+
MeOH
CO2Me
R1
O
Me
CO2Me
R3
t-Bu
t-Bu
t-Bu
t-Bu
R3
CHO
pyranose oxetane
SiO2 unstable
O
OH
R2
'corset effect'
t-Bu4-T
36calc: R1 = OMe, R2 = H
37calc: R1 = H, R2 = OMe
39a–d
O
O
DMSO
THF
36calc
H2O
36calc 37calc
HO
R3
CO2Bn
CO2Bn
36calc
37calc
37calc
a: t-Bu
b: i-Pr
c: Me
d: H
+2.2
-0.2
-2.0
-9.4
+2.1
-0.1
-2.4
-9.1
+2.0
-0.5
-2.3
-9.5
+2.1
-0.2
-2.4
-9.0
+5.6
+1.1
+1.2
-5.5
+5.2
+3.3
+1.0
-6.1
HO
OMe
t-Bu
O
C5H11 C4H7CO2H
H
TXA2
t1/2 = 32 s
@ pH 7.4, 37 °C
‘endo-oxetane’
36 (x-ray)
36
Figure 13. Calculated reaction free energies (kcal/mol) for the
hydrolysis of oxetane acetals at the ωB97X-D/6-311++G(d,p),
SMD//ωB97X-D/6-31+G(d,p) level of theory.
Figure 12. Unexpected trans-acetalization to an acid-stable
oxetane acetal driven by Maier’s hypothetical ‘corset’ effect,
where bulky groups destabilize the bond-lengthening decom-
position.
Density functional theory (DFT) calculations probed wheth-
er the unusual stability of the oxetane acetal was kinetic or
thermodynamic in origin. Whereas a significant barrier to pro-
tolytic ring-opening could not be found, hydrolysis was calcu-
lated to be thermodynamically uphill for 36 and 37 in DMSO,
THF and water (Figure 13), despite the expected release of
considerable ring strain. Replacement of the tert-butyl group
with progressively smaller alkyl substituents revealed its key
role in tipping the thermodynamic balance against the hydro-
lyzed free aldehyde 39a–d. With an isopropyl substituent, the
proximal ester group was free to assume a conformation that
avoided clash with the distal ester group. In contrast, a tert-
butyl substituent in the same position forced the two ester
moieties into close proximity with each other, creating repul-
sion that destabilized ring-opened form 39a (Figure 13).
These effects were analogous to Maier’s corset ef-
The oxetane acetal motif is rare and represented by only a
handful of examples in the literature. For example, the human
derived platelet aggregation factor thromboxane A2 (TXA2)68
contains a cyclic oxetane acetal, but degrades with a half-life
of 32 s at 37 °C in pH 7.4 Krebs media. Its instability compli-
cates isolation and requires storage in basic methanol or as the
alkali metal salt; the free acid decomposes within hours via
oxetane ring opening. This instability is not surprising given
the strain energy of oxetanes (Estrain ~ 25 kcal/mol),69 similar to
epoxides (27),69 cyclobutanes (26) and cyclopropanes (28), but
substantially less stable than tetrahydrofurans (6 kcal/mol).
The oxetane acetals of both TXA2 and dehydrated rhamnopy-
ranoses70 are formed under basic conditions as a result of acid
lability. In addition to TXA2, two other naturally-occurring
oxetane acetals have been isolated: oxetanocin A (a hemi-
aminal)71 and maoecrystal I;72 however neither compound ap-
fect: strain-release through small ring-opening was offset by
8
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Journal of the American Chemical Society
HO
HO
Figure 15. The alternative hypothesis of an intermediate oxo-
carbenium that persists in acidic media, but reverts to oxetane
CO2Me
CO2Me
CO2Me
CO2Me
1
2
3
4
5
6
7
8
9
10
in basic media, was excluded.
i-Pr
t-Bu
CHO
CHO
H
OH
O
OH
O
O
CO2Bn
CO2Bn
TMS-EBX,
TBAF
H
39a
39b
CO2Bn
CO2Bn
CO2Bn
+
THF,
H
t-Bu
-78 to -20°C
(53%, 2 steps
w/ DMP [O])
O
H
t-Bu
OCO2Bn
t-Bu
MeO
O
OMe
OMe
38: from 'exo' oxetane
42: not observed
43: not expected!
HO
O
CO2Bn
CO2Bn
CO2Bn
CO2Bn
Figure 14. Calculated structures of aldehydes 39a and 39b at
the ωB97X-D/6-31+G(d,p) level of theory.
IBX
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
DMSO, 22 °C
OMe
OMe
t-Bu
t-Bu
destabilizing repulsion between substituents. An alternative
hypothesis suggested by calculated Intrinsic Reaction Coordi-
nates (IRCs) involved formation of stable oxocarbenium in-
termediates 40 or 41, which might persist in acidic media but
revert to oxetane on basic work-up. This hypothesis was
probed by monitoring 36 in situ. In fact, the oxetane acetals
themselves were remarkably persistent, even in concentrated,
strong acid. As shown in Figure 15, both 36 and later interme-
diate 48 were stored in 1.5 M DCl (1:1 THF-d8/ D2O) for over
O
O
H
H
36
44
O
O
I
'TMS-EBX' THF,
TBAF -78 °C to -20°C
TMS
'TMS-EBX'
target cyclopentane
O
HO
CO2Bn
CO2Bn
CO2Bn
CO2Bn
SmI2
OMe
THF/H2O, 0 °C
(60%,3 steps)
1
OMe
t-Bu
t-Bu
24 h with no change in their H NMR spectra. The lack of
O
O
H
H
epimerization, oxocarbenium formation or hydrolysis indicat-
ed either a high kinetic stability or rapid internal return of a
5
45
NaBH4, MeOH
(52%, 2 steps)
thermodynamically less stable oxocarbenium 40 or 41.
a. Alternative hypothesis: stable oxocarbenium and return to oxetane
HO
• fully substituted
cyclopentane
(BB core)
HO
HO
CO2Bn
OBn
CO2Bn
OBn
CO2Bn
HO
H
CO2Bn
CO2Bn
O
or
O
OBn
t-Bu
t-Bu
• 7 steps
basic
workup
OMe
t-Bu
O
OMe
O
O
HO
MeO
OMe
H
t-Bu
• stereoselective
H
O
5 (x-ray)
36•H+
40
41
H
b. Pathway suggested by calculated Intrinsic Reaction Coordinates (IRCs)
46
‡
DNBA, EDCI,
DMAP
84% yield
O
O2N
O
CO2Bn
CO2Bn
O2N
OMe
t-Bu
47
c. Exclusion of persistant oxocarbenium by in situ monitoring
O
H
1.5 M DCl (aq.)
24 h, 22 °C
47 (x-ray)
Figure 16. endo-Oxetane 36 allows stereoselective alkynyla-
tion whereas exo-oxetane 37 rearranges. Hydride reductants
deliver the wrong stereoisomer but SmI2 accesses the fully
substituted cyclopentane 48, a total of 7 steps from commer-
cial materials.
36
1.5 M DCl (aq.)
0 h, 22 °C
1.5 M DCl (aq.)
24 h, 22 °C
The oxetane acetal motif proved more than an idiosyncrasy:
its deformation of the bilobalide ring system ultimately al-
lowed for completion of the synthesis, with one caveat. Only
endo-methoxy oxetane 36, the minor isomer (20% isolated,
using TsOH•H2O), could be advanced to bilobalide.
We had hoped that the oxetane would effectively shield the
enolate Si-face from attack to produce alkyne 42. Unfortunate-
ly, treatment of exo-methoxy oxetane 37 with IBX followed
by Waser’s reagent only afforded rearrangement product 43,
possibly derived from the desired, albeit unstable isomer 42.
Minor isomer 36, on the other hand, could be oxidized to 44
48
1.5 M DCl (aq.)
0 h, 22 °C
d. Conclusion: high barrier or rapid "internal return"
HO
HO
CO2Bn
CO2Bn
k1 ~ 0
CO2Bn
40
36•H+
CO2Bn
or
t-Bu
t-Bu
OMe
OMe
O
k-1 >> k1
O
H
H
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Journal of the American Chemical Society
Page 10 of 21
and alkynylated to 45 in good yield (≥61%) and as a single
diastereomer.
(9→8) was under development and (+)-17 was not readily
available.
1
2
3
4
5
6
7
8
a
Workflow to determine dr imparted by acid on each enantiomer of a rac-17
36
After IBX-mediated oxidation and TMS-EBX-mediated al-
kynylation, the synthesis required return to the alcohol oxida-
tion state. This task was complicated by the instability of β-
ketoester 45, which decomposed on silica or prolonged stand-
ing, and the tendency of the ketone to undergo reduction to 46
from the Si-face of C6 by hydridic reagents, as confirmed by
X-ray crystallography (47 X-ray). Inspired by a proposed di-
rected reduction in Evans’ synthesis of cytovaricin,75 we ex-
plored SmI2, which had been proposed to favor stereoisomers
that could accommodate a chelated organosamarium(III) in-
termediate. In this case, chelation to the Lewis basic oxetane
acetal might direct the reduction. In accordance with Evans’
model, stereoisomer 5 was formed with high (>20:1) selectivi-
ty (see 5, X-ray). This key sub-target (see Figure 3) contained
all the carbon-carbon bonds of 1 and required seemingly sim-
ple hydration/dehydration chemistry to complete the target, yet
derived from the minor diastereomer (36) of oxetane.
Attempts to reverse the stereoselectivity of oxetane acetal
formation (17→36/37) were initially unsuccessful. Neither
solvents, simple Brønsted acids or Lewis acids led to an ex-
cess of 36. However, we observed that certain chiral phos-
phoric acids afforded a modest dr of 1.7 : 1.0 in favor of 36.
Given the cost of these catalysts and the low selectivity, such a
solution seemed impractical. However, when working with
racemic material (i.e. rac-17), we observed by chiral super-
critical fluid chromatography (SFC) that each diastereomer
was enriched in opposing enantiomers: chiral phosphoric acid
(–)-A in a mixture of THF and H2O led to exo-oxetane 37 in
69:31 er and endo-oxetane 36 in 39:61 er. In other words, each
enantiomer of rac-17 reacted with catalyst (–)-A to give op-
posing diastereomeric ratios (see Figure 17): (–)-17 favored
exo-isomer 37 (29:21) and (+)-17 favored endo-isomer 36
(39:11).
These observations suggested that a parallel kinetic resolu-
tion had occurred during oxetane acetal formation. 76 This ef-
fect is consistent with enantio-differentiation of prochiral ace-
tals by chiral phosphoric acids reported by the List group.77
The same report noted that a partial parallel kinetic resolution
occurred with a substrate bearing a tertiary alcohol.
In collaboration with the Scripps Automated Synthesis Fa-
cility, we developed a two-dimensional (2D) liquid chroma-
tography (LC) / chiral SFC separation to directly probe effects
of solvent, temperature and catalyst structure on the efficiency
of PKR (Figure 17). Consequently, crude racemic reaction
mixtures divulged their selectivity profile (dr, er) without puri-
fication.78 Pure, internal ‘slices’ of each 1st dimension (LC)
peak were internally directed to a chiral SFC system that sepa-
rated the enantiomers of each diastereomer, quickly providing
the user with ratios of all four stereoisomers of interest. A
similar 2D method was developed during optimization of the
Reformatsky reaction.
1H NMR
or LC
37
dr obtained by
crude NMR or LC
no purification required
1.7 : 1
(36 : 37)
(–)-A
scalemic
36 + 37
crude
rac-17
THF/H2O
50 °C
pure slice
of 36 / 37
2D
analysis
36
37
9
3 representative datasets
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
directly
to SFC
36
37
+
LCMS
chromatogram
SFC traces
er
by 2D
39 : 61 (36(–) : 36(+)
)
(–), (+) = (dr)•(er)
normalized to 50→dr ent(–) & ent(+)
LCMS /
SFC
69 : 31 (37(–) : 37(+)
)
b
Parallel kinetic resolution of rac-17 by Brønsted acids
36 37
1
2
3
4
5
6
7
8
9
10
c
Diastereoselectivity imparted by Brønsted acids using pure (+)-17
36 : 37
from (+)-17
R
Entry
Conditions
PTSA, THF/H2O, rt
(R)-A, THF/H2O, 50 °C
(R)-A, PhMe, 23 °C
(R)-A, C6H12, 23 °C
(R)-A, DCM, 23 °C
(S)-B, C6H12, 23 °C
(S)-C, C6H12, 23 °C
(R)-C, THF/H2O, 50 °C
(S)-D, C6H12, 23 °C
(S)-E, C6H12, 23 °C
1
2
1.0 : 2.8
2.8 : 1.0
4.6 : 1.0
5.3 : 1.0
2.6 : 1.0
0.0 : 1.0
0.0 : 1.0
2.8 : 1.0
1.0 : 7.3
1.0 : 3.5
OH
O
O
O
P
3
4
R
5
6
A: R = 9-phenanthryl
B: R = 2,4,6-(C6H12)3Ph
C: R = 2,4,6-(i-Pr)3Ph
D: R = H
7
8
9
E: R = 3,5-(CF3)2Ph
10
Figure 17. Optimization of acetalization dr via parallel kinetic
resolution (PKR): workflow and data analysis (SI22).
The data in Figure 17 indicated that lower temperature, non-
aqueous solvent (cyclohexane) and phenanthryl substituents
on the chiral acid provided the highest selectivity (5.3 : 1.0)
for endo-isomer (+)-36 from (+)-17, whereas the exo-isomer (-
–)-37 was produced from (–)-17 with 1.0 : 3.2 selectivity. Dia-
stereoselectivity combined with product enantioenrichment
allowed prediction of dr using a one enantiomer of 17. This
singular antipode was available from the catalytic enantiose-
lective Reformatsky discovered in step two of the synthesis,
combined with purification by crystallization, which enriched
enantiomeric excess to >99% ee. In the event, (+)-17 was con-
verted with 5.3 : 1.0 selectivity to (+)-36 under optimal condi-
tions.
Multiplication of dr by er of each diastereomer derived from
rac-17 delivered the ratio of endo and exo diastereomers for
each enantiomer of 17. Reactions of enantiopure (+)-17 pro-
duced diastereoselectivities that corresponded well to those
derived from 2D analysis and deconvolution. This method
allowed us to work in parallel: we optimized the diastereose-
lectivity for 17→36 while the enantioselective Reformatsky
The alkyne subunit had been installed as a surrogate for an
acetate appendage, however its hydration and oxidation
10
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Journal of the American Chemical Society
proved non-trivial. We planned to oxidize the alkyne using A) led to a mechanistic morass that would consume over nine-
standard conditions (Figure 18).79 Unfortunately, only hydrob-
oration (BH3•THF) and iterative oxidation (1. Na2B2O3; 2.
TEMPO, PIDA) successfully generated the target lactone 48,
and this only in 23% yield. The yields severely diminished
when the reaction was run on scales larger than 0.1 mmol. An
alternative solution was inspired by a report by Julia that al-
kyne oxidation could be carried out directly using LiOOt-
Bu/LiHMDS by way of an intermediate lithium ynolate.80 Ap-
plication of these conditions led to recovered starting material
and a trace amount of the TMS-protected alkyne, which impli-
cated at least partial deprotonation of 5. We hypothesized that
the alkynyl lithium could not efficiently coordinate LiOOtBu,
whereas a more Lewis acidic substrate might. Ultimately, we
found that the alkynyl lithium intermediate reacted efficiently
with trimethylborate, B(OMe)3 to generate a transient alkynyl-
borate anion. Only m-CPBA effected oxidation to the lactone,
presumably by protonation of one methoxide ligand and cap-
ture of the trivalent boron by the peroxybenzoate,81 as suggest-
ed by early work on alkoxyboronates by Brown.82 Migration
of the alkyne into the adjacent O–O σ* orbital would generate
an intermediate boron ynolate, which could cyclize to lactone
48 via several pathways. Protonation with benzoic acid or
perbenzoic acid could generate a ketene or, if captured, a
mixed anhydride. Alternatively, the boron ynolate might be
stable until addition of aqueous NH4Cl. No byproducts were
isolated that provided insight into mechanistic details, but an
upcoming report from the Ohtawa lab seeks to shed light on
1
months of research.
2
3
4
5
6
7
8
9
Because oxetane 48 proved stable to Brønsted acid, a
dealkylating Lewis acid, BBr3 (>10 equivalents), was added to
48 to decompose both the methoxy acetal and the benzyl es-
ters. To our surprise, des-hydroxybilobalide was produced on
the first attempt, albeit in apparent low (ca. 30%) yield. This
result dispelled the uncertainty of whether the bilobalide di-
lactone acetal would serve as a thermodynamic sink to which
the acyclic carboxylates might funnel. Flush with success, the
single milligram of crude 2 was subjected to conditions that
might hydroxylate either lactone, with the hope that luck was
now on our side. Treatment of crude 2 with NaHMDS,
P(OEt)3, and O2 delivered a new product by TLC (with pro-
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1
longed heating for visualization, see below) and H NMR of
material recovered after workup showed unambiguous peaks
of bilobalide as the major product (see Figure 19a).84
a. enolate hydroxylation using crude 2 yielded bilobalide (1), but only once
O
O
O
O
O
O
O
O
1
c,d
H
Hh
H
BBr3
NaHMDS
CO2Bn
CO2Bn
OMe
O
f,g
O
O2
P(OEt)3
CH2Cl2
-78 °C
HO
HO
HO
H
Hj
10
Hi
tBu
O
t-Bu
O
t-Bu
O
H
O
O
48
2: crude material
natural BB
1
Hd Hc
Hg
Hf
t-Bu
Hj
Hh Hi
this question.83
O
HO
O
LiHMDS; B(OMe)3;
CO2Bn
CO2Bn
H
m-CPBA
H
CO2Bn
CO2Bn
OMe
THF, -78 to 22 °C
(55%)
OMe
crude synthetic BB
t-Bu
O
t-Bu
O
H
b. enolate hydroxylation using pure 2 yields a bilobalide isomer (49) reproducibly
H
O
O
O
5
48
H
O
O
O
O
Entry
Variations
Product
O
O
KHMDS
THF, -78°C
1
OH
1
H
H
H
1
2
3
LiHMDS, LiOOtBu
CpRuCl(bpy); PIDA
PPh3AuNTf2. m-CPBA
N.R.
<5%
+
O
O
O
H
Ph
HO
HO
t-Bu
HO
t-Bu
HO
t-Bu
H
H
H
10
H
10
O
O
O
decomposition
O
NSO2Ph
50
O
O
4
5
Cy2BH; Na2B2O8
<10% conversion
23%
O
2
1: bilobalide
(not observed)
49: neo-bilobalide
(exclusive product)
BH3 THF; Na2B2O8; TEMPO, PIDA
(pure material)
then 3M HCl
O
O
Figure 19a. BBr3 deprotection/ O2 oxidation of crude material
delivered bilobalide (1) on the first attempt and b. never again.
O
O
HO
CO2H
O
-2 Bn
-1 OMe
H
CO2H
H
H
-3 H2O
CO2Bn
CO2Bn
OMe
O
This result was extraordinary given the apparent inaccessi-
bility of the C10 protons. Our delight was fleeting: the reac-
tion has never been reproduced even to this day.
CHO
HO
t-Bu
HO
t-Bu
H
CO2H
t-Bu
O
O
H
O
Repeated attempts to effect the oxidation of 2 to 1 under the
original conditions by the same or different personnel either
returned starting material or produced the undesired (but ex-
pected) isomer, designated neo-bilobalide (49, Figure 19b), a
result of oxidizing the more accessible ‘outer’ lactone. Alt-
hough the adage ‘garbage-in, garbage-out’ proved partly true,
a reproducible procedure for the final oxidation may never
have been discovered without this crucial reconnaissance. The
process of invention now became archaeology: what had hap-
pened the first time?
48: synthetic
equivalent
3: retrosythetic
2: thermodynamic
tgt
sink?
Figure 18. Alkyne to lactone conversion via sp-
borylation/oxidation versus other tactics. The resulting syn-
thetic equivalent to 2 did not obviously resemble the retrosyn-
thetic target.
The question of whether the bilobalide ring system repre-
sented a thermodynamic sink and whether the unusually stable
oxetane would deprotect under suitable conditions now lay
before us. Impatience to answer this question and attempt the
final, deep oxidation (‘inner’ versus ‘outer,’ lactone D versus
11
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Journal of the American Chemical Society
Page 12 of 21
a. isomerization at cryogenic temperatures
observed by 1H NMR
quantitative conversion to translactonized products 51 or 52.
Similarly, we found that treatment of 2 at -78 °C with 1 equiv-
1
2
3
4
5
6
7
8
O
O
O
KHMDS
THF, -78°C
alent of the strong base KHMDS—the same base used for
attempted lactone hydroxylation—followed by 1 equivalent of
1M methanolic HCl quantitatively forms iso-2, which is un-
stable in acid, difficult to isolate and slowly reverts to 2. Since
rearrangement to the iso-bilobalide skeleton (iso-2) occurred
first, it clearly played a role in the irreproducible deep oxida-
O
O
O
O
1
H
1
1
H
H
1 equiv. HCl
-78 C
OH
O
H
O
O
O
O
8
CHO
HO
8
H
R
t-Bu
10
10
10
O
O
t-Bu
O
t-Bu
CDCl3
22 °C
O
DBU–H
O
O
R = OH (51)
R = H (52)
tion.
2
iso-2
a. Pd catalyzed hydrogenolysis is superior to BBr3 dealkylation
b. isomerization appears to change inner lactone accessibility
O
9
O
O
O
O
O
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
O
O
1
O
H2
Pd/C
H2→N2
3 M HCl
O
O
O
H
1
O
H
H
H
H
O
H
CO2Bn
CO2Bn
CO2H
CO2H
OMe
O
Hk
Me
Me
O
MeOH
22 °C
HO
t-Bu
80 °C
(90%)
8
H
10
HO
Hi
OMe
H
t-Bu
10
O
H
t-Bu
O
O
O
Me
O
t-Bu
O
H
H
O
2
O
48
persistent in 3M HCl
53
reactive in 3M HCl
2
O
inner lactone protons (Hi, Hk) inaccessible
b. Boron Lewis acids do not affect oxidation regioselectivity, but a silicon group helps
O
O
OH
Hi
O
O
1
O
O
H
H
O
OH
H
O
Me
Me
O
O
O
O
X
KHMDS
50
1
1
1
H
H
O
H
H
O
KHMDS
B
8
Hk
O
10
O
O
O
Me
X
O
t-Bu
O
O
BBr3,
B(OMe)3
EtB(OH)2
etc.
then
3M HCl
HO
t-Bu
HO
HO
t-Bu
H
H
H
10
10
H
O
O
t-Bu
O
O
iso-2
inner lactone protons (Hi, Hk) more accessible
O
O
O
Figure 20. Does rearrangement to the iso-bilobalide skeleton
allow access to the inner lactone protons Hi and Hk? a. isomer-
ization can be effected with strong base at low temperature or
weak base at room temperature (quantitative in 10 min.); b.
isomerization changes proton accessibility on 180° trajectories
of approach.
1: bilobalide
(not observed)
2
54
TBSOTf, 2,6-lutidine
(100%)
CH2Cl2
22 °C
O
O
O
H
O
H
O
O
O
O
KHMDS
THF, -78°C
1
1
OH
H
H
OTBS
+
O
O
O
O
H
The base lability of bilobalide provided the first evidence
for a complex mechanism of oxidation. Its instability has led
to many optimizations of extraction and purification from G.
biloba leaves.85 At pH 3, the trilactone fraction (ginkgolides
and bilobalide) appears stable, but at pH 6.5 mono-lactone
saponification of ginkgolide B occurs. Strongly basic (pH
10.4) aqueous washes of crude G. biloba extract removes BB
and ginkgolide C from the organic phase. However, acidifica-
tion of the aqueous phase with 1N HCl, followed by re-
extraction with EtOAc only leads to the recovery of gink-
golide C whereas BB decomposes and cannot be recovered.85a
BB has been found to irreversibly decompose at pH values
above 7,86 likely through irreversible lactone opening.87 To
date, these decomposition byproducts have not been character-
ized and the pathway for irreversible BB decomposition re-
mains unknown. This information is important for both isola-
tion and pharmacokinetics: BB degrades in rat heparinized
plasma (k = 0.5465 h-1) and even in pH 7.4 buffer (k = 0.1868
h-1).88
The base instability of bilobalide might derive from its rapid
rearrangement to iso-bilobalide. A similar rearrangement had
been observed by Weinges and co-workers upon treatment of
bilobalide with acetic anhydride and pyridine, which delivered
diacetoxy-iso-bilobalide.89 However, we found rearrangement
occurred simply upon treatment with base (Figure 20a). The
C8 tertiary alcohol is positioned within the Burgi–Dunitz an-
gle of lactone A, which is rendered more electrophilic by lone
pair hyperconjugation, and its translactonization pulls the tert-
butyl away from C10. For example, addition of 1 equivalent
of the weak base DBU to a CDCl3 solution of 1 or 2 effects
H
O
Ph
HO
HO
t-Bu
HO
t-Bu
H
H
10
10
H
t-Bu
O
O
O
NSO2Ph
50
O
O
O
55
1: bilobalide
49: neo-bilobalide
then 3M HCl
:
1
16
Figure 21a. Replacement of BBr3 in a global deprotection
increases yield but removes the boron impurities that may
have facilitated C10 hydroxylation; b. boron Lewis acids are
ineffectual, but an isomerized silyl ether provides a trace of 1.
The original, successful oxidation (2 to 1) must have oc-
curred via the base-induced iso-bilobalide rearrangement, giv-
en its facility in the presence of base. This hypothesis benefit-
ed from molecular models that indicated the iso-bilobalide
skeleton increased access to C10 of the ‘inner’ lactone (see
Figure 20b). However, if rearrangement alone were necessary,
any successful oxidation should deliver 1. In reality, any con-
ditions for α-hydroxylation including KHMDS/ Davis’ oxa-
ziridine (50) yielded exclusively the C1-oxidized isomer 49
via oxidation of the outer lactone.
Recall that global deprotection of 48 was carried out using
BBr3. This procedure reliably provided low yields of 2, but
when oxidation to 1 failed repeatedly (with crude or purified
material), we took the opportunity to improve the deprotec-
tion. To our surprise, 48 was completely stable to 3 M HCl at
80 °C for over 12 h, reflecting the remarkable ‘corset’ fortifi-
cation of the oxetane acetal (see Figure 15). However, hydro-
genolysis of the benzyl esters (Figure 21) significantly en-
hanced the reactivity of the oxetane: now 3 M HCl over 12 h
produced 2 cleanly, which supports the equilibrium ‘corset’
effect proposed above.
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Perhaps a boron impurity led to formation of 1 by capturing
carbonyl more ketone-like and result in acidification of the α-
protons.
We were unable to isolate a boric acid or boric ester deriva-
1
2
3
4
5
6
7
8
the opened isomer as 54, the borate of iso-2. Unfortunately,
addition of boron Lewis acids—BBr3, B(OMe)2, EtB(OH)2,
etc.—either prevented reaction or had no effect. However,
attempts to mimic the borylated iso-bilobalide intermediate
using a tert-butyl-dimethylsilyl (TBS) group (55) met with
modest success: for the first time in nine months, bilobalide
was observed in the crude 1H NMR, albeit as the minor isomer
tive of des-hydroxy-isobilobalide iso-2, consistent with the
absence of borylated g-hydroxy lactones in the literature, so
we turned to an electronically equivalent group. Whereas si-
lylated g-hydroxy lactone 55 would not benefit from the same
acidification of a borylated g-hydroxy lactone (cf. TBSOH
pKa = 15.36 versus B(OH)3 pKa = 9.24),94 a benzoyl group
(BzOH pKa = 4.20) might prove suitable. This derivative (56)
was synthesized from 2 with DMAP, EDCI and benzoic acid
(see Figure 22a/b). Drawing analogy to haloglycosides (see
above), we compared the bond lengths at the anomeric carbon
of 56 to other substituted g-hydroxy lactones (see Supporting
Information S38) and observed a similar trend. Electron with-
drawing groups adjacent to the ethereal oxygen of g-hydroxy
lactones cause a shortening of the anomeric C–O bond and a
lengthening of the lactone C–O bond (see Figure 22c). These
data derived from crystal structures, and DFT calculations
found similar trends (Figure 22d). Compared to 4-hydroxy-γ-
lactone (pKa = 22) and its tert-butyldimethylsilyl ether (21.8),
the borate (18.6), acetate (17.3) and benzoate (17.8) were sig-
nificantly more acidic and exhibited bond length changes con-
sistent with O lone pair delocalization into the σ* orbital of the
substituent; overall this decrease in the C–O π character might
acidify the C-10 α-protons.
(1:16) versus neo-bilobalide.
a
O
O
O
9
O
O
O
X
O
1
1
1
H
H
H
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
B
OTBS
O
O
X
R
O
O
O
O
O
O
H
H
H
10
10
10
t-Bu
O
t-Bu
O
t-Bu
O
O
O
O
55
54
56
H
H
H
σ*
H
σ*
H
σ*
H
H
H
H
O
O
O
O
O
O
π
π
π
X
R
O
R
O
O
Si
B
X
π
π
R
R
O
weak π_σ* hyperconjugation?
strong π_σ* hyperconjugation?
O
O
O
b
O
O
O
O
O
Ph
BzOH
EDCI
1
1
1
H
H
H
KHMDS
O
O
O
O
O
O
H
50
DMAP
CH2Cl2
(85%)
HO
HO
H
H
10
10
THF, -78°C HO
H
t-Bu
O
O
O
t-Bu
t-Bu
R
a
b
ΔGdepr
pKa
ΔpKa
then 3M HCl
(73% brsm by NMR)
O
O
O
H
H
1.35
1.36
1.36
1.36
1.37
1.44
1.44
1.43
1.41
1.41
27.5
27.2
23.2
21.6
22.2
22
0
2
56
1: bilobalide
H
OTBS
OB(OH)2
21.8
18.6
17.3
17.8
0.2
3.4
4.7
4.2
c
O
O
a
b
R
short CA–O, long CB–O
γ-alkoxy lactones (avg.)
H
long CA–O, short CB–O
γ-acyloxy lactones (avg.)
H
56 (inner lactone)
OCOMe
OCOPh
H
Figure 22d. Calculated bond lengths (ångströms) and free
energies of deprotonation (kcal/mol) by acetate anion at the
ωB97X-D/6-311++G(d,p),SMD(THF)//ωB97X-D/6-
31+G(d,p) level of theory.
H
H
H
H
H
H
A
A
A
O
O
O
B
B
B
O
O
O
Ph
R
O
O
O
alkyl
O
O
1.350 Å 1.460 Å
1.360 Å 1.433 Å
1.361 Å 1.422 Å
(X-ray avg.)
(X-ray)
(X-ray avg.)
In the event, subjection of benzoylated intermediate 56 to
KHMDS and Davis oxaziridine yielded, to our immense relief,
bilobalide (1) in 91:9 preference to its isomer, neo-bilobalide
(49). This acidification of α-protons through the anomeric
effect appeared to be unique and warranted further investiga-
tion. Acidification of the α-protons might open two pathways
to C10 oxidation: 1) direct deprotonation of the C10 position
(56•KD, Figure 23) or 2) indirect deprotonation via proton
exchange with the enolate of the more accessible outer lactone
A (56•KA).
Experiment effectively differentiated these pathways. First,
we found that deprotonation occurred very quickly: incubation
of 56 with base for 60, 30 or 10 minutes did not change con-
version to 1. Second, based on recent reports that Davis’ oxa-
ziridine (50) does not react with KHMDS at cryogenic tem-
peratures,95 we probed the possibility of enolate-lactone proton
exchange by addition of KHMDS to a mixture of 50 and 56.
The result was a 60:40 mixture of 1:49 (bilobalide : neo-
bilobalide), revealing that deprotonation occurs at C1 (lactone
A, 56•KA) followed by an enolate equilibration to C10 in the
anomerically-stabilized lactone D (56•KD). Whether this equi-
libration occurred by intermolecular or intramolecular proton
Figure 22a–c. Did hyperconjugation from an intermediate
borate ester acidify the inner lactone? A stable benzoate ester
causes selective oxidation of C10 to generate bilobalide (1).
A major difference between silicon and boron is the effect
of valence bonding on electron delocalization. In this case, a
trivalent boron (e.g. 54) might acidify the α-protons (i.e. stabi-
lize the corresponding enolate) via B–O π-bonding90 and delo-
calization of the lactone π-system into an adjacent C-O σ*
orbital, more so than the corresponding Si–O substituent (e.g.
55). Analogous delocalization is common in glycosides and
reflected by changes in bond lengths to the anomeric carbon.91
In the case of chloro-glycosides, the axial conformer has a
0.62 Å shorter C–O bond and a 0.105 Å longer C–Cl bond
than its equatorial isomer (see Supporting Information), as a
result of oxygen lone-pair delocalization into the antiperipla-
nar antibonding orbital of the adjacent carbon halogen bond..92
In the case of an α-hydroxy lactone, this lone pair delocaliza-
tion into the anomeric antibonding orbital could decrease the
lone pair donation into the adjacent carbonyl, making it more
electron deficient.93 In turn, this deficiency should make the
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Page 14 of 21
transfer was determined by varying the concentration of 56 at )-jiadifenolide and (–)-11-O-debenzoyltashironin caused hy-
perexcitation of neurons,22b consistent with a model of excita-
tory neurite outgrowth. This hypothesis was formulated based
on structural overlap between jiadifenolide (J, 239 Å3) and
picrotoxinin (PXN, 229 Å3),97 an overlap shared by bilobalide
(BB, 260 Å3) (see Figure 1).17a We have begun to annotate
similarities and differences between these small, dense ses-
quiterpenes.
1
2
3
4
5
6
7
8
a constant concentration of 50. A positive correlation between
1:49 and [56] revealed an intermolecular exchange: as the rate
of productive collisions between substrates increased, the con-
centration of inner enolate increased.
O
O
O
Ph
O
Ph
O
Ph
O
O
O
O
Me
Me
O
Me
O
Me
KHMDS
O
Me
O
Me
KHMDS
O
Me
O
D
A
Me
D
Me
A
O
H
O
H
O
H
H
H
K
O
O
9
H
O
inner outer
O
K
O
O
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
56•KA
56
56•KD
1/49
O
O
Ph
Ph
2
O
2:1
O
O
O
Me
Me
O
O
Me
Me
1.5
1
1.5:1
1.2:1
O
O
Me
Me
O
O
H
H
H
0.75:1
O
H
H
H
O
0.5
5
10
15
20
O
O
K
[56] (mM)
K
N(TMS)2
direct
deprotonation
equilibration from
kinetic enolate (C1):
inter- or intramolecular
conclusion
intermolecular equilibration
kinetic vs. thermodynamic acidity
Figure 23. Addition of 56 (varying concentrations) to a solu-
tion of KHMDS and 50 (constant concentration) indicates
intermolecular proton transfer and equilibration to a thermo-
dynamic, inner enolate from the kinetic, outer enolate.
Consistent with this data, deprotonation at C10 (inner lac-
tone) was calculated to be 4.3 kcal/mol more favorable than
deprotonation at C1 (outer lactone), meaning that the inner
lactone is about 3 pKa units more acidic. This acidification is
primarily due to the anomeric and inductive effects of the pen-
dant ester moiety, which is absent in the outer lactone ring.96
We suspect that the original, irreproducible oxidation occurred
from an iso-bilobalide borate (54, Figure 21a, 22a) similar to
56.
O
Ph
O
Figure 25. Effects on spontaneous GABAA receptor mediated
neurotransmission in acute brain slices. a. representative min-
iature inhibitory postsynaptic current (mIPSC) recordings in
central nucleus of the amygdala (CeA) neurons from naive rats
during control (left panel), during superfusion of either jia-
difenolide, (–)-bilobalide, or (+)-bilobalide (middle panel; all
at 1 µM), and in continued presence of either jiadifenolide, (-)-
bilobalide, or (+)-bilobalide, in presence of bicuculline
(30µM) confirming the GABAergic nature of the mIPSC are
depicted; b. scaled mIPSC averages from the traces depicted
in panel A before (black trace) and in presence of the indicated
compound (red trace) are shown; c. cumulative frequency
distributions for CeA neurons before (black) and in the pres-
ence of the indicated drug are shown; d. bars represent means
± S.E.M. from 8-9 individual CeA neurons summarizing the
effects of jiadifenolide, (–)-bilobalide, and (+)-bilobalide on
mIPSC frequencies, amplitudes, rise and decay times. Differ-
ences to baseline control conditions (indicated as dashed line)
were calculated using one-sample t-tests (*/** = P <
0.05/0.01).
O
Me
O
Me
O
Me
O
H
H
H
H
O
O
11.7 16.0
Figure 24. Calculated free energies (kcal/mol) of deprotona-
tion by acetate anion at the ωB97X-D/6-311++G(d,p),
SMD(THF)//ωB97X-D/6-31+G(d,p) level of theory.
All the data taken together indicates that the inner lactone is
inaccessible to base for direct deprotonation. Even in the un-
folded iso-bilobalide analogs (55) and (56), deprotonation
occurs initially from the outer lactone. Fortunately, acidifica-
tion by the benzoate allows the conjugate base of the outer
lactone—the kinetic acid—to intermolecularly deprotonate the
inner lactone—the thermodynamic acid. This funneling of
reactivity from an outer to an inner site allows deep oxidation
and completion of the target.
Effects on spontaneous GABAA receptor-mediated synap-
tic transmission. Our interest in the synthesis of bilobalide (1)
stemmed from our hypothesis that the so-called ‘neurotropic’
sesquiterpenes shared the functional properties of 1 and other
prototypical GABAA receptor antagonists,97 and therefore may
hold similar therapeutic potential. We recently showed that (–
PXN and BB exhibited strong antagonism in functional
and binding assays, whereas J reduced GABA-evoked currents
about 300X more weakly than PXN and did not displace
channel ligand [3H]-BOB; molecular dynamics suggested a
binding site for J closer to the intracellular pore.23 Neverthe-
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less, J caused robust hyperexcitation of neurons at 30 μM.22b
greater-than-additive products. Our group (R.M.D, M.A.B,
1
2
3
4
5
6
7
8
Whereas PXN and BB shared similar potencies, only PXN
was a strong convulsant (CF-1 mice; 3 mg/kg SC). BB was
only convulsant at high dose (100-300 mg/kg PO) and J was
nonconvulsive (up to 150 mg/kg IP).23 Nevertheless, intraperi-
toneally-dosed J reached total brain concentrations of 26 μM
(13 μM unbound) and attenuated amphetamine-induced hyper-
locomotion, similar to picrotoxin at nonconvulsive dose.23
M.O., R.A.S.) is interested in objectively measuring this pro-
cess and has turned to Böttcher’s complexity index (Cm)25 to
formalize intrinsic information content.104 Böttcher scores as-
sign information content (mcbits) to each non-hydrogen atom,
which are added to measure molecular complexity, a compara-
tive index given as Cm. Representative Böttcher indices corre-
spond, in part, to the results of Bertz’s graph theory-based
C(η,ε) complexity index,105 yet can be done with pencil and
paper, albeit slowly. The Supporting Information for this paper
includes a Python script106 (written by S. F.) that annotates
molecules with Böttcher scores using a SMILES string input
at a rate of ca. 132,000 compounds per minute, simplifying
use of this metric. Cm does not include any aspect of synthetic
difficulty, a changing and subjective measure.107 Its ease and
objectivity make Cm an appealing measurement of complexity.
Given the in vitro effects on GABAA receptor-mediated
currents by BB and J, we aimed to explore whether BB and J
would also affect GABAA receptor mediated synaptic trans-
mission in ex vivo brain slice preparations. The central nucleus
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
,
,
of the amygdala (CeA) is highly GABAergic,98 99 100 thus we
compared BB and J directly in acute CeA brain slices from
male Wister rats at low concentrations to study their effects on
pharmacologically isolated miniature inhibitory postsynaptic
current (mIPSC). The brevity of our synthesis also enabled
easy access to ent-BB (i.e. (+)-bilobalide), which served as a
negative control and also allowed us to exclude the possibility
that achiral metabolites of BB cause excitatory effects. Re-
markably, despite profound differences in potency at recombi-
nant GABAA receptors, BB and J exhibited very similar ef-
fects at 1 μM to reduce mIPSC frequencies and amplitudes
(Figures 25a-c, summarized in 25d).
In Figure 26, we view the three syntheses of bilobalide
through the lens of Cm changes. Each intermediate was as-
signed a Cm value and scores were averaged per atom to miti-
gate the effect of protecting groups, which can carry high in-
formation content without contributing to core complexity.
After normalizing the Cm scores, all three syntheses started
from similar complexity scores, but advanced to the target at
different rates and different sacrifices in complexity, reflecting
different step counts.
Importantly, these data underline a direct interaction of (–
)-jiadifenolide and (–)-bilobalide with native GABAA recep-
tors and GABAA receptor-mediated synaptic transmission,
further supported by lack of activity of the enantiomer (+)-
bilobalide. To the best of our knowledge, these are the first
studies to show the effects of bilobalide on action-potential
independent mIPSCs. The activity of safe GABAAR antago-
nists (–)-jiadifenolide and (–)-bilobalide may indicate transla-
tion potential, given the importance of GABAergic transmis-
sion in the CeA in fear processing, anxiety, negative emotions
We had qualitatively rationalized our choices in synthesis
design as efficient through minimization of redox reactions
and sought a Böttcher analysis to quantitatively interrogate
this idea. Summation of the raw scores of carbons that have an
oxygen atom in bilobalide (C2, 4, 6, 8, 10, 11, 12) revealed
three trends: 1) our work began from a higher total score,
largely due to initial incorporation of bislactone oxygens in
their final oxidation state, and therefore minimized the total
oxidative distance to product, significantly lowering necessary
operations; 2) the percentage of steps in a synthesis that result
in an overall decrease in complexity at the carbons of interest
was generally consistent across all three syntheses regardless
of step count: this work: 25%, Crimmins: 35%, Corey: 27%,
therefore future strategies might try to minimize these sacrifi-
cial maneuvers; 3) a direct correlation existed between step
count and complexity score changes at carbons bearing a het-
eroatom in a particular synthesis: this work = 10 (83% of 12
total steps), Crimmins = 16 (94% of 17 total steps, racemic),
and Corey = 20 (83% of 24 total steps, asymmetric). The most
telling metric of synthetic efficiency here is the total number
of times these oxidized carbon atoms change complexity
scores throughout the synthesis. Ideally, a carbon atom is in-
troduced in the synthesis at its final value (oxidation state,
hybridization, stereochemistry) or undergoes minimal trans-
formations to reach the target score.
,
and addictive disorders such as alcohol use disorder.98,
101 102
The most surprising finding here may be the large difference
in relative potencies of (–)-jiadifenolide and (–)-bilobalide
between recombinant GABAA receptors (α1-5β3γ2) (where jia-
difenolide is about 50X lower in potency than bilobalide) and
GABAA receptors in native brain tissue (where jiadifenolide is
equipotent to bilobalide). Such a discrepancy may indicate
either subtype-selective antagonism or potential mechanistic
differences in antagonism at submaximal GABA concentra-
tions (recombinant expression system) versus millimolar
GABA concentrations in the synaptic cleft required for mIP-
SCs. Furthermore, this discrepancy could also arise from using
isolated systems (HEK cells) versus brain slices representing a
more physiological preparation. In fact, in acute brain slices,
the neurons express a variety of different GABAA receptor
subunits and maintain their physiological intracellular signal-
ing and in part their connectivity.
Information content is easy to buy; information density is
not. The dense packaging of information in natural product
space is not unique to bilobalide; in fact, high information
density may describe much of natural product space. Bilobal-
ide uniquely fills a biologically-privileged and poorly-
explored region of chemical space characterized by unusually
information-dense structures. Our synthesis addresses the
challenge of encoding dense content through several features:
1) embedding oxygenation, largely of the correct oxidation
state, in starting materials; 2) development of a catalytic,
Conclusion. Chemical synthesis can be viewed as the encod-
ing of information content in a molecular structure. Secondary
metabolites tend to encode information differently than other
types of complex structures (dendrimers, dyes, drugs) and
include a greater fraction of sp3 hybridized atoms (Fsp3),103
high chiral atom content, high heteroatom content and low
aromaticity. Total synthesis encodes this complexity by join-
ing together low information content reactants, but aims for
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,
asymmetric Reformatsky reaction to merge these starting ma-
finity.108 109 Both strategies require close collaboration between
1
2
3
4
5
6
7
8
terials with high enantioselectivity; 3) use of radical reactions
that benefit from early transition states to establish sterically
congested centers, including 4) a solvent-controlled stereose-
lective Mukaiyama hydration that tolerates steric bulk via an
outer-sphere MHAT elementary step; 5) use of extreme steric
repulsion to leverage an unusually stable oxetane acetal in
stereoselective C–C bond formation via 6) the sterically small,
but highly electrophilic Waser’s reagent (TMS-EBX). Finally,
the hypothesis that the bilobalide scaffold represented a ther-
modynamic sink allowed a late-stage, global deprotection to
reach penultimate intermediate des-hydroxy-bilobalide (2).
Crucial reconnaissance revealed the potential to effect selec-
tive oxidation of the hindered, inner lactone, which was repro-
ducibly executed after significant interrogation and mecha-
nism-based experimentation.
chemistry, computation and biology, and for the fields to un-
derstand one another with clarity. The push to develop retro-
synthetic software superior to original iterations110 requires a
clear-eyed understanding of how synthesis is actually con-
ceived and executed. Here we have delineated in granular de-
tail the recursive feedback of experimental results into synthet-
ic design. In addition to the provision of 1) tools for GABAAR
biology, 2) code for molecular complexity, 3) methods for
catalytic enantioselective synthesis and 4) combined computa-
tional/ experimental insight, we hope this work provides a
clear illustration of the interplay between synthesis, mecha-
nism and computation in the current era.
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ACKNOWLEDGMENT
Support was provided by the National Institutes of Health (R35
GM122606, R.A.S.; P60 AA006420 and AA015566, M.R.),
Jiangsu Industrial Technology Research Institute (M.A.B.), the
Uehara Memorial Foundation (M. O.) and the National Science
Foundation (CHE-1955922, R.A.S; CHE-1764328, K.N.H). Cal-
culations were performed on the Hoffman2 cluster at the Univer-
sity of California, Los Angeles, and the Extreme Science and
Engineering Discovery Environment (XSEDE), supported by the
National Science Foundation (OCI-1053575). C.C.L. is grateful to
the Imperial College Bursary Scheme for funding. We thank Dr.
Jason S. Chen and Brittany Sanchez of the Scripps Automated
Synthesis Facility for help with separations and analysis. We
gratefully acknowledge Brandon Wright, Robert Lusi and Profes-
sor Richmond Sarpong for beta-testing the Böttcher score code,
and Professor Böttcher for helpful correspondence.
2
5
2
ASSOCIATED CONTENT
Supporting Information
4
2
The Supporting Information is available free of charge on the
ACS Publications website.
1
2
3
3
Experimental materials and methods; computational methods; X-
ray crystallography data; python script for Böttcher analysis.
3
2
2
1
2
AUTHOR INFORMATION
5
2
Corresponding Author
3
* rshenvi@scripps.edu
1
1
Present Addresses
20
16
†Graduate School of Pharmaceutical Sciences, Kitasato Universi-
ty, Tokyo, Japan
10
Author Contributions
Figure 26. Different uses of Böttcher scores to annotate
changes in complexity over the course of synthesis. a. Over-
shooting complexity is correlated to longer syntheses; b. num-
ber of complexity changes at each oxidized carbon is directly
related to number of total steps.
Information dense (high mcbits/ Å3) natural products can be
efficiently ‘encoded’ by incorporation of heteroatoms and
oxidation states into starting materials. In principle and in
practice,24 this approach allows a simple, combinatorial route
to dense, polyoxygenated libraries. When paired with a mac-
romolecular target, we imagine that oxygenation patterns of
bioactive natural products can be altered to uncover simpler
paths to oxygenated starting materials yet maintain target af-
‡These authors contributed equally.
REFERENCES
(1) Seeff, L.; Bonkovsky, H. L.; Navarro, V. J.; Wang, G.
Herbal products and the liver: A review of adverse effects and
mechanisms. Gastroenterology 2015, 148, 517.
(2) https://www.fda.gov/food/dietary-supplements
16
ACS Paragon Plus Environment

Page 17 of 21
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
(3) For modest, positive effects, see: (a) Le Bars, P. L.; Katz
(7) NTP Technical Report on the Toxicology and carcino-
genesis studies of Ginkgo bilobaextract (CAS No. 90045-36-6) in
F344/N rats and B6C3F1/N mice (Gavage studies). Natl. Toxicol.
Program Tech. Rep. Ser. 2013, 578, 1.
(8) Fernandez, F.; Morishida, W.; Zuniga, E.; Nguyen, J.;
Blank, M.; Malenka, R. C.; Garner, C. C. Pharmacotherapy for
cognitive impairment in a mouse model of Down syndrome. Na-
ture Neurosci. 2007, 10, 411.
(9) Kiewert, C.; Kumar, V.; Hildmann, O.; Rueda, M.; Hart-
mann, J.; Naik, R. S.; Klein, J. Role of GABAergic antagonism in
the neuroprotective effects of bilobalide. Brain Res. 2007, 1128,
70.
(10) Masiulis, S. Desai, R.; Uchanski, T.; Martin, I. S.;
Laverty, D.; Malinauskas, T.; Zivanov, J.; Pardon, E.; Kotecha,
A.; Steyaert, J.; Miller, K. W.; Aricescu, A. R. GABAA receptor
signaling mechanisms revealed by structural pharmacology. Na-
ture 2019, 565, 454.
(11) (a) Atzori, C.; Bruno, A.; Chichino, G.; Bombardelli, E.;
Scaglia, M.; Ghione, M. Activity of Bilobalide, a Sesquiterpene
from Ginkgo biloba, on Pneumocystis carinii. Antimicrob. Agents
Chemother. 1993, 37, 7, 1492; (b) Bombardelli, E.; Ghione, M.
Pharmaceutical Uses of Bilobalide and Derivatives Thereof and
Pharmaceutical Compositions Adapted for Such Use. U.S. Pat.
Appl. US005264216A, 1993; (c) Bombardelli, E.; Ghione, M.
Bilobalide Derivatives, Their Applications and Formulations Con-
taining Them. US005202313A, 1993; (d) Bombardelli, E.;
Ghione, M. Use of bilobalide and derivatives thereof for treating
an infection in an individual and pharmaceutical compositions
adapted for such use. EP0556051B1, 1997.
(12) (a) Pan, L.; Ren, L.; Chen, F.; Feng, Y.; Luo, Y. Anti-
feedant Activity of Ginkgo biloba Secondary Metabolites against
Hyphantria cunea Larvae: Mechanisms and Applications. PLoS
ONE 2016, 11, 1; (b) Pszczolkowski, M. A.; Durden, K.; Sellars,
S.; Cowell, B.; Brown, J. J. Effects of Ginkgo biloba Constituents
on Fruit-Infesting Behavior of Coding Moth (Cydia pomonella) in
Apples. J. Agric. Food Chem. 2011, 59, 10879-10886. (c) Ahn,
Y. J.; Kwon, M.; Park, H. M., Han, C. K. Potent Insecdticidal
Activity of Ginkgo biloba Derived Trilactone Terpenes. ACS
Symposium Series 1997, Ch. 7, 90; (d) Ozoe, Y. γ-aminobutyrate-
and glutamate-gated chloride channels as targets of insecticides.
Advances in Insect Physiology, 2013, 44, 211.
(13) Major, R. T. The Ginkgo, the most ancient living tree,
Science 1967, 157, 1270.
(14) Koch, E. Inhibition of platelet activating factor (PAF)-
induced aggregation of human thrombocytes by ginkgolides: con-
siderations on possible bleeding complications after oral intake of
Ginkgo biloba extracts. Phytomedicine 2005, 12, 10.
(15) Lee, D.; Su, H.; O’Dowed, D. K. GABA receptors Con-
taining Rdl Subunits Mediate Fast Inhibitory Synaptic Transmis-
sion in Drosophola Neurons. J. Neuroscience 2003, 23, 4625.
(16) (a) Constant, R. H. F.; Steichen, J. C.; Rocheleau, T. A.;
Aronstein, K.; Roush, R. T. A single-amino acid substitution in a
γ-aminobutyric acid subtype A receptor locus is associated with
cyclodiene insecticide resistance in Drosophila populations.
PNAS, 1993, 90, 1957; (b) Thompson, A. J.; McGonigle, I.; Duke,
R.; Johnston, G. A. R.; Lummis, S. C. R. A single amino acid
determines the toxicity of Ginkgo biloba extracts. FASEB J. 2012,
26, 1884.
M. M.; Bermann N.; Itil, T. M.; Freedman, A. M; Schatzberg, A.
F. A placebo-controlled, double-blind, randomized trial of an
extract of Ginkgo biloba for dementia, North American EGb
Study Group. JAMA 1997, 278, 1327; (b) Herrschaft, H.; Nacu,
A.; Likhachev, S.; Sholomov, I.; Hoerr, R.; Schlaefke, S. Ginkgo
biloba extract EGb 761® in dementia with neuropsychiatric fea-
tures: a randomised, placebo-controlled trial to confirm the effica-
cy and safety of a daily dose of 240 mg. J. Psychiatr. Res. 2012,
46, 716; (c) Tan, M. S.; Yu, J. T.; Tan, C. C.; Wang, H. F.; Meng,
X. F.; Wang, C.; Jiang, T.; Zhu, X.C.; Tan, L. Efficacy and ad-
verse effects of ginkgo biloba for cognitive impairment and de-
mentia: a systematic review and meta-analysis. J. Alzheimers Dis.
2015, 43, 589; (d) Yuan, Q.; Wang, C.W.; Shi, J.; Lin, Z. X. Ef-
fects of Ginkgo biloba on dementia: An overview of systematic
reviews. J. Ethnopharmacol. 2017, 195, 1. For no effect, see: (e)
Schneider, L.S.; DeKosky, S.T.; Farlow, M.R.; Tariot. P.N.; Ho-
err, R.; Kieser, M. A randomized, double-blind, placebo-
controlled trial of two doses of Ginkgo biloba extract in dementia
of the Alzheimer's type. Curr Alzheimer Res. 2005, 5, 541; (f)
Butler, M.; Nelson, V. A.; Davila, H.; Ratner, E.; Fink, H. A.;
Hemmy, L. S.; McCarten, J. R.; Barclay, T. R.; Brasure, M.;
Kane, R. L. Over-the-Counter Supplement Interventions to Pre-
vent Cognitive Decline, Mild Cognitive Impairment, and Clinical
Alzheimer-Type Dementia: A Systematic Review. Ann. Intern.
Med. 2018, 168, 52; (g) DeKosky, S. T.; Williamson, J. D.; Fitz-
patrick, A. L.; Kronmal, R. A.; Ives, D. G.; Saxton, J. A.; Lopez,
O. L.; Burke, G.; Carlson, M. C.; Fried, L. P.; Kuller, L. H.; Rob-
bins, J. A.; Tracy, R. P.; Woolard, N. F.; Dunn, L.; Snitz, B. E.;
Nahin, R. L.; Furberg, C. D. Ginkgo Evaluation of Memory
(GEM) Study Investigators, Ginkgo biloba for prevention of de-
mentia: a randomized controlled trial. JAMA 2008, 300, 2253; (h)
Vellas, B.; Coley, N.; Ousset, P. J.; Berrut, G.; Dartigues, J. F.;
Dubois, B.; Grandjean, H.; Pasquier, F.; Piette, F.; Robert, P.;
Touchon, J.; Garnier, Mathiex-Fortunet, H.; Andrieu, S.; GuidAge
Study Group. Long-term use of standardised Ginkgo biloba ex-
tract for the prevention of Alzheimer's disease (GuidAge): a ran-
domised placebo-controlled trial. Lancet Neurol. 2012, 10, 851;
(i) Charemboon, T.; Jaisin, K. Ginkgo biloba for prevention of
dementia: a systematic review and meta-analysis. J. Med. Assoc.
Thai. 2015, 5, 508; (j) Laws, K. R.; Sweetnam, H.; Kondel, T. K.
Is Ginkgo biloba a cognitive enhancer in healthy individuals? A
meta-analysis. Hum Psychopharmacol. 2012, 6, 527; (k) For a
summary, see: Nguyen, T.; Alzahrani, T. Ginkgo Biloba. [Updat-
ed 2019 Oct 3]. In: StatPearls [Internet]. Treasure Island (FL):
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
StatPearls
Publishing;
2019
Jan-. Available
from:
https://www.ncbi.nlm.nih.gov/books/NBK541024/
(4) Mei, N.; Guo, X.; Ren, Z.; Kobayashi, D.; Wada, K.;
Guo, L. Review of Ginkgo biloba-induced toxicity, from experi-
mental studies to human case reports. J. Environ. Sci. Health Part
C 2017, 35, 1.
(5) Clarke, T. C.; Black, L. I.; Stussman, B. J.; Barnes, P. M.;
Nahin, R. L. Trends in the use of complementary health ap-
proaches among adults: United States, 2002–2012. National
health statistics reports 2015, no. 79. Hyattsville, MD: National
Center for Health Statistics.
(6) Madgula V. L.; Avula, B.; Yu, Y. B.; Wang, Y. H.;
Tchantchou, F.; Fisher, S.; Luo, Y.; Khan, I. A.; Khan, S. I. Intes-
tinal and blood-brain barrier permeability of ginkgolides and bi-
lobalide: in vitro and in vivo approaches. Planta Med. 2010, 76,
599.
(17) (a) Huang, S. H., Duke, R. K., Chebib, M., Sasaki, K.,
Wada, K., and Johnston, G. A. Bilobalide, a sesquiterpene tri-
lactone from Ginkgo biloba, is an antagonist at recombinant
17
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 18 of 21
1
2
3
4
5
6
7
8
9
α1β2γ2L GABAA receptors. Eur. J. Pharmacol. 2003, 464, 1.; (b)
(31) Baker, M. A.; Demoret, R. M.; Ohtawa, M.; Shenvi, R.
A. Concise asymmetric synthesis of (–)-bilobalide. Nature 2019,
575, 643.
Ng, C. C.; Duke, R. K.; Hinton, T.; Johnston G. A. R. GABAA
receptor cysteinyl mutants and the ginkgo terpenoid lactones bi-
lobalide and ginkgolides. Eur. J. Pharmacol. 2016, 777, 136.
(18) Kalueff, A. V. Mapping convulsants’ binding to the
GABA-A receptor chloride ionophore: A proposed model for
channel binding sites Neurochem. Int. 2007, 57, 61.
(19) A note on nomenclature: residues in the transmembrane
domain (TMD)-spanning α-helices (see Figure 1) are numbered
from 0’, a conserved charged residue near the cytoplasm. Nega-
tive numbered residues (e.g. -2’) are farther from the extracellular
domain and positive numbered residues are closer. See Ref. 16b.
(20) Ng, C. C.; Duke, R. K.; Hinton, T.; Johnston, G. A. R.
Effects of Bilobalide, Ginkgolide B and Picrotoxinin on GABAA
receptor Modulation by Structurally Diverse Positive Modulators.
Eur. J. Pharmacol. 2017, 806, 83.
(21) (a) Jorgensen, J. L. Aldrin and Dieldrin: A Review of
Research on Their Production, Environmental Deposition and
Fate, Bioaccumulation, Toxicology, and Epidemiology in the
United States. Environ. Health Perspect. 2001, 109, 113. (b)
Agency for Toxic Substances and Disease Registry (ATSDR).
2002. Toxicological profile for Aldrin/Dieldrin. Atlanta, GA: U.S.
Department of Health and Human Services, Public Health Ser-
vice. (c) Soto, A. R.; Deichman W.B.; Major Metabolism and
Acute Toxicity of Aldrin, Dieldrin, and Endrin. Environmental
Research 1967, 1, 307.
(22) (a) Lu, H. H.; Martinez, M. D.; Shenvi, R. A. An eight-
step- gram-scale synthesis of (–)-jiadifenolide. Nature Chem.
2015, 7, 604; (b) Ohtawa, M.; Krambis, M. J.; Cerne, R.;
Schkeryantz, J. M.; Witkin, J. M., Shenvi, R. A. Synthesis of (–)-
11-O-Debenzoyltashironin: Neurotrophic Sesquiterpenes Cause
Hyperexcitation. J. Am. Chem. Soc. 2017, 139, 9637.
(23) Witkin et. al. Pharmacological characterization of the
neurotrophic sesquiterpenejiadifenolide reveals a non-convulsant
signature and potential forprogression in neurodegenerative dis-
ease studies. Biochem. Pharmacol. 2018, 155, 61.
(24) Huffman, B. J.; Chen, S.; Schwarz, J. L; Plata, R. E.;
Chin, E.; Houk, K. N.; Lairson, L. L.; Ryan A. Shenvi Electronic
Complementarity Permits Hindered Butenolide Heterodimeriza-
tion and Discovery of Novel cGAS/STING Pathway Antagonists,
Nature Chem. 2020, 12, 310.
(25) Böttcher, T. An Additive Definition of Molecular Com-
plexity. J. Chem. Inf. Model. 2016, 56, 462.
(26) Herzon, S. B. Emergent Properties of Natural Products.
SynLett 2018, 29, 1823.
(32) Schuster, D. I.; Lem, G.; Kaprinidis, N. A. New Insights
into an old Mechanism: [2+2] Photocycloadditions of Enones to
Alkenes. Chem. Rev. 1993, 93, 3.
(33) (a) Iwasaki, K.; Wan, K. K.; Oppedisano, A.; Crossley,
S. W. M.; Shenvi R. A. Simple, Chemoselective Hydrogenation
with Thermodynamic Stereocontrol. J. Am. Chem. Soc. 2014, 136,
1303; b) Crossley, S. W. M.; Barabé, F.; Shenvi, R. A. Simple,
Chemoselective, Catalytic Olefin Isomerization, J. Am. Chem.
Soc. 2014, 136, 16788.
(34) Crossley, S. W. M.; Martinez, R. M.; Obradors, C.;
Shenvi, R. A. Mn, Fe, and Co-Catalyzed Radical Hydrofunction-
alizations of Olefins, Chem. Rev. 2016, 116, 8912.
(35) Evans, D. A.; Tedrow, J. S.; Shaw, J. T.; Downey, C.
W. Diastereoselective Magnesium Halide-Catalyzed anti-Aldol
Reactions of Chiral N-Acyloxazolidinones. J. Am. Chem. Soc.
2002, 124, 392.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(36) Nicolaou, K. C.; Ellery, S. P.; Chen, J. S. Samarium
Diiodide-Mediated Reactions in Total Synthesis. Angew. Chem.
Int. Ed. 2009, 48, 39, 7140.
(37) Thankachan, A. P.; Asha, S.; Singhu, K. S.; Anilkumar,
G. An overview of Zn-catalyzed enantioselective aldol type C-C
bond formation. RSC Adv. 2015, 5, 62179.
(38) Kern, N.; Plesniak, M. P.; McDouall, J. J. W.; Procter,
D. J. Enantioselective cyclizations and cyclization cascades of
samarium ketyl radicals. Nature Chem. 2017, 9, 1198.
(39) Ribeiro, C. M. R.; Cordeiro de Farias, F. M. Chiral Lig-
ands in the Asymmetric Reformatsky Reaction. Mini-Reviews in
Organic Chemistry. 2006, 3, 1.
(40) Ojida, A.; Yamano, T.; Taya, N.; Tasaka, A. Highly
Enantioselective Reformatsky Reaction of Ketones: Chelation-
Assisted Enantioface Discrimination. Org. Lett. 2002, 4, 18, 3051.
(41) Kloetzing, R. J.; Thaler, T.; Knochel, P. An Improved
Asymmetric Reformatsky Reaction Mediated by (–)-N,N-
Dimethylaminoisoborneol. Org. Lett. 2006, 8, 1125.
(42) Wolf, C.; Moskowitz, M. Bisoxazolidine-Catalyzed
Enantioselective Reformatsky Reaction. J. Org. Chem. 2011, 76,
6372.
(43) Fernández-Ibáñez, M. A.; Maciá, B.; Minnaard, A. J.;
Feringa, B. L. Catalytic Enantioselective Reformatsky Reaction
with Aldehydes. Angew. Chem. Int. Ed. 2008, 47, 1317.
(44) Lin, N.; Chen, M-M.; Luo, R-S.; Deng, T-Q.; Lu, G.
Indolinylmethanol catalyzed enantioselective Reformatsky reac-
tion with ketones. Tetrahedron: Asymm. 2010, 21, 2816.
(45) Cozzi, P. G.; Mignogna, A.; Zoli, L. Catalytic enanti-
oselective Reformatsky reactions. Pure Appl. Chem. 2008, 80,
891.
(46) Kanai, K.; Wakabayashi, H.; Honda, T. Rhodium-
Catalyzed Reformatsky-Type Reaction. Org. Lett. 2000, 2, 2549.
(47) Li, Y.; He, B. Asymmetric Reformatsky Reaction of
Aldehydes Catalyzed by Novel b-amino Alcohols and Zinc
Complexes. Synth. Commun. 2014, 44, 13, 1938.
(27) Weir, C. J.; Ling, A. T. Y.; Belelli, D.; Wildsmith, J. A.
W.; Peters, J. A.; Lambert, J. J. The interaction of anaesthetic
steroids with recombinant glycine and GABAA receptors. Br. J.
Anaesth. 2004, 92, 704.
(28) (a) Corey, E. J.; Guo, S. Total synthesis of a C15 gink-
golide, (±)-bilobalide. J. Am. Chem. Soc. 1987, 109, 7534. (b)
Corey, E. J.; Su, W. Enantioselective Total Synthesis of Bilobal-
ide, A C15 Ginkgolide. Tetrahedron Lett. 1988, 29, 3423.
(29) Crimmins, M. T.; Jung, D. K.; Gray, J. L. Synthetic
Studies on the Ginkgolides: Total Synthesis of (±)-Bilobalide. J.
Am. Chem. Soc. 1993, 115, 3155.
(48) Fornalczyk, M.; Singh, K.; Stuart, A. M. Enantioselec-
tive Reformatsky reaction of ethyl iododifluoroacetate with ke-
tones. Org. Biomol. Chem. 2012, 10, 3332.
(30) Normalization by dividing by atom count mitigates the
influence of protecting groups which often encode large amounts
of information that is irrelevant to building target complexity.
18
ACS Paragon Plus Environment

Page 19 of 21
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
mote Asymmetric Induction in the Amino Acid Series. J. Am.
(49) Fernández-Ibáñez, M. A.; Maciá, B.; Alonso, D. A.;
Pastor, I. M. Recent Advances in the Catalytic Enantioselective
Reformatsky Reaction. Eur. J. Chem. 2013, 7028.
(50) Choppin, S.; Ferreiro-Medeiros, L.; Barbarotto, M.;
Colobert, F. Recent advances in the diastereoselective Refor-
matsky-type reaction. Chem. Soc. Rev. 2013, 42, 937.
(51) Li, F.; Castle, S. L. Synthesis of the Acutumine Spiro-
cycle via a Radical−Polar Crossover Reaction. Org. Lett. 2007, 9,
4033.
Chem. Soc. 1996, 118, 2507; (c) Pedrosa, R.; Andrés, C.; Duque-
Soladana, J. P.; Mendiguchía, P. A Novel Case of Diastereoselec-
tion in 5-exo Radical Cyclization Promoted by Hydrogen Bonding
Eur. J. Org. Chem. 2000, 3727; (d) Guindon, Y.; Guérin, B.;
Landry, S.R., Intramolecular Aminyl and Iminyl Radical Addi-
tions to α,β-Unsaturated Esters. Diastereoselective Tandem Cy-
clofunctionalization and Hydrogen Transfer Reactions. Org. Lett.
2001, 3, 2293.
(63) Heathcock, C.H.; Kath, J.C; Ruggeri, R. B. Daphniphyl-
lum Alkaloids. 16. Total Synthesis of (+)-Codaphniphylline J.
Org. Chem. 1995, 60, 1120.
(64) Xue, F.; Gu, W.; Silverman, R.B. Concise Route to the
Chiral Pyrrolidine Core of Selective Inhibitors of Neuronal Nitric
Oxide. Org. Lett. 2009, 11, 5194.
(65) González, D. F.; Brand, J. P.; Waser, J. Ethynyl-1,2-
benziodoxol-3(1H)-one (EBX): An Exceptional Reagent for the
Ethynylation of Keto, Cyano, and Nitro Esters. Chem. Eur. J.
2010¸16, 9457.
(66) Yudasaka, M.; Shimbo, D.; Maruyama, T.; Tada, N.;
Itoh, A. Synthesis, Characterization, and Reactivity of an Ethynyl
Benziodoxolone (EBX)–Acetonitrile Complex. Org. Lett. 2019,
21, 1098.
(67) 3,5-dinitrobenzoate 33 was isolated and characterized,
but it decomposed via facile benzoate hydrolysis during multiple
crystallization attempts. The bis-(ethyl ester) corresponding to 33
was much more stable and easily crystallized (see Supporting
Information).
(68) (a) Bhagwat, S. S., Hamann, P. R., Still, W. C. Synthe-
sis of Thromboxane A2. J. Am. Chem. Soc. 1985, 107, 6372; (b)
for a beautiful structural modification to impart acid stability, see:
Jing, C.; Mallah, S.; Kriemen, E.; Bennett, S. H.; Fasano, V.;
Lennox, A. J. J.; Hers, I.; Aggarwal, V. K. Synthesis, Stability,
and Biological Studies of Fluorinated Analogues of Thromboxane
A2 ACS Cent. Sci. 2020, 6, 995.
(69) Bull, J. A.; Croft, R. A.; Davis, O. A.; Doran, R.; Mor-
gan, K. F. Oxetanes: Recent Advances in Synthesis, Reactivity,
and Medicinal Chemistry. Chem. Rev. 2016, 116, 12150.
(70) Wu, E., Kong, F., Synthesis and Conformational Analy-
sis of Substituted 2,6-dioxabicyclo[3.3.1]heptanes: 1,3-anhydro-
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(52) Va, P.; Campbell, E. L.; Robertson, W. M.; Boger, D. L.
Total Synthesis and Evaluation of a Key Series of C5-Substituted
Vinblastine Derivatives. J. Am. Chem. Soc. 2010, 132, 8489.
(53) Schröder, M. Osmium Tetraoxide Cis Hydroxylation of
Unsaturated Substrates. Chem. Rev. 1980, 80, 187.
(54) (a) Abelman, M. M.; Oh, T.; Overman, L. E. Intramo-
lecular Alkene Arylations for Rapid Assembly of Polycyclic Sys-
tems Containing Quaternary Centers. A New Synthesis of Spi-
rooxindoles and Other Fused and Bridged Ring Systems. J. Org.
Chem. 1987, 52, 4130; (b) Ludlow, B. S.; Danielson, J.; Somfai,
P. Comino Carbopalladation-Carbonylation: Investigation Sub-
strate Scope. Adv. Synth. Catal. 2012, 354, 206; (c) Liu, P.; Seo, J.
H.; Weinreb, S. M. Total Synthesis of the Polycyclic Fungal Me-
tabolite (±)-Communesn F. Angew. Chem. Int. Ed. 2010, 49,
2000.
(55) (a) Zombeck, A.; Hamilton, D. E.; Drago, R. S. Novel
Catalytic Oxidations of Terminal Olefins by Cobalt(II)-Schiff
Base Complexes. J. Am. Chem. Soc. 1982, 104, 6782. (b) Mukai-
yama, T.; Isayama, S.; Inoki, S.; Kato, K.; Yamada, T.; Takai, T.
Oxidation-reduction hydration of olefins with molecular oxygen
and 2-propanol catalyzed by bis(acetylacetonato)cobalt(II). Chem.
Lett. 1989, 18, 449.
(56) Green, S. A.; Crossley, S. W. M.; Matos, J. L. M.; Vás-
quez-Céspedes, S.; Shevick, S.; Shenvi, R. A. The High Chemofi-
delity of Metal-Catalyzed Hydrogen Atom Transfer. Acc. Chem.
Res. 2018, 51, 2628.
(57) Yaremenko, I. A.; Vil’, V. A.; Demchuk, D. V.; Ter-
ent’ev, A. O. Rearrangements of organic peroxides and related
processes. Beilstein J. Org. Chem. 2016, 12, 1647.
(58) Nishinaga, A.; Yamada, T.; Fujisawa, H.; Ishizaki, K.;
Ihara, H.; Matsuura, T. Catalysis of Cobalt-Schiff Base Complex-
es In Oxygenation of Alkenes: On the Mechanism of Ketoniza-
tion. J. Mol. Catal. 1988, 48, 249.
2,4-di-O-benzyl-
and
-2,4-di-O-(p-bromobenzyl)-β-L-
rhamnopyranose, Carbohydrate Res. 1987, 161, 235.
(71) Shimada, N.; Hasegawa, S.; Harada, T.; Tomisawa, T.;
Fujii, A.; Takita, T. Oxetanocin, a Novel Nucleoside from Bacte-
ria J. Antibiotics 1986, 39, 1623.
(72) Han, Q.; Zhang, J.; Lu, Y.; Wu, Y.; Zheng, Q.; Sun, H.
A Novel Cytotoxic Oxetane ent-Kauranoid from Isodon japoni-
cus. Planta Med. 2004, 70, 581.
(73) Emsermann, J.; Opatz, T.; Photochemical Approaches
to the Bilobalide Core. Eur. J. Org. Chem. 2017, 3362.
(74) Maier, G.; Pfriem, S.; Schäfer, U.; Matusch, R. Tetra-
tert-butyltetrahedrane. Angew. Chem. Int. Ed. Engl. 1978, 17, 520.
(75) Evans, D.A.; Kaldor, S.W.; Jones, T. K; Clardy, J.;
Stout, T. J. Total synthesis of the macrolide antibiotic cytovaricin
J. Am. Chem. Soc. 1990, 112, 7001.
(76) Vedejs, E.; Chen, X. Parallel Kinetic Resolution. J.
Am. Chem. Soc. 1997, 119, 2584.
(77) (a) Coric, I., Muller, S., List, B. Kinetic Resolution of
Homoaldols via Catalytic Asymmetric Transacetalization. J. Am.
Chem. Soc. 2010, 132, 17370; (b) Sun, Z.; Winschel, G. A.;
Borokiva, A.; Nagorny, P. Chiral phosphoric acid-catalyzed enan-
(59) Only recently has absolute stereocontrol emerged. See:
(a) Discolo, C. A.; Touney, E. E.; Pronin, S. V. Catalytic Asym-
metric Radical–Polar Crossover Hydroalkoxylation. J. Am. Chem.
Soc. 2019, 44, 17527; (b) Ebisawa, K.; Izumi, K.; Ooka, Y.; Kato,
H.; Kanazawa, S.; Komatsu, S.; Nishi, E.; Shigehisa, H. Catalyst-
and Silane-Controlled Enantioselective Hydrofunctionalization of
Alkenes by Cobalt-Catalyzed Hydrogen Atom Transfer and Radi-
cal-Polar Crossover. J. Am. Chem. Soc. 2020, 142, 13481.
(60) Zuckerman, D. S.; Woerpel, K. A. Diastereoselective
peroxidation of derivatives of Baylis-Hillman adducts. Tetrahe-
dron 2019, 75, 4118.
( 61 ) Obradors, C.; Martinez, R. M.; Shenvi, R. A.
Ph(i-PrO)SiH2: An Exceptional Reductant for Metal-Catalyzed
Hydrogen Atom Transfers. J. Am. Chem. Soc. 2016, 138, 4962.
(62) Parsons, A. F.; Bar, G. Stereoselective radical reactions
Chem. Soc. Rev. 2003, 32, 251; (b) Hanessian, S.; Yang, H,;
Schaum, R. Hydrogen-Bonding as a Stereocontrolling Element in
Free-Radical C-Allylation Reactions: Vicinal, Proximal, and Re-
19
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Page 20 of 21
1
2
3
4
5
6
7
8
9
tioselective and diastereoselective spiroketalizations. J. Am.
Chem. Soc. 2012, 134, 8074.
(78) We validated dr by NMR to avoid LC errors based on
differential UV-activity, ionization of diastereomers, or impurity
co-elution.
(89) Weinges, K.; Hepp, M.; Huber-Patz, U.; Rodewald, H.;
Irngartinger, H. Chemie Der Ginkgolide, I. 10-Acetyl-1-
Methoxycarbonyl-2,3,14,15,16-Pentanorginkgolid A, Ein
Zwischenprodukt Zur Herstellung von Bilobalid. Liebigs Ann.
der Chemie 1986, 6, 1057.
(90) Lanthier, G. F.; Graham, W. A. G. π-Bonding in the
Boron–Oxygen Bond. Chem. Comm. 1968, 715.
(91) Juaristi, E.; Cuevas, G. Recent Studies of the Anomeric
Effect. Tetrahderon 1992, 48, 5019.
(79) (a) Shu, C.; Liu, M. Q.; Sun, Y. Z.; Ye, L. W. Efficient
Synthesis of γ-Lactones via Gold-Catalyzed Tandem Cycloisom-
erization/Oxidation. Org. Lett. 2012, 14, 4958; (b) Zeng, M.; Her-
zon, S. B. Synthesis of 1,3-Amino Alcohols, 1,3-Diols, Amines,
and Carboxylic Acids from Terminal Alkynes. J. Org. Chem.
2015, 80, 8604; (c) Asao, N.; Ohishi, T.; Sato, K.; Yamamoto, Y.
Lewis acid catalyzed stereoselective hydrosilylation of ketones
under the control of σ-π chelation. Tetrahedron, 2002, 58, 8195.
(80) Julia, M.; Saint-Jalmes, V. P.; Verpeaux, J.-N. Oxida-
tion of Carbanions with Lithium Tert-Butyl Peroxide. Synlett
1993, 233.
(81) Ohtawa, O.; Ogihara, S.; Sugiyama, K.; Shiomi, K.;
Harigaya, Y.; Nagamitsu, T.; Ōmura, S. Enantioselective total
synthesis of atpenin A5. J. Antibiot. 2009, 62, 289.
(82) Brown, H.C.; Bhat, N. G.; Srebnik, M. A Simple, Gen-
eral Synthesis of 1-Alkynyldiisopropoxyboranes. Tet. Lett. 1988,
29, 2631.
(83) This reaction was conceived and executed in October
2017 by Dr. Masaki Ohtawa, presented in November 2017 at
ISONIS-11, posted to ChemRxiv (May 30, 2019) and published in
Ref. 31 (October 16, 2019). A variation on this same reaction was
published recently (March 27, 2020): Li, C.; Zhao, P.; Li, R.;
Zhang, B.; Zhao, W. Oxidation of Alkynyl Boronates to Carbox-
ylic Acids, Esters, and Amides. Angew. Chem. Int. Ed. 2020, 59,
10913. A revised manuscript now cites our work.
(84) Nakanishi, K.; Habaguchi, K.; Nakadaira, Y.; Woods,
M. C.; Maruyama, M.; Major, R. T.; Alauddin, M.; Patel, A. R.;
Weinges, K.; Baehr, W. Structure of bilobalide, a rare tert-butyl
containing sesquiterpenoid related to the C20-ginkgolides. J. Am.
Chem. Soc. 1971, 93, 3544.
(85) (a) Lichtblau, D.; Berger, J. M.; Nakanishi, K. Efficient
Extraction of Ginkgloides and Bilobalde from Ginkgo biloba
Leaves J. Nat. Prod. 2002, 65, 1501; (b) van Beek, T. A. Chemi-
cal analysis of Ginkgo biloba leaves and extracts J. Chromatogr.
A. 2002, 967, 21; (c) Jaracz, S.; Malik, S.; Nakanishi, K. Isolation
of ginkgolides A, B, C, J and bilobalide from G. biloba extracts.
Phytochemistry 2004, 65¸2897; (d) Choi, Y. H.; Kim, J.; Yoo, K.P
Eupercritical-Fluid Extraction of Bilobalide and Ginkgolides from
Ginkgo biloba Leaves by Use of a Mixture of Carbon Dioxide,
Methanol, and Water Chromatographia 2002, 56, 753; (e) Lang,
Q.; Wai, C. M., Pressurized water extraction (PWE) of terpene
trilactones from Ginkgo biloba leaves Green Chemistry 2003, 5,
415.
(92) Luger, P.; Kothe, Paulsen, H. Krisallstructur von 2,3,4-
tri-O-acetyl-β-D-xylopyranosylchloride. Carbohydr. Res. 1974,
37, 283.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(93) For example, acetylation of amides to imides lowers α-
CH acidity: Bordwell, F. G.; Fried, H. E. Heterocyclic Aromatic
Anions with 4n + 2 π-electrons. J. Org. Chem. 1991, 56, 4218.
(94) Kochkodan, V.; Darwish, N .B.; Hilal, N. The chemistry
of boron in water, in: Kabay, N.; Bryjak, M.; Hilal, N. (Eds.),
Boron Separation Processes, Elsevier, Amsterdam, 2015.
(95) Chuang, K. V.; Xu, C.; Reisman, S. E. A 15-step syn-
thesis of (+)-ryanodol. Science 2016, 353, 912.
(96) Related derivatives of 2 yielded similar pKa difference
between inner/outer lactones (details in SI).
(97) Shenvi, R. A. Neurite Outgrowth Enhancement by Jia-
difenolide: Possible Targets. Nat. Prod. Rep. 2016, 33, 535.
(98) Roberto, M., Kirson, D. & Khom, S. The Role of the
Central Amygdala in Alcohol Dependence. Cold Spring Harb
Perspect. Med. 2020 doi:10.1101/cshperspect.a039339.
(99) Herman, M. A.; Contet, C.; Justice, N. J.; Vale, W.;
Roberto, M. Novel subunit-specific tonic GABA currents and
differential effects of ethanol in the central amygdala of CRF
receptor-1 reporter mice. J. Neurosci. 2013, 33, 3284.
(100) Pirker, S.; Schwarzer, C.; Wieselthaler, A.; Sieghart,
W.; Sperk, G. GABA(A) receptors: immunocytochemical distri-
bution of 13 subunits in the adult rat brain. Neuroscience 2000,
101, 815.
(101) Gilpin, N. W.; Herman, M. A.; Roberto, M. The cen-
tral amygdala as an integrative hub for anxiety and alcohol use
disorders. Biol. Psychiatry 2015, 77, 859.
(102) Roberto, M.; Cruz, M. T.; Gilpin, N. W.; Sabino, V.;
Schweitzer, P.; Bajo, M.; Cottone, P.; Madamba, S. G.; Stouffer,
D. G.; Zorrilla, E. P.; Koob, G. F.; Siggins, G. R. Parsons, L. H.
Corticotropin releasing factor-induced amygdala gamma-
aminobutyric Acid release plays a key role in alcohol dependence.
Biol. Psychiatry 2010, 67, 831.
(103) Lovering, F.; Bikker, J.; Humblet, C. Escape from
Flatland: Increasing Saturation as an Approach to Improving Clin-
ical Success. J. Med. Chem. 2009, 52, 6752.
(104) For use in complexity-generation and biological anno-
tation by ‘cell-painting,’ see: Gerry, C. J.; Hua, B, K.; Wawer, M.
J.; Knowles, J. P.; Nelson Jr., S. D.; Verho, O.; Dandapani, S.;
Wagner, B. K.; Clemons, P. A.; Booker-Milburn, K. I.; Boskovic,
Z. V.; Schreiber, S. L. Real-Time Biological Annotation of Syn-
thetic Compounds. J. Am. Chem. Soc. 2016, 138, 8920.
(105) Bertz, S. H. The first general index of molecular com-
plexity” J. Am. Chem. Soc. 1981, 103, 3599.
(86) Van Beek, T.A. Comments on ‘An Extraction Method
for Determination of Ginkgolides ad Bilobalide in Ginkgo Leaf
Extracts’ Anal. Chem. 2000, 72, 3396.
(87) Fourtillan, J. B.; Brisson, A. M.; Girault, J.; Ingrand, I.;
Decourt, J. P.; Drieu, K.; Jouenne, P.; Biber, A. Pharmacokinetic
properties of Bilobalide and Ginkgolides A and B in healthy sub-
jects after intravenous and oral administration of Ginkgo biloba
extract (EGb 761). Thérapie 1995, 50, 137.
(88) Wang, J.; Ouyang, J.; Liu, Y.; Jia, X.; You, S.; He, S.;
Di, X. Development of a sensitive LC–MS/MS method for the
determination of bilobalide in rat plasma with special considera-
tion of ex vivo bilobalide stability: Application to a preclinical
pharmacokinetic study. J. Pharm. Biomed. Anal. 2014, 95, 238.
(106) Code
and
updates
can
be
found
at:
https://github.com/ccsb-scripps/bottchscore
(107) Li, J.; Eastgate, M. D. Current complexity: a tool for
assessing the complexity of organic molecules, Org. Biomol.
Chem. 2015, 13, 7164.
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(108) Crossley, S. W. M.; Tong, G.; Lambrecht, M.; Burdge,
H. E.; Shenvi, R. A. Synthesis of Picrotoxinin via Late-Stage
Strong Bond Activations, J. Am. Chem. Soc. 2020, 142, 11376.
(109) Huffman, B.; Shenvi, R. A. Natural Products in the
‘Marketplace’: Interfacing Synthesis and Biology. J. Am. Chem.
Soc. 2019, 141, 3332.
(110) Corey, E. J.; Wipke, W. T. Computer-assisted design
of complex organic synthesis, Science 1969, 166, 178.
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