Synthesis and structure-activity studies of the V-ATPase inhibitor saliphenylhalamide (SaliPhe) and simplified analogs

Article abstract of DOI:10.1016/j.bmcl.2015.09.021

An efficient total synthesis of the potent V-ATPase inhibitor saliphenylhalamide (SaliPhe), a synthetic variant of the natural product salicylihalamide A (SaliA), has been accomplished aimed at facilitating the development of SaliPhe as an anticancer and antiviral agent. This new approach enabled facile access to derivatives for structure-activity relationship studies, leading to simplified analogs that maintain SaliPhe's biological properties. These studies will provide a solid foundation for the continued evaluation of SaliPhe and analogs as potential anticancer and antiviral agents.

Full text of DOI:10.1016/j.bmcl.2015.09.021

Bioorganic & Medicinal Chemistry Letters 25 (2015) 4393–4398
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis and structure–activity studies of the V-ATPase inhibitor
saliphenylhalamide (SaliPhe) and simplified analogs
Jose Garcia-Rodriguez a, Saurabh Mendiratta b, Michael A. White b, Xiao-Song Xie c, Jef K. De Brabander a,
⇑
a Department of Biochemistry, UT Southwestern Medical Center, 5323 Harry Hines Blvd., Dallas, TX 75390-9038, USA
b Department of Cell Biology, UT Southwestern Medical Center, 5323 Harry Hines Blvd., Dallas, TX 75390-9039, USA
c Eugene McDermott Center for Human Growth & Development, UT Southwestern Medical Center, 5323 Harry Hines Blvd., Dallas, TX 75390-8591, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 July 2015
Revised 2 September 2015
Accepted 7 September 2015
Available online 8 September 2015
An efficient total synthesis of the potent V-ATPase inhibitor saliphenylhalamide (SaliPhe), a synthetic
variant of the natural product salicylihalamide A (SaliA), has been accomplished aimed at facilitating
the development of SaliPhe as an anticancer and antiviral agent. This new approach enabled facile access
to derivatives for structure–activity relationship studies, leading to simplified analogs that maintain
SaliPhe’s biological properties. These studies will provide a solid foundation for the continued evaluation
of SaliPhe and analogs as potential anticancer and antiviral agents.
Keywords:
V-ATPase
Ó 2015 Elsevier Ltd. All rights reserved.
Antiviral
Anticancer
Benzolactone
Salicylihalamide
Vacuolar H+-ATPases are emerging as important cellular targets
for the identification of potential novel therapeutics. This class of
enzymes plays a key role in controlling the acidity of intracellular
compartments and the extracellular environment, and its role has
been implicated in various diseases including cancer and antiviral
infections.1 The pharmacological modulation of the activity of
these enzymes has contributed to a better understanding of their
cellular function and has led to the discovery and identification
of a wide variety of structurally diverse inhibitors.2 Among them,
the benzolactone enamide family emerged due to its potency and
selectivity towards the mammalian V-ATPases.3 A prominent
example of this family is the macrolactone salicylihalamide A
(SaliA, 1, Fig. 1). SaliA was isolated in 1997 from an unidentified
species of the marine sponge Haliclona sp. and showed a unique
and selective cytotoxicity profile against the NCI 60 cell line human
tumor panel.4,5 This activity was subsequently ascribed to the abil-
ity of SaliA to inhibit the V-ATPase (IC50 <1 nM).6,7 The promising
biological properties of SaliA encouraged our research group and
others to develop synthetic approaches to access SaliA and more
potent derivatives to further evaluate the therapeutic potential of
this natural product.8–10 In this context, saliphenylhalamide
(SaliPhe, 2, Fig. 1)10c was selected from an extensive structure–
activity study published by our group due to its potent inhibitory
activity, selectivity, stability and synthetic access.7,8e,10b,c
SaliPhe has not only been useful for the study of V-ATPase func-
tion and its role in a variety of normal and pathogenic cellular pro-
cesses,7,10c,11 but also enabled in vivo modulation of V-ATPase
activity as a means to inhibit the growth of human lung cancer
xenografts with a KRASmut/LKB1mut oncogenotype (present in
ꢀ6% of human adenocarcinomas) in mice,12 to attenuate wear par-
ticle-induced osteolysis in a mouse calvarial model (for the poten-
tial treatment of peri-implant osteolysis),13 or to protect mice
against a lethal challenge of a mouse-adapted influenza strain.14
In the context of the continued promise for pharmacological
V-ATPase modulation as a potential means to treat a wide host
of diseases, we sought to develop a more efficient synthesis of
SaliPhe that also would enable access to additional analogs that were
not explored previously. The results of these studies and in vitro
biological activities of new SaliPhe analogs are reported herein.
Although we already formulated a synthetic approach that sup-
ported multi-gram access to SaliPhe,10c one of the drawbacks was
the rather lengthy 8-step sequence required to install the enamide
side-chain after formation of the macrolactone was complete. As
shown in Scheme 1, after ring-closing metathesis of precursor III,
4 steps were required to transform the primary protected alcohol
into the homologated unsaturated ester II. An additional 4-step
sequence, involving
trapping, completed the synthesis of SaliPhe. We reasoned that a
Buchwald-type Cu-catalyzed amidation of vinyl iodide
a key Curtius rearrangement/isocyanate-
⇑
Corresponding author. Tel.: +1 214 648 7808; fax: +1 214 648 0320.
E-mail address: jef.debrabander@utsouthwestern.edu (J.K. De Brabander).
a
http://dx.doi.org/10.1016/j.bmcl.2015.09.021
0960-894X/Ó 2015 Elsevier Ltd. All rights reserved.

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J. Garcia-Rodriguez et al. / Bioorg. Med. Chem. Lett. 25 (2015) 4393–4398
Ph
The synthesis of the alcohol fragments VI for SaliPhe and ana-
logs is depicted in Scheme 2 below. Because our short and scalable
synthesis of the related fragment V (Scheme 1) was nevertheless
not fully stereoselective, we designed an improved, highly stereos-
elective approach that initiated with our previously synthesized
aldehyde 3a and the corresponding aldehyde 3b lacking the
C6-methyl group.10c,15 In the event, aldol addition to 3a,b
followed the protocol described by Crimmins et al. using the Z
(O)-titanium enolate of thiazolidinethione 4 generated with (À)-
sparteine as a base.16 The aldol products 5a,b were obtained in
60–75% yield and high selectivity (dr >10–15:1). Subsequent
silylation and reductive (DIBAL) cleavage of the auxiliary rendered
aldehydes 6a,b in ꢀ70% yield. An alternative route to 6b began
from known epoxide 7 (readily available in three steps from
HN
HN
O
O
OH
OH
O
O
OH
Me
OH
Me
O
O
Salicylihalamide A
Saliphenylhalamide
(1, SaliA)
(2, SaliPhe)
Figure 1. Structures of salicylihalamide A (1, SaliA) and saliphenylhalamide (2,
SaliPhe).
inexpensive L
-aspartic acid).17 After protecting the primary alcohol
macrocycle precursor such as IV with 3-phenylpropiolamide,
followed by ring-closing metathesis, would significantly
of 7 as a tetrahydropyranyl ether, the epoxide was opened with
allylmagnesium bromide (cat. CuI) to yield homoallylic alcohol 8
(55%, two steps). Silylation, followed by removal of the ether pro-
tecting group and oxidation afforded aldehyde 6b.
streamline the side-chain installation to
(including final deprotection). Alternatively,
a
3-step sequence
Pd-catalyzed
a
carbonylation would provide a 2-step access to unsaturated ester
precursor II instead of our previous 5-step sequence. Moreover,
having the vinyl iodide functionality installed prior to
esterification with a benzoic acid coupling partner VII (cf. alcohol
VI) will increase convergency and facilitate analog development.
The functional groups of SaliPhe selected for modification are
indicated in red in Scheme 1. We did not modify the N-acyl
enamide side-chain of SaliPhe because we had previously shown
that it keeps the potency while being chemically more stable and
easier to synthesize than the homologous side chain of SaliA. We
hypothesized that the methyl group at the C6 position could be
removed (R = H) without affecting the activity and aimed to
explore the substituents at the phenol ring by introducing other
groups (Y = F, CHF2, NO2, NH2) at the ortho-, meta-, and para-
positions (in relation to the benzoate position), and to replace
the lactone by a chemically more robust lactam (X = NH).
An asymmetric Barbier-type propargylation of aldehydes 6a,b
was performed using indium powder, pyridine and (1R,2S)-2-
amino-1,2-diphenylethanol 10 as a chiral source to provide the
corresponding homopropargylic alcohols 11a,b as single stereoiso-
mers (>20:1 dr).18–20 A subsequent radical-mediated hydrostanny-
lation (AIBN, Bu3SnH) was followed by a transmetallation with
iodine to afford vinyl iodides 12a,b in 70–72% yield. To enable
the synthesis of a SaliPhe lactam analog, alcohol 12b was treated
with mesyl chloride followed by SN2-inversion with sodium azide
(63%, two steps). The corresponding azide could be reduced to the
primary amine 12c with trimethyl phosphine, but this was best
done immediately prior to coupling with the benzoic acid partner
(vide infra).21
The synthesis of SaliPhe (2) and the corresponding des-Me ana-
log 17 via late stage Cu-mediated C–N bond formation is depicted
in Scheme 3. The above prepared alcohols 12a,b were esterified
Ph
HN
O
CO2R
4 steps
OP
O
O
Y
OH
OTBS
Me
X
O
I
• Curtius
R
rearrangement
R = H, Me; X = O, NH
II
Y = OH, CHF2, F, NO2, NH2
• RCM
5 steps
• homologation
• RCM
2 steps
3 steps
• amidation
I
• carbonylation
• RCM
OPMB
OAc
O
O
Y
OTBS
OTBS
Me
X
O
R
IV
III
I
OPMB
Y
VI
V
HO
CO2H
HO
VII
OTBS
OTBS
R
Me
Scheme 1. Retrosynthetic analysis.

J. Garcia-Rodriguez et al. / Bioorg. Med. Chem. Lett. 25 (2015) 4393–4398
4395
d. DHP, TsOH
e. allylMgBr, CuI
f. TBSCl
g. MgBr2
HO
THPO
HO
O
OH
OTBS
OTBS
55%
63%
7
8
9
h. Swern ox.
80%
a. TiCl4, (-)-sparteine
6b
OHC
S
OH
O
b.TBSCl
c. DIBAL
S
O
CHO
N
S
Me
N
S
R
R
4
i
Pr
iPr
R
60%, dr >15:1
75%, dr >10:1
3a: R = Me
3b: R = H
5a: R = Me
5b: R = H
70%
71%
6a: R = Me
6b: R = H
HO
Ph
NH2
Ph
i. In, 10,
propargylBr
10
I
I
k. MsCl
l. NaN3
m. Me3P
dr >20:1
j. Bu3SnH,
AIBN, I2
H2N
HO
HO
OTBS
OTBS
OTBS
R
R
63% (crude)
72%
70%
86%
85%
12c
12a: R = Me
12b: R = H
11a: R = Me
11b: R = H
Scheme 2. Reagents and conditions: (a) TiCl4 (6.0 equiv), (À)-sparteine (3.0 equiv), 4 (3.0 equiv), CH2Cl2, À78 °C, 30 min, (5a, 60%; 5b, 75%.); (b) TBSOTf (1.2 equiv), 2,6-
lutidine (1.5 equiv), CH2Cl2, 23 °C, 2 h; (c) DIBAL-H (1.8 equiv), toluene, À78 °C, 3 h, (6a, 70%; 6b, 71%; two steps); (d) p-TsOH (0.05 equiv), dihydropyran (DHP, 2.0 equiv),
THF, 23 °C, 4 h, 88%; (e) allylmagnesium bromide (3.0 equiv), CuI (0.1 equiv), THF, À20 °C, 1 h, 62%; (f) imidazole (5.0 equiv), TBSCl (1.2 equiv), DMF, 23 °C, 12 h, 87%; (g)
MgBr2ÁEt2O (3.0 equiv), Et2O, 23 °C, 2 h, 72%; (h) DMSO (2.0 equiv), oxalyl chloride (1.5 equiv), Et3N (5.0 equiv), CH2Cl2, À78 °C, 1 h, 80%; (i) propargyl bromide (4.5 equiv), 10
(3.0 equiv), In (3.0 equiv), pyridine (3.0 equiv), THF, À78 °C to 23 °C, 10 h, (11a, 86%; 11b, 85%); (j) Bu3SnH (1.2 equiv), AIBN (0.5 equiv) toluene, 110 °C, 2 h; then I2 (1.0 equiv),
THF, À78 °C, 30 min (12a, 72%; 12b, 70%); (k) MsCl (3.5 equiv), Et3N (3.5 equiv), Et2O, 23 °C, 1 h; (l) NaN3 (3.0 equiv), DMF, 60 °C, 2 h, (63%, two steps); (m) Me3P (3.0 equiv),
9:1 THF/H2O, 60 °C, 1 h, then MW, 100 °C, 30 min (used crude).
with substituted salicylic acid 1322 under Mitsunobu conditions
(DEAD/PPh3).10c For the esterification of 12a, a stoichiometric
amount of pyridine was required to prevent competing
elimination of the alcohol under the reaction conditions.23 The
N-acyl enamide side chain was installed by Cu-catalyzed amida-
tion of 14a,b with amide 15 under the conditions described by
Buchwald et al., providing enamides 16a,b in 60–78% yield after
re-installation of the acetate group.24,10f The ring-closing metathe-
sis of 16a,b with Grubbs I catalyst provided the benzolactones 2
and 17 in 31–36% yields after global deprotection.25,26 The E/Z iso-
mers could be separated by flash chromatography after removing
the acetate group. Of note is that the des-Me RCM-precursor 16b
cyclized with significant lower E/Z selectivity (4:1) compared to
the parent 16a (8:1), indicating a beneficial role for the Me-sub-
stituent in pre-organizing the substrate for an E-selective metathe-
sis.27 This lowered selectivity is observed for all des-methyl analogs
prepared (vide supra, Chart 1). Final desilylation with buffered
HFÁpyridine provided SaliPhe 2 and analog 17 in ten steps from
known aldehydes 3a,b.
When assayed for V-ATPase inhibitory activity, it was found
that des-Me analog 17 displayed potency comparable to SaliPhe
(IC50 = 5 nM, 17; 1 nM, 2), indicating only a marginal role for the
Me-group on V-ATPase inhibitory activity.28 Based on these results,
we decided to carry out the subsequent structural modifications in
the des-Me series, and survey the influence of various aromatic
substituents on biological activity. We envisioned exploring diflu-
oromethyl, fluoro, amino, nitro and hydroxyl substituents at the
ortho-, meta-, or para-position. Analogs 21–24 (see Chart 1) were
readily obtained using the chemistry outlined in Scheme 3, starting
from Mitsunobu reaction of alcohol 12b with various benzoic
acids.29,30 Interestingly, the RCM-product of an intermediate with
a meta-OH or final product with a meta-fluoro substituent were
unstable and could not be further processed or evaluated, whereas
the o-NO2 substituted analog 19 (Chart 1) could not be prepared
using this route due to failure of the corresponding bis-olefin inter-
mediate to undergo the ring closing metathesis.29
As discussed during the retrosynthetic analysis, we also investi-
gated whether we could intercept vinyl iodide intermediates in a
Pd-catalyzed carbonylation event to produce a,b-unsaturated ester
intermediates reminiscent of those utilized in our first-generation
synthetic approach (cf. IV ? II, Scheme 1). As shown in Scheme 4,
the palladium-catalyzed carbonylation of vinyl iodides 26a–c (pre-
pared via Mitsunobu reaction of alcohol 12b with the correspond-
ing benzoic acids)29 with catalytic PdCl2(PPh3)2 (Et3N, 1 atm of CO,
MeOH, rt) proceeded smoothly to yield, after ring-closing metathe-
sis as before, the desired unsaturated esters 27a–c in good overall
yields. Of note is that the ring-closing metathesis to o-NO2 substi-
tuted benzolactone 27c occurred with the best yield and E:Z-selec-
tivity (94%, 5.6:1) of all des-Me analogs evaluated in this study,
whereas the corresponding ring-closing metathesis with o-NO2
as implemented in the route depicted in Scheme 3 did not occur
at al. The remainder of the synthesis (enamide side chain forma-
tion) essentially followed our published sequence of ester hydrol-
ysis, acyl azide formation (28a–c) followed by Curtius
rearrangement and trapping of the resulting isocyanate with phe-
nylethynyl lithium, and final deprotection.8e,10c In addition to
providing alternate access to analog 17, this route also furnished
new analogs 18–20 (20 from reduction of 19 with Fe).
We tested the ability of all new analogs for their ability to inhi-
bit the activity of a purified reconstituted V-ATPase from bovine
brain (Chart 1, IC50-values shown in red).28 Analysis of the data
indicated that the des-Me analog 17, and the corresponding lactam
2531,32 remain potent V-ATPase inhibitors with IC50’s of 5 and
30 nM, respectively (versus 1 nM for SaliPhe 2). The modifications
around the phenol ring proved to be detrimental for activity.
Replacement of the o-OH with
a difluoromethyl bioisostere
resulted in a 100-fold reduction in activity of analog 21 (500 nM)
versus the corresponding phenol 17 (5 nM).33 A similar loss of

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J. Garcia-Rodriguez et al. / Bioorg. Med. Chem. Lett. 25 (2015) 4393–4398
I
I
OAc
a. DEAD, PPh3, py
(for 12a only), PhMe
OAc
O
CO2H HO
OTBS
O
OTBS
60% (14a)
67% (14b)
R
R
13
12a: R = Me
12b: R = H
14a,b
b. CuI, Cs2CO3,
diamine, 15
O
Ph
Ph
15
c. Ac2O, py
H2N
Ph
HN
HN
O
O
d. RCM
e. K2CO3, MeOH
f. HF⋅py
OH
OAc
O
O
O
OH
OTBS
O
36%; 8:1 E:Z (2)
31%; 4:1 E:Z (17)
R
R
2: R = Me (SaliPhe)
17: R = H
16a 78%
16b 60%
Scheme 3. Reagents and conditions: (a) DEAD (2.0 equiv), 13 (1.8 equiv), PPh3 (2.0 equiv), pyridine (1.0 equiv, for 12a only), toluene, 0 °C (14a, 62%; 14b, 68%); (b) Cs2CO3
(5.0 equiv), CuI (2.0 equiv), 15 (5.0 equiv), MeHN(CH2)2NHMe (5.0 equiv), DMF, 23 °C, 2–4 h; (c) Ac2O (1.1 equiv), pyridine (1.1 equiv), DMAP (0.1 equiv), CH2Cl2, 23 °C, 1 h
(16a, 78%; 16b, 60%); (d) Grubbs’ I (0.6 equiv), CH2Cl2, 23 °C, 6 h; (e) K2CO3 (10.0 equiv), MeOH, 23 °C, 30 min (a, 45%; b, 40%); (f) HFÁpy, 23 °C, 2–5 days (2, 80%; 17, 78%).
NHC(O)R
NHC(O)R
F
F
OH
OH
F
O
O
O
O
O
O
OH
OH
O
O
O
O
O
O
2
21
1 nM
500 nM
Me
NHC(O)R
NHC(O)R
O
OH
OH
O
17
22
5 nM
500 nM
F
HO
O2N
NHC(O)R
NHC(O)R
O
OH
OH
O
18
23
1,000 nM
not active
@ 2 μM
NHC(O)R
NHC(O)R
NO2
O
OH
OH
O
19
24
1,000 nM
300 nM
NHC(O)R
NHC(O)R
NH2
OH
O
OH
OH
N
H
20
25
30 nM
300 nM
Ph
R =
Chart 1. Structure of analogs prepared under this study. The IC50’s for inhibition of V-ATPase activity are shown in red (all compounds were stable to the assay buffer).28
activity was noted for the o-amino, p-fluoro, and p-nitro analogs
20, 22, and 24 (IC50’s 300–500 nM), and even worse for the o-fluoro
and o-nitro congeners 18 and 19 (1000 nM). Finally, the p-phenol
regioisomer of potent des-Me analog 17, compound 23, did not
register any inhibitory activity up to 2 lM concentrations. In
conjunction with previous SAR-studies,8b,10a–c we conclude that
the o-phenol is critical for activity, whereas the alicyclic and
side-chain fragments are more tolerant towards modifications.

J. Garcia-Rodriguez et al. / Bioorg. Med. Chem. Lett. 25 (2015) 4393–4398
CO2Me
4397
I
a. Pd(II), CO, MeOH
b. RCM
Y
Y
O
O
OTBS
OTBS
O
O
E/Z ratios:
2:1 (27a)
4:1 (27b)
5.6:1 (27c)
26a-c
27a-c
Y = OMe (a); F (b); NO2 (c)
[for 27a: c. BBr3; d. TBSOTf]
e. tributyltin oxide
Ph
f. (PhO)2P(O)N3, NEt3
HN
O
CON3
Y
Y
O
O
g. Δ; then LiC
h. HF⋅py
≡CPh
OH
OTBS
O
O
19 (Y = NO2)
20 (Y = NH2)
28a-c
17 (Y = OH)
18 (Y = F)
i. Fe, NH4Cl
Y = OTBS (a); F (b); NO2 (c)
Scheme 4. Reagents and conditions: (a) PdCl2(PPh3)2 (10 mol %), CO, Et3N (1.5 equiv), MeOH, 23 °C, 4 h (a, 66%; b, 77%; c, 57%); (b) Grubbs’ I (10 mol %), CH2Cl2, 23 °C, 3 h (a,
crude; b, 81%, c, 94%); (c) BBr3 (2.0 equiv), CH2Cl2, À78 °C, 1 h (a, 40%, two step); (d) TBSOTf (1.2 equiv), 2,6-lutidine (1.5 equiv), CH2Cl2, 23 °C, 1 h (a, 98%); (e) tributyltin oxide
(4.0 equiv), toluene, 120 °C, 10 h; (f) Et3N (4.5 equiv), DPPA (4.0 equiv), benzene, 23 °C, 10 h (a, 57% two steps; b, 44% two steps; c, 50% two steps); (g) toluene, 120 °C, 3 h;
evaporate solvent; then n-BuLi in THF (5.0 equiv), phenyl acetylene (6.0 equiv), THF, À78 °C, 1 h (a, crude; b, 51%; c, 60%); (h) HFÁpy, 23 °C, 2 days (17, 55% three steps; 18,
75%; 193, 83%); (i) NH4Cl (16.0 equiv), Fe (8.0 equiv), EtOH, 23 °C, 4 h, 40%.
Acknowledgments
We thank the Robert A. Welch Foundation (I-1422) and CPRIT
(RP120718) for financial support. J.K. De Brabander holds the Julie
and Louis Beecherl, Jr., Chair in Medical Science; M.A. White holds
the Grant A. Dove Chair for Research in Oncology and the Sherry
Wigley Crow Cancer Research Endowed Chair in Honor of Robert
Lewis Kirby, M.D.
Supplementary data
Supplementary data (experimental procedures and characteri-
Figure 2. Dose–response curves for inhibition of the HCC-4017 lung cancer cell line
zation data for new compounds and intermediates, copies of
with SaliPhe (2) and analogs 17–25. Error bars indicate SD, n = 3.34
NMR spectra) associated with this article can be found, in the
online version, at http://dx.doi.org/10.1016/j.bmcl.2015.09.021.
In previous work,12 we found that co-occurring mutations in
References and notes
KRAS and LKB1, present in 6% of lung adenocarcinoma patients,
are sufficient to drive addiction to the coatomer complex I
(COPI)-dependent lysosome acidification, and that this vulnerabil-
ity could be exploited pharmacologically by inhibition of
V-ATPase activity with SaliPhe (2) in vitro and in vivo. We there-
fore evaluated the growth inhibitory effects of the SaliPhe analogs
on the human HCC-4017 lung cancer cell line, which has the
co-occurring KRASmut/LKB1mut oncogenotype (Fig. 2).34 Whereas
analog 17 only showed a reduction in growth-inhibitory activity
of ꢀ5–10 fold versus SaliPhe, lactam analog 25 was >100-fold less
effective, and all other analogs tested did not register any
anti-proliferative activity. These results are therefore consistent
with the V-ATPase inhibitory activities observed for these
compounds.
In conclusion, we have reported a short, convergent and stere-
oselective synthesis of the potent V-ATPase inhibitor SaliPhe.
Two complementary synthetic sequences based on late stage func-
tionalization of a common intermediate have been fully evaluated
to produce and diversify SaliPhe. We have discovered potent ana-
logs that simplify the chemical structure of SaliPhe while keeping
the potent V-ATPase inhibitory and anticancer properties. Our ini-
tial structure–activity studies revealed that the o-phenol group is
crucial for biological activity.
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enamide side chain. Although Balan et al. have used a similar Buchwald
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homologation, and final Buchwald amidation.
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29. See Supplementary Material for experimental procedures, characterization
data, and copies of NMR spectra for all new analogs and intermediates.
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31. The lactam was prepared via amide-formation between acid 13 and amine 12c
(Scheme 2) under peptide coupling conditions, followed by a similar sequence
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of reactions as depicted in Scheme
4 for the corresponding lactones
(carbonylation route).29
32. For the synthesis of a different aza-analog of salicylihalamide, see Ref. 10f.
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34. See Ref. 12 for detailed assay conditions.
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