Enantioselective catalytic intramolecular cyclopropanation using modified cinchona alkaloid organocatalysts

Article abstract of DOI:10.1002/anie.200602129

(Chemical Equation Presented) A mechanism-guided design process was used to develop cinchona alkaloids that are substituted at the C2′ position of the quinoline moiety. These compounds prove to be excellent organocatalysts for an enantioselective intramol

Full text of DOI:10.1002/anie.200602129

Communications
Asymmetric Synthesis
DOI: 10.1002/anie.200602129
Enantioselective Catalytic Intramolecular
Cyclopropanation using Modified Cinchona
Alkaloid Organocatalysts**
Carin C. C. Johansson, Nadine Bremeyer, Steven V. Ley,
Dafydd R. Owen, Stephen C. Smith, and
Matthew J. Gaunt*
New asymmetric methods to generate the cyclopropane motif
have attracted widespread attention from the synthetic
community owing to their ubiquitous presence in a diverse
range of natural products and their crucial role in the mode of
action of many therapeutic agents.[1] Furthermore, the rigid
structure and strain-driven reactivity make them attractive
[2]
intermediates in complexmolecule synthesis.
Because of
these important properties and the need for efficient methods
for their stereoselective formation, the synthesis of cyclo-
propane-containing molecules has become a platform for the
development of new asymmetric catalytic processes.[3] Nota-
bly, in the last few years numerous metal-catalyzed and
organocatalytic intermolecular cyclopropanation reactions
have been reported that enable the generation of discrete
three-membered ring systems with high diastereo- and
enantioselectivity.[4] In contrast, there are few corresponding
catalytic asymmetric intramolecular reactions. Although
recently a number of catalytic diastereoselective intramolec-
ular cyclopropanation processes have been reported,[5] only
methods that exploit the metal-catalyzed decomposition of a-
diazo-carbonyl compounds lead to a general enantioselective
assembly of [n.1.0]-bicycloalkane frameworks.[6] The develop-
ment of a new catalytic enantioselective intramolecular
cyclopropanation method would be a valuable tool for the
synthetic chemist and would further aid the quest for novel
[*] C. C. C. Johansson, Dr. N. Bremeyer, Prof. Dr. S. V. Ley,
Dr. M. J. Gaunt
Department of Chemistry
University of Cambridge
Lensfield Road, Cambridge CB21EW (UK)
Fax: (+44)1223-336362
E-mail: mjg32@cam.ac.uk
Dr. D. R. Owen
Pfizer Global Research and Development
Sandwich, Kent CT139NJ (UK)
Dr. S. C. Smith
Syngenta Jealotts Hill International Research Centre
Bracknell, Berkshire RG426EY (UK)
[**] We gratefully acknowledge Pfizer Ltd (Sandwich) for a PhD
studentship (C.C.C.J.), Syngenta for a Case Award (N.B.), Novartis
for a Research Fellowship (S.V.L.), the Royal Society for University
Research Fellowship (M.J.G.), and the EPSRC Mass Spectrometry
service at the University of Swansea.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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ways to efficiently assemble architecturally complexmole-
cules.[7]
Herein, we describe the development of an organocata-
lytic enantioselective intramolecular cyclopropanation reac-
tion via ammonium ylides that forms [4.1.0]-bicycloalkanes
with high stereoselectivity. The process is catalyzed by novel
cinchona alkaloids that were developed through a mecha-
nism-guided design strategy and produce the products in
excellent yields and as single diastereomers and with ee values
usually over 95% (Scheme 1).
Scheme 1. Enantioselective organocatalytic cyclopropanation. EWG=
electron-withdrawing group.
Scheme 2. Catalytic cycle for intramolecular cyclopropanation.
We recently reported that an intramolecular cyclopropa-
nation reaction could be catalyzed by the nucleophilic amine
1,4-diazabicyclo[2.2.2]octane (DABCO). When we replaced
DABCO with a catalytic quantity of the chiral quinidine
derivative 9-O-methylquinidine (MQD; MQ = 9-O-methyl-
quinine), the cyclopropane was formed with an excellent
ee value of 94%, although in poor yield.[8] The corresponding
quinine catalyst gave similar results when producing the
opposite enantiomer, and frustratingly all attempts to
improve the yield and maintain the ee value through variation
of the conditions were met with failure. It was notable during
these optimization studies that in the reaction of 1a with
20 mol% of the MQD catalyst it was not possible to
quantitatively reisolate the catalyst at the end of the process.
We speculated the catalyst was being derailed from its
desired mechanistic pathway, thus resulting in a deleterious
effect on the yield of the cyclopropane (À)-2a. The catalytic
cycle for the enantioselective intramolecular cyclopropana-
tion initially involves Finkelstein substitution at the chloro-
ketone unit with sodium bromide (Scheme 2). Displacement
of the bromide with the alkaloid catalyst forms the quaternary
ammonium salt I, which undergoes deprotonation with
sodium carbonate to the ylide-type species II. Intramolecular
conjugate addition forms a new intermediate III and sub-
sequently the cyclopropane 2, thus expelling the catalyst
MQD to restart the cycle. Therefore, to investigate the poor
yield in this process we analyzed a reaction that was carried
out until 60% conversion of the starting material was
achieved. From this we could identify the expected salt I;
however, a complexcombination of salts was also obtained.
Although we were not able to purify them, we speculate these
intermediates result from alkylation at the quinoline nitrogen
atom of IV and both nitrogen atoms of the alkaloid catalyst
V.[9a] Importantly, no further cyclopropanation was observed
when these salts were resubjected to the reactions conditions.
This behavior suggested that the proposed side reaction at the
quinoline nitrogen atom consumed both the catalyst and
starting material and was responsible for the poor yield in the
reaction.[9b]
We therefore speculated that a modified catalyst with
substitution at the C2’ position on the quinoline ring would
render the nitrogen atom inert and should inhibit this
problematic reaction.[10] Towards the synthesis of these
catalysts, we found that treatment of the cinchona alkaloid
catalysts MQD and MQ with methyl lithium in diethyl ether
at room temperature followed by exposure to air or iodine
afforded 2’-methyl-9-O-methylquinine (Me-MQ) and 2’-
methyl-9-O-methylquinidine (Me-MQD) in good yield
(Scheme 3).
The modified cinchona catalysts were tested in the
intramolecular cyclopropanation, and to our delight use of
the Me-MQD catalyst resulted in the formation of (À)-2a in a
high yield (88%) with 97% ee (Scheme 4). The Me-MQ
catalyst also improved the yield of the reaction to 84%, again
with 97% ee of the opposite isomer (+)-2a.[11] We found that
the reaction was most effective when 20 mol% of the
cinchona alkaloid catalyst was used.
We next investigated the scope and limitations of this new
catalytic asymmetric process. The reaction worked on a range
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Figure 1. X-ray crystal structure of a Me-MQD salt, Ic.
Scheme 3. Synthesis of modified cinchona alkaloid organocatalysts.
a conformation that would give a Z enolate on deprotonation
with the sodium carbonate. Although we are assuming that
the structure suggested by the X-ray diffraction is indeed
related to the active conformation in solution, it does allow us
to propose a model that is consistent with the formation of the
observed enantiomer (Scheme 5). Interestingly, from this
model the required enantiomer will form from intramolecular
conjugate addition through a boat-type transition state.
Scheme 4. Cyclopropanation with modified cinchona catalysts.
of systems as shown in Table 1.[12] Enones were the best
substrates for this reaction, and a range of substituents could
be tolerated, including alkyl (branched and unbranched;
entries 1–3 and 10–12), aryl (electron-rich and electron-
deficient substituents; entries 4–6 and 13–15) and heteroaryl
motifs (entries 7 and 16). In all cases, a single diastereomer
was observed. Yields were generally greater than 70%, the
enantiomeric excess was > 95% in almost all cases, and the
bicycloalkanes could be accessed as either enantiomer
depending on whether the Me-MQ or Me-MQD catalyst
was used. Although the catalytic process works well for the
formation of [4.1.0]-bicycloalkanes, the process is unsuccess-
ful under these conditions for the corresponding [3.1.0]
system. Formation of a heterocyclic amide product could be
achieved with excellent enantiomeric excess, although the
turnover in the reaction was surprisingly poor (entry 8). The
reaction also works well when the enone is replaced with an
a,b-unsaturated diimide (entries 9 and 18), thus providing
convenient access to substrates with higher oxidation states.
This new enantioselective organocatalytic process represents
a facile and efficient method for the synthesis of these useful
functionalized molecules in a high-yielding and stereocon-
trolled manner.
With an efficient new process in hand we investigated the
origin of the enantioselectivity. A crystal structure of (+)-2e
provided the absolute configuration of the cyclopropanes
from the quinine series, and the opposite rotation of (À)-2e
confirmed that the quinidine-based catalysts gave the other
enantiomer.[12] Stirring bromoketone 1c with one equivalent
of Me-MQD in acetonitrile afforded the corresponding salt
intermediate Ic, which was crystallized from the same solvent,
thus enabling analysis of the structure by X-ray diffraction
(Figure 1). This study showed that the ammonium salt adopts
Scheme 5. Proposed origin of enantioselectivity.
Having demonstrated the effectiveness of our new
enantioselective organocatalytic cyclopropanation process
we turned our attention towards its application in complex
molecule synthesis. We were intrigued by the possibility of
using these readily accessible cyclopropanes as immediate
precursors to polycyclic natural products. Of particular
interest were structures such as guanacastapene A[13] and
rameswaralide,[14] targets of significant biological interest.
Furthermore, these natural products have a common 5–7–6
tricyclic framework, and we envisioned that these structures
could be rapidly accessed through our organocatalytic cyclo-
propanation tactic and an intramolecular metal-catalyzed
[5+2] cycloaddition reaction similar to that reported by Trost
et al.[15] Accordingly, alkynyl vinyl cyclopropane 5 was
synthesized from the corresponding aldehyde through a
Wittig olefination. We were able to isolate both the Z and
E isomers from this olefination, which was important as we
speculated that the olefin isomers would lead to different
diastereomers in the [5+2] cycloadditon. Accordingly, the
treatment of (Z)-5 with the ruthenium catalyst 6[15a,b,16]
surprisingly afforded none of the [5 + 2] cycloaddition prod-
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Table 1: Scope of the catalytic enantioselective intramolecular cyclopropanation reaction.
Entry[a]
Product
Yield[b] [%]
84
ee [%]
Entry[a]
10
Product
Yield[b] [%]
75
ee [%]
1
(+)-2a
(+)-2b
97
(À)-2a
(À)-2b
93
2
3
68
79
98
95
11
12
65
71
99
99
(+)-2c
(+)-2d
(À)-2c
(À)-2d
4
5
78
83
98
96
13
14
85
88
98
99
(+)-2e
(À)-2e
6
7
(+)-2 f
(+)-2g
77
95
99
98
15
16
(À)-2 f
(À)-2g
83
85
99
99
8[c]
(+)-2h
(+)-2i
27
77
97
99
17[d]
(À)-2h
(À)-2i
n.d.
87
n.d.
99
9
18
[a] Reactions run for 36 h. [b] Yield of the isolated product after chromatography. [c] One equivalent of catalyst was used. [d] Reaction not carried out.
À
uct, thus returning only the starting material. However,
treatment of (E)-5 with the same catalyst 6 led to the 5–7–6
ring system as the all syn-diastereomer 7, which resembles the
core of rameswaralide.[15a,b] We are further investigating the
[5+2] cycloaddition of (Z)-5, as this process would form the
anti relationship present in guanacastapene A. However, it is
noteworthy that we can assemble the polycyclic framework
present in many natural products from readily available
starting materials in only four steps[17] by using three catalytic
operations that control the installation of five C C bonds and
three stereocenters (Scheme 6).
In summary, we have developed a highly enantioselective
catalytic intramolecular cyclopropanation process that uses
modified cinchona alkaloids to generate the desired function-
alized [4.1.0]-bicycloheptanes in excellent yields and enantio-
selectivities. The new catalysts contain an alkyl substituent at
the C2’ position, thus preventing the quinoline nitrogen atom
from interfering in the reaction. We are currently investigat-
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Communications
Org. Lett. 2002, 4, 3271; e) T. Miura, T. Sasaki, T. Harumashi, M.
Murakami, J. Am. Chem. Soc. 2006, 128, 2516.
[6] a) G. Maas, Chem. Soc. Rev. 2004, 3, 183; b) M. Honma, T.
Sawada, Y. Fujisawa, M. Utsugi, H. Watanabe, A. Umino, T.
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Soc. 2003, 125, 2860; c) it is also possible to form [n.1.0]-bicyclic
motifs through the intermolecular cyclopropanation of cyclic
alkenes; for examples, see: M. P. Doyle, D. C. Forbes, Chem.
Rev. 1998, 98, 911.
[7] For examples of using strain-driven reactivity, see: a) R.
Sarpong, J. T. Su, B. M. Stoltz, J. Am. Chem. Soc. 2003, 125,
13624; b) P. A. Wender, C. O. Husfild, E. Langkopf, J. Love, J.
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[9] a) Based on LC–mass-spectrometric analysis of the mixture;
b) quinoline A, when treated with 1a and NaBr in MeCN at
808C, leads to an intractable mixture of products and consumes
all the starting material; quinoline B does not react with 1a
under the same conditions, thus both materials are returned
untouched; no cyclopropanantion was observed in these
reactions.
Scheme 6. Catalytic elaboration of [4.1.0]-bicycloalkenes in complex
molecule synthesis. Cp=cyclopentadienyl, TMS=trimethylsilyl.
ing the application of this new catalytic reaction in the
synthesis of architecturally complexnatural products.
Experimental Section
The cinchona alkaloid catalyst (20 mol%) was added to a mixture of
the chloroketone 1 (1.0 equiv), sodium bromide (0.25 equiv), and
sodium carbonate (1.3 equiv) in anhydrous acetonitrile (0.1m), and
the reaction mixture was stirred at 808C for 24–36 h until complete, as
shown by TLC and LC–mass-spectrometric analysis. The reaction was
concentrated and diluted with diethyl ether before filtering through a
pad of silica. After concentration of the filtrate the residue was
purified by flash column chromatography on silica gel to afford the
[4.1.0]-bicycloalkane.
[10] 1’-deazo cinchona alkaloids have been synthesized previously
through a nontrivial route: a) C. Exner, A. Pfaltz, M. Studer, H.-
U. Blaser, Adv. Synth. Catal. 2003, 345, 1253; b) E. V. Dehmlow,
S. Düttmann, B. Neumann, H.-G. Stammler, Eur. J. Org. Chem.
2002, 2087.
[11] We also tested catalysts containing C2’-Ph (83% yield, 96% ee),
iPr (59% yield, 98% ee), and tBu (46% yield, 93% ee), but the
Me-MQ/ Me-MQD catalysts gave the best results.
Received: May 28, 2006
Published online: August 4, 2006
Keywords: asymmetric synthesis · cinchona alkaloids ·
.
cyclopropanation · intramolecular reactions · organocatalysis
[12] See the Supporting Information for details.
[13] For isolation, see: a) S. F. Brady, M. P. Singh, J. E. Janso, J.
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[15] For a related example of intramolecular [5+2] cycloadditions,
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[17] The chloroketone precursors are made through an alkene cross
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[5] For examples of catalytic intramolecular cyclopropanation
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