Asymmetric approach toward chiral cyclohex-2-enones from anisoles via an enantioselective isomerization by a new chiral diamine catalyst

Article abstract of DOI:10.1021/ja308623n

A 3-step asymmetric approach toward the optically active chiral cyclohex-2-enones from anisoles has been developed. The crucial asymmetric induction step is an unprecedented catalytic enantioselective isomerization of β,γ-unsaturated cyclohex-3-en-1-ones

Full text of DOI:10.1021/ja308623n

Communication
pubs.acs.org/JACS
Asymmetric Approach toward Chiral Cyclohex-2-enones from
Anisoles via an Enantioselective Isomerization by a New Chiral
Diamine Catalyst
Jung Hwa Lee and Li Deng*
Department of Chemistry, Brandeis University, Waltham, Massachusetts 02454-9110, United States
S
* Supporting Information
ABSTRACT: A 3-step asymmetric approach toward the
optically active chiral cyclohex-2-enones from anisoles has
been developed. The crucial asymmetric induction step is
an unprecedented catalytic enantioselective isomerization
of β,γ-unsaturated cyclohex-3-en-1-ones to the correspond-
Figure 1. Some H-bonding organocatalysts from cinchona alkaloids.
ing α,β-unsaturated chiral enones. This new asymmetric
transformation was realized by cooperative iminium-base
catalysis with an electronically tunable new organic
Table 1. Asymmetric Isomerization of 2A to 3A
catalyst. The synthetic utility of this methodology is
highlighted by the enantioselective total synthesis of
(−)-isoacanthodoral.
hiral 6-membered carbocycles are among the most
common structural motifs in functional organic mole-
C
cules.1 Consequently, the development of efficient methods for
the construction of 6-membered carbocycles represents a
fundamentally important task in organic synthesis. Chiral
cyclohex-2-enones1k−p,2 constitute arguably the most versatile
synthon for 6-membered carbocycles due to their ability to
participate in stereoselective 1,4- and 1,2-additions with
numerous nucleophiles and pericyclic reactions with a broad
range of partners. Here we report the development of a concise
catalytic enantioselective route for the transformation of
substituted anisoles to optically active cyclohex-2-enones and
its application to the first asymmetric synthesis of the natural
product (−)-isoacanthodoral.
We became interested in the application of asymmetric proton
transfer catalysis to the development of an isomerization of alkyl
cyclohex-3-enones 2 to cyclohex-2-enones 3, as it would provide
the stereochemistry-defining transformation for a 3-step route3
from anisoles 1 to optically active enones 3 (Scheme 1).
Although the isomerizations of β,γ-unsaturated ketones to their
α,β-unsaturated counterparts have been reported to be catalyzed
by both enzymes and small-molecule-based acids, bases, and
amines,4 an enantioselective variant has not been reported to
date.5a,b Building on our recent progress in the development of
a
Unless noted, reactions were run with 0.1 mmol of 2A at rt in CHCl3
b
c
(1.0 M). Determined by 1H NMR. Determined by GC.
d
f
Determined by HPLC. Data in parentheses were obtained when
g
the reactions were conducted for 24 h. Absolute configuration was
determined as R.9 Reaction was run at 0 °C. Reaction was run at
h
i
−25 °C.
Scheme 1. Asymmetric Synthesis of Cyclohex-2-enones from
Anisoles 1
an asymmetric olefin isomerization of β,γ-butenolides,5b we
initially attempted the isomerization of β,γ-unsaturated cyclo-
hexenone 2A with known cinchona alkaloid derivatives bearing a
H-bond donor (4−6, Figure 1).5b−f Although these cinchona
Received: August 30, 2012
Published: October 8, 2012
© 2012 American Chemical Society
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Communication
alkaloid derivatives showed considerable catalytic activity for the
isomerization of β,γ-butenolides, they failed to afford meaningful
activity for the isomerization of the enone 2A (Table 1, entries
1−3) at room temperature. The failure of these catalysts in
activating enone 2A for the isomerization could be attributed to
the low acidity of the α-H of enone 2A relative to that of 5-
membered β,γ-butenolides.6
Unfortunately, attempts to improve the enantioselectivity of
QD-7 were met with no success.
Although catalyst 7 has been successfully applied as an efficient
catalyst for a range of asymmetric transformations,7 one
disadvantage of 7 is the lack of a handle for catalyst tuning. In
order to develop an effective catalyst to afford highly
enantioselective cooperative iminium-base catalysis for the
isomerization of 2A, we designed a novel class of cinchona
alkaloid-derived catalysts 8 that bears an aniline moiety at the C9
position. This design intended to take advantage of the
electronically tunable features of the aniline for catalyst
optimization, which we expect to have an impact on how facile
the deprotonation step would occur and on how enantioselective
the protonation step could be along the enamine pathway.
Importantly, as described in detail in the Supporting
Information, cinchona alkaloid derivatives 8a−c presenting
differing substituted anilines could be readily prepared in two
steps from 7.8
The tunable nature of 8 proved to be critical to the
development of an efficient catalyst for the enantioselective
isomerization of 2A. Compared to QD-7, catalyst QD-8a was
found to be less active but afford significantly higher
enantioselectivity (Table 1, entry 5, 18 h, 9% conv., 65% ee).
Importantly, in comparison to QD-8a, catalyst QD-8b
presenting an aniline substituted with an electron-withdrawing
trifluoromethyl group at the para position provided much
improved activity without deterioration in enanioselectivity
(Table 1, entry 6, 18 h, 65% conv., 67% ee). Bearing an even
more electron-deficient aromatic amine, QD-8c furnished the
highest activity but also the lowest enantioselectivty (Table 1,
entry 7, 18 h, 99% conv., 40% ee). Building on the overall most
promising result obtained with QD-8b, we next focused on the
identification of the optimal acid cocatalyst, which led to the
identification of chiral R-2-chloropropionic acid (R-9) as the
optimal cocatalyst for both reactivity and selectivity (Table 1,
entry 12, 2 h, 98% conv., 73% ee).10
Although cooperative acid−base catalysis by the Δ5-3-
ketosteroid isomerase is implicated by extensive mechanistic
studies for the isomerization of β,γ- to α,β-unsaturated steroidal
ketones,4a−d the same isomerization has also been shown to
readily occur with cooperative iminium-base catalysis by primary
amines and catalytic antibodies, respectively.4g−i With respect to
the former, Kayser and Pollack first demonstrated that an
aqueous solution of 2,2,2-trifluoroethylamine could promote the
isomerization of 3-methyl cyclohex-3-enone to its α,β-unsatu-
rated isomer. The same authors also reported a particularly
insightful kinetic result, namely the determination of a second-
order dependence of the rate of isomerization on the amine
concentration, which provided strong support for an enamine
pathway initiated by deprotonation of the α-H in 3-methyl-
cyclohex-3-enone via cooperative iminium-base catalysis.4g
We thus initiated an investigation of the 9-NH2 cinchona
alkaloid QD-7 (Figure 2) as a catalyst for the isomerization of the
Figure 2. Electronically tunable primary amine catalysts derived from
cinchona alkaloids.
Similar to our observations in the studies of other catalytic
asymmetric isomerizations,5b erosion of optical purity of the
product via racemization occurred gradually at room temper-
ature (Table 1, entry 12, 24 h, 55% ee from 73% ee). Fortunately,
this undesirable racemization could be nearly eliminated when
the reaction was carried out at 0 °C (Table 1, entry 13, 79% ee).
Upon further optimization of reaction temperature, solvent and
concentration, a highly enantioselective isomerization of 2A
could be attained in toluene at −25 °C (Table 1, entry 16).10 To
our surprise, the quinine-derived Q-8b under the optimized
condition did not perform nearly as well as QD-8b did (Table 1,
entry 17). Fortunately, after substantial catalyst screening and
reaction optimization studies, we established that Q-8c catalyzed
the isomerization of 2A to afford the other enantiomer of the
chiral product in high optical purity (Table 1, entry 18).10
Having achieved a highly enantioselective isomerization with
enone 2A, we turned our attention to the investigation of the
substrate scope. The catalyst demonstrated a great deal of
latitude in tolerating the alkyl groups at both the β- and γ-
positions. Variations of the length of the alkyl chain at either
position have no or little influence on either the catalytic activity
or enantioselectivity. The sterically bulkier isopropyl group was
also readily tolerated at both positions (Table 2, entries 1−5, 79−
94% yield, 87−90% ee). It is noteworthy that the presence of the
corresponding cyclohexanones of 2, a commonly observed side
product from the Birch reduction,3b showed no negative impact,
as β,γ-unsaturated cyclohexenones 2 ranging from 70 to 98% in
enone 2A. Presumably, the chiral diamines such as QD-7 could
mediate the enantioselective isomerization of the enone 2A via
the enamine pathway with cooperative iminium-base catalysis
(Scheme 2). In the presence of TFA as a cocatalyst, the
isomerization of 2A progressed to near completion and produced
the desired α,β-unsaturated enone 3A in 30% ee (Table 1, entry
4). These results verified that chiral diamines such as QD-7 could
afford the desired activity for the isomerization of 2A to 3A.
Scheme 2. Proposed Mechanism for Isomerization of 2A via
an Enamine Pathway by a Chiral Primary−Tertiary Diamine
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Communication
to our knowledge, only two racemic syntheses of 11 have been
reported, requiring 15 and 11 steps, respectively.13a,b
Table 2. Isomerization of β,γ-Unsaturated to α,β-Unsaturated
Enones Catalyzed by QD-8b and Q-8c
Scheme 3. Retrosynthetic Analysis
a
Scheme 4. Asymmetric Synthesis of (−)-Isoacanthodoral
a
Reagents and conditions: (a) Mg; CuI, THF, −20 °C; 16, 91%. (b)
Polyphosphoric acid, 10 °C, warm to rt, 70%, 13% regioisomer. (c) Li/
NH3/Et2O; (CO2H)2, 74%. (d) QD-8b (10 mol%), R-2-chloro-
propionic acid (20 mol%), PhCH3/PhF (5/1, 0.33 M), −20 °C, N2(g),
5 d, 69%, 86% ee, 13 (16%). (e) MeLi·LiBr, −78 °C, 70%. (f)
Dodecane (3.5 M), 65 °C, 5 d; dodecane (0.4 M), 140 °C, 20 h, 65%.
a
Unless noted, reactions were run with 0.1 mmol of 2 with QD-8b by
method A. Results in parentheses were obtained with Q-8c by method
B. Isolated yield. Determined by HPLC. Reaction was run with
QD-8b in PhF. 10 mol% acid used. Determined by GC. Only one
diastereomer was obtained.
b
c
d
e
f
g
Our synthesis commenced with nucleophilic addition of the
cuprate derived from 17 to dimethyl oxirane 16 (Scheme 4).14
The resulting alcohol 15 readily underwent Freidel−Crafts
cyclization in the presence of polyphosphoric acid15 to furnish
anisole 14 as the major isomer, which was subjected to Birch
reduction followed by hydrolysis to form β,γ-unsaturated enone
13. To our delight, upon minor modifications of the standard
protocol,10 the key asymmetric isomerization was accomplished
in a highly enantioselective manner to afford the chiral bicyclic
enone 12 in 86% ee and 69% yield. However, attempted 1,4-
additions with vinyl cuprate to 12 invariably produced the 1,2-
addition product along with unreacted enone 12.16 We then
explored an alternative strategy featuring an intramolecular
Claisen rearrangement to install the cis-fused bicyclic γ,δ-
unsaturated aldehyde. Accordingly, enone 12 was subjected to
1,2-addition with MeLi·LiBr to yield alcohol 18 in 70% yield.17
After considerable experimentations, a tandem vinylation−
Claisen rearrangement reaction sequence following modifica-
tions of the protocol reported by Wei18 was realized for the direct
conversion of alcohol 18 into (−)-isoacanthodoral 11 in 65%
yield. Thus, enantioselective total synthesis of (−)-isoacantho-
doral was achieved in seven steps and 18% overall yield from
commercially available starting materials 17 and 16.
purity were successfully applied in the asymmetric isomerization.
Moreover, enones 2 bearing functional groups such as prenyl and
carboxylic ester were converted into the desired product in useful
yield and enantioselectivity (Table 2, entries 6−8, 75−79% yield,
87−89% ee). Consequently, the asymmetric synthesis of the
optically active γ-prenyl cyclohex-2-enone 3G (Table 2, entry 7),
an intermediate for the total synthesis of garsubellin A,1l was
readily accomplished from the corresponding anisole precursor.
In addition, the presence of an unprotected hydroxyl group is
also compatible with the reaction, rendering it possible to directly
generate the chiral chromen-7-one 10I from 2I via an asymmetric
tandem isomerization-conjugate addition reaction. To our
delight, the bicyclic substrate 2J was found to undergo the
QD-8b catalyzed isomerization to furnish the Δ1,9-octalone-2 3J
in 85% ee and 67% yield (Table 2, entry 10).11 We also examined
a γ-substituted cyclohex-3-en-1-one 2K, which was converted
into the expected product 3K in 44% yield and 72% ee (Table 2,
entry 11).
To illustrate the synthetic utility of this new asymmetric
transformation, we embarked on the first asymmetric total
synthesis of (−)-isoacanthodoral (11),12 featuring a new strategy
for the construction of the cis-fused bicyclo[4.4.0]dec-1-ene ring
via the enantioselective isomerization of enone 13. The enone 13
was to be prepared from a Birch reduction of anisole 14, which in
turn could be synthesized from anisole 17 (Scheme 3). Notably,
In summary, by exploring cooperative iminium-base catalysis
with a newly designed class of electronically tunable organic
catalysts derived from cinchona alkaloid, we have realized an
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Journal of the American Chemical Society
Communication
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accessible from anisoles. The synthetic potential of this new
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of the first enantioselective synthesis of (−)-isoacanthodoral 11.
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for the current asymmetric isomerization. More importantly, this
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the development of useful catalysts for other asymmetric
transformations.
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ASSOCIATED CONTENT
* Supporting Information
Experimental procedures, additional optimization studies,
analytical data, and spectra for new compounds. This material
is available free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
deng@brandeis.edu
■
(6) (a) pKa of the α proton of cyclohex-3-enone in water was
determined to be 15.2: Dzingeleski, G. D.; Blotny, G.; Pollack, R. M. J.
Org. Chem. 1990, 55, 1019. (b) pKa of the α proton of α-angelica lactone
was calculated to be 10.8: Gom
Perez-Prior, M. T.; Calle, E.; Casado, J. J. Org. Chem. 2009, 74, 4943.
́
́ ́ ́
ez-Bombarelli, R.; Gonzalez-Perez, M.;
Notes
The authors declare no competing financial interest.
(7) Selected reports of 7-catalyzed asymmetric reactions: (a) Xie, J.-
W.; Chen, W.; Li, R.; Zeng, M.; Du, W.; Yue, L.; Chen, Y.-C.; Wu, Y.;
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ACKNOWLEDGMENTS
We are grateful for the generous financial support from National
Institutes of Health (GM-61591).
■
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