Gold-Catalyzed Synthesis of Chiral Cyclopentadienyl Esters via Chirality Transfer

Article abstract of DOI:10.1021/acs.orglett.0c02293

Efficient access to chiral cyclopentadienyl esters from readily accessible chiral enynyl ester substrates is developed. Typically high levels of chirality transfer realized in this homogeneous gold catalysis are attributed to the intermediacy of a chiral bent allene gold complex. Cyclopentadienyl esters can be prepared in good yields and with excellent enantiomeric excesses. The synthetic utilities of the chiral cyclopentadienyl esters are demonstrated by the Diels-Alder reactions, fluorination, alkylation, and epoxidation without any notable erosion of enantiopurity.

Full text of DOI:10.1021/acs.orglett.0c02293

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Letter
Gold-Catalyzed Synthesis of Chiral Cyclopentadienyl Esters via
Chirality Transfer
Ke Zhao,§ Yu-Chen Hsu,§ Ziguang Yang, Rai-Shung Liu,* and Liming Zhang*
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ABSTRACT: Efficient access to chiral cyclopentadienyl esters from
readily accessible chiral enynyl ester substrates is developed.
Typically high levels of chirality transfer realized in this homogeneous
gold catalysis are attributed to the intermediacy of a chiral bent allene
gold complex. Cyclopentadienyl esters can be prepared in good yields
and with excellent enantiomeric excesses. The synthetic utilities of
the chiral cyclopentadienyl esters are demonstrated by the Diels−
Alder reactions, fluorination, alkylation, and epoxidation without any
notable erosion of enantiopurity.
yclopentadienes are important synthetic intermediates
Cwidely used as reactive dienes in the Diels−Alder
reaction1 and as ligand precursors in organometallic chemistry,
and the enantiomerically enriched ones are highly valued in
complex molecular synthesis.2 Aside from modification from
chiral five-membered ring precursors,2a−d,3 which can be of
limited scope, asymmetric synthesis of this class of chiral cyclic
dienes from acyclic substrates offers a potentially more efficient
and flexible synthetic alternative, but there have been only a
few reported studies of limited scope and/or moderate
enantiopurity.4 Moreover, few reports have documented such
an approach to cyclopentadienes with heterofunctionalization
at the ring. In this work, we report a study of this nature based
on gold-catalyzed efficient chirality transfer,4b,5 which permits
the synthesis of cyclopentadienyl esters with generally excellent
enantiomeric access from readily accessible chiral substrates.
In 2006, one of us reported the gold-catalyzed cyclo-
isomerization of enynyl esters into cyclopentenones.6,7 An
example is shown in Scheme 1A. The proposed reaction
mechanism entails a gold-catalyzed 3,3-sigmatropic rearrange-
ment to deliver enallenyl acetate 3, its subsequent gold-
catalyzed Nazorov-type cyclization, and hydrolysis. The
intermediacy of cyclopentadienyl acetate 2 was confirmed by
its detection when the reaction was performed in dry DCM
instead of wet DCM. Pentadienyl cation A was invoked as the
reactive intermediate in the conversion of 3 to 1. However, the
subsequent studies by Gandon, Fensterbank, and Malacria8
Scheme 1. Previous Work on the Gold-Catalyzed
Cycloisomerizations of Enynyl Esters and This Chemistry
Received: July 10, 2020
https://dx.doi.org/10.1021/acs.orglett.0c02293
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

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a
Table 1. Initial Trial and Condition Optimization
Table 2. Scope of with Substrates Featuring Cyclic Alkenes
catalyst (5 mol %/
5 mol %)
yield (%) of
ee
c
entry
1
solvent
wet
R
6a/6a′
(%)
PPh3AuCl/
a
CH3
0/70
67
70
74
77
94
92
93
94
−
AgSbF6
DCM
wet
DCM
wet
DCM
wet
DCM
wet
DCM
wet
DCM
wet
DCM
wet
DCM
wet
2
3
4
5
6
7
8
9
PPh3AuCl/
tBu
0/76
0/75
0/74
0/84
0/70
0/84
0/78
0/trace
a
AgSbF6
JohnPhosAuCl/
tBu
a
AgSbF6
a
a
a
a
a
a
,
,
,
,
,
,
,
b
b
b
b
b
b
b
L1AuCl/AgSbF6
L1AuCl/AgSbF6
L1AuCl/AgSbF6
L1AuCl/AgSbF6
L1AuCl/AgSbF6
L1AuCl/AgSbF6
tBu
Ph
mesityl
4-MeOPh
4-BrPh
4-NO2Ph
DCM
toluene
toluene
toluene
a
b
10
11
12
13
14
15
L1AuCl/AgSbF6
L1AuCl/AgSbF6
L1AuCl/AgNTf2
Ph
Ph
Ph
Ph
Ph
Ph
49/29
52/33
48/44
91/0
87/0
82/0
93
93
91
95
95
96
b
b
L1AuCl/NaBARF toluene
L1AuCl/NaBARF PhCF3
L1AuCl/NaBARF DCM
b
b
a
b
c
With 10 mol % AgSbF6. L1 = tBuBrettPhos. Of the major product.
that trap the gold carbene intermediate of type B by a tethered
alkene revealed that gold-coordinated bent allene structures C
and C′ are the alternative intermediates that account for the
observed efficient chirality transfer (Scheme 1B). These works
explored only a few substrates. We reasoned that this chirality
transfer would similarly permit efficient access to chiral
cyclopentadienyl esters from chiral enynyl esters (Scheme
1C), and expanded exploration of the substrate scope would
shed more light on chirality transfer. During our study,
Carreira reported a related efficient chirality transfer process, in
which an intramolecular aldol reaction serves to terminate the
tandem gold-catalyzed 3,3-sigmatropic rearrangement and the
Nazarov reaction (Scheme 1D).9
At the outset, we employed (S)-1-(cyclohex-1-en-1-yl)oct-1-
yn-3-yl ester 4a as the substrate. It is readily prepared from the
corresponding enynone via Noyori’s asymmetric transfer
hydrogenation10 and subsequent esterification. Anticipating
the nature of the acyl group in 4a might influence the desired
chirality transfer, and to simplify chiral HPLC analysis, we
opted to first optimize the chirality transfer by examining the
hydrolyzed bicyclic cyclopentenone product 6a′, which could
be directly obtained with wet methylene chloride as the
reaction solvent. In the presence of catalytic Ph3PAuCl (5 mol
%) and AgSbF6 (10 mol %), the acetate version of 4a indeed
underwent smooth cycloisomerization and hydrolysis to afford
6a′ in 70% yield (entry 1). Moreover, the enantiomeric excess
of the product was 67%, confirming a significant level of
chirality transfer from the chiral enyne substrate. The
configuration of 6a′ is assigned on the basis of mechanistic
consideration and supported by the prior report.9 Switching
a
Reaction conditions: 5 mol % tBuBrettPhosAuCl, 5 mol % NaBARF,
b
0.2 mmol of 4 (0.05 M in toluene), rt, 3 h. ee was determined after
hydrolysis. The standard condition was applied and followed by
c
d
hydrolysis. The standard condition but at 60 °C for 4 h was applied
e
and followed by hydrolysis. With 5% tBuBrettPhosAuCl, 10%
f
AgSbF6, and wet DCM applied. cis:trans ratio of 14:1.
the acyl group of 4a to a pivaloyl led to slight improvements in
the reaction yield and the product ee (entry 2). The product ee
was improved to 74% by using the sterically hindered
JohnPhos as the metal ligand8b,9 (entry 3) and further to
t
77% by using the even bulkier BuBrettPhos (entry 4). Much
to our delight, when the benzoate of 4a was employed as the
substrate, the yield was improved to 84% and the ee to 94%
(entry 5). Modifications on the benzoate benzene ring did not
further improve the reaction yield or the efficacy of chirality
transfer (entries 6−8) or in the case of 4-NO2 led to little
reaction (entry 9). With the optimal substrate and catalyst in
hand, we then turned our attention to avoiding the hydrolysis
of 6a to 6a′ during the gold catalysis. To this end, anhydrous
toluene was used as the reaction solvent. To our delight, the
desired cyclopentadienyl benzoate 6a was formed with 93% ee,
albeit accompanied by a substantial amount of hydrolyzed
product 6a′. Decreasing the amount of AgSbF6 to 5 mol %
(entry 11) or switching it to 5 mol % AgNTf2 (entry 12) did
not prevent the formation of a substantial amount of 6a′.
Remarkably, when NaBARF (5 mol %) was used as the
B
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a
Table 3. Scope of Substrates Containing Acyclic Alkenes
Scheme 2. Synthetic Transformations of Cyclopentadienyl
Benzoate 6a
conditions, affording cyclopentenone 6h′ in a much improved
61% yield and with a slightly lower ee (87%). For cycloheptene
substrate 4i, cyclopentadienyl ester 6i was unstable. As such,
the hydrolytic conditions in entry 5 of Table 1 were employed
to afford 5,7-fused enone 6i′ in 84% yield (entry 8). However,
the ee value is moderate, which may be due to a relatively slow
cyclization of the bent allene intermediate of type C and hence
its increased level of racemization8a or conversion to achiral
pentadienyl cations of type A.
To further explore the reaction scope, we turned our
attention to substrates featuring acyclic CC bonds for the
synthesis of chiral cyclopentadienyl esters without ring fusion.
As shown in Table 3, under the optimized conditions, these
reactions proceeded smoothly, affording the tetrasubstituted
cyclopentadienes in good to excellent yields and with excellent
enantiomeric excesses (entries 1−7). Electron-withdrawing
groups on the substrate phenyl ring (entries 6 and 7) are
tolerated, albeit in lower yet serviceable yields. Under the
standard conditions, the reaction of 4q resulted in a
complicated mixture, which is attributed to 1,5-hydride shifts
of the cyclopentadiene moiety. When 4q was subjected to the
hydrolytic conditions, to our surprise, the cyclopentenone
product 6q′ barely exhibits any ee. This result indicates the
steric hindrance offered by the R1 group is essential for
hindering the formation of the corresponding achiral
pentadienyl cation and/or the allene racemization.
To demonstrate the synthetic utilities of this chemistry, we
carried out a scale-up synthesis of 6a (Scheme 2). Hence, with
only 1 mol % catalyst loading, 0.51 g of the product of high
enantiomeric purity (94% ee) was isolated, although the
reaction required overnight and the yield was slightly
decreased due to the much lower catalyst loading. The
reactions of 6a were then pursued first with the isolated
material. For example, it underwent the Lewis acid-promoted
Diels−Alder reaction with methyl acrylate at −40 °C to deliver
bridged tricycle 7 in 84% yield while maintaining the ee value,
and its epoxidation by DMDO smoothly afforded the cis-fused
α′-hydroxycyclopentenone 8 upon hydrolytic workup. The
endo nature of the major isomer of 7 is confirmed by two-
dimensional NMR studies. One-pot processes without the
isolation of 6a were also demonstrated. For example, the
Diels−Alder reaction with N-phenylmaleimide smoothly
a
Reaction conditions: 5 mol % tBuBrettPhosAuCl, 5 mol % NaBARF,
b
0.2 mmol 4 (0.05 M in toluene), rt, 3 h. With 5% tBuBrettPhosAuCl,
10% AgSbF6, and wet DCM applied.
chloride abstractor, the undesired hydrolysis was completely
shut down, and 6a was formed in 91% yield while maintaining
the excellent ee value (entry 13). Similar phenomena were
detected with PhCF3 or DCM as the solvent, albeit the yields
were slightly lower (entry 14 or 15, respectively).
With the optimized reaction conditions in hand, we first
probed the reaction scope by varying the n-pentyl group of 4a.
As shown in Table 2, the sterically more demanding cyclohexyl
(entry 1), a methyl (entry 2), a benzyl (entry 3), and an
oxygenated alkyl group (entry 4) are all readily accommodated,
and the cyclopentadienyl esters were isolated in good to
excellent yields and with ≥92% ee. Incorporation of a
heteroatom such as O (entry 5) and N (entry 6) in the
cyclohexene ring led to lower yields, but the chirality transfer
remained efficient. When 4h contained a dihydronaphthalene
ring as the substrate, the reaction was quite slow under
standard conditions (entry 7). It was accelerated at 60 °C. Due
to the contamination of side products, the cyclopentadienyl
ester product was subsequently hydrolyzed under acidic
C
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delivered the tetracycle 9 in 86% overall yield and with 93% ee,
demonstrating a rapid increase in structural complexity.11
Fluorination at the enone α′-position by Selectfluor afforded
selectively cis-fused 10 in a serviceable yield and without
erosion of ee, and diastereoselective installation of a methyl
group at the same position was achieved with similar results.
In conclusion, we have developed efficient access to chiral
cyclopentadienyl esters from readily accessible chiral enynyl
ester substrates. Typically high levels of chirality transfer
realized in this homogeneous gold catalysis can be attributed to
the intermediacy of chiral bent allene gold complexes.
Cyclopentadienyl esters with or without ring fusion can be
prepared in good yields and with excellent enantiomeric excess.
Benzoates are shown to be more conducive to chirality transfer
than aliphatic esters, and a trisubstituted CC bond in the
substrate is found to be essential to minimize the racemization
of the bent allene gold complex or the access to pentadienyl
cations of type A. The synthetic utilities of the chiral
cyclopentadienyl esters are demonstrated by one-pot or
sequential Diels−Alder reactions, fluorination, alkylation, and
epoxidation without any notable enantiopurity erosion.
ACKNOWLEDGMENTS
■
L.Z., K.Z., and Z.Y. thank the National Science Foundation
(NSF) (CHE 1800525) and National Institute of General
Medical Sciences (R01GM123342) for financial support. NSF
Grant MRI-1920299 is acknowledged for the purchase of
NMR instruments.
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ASSOCIATED CONTENT
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* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c02293.
Experimental procedures, compound characterization
1
data, and H and 13C NMR spectra (PDF)
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AUTHOR INFORMATION
■
Corresponding Authors
Liming Zhang − Department of Chemistry and Biochemistry,
University of California, Santa Barbara, California 93106,
United States; orcid.org/0000-0002-5306-1515;
Email: zhang@chem.ucsb.edu
Rai-Shung Liu − Department of Chemistry, National Tsing-Hua
University, Hsinchu 30013, Taiwan, Republic of China;
orcid.org/0000-0002-2011-8124; Email: rsliu@
mx.nthu.edu.tw
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University of California, Santa Barbara, California 93106,
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Yu-Chen Hsu − Department of Chemistry and Biochemistry,
University of California, Santa Barbara, California 93106,
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Complete contact information is available at:
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Author Contributions
§K.Z. and Y.-C.H. contributed equally to this work.
Notes
The authors declare no competing financial interest.
D
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