Functionalized cyclopentenes through a tandem NHC-catalyzed dynamic kinetic resolution and ambient temperature decarboxylation: Mechanistic insight and synthetic application

Article abstract of DOI:10.1039/c4cc09308a

An unusual room temperature β-lactone decarboxylation facilitated a five-step enantioselective formal synthesis of the cyclopentane core of an estrogen receptor β-agonist. A computational study probed the underlying factors facilitating unprecedented, rapid decarboxylation. Aryl substitution promotes faster reaction in the retro-[2+2] as a result of conjugative stabilization with the forming olefin. Additionally, the configuration of the α-ester in these fused β-lactones leads to differential decarboxylation rates. This journal is

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Functionalized Cyclopentenes Through a Tandem
NHCꢀCatalyzed Dynamic Kinetic Resolution and
Ambient Temperature Decarboxylation: Mechanistc
Insight and Synthetic Application
Cite this: DOI: 10.1039/x0xx00000x
Received 00th January 2012,
Accepted 00th January 2012
Daniel T. Cohen,a Ryne C. Johnston,b Nicholas T. Rosson,b Paul HaꢀYeon Cheong*b
and Karl A. Scheidt*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
furnished the respective products in 20:1 dr. Therefore, the
rapid decarboxylation before the separation of the diastereomer
lactones is inconsequential, allowing for the isolation of the
desired cyclopentene in high yield while maintaining the high
enantioselectivity (90% ee). In this communication, we apply
this DKR to construct the cyclopentane ring of known bioactive
benzopyrans. This report demonstrates that this DKR can be
applied to the enantioselective synthesis of different analogues
of these bioactive benzopyrans. Additionally, we delve into the
mechanism of the rapid decarboxylation for this DKR process
using computational methods and explain why certain
substrates are more prone to rapid decarboxylation, even under
these mild conditions.
An unusual room temperature
facilitated a fiveꢀstep enantioselective formal synthesis of the
cyclopentane core of an estrogen receptor ꢀagonist. A
computational study probed the underlying factors
facilitating unprecedented, rapid decarboxylation. Aryl
substitution promotes faster reaction in the retroꢀ[2+2] as a
result of conjugative stabilization with the forming olefin.
Additionally, the configuration of the αꢀester in these fused βꢀ
lactones leads to differential decarboxylation rates resulting
in enantioselectivity.
βꢀlactone decarboxylation
β
Catalytic methods to assemble bioactive molecules and
privileged motifs are onꢀgoing pursuits in synthesis. Many have
sought to achieve this goal by employing organic catalysts,
which allows for a greener approach.1
NꢀHeterocyclic carbenes
(NHC) have emerged as powerful class of organic catalysts that
can be used to construct important structural motifs, bioactive
molecules, and natural products.2 Our group and others have
shown that NHCs react with aldehydes to generate catalytically
competent enolate,3 homoenolate,4 and acyl anion
intermediates,5 which have been trapped with various
electrophiles. In 2012, we reported the first NHCꢀcatalyzed
dynamic kinetic resolution (DKR) of αꢀsubstitutedꢀβꢀketoesters
(1) to furnish bicyclic βꢀlactones (2) and cyclopentenes (3)
(Scheme 1 eq. 1).6 Computational studies7 shed light on the
rare, nonꢀclassical nature of this specific organocatalytic DKR
and on the origins of stereoselectivity in the βꢀlactone
formation. While examining this NHCꢀDKR process we
determined that aryl ketones with electronꢀdonating substitution
or heteroaromatic ketones normally resulted in complete
spontaneous decarboxylation to the cyclopentene (3) under the
reaction conditions (eq. 2). Unfortunately, this rapid
decarboxylation led to cyclopentenes with diminished
enantioselectively, due the moderate diastereoselectivity (5ꢀ7:1
d.r.) for the lactone forming process. However, substrates
possessing an orthoꢀsubstitution on the aryl ring of the ketone
Scheme 1. NHC-Catalyzed Dynamic Kinetic Resolution
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Scheme 2. Enantioselective formal synthesis of estrogen receptor
β
-agonist 11.
In 2006, Eli Lilly disclosed a new class of hydroxylated benzopyrans decarboxylative [2+2]ꢀcycloreversion of βꢀlactones (Scheme 3).11
that were highly potent and selective estrogen receptor βꢀagonist Reduced temperatures have traditionally only been achieved through
with nanomolar activity in models of benign prostatic hyperplasia, radical decarboxylation.12 Electronꢀneutral (2b) and ꢀpoor (2c) βꢀ
otherwise known as enlarged prostate (Figure 1).8 A subsequent lactones are stable to Scheme 1 conditions; decarboxylation only
structureꢀactivity relationship study revealed that the cyclopentane occurs after heating in SiO2 (Scheme 3), as previously observed.
ring was essential to provide nanomolar activity while maintaining
To probe the apparent electronic effects controlling this unusual
the great selectivity.8e Previous reported syntheses to these
decarboxylation and the origins of enantioselectivity, we conducted
enantioenriched benzopyrans relied on an eightꢀstep linear reaction
a computational study using quantum mechanical computations
sequence and a subsequent preparative chrial HPLC separation of
(SCSꢀMP213/def2∞ZVP14//B3LYP15/6ꢀ31G(d)16 with B3LYP/ 6ꢀ
the final benzopyran to obtain enantiopure products. To the best of
31+G(d,p)/PCM(DCE)17 solvation corrections).18 The computed
our knowledge, a general catalytic asymmetric variant to these
retroꢀ[2+2] transition state structures (TSs) are shown in Figure 1.
compounds has not been reported. The formal synthesis began with
The anti and syn refer to the diastereomeric relationship between the
the treatment of literature known9 acetophenone 4 with dimethyl
lactone and the loss of CO2. All transition structures are concerted
carbonate and sodium hydride in toluene afforded βꢀketo ester 5. A
TsujiꢀTrost allylation of βꢀketoester and with allyl carbonate 6 and
subsequent PCC oxidation provided aldehyde 7 in 36% yield over
three steps. In the key enantioselective, catalytic DKR step, the
exposure of aldehyde 7 to 7 mol % of azolium 8 with 30 mol % of
asynchronous,19 and our computed barriers match the observed
relative rates of decarboxylation. The anti configuration reacts faster
than the syn (Scheme 3).
Cs2CO3 gave cyclopentene
9 in 86% yield with excellent
enantioselectivity (92% ee). A subsequent hydrogenation (Pd/C)
gave cyclopentane 10 in 48% yield as a sole diastereomer based on
comparison with reported structures.10 This cyclopentane (10) in
racemic form has previously been converted to selective estrogen
receptor benzopyran 11 in three steps by scientists at Eli Lilly.8e
Thus we have been able to employ our NHCꢀDKRꢀdecarboxylation
strategy toward an efficient, asymmetric route to these valuable
benzopyrans and related structures. Most notably this synthetic
sequence avoids the previously described chiral separation approach
and provides a stereocontrolled and efficient method to rapidly
assemble a diverse library of these bioactive benzopyrans.
βꢀlactones with an electronꢀrich aryl ring, such as 4ꢀMeOPh (2a,
Table 1, entry 1), synthesized under Scheme 1 conditions undergo
unprecedented and rapid decarboxylation to the cyclopentene, often
leaving no trace of the lactone.6 This outcome is in sharp contrast to
the thermolytic conditions (>60 ºC) typically required for
Scheme 3. β-Lactone decarboxylation to cyclopentene.20
CO2 ejection is accelerated by conjugation to the planar aryl group.
The configuration of the αꢀester affects the conjugation by affecting
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the degree of planarity of the aryl group with the forming olefin. In
the anti TS, the aryl groups are planar to the forming olefin,
facilitating the ejection. This is in contrast to the syn TS, where the
aryl group is slightly twisted out of the plane, costing an energetic
penalty of 0.3 kcal molꢀ1.
Table 2. Test for origins of selectivity by α group modulation.
As a consequence of the conjugation, the electronic nature of the
aryl group has a direct effect on the reactivity. Initial cleavage of the
C–O bond leads to a transient benzylic carbocation character, which
is sensitive to the electronics of the aryl group. Electronꢀdonating
para and/or ortho substituted aryl groups dramatically lowers the
decarboxylation TS barriers, for example, pꢀOMe aryl substitution
(Table 1) reacts rapidly anti at 19.1 kcal molꢀ1. This is in contrast to
electronꢀneutral and electronꢀpoor substituted aryl lactones that have
higher barriers of 27.5 and 31.4 kcal molꢀ1, respectively, in
agreement with experimentally observed higher reaction
temperatures. In all aryl cases, the syn barriers are higher, but still
follow the general trend. This trend echoes limited reports on the
decarboxylation in a series of related βꢀlactones at ambient and
cryogenic temperatures, with rapid decarboxylation for electronꢀ
donating (pꢀOMePh) substituents and none for electronꢀneutral or ꢀ
poor substituents.21 In the absence of conjugating aryl groups, e.g.,
aliphatic (∆G‡ = 33.3, ∆∆G‡ = 0.1) and hydro (∆G‡ = 38.2, ∆∆G‡ = –
1.0), barriers are higher and the enantioselectivity is diminished and
reversed, respectively.
antiꢀTS12
∆G‡a
33.9
synꢀTS12
∆G‡ (∆∆G‡)a
34.3 (0.4)
29.7 (–0.8)
21.0 (2.6)
Entry
R’
4ꢀOMePh
H
4ꢀOMePh
H
R
1
2
3
4
CN
CN
Me
Me
2f
2g
2h
2i
30.5
18.4
35.8
34.6 (–1.2)
a Energies reported in kcal molꢀ1.
Enantioselectivity is inherent to the molecule by virtue of the αꢀ
group configuration. We modulated the size and electronics of the αꢀ
group to probe whether steric or electronic factors (measured in A
+ 23
values22 and σp , respectively) give rise to the different anti/syn
+
barriers. To test for steric control, the ester (σp = 0.49, A = 1.27)
was replaced by the similarly electronꢀwithdrawing but smaller
cyano group (σp = 0.66, A = 0.17). CN αꢀsubstitution led to a loss
of selectivity. Electronic control was investigated by replacement
with the equivalently sized, but electron donating methyl group (σp
+
+
=
–0.31, A = 1.7). Retention of selectivity was observed
computationally with methyl αꢀsubstitution and loss with cyano,
indicating that the ester controls selectivity through its steric
component. This observed steric effect originates from the strain
afforded by decarboxylation syn to the α group (Figure 1). The
pseudoꢀequatorial (R)ꢀester in the minor syn-TS12 diastereomer
leads to a repulsive interaction (2.9 Å) with the departing CO2. The
absence of this repulsion in the favored anti-TS12 lowers the barrier
by ~2–3 kcal/mol.
In conclusion, we have demonstrated the NHCꢀcatalyzed
DKR/decarboxylation process can be utilized as the key step in the
enantioselective formal synthesis of a benzopyran estrogen receptor
βꢀagonist. Using an orthoꢀsubstituted aryl ketone allowed us to
achieve high enantioselectivity (92% ee) without prior separation of
the diastereomers. This processes alleviates the need for chiral
HPLC as previously used in a Lilly campaign to separate the
enantiomers of the benzopyrans. Utilizing computational studies we
uncovered the principles controlling the concerted retroꢀ[2+2]
decarboxylation mechanism from this DKR, including the electronic
effects on rate. Aryl ketones with electron rich substitution likely
decarboxylate through ortho/paraꢀquinone methide resonance like
intermediates. The antiꢀselective decarboxylation observed for aryl
ketones is the result of steric interactions between the α group and
the carboxylate groups in the syn-TSs.
Figure 1. Decarboxylation TSs of synꢀ and antiꢀdiastereomers of 2a
.
Table 1. Electronic effects on decarboxylation rate and selectivity.
Yield
(% ee)a
71 (70)
95 (98)c,d
64 (99)c
antiꢀTS12
∆G‡b
19.1
synꢀTS12
∆G‡ (∆∆G‡)b
22.1 (3.0)
27.5 (2.8)
31.4 (2.3)
33.4 (0.1)
37.2 (–1.0)
Entry
R
4ꢀOMePh
Ph
4ꢀCNPh
Me
1
2
3
4
5
2a
2b
2c
2d
2e
24.7
29.1
33.3
38.2
e
–
e
H
–
a Scheme 1 conditions, (S)ꢀ3 major product. b Energies reported in
kcal molꢀ1. c No decarboxylation observed under Scheme 1
conditions. d Scheme 2 conditions. e Theoretical values only.
Substrates not accessible through Scheme 1.
DTC and KAS acknowledge financial support generously provided
by the NIH (NIGMS RO1ꢀGM073072). RCJ and PHYC
acknowledge financial support generously provided by Oregon State
University and computing infrastructure in part provided by the NSF
Phaseꢀ2 CCI, Center for Sustainable Materials Chemistry (NSF
CHEꢀ1102637). DTC thanks the ACS Division of Organic
Chemistry for a 2011ꢀ12 Graduate Fellowship, sponsored by
Organic Syntheses/Organic Reactions. RCJ thanks Oregon State
University for the Tartar Research Fellowship. Professor Chad
Eichman (Loyola University) is thanked for helpful discussions. Dr.
Elizabeth O. McCusker is acknowledged for help in preparation of
acetophenone 4.
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