Asymmetric Multifunctional Modular Organocatalysis: One-Pot Direct Strategy to Enantiopure α,β-Disubstituted γ-Butyrolactones

Article abstract of DOI:10.1021/acs.orglett.9b02094

A simple and efficient approach to enantioenriched α,β-disubstituted γ-butyrolactones has been developed through multifunctional modular organocatalysis in a highly enantioselective (>99% ee) and diastereoselective (>30:1) manner following a one-pot sequential Michael-hemiacetalization-oxidation reaction. The catalytic process has great substrate compatibility, and the products have been transformed to synthetically useful molecules. The methodology has also been applied to the formal synthesis of (+)-Pilocarpine.

Full text of DOI:10.1021/acs.orglett.9b02094

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Asymmetric Multifunctional Modular Organocatalysis: One-Pot
Direct Strategy to Enantiopure α,β-Disubstituted γ‑Butyrolactones
Pratibha Mahto,† Nirmal K. Rana,‡ Khyati Shukla,† Braja G. Das,† Harshit Joshi,†
and Vinod K. Singh*,†
†Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur - 208 016, UP, India
‡Department of Chemistry, Indian Institute of Technology Jodhpur, Jodhpur - 342 037, Rajasthan, India
S
* Supporting Information
ABSTRACT: A simple and efficient approach to enantioenriched α,β-disubstituted γ-butyrolactones has been developed
through multifunctional modular organocatalysis in a highly enantioselective (>99% ee) and diastereoselective (>30:1) manner
following a one-pot sequential Michael−hemiacetalization−oxidation reaction. The catalytic process has great substrate
compatibility, and the products have been transformed to synthetically useful molecules. The methodology has also been
applied to the formal synthesis of (+)-Pilocarpine.
ne-pot sequential reaction is one of the most dynamic
Oand powerful synthetic strategies especially in the field of
asymmetric organic synthesis. Endowed with the advantages of
sustainability, cost, time effectiveness, step economy, and low
waste production, one-pot operation provides efficient path-
ways for the execution of multiple transformations involving
the formation of multiple C−C and/or C−X bonds.1 Owing to
the numerous advantages, asymmetric one-pot reactions have
been frequently utilized in manufacturing small to complex
chiral molecules including heterocycles.2
γ-Butyrolactones, oxygen-rich five-membered heterocycle
scaffolds, are found in a diverse range of natural products
and pharmaceutically significant molecules.3 Furthermore, they
serve as versatile intermediates in synthesizing complex
molecules.4 A few selected examples are shown in Figure 1.
Therefore, the ubiquity of a γ-butyrolactone motif and its
significant bioactivities makes it quite imperative to explore
Figure 1. Selected examples of biologically relevant molecules having
γ-butyrolactone core.
and map new methodologies. Thus, in recent years, with the
advent of organocatalytic tandem/cascade reactions,5 a wide
Other significant developments involving amine catalysis
include Michael−cyclization cascades of several active
nucleophiles such as indoles,9 boronic acid,10 and α-hydroxy
ketones.11 However, direct asymmetric organocatalytic meth-
ods to construct chiral α,β-disubstituted γ-butyrolactones
remain elusive.12 Therefore, a straightforward approach
variety of stereoselective processes have been established to
access optically active butyrolactones.6 The majority of
organocatalytic methods to construct γ-butyrolactones are
directed toward the use of NHC catalysis.7 In 2015, Xu et al.
reported an enantioselective route to trisubstituted γ-lactones
via addition of aldehydes to activated α,β-unsaturated ketones
bearing a carboxylic acid group at β-position which proved to
be essential for reactivity as well as selectivity (Scheme 1a).8
Received: June 18, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02094
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Organic Letters
Letter
direct asymmetric one-pot synthesis of α,β-disubstituted γ-
butyrolactone via Michael−hemiacetalization−oxidation.
At the outset, model substrates valeraldehyde (1a) and (E)-
4-hydroxy-1-phenyl-2-en-1-one (2a) were stirred at room
temperature with 10 mol % of Hayashi−Jørgensen catalyst18
(3a) in dichloroethane. Despite some expectations, only a 12%
yield of desired γ-butyrolactone 4aa was isolated after
continuing the reaction for 7 days followed by in situ oxidation
with pyridinium chlorochromate (PCC). Pleasingly, the
enantiomeric excess of the γ-lactone 4aa was determined to
be >99% (Table 1, entry 1). In a traditional way,19 reaction
conditions were varied to increase the yield, but the results
were disappointing.20 Concerned about the yield of the
reaction and to check the feasibility of our hypothesis, we
Scheme 1. Background
Table 1. Screening of Multifunctional Modular
a
Organocatalysts
utilizing simple starting materials under mild reaction
conditions for creating chiral γ-butyrolactones in a highly
enantio- and diastereoselective as well as one-pot fashion is still
desired. Following the pioneering work of Falck et al. on
intramolecular oxy-Michael reaction (Scheme 1b),13 several
groups have utilized the bidentate substrate γ-hydroxy α,β-
unsaturated ketone as a potential synthon in many catalytic
asymmetric transformations.14
In our continuous effort to develop efficient and novel
asymmetric approaches for the synthesis of chiral molecules,15
we envisioned that the γ-hydroxy α,β-unsaturated ketone could
be employed as a Michael acceptor in an enamine catalysis
with simple aldehyde to efficiently produce substituted γ-lactol
which could in principle be oxidized in situ to γ-butyrolactone
(Scheme 1c). However, this would be challenging because of
the following: (i) low electrophilicity at the β-carbon of the
α,β-unsaturated ketone toward the addition of enamine, (ii)
formation of unwanted acetal in the presence of an aldehyde,
(iii) dimerization and control of enantiofacial selectivity. Thus,
the choice of catalysts is of utmost importance. Cooperative
hydrogen bond donor catalytic systems could be suitable here,
and these have been proven to be efficient tools for several
asymmetric transformations.16 However, these are mainly
limited to anion-binding catalysis in which substrates are
capable of forming stabilized, conjugate ionic intermediates.17
It was hypothesized that a multifunctional modular assembly of
two distinct catalysts could successfully be engaged to
overcome these challenges. As depicted in Scheme 1d, a
secondary amine catalyst would form enamine while the H-
bond donor cocatalyst would activate the γ-hydroxy α,β-
unsaturated ketone to increase its reactivity as well as
selectivity. Since these two catalyst modules are not joined
by any covalent bond, each module could individually be tuned
easily as desired. Further, this would provide more scope to
build a modular catalysts library and reduce both the time and
the cost of the screening process. Herein, we report an efficient
b
c
entry
cat. 3
cat. 5
time (d) yield (%)
dr
ee (%)
1
2
3
4
5
6
7
8
9
10
3a
3a
3a
3a
3a
3a
3b
3d
3e
3f
3c
3a
3a
ent-3a
ent-3a
3a
−
5a
5b
5c
5d
5e
5e
5e
5e
5e
−
5e
ent-5e
5e
ent-5e
5e
7
3
3
3
3
2
5
5
3
5
7
2
2
2
2
4
2.5
12
29
32
62
64
78
trace
trace
52
trace
12
62
74
72
72
20:1
20:1
20:1
>30:1
>30:1
>30:1
−
>99
98
98
>99
99
>99
−
−
96
−
−
>30:1
−
d
11
12
16:1
>30:1
>30:1
>30:1
>30:1
−
17
e
>99
>99
>99
>99
>99
>99
13
14
15
16
f
f
g
trace
70
h
17
3a
5e
>30:1
a
Reaction conditions: 1a (34.4 mg, 0.4 mmol), 2a (32.4 mg, 0.2
mmol), 3 (0.02 mmol), 5 (0.02 mmol), DCE (1 mL), PCC (129.3
b
mg, 0.6 mmol), unless noted otherwise. Determined by the 1H NMR
c
analysis of crude reaction mixture. Determined by chiral HPLC
analysis. 10 mol % DMAP was used in place of cat. 5. The
intermediate lactol was isolated and then oxidized using PCC. The
opposite enantiomer was obtained. Conditions for Anelli oxidation
d
e
f
g
h
were used instead of PCC. The reaction was conducted in 2.5 mmol
scale.
B
DOI: 10.1021/acs.orglett.9b02094
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Organic Letters
Letter
thought to employ the multifunctional modular catalysts
assemblies. Initially, the combination of catalyst 3a with a
series of cocatalyst modules 5a−e was tested. To our delight,
the yield of the reactions improved significantly and the
combination of catalyst 3a and 5e proved to be the best and
delivered γ-butyrolactone 4aa in 78% yield in just 2 days
(Table 1, entry 6). It is worth noting that the use of combined
catalysts proved advantageous in improving the diastereomeric
ratio of the product. These observations clearly pointed out the
significant role of multifunctional modular catalysis in the rate
enhancement as well as diastereoselectivity of the reaction.
After resolving the yield crunch, we studied the effect of
various secondary amine based catalysts 3b and 3d−f. In sharp
contrast to the accelerated rate observed with combined
catalysts 3a and 5e, chiral secondary amines 3b and 3d
screened in combination with 5e in the Michael−hemi-
acetalization reaction resulted in only trace amount of the
product. Notably, the combined catalysts 3e and 5e gave a
good yield (52%) with slightly lower enantioselectivity.
However, imidazolidinone catalyst 3f was found to be
inefficient for this reaction. In a control experiment, when
the reactions were conducted in a two-pot operation, the
isolated yield of 4aa was slightly lower but stereoselectivities
were comparable to the one-pot process (Table 1, entry 12).
Encouraged by the profound success of synergistic catalysis,
the effect on stereoselectivity by employing different
combinations of enantiomeric catalyst partners was inves-
tigated. It was observed that the absolute stereochemistry of 5e
(or ent-5e) had no influence on the outcome of the absolute
stereochemistry of the product and the stereoselectivity as well.
On the other hand, employing ent-3a in combination with
cocatalyst 5e, as expected, provided access to the opposite
enantiomer of γ-butyrolactone 4aa in synthetically viable yield
with excellent stereoselectivities (>30:1 dr, 99% ee). These
observations indicate that there was no matched and
mismatched combination of chiral catalyst modules. Although
the oxidation step was successfully performed by PCC, a green
alternative, e.g., Anelli oxidation has also been tested.
Unfortunately, the conversion in the oxidation step was very
low (Table 1, entry 16), but the enantioselectivity remained
unaltered (>99%). The model reaction was scaled up to 2.5
mmol providing 4aa in 70% yield and >99% ee (Table 1, entry
17).
Scheme 2. Scope of One-Pot Sequential Michael−
a b
,
Hemiacetalization−Oxidation Reaction
After establishing the optimized reaction conditions and the
best combination of catalysts, we proceeded to explore the
substrate scope of the one-pot sequential Michael−hemi-
acetalization−oxidation reaction. A series of primary aldehydes
were first investigated with (E)-4-hydroxy-1-phenyl-2-en-1-one
(2a) (Scheme 2). In general, short- (R = Me) to long-chain (R
= n-hex) aldehydes smoothly underwent the one-pot
sequential reaction to afford the desired α,β-disubstituted γ-
butyrolactones 4aa−fa in high yields with excellent stereo-
selectivities (up to >30:1 dr, >99% ee). Importantly, sterically
demanding isovaleraldehyde (1g) was also compatible with the
catalytic process. Albiet with a prolonged reaction time,
corresponding product 4ga was isolated in 51% yield and
>99% ee. Dihydrocinnamaldehydes 1h−k bearing different
substituents (4-F-, 4-MeO-, 2-MeO-) on the aromatic ring
successfully reacted with 2a, affording diverse γ-butyrolactones
4ha−ka in almost enantiomerically pure form. Interestingly, an
optically pure γ-butyrolactone 4la featuring a piperonyl group
at the α-carbon could also be synthesized using our current
protocol. Additionally, 3-(furan-2-yl)propanal was well tol-
a
Reaction conditions: 1a (0.4 mmol), 2a (0.2 mmol), 3 (0.02 mmol),
5 (0.02 mmol), DCE (1 mL), PCC (129.3 mg, 0.6 mmol), unless
b
noted otherwise. Dr was determined to be >30:1 for all cases by 1H
NMR of the crude reaction mixture, unless specified.
erated, offering the corresponding product 4ma in moderate
yield with excellent enantioselectivity.
The one-pot sequential Michael−hemiacetalization−oxida-
tion strategy was further explored by incorporating a number
of different substituents on the aryl ring of Michael acceptors
2. As presented in Scheme 2, γ-hydroxy-α,β-unsaturated
ketones having halogens (fluoro, chloro, bromo) at para- and
ortho-positions of the aryl ring were successfully employed with
dihydrocinnamaldehyde (1h), producing the disubstituted γ-
lactones 4hb−he in high yields with excellent stereo-
C
DOI: 10.1021/acs.orglett.9b02094
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Organic Letters
Letter
selectivities. In addition, an electron-withdrawing (nitrile) and
moderate to strong electron-releasing groups (methyl,
methoxy) on the aromatic ring of the Michael acceptor were
also compatible under current catalytic system. Moreover, γ-
butyrolactones containing biphenyl 4hj and 3,4-dichlorophenyl
4hk ketone units were achieved in synthetically viable yields
and excellent enantioselectivities. To understand the versatility
of the methodology toward alkyl enone, (E)-5-hydroxypent-3-
en-2-one (2l) was allowed to react with p-methoxy hydro-
cinnamaldehyde (1j) under optimized reaction conditions.
However, only a trace amount of lactone product 4jl was
detected. Thus, it may be concluded that the present method is
only suitable for aryl ketones.
The absolute stereochemistry of compound 4hd (R =
−CH2C6H5; Ar = 4-Cl-C6H4−) (CCDC: 1923223) was
unambiguously determined by the single-crystal X-ray
analysis.20 The stereochemistry of related γ-butyrolactone
products were assigned by analogy.
To illustrate the synthetic utility of our catalytic method, the
γ-butyrolactone or γ-lactol was further subjected to several
organic transformations (Scheme 3). Initially, the γ-lactone
one step to (+)-Homopilopic acid 9, a synthetic precursor21 of
(+)-Pilocarpine.
In summary, a one-pot sequential Michael−hemiacetaliza-
tion−oxidation strategy has been developed using multifunc-
tional modular organocatalysts, enabling a simple yet highly
diastereo- and enantioselective synthesis of γ-butyrolactones
possessing two contiguous stereogenic centers. The present
catalytic method is compatible with a diverse range of
substrates. The synthetic utility of this methodology was
demonstrated by functional group elaboration and its
implementation in the formal asymmetric synthesis of
(+)-Pilocarpine. Further studies toward the understanding of
reactivity and selectivity enhancement by multifunctional
modular catalysis and the development of newer approaches
for synthesizing complex molecular architectures are currently
underway in our laboratory.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b02094.
Scheme 3. Synthetic Transformations and Formal Synthesis
of (+)-Pilocarpine
Experimental procedures and analytical data (PDF)
Accession Codes
CCDC 1923223 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: vinodks@iitk.ac.in.
ORCID
Vinod K. Singh: 0000-0003-0928-5543
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
V.K.S. acknowledges DST/SERB, India for a J. C. Bose
fellowship (SR/S2/JCB-17/2008) and SERB (CRG/2018/
000552) for a research grant. P.M., K.S., and H.J. thank IIT
Kanpur, CSIR and UGC, India, respectively, for a doctoral
fellowship. N.K.R. and B.G.D. thank DST, India for an
INSPIRE Faculty award. We thank Vierandra Kumar, IIT
Kanpur, for assistance with X-ray crystallography and IIT
Kanpur for instrumentation facilities.
4cg bearing a para-methoxy group was transformed to the
corresponding ester 6 without erosion of any enantiopurity via
Baeyer−Villiger oxidation. The intermediate hemiacetal 7
formed by the reaction of butyraldehyde (1c) with 2a using
the combination of catalysts ent-3a and 5e under optimal
reaction conditions was successfully transformed to disub-
stituted tetrahydrofuran 8 with high enantioselectivity of >99%
and 70% yield. It is interesting to note that the ketone moiety
remained intact during this transformation. Finally, the present
one-pot protocol was applied to the formal asymmetric
synthesis of (+)-Pilocarpine. (+)-Pilocarpine (I) is found in
the pilocarpus Jaborandi species and is frequently prescribed
for the treatment of narrow and wide angle glaucoma. The
hemiacetal 7 on in situ oxidation gave the corresponding γ-
butyrolactone 4ca which was then successfully converted in
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