Stereodivergent Synthesis of Chiral Paraconic Acids via Dynamic Kinetic Resolution of 3-Acylsuccinimides

Article abstract of DOI:10.1021/acs.orglett.9b01445

A direct N-heterocyclic carbene (NHC) catalysis of maleimides with alkyl aldehydes is established for the synthesis of 3-acylsuccinimides. The first dynamic kinetic resolution of 3-acylsuccinimides is accomplished through asymmetric transfer hydrogenation. These two catalytic methodologies are utilized for the synthesis of each enantiomer of trans-paraconic acids in three steps and cis-paraconic acids in four steps with good yields and high stereoselectivities. This stereodivergent synthetic methodology is applied for the synthesis of seven bioactive paraconic acid natural products.

Full text of DOI:10.1021/acs.orglett.9b01445

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Stereodivergent Synthesis of Chiral Paraconic Acids via Dynamic
Kinetic Resolution of 3‑Acylsuccinimides
Abhijeet M. Sarkale, Vidyasagar Maurya, Sachin Giri, and Chandrakumar Appayee*
Discipline of Chemistry, Indian Institute of Technology Gandhinagar, Palaj, Gandhinagar, Gujarat 382355, India
*
S Supporting Information
ABSTRACT: A direct N-heterocyclic carbene (NHC) catalysis
of maleimides with alkyl aldehydes is established for the synthesis
of 3-acylsuccinimides. The first dynamic kinetic resolution of 3-
acylsuccinimides is accomplished through asymmetric transfer
hydrogenation. These two catalytic methodologies are utilized for
the synthesis of each enantiomer of trans-paraconic acids in three
steps and cis-paraconic acids in four steps with good yields and
high stereoselectivities. This stereodivergent synthetic method-
ology is applied for the synthesis of seven bioactive paraconic acid
natural products.
he stereoisomers of a drug exhibit a difference in their
T
biological properties due to the building blocks such as
amino acids and carbohydrates in the biological systems
existing in homochiral forms. Hence, studying the structure−
activity relationship of each stereoisomer of a drug in a specific
1
target is an important area of research in medicinal chemistry.
Asymmetric synthesis of all the possible stereoisomers
(
stereodivergent synthesis) in pure form is a challenging task
2
in synthetic organic chemistry. Asymmetric catalysis has been
developed as a modern tool for the stereodivergent synthesis of
chiral molecules.
2
b−f
But, direct application of the asymmetric
catalysis for the stereodivergent synthesis of bioactive natural
3
products is rarely studied in literature.
Figure 1. Bioactive paraconic acid natural products containing
stereodivergent chiral centers.
On the other hand, stereodivergent paraconic acid natural
4
products 1−7 show different biological properties, such as
4
a
4b,c
4d
antibiotic, anti-HIV-1, and antimicrobial, with respect to
their substitution and stereochemistry (Figure 1). Some of
these natural products have been synthesized using a chiral
generated by the N-heterocyclic carbene (NHC) catalysis of
maleimide 8 and alkyl aldehyde 9. Synthesis of the other
enantiomers of paraconic acids 12 (ent-12) was achieved
through the alcohol ent-11, which could be obtained from 10
using the other enantiomer of a chiral catalyst in the ATH
accompanied by DKR. Diastereomer 13 and its enantiomer
(ent-13) could be accessed from alcohols 11 and ent-11,
respectively, through hydrolysis, followed by cyclization
without epimerization.
5
6
7
pool approach, chiral auxiliaries, chiral reagents, and
asymmetric catalysis. To the best of our knowledge, there is
8
no report in the literature for the stereodivergent synthesis of
these natural products.
8c
5d
The α-methylene or α-methyl group could be con-
veniently installed at the later stage of the synthesis of these
natural products. Hence, we envisaged a stereodivergent
synthesis of the paraconic acids 12 and 13 and their
enantiomers (ent-12 and ent-13), as depicted in Scheme 1.
trans-Paraconic acid 12 was synthesized from alcohol 11
through epimerization using 1,8-diazabicyclo[5.4.0]undec-7-
ene (DBU), followed by hydrolysis and cyclization.
To initiate the stereodivergent synthesis of paraconic acids,
the existing methods for the synthesis of 3-acylsuccinimide 10
9
were explored. Unlike the synthesis of 3-aroylsuccinimide,
there are very few methodologies
9b,c
for the synthesis of 3-
acylsuccinimide 10 reported in the literature. Moderate to poor
The alcohol 11 was obtained through asymmetric transfer
hydrogenation (ATH) accompanied by the dynamic kinetic
resolution (DKR) of 3-acylsuccinimide 10, which could be
Received: April 25, 2019
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01445
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 1. Retrosynthetic Plan for the Stereodivergent
Synthesis of Chiral Paraconic Acids
Table 1. Reaction Optimization for the Synthesis of 3-
a
Acylsuccinimide 10a
b
yield (%)
entry
precat
base
solvent
temp
10a
14a
1
2
3
4
5
cat-I
K CO3
THF
THF
THF
THF
toluene
toluene
toluene
rt
rt
rt
rt
rt
rt
reflux
−
0
0
0
0
−
72
2
c
cat-II
cat-II
cat-II
cat-II
cat-II
cat-II
K CO3
2
NaOAc
DIPEA
DIPEA
DIPEA
DIPEA
60
76
86
60
0
d
6
e
25
85
c
7
a
9
b,c
Reactions were performed with 8a (38 mg, 0.2 mmol, 1.0 equiv), 9a
74 μL, 0.6 mmol, 3.0 equiv), NHC precatalyst (0.02 mmol, 10 mol
%), and base (0.02 mmol, 10 mol %) in solvent (2 mL) unless
yields were reported for the direct acylation of succinimide.
(
Very recently, 3-acylsuccinimide was synthesized in three steps
starting from alkyl aldehyde. But, a NHC-catalyzed reaction
of alkyl aldehyde with maleimide was found to be
unsuccessful. Therefore, we attempted to develop a method-
ology for a one-step synthesis of 3-acylsuccinimide 10 starting
from the alkyl aldehyde 9 and maleimide 8.
9d
b
1
otherwise mentioned. Determined by H NMR analysis using 1,3-
dinitrobenzene as an internal standard. Isolated yield after column
chromatography. Catalyst (30 mol %) and DIPEA (30 mol %) were
used. Catalyst (20 mol %) and DIPEA (20 mol %) were used.
c
9
a
d
e
Accordingly, hexanal 9a and N-benzylmaleimide 8a were
treated with NHC precatalyst cat-I and K CO in THF at rt,
Scheme 2. Synthesis of 3-Acylsuccinimides 10a−e
2
3
and no reaction was observed (Table 1, entry 1). When a
precatalyst cat-II was used under similar reaction conditions,
the product 14a was isolated in 72% yield and 2:1 dr (entry 2).
But, the expected 3-acylsuccinimide 10a was not obtained even
with the use of NaOAc or N,N-diisopropylethylamine
(
DIPEA) as the base additive (entries 2−5).
During the reaction at a higher catalytic loading (30 mol %)
in toluene, the product 10a was noticed in 25% yield along
with 14a (entry 6). The formation of 14a was predicted to be
an outcome of the Michael addition reaction of 10a to 8a.
Therefore, the reaction was planned at a higher temperature to
facilitate the retro-Michael reaction to obtain 10a as the major
product. Upon heating the reaction to reflux temperature in
toluene, we were delighted to see the formation of 3-
acylsuccinimide 10a in 85% isolated yield (entry 7).
cat-IV). On the other hand, an alcohol 11a was obtained in
70% ee but with a lower dr (Table 2, entry 3). Further reaction
optimization with the precatalyst cat-IV in different solvents
did not improve the stereoselectivities (dr and ee) of the
alcohol 15a (entries 4−6). However, a reversal of diaster-
eoselectivity (3:1) was noticed in EtOAc to form 11a as a
major diastereomer with 81% ee (entry 6). As the precatalyst
cat-V produced the alcohol 11a with a higher ee in CH Cl
After the successful reaction optimization for the synthesis of
3
-acylsuccinimide 10a, the reaction of aliphatic aldehyde 9a−c
2
2
with maleimide 8a−b to form 3-acylsuccinimides 10a−e was
achieved in excellent isolated yields under the optimized
reaction conditions (Scheme 2).
(entry 3), it was tested again in EtOAc, and 11a was obtained
in 95% yield with 3:1 dr and 94% ee (entry 7).
Upon further solvent screening, DMF was found to be a
suitable solvent to form 11a in 98% yield with 9:1 dr and 97%
ee (entry 9). A lower catalytic loading (2 mol % of cat-V) at
the higher reaction concentration in DMF (0.4 M) was
successfully employed to achieve the alcohol 11a in 97% yield
with 9:1 dr and 97% ee through the ATH accompanied by
DKR (entry 10). A further attempt to decrease the catalytic
loading (1 mol % of cat-V) was not fruitful (entry 11).
Using cat-V under the optimized reaction conditions (Table
2, entry 10), 3-acylsuccinimides 10a and 10c were converted
to the corresponding alcohols 11a (78%) and 11c (80%),
respectively, as the major diastereomers with 9:1 dr and 97%
As 3-acylsuccinimide 10 possesses an easily epimerizable
chiral center, the DKR was envisaged during the reduction of
1
0
11
12
1
0 using ATH. The DKR of ketones, imines, keto esters,
13 14
keto amides, and keto phosphonates has been reported
using ATH. To the best of our knowledge, there is no report in
the literature for the DKR of keto imides (3-acylsuccinimides
10). Hence, we initiated the reaction optimization for the ATH
accompanied by DKR of 3-acylsuccinimides 10 (Table 2).
During the catalyst screening for the ATH of 10a in CH Cl ,
2
2
an alcohol 15a was observed as a major diastereomer (Table 2,
entries 1−3) but with a lower ee (61% ee with the precatalyst
B
DOI: 10.1021/acs.orglett.9b01445
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Table 2. Reaction Optimization for the Asymmetric
Transfer Hydrogenation of 3-Acylsuccinimide 10a
Scheme 3. ATH of 3-Acylsuccinimides 10a−c Using
Precatalyst cat-V or ent-cat-V
a
c
d
dr
ee (%)
precat (mol time
yield
b
entry
solvent
%)
(h)
(%)
11a:15a 11a 15a
1
2
3
4
5
6
7
8
CH Cl2
cat-III (5)
cat-IV (5)
cat-V (5)
cat-IV (5)
cat-IV (5)
cat-IV (5)
cat-V (5)
cat-V (5)
cat-V (5)
cat-V (2)
cat-V (1)
24
24
24
24
24
24
12
12
24
18
40
93
95
94
86
−
89
95
92
98
97
86
1:3
1:2
1:2
1:2
9
49
70
57
−
81
94
91
97
97
97
41
61
36
52
−
27
60
18
20
20
nd
2
CH Cl2
2
CH Cl2
2
DCE
CHCl3
EtOAc
EtOAc
−
3:1
3:1
3:1
9:1
9:1
5:1
Scheme 4. Synthesis of trans-Paraconic Acids 12
CH CN
3
9
DMF
DMF
DMF
e
1
1
0
1
f
a
Reactions were performed with 10a (58 mg, 0.2 mmol),
HCO H:NEt (188 μL, 5:2), and precatalyst in solvent (2 mL, 0.1
2
3
b
1
M) unless otherwise mentioned. Determined by H NMR analysis
using 1,3-dinitrobenzene as an internal standard. Determined by H
NMR analysis of the crude reaction mixture. Determined by HPLC
c
1
d
e
analysis on a chiral stationary phase. DMF (0.5 mL, 0.4 M) was used.
f
DMF (0.2 mL, 1M) was used.
ee. On the other hand, the enantiomer of cat-V (ent-cat-V)
was used for the synthesis of other enantiomers of alcohols
1
1a−c (ent-11a−c) under the optimized reaction conditions in
Accordingly, the alcohol 11a was treated with 5% aqueous
KOH at 0 °C to facilitate hydrolysis of imide, and the crude
excellent isolated yields and stereoselectivities (Scheme 3).
When 3-acylsuccinimides 10d or 10e were subjected to the
ATH using cat-V, an inseparable diastereomeric mixture (9:1
dr) of alcohols was observed.
reaction mixture was further treated with NaNO and Ac O in
2 2
AcOH to achieve the cis-paraconic acid 13a as a major
diastereomer in 80% isolated yield with 10:1 dr and 94% ee
(Scheme 5). Similarly, the alcohols 11c and ent-11a were
converted to 13c and ent-13a, respectively, in excellent isolated
yields and stereoselectivities.
After the stereodivergent syntheses of paraconic acids (12,
ent-12, 13, and ent-13), their application to the syntheses of
Once the alcohols 11 were synthesized from the ATH
accompanied by DKR of 10, we turned our attention toward
the conversion of 11 to paraconic acids 12 or 13 with or
without epimerization of the chiral center at the 3-position.
When the alcohol 11a was heated in acidic (conc. HCl) or
basic conditions (aq. KOH), a diastereomeric mixture of
paraconic acids 12a and 13a was noticed (see the Supporting
Information). Hence, the alcohol 11a was epimerized to its
diastereomer 15a (4:1 dr) using DBU (30 mol %) in 1,4-
dioxane at 55 °C, followed by the treatment with 10% aqueous
KOH and then 30% aqueous KOH at reflux temperature to
obtain trans-paraconic acid 12a as the major diastereomer in
Scheme 5. Synthesis of cis-Paraconic Acids 13
6
9% isolated yield and 4:1 dr (Scheme 4). Similarly, ent-12b
(
70%) and ent-12c (61%) were synthesized from ent-11b and
ent-11c, respectively. The enantiomeric excess of trans-
paraconic acids 12 was determined after converting them
into the corresponding benzyl esters (see the Supporting
Information).
For the synthesis of cis-paraconic acid 13a as the major
diastereomer, an alternate method was considered in two steps.
C
DOI: 10.1021/acs.orglett.9b01445
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
bioactive natural products 1−7 was planned (Scheme 6).
Accordingly, the natural products 1−3 were synthesized by the
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.9b01445.
Scheme 6. Syntheses of Bioactive Paraconic Acid Natural
Products 1−7
Complete experimental procedures and characterization
of new products, NMR spectra, and HPLC chromato-
grams (PDF)
AUTHOR INFORMATION
Corresponding Author
■
*
E-mail: a.chandra@iitgn.ac.in.
ORCID
Chandrakumar Appayee: 0000-0003-1165-4918
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The authors are grateful to Indian Institute of Technology
Gandhinagar for facilities and financial support.
■
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