Carbene-Catalyzed α-Carbon Amination of Chloroaldehydes for Enantioselective Access to Dihydroquinoxaline Derivatives

Article abstract of DOI:10.1021/acs.orglett.9b01520

An NHC-catalyzed α-carbon amination of chloroaldehydes was developed. Cyclohexadiene-1,2-diimines are used as amination reagents and four-atom synthons. Our reaction affords optically enriched dihydroquinoxalines that are core structures in natural products and synthetic bioactive molecules.

Full text of DOI:10.1021/acs.orglett.9b01520

Letter
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Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Carbene-Catalyzed α‑Carbon Amination of Chloroaldehydes for
Enantioselective Access to Dihydroquinoxaline Derivatives
Ruoyan Huang,†,∥ Xingkuan Chen,§,∥ Chengli Mou,‡,∥ Guoyong Luo,‡ Yongjia Li,§ Xiangyang Li,†
Wei Xue,*,† Zhichao Jin,† and Yonggui Robin Chi*,†,§
†Laboratory Breeding Base of Green Pesticide and Agricultural Bioengineering, Key Laboratory of Green Pesticide and Agricultural
Bioengineering, Ministry of Education, Guizhou University, Huaxi District, Guiyang 550025, China
‡Guizhou University of Traditional Chinese Medicine, Huaxi District, Guiyang 550025, China
§Division of Chemistry & Biological Chemistry, School of Physical & Mathematical Sciences, Nanyang Technological University,
Singapore 637371, Singapore
S
* Supporting Information
ABSTRACT: An NHC-catalyzed α-carbon amination of
chloroaldehydes was developed. Cyclohexadiene-1,2-diimines
are used as amination reagents and four-atom synthons. Our
reaction affords optically enriched dihydroquinoxalines that
are core structures in natural products and synthetic bioactive
molecules.
ihydroquinoxalines and their derivatives are frequently
catalysis (Figure 1c). The cyclohexadiene-1,2-diimine reacts as
Dfound as core structures in natural and non-natural a four-atom synthon and leads to a cyclic “C−N−C−C−N−
molecules with proven biological activities.1 They have been
C” fragment fused with a benzene that is exactly the core
component in dihydroquinoxaline derivatives (Figure 1a). The
dihydroquinoxaline products were all obtained with excellent
yields and er values. It is worth noting that Lactka and co-
workers have pioneered the asymmetric construction of the
cyclic “C−N−C−C−N−C” fragment with ketene enolates and
the cyclohexadiene-1,2-diimines through a cooperative cata-
lytic strategy with chiral cinchona alkaloid catalysts and achiral
Lewis acid cocatalysts.3a The key enolate intermediates in
Lectka’s and our studies are different. Our approach involves
acylazolium enolate generated from aldehyde substrates under
NHC catalysis, which possesses different reactivities from
Lectka’s enolate intermediates generated from acyl chlorides
(via ketenes) and cinchona alkaloids.3a The Lewis acid
cocatalysts used in Lectka’s approach are not required in our
approach.
Different NHC precatalysts were first examined for this aza-
[2 + 4] cycloaddition reaction between α-chloroaldehyde 1a
and cyclohexadiene-1,2-diimine 2a (Table 1, entries 1−6). The
N-mesityl substituted triazolium NHC precatalyst A13 derived
from an amino-indanol skeleton could give the product 3a in
better yield and er value than those bearing an N-Ph14 or N-
pentafluorobenzene15 group (entry 1 vs entries 2−3). The
product yield could be further increased by switching the NHC
precatalyst A into the corresponding chloride salt D16 (entry
4). Other NHC precatalysts we tested did not provide any
better results in this transformation (e.g., entries 5−6).
Organic bases generally gave the enantioenriched dihydroqui-
intensively studied as potential drugs for the treatment of
multiple diseases such as HIV infections,1a atherosclerosis,1b
and allergies1c (Figure 1a). Considerable attention has
therefore been focused toward the synthesis of dihydroqui-
noxalines especially in enantiomerically enriched forms.2−5
Main approaches include chiral amine catalyzed hetero-Diels−
Alder reactions of o-benzoquinone diimides with various
carbonyl compounds,3 copper-catalyzed cross-couplings of α-
amino acids with substituted anilines,4 and asymmetric
hydrogenation of substituted quinoxaline derivatives enabled
by either transition metals or chiral Brønsted acids.5 Despite
these elegant advancements, metal-free and operationally
simple methods are still needed for the asymmetric synthesis
of these molecules.
N-Heterocyclic carbene (NHC) organic catalysis6 can in
principle offer effective solutions in this direction. Studies have
shown that asymmetric carbon−nitrogen bonds can be
constructed via NHC catalysis by using diazenes as the
electronic amination reagents (Figure 1b), as reported by
Scheidt,7 Ye,8 Smith,9 and Zhong.10 These methods provide
pyrazolidinones,7 1,3,4-oxadiazin-6-ones,8a,10 α-hydrazino es-
ters,9 and dihydropyridazinones8b as the products. We recently
reported nucleophilic β-carbon-atom amination of enals, in
which a protected hydrazine behaved as a nucleophilic
nitrogen reaction center.11 In these studies from us and
others,12 the amination reagents behave as two-atom synthons
and end up with a “C−N−N−C” fragment in the final
products (Figure 1b). Here we disclose that cyclohexadiene-
1,2-diimine can behave as an effective amination reagent that
functionalizes the α-carobn of chloroaldehydes under NHC
Received: April 30, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01520
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
noxaline products in lower yields, while the inorganic bases we
tested could provide us with the products in better yields
without erosion of the enantioselectivities (e.g., entries 7−9).
To our delight, NaOAc could give the product 3a in up to 88%
isolated yield with an excellent 98:2 er value (entry 9).
Additional screening of the reaction solvents could not further
increase either the product yield or er value (entries 10−12).
With the optimized reaction conditions in hand, we next
examined the reaction scope using α-chloroaldehydes with
different substituents or substitution types (Scheme 1). Both
a
Scheme 1. Scope of α-Chloroaldehydes 1
Figure 1. Bioactive dihydroquinoxalines and construction of two C−
N bonds with NHC organocatalysis.
a
Table 1. Optimization of Reaction Conditions
b
c
entry
NHC
base
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
DBU
Cs2CO3
NaOAc
NaOAc
NaOAc
NaOAc
solvent
yield (%)
er
a
Reaction conditions as stated in Table 1, entry 9. Yields are isolated
1
2
3
4
5
6
7
8
A
B
C
D
E
THF
THF
THF
THF
THF
THF
THF
THF
THF
CH3CN
Toluene
EA
52
31
43
62
27
29
20
67
88
29
47
65
98:2
96:4
72:28
98:2
99:1
97:3
89:11
98:2
98:2
92:8
96:4
97:3
yields after purification via SiO2 column chromatography. Er values
b
were determined via HPLC on chiral stationary phase. The reaction
c
was carried out at 1.0 mmol scale. α-Chloroacetaldehyde (40% aq.)
was used as the reaction starting material.
F
electron-donating and electron-withdrawing groups could be
installed on the β-phenyl ring of the α-chloroaldehyde 1, with
the corresponding dihydroquinoxaline products 3 afforded in
good to excellent yields with excellent enantioselectivities (3b
to 3k). The β-phenyl ring of the α-chloroaldehyde 1a could
also be switched to aliphatic groups, with both of the product
yields and er values remaining high (3l to 3m). The benzyl
group on the α-chloroaldehyde 1a could be replaced with a
phenyl group (3n), and α-chloroacetaldehyde could react fine
as well (3o), albeit with low yields under the standard
conditions without additional optimizations. As a technical
note, the formation of chiral products 3n and 3o was not
reported in Lectka’s study.3
D
D
D
D
D
D
9
10
11
12
a
General conditions (unless otherwise specified): 1a (0.15 mmol), 2a
(0.10 mmol), NHC (0.02 mmol), base (0.15 mmol), 4 A MS (50
b
c
mg), solvent (1.0 mL), rt, 24 h. Isolated yield of 3a. Er was
determined via HPLC on chiral stationary phase.
B
DOI: 10.1021/acs.orglett.9b01520
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Various substituted cyclohexadiene-1,2-diimine 2 also
worked well in this asymmetric cycloaddition reaction
(Scheme 2). Cyclohexadiene-1,2-diimines bearing electron-
3n with excellent enantioselectivity, albeit with a low yield
under current conditions.
To demonstrate the operational simplicity of our NHC-
catalyzed enolate reaction for the synthesis of chiral
dihydroquinoxaline derivatives, we next combined the
substrate preparation step and the NHC catalytic reaction
step in a one-pot operation. The cyclohexadiene-1,2-diimine
substrates (2) used in our reactions (Schemes 1 and 2) were
preprepared via oxidation of the corresponding diamide
substrates (e.g., 5a and 5b, Scheme 4). This substrate
preparation step can be combined with the NHC catalytic
step in a one-pot operation to furnish the cycloaddition
products (3a and 3q, Scheme 4).
a
Scheme 2. Scope of Cyclohexadiene-1,2-diimine 2
Scheme 4. One-Pot Operation by Combining Substrate
Preparation with NHC-Catalyzed Cycloaddition
a
Reaction conditions as stated in Table 1, entry 9. Yields are isolated
yields after purification via SiO2 column chromatography. Er values
b
The Bz group on the lactam moiety of the chiral
dihydroquinoxaline 3a could be selectively removed under
basic conditions to give product 6 in good yield with little
erosion of the product er value.17 The lactam moiety of 3a
could also be stereoselectively reduced by BH3·THF to afford
the secondary amine product 7 in good yield with a slight
increase of the optical purity.18 Both of the Bz protecting
groups could be removed with the assistance of Red-Al, and
the corresponding amide 8 could be formed in 78% yield with
retention of the enantioselectivity.19 Lactam 8 could be further
reduced by BH3·THF and give the diamine product 9 in good
yield with an excellent er value (Scheme 5).
were determined via HPLC on chiral stationary phase. Regioselective
1
ratios of the products 3u/3u′ were determined by H NMR of the
isolated mixture of both products and confirmed by X-ray analysis.
withdrawing groups gave the corresponding products in lower
yields, which were probably due to the decomposition of the
unstable starting materials under the current reaction
conditions (e.g., 3p). However, electron-rich cyclohexadiene-
1,2-diimines 2 could give the desired products in good to
excellent yields with excellent enantioselectivities, regardless of
the substitution patterns existing on the α-chloroaldehyde 1
used in these transformations (3q to 3t, 3u/3u′). It is worth
noting that the electron-donating group existing on the
cyclohexadiene-1,2-diimine 2 could decrease the electro-
philicity of the imine group on its para-position due to the
conjugate effect, which might result in the good regioselectiv-
ities observed in the formation of the products 3u/3u′. The
absolute configurations of the products 3 were assigned based
on the X-ray analysis on the single crystal of the product 3b.
In addition to α-chloroaldehydes, saturated aldehydes (such
as 3-phenylpropionic aldehyde) could also be used as
substrates to generate the azolium enolate intermediates
under oxidative NHC catalysis (Scheme 3). For example,
with DQ used as an oxidant, dihydroquinoxaline product 3a
could be formed from 3-phenylpropionic aldehyde (4a) with a
47% yield and excellent enantioselectivity by using the same
NHC catalyst (D). Phenyl acetaldehyde could also be used in
this oxidative [2 + 4] cycloaddition reaction to form product
Scheme 5. Synthetic Transformations of 3a
In summary, we have developed an NHC-catalyzed
amination and cycloaddition reactions of α-chloroaldehydes.
In our reaction, cyclohexadiene-1,2-diimine compounds were
used as amination reagents and four-atom synthons, with two
C−N bonds formed. The reactions afford substituted
dihydroquinoxaline derivatives with excellent enantioselectiv-
ities. The chiral dihydroquinoxaline products afforded in this
reaction could be efficiently transformed to various functional
molecules via simple protocols. Further synthetic trans-
formation and bioactivity evaluation of the dihydroquinoxa-
lines are in progress in our laboratory.
Scheme 3. Saturated Aldehyde as Enolate Precursor under
Oxidative NHC Catalysis
C
DOI: 10.1021/acs.orglett.9b01520
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
A.; Paton, R. S.; Mu
̈
ller, S.; Knapp, S.; Brennan, P. E.; Conway, S. J.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b01520.
■
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AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: robinchi@ntu.edu.sg.
*E-mail: wxue@gzu.edu.cn.
ORCID
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Zhichao Jin: 0000-0003-3003-6437
Yonggui Robin Chi: 0000-0003-0573-257X
Author Contributions
∥R.H., X.C., and C.M. contributed equally to this work.
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge financial support from the National Natural
Science Foundation of China (Nos. 21772029, 21472028),
National Key Technologies R&D Program (No. 2014BA-
D23B01), “Thousand Talent Plan”, The 10 Talent Plan
(Shicengci) of Guizhou Province, Guizhou Province Returned
Oversea Student Science and Technology Activity Program,
Guizhou University (China), Singapore National Research
Foundation (NRF-NRFI2016-06), the Ministry of Education
of Singapore (MOE2013-T2-2-003; MOE2016-T2-1-032;
RG108/16), A*STAR Individual Research Grant
(A1783c0008), Nanyang Research Award Grant, and Nanyang
Technological University.
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