Asymmetric Multicomponent Sulfa-Michael/Mannich Cascade Reaction: Synthetic Access to 1,2-Diamino-3-Organosulfur Compounds and 2-Nitro Allylic Amines

Article abstract of DOI:10.1021/acs.orglett.5b02423

A novel catalytic asymmetric three-component intermolecular sulfa-Michael/Mannich cascade reaction has been developed using a chiral multifunctional catalyst. This reaction provides facile access to 1-amino-2-nitro-3-organosulfur compounds bearing three c

Full text of DOI:10.1021/acs.orglett.5b02423

Letter
pubs.acs.org/OrgLett
Asymmetric Multicomponent Sulfa-Michael/Mannich Cascade
Reaction: Synthetic Access to 1,2-Diamino-3-Organosulfur
Compounds and 2‑Nitro Allylic Amines
Wenduan Hou, Qi Wei, Guisheng Liu, Jing Chen, Jing Guo, and Yungui Peng*
Key Laboratory of Applied Chemistry of Chongqing Municipality, School of Chemistry and Chemical Engineering, Southwest
University, Chongqing 400715, China
S
* Supporting Information
ABSTRACT: A novel catalytic asymmetric three-component
intermolecular sulfa-Michael/Mannich cascade reaction has been
developed using a chiral multifunctional catalyst. This reaction
provides facile access to 1-amino-2-nitro-3-organosulfur compounds
bearing three consecutive stereocenters in high yields (up to 96%)
with good diastereo- (up to 91:4:4:1 dr) and excellent enantioselectivities (93−99% ee). Furthermore, the products of this
reaction could be facilely transformed into potentially bioactive 1, 2-diamino-3-organosulfur compounds and 2-nitro allylic
amines.
he prevalence of chiral organosulfur compounds in natural
Tproducts and other bioactive compounds has provided a
major impetus for the development of efficient methods for their
construction.1 In this regard, the catalytic asymmetric sulfa-
Michael addition has distinguished itself as a powerful tool for the
synthesis of organosulfur compounds, and significant progress
has been achieved in this area.2 In contrast, reports pertaining to
the development and application of sulfa-Michael addition-
triggered catalytic asymmetric multicomponent reactions, which
could be used as an efficient method for the synthesis of diverse
sets of relatively complex chiral structures,3 have been scarce.4
Chiral 1,2-diamino-3-organosulfur compounds are ubiquitous in
living organisms (e.g., glutathione and biotin),1 and subunits of
this type can also be found in a wide range of bioactive natural
products5 and drugs.6 It was envisaged that the use of a catalytic
asymmetric multicomponent sulfa-Michael/Mannich reaction
would provide efficient access to 1-amino-2-nitro-3-organosulfur
compounds as the precursor of 1,2-diamino-3-organosulfur
compounds. Notably, the products of this reaction could also
be readily converted7 to the corresponding chiral bioactive 2-
nitro allylic amines,8 which are otherwise difficult to prepare with
a high level of enantioselectivity via the direct asymmetric aza-
Baylis−Hillman reaction of nitroalkenes.9 The potential retro-
sulfa-Michael10 or retro-Mannich11 reaction constitutes a major
obstacle for achieving the catalytic asymmetric sulfa-Michael/
Mannich reaction.12 The difficulties associated with these issues
could also be the main reason why only three examples of a sulfa-
initiated catalytic asymmetric three-component sulfa-Michael/
Michael intermolecular domino reaction have been reported to
date.4a−c Recently, our group reported the enantioselective
three-component intermolecular sulfur-Michael/Mannich dom-
ino reaction in the catalysis of chiral quaternary ammonium salts
using chalcones as Michael acceptors.4d However, when we tried
to expand this catalytic system to nitroolefins as Michael
acceptors, only racemic products resulted. Thus, the develop-
ment of an efficient catalytic system for the asymmetric
multicomponent sulfa-Michael/Mannich reaction that accom-
modates other substrates is therefore still highly desired and
represents a significant challenge to synthetic chemistry.
As part of ongoing work toward the development of new
methods for the asymmetric construction of biologically active
sulfur-containing complex compounds, herein, we report the
development of a novel enantioselective asymmetric multi-
component sulfa-Michael/Mannich reaction for the construction
of 1-amino-2-nitro-3-organosulfur compounds using a multi-
functional organocatalyst (Scheme 1).
Scheme 1. Asymmetric Multicomponent Sulfa-Michael/
Mannich Cascade Reaction
The reaction of thiophenol 1a with nitrostyrene 2a and N-
sulfonylaldimine 3a was selected as a model reaction. Following a
series of preliminary experiments, we investigated the use of
bifunctional catalysts 5a−5c13 to evaluate their performance on
the reaction in DCM at −40 °C. Pleasingly, all three of these
catalysts could promote the model reaction to give the desired
products in excellent yields (87−97%) (Table 1, entries 1−3).
Takemoto catalyst 5a13a afforded the desired product 4aaa with
good diastereoselectivity (87:13 dr), but only moderate
Received: August 21, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b02423
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Organic Letters
Letter
provide better results than catalyst 5i. A variety of solvents were
also screened, and MTBE was found to provide the best
performance in terms of the diastereoselectivity (74:26 dr) and
enantioselectivity (99% ee) (see SI). The addition of a small
amount of tBuOH led to a significant improvement in the yield
and selectivity of the reaction (90% yield, 99% ee and 82:18 dr)
(Table 1, entry 10), although the reason for these improvements
in the catalytic performance remains unclear. The addition of
tBuOH could lead to the formation of synergistic hydrogen
bonding interactions between the catalyst, reactive substrates,
Table 1. Catalytic Asymmetric Sulfa-Michael/Mannich
Reaction under Various Conditions
a
15
t
and BuOH. It was also noted that lowering the reaction
temperature to −50 °C led to a slight improvement in the
diastereoselectivity of the desired product (83:17 dr) (Table 1,
entry 11). To gain insight into the effect of the multiple hydrogen
bonds of the catalyst, catalysts (5j−5l) were synthesized and
evaluated. When the sulfonamide moiety was methylated (5j),
the catalytic performance of the reaction was maintained to some
extent, albeit with a slightly reduced diastereoselectivity (Table 1,
entry 13 vs 11). In sharp contrast, once the nitrogen of the
thiourea moiety of the catalyst was methylated (5k and 5l), the
reactivities and enantioselectivities decreased dramatically and
only 10% yields were obtained (Table 1, entries 14 and 15).
Apparently these results show the thiourea and the sulfonamide
of the catalyst 5i may synergistically fix and activate the substrate
via multiple hydrogen bonding.
b
cd
,
d
entry
cat.
solvent
t (h)
yield (%)
dr
ee (%)
1
5a
5b
5c
5d
5e
5f
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
MTBE
MTBE
MTBE
MTBE
MTBE
MTBE
28
43
43
43
43
43
43
43
43
24
67
70
67
67
67
97
94
87
77
71
75
79
77
79
90
90
83
90
10
10
87:13
76:24
64:36
65:35
65:35
47:53
57:43
62:38
69:31
82:18
83:17
83:17
72:28
78:22
67:33
45
35
65
98
80
92
95
94
98
99
99
99
96
76
75
2
3
4
5
6
7
5g
5h
5i
With the optimized reaction conditions in hand, a series of
substituted thiophenols (1a−1k) as the nucleophile was
investigated (see SI). The use of 4-tert-butylbenzenethiol (1b)
as a nucleophile gave the best diastereoselectivity (91:4:4:1 dr) of
all of the thiophenols tested, and the corresponding product was
obtained in 80% yield with 99% ee.
8
9
e
e
e
e
e
e
,
,
,
,
,
,
f
f
f
f
f
f
10
11
12
13
14
15
5i
,
,
,
,
,
g
g
g
g
g
5i
,
h
5i
5j
4-tert-Butylbenzenethiol was selected for study to further
expand the substrate scope (Table 2). Various nitrostyrenes 2
were investigated in this cascade reaction. The results revealed
that the cascade reaction proceeded smoothly, regardless of the
position (Table 2, entries 5−9) or the electronic nature (Table 2,
entries 3−6 or 7−13) of the substituents on the aromatic ring of
the nitrostyrene substrate to give the desired products in good
yields (65−95%), diastereoselectivities (72:5:16:7−90:5:5:0 dr),
and excellent enantioselectivities (98−99% ee) (Table 2, entries
1−13). Nitroalkenes bearing heteroaromatic rings also reacted
smoothly under the optimized reaction conditions to give the
corresponding 1-amino-2-nitro-3-organosullfur products in 85−
90% yields, 88:4:8:0−89:7:4:0 dr, and 98% ee (Table 2, entries
14−16). Subsequently, we turned our attention to the scope of
the imine 3 (Table 2, entries 17−23). The results of these
experiments revealed that the position of the substituent on the
aryl ring of the imine had a significant effect on the outcome of
the reaction. For example, imines with a meta- or para-
substituted phenyl ring gave better results than those with an
ortho-substituted phenyl ring (Table 2, entries 18 and 19 vs 20).
It is noteworthy that the diastereoselectivity of the products
could be improved by recrystallization. For example, a single
recrystallization of 4aaa (99% ee, 83:17 dr) from methanol gave
almost optically pure material (70% yield, >99% ee, >99:1 dr).
The absolute and relative configurations of the products were
unambiguously determined to be (1R, 2S, 3S), (1R, 2S, 3R) and
the mixture of (1R, 2R, 3R) and (1S, 2S, 3S) by X-ray
crystallographic analysis of 4aaa-A, 4aaa-B, and ( )-4aaa-C,
respectively (see Figure 1).
5k
5l
a
Unless noted, all of the reactions were carried out with 1a (0.15
mmol), 2a (0.15 mmol), 3a (0.1 mmol), and the cat. (10 mol %) in
0.5 mL of solvent at −40 °C. Combined isolated yields of the
isomers. Ratio of the major diastereomer to the total of the other
three pairs. Determined by chiral HPLC of the major stereoisomer.
Reactions were carried out with 1a (0.3 mmol), 2a (0.3 mmol), 3a
b
c
d
e
f
(0.2 mmol), and the cat. (10 mol %) in 0.5 mL of solvent. 38 μL of
g
h
tBuOH were added. Performed at −50 °C. 5 mol % 5i was used as
the catalyst.
enantioselectivity (45% ee). Based on this result and the
structural framework of catalyst 5a, a series of bi- and
multifunctional catalysts14 were also screened in an attempt to
increase the enantioselectivity. Unfortunately, however, none of
these catalysts provided the desired product with a satisfactory
yield and enantioselectivity (see the Supporting Information
(SI)). Good enantioselectivity was achieved using catalyst 5c,
which indicated that the presence of a C-6′-thiourea substituted
quinine moiety was important for controlling the stereo-
chemistry of the reaction. Based on this result, we designed
and synthesized a series of trifunctional C-6′ substituted quinine
derivatives 5d−5i, bearing tertiary amine, thiourea, and
sulfonamide functional groups. These catalysts were then
evaluated in terms of their efficiency (Table 1, entries 4−9).
The results of these experiments revealed that catalyst 5i bearing
a bulky tert-butyl ether moiety gave the best results, which were
attributed to the steric hindrance of this group (79% yield, 69:31
dr, 98% ee) (Table 1, entry 9). Several other multifunctional
catalysts were also investigated (see SI), but they all failed to
A series of control experiments were conducted to achieve a
deeper understanding of the mechanism of this reaction (Scheme
2). Under the optimum reaction conditions, the Michael
B
DOI: 10.1021/acs.orglett.5b02423
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Organic Letters
Letter
addition reaction of thiophenol 1a with nitrostyrene 2a proceeds
smoothly to give adduct 6aa in good yield with poor
enantioselectivity (87% yield, 32% ee), which contrasted sharply
with the excellent enantioselectivity observed for the catalytic
asymmetric multicomponent domino reaction (90% yield, 99%
ee, and 83:17 dr) (Table 1, entry 11). The differences in the
outcome of these reactions could be attributed to the occurrence
of a dynamic kinetic resolution (DKR) in the multicomponent
reaction in the presence of catalyst 5i. Further evaluation
revealed that when DCM was used as solvent, racemic 6aa could
smoothly react with imine 3a to afford 4aaa with almost the same
stereoselectivities that were observed from the direct reaction of
1a, 2a, and 3a (Table 1, entry 9); this is similar to literature
reported for the DKR model.10d−f While under the optimum
reaction conditions, the alkalinity of the tertiary amine moiety of
the catalyst was not strong enough to deprotonate the acidic
nitroalkane 6aa. The treatment of a mixture of nitroalkane 6aa
and imine 3a under the same reaction conditions resulted in
almost no reaction (Scheme 2). Based on the experiment results,
we proposed a possible mechanism of this reaction. As shown in
Figure 2, the reaction proceeded in two steps. First, the tertiary
Table 2. Asymmetric Sulfa-Michael/Mannich Reactions of
p-tBuC6H4SH (1b) with Nitrostyrenes 2 and Imines 3 in the
a
Presence of Catalyst 5i
yield
b
dr
ee (%)
d
c
entry
Ar2
Ar3
(%)
(A:B:C:D)
(A/B)
1
Ph
Ph
Ph
4baa, 80
4bab, 88
91:4:4:1
89:6:4:1
99/99
98/98
98/98
98/99
98/98
98/98
98/98
98/98
99/98
98/98
98/99
98/98
98/98
2
m-CH3C6H4
m-CH3C6H4
m-CH3C6H4
m-CH3C6H4
m-CH3C6H4
m-CH3C6H4
m-CH3C6H4
m-CH3C6H4
m-CH3C6H4
m-CH3C6H4
m-CH3C6H4
m-CH3C6H4
m-CH3C6H4
3
p-MeC6H4
m-MeC6H4
p-OMeC6H4
m-OMeC6H4
p-FC6H4
4bbb, 91 89:6:4:1
4bcb, 76 83:8:6:3
4bdb, 91 90:5:5:0
4
5
6
4beb, 95
4bfb, 83
4bgb, 72
81:7:11:1
86:8:5:1
7
8
m-FC6H4
o-FC6H4
84:10:5:1
9
4bhb, 83 72:5:16:7
10
11
12
p-ClC6H4
m-ClC6H4
p-BrC6H4
m-BrC6H4
Furyl
4bib, 96
4bjb, 72
83:9:5:3
79:13:6:2
4bkb, 85 83:12:4:1
f
13
4blb, 65
78:12:7:3
89:2:9:0
e
14
15
16
4bmb,
89
98/−
98/−
98/−
98/97
93/93
98/98
98/97
98/99
e
e
Furyl
Ph
4bma,
90
88:4:8:0
Thiophen
p-MeC6H4
p-MeC6H4
p-MeC6H4
p-MeC6H4
p-MeC6H4
p-MeC6H4
p-MeC6H4
Ph
4bna, 85
4bbc, 64
89:7:4:0
85:6:6:3
f
17
p-CH3C6H4
p-FC6H4
m-FC6H4
o-FC6H4
m-ClC6H4
m-BrC6H4
f
18
4bbd, 72 83:8:5:4
f
19
4bbe, 93
4bbf, 28
4bbg, 76
85:10:4:1
82:11:5:2
83:10:5:2
20
21
22
23
e
4bbh, 93 83:11:5:1
89:6:4:1
97/−
98/91
m-MeOC6H4 4bbi, 90
a
Unless noted, all of the reactions were carried out with 1b (0.3
Figure 2. Proposed mechanism and main transition-state model.
t
mmol), 2 (0.3 mmol), 3 (0.2 mmol), 5i (10 mol %), and BuOH (38
uL) in 0.5 mL of MTBE for the specified reaction times, which are
shown in the Supporting Information. Combined isolated yields of
the isomers. Determined by H NMR analysis of the crude reaction
mixture; A−D represent the four pairs of enantiomers of the product,
respectively. Determined by chiral HPLC of the stereoisomers A and
b
amine moiety of the catalyst would initially activate the
thiophenol 1 by deprotonation, and the resulting thiolate
anion would undergo an asymmetric sulfa-Michael addition
with low enantioselectivity, which is a reversible reaction. The
resulting intermediate carbanion 6′ would then undergo a DKR
process, with the S configuration favoring the nucleophilic attack
of the imine from the Si-face to give the desired product with the
observed high level of stereoselectivity. The sulfonamide
hydrogen of the catalyst strengthens the hydrogen bond
interaction with the nitro group of carbanion 6′.
c
1
d
e
f
B. Not determined. Performed at −40 °C.
To demonstrate the synthetic potential of the 1-amino-2-
nitro-3-organosulfur products formed by this unprecedented
catalytic asymmetric multicomponent sulfa-Michael/Mannich
reaction, we converted a selection of these products to the
corresponding 1,2-dimino-3-organosulfur and 2-nitro allylic
amine compounds, as shown in Scheme 3. The chiral bioactive
1,2-diamino-3-organosulfur compounds were readily obtained
by the reduction of the 2-nitro group in 4aaa with Zn/HCO2H at
−5 °C, which gave 7aaa in 63% yield without any discernible loss
in the enantiomeric purity. This novel asymmetric multi-
component reaction also provided facile access to highly
enantioselective chiral bioactive 2-nitro allylic amines, which
would be otherwise difficult to access via the direct asymmetric
aza-Baylis−Hillman reaction of nitroalkene. Oxidation of 4aaa
(>99% ee) with m-CPBA (1 equiv) gave the corresponding
sulfoxide in quantitative yield, which subsequently underwent an
elimination reaction under thermal conditions to afford the
Figure 1. X-ray crystal structures of 4aaa-A, 4aaa-B, and ( )-4aaa-C.
Scheme 2. Mechanistic Investigations
C
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Organic Letters
Letter
Scheme 3. Conversion of Cascade Adducts into 1,2-Diamino-
3-organosulfur Compound and 2-Nitro Allylic Amines
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.5b02423.
Experimental procedures and detailed characterization
data of all new compounds (PDF)
X-ray crystal details for 4aaa-A (CIF)
X-ray crystal details for 4aaa-B (CIF)
X-ray crystal details for ( )-4aaa-C (CIF)
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AUTHOR INFORMATION
Corresponding Author
■
*E-mail: pengyungui@hotmail.com; pyg@swu.edu.cn.
Notes
The authors declare no competing financial interest.
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2005, 127, 12224. (b) Zuend, S. J.; Coughlin, M. P.; Lalonde, M. P.;
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ACKNOWLEDGMENTS
■
We are grateful for financial support from the National Science
Foundation of China (21172180) and the Specialized Research
Fund for the Doctoral Program of Higher Education
(20110182110006).
D
DOI: 10.1021/acs.orglett.5b02423
Org. Lett. XXXX, XXX, XXX−XXX