Highly enantioselective [3+2] coupling of indoles with quinone monoimines promoted by a chiral phosphoric acid

Article abstract of DOI:10.1002/anie.201405689

Highly enantioselective [3+2] coupling of 3-substituted indoles with quinone monoimines promoted by a chiral phosphoric acid has been reported. A large variety of benzofuroindolines were prepared in moderate to good yields (up to 98 %) with generally excellent enantioselectivities (up to 99 % ee). Benzofuroindolines: The title reaction has been realized. A wide variety of benzofuroindolines have been prepared with moderate to good yields in moderate to excellent enantioselectivities. Ts=4-toluenesulfonyl.

Full text of DOI:10.1002/anie.201405689

Angewandte
Chemie
DOI: 10.1002/anie.201405689
Asymmetric Catalysis
Highly Enantioselective [3+2] Coupling of Indoles with Quinone
Monoimines Promoted by a Chiral Phosphoric Acid**
Lihua Liao, Chang Shu, Minmin Zhang, Yijun Liao, Xiaoyan Hu, Yonghong Zhang, Zhijun Wu,
Weicheng Yuan, and Xiaomei Zhang*
Abstract: Highly enantioselective [3+2] coupling of 3-sub-
stituted indoles with quinone monoimines promoted by a chiral
phosphoric acid has been reported. A large variety of
benzofuroindolines were prepared in moderate to good
yields (up to 98%) with generally excellent enantioselectivities
(
up to 99% ee).
F
used indolines are important building blocks for a large
[
1]
number of natural products and phamarceuticals. Conse-
quently, diverse successful strategies have been developed to
Figure 1. Representative benzofuroindoline natural products. Ac=ace-
tyl.
[
2]
construct fused indolines. Among these transformations,
dearomative annulation of indoles is the most straightforward
method and has attracted much attention.
The benzofuroindoline core is a unique motif found in
some important natural alkaloids,
and azonazine (Figure 1). Since diazonamide A has been
found to be a very potent anticancer agent owing to its high
antitumor activity (IC50 < 5 nm),
erable attention from chemists for the synthesis of these
benzofuroindoline skeletons.
[
3]
[3+2] coupling of 3-substituted indoles with quinone deriv-
[
6c–m]
atives were also demonstrated by several groups.
For an
[
4–6]
[4]
such as diazonamides
example, in 2011, Chen and co-workers employed a Brønsted
acid catalyzed direct [3+2]coupling of b-carbolines with
qinones as a key strategy in the formal synthesis of
[
5]
[4b,c]
[6j]
it has received consid-
(+)-haplophytine. In 2003, MacMillan et al. described an
enantioselective synthesis of a benzofuroindoline by an
addition/cyclization cascade of 3-phenol-substituted indole
[4f–k]
[
6a]
[6l]
In 2012, Bisai et al.
reported Lewis acid catalyzed
with acrolein promoted by a chiral imidazolinone catalyst.
Friedel–Crafts alkylations of 3-hydroxy-2-oxindoles, and it
provides access to benzofuroindolines in three steps. Soon
Recently, we employed a Lewis acid to promote [3+2]
coupling of 3-substituted indoles with quinone monoacetals
and a quinone imine ketal through which various benzofur-
oindolines and tetrahydroindolo[2,3-b]indoles were prepared
in moderate to good yields. Meanwhile, we also attempted
an enantioselective variant using a chiral phosphoric acid as
a catalyst. However, only 33% ee was obtained. To the best of
our knowledge, these are the only two examples of the
enantioselective construction of benzofuroindolines.
[6b]
after, Vincent
and co-workers reported FeCl -mediated
3
Friedel–Crafts hydroarylation of electrophilic N-acetyl
indoles with phenols to afford 3,3-disubstituted indolines,
followed by oxidation to yield the desired benzofuroindolines.
In addition, direct construction of benzofuroindolines by
[
6m]
[
*] Dr. L. Liao, Dr. C. Shu, Dr. M. Zhang, Dr. Y. Liao, Dr. X. Hu,
Dr. Y. Zhang, Prof. Dr. W. Yuan, Prof. Dr. X. Zhang
Key Laboratory for Asymmetric Synthesis and Chiral Technology of
Sichuan Province, Chengdu Institute of Organic Chemistry, Chinese
Academy of Sciences, Chengdu, 610041 (China)
Quinones and quinone monoimines are excellent electro-
philes. In recent years, quinones and quinone monoimines
have attracted increasing attention in asymmetric reactions.
In 2010, Jørgensen et al. reported a highly enantioselective a-
arylation of aldehydes through nucleophilic addition of
aldehydes to quinones or quinone monoimines followed by
[7]
E-mail: xmzhang@cioc.ac.cn
Dr. L. Liao, Dr. C. Shu, Dr. M. Zhang, Dr. Y. Liao, Dr. X. Hu,
Dr. Y. Zhang
University of Chinese Academy of Sciences
Beijing, 100049 (China)
[
7a]
aromatization.
involving activation of quinones by chiral organocatalysts
In addition, asymmetric transformations
[7b–j]
were also demonstrated.
Chiral phosphoric acids are versatile catalysts for many
asymmetric reactions. In particular, many efficient strat-
egies for asymmetric C3 functionalization of indoles and
Dr. Z. Wu
[8]
Chengdu Institute of Biology, Chinese Academy of Sciences
Chengdu, 610041 (China)
[
**] We thank the National Basic Research Program of China (973
Program) (2010CB833300) and the National Natural Science
Foundation of China (21372218 and 21172217). We also thank Prof.
Qilin Zhou and Jianhua Xie at Nankai University for a gift of the
chiral phosphoric acid PA13. Finally we thank Prof. Yingchun Chen
at Sichuan University for a gift of 2-methyl-3-ethyl-indole (1v).
functionalization of enamines (enamides) by phosphoric acids
[3a,g,j,k,o,9]
have been well established.
Herein we report a highly
enantioselective [3+2] coupling of 3-substituted indoles with
quinone monoimines promoted by a chiral phosphoric acid.
This reaction provides a large variety of benzofuroindolines
with moderate to high yields in generally excellent enantio-
selectivities.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201405689.
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.
Angewandte
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First, various chiral phosphoric acids were evaluated in
Table 2: Optimization of the reaction conditions.
the [3+2] coupling of 3-methyl indole (1a) with 4-methyl-N-
(
4-oxocyclohexa-2,5-dienylidene)benzenesulfonamide (2a)
in toluene at 258C. As can be seen in Table 1, the chiral
phosphoric acid PA8, which bears a 2,4,6-triisopropylphenyl
group at the 3,3’-positions of binol, promoted the reaction
efficiently to provide the benzofuroindoline 3a in nearly
quantitative yield with the highest ee value of 87% (Table 1,
entry 8). Thus PA8 was determined to be the optimal catalyst
and used in subsequent investigations.
Next, the other reaction parameters were modified. When
the temperature was lowered to 08C, the reaction proceeded
for 5 hours to afford the desired product with almost
[
a]
Entry
Solvent
PA8
x mol%)
T
[8C]
t
[h]
Yield
[%]
ee
[%]
[
b]
[c]
(
1
2
3
4
5
6
7
8
toluene
toluene
CH Cl
10
10
10
10
10
10
5
RT
0
0
0
0
0
0
0
2
5
5
5
6
5
5
7
99
99
98
99
80
98
98
90
87
89
93
93
90
98
98
87
2
2
Cl CH CH Cl
2
2
2
2
THF
Table 1: Screen of the chiral phosphoric acids.
CH CN
3
CH CN
3
CH CN
2
3
[
0
[
a] Unless otherwise specified, the reactions were carried out with
.20 mmol of 1a, 0.30 mmol of 2a, and PA8 in 2 mL of the solvent.
b] Yield of isolated product based on 1a. [c] Enantiomeric excess was
determined by HPLC analysis on a chiral stationary phase.
quantitative yield and a slightly higher ee value (Table 2,
entry 2). Then several other solvents were screened at 08C.
Reactions in two chlorinated solvents gave good yields as well
as good enantioselectivities (Table 2, entries 3 and 4). Mean-
while THF delivered a lower yield, although the ee value is
good (Table 2, entry 5). To our delight, when acetonitrile was
used, excellent yield and excellent enantioselection were
obtained (Table 2, entry 6). Hence acetonitrile was identified
as the most favorable solvent in the reaction. Moreover,
lowering the catalyst loading to 5 mol% did not affect the
result (Table 2, entry 7). However, further lowering the
catalyst loading to 2 mol% led to a decrease in both the
yield and ee value (Table 2, entry 8).
With the optimized reaction conditions in hand, the scope
of the reaction was investigated. In the presence of 5 mol% of
PA8, various 3-substituted indoles (1) were subjected to the
[
a]
[b]
[c]
Entry
Cat*
Solvent
t [h]
Yield [%]
ee [%]
1
2
3
4
5
6
7
8
9
PA1
PA2
PA3
PA4
PA5
PA6
PA7
PA8
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
2
2
2
6
24
2
12
2
6
48
5
12
2
50
73
93
50
90
77
65
99
49
56
36
43
90
95
33
71
57
17
47
13
67
28
49
87
37
43
17
21
83
47
21
23
[
3+2] coupling with the quinone monoimines 2. The results
are summarized in Scheme 1. Generally, indoles bearing
various electron-donating and electron-withdrawing groups
on the benzene part are well tolerated in the reaction and
afforded the corresponding benzofuroindolines in moderate
to good yields with high ee values, except for 3-phenyl indole
(
1t), which exhibited lower enantioselection (80% ee). A
better ee value (86% ee) was observed by employing
0 mol% of the catalyst PA8. For some indoles with bulkier
PA9
10
11
12
13
14
15
16
PA10
PA11
PA12
PA13
PA14
PA15
PA16
1
substituents, lower yields were obtained even after a pro-
longed reaction time. When the quinone monoimine having
an ortho-methyl substituent was employed, no desired
reaction was observed, perhaps because of the decreased
electrophilicity of this quinone monoimine. We also tried to
prepare quinone monoimines with electron-withdrawing
groups such as a chloro group in the ortho/meta position.
However, we failed to synthesize these substrates. Finally, we
tried to replace N-Ts by N-Boc and N-CBz. However, these
quinone monoimines are very unstable and decomposed
12
24
72
[
0
a] Unless otherwise specified, the reactions were carried out with
.20 mmol of 1a, 0.30 mmol of 2a, and 0.02 mmol of the chiral
phosphoric acid in 2 mL of toluene at room temperature. [b] Yield of
isolated product based on 1a. [c] Enantiomeric excess was determined
by HPLC analysis on a chiral stationary phase. Ts=4-toluenesulfonyl,
Tf =trifluoromethanesulfonyl.
1
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Scheme 1. Substrate scope. Reaction conditions: Unless otherwise specified, the reactions were carried out with 0.20 mmol of 1, 0.30 mmol of 2,
and 0.01 mmol of PA8 in 2 mL of acetonitrile at 08C. Yields shown are of the isolated products and based on 1. Enantiomeric excesses were
determined by HPLC analysis on a chiral stationary phase. [a] 10 mol% of PA8 was employed. TBS=tert-butyldimethylsilyl.
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quickly. Hence, it can be concluded that the scope of indoles
was very good, but the scope of quinone monoimines was
rather poor.
detected by TLC. The solvent was evaporated and the residue was
subjected to chromatography (silica gel, petroleum ether/EtOAc
5
:1!4:1) to afford the desired product 3.
The absolute configuration of the benzofuroindoline 3b
was determined to be 2R,3S by an X-ray crystal structural
analysis (see the Supporting Information). Consequently,
all of the other benzofuroindolines can be assigned absolute
configurations by analogy.
Based on the absolute configuration of the product 3b,
a plausible reaction mechanism is proposed. As outlined in
Scheme 2, first, the bifunctional phosphoric acid activated
both the indole and quinone monoimine. 3-Methylindole
attacked the quinone monoimine to give the intermediate
Received: May 27, 2014
Revised: June 12, 2014
Published online: August 1, 2014
[
10]
Keywords: asymmetric catalysis · enantioselectivity ·
.
heterocycles · indoles · organocatalysis
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monoimines promoted by a chiral phosphoric acid. Through
this transformation, a wide variety of benzofuroindolines
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3
0
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a magnetic stirring bar. Then 2 mL of CH CN was added to dissolve
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3
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