Asymmetric michael addition of 1-acetylindolin-3-ones to β-nitrostyrenes catalyzed by bifunctional thioureas: A simple access to 2-functionalized indoles

Article abstract of DOI:10.1021/jo102022g

The first asymmetric Michael addition of 1-acetylindolin-3-ones to β-nitrostyrenes has been developed. 2-Substituted indolin-3-one derivatives were obtained with excellent yields (up to 99%) and good stereoselectivities (up to 28:1 dr and 92% ee), which could be transformed into 2-functionalized indoles easily without racemization. This achievement might further contribute to the chemistry and pharmacology of indole-related compounds.

Full text of DOI:10.1021/jo102022g

NOTE
pubs.acs.org/joc
Asymmetric Michael Addition of 1-Acetylindolin-3-ones to
β-Nitrostyrenes Catalyzed by Bifunctional Thioureas: A Simple
Access to 2-Functionalized Indoles
Yao-Zong Liu, Rui-Ling Cheng, and Peng-Fei Xu*
State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou
730000, P. R. China
S Supporting Information
b
ABSTRACT: The first asymmetric Michael addition of 1-acet-
ylindolin-3-ones to β-nitrostyrenes has been developed. 2-Sub-
stituted indolin-3-one derivatives were obtained with excellent
yields (up to 99%) and good stereoselectivities (up to 28:1 dr
and 92% ee), which could be transformed into 2-functionalized
indoles easily without racemization. This achievement might
further contribute to the chemistry and pharmacology of indole-
related compounds.
he catalytic asymmetric Michael addition is one of the most
important approaches to carbon-carbon bond formation in
4a in 44% yield almost without stereoselectivity (Table 1, entry 1).
(S)-1-(2-Pyrrolidinylmethyl)pyrrolidine 1b,11 thiourea-primary
amine catalyst 1c, and thiourea-secondary amine catalyst 1d also
gave almost racemic product.
To our delight, thiourea-cyclic tertiary amine catalysts (1e
and 1f) provided good yields, diastereoselectivities, and enantio-
selectivities.12 The best outcome was obtained with bifunctional
catalyst 1e derivated from quinine (Table 1, entry 5).
T
asymmetric synthesis. Remarkable progress has been made in the
organocatalytic version of this reaction in recent years.1,2 Many
chemists in the world continue to investigate new substrates,
catalysts, and catalytic systems. Indole is a common building unit,
and its interesting chemical properties have inspired chemists to
design and synthesize a variety of its derivatives. Since its
discovery in 1866, more than 80000 papers related to indole
chemistry have been published.3
Because of the reactive difference between the 2- and 3-posi-
tions of indole, much more successful examples have been
applied to the formation of 3-substituted indoles than to the
formation of 2-substituted indoles.4 Thus, the preparation of
2-substituted indoles, particularly optically active ones, is still a
challenge.5 Although You et al. and Wang et al. have reported
organocatalyzed asymmetric Friedel-Crafts reactions of 4,7-dihy-
droindoles to provide 2-substituted indole derivatives after subse-
quent oxidation, these reactions were limited by substrate scope
and harsh reaction conditions.6 Recently, the Takenaka group has
also reported an efficient approach to C2-alkylated indole deriva-
tives with the asymmetric conjugate addition of 4,7-dihydroindoles
to β-nitrostyrenes catalyzed by 2-aminopyridinium ions.7 Indolin-
3-ones, a class of important building blocks, are used in total syn-
thesis frequently.8 However, compared to 2-oxindoles,9 the appli-
cation of indolin-3-ones as nucleophiles is limited.10 Herein, we
report the first highly enantio- and diastereoselective addition of
1-acetylindolin-3-ones to nitroolefins catalyzed by thioureas and its
application to the synthesis of 2-functionalized indoles.
The screening of different organic solvents with 5% catalyst 1e
was carried out. The reaction proceeded faster in chlorinated
solvents (Table 2, entries 1 and 3). In polar solvents such as
EtOH and THF, the reaction occurred in high yields but lower
enantioselectivities (Table 2, entries 4 and 5). When the reaction
temperature was dropped to -40 °C in DCM, the best result was
obtained with 99% yield, 12:1 dr, and 91% ee (Table 2, entry 8).
Increasing the catalyst loading to 10% did not show any beneficial
effect to the diastereo- and enantioselectivity (Table 2, entry 9).
Under optimized reaction conditions, a wide range of nitro-
olefins 3 and 1-acetylindolin-3-ones 2 were investigated in this
asymmetric Michael addition reaction. The results are summar-
ized in Table 3. In most cases, excellent yields and good dr and ee
values were obtained for the desired products.13 Structural
variation of nitroolefins could be tolerated (Table 3). The elec-
tron-withdrawing (Table 3, entries 4-10), electron-donating
(Table3, entries 2 and 3), and neutral (Table 3, entry 1) systems
could participate in the reactions. And the electronic nature of
the aromatic ring of nitroolefin had limited influence on the
stereochemical outcome. 2-Furyl-substituted nitroolefin 3k was
also well tolerated under the optimized reaction conditions (97%
yield, 11:1 dr and 89% ee; Table 3, entry 11). Unfortunately,
We initially investigated the reaction of 1-acetylindolin-3-one
2a with trans-β-nitrostyrene 3a in the presence of readily
available diarylprolinol trimethylsilyl ether 1a (20%) in DCM.
The Michael addition proceeded slowly to afford desired product
Received: October 25, 2010
Published: March 11, 2011
r
2011 American Chemical Society
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|

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NOTE
Table 1. Screening of the Catalysts
Table 3. Catalytic Asymmetric Michael Addition of 1-Acet-
ylindolin-3-ones to Nitroolefins
entrya
R1
R2
yield (%)
drb
eec (%)
1
H
H
H
H
H
H
H
H
H
H
H
H
Ph
99 (4a)
90 (4b)
96 (4c)
92 (4d)
75 (4e)
97 (4f)
94 (4g)
98 (4h)
95 (4i)
91 (4j)
97 (4k)
96 (4l)
93 (4m)
91 (4n)
89(4o)
80(4p)
12:1
13:1
26:1
4:1
91
2
4-MeC6H4
2-MeOC6H4
2-BrC6H4
3-BrC6H4
4-BrC6H4
3-NO2C6H4
2-ClC6H4
3-ClC6H4
4-ClC6H4
2-furyl
88
3
91
4
87
5
6d
10:1
28:1
12:1
18:1
15:1
18:1
11:1
1.3:1
8:1
80
86
entrya
1
time (h)
yield (%)
drb
5:1
eec (%)
7
78
8
90
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1f
24
24
24
24
1
44
91
83
99
96
94
76
62
-7
-6
13
9
89
2.1:1
2.2:1
2:1
10
11
12e
13
14
15
16
90
89
67 (80f)
8
cyclohexyl
Ph
9:1
88
5-Me
91
1
8:1
-84
-78
-77
4-Br
Ph
13:1
12:1
11:1
92
1g
1h
2
8:1
5-Br
Ph
92
2
2.2:1
6-COOMe
Ph
85
a Unless otherwise noted, reactions were carried out with 2a (0.2 mmol),
3a (0.3 mmol), and catalyst 1 (20%) in 0.4 mL DCM at -15 °C.
b Determined by 1H NMR spectroscopic analysis of the crude product
mixture. c Determined by chiral HPLC analysis. Ar = 3,5-(CF3)2C6H3
a Unless otherwise noted, the reactions were carried out with 2 (0.2
mmol), 3 (0.3 mmol), and catalyst 1e (5%) in 0.4 mL of DCM at -40
°C. b Determined by 1H NMR spectroscopic analysis of the crude
product mixture. c Determined by chiral HPLC analysis. d The reaction
needed 24 h to complete. e The reaction was performed at 16 °C for 36 h,
and 20% 1e was used. f The ee value of the minor diastereomer.
Table 2. Optimization of the Reaction Conditions
Table 4. Synthesis of 2-Substituted Indole Derivatives
entrya
4
yield (%)
eeb (%)
1
2
3
4
4a (Ph)
57 (5a)
55 (5c)
61 (5f)
54 (5i)
91
90
86
88
4c (2-MeOC6H4)
4f (4-BrC6H4)
4i (3-ClC6H4)
entrya 1e (mol %) solvent T (°C) time (h) yield (%) drb eec (%)
1
2
3
4
5
6
7
8
9
5
5
DCM
-15
3
8
91
92
88
98
99
97
92
99
97
10:1
6:1
84
88
85
73
35
90
84
91
91
a Reaction conditions: see the Supporting Information for details.
toluene -15
CHCl3 -15
b Determined by chiral HPLC analysis; see the Supporting Information.
5
3
6:1
5
THF
-15
-15
24
24
24
10
12
6
10:1
5:1
5
EtOH
In order to apply this reaction in the synthesis of 2-substituted
indole derivatives, 2-substituted 1-acetylindolin-3-ones 4 were
reduced with NaBH4 in MeOH followed by dehydration in the
presence of p-TSA at 80 °C. Fortunately, although one stereo-
genic center was destroyed, the desired 2-substituted indoles 5
were obtained in moderate yields and good enantioselectivities.
This transformation provided a convenient access to 2-substi-
tuted indoles.
5
toluene -40
CHCl3 -40
9:1
5
8:1
5
DCM
-40
-40
12:1
10
DCM
10:1
a Reaction conditions: 0.2 mmol of 2a, 0.3 mmol of 3a in 0.4 mL of
solvent. b Determined by 1H NMR spectroscopic analysis of the crude
product mixture. c Determined by chiral HPLC analysis.
To determine the absolute configuration of the asymmetric
Michael adduct, a single crystal of compound 4f bearing a
bromine atom was obtained for X-ray crystallographic analysis.15
As shown in Figure 1, the newly formed stereogenic centers in 4f
were confirmed as S,S.
On the basis of the observed reactivity and experimental
results of the Michael addition reactions, we propose that the
reaction proceeds via a dual activation model. As shown in
Figure 2, a thiourea moiety of the catalyst 1e interacts through
cyclohexyl-substituted nitroolefin 3l (Table 3, entry 12) took no
reaction under the optimized reaction conditions. When the
reaction temperature was raised to 16 °C, the reaction proceeded
well in the presence of 20% 1e to give the product in 96% yield,
1.3:1 dr, and 67% ee. Substituted 1-acetylindolin-3-ones were
also well tolerated (Table 3, entries 13 -16).14 With this protocol,
highly enantioenriched 2-substituted 1-acetylindolin-3-ones could
be easily obtained.
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NOTE
(n-hexane/i-PrOH = 80:20), 1.0 mL/min; minor enantiomer, tR
31.9 min, major enantiomer, tR = 41.0 min.
=
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental details, spectral
b
data for the products, and X-ray crystallographic data (CIF file of
4f: CCDC 787026). This material is available free of charge via
the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Figure 1. X-ray crystal structure of 4f.
Corresponding Author
*E-mail: xupf@lzu.edu.cn.
’ ACKNOWLEDGMENT
We are grateful for the National Basic Research Program
of China (Nos. 2009CB626604, 2010CB833203), the NSFC
(20972058, 21032005), and the “111” program from MOE of
P. R. China
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Figure 2. Proposed transition state model in the Micheal addition.
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In conclusion, we have developed a bifunctional thiourea-
tertiary amine-catalyzed asymmetric Michael addition of 1-acet-
ylindolin-3-ones to β-nitrostyrenes in excellent yields and high
diastereo- and enantioselectivities. The corresponding products
could be subsequently converted to 2-functionalized indoles
without loss of enantioselectivities. Further investigation and
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’ EXPERIMENTAL SECTION
Representative Procedure for the Michael Addition of
1-Acetylindolin-3-one to β-Nitrostyrene (Table 3, Entry 1).
To a solution of 1-acetylindolin-3-one 2a (35 mg, 0.2 mmol, 1.0 equiv)
and thiourea catalyst 1e (6 mg, 0.01 mmol, 0.05 equiv) in freshly distilled
DCM (0.4 mL) at -40 °C was added trans-β-nitrostyrene 3a (45 mg,
0.3 mmol, 1.5 equiv). The resulting solution was stirred at -40 °C for
12 h. The reaction was quenched with saturated aqueous ammonium
chloride solution. The aqueous layer was separated and extracted with
ethyl acetate (3 times). The combined organic layers were dried over
Na2SO4, filtered, and concentrated in vacuo. The crude material was
purified by flash column chromatography (PE/EA = 2:1) to afford 64
mg (99% yield) of 4a (91% ee) and its minor diastereomer as a colorless
oil (12:1 dr). Analytical data for 4a: [R]2D0 = -59 (c 1.0, CH2Cl2); 1H
NMR (400 MHz, DMSO-d6) δ 8.01 (s, 1H), 7.51-7.44 (m, 2H), 7.07-
7.00 (m, 6H), 5.62 (dd, J = 6.0, 14.0 Hz, 1H), 5.32 (dd, J = 8.0, 14.0 Hz,
1H), 4.94 (d, J = 4.0 Hz, 1H), 4.39 (s, 1H), 2.52 (s, 1H); 13C NMR
(100 MHz, DMSO-d6) δ 180.0, 168.7, 136.9, 133.0, 128.2, 128.0, 127.9,
127.8, 124.7, 123.7, 122.7, 73.8, 65.5, 43.8, 24.0; HRMS (ESI) m/z calcd
for C18H20N3O4 [M þ NH4]þ:342.1448, found 342.1444. The en-
antiomeric excess was determined by HPLC with an AS-H column.
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NOTE
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(15) CCDC 787026 contains the supplementary crystallographic
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Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
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