Asymmetric construction of quaternary stereocenters by direct conjugate addition of oxindoles to enone

Article abstract of DOI:10.1016/j.tetasy.2010.09.016

Chiral cinchona-based primary amine A was found to catalyze the asymmetric direct conjugate addition of prochiral 3-oxindoles with enones to afford 3,3-disubstituted oxindoles in good yields, moderate to high diastereoselectivities, and excellent enantios

Full text of DOI:10.1016/j.tetasy.2010.09.016

Tetrahedron: Asymmetry 21 (2010) 2493–2497
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Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Asymmetric construction of quaternary stereocenters by direct conjugate
addition of oxindoles to enone
Wangsheng Sun a, Liang Hong a, Chunxia Liu a, Rui Wang a,b,
⇑
a Key Laboratory of Preclinical Study for New Drugs of Gansu Province, State Key Laboratory of Applied Organic Chemistry and Institute of Biochemistry
and Molecular Biology, Lanzhou University, Lanzhou 730000, China
b Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Kowloon, Hong Kong
a r t i c l e i n f o
a b s t r a c t
Article history:
Chiral cinchona-based primary amine A was found to catalyze the asymmetric direct conjugate addition
of prochiral 3-oxindoles with enones to afford 3,3-disubstituted oxindoles in good yields, moderate to
high diastereoselectivities, and excellent enantioselectivities.
Received 26 August 2010
Accepted 29 September 2010
Available online 2 November 2010
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
selective transformations based on the efficient activation of
aldehydes. However, minor progress has been achieved in the cor-
Oxindoles bearing a quaternary carbon stereocenter at the C3
position are a versatile structural motif found in a variety of biolog-
ically significant natural products and utilized as building blocks
for alkaloid synthesis, as well as the development of potential ther-
apeutic agents.1 The interesting chemical properties of oxindoles
have inspired chemists to design and synthesize a variety of oxin-
dole derivatives.2 However, the catalytic asymmetric synthesis of
all carbon oxindole quaternary centers has remained a difficult
challenge.3 Their asymmetric construction is mainly achieved by
chiral resolution or diastereoselective methods using a stoichiom-
etric amount of starting materials, that is, chiral auxiliaries.4 There-
fore, the catalytic enantioselective synthesis of such oxindoles is
particularly appealing.
Among the established strategies for the synthesis of chiral 3,
3-disubstituted oxindoles, the transition metal-catalyzed asym-
metric reaction has been intensively studied.5 Recently, organoca-
talysis has emerged as a powerful and promising synthetic field
that is complementary to metal-catalyzed transformations and
has accelerated the development of novel methodologies for syn-
thesizing diverse chiral molecules.6 Although organocatalytic
enantioselective conjugate addition reactions of oxindoles with en-
als and nitroolefins, leading to highly valuable enantio-enriched
3,3-disubstituted oxindoles, have been reported,7 the use of enones
as Michael acceptors remains elusive.8 Toward this end, we herein
report a study which results in a chiral amine promoted direct
Michael addition of oxindoles to enones in good yields, moderate
to high diastereoselectivities and excellent enantioselectivities.
In the past few years, aminocatalysis has proven to be a power-
ful procedure within the field of organocatalysis for many enantio-
responding transformations of ketones, probably due to the inher-
ent difficulties in generating congested covalent intermediates
from chiral secondary amines and ketones. Recently, chiral primary
amines have emerged as general and powerful catalysts for ketone
activations, and proved high reactivity and selectivity in the enan-
tioselective conjugate additions of various nucleophiles to enones
through iminium catalysis.9 Despite recent progress on chiral pri-
mary amines catalyzed asymmetric 1,4-additions of various nucle-
ophiles to enones, there are no examples using prochiral oxindoles
as nucleophiles. In this context, we are interested in the possibility
of developing an enantioselective conjugate addition of 3-substi-
tuted oxindoles to enones in the presence of chiral primary amine,
since an enantioselective conjugate addition of 3-substituted oxin-
doles to Michael acceptors is currently one of the hot topics in cat-
alytic asymmetric synthesis. However, the utility of 3-substituted
oxindoles as nucleophiles is still found to be difficult because the
creation of a stereogenic quaternary center at C3, which is steri-
cally congested, as the Michael reaction is generally sensitive to
steric hindrance. Herein, we report our preliminary results for this
reaction.
2. Results and discussion
As part of our research program toward the development of chi-
ral amine-based asymmetric catalysis,10 we became interested in
the design of novel organocatalytic reactions utilizing primary
amine-induced iminium activation. In light of the capacity of chiral
primary amines in promoting efficient conjugate addition reac-
tions with enones, we initially probed the reaction between enone
1a and oxindole 2a in the presence of cinchona-based primary
amine A (Fig. 1),11 in combination with CF3CO2H, in toluene at
room temperature for 48 h. The reaction proceeded smoothly to
⇑
Corresponding author.
E-mail address: wangrui@lzu.edu.cn (R. Wang).
0957-4166/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2010.09.016

2494
W. Sun et al. / Tetrahedron: Asymmetry 21 (2010) 2493–2497
afford the desired product 3aa in excellent yield and moderate
enantioselectivity (Table 1, entry 1). Encouraged by this result,
we studied various acid additives for reaction under the same reac-
tion conditions (Table 1, entries 1–8). The results showed that N-
N
H2N
Boc-D
-phenylglycine (NBDP)12 was an effective acid additive for
the reaction. To further optimize the process, we probed the effects
of the reaction medium on the reaction. Solvent screening revealed
that toluene was the solvent of choice, in which good yield (97%)
and enantioselectivity (97%) were achieved (Table 1, entry 6).
With the optimal reaction conditions in hand, we studied the
generality of the method with regards to the enone and oxindole
substrates (Tables 2 and 3). In general, the products were obtained
MeO
N
A
Figure 1. The structure of catalyst A.
Table 1
Screening the optimal reaction conditionsa
Ph
O
Ph
O
Ph
O
A (20 mol %)
O
+
Ph
additive (20 mol %)
toluene, rt. 48 h
N
H
N
H
1a
3aa
d.r.d (anti/syn)
2a
Additiveb
CF3CO2H
2-NO2PhCO2H
CH3CO2H
PhCO2H
NBDP
Entry
Solvent
Yieldc (%)
eee (%)
1
2
3
4
5
6f
7
Tol
Tol
Tol
Tol
Tol
Tol
Tol
98
97
93
94
96
2:1
2:1
2:1
2:1
2:1
2:1
—
83
90
77
85
94
97
—
NBDP
97
<10
D-CSA
8
9
10
11
12
13
Tol
PTS
<10
97
93
81
<10
73
—
—
96
89
95
—
CHCl3
CH2Cl2
Ether
CH3OH
THF
NBDP
NBDP
NBDP
NBDP
NBDP
2:1
2:1
2:1
—
2:1
88
a
b
c
d
e
f
All reactions were performed with 0.2 mmol of 1a, 0.24 mmol of 2a, 0.04 mmol of catalyst A, and 0.04 mmol of acid in 1 mL of solvent at room temperature for 48 h.
Abbreviations: NBDP = N-Boc-D-phenylglycine; CSA = camphor-10-sulfonic acid; PTS = p-toluenesulfonic acid.
Isolated yield.
Determined by 1H NMR analysis of the products.
Enantiomeric excess was determined by HPLC analysis using a chiral column.
0.08 mmol of acid were used.
Table 2
Conjugate addition of oxindole 2a to enones 1a
R1
O
Ph
O
Ph
O
R2
A (20 mol %)
O
R1
R2
+
NBDP (40 mol %)
toluene, rt. 48 h
N
H
N
H
1
2a
3
Entry
1, R1
R2
Product
Yieldb (%)
d.r.c
eed (%)
1
2
3
4
5
6
7
8
9
1a, Ph
Me
Me
Me
Me
Me
Me
Me
Me
Et
3aa
3ba
3ca
3da
3ea
3fa
3ga
3ha
3ia
97
91
94
94
96
93
97
97
80
99
75
71
2:1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
9:1
97
98
94
98
96
98
97
98
>99
>99
>99
99
1b, 2-MeOPh
1c, 3-MeOPh
1d, 3-ClPh
1e, 4-MeOPh
1f, 4-FPh
1g, 4-ClPh
1h, 4-BrPh
1i, Ph
10
11
12
1j, –(CH2)4–
1k, Pr
1l, Pent
3ja
3ka
3la
Me
Me
>20:1
>20:1
a
b
c
Reactions were performed with enone 1 (0.2 mmol), oxindole 2a (0.24 mmol), A (0.04 mmol), and NBDP (0.08 mmol) in toluene at room temperature for 48 h.
Isolated yield.
Determined by 1H NMR analysis of the products.
Enantiomeric excess was determined by HPLC analysis using a chiral column.
d

W. Sun et al. / Tetrahedron: Asymmetry 21 (2010) 2493–2497
2495
Table 3
Conjugate addition of oxindoles 2 to enone 1aa
Ph
O
R3
R3
O
A (20 mol %)
+
O
O
NBDP (40 mol %)
toluene, rt, 48 h
Ph
N
N
H
H
2
1a
3
Entry
2, R3
Product
Yieldb (%)
d.r.c
eed (%)
1
2
3
4
5
6
2a, Bn
3aa
3ab
3ac
3ad
3ae
3af
97
95
95
96
95
96
2:1
2:1
2:1
2:1
2:1
2:1
97
97
>99
98
95
Rac
2b, 2-MeOBn
2c, 3-ClBn
2d, 4-ClBn
2e, Pr
2f, Ph
a
b
c
Reactions were performed with enone 1a (0.2 mmol), oxindoles 2 (0.24 mmol), A (0.04 mmol), and NBDP (0.08 mmol) in toluene at room temperature for 48 h.
Isolated yield.
Determined by 1H NMR analysis of the products.
Enantiomeric excess was determined by HPLC analysis using a chiral column.
d
in excellent yields and enantioselectivities. For enones 1a–h carry-
almost enantiomerically pure products were obtained in high
ing aromatic substituents, the diastereoselectivities were low, but
yields, irrespective of the steric and electronic demands of the b-
Ph
Ar
Ar
OTMS
O
Ph
3
N
H
Ph
3
(20 mol %)
Ph
N
H
Ph
CHO
CHO
2a
Ar = 3,5-(CF3)2-Ph
3'
O
O
+
ref. 7b
PhCOOH (20 mol %)
Tol, r.t. 5 d
+
N
N
H
H
(3S,3'R)-5
5
(3S,3'S)-
CHO
Ph
1. MeLi (3 equiv)
THF, 3h
4
2. MnO2 (20 equiv)
CH2Cl2, 48 h
Ph
3
O
Ph
3
O
Ph
Ph
+
3'
3'
O
O
N
N
H
H
3aa'
(3S,3'R)-
3aa'
(3S,3'S)-
Ph
3
O
Ph
3
O
Ph
Ph
Ph
O
A
(20 mol %)
3'
3'
O
O
O
+
+
Ph
NBDP (40 mol %)
toluene, rt. 48 h
N
H
N
N
H
H
2a
1a
3aa
3aa
(3S,3'S)-
(3S,3'R)-
major product
minor product
Scheme 1. Establishment of the absolute configuration.

2496
W. Sun et al. / Tetrahedron: Asymmetry 21 (2010) 2493–2497
olefin substituents (Table 2, entries 1–8). No decrease in yield or ee
value was observed for the slightly sterically hindered enone 1i
(Table 2, entry 9). Cyclic enone 1j was also an excellent acceptor
for the conjugation addition, affording the desired product in high
yield, improved diastereoselectivity and excellent enantioselectiv-
ity (Table 2, entry 10). For the less reactive alkyl-substituted enon-
es 1k–l, the reaction showed slightly lower reactivities but greatly
improved diastereoselectivities and excellent enantioselectivities
(Table 2, entries 11 and 12).
spectrometer and only major peaks were reported in cmꢀ1. Optical
rotations were reported as follows: ½a Drt
ꢁ
(c: g/100 mL, in solvent).
High resolution mass spectra (HRMS) were obtained by the ESI ion-
ization sources. The ee value determination was carried out using
chiral HPLC with Daicel Chiracel OD-H or AD column on Waters
with a 996 UV-detector.
4.2. General procedure for the synthesis 3
Next, we decided to investigate the scope of the reaction with
different 3-substituted oxindoles. The present organocatalytic tac-
tic is also suitable for 3-benzyloxindole architectures, as electronic
modification of the aromatic rings can be accomplished without
affecting the efficiency of the system (Table 3, entries 1–4). When
the 3-alkyloxindole 2e was used, excellent yield and enantioselec-
tivities were achieved (Table 3, entry 5). Unfortunately, when the
3-phenyloxindole 2f was used, an almost racemic product was ob-
tained (Table 3, entry 6).
In an ordinary flask, organocatalyst A (Fig. 1) (0.04 mmol) was
dissolved in 1 mL of toluene. After addition of 0.08 mmol of D-N-
Boc-phenylglycine, the solution was stirred for 5 min at room tem-
perature. After the addition of enone (0.20 mmol), the mixture was
stirred for 10 min. Then oxindole (0.24 mmol) was added in one
portion, and stirred for 48 h at room temperature. The reaction
mixture was directly purified by silica gel chromatography without
workup and fractions were collected and concentrated in vacuo to
provide the pure desired product 3.
In the case of aryl-substituted enones, although the diastereose-
lectivities were low, the two diastereomers can be easily separated
by column chromatography, which renders this catalytic system a
useful synthetic route to valuable chiral oxindoles with contiguous
quaternary and tertiary stereocenters.
4.2.1. 3-Benzyl-3-(3-oxo-1-phenylbutyl)indolin-2-one (3aa)
Compound 3aa was isolated by column chromatography in
97% yield as a mixture of diastereomers (2:1); Major diastereo-
mer: 1H NMR (300 MHz, CDCl3) d 7.63 (s, 1H), 7.35 (dd, J = 8.4,
1.5 Hz, 1H), 7.14–6.78 (m, 12H), 6.47 (dd, J = 6.9, 1.8 Hz, 1H),
3.82 (dd, J = 11.1, 3.6 Hz, 1H), 3.21–3.12 (m, 3H), 2.98 (dd,
J = 15.9, 3.6 Hz, 1H), 1.95 (s, 3H); 13C NMR (75 MHz, CDCl3) d
206.6, 179.4, 141.1, 138.7, 135.5, 130.1, 129.9, 129.4, 128.3
127.8, 127.5, 127.1, 126.4, 124.6, 121.8, 109.5, 58.1, 47.9, 44.5,
41.2, 30.6; IR: 3212, 3031, 1714, 1620, 1472, 911, 732,
700 cmꢀ1; HRMS (ESI): C25H23NO2+H, calcd: 370.1802, found:
The establishment of the absolute configuration of the 3,30-
disubstituted oxindole 3aa was achieved by chemical correlation
following Bandini’s methodology(Scheme 1).13 Following the
asymmetric organocatalytic conjugate addition of oxindole 2a with
cinnamaldehyde 4 described by Melchiorre,7b we prepared two
diastereomeric (3S,30R)-5 and (3S,30S)-5, respectively. Compound
5 was then reacted with MeLi at 0 °C to afford a mixture of two dia-
stereomeric alcohols. The alcohols mixtures were used without
purification and oxidized with MnO2 to give two diastereomeric
(3S,30R)-3aa0 and (3S,30S)-3aa0, respectively. Comparison of both
the 1H NMR and HPLC retention times allowed us to establish
the absolute stereochemistry of the major product 3aa to be
(3S,30S).
370.1809; ½a rDt
ꢁ ¼ þ142 (c 0.57, CHCl3); HPLC: DAICEL CHIRALCEL
AD, Hexane/iPrOH = 90:10, flow rate = 1.0 mL/min, retention time:
tminor = 21.1, tmajor = 23.7, 97% ee.
Acknowledgments
We gratefully acknowledge the financial support from NSFC
(20932003, 90813012) and the National S&T Major Project of China
(2009ZX09503-017).
3. Conclusion
In conclusion, we have developed a highly enantioselective con-
jugate addition of oxindoles to enones. The reaction is efficiently
catalyzed by a commercially available chiral cinchona-based pri-
mary amine and gives the corresponding adducts in good yields,
moderate to high diastereoselectivities, and excellent enantiose-
lectivities. The high enantioselectivity and broad substrate scope
of this transformation make it potentially useful in the synthesis
of chiral quaternary oxindole derivatives. The synthetic applica-
tions of this transformation are currently ongoing in our
laboratory.
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