Highly enantioselective Michael addition of 2-oxindole- 3-carboxylate esters to nitroolefins promoted by cinchona alkaloid-thiourea-Bronsted acid cocatalysts

Article abstract of DOI:10.1002/adsc.201200206

A highly efficient organocatalyzed Michael addition of 2-oxindole-3- carboxylate esters to nitroolefins using a Cinchona alkaloid-thiourea and an achiral Bronsted acid as cooperative organocatalysts is reproted that affords significantly improved enantioselectivity and diastereoselectivity. It also provides an efficient approach to the synthesis of spirooxindole derivatives with high enantioselectivity. Copyright

Full text of DOI:10.1002/adsc.201200206

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DOI: 10.1002/adsc.201200206
Highly Enantioselective Michael Addition of 2-Oxindole-
3-carboxylate Esters to Nitroolefins Promoted by Cinchona
Alkaloid-Thiourea-Brønsted Acid Cocatalysts
Xianjie Chen,a,b Wei Zhu,b Wangke Qian,b Enguang Feng,b Yu Zhou,b
Jinfang Wang,b Hualiang Jiang,a,b Zhu-Jun Yao,a,* and Hong Liub,*
a
School of Chemistry and Chemical Engineering, State Key Laboratory of Coordination Chemistry and Nanjing National
Laboratory of Microstructures, Nanjing University, 22 Hankou Road, Nanjing 210093, Peopleꢀs Republic of China
E-mail: yaoz@nju.edu.cn
State Key Laboratory of Drug Research, Shanghai Institute of Materia Medica, Shanghai Institutes for Biological
b
Sciences, Chinese Academy of Sciences, 555 Zu Chong Zhi Road, Shanghai, Peopleꢀs Republic of China
Fax : (+86)-21-5080-7042; phone: (+86)-21-5080-7042; e-mail: hliu@mail.shcnc.ac.cn
Received: March 13, 2012; Revised: April 24, 2012; Published online: July 31, 2012
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200206.
Abstract: A highly efficient organocatalyzed Mi-
chael addition of 2-oxindole-3-carboxylate esters to
nitroolefins using a Cinchona alkaloid-thiourea and
an achiral Brønsted acid as cooperative organocata-
lysts is reproted that affords significantly improved
enantioselectivity and diastereoselectivity. It also
provides an efficient approach to the synthesis of
spirooxindole derivatives with high enantioselectivi-
ty.
egies are limited to 3-alkyl- or 3-aryl-substituted oxin-
doles.[2–9] These limitations have renewed the impetus
to develop alternative strategies for the asymmetric
synthesis of versatile oxindole scaffolds.
In recent years, important contributions have been
made to the development of bifunctional thiourea cat-
alysts, which have been widely applied in the enantio-
Keywords: nitroolefins; 2-oxindole-3-carboxylate
esters; spirooxindoles; thiourea-Brønsted cocataly-
sis
Oxindole alkaloids bearing a quaternary carbon ste-
reocenter, especially spirooxindoles, represent a preva-
lent scaffold found in natural products and biological-
ly active compounds,[1] such as horsfiline, coerulescine,
rychnophylline, spirotryprostatin A and B, CPC-1 and
the orally bioavailable CRTH2 (DP2) receptor
antagonistACHTUNGTRENNUNG(Figure 1).
An impressive number of organocatalyzed asym-
metric reactions have been developed based on the
oxindole motif, including conjugate addition,[2] Henry
reaction,[3] aldol reaction,[4] Mannich reaction,[5] alky-
lation,[6] cycloaddition[7] and domino reactions.[8]
Among these reactions, asymmetric Michael additions
are one of the most powerful tools in synthetic organ-
ic chemistry. The pioneering work on the Michael ad-
dition of oxindole to nitroolefin was undertaken by
Barbas III,[2a,k] Shibasaki,[9] Cheng,[2b] Yuan[2c] and
Figure 1. Examples of quaternary carbon-bearing oxindole
Zhou.[2d,e] Despite these advances, most of these strat- alkaloids.
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troolefins which could be applied to the synthesis of
spirooxindole derivatives.
A variety of catalysts 3a–3j (Figure 2) was evaluat-
ed for the Michael addition of oxindole 1a to nitroole-
fin 2a and the results are summarized in Table 1. Use
of catalysts 3a–3e gave high yields (up to 99%) and
enantiomeric excesses (ee) (up to 99%) when the re-
actions were performed in CH2Cl2 at À708C (Table 1,
entries 1–5). Chiral squaramide derivatives 3f and 3g
were also investigated but the results proved disap-
pointing (entries 6 and 7). To further optimize the re-
action conditions with the aim to improve the diaste-
reoselectivity, multifunctional catalysts 3h and 3i de-
Scheme 1. An alternative approach to spirooxindoles.
selective Michael addition of malonate esters to nitro- rived from chiral trans-cyclohexane-1,2-diamine and
olefins.[10] However, to the best of our knowledge, Cinchona alkaloid thiourea were tested (entries 8 and
ethyl 2-oxoindoline-3-carboxylate (easily derived 9). Unfortunately, both of them gave unsatisfactory
from oxindole)[11] has been less used as a nucleophile results. Quinidine-derived sulfonamide 3j also turned
in the organocatalysis field. As an alternative of the out to be a poor catalyst (entry 10). Examination of
known protocols, variously substituted spirooxindole different solvents indicated that CH2Cl2 was the opti-
products could be obtained via the asymmetric Mi- mal choice (see the Supporting Information, Table 1).
chael addition of ethyl 2-oxoindoline-3-carboxylates
On the basis of the results, four additional nitroole-
to nitroolefins using an organocatalyst (Scheme 1). fin substrates were examined using the catalyst 3d.
Here, we wish to report a new valuable example of Unfortunately, the level of stereocontrol decreased
cooperative thiourea-Brønsted acid organocatalyzed obviously (entries 11–14). It is presumed that the dis-
Michael addition of 2-oxindole-3-carboxylates to ni- tance between the two oxygen atoms of the dicarbon-
yl was shortened by the oxindole skeleton and cata-
lyst 3d bearing the pyrrolidine unit. Such a change
may have led to a crowd-structure interaction. For
this reason, two other suitable catalysts, 3b and 3c,
were both employed on substrate 2c with little modifi-
cation. Gratifyingly, higher ees (88% and 84%) were
obtained with catalysts 3b and 3c than that with 3d
(entries 15 and 16). Inspired by previous reports of
thiourea-Brønsted acid cooperative organocatalysis,[13]
we also evaluated a number of Brønsted acids as co-
catalysts in this reaction (entries 17–22). Rewardingly,
the enantioselectivity increased to 96% when both 3b
(10 mol%) and benzoic acid (10 mol%) were em-
ployed, and the corresponding diastereoselectivity
was improved as well. The result was almost un-
changed upon decreasing the catalyst loading to
5 mol% (entry 23).
Encouraged by the above excellent results of sub-
strate 2c (Table 1, entry 22), we further examined the
substrate generality with various substituted nitroole-
fins. As depicted in Table 2, nitroolefins 2b–2l with
either electron-donating or electron-withdrawing
groups worked efficiently and afforded the corre-
sponding products in excellent yields (91–99%) and
high enantioselectivities (90–98%) with moderate to
good diastereoselectivities (79:21 to 90:10) (Table 2,
entries 2–12). However, use of 4-nitrostyrene 2m re-
sulted in decreased enantioselectivity (Table 2,
entry 13), which may be due to a direct inhibitory
effect on the formation of favourable hydrogen bonds
by the nitro group of 2m. Heteroaromatic nitroolefin
2n also proved to be a feasible substrate generating
Figure 2. Screened catalysts.
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Highly Enantioselective Michael Addition of 2-Oxindole-3-carboxylate Esters to Nitroolefins
Table 1. Catalyst screening and reaction optimization.[a]
Entry
2
Catalyst (mol%)
Additive (mol%)
Time [h]
Yield of 4 [%][b]
dr[c]
ee [%][d]
1
2
3
4
5
6
7
8
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2b
2c
2d
2e
2c
2c
2c
2c
2c
2c
2c
2c
2c
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3d
3d
3d
3d
3b
3c
3b (20)
3b
3b
3b
3b
3b
3b (5)
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
12
12
12
12
12
12
12
12
12
12
24
12
12
12
12
12
24
24
24
24
24
24
24
99 (4a)
99 (4a)
95 (4a)
99 (4a)
99 (4a)
95 (4a)
72 (4a)
99 (4a)
99 (4a)
78 (4a)
95 (4b)
99 (4c)
98 (4d)
98 (4d)
99 (4c)
99 (4c)
99 (4c)
99 (4c)
94 (4c)
99 (4c)
57 (4c)
91 (4c)
99 (4c)
80:20
79:21
78:22
80:20
72:28
60:40
67:33
67:33
77:23
62:38
67:33
75:25
80:20
80:20
81:19
67:33
79:21
84:16
83:17
82:18
84:16
81:19
81:19
85
94
97
99
82
88
40
82
91
65
73
74
82
85
88
84
93
96
85
92
89
87
94
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
PhCO2H (10)
PhCO2H (10)
o-NO2-PhCO2H (10)
CH3CO2H (10)
CCl3CO2H (10)
CF3CO2H (10)
PhCO2H (5)
[a]
Unless otherwise noted, reactions were performed with oxindoles 1a (0.1 mmol) and nitroolefins 2 (2.0 equiv.) in the pres-
ence of 10 mol% catalyst 3 in 2 mL of CH2Cl2.
[b]
[c]
[d]
Isolated yields.
Determined by H NMR or HPLC analysis.
1
Determined by chiral HPLC analysis for the major diastereomer.
the desired product 4n (Table 2, entry 14) with high cantly under our optimized conditions (Table 3,
yield (95%) and good enantioselectivity (86%). Fur- entry 9).
thermore, the reaction was found to proceed smoothly
The benzoic acid additive played an important role
with an alkyl-substituted nitroolefin, giving 4o in 90% in this process due to its enforcement effect in the
yield with high enantioselectivity (82/94% ee) but enantioselectivities and diastereoselectivities. Schrei-
poor diastereoselectivity (60:40).
ner,[13b] Najera,[13c] Jacobsen,[13d,e] and Wang[2i,3] also re-
To further expand the substrate scope, a series of ported similar cooperative catalysis systems. Schreiner
oxindoles 1b–1k and nitroolefin 2g were examined found that hydrogen-bonding interactions were
(Table 3). The processes were highly efficient for all formed in the bifunctional thiourea and Brønsted acid
the cases with different substituents on the aromatic cooperative system using NMR techniques. Similar
ring (1b–1f) and ester carboxylate (1g) oxindoles, af- 1H NMR experiments were also reproduced in our
fording products 4p–4u with high yields and enantio- system (see the Supporting Information, Figure 1).
selectivities (Table 3, entries 1–6). It is worth noting We envisaged that the tertiary amine of thiourea 3b
that N-Bn and N-MOM protecting groups had no in- may be protonated by the weak Brønsted acid which
fluence on the enantioselectivity (Table 3, entries 7 was similar to organic ion-pair formation.[14] The pro-
and 8). Use of unprotected oxindole 1k also led to an tonated catalyst 3b may result in a slower deprotona-
excellent yield and a good enantioselectivity (Table 3, tion of the 1,3-dicarbonyl moiety leading to the im-
entry 10). However, when the protecting group on the provement of the enantioselectivity in this reaction.
nitrogen atom was CO2Et, the yield decreased signifi- Unfortunately, no clear evidence colud explain wheth-
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Table 2. Asymmetric Michael addition of ethyl 1-methyl-ox-
Table 3. Substrate scope of asymmetric Michael additions of
a variety of oxindoles with nitroolefin 2g.[a]
oindoline-3-carboxylate to a series of nitroolefins.[a]
Entry
R
Yield of 4 [%][b] dr[c]
ee [%][d]
1
C6H5 (2a)
4-MeO-C6H4 (2b) 98 (4b)
4-CH3-C6H4 (2c) 99 (4c)
4-Br-C6H4 (2d)
2-Br-C6H4 (2e)
3-Br-C6H4 (2f)
4-Cl-C6H4 (2g)
2-Cl-C6H4 (2h)
99 (4a) 88:12 97
2[e]
3
79:21 90
Entry R1, R2, R3
Yield [%][b] dr[c]
ee [%][d]
84:16 96
87:13 96
85:15 97
87:13 96
89:11 98
86:14 97
82:18 94
86:14 93
92:08 97
90:10 94
89:11 72
67:33 86
60:40 82/94[g]
4
6
5
7
8
9
10
11
12
13
14
15[f]
98 (4d)
99 (4e)
99 (4f)
99 (4g)
99 (4h)
1
2
3
4
5
6
7
8
9
10
5-OMe, Me, Et (1b 99 (4p)
95:5
82:18 94
81:19 95
80:20 94
84:16 95
86:14 95
67:33 93
78:22 95
n.d.[e] n.d.[e]
70:30 79
97
5-Cl, Me, Et (1c)
7-Cl, Me, Et (1d)
5-Br, Me, Et (1e)
5-F, Me, Et (1f)
H, Me, i-Bu (1g)
H, Bn, Et (1h)
99 (4q)
99 (4r)
98 (4s)
99 (4t)
98 (4u)
96 (4v)
96 (4w)
48 (4x)
99 (4y)
2,4-Cl2-C6H3 (2i) 99 (4i)
2-naphthyl (2j)
4-F-C6H4 (2k)
4-CF3-C6H4 (2l)
4-NO2-C6H4 (2m) 93 (4m)
2-furanyl (2n)
n-Pr (2o)
97 (4j)
91 (4k)
98 (4l)
H, MOM, Et (1i)
H, CO2Et, Et (1j)
H, H, Et (1k)
95 (4n)
90 (4o)
[a]
Unless otherwise noted, reactions were performed with
oxindoles 1a–k (0.1 mmol) and nitroolefin 2g (2.0 equiv.)
in the presence of 10 mol% catalyst 3b and 10 mol%
benzoic acid in 2 mL of CH2Cl2.
[a]
Unless otherwise noted, reactions were performed with
oxindoles 1a (0.1 mmol) and nitroolefins 2a–2o
(2.0 equiv.) in the presence of 10 mol% catalyst 3b and
10 mol% benzoic acid in 2 mL of CH2Cl2.
[b]
[c]
[d]
Isolated yields.
Determined by H NMR or HPLC analysis.
1
[b]
[c]
[d]
Isolated yields.
Determined by H NMR or HPLC analysis.
Determined by chiral HPLC analysis for the major dia-
1
stereomer.
Not determined.
Determined by chiral HPLC analysis for the major dia-
stereomer. The absolute configuration was determined
by X-ray analysis of the major diastereomer 4e (see the
Supporting Information). The other major diastereomers
were assigned accordingly.[12]
[e]
were reported by Zhang.[15] Finally, the Boc-protected
product 6 was obtained with 99% ee by treatment
with (Boc)2O at 08C (Scheme 3).
[e]
[f]
Performed for 36 h.
Performed at À608C for 36 h.
[g]
The ee value of the minor diastereomer.
er the hydrogen-bonding interactions occurred in our
1H NMR experiments.
To further investigate the potential applications of
this method, the asymmetric Michael addition of sub-
strate 1a to nitroolefin 2a on a gram-scale was exam-
ined. The desired product 4a was obtained with 96%
yield, 86:14 dr, and 95% ee (Scheme 2).
The compound 4a could be converted into spiroox-
indole derivative 5 with 99% ee in one step using Zn/
HOAc reduction under heated conditions. In addi-
tion, reduction of the nitro group was readily accom-
plished with NaBH4/NiCl2·6H2O. However, the un-
protected amine led to epimerization at the C-3 posi-
tion by an easy rearrangement reaction under neutral
or basic conditions. The mechanism of rearrangement
Scheme 2. The asymmetric Michael addition of 1a to 3a on
and conversion of compound 7 to CPC-1 derivative a gram scale.
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Highly Enantioselective Michael Addition of 2-Oxindole-3-carboxylate Esters to Nitroolefins
Scheme 3. Synthesis of spirooxindole derivatives from compound 4a.
In conclusion, we have developed a highly efficient S&T Major Projects (2012ZX09103-101-072) and Silver
Project (260644).
organocatalyzed Michael addition reaction of 2-oxin-
dole-3-carboxylates to nitroolefins. When a Cinchona
alkaloid-thiourea and a weak Brønsted acid were
used as cooperative organocatalysts, both enantiose-
lectivity and diastereoselectivity were improved. This
catalytic system displayed broad substrate scope and
gave the corresponding adducts in high yields (up to
99%) with high enantioselectivities (up to 98%) and
good diastereoselectivities (up to 95:5). This new
strategy will be applicable to other asymmetric cata-
lytic systems as well as efficient construction of bio-
logically active spirooxindoles.
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We gratefully acknowledge financial support from National
Basic Research Program of China (Grants 2009CB940903
and 2012CB518000), the National Natural Science Founda-
tion of China (Grants 20721003 and 81025017), National
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