Calcium-catalyzed asymmetric synthesis of 3-tetrasubstituted oxindoles: Efficient construction of adjacent quaternary and tertiary chiral centers

Article abstract of DOI:10.1021/acs.orglett.5b00749

Chiral Ca-catalyzed asymmetric addition reactions of 3-substituted oxindoles with N-Boc-imines afford 3-tetrasubstituted oxindole derivatives bearing adjacent quaternary and tertiary chiral centers, which are key structures for biological activities. Ubiq

Full text of DOI:10.1021/acs.orglett.5b00749

Letter
pubs.acs.org/OrgLett
Calcium-Catalyzed Asymmetric Synthesis of 3‑Tetrasubstituted
Oxindoles: Efficient Construction of Adjacent Quaternary and
Tertiary Chiral Centers
Shota Shimizu, Tetsu Tsubogo, Pengyu Xu, and Shu Kobayashi*
̅
Department of Chemistry, School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
S
* Supporting Information
ABSTRACT: Chiral Ca-catalyzed asymmetric addition reac-
tions of 3-substituted oxindoles with N-Boc-imines afford 3-
tetrasubstituted oxindole derivatives bearing adjacent quater-
nary and tertiary chiral centers, which are key structures for
biological activities. Ubiquitous and nontoxic Ca catalysts (1−
10 mol %) work well in this reaction, and high yields (up to
99%) and selectivities (up to >99% ee) of the products with
wide substrate scope have been attained. The structures of the
chiral Ca catalysts and intermediary Ca enolates are also
discussed.
xindole derivatives are an interesting class of compounds
and no enantioselective reactions with aliphatic imines were
1
Ofor several biological activities, such as anticancer, anti- reported. In addition, organocatalysts (nonmetal catalysts) are
HIV,2 antimalarial,3 antitubercular,4 progesterone receptor
used in all of the previous examples, and relatively high loading
agonist,5 and MDM2 inhibitor,6 etc.7 For asymmetric synthesis
amounts of catalysts (10 mol %) are used except for
phosphonium salts (3 mol %). Here, we report the reactions of
3-substituted oxindoles with N-Boc-imines using chiral calcium
catalysts. Not only aromatic imines but also aliphatic imines work
well, and high yields and high diastereo- and enantioselectivities
have been attained using 1−10 mol % loadings of the calcium
catalysts with wide substrate scope.
of these compounds, construction of the quaternary chiral
centers often observed in their structures is one of the most
challenging tasks.8 While the reactions of 3-substituted oxindoles
with imines potentially provide an efficient method for making
adjacent quaternary and tertiary chiral centers,9 control of such
stereogenic centers is known to be very difficult presumably
because of steric congestion. Up to now, only four reports appear
for this reaction, and yields, selectivities, catalyst efficiency, and/
or substrate generality are not satisfactory.10 In 2008, Chen and
co-workers reported the first example of the reaction of 3-
substituted oxindoles with N-Boc-imines using bifunctional
thiourea−tertiary amine organocatalysts, and high enantioselec-
tivities (up to 95% ee) were obtained for several 3-substituted
oxindoles with aromatic and heteroaromatic N-Boc-imines.10a
However, a racemic compound was obtained in a reaction with
an aliphatic N-Boc-imine. In 2009, Maruoka and co-workers
reported phosphonium salts as chiral phase-transfer catalysts and
demonstrated the reactions of 3-aryloxindoles with aromatic N-
Boc-imines and good enantioselectivities (up to 88% ee) were
obtained.10b Liu, Chen, and co-workers reported modified
cinchona alkaloid-catalyzed reactions of N-unprotected 3-
substituted oxindoles with aromatic N-Ts-imines, and good
enantioselectivities (up to 89% ee) were obtained.10c Finally, in
2013, Zhang and Peng reported the reactions of N-unprotected
3-bromooxindoles with aromatic N-Ts-imines catalyzed by
bifunctional thiourea derivatives, and high enantioselectivities
(up to 99% ee) were obtained.10d In those four reports, while
high yields and high selectivities were obtained in some cases,
substrates were limited to aromatic and heteroaromatic imines,
Recently, we have been interested in calcium compounds as
catalysts in organic transformations. Calcium is a ubiquitous
element, nontoxic, and readily available.11 Previously, we have
investigated chiral calcium alkoxide-catalyzed asymmetric
reactions,12 such as asymmetric Michael reactions,12a−e,h,i
asymmetric [3 + 2] cycloaddition reactions,12a,b,i and asymmetric
protonation reactions.12f More recently, we have developed
chiral calcium chloride12h,i and chiral calcium iodide (CaI2)13 for
catalytic asymmetric transformations. We chose 3-methyl-
substituted oxindole 1a and N-Boc-imine 2a as models, and
several reaction conditions were examined. It was found that a
combination of CaI2 with Pybox L gave promising results. When
CaI2 and L1 were used, the reaction proceeded smoothly to
afford the desired product 3aa in 85% yield with high
diastereoselectivity and moderate enantioselectivity (Table 1,
entry 1). Ligand screening showed that L5 gave the best
enantioselectivity (entries 2−5). With the optimized ligand L5,
several solvents were screened. Slightly higher enantioselectiv-
ities were observed when diethyl ether (Et2O) and dichloro-
methane (DCM) were employed (entries 6−8). By lowering the
Received: March 17, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b00749
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Organic Letters
Letter
a
Table 1. Optimization of Reaction Conditions
Table 2. Substrate Scope
b
d
yield
ee
c
entry
R1
R2
R3
Ph (2a)
(%)
3:4
(%)
1
Me (1a)
Me (1a)
Me (1a)
Me (1a)
Me (1a)
H
H
H
H
H
87 (3aa)
95:5
97:3
95:5
96:4
96:4
98
97
97
95
97
2
3
4
5
o-MeC6H4 (2b) 93 (3ab)
m-MeC6H4 (2c) 80 (3ac)
p-MeC6H4 (2d) 95 (3ad)
e
f
m-CF3C6H4
(2e)
93 (3ae)
6
Me (1a)
H
p-OMe C6H4
(2f)
91 (3af)
94:6
95
7
8
9
Me (1a)
Me (1a)
Me (1a)
Me (1a)
Me (1a)
Me (1a)
Me (1a)
Bn (1b)
Bn (1b)
n-Pr (1c)
n-Pr (1c)
n-Pr (1c)
i-Pr (1d)
i-Pr (1d)
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
p-ClC6H4 (2g)
2-furyl (2h)
92 (3ag)
89 (3ah)
91 (3ai)
97:3
97:3
98:2
97:3
97:3
93:7
96
96
98
97
90
72
84
95
98
98
72
74
>99
73
93
95
97
91
95
93
2-thienyl (2i)
10
f g
2-naphthyl (2j) 84 (3aj)
11 ,
f g
Et (2k)
96 (3ak)
95 (3al)
12 ,
n-C5H11 (2l)
i-Bu (2m)
Ph (2a)
,h,i
e
e
e
,
h,
i
13
14
15
16
17
18
19
20
21
22
23
24
25
26
99 (3am) 94:6
81 (3ba)
84 (3bi)
83 (3ca)
43 (3ck)
94:6
96:4
94:6
97:3
2-thienyl (2i)
Ph (2a)
a
b
Isolated yield. Determined by 1H NMR analysis of the crude
Et (2k)
c
d
reaction mixture. Determined by chiral HPLC analysis. Et3N was
added at −78 °C. Et3N was added at rt. Slow addition of imine over
1 h; the reaction was further continued for 17 h. 1a (1.0 equiv), 2a
(1.5 equiv). Slow addition of imine over 10 h; the reaction was
continued for 8 h. Slow addition of imine over 20 h; the reaction was
continued for 22 h. 0.4 M. CaCl2 was used instead of CaI2. Ca(O-i-
e
,
,
h
,
,
i
i
i-Bu (2m)
Ph (2a)
93 (3cm) 97:3
80 (3da) 95:5
77 (3dm) 95:5
e
f
g
e
h
h
i-Bu (2m)
Ph (2a)
j
i
1e
80 (3ea)
95:5
94:6
96:4
96:4
94:6
94:6
f
j
j
k
l
1e
p-MeC6H4 (2d) 82 (3ed)
f
j
1e
p-ClC6H4 (2g)
2-furyl (2h)
88 (3eg)
93 (3eh)
93 (3fa)
82 (3ga)
Pr)2 was used instead of CaI2 without Et3N.
e
j
1e
k
1f
Me Ph (2a)
F Ph (2a)
e
l
1g
a
b
2 was added over 1 h unless otherwise noted. Isolated yield.
c
Determined by 1H NMR analysis of the crude reaction mixture.
d
e
Determined by chiral HPLC analysis. Slow addition of imine over 10
f
h; the reaction was further continued for 8 h. Slow addition of imine
over 5 h; the reaction was further continued for 13 h. Imine (3 equiv)
g
h
i
j
was used. 10 mol % of the catalyst was used. Pybox L3 was used. 1e:
k
R1 = CH2CH(O(CH2)2O), R2 = H. 1f: R1 = CH2CH(O(CH2)2O),
reaction temperature, the enantioselectivity was further
improved to 93% ee (entry 9). To decrease the catalyst loading,
several conditions were investigated (entries 10−15). When the
ratio of CaI2 to L5 was 1:1.5, the desired product 3aa was
obtained in 87% yield with 95:5 dr and 98% ee (entry 12).
Further decreasing the catalyst loading showed promising results
(entries 13−15). Finally, the reaction proceeded smoothly to
afford the desired adduct in 97% yield with high diastereose-
lectivity (3aa/4aa = 94/6), and the enantioselectivity of 3aa was
97% using 1 mol % of the catalyst (entry 15). On the other hand,
while the enantioselectivity decreased slightly using CaCl2
instead of CaI2 (entry 16), a significant drop in the
enantioselectivity was observed when calcium isopropoxide
(Ca(O-i-Pr)2) was used as a calcium source (entry 17).
l
R2 = Me. 1g: R1 = CH2CH(O(CH2)2O), R2 = F.
with imines 2e, 2f, and 2g, bearing electron-withdrawing and
electron-donating substituents at the benzene rings, the reactions
also proceeded smoothly to afford the desired products 3ae, 3af,
and 3ag in excellent yields with high diastereo- and
enantioselectivities (entries 5−7). When heteroaromatic imines
2h and 2i were examined, excellent enantioselectivities were
obtained (entries 8 and 9). The 2-naphthylaldehyde-derived
imine 2j also gave excellent enantioselectivity (entry 10). For
aliphatic imines, imine 2k derived from propionaldehyde reacted
with oxindole 1a to afford the desired product 3ak in excellent
yield with excellent diastereo- and enantioselectivities (entry 11).
n-C5H11 imine 2l and i-Bu imine 2m also reacted smoothly to
provide the desired products 3al and 3am, respectively in
excellent yields with good to high diastereo- and enantiose-
Next, we investigated the substrate scope of this asymmetric
reaction (Table 2). When we applied tolyl imines 2b−d, high
yields, high diastereoselectivities, and excellent enantioselectiv-
ities were observed in all cases (entries 2−4). In the reactions
B
DOI: 10.1021/acs.orglett.5b00749
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Letter
lectivities using Pybox L3 as the ligand (entries 12 and 13). It
should be noted that aliphatic imines reacted smoothly to give
the desired adducts in excellent yields with good to excellent
diastereo- and enantioselectivies. In previous reports, as
mentioned, there were no examples of enantioselective reactions
using aliphatic imines.10 The substrate scope of oxindoles 1 was
also surveyed. 3-Benzyl-substituted oxindole 1b reacted with
imines 2a and 2i smoothly to afford the desired adducts in high
yields with excellent diastereo- and enantioselectivities (entries
14 and 15). The absolute configuration was determined by
comparing the optical rotation of 3bi with that of the
literature.10a 3-n-Propyl- and 3-isopropyl-substituted oxindoles
1c and 1d also reacted with imine 2a smoothly, and almost
perfect enantioselectivities were obtained (entries 16 and 19).
Aliphatic imines 2k and 2m also worked well (entries 17, 18, and
20). We also examined the reactions of substituted oxindoles 1e,
1f, and 1g (entries 21−26).14 In all cases, the reactions proceeded
smoothly to afford the desired adducts in high yields with
excellent diastereo- and enantioselectivities. The adduct 3ea was
readily converted to spirooxindole derivative 5ea in good yield
without loss of any optical purity (Scheme 1).15−17 Thus, we
ligand (vide infra), and 7 attacks imine 2’s Re face smoothly to
give intermediate 8. There may be two possible ways to complete
the catalytic cycle. In the first, the intermediate 8 is protonated by
Et3NH+I− to give product 3 along with regeneration of the chiral
Ca catalyst 6 (path A). In the second, the intermediate 8
deprotonates 1 to form the Ca enolate 7 (path B). Given the
steric bulkiness of 1 and 8, path A may be more likely; however,
path B cannot be ruled out at this moment. It is noteworthy that
the Ca catalysts, which have both mild Lewis acidity and
Brønsted basicity,11 may play a key role in obtaining high yields
and stereoselectivities and also efficient catalyst turnover.
We further investigated stereochemical structures of the chiral
Ca catalyst and the Ca enolate. While X-ray crystallographical
analysis of Ca(ClO4)2−Pybox was conducted,20 we carried out
DFT calculation of CaCl2−L5 (Scheme 3).21 The optimized
Scheme 3. Stereochemical Model of CaCl2−L5
Scheme 1. Transformation to Spirooxindole
structure has C2-symmetry, and the three nitrogen atoms of L5
coordinate to Ca. On the other hand, the length between Ca and
O of the methoxy group of L5 is 3.8 Å, suggesting weak
coordination of O to Ca. We also investigated the structure of the
Ca enolate (Scheme 4). The DFT calculation showed that the
have succeeded in providing oxindole derivatives that have
adjacent quaternary and tertiary chiral centers in satisfactory
yields and stereoselectivities with wide substrate scope.
An assumed catalytic cycle of this asymmetric reaction is
shown in Scheme 2. CaI2 and ligand L5 may form a 1:1 complex
(6) and a 1:2 complex in solution; that is supported by NMR
experiments.13,18 Chiral Ca catalyst 6 may work as a monomeric
form because a linear correlation between ligand ee’s and product
ee’s was observed.18,19 Ca catalyst 6 activates oxindole 1 to form
Ca enolate 7, which was detected by ESI HRMS analysis.18 One
enantioface (the Re face) of 7 is effectively shielded by the chiral
Scheme 4. Stereochemical Model of Ca Enolate
a
Scheme 2. Assumed Catalytic Cycle
three nitrogen atoms and the carbonyl oxygen of the Boc group
coordinated to Ca. The length between Ca and O of the methoxy
groups of L5 is 3.9 and 4.0 Å, also suggesting weak interactions
between O and Ca. The stereochemical model of the Ca enolate
indicates that the plane of chiral ligand L5 and the plane of the
enolate are almost perpendicular and that the Re face of the
enolate is efficiently shielded by the CH2OMe arm of L5.
In conclusion, we have developed a new catalytic method for
the synthesis of optically active 3-tetrasubstituted oxindoles using
chiral Ca catalysts. Adjacent quaternary and tertiary chiral centers
have been created efficiently by the Ca-catalyzed reactions of 3-
substituted oxindoles with imines. High diastereo- and
enantioselectivities have been realized with wide substrate
scope. This is the first example of a chiral metal-catalyzed
reaction of a 3-substituted oxindole with an imine. The
a
Iodide ions are omitted for clarification.
C
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Letter
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ubiquitous and nontoxic Ca catalyst worked well, and even 1 mol
% of the catalyst gave a high yield and high selectivities of the
product. This reaction may provide an efficient approach to the
synthesis of biologically important oxindoles, including spiroox-
indole derivatives. Further studies on this topic are underway in
our laboratories.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental section and copies of NMR spectra. This material is
available free of charge via the Internet at http://pubs.acs.org.
(10) (a) Tian, X.; Jiang, K.; Peng, J.; Du, W.; Chen, Y. C. Org. Lett.
2008, 10, 3583. (b) He, R.; Ding, C.; Maruoka, K. Angew. Chem., Int. Ed.
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Org. Chem. 2009, 74, 4650. (d) Li, J.; Zhang, D. G.; Peng, Y. Chem.
Commun. 2013, 49, 1330.
(11) (a) Harder, S. Chem. Rev. 2010, 110, 3852. (b) Yamashita, Y.;
Kobayashi, S. Acc. Chem. Res. 2011, 44, 58. (c) Yamashita, Y.; Tsubogo,
T.; Kobayashi, S. Chem. Sci. 2012, 3, 967. (d) Tsubogo, T.; Yamashita,
Y.; Kobayashi, S. Top. Organomet. Chem. 2013, 45, 243.
(12) (a) Saito, S.; Tsubogo, T.; Kobayashi, S. J. Am. Chem. Soc. 2007,
129, 5364. (b) Tsubogo, T.; Saito, S.; Seki, K.; Yamashita, Y.; Kobayashi,
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Our current contribution of chiral calcium salt-catalyzed asymmetric
reactions: (h) Tsubogo, T.; Yamashita, Y.; Kobayashi, S. Chem.Eur. J.
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AUTHOR INFORMATION
Corresponding Author
■
*E-mail: shu_kobayashi@chem.s.u-tokyo.ac.jp.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was partially supported by a Grant-in-Aid for Science
Research from the Japan Society for the Promotion of Science
(JSPS), Global COE Program, The University of Tokyo, MEXT,
Japan, Japan Science Technology Agency (JST). We thank Mr.
K. Takasugi (The University of Tokyo and JEOL Ltd.) for
helpful discussions about NMR experiments.
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