Catalytic asymmetric synthesis of spirooxindoles via addition of isothiocyanato oxindoles to aldehydes under dinuclear nickel schiff base catalysis

Article abstract of DOI:10.1002/asia.201300251

One-step approach: A homodinuclear nickel Schiff base catalyst was utilized for the title reaction, thus providing easy access to spirocyclic oxindole compounds in one step. High enantioselectivity as well as high catalyst turnover number (TON, up to 850) were achieved at ambient temperature. Copyright

Full text of DOI:10.1002/asia.201300251

COMMUNICATION
DOI: 10.1002/asia.201300251
Catalytic Asymmetric Synthesis of Spirooxindoles via Addition of
Isothiocyanato Oxindoles to Aldehydes Under Dinuclear Nickel Schiff Base
Catalysis
Shota Kato,[a] Motomu Kanai,*[a] and Shigeki Matsunaga*[a, b]
Spirocyclic oxindole units are privileged structural motifs
found in natural and unnatural compounds with diverse and
important biological activities.[1] Inspired by their important
scaffolds, various synthetic methods for producing chiral spi-
rooxindoles, especially in syntheses of natural products, have
been investigated over the last decade.[2] As a step-economi-
cal method, catalytic asymmetric one-step construction of
spirooxindoles has been intensively studied using either
a metal catalyst or an organocatalyst.[3–5] Most studies re-
ported to date, however, have focused on the catalytic asym-
metric synthesis of spirooxindoles bearing an all-carbon qua-
ternary stereocenter at the C-3’ position of oxindole, such as
a spirocycloalkaneoxindole core[4] and a spiro[pyrrolidin-
3,3’-oxindole] core.[5] By contrast, studies on catalytic asym-
metric synthesis of spirooxindoles bearing a nitrogen atom
at the C3’-position of the oxindole unit have been, until re-
cently, rather limited.[6–10] Considering their importance for
Figure 1. Structures of biologically active spirooxindoles bearing a nitro-
gen atom at the C3’ position of the oxindole unit and isothiocyanato ox-
indole 1.
the design of medicinally important compounds as exempli-
fied in Figure 1,[11] further expansion of the scope of avail-
able chiral spirooxindole cores bearing a nitrogen atom at
the C3’-position is highly desirable.
For rapid access to the spirooxindole core bearing a nitro-
gen atom at the C3’-position, the use of isothiocyanato oxin-
doles 1 (Figure 1) as donors in the reaction of electrophiles
is an attractive strategy.[7–9,12] In 2011, Yuan and coworkers
were the first to demonstrate the utility of 1 as donors in the
catalytic asymmetric addition to ketones, thus affording spi-
rooxindoles with vicinal quaternary carbon stereocenters.[7]
Later, in collaboration with Shibasaki, we utilized 1 in the
reaction of aldimines with the SrACHTNURGTNEUNG(O-iPr)2/Schiff base 2a
(Figure 2) complex.[8] Other groups have performed the re-
action with other electrophiles in organocatalytic Michael
Figure 2. Structures of Schiff bases 2 and homobimetallic Schiff base 2b
complexes.
reaction/cyclization sequences.[9] To the best of our knowl-
edge, however, catalytic asymmetric addition of the isothio-
cyanato oxindoles to aldehydes has never been achieved.
Herein, we report the utility of homodinuclear Schiff base
catalysis for promoting the asymmetric addition to alde-
hydes. A dinuclear Ni2-Schiff base 2b (Figure 2) complex
promoted the aldol-type reaction of isothiocyanato oxin-
doles 1 with aldehydes 3 at an ambient temperature, thereby
giving spirooxindole products in up to 91:9 d.r. and 99% ee.
Optimization studies using isothiocyanato oxindole 1a
and aldehyde 3a as model substrates are summarized in
[a] S. Kato, Prof. Dr. M. Kanai, Dr. S. Matsunaga
Graduate School of Pharmaceutical Sciences
The University of Tokyo
Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
Fax : (+81)3-5684-5206
E-mail: kanai@mol.f.u-tokyo.ac.jp
smatsuna@mol.f.u-tokyo.ac.jp
[b] Dr. S. Matsunaga
ACT-C
Japan Science and Technology Agency
Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201300251.
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Shigeki Matsunaga et al.
Table 1. Optimization studies.
tivity was also achieved with oxindole donor 1d,
bearing
a removable N-allyl protecting group
(entry 11, 89% ee). In contrast to aliphatic alde-
hydes, the present system afforded poor results for
aromatic aldehydes. For instance, the reaction of
benzaldehyde with 1a under the optimized condi-
tions for aliphatic aldehydes resulted in both poor
diastereo- and enantioselectivity (55:45 d.r., 33%
ee). Further studies to expand the scope of alde-
hydes to aromatic aldehydes by using some other
chiral catalysts are ongoing. Trials to reduce catalyst
loading are summarized in entries 12 and 13. The
reaction was promoted by 1 mol% of the Ni2-2b
catalyst without loss of selectivity, and product 4ae
was obtained in 90% yield (TON=90), 89:11 d.r.,
and 99% ee (entry 12). Good yield and enantiose-
lectivity were obtained with as little as 0.1 mol%
catalyst loading (TON=850, 98% ee), although the
diastereoselectivity decreased to some extent (71:29
d.r., entry 13).
Entry Catalyst
Solvent
Additive[a] Yield [%][b] d.r.[b]
ee [%][c]
1[d]
2
3
4
5
6
7
8
9
Sr
G
MS 5 ꢁ
MS 5 ꢁ
MS 5 ꢁ
MS 5 ꢁ
MS 5 ꢁ
MS 5 ꢁ
MS 5 ꢁ
MS 5 ꢁ
>95
94
>95
80
>95
>95
>95
>95
>95
>95
>95(91)[e]
>95:5
87:13
88:12
59:41
67:33
67:33
64:36
74:26
88:12
89:11
89:11
3
Co2-2b
Ni2-2b
Cu2-2b
Ni2-2b
Ni2-2b
Ni2-2b
Ni2-2b
Ni2-2b
Ni2-2b
Ni2-2b
THF
THF
THF
21
84
2
80
70
77
80
91
90
91
toluene
CHCl3
tBuOMe
DME
1,4-dioxane MS 5 ꢁ
1,4-dioxane MS 4 ꢁ
1,4-dioxane MS 3 ꢁ
10
11
[a] 200 mg molecular sieves per 1 mmol of 3a was used. [b] Determined by 1H NMR
spectroscopic analysis of the crude reaction mixture. [c] Determined by chiral station-
ary-phase HPLC analysis. [d] Reaction was run at À408C. [e] Number in parenthesis
is the combined isolated yield of product 4aa and its diastereomer after purification
by silica gel column chromatography.
In summary, we achieved catalytic asymmetric
addition of isothiocyanato oxindoles to aliphatic al-
dehydes. The use of 10 mol% of a dinuclear Ni2-
Table 1. Although the SrACHTUNTRGNEUNG(O-iPr)2/Schiff base 2a (and its bi-
phenyldiamine analogue) 1:1 complex was suitable for the
reaction of 1a with aldimine,[8] the Sr-2a catalyst resulted in
poor enantioselectivity for aldehyde 3a, even at low temper-
ature (entry 1, 3% ee). Because we have previously reported
the utility of a couple of other dinuclear transition metal/
Schiff base 2b complexes for the enantioselective reaction
with related oxindoles and isoindolinones as donors,[13] we
screened dinuclear Schiff base 2b complexes for the present
reaction (entries 2–4). Among the catalysts screened,[14] the
Ni2-2b complex gave promising results, and product 4aa was
obtained in 88:12 d.r. and 84% ee (entry 3). After further
optimization of the solvent (entries 5–9) and molecular
sieves (entries 10–11), the best stereoselectivity was ach-
ieved in 1,4-dioxane at ambient temperature in the presence
of molecular sieves 3 ꢁ, and 4aa was obtained in 91%
yield, 89:11 d.r., and 91% ee (entry 11). The relative and ab-
solute stereochemistry of 4aa were unequivocally deter-
mined by single-crystal X-ray analysis after 4-bromobenzoy-
lation (Figure 3).[15]
The substrate scope of the reaction under the optimized
conditions is summarized in Table 2. a-Branched aliphatic
aldehydes 3a–3d gave spirooxindole products in 83:17–
89:11 d.r. and 80–92% ee (entries 1–4). Linear aliphatic al-
dehydes 3e–3g showed slightly higher enantioselectivity
than the a-branched aldehydes, and products were obtained
in 90:10–91:9 d.r. and 88–99% ee (entries 5–7). Aldehyde
3h, bearing a silyl ether moiety, also gave product 4ah with
high enantioselectivity and yield, albeit with only moderate
diastereoselectivity (81:19 d.r., entry 8). In addition to 1a,
oxindole donors with either a Me- (1b) or Cl-substituent
(1c) were applicable, and products were obtained in 98% ee
and 92% ee, respectively (entries 9–10). Good stereoselec-
Figure 3. ORTEP plots of 4-bromobenzoylated 4aa set at the 50% prob-
ability level.
Schiff base 2b complex smoothly promoted the reaction at
ambient temperature, giving spirooxindoles in 81:19–91:9
d.r. and 80–99% ee. The catalyst loading was successfully re-
duced to 1 and 0.1 mol%, and high TON, up to 850, was
achieved.
Acknowledgements
We thank Prof. Dr. Masakatsu Shibasaki at the Institute of Microbial
Chemistry for his fruitful advice at the initial stage of this project. This
work was supported in part by ACT-C program from JST, Grant-in-Aid
for Young Scientist (A) from JSPS, and the Naito Foundation.
Keywords: asymmetric catalysis · asymmetric synthesis ·
oxindole · salen · spiro compounds
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Shigeki Matsunaga et al.
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Entry
1
1
Aldehyde: R
3
Cat.
AHCTUNGTRENGN[UN x mol%]
4
Yield [%][b]
91
d.r.[c]
ee [%][d]
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3a
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4aa
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9
1b
1c
1d
1a
1a
3e
3e
3e
3e
3e
10
10
10
1
4be
4ce
4de
4ae
4ae
90
84
96
90
85
89:11
82:18
88:12
89:11
71:29
98
92
89
99
98
10
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Received: February 27, 2013
Published online: April 15, 2013
Chem. Asian J. 2013, 8, 1768 – 1771
1771
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