Enantioselective Vinylogous Michael/Cyclization Cascade Reaction of Acyclic β,γ-Unsaturated Amides with Isatylidene Malononitriles: Asymmetric Construction of Spirocyclic Oxindoles

Article abstract of DOI:10.1002/adsc.201500469

The first direct and enantioselective vinylogous Michael/cyclization cascade reaction between acyclic β,γ-unsaturated amides and isatylidene malononitriles has been developed. Optically active spirocyclic oxindoles have been obtained in good-to-excellent

Full text of DOI:10.1002/adsc.201500469

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DOI: 10.1002/adsc.201500469
Enantioselective Vinylogous Michael/Cyclization Cascade Reac-
tion of Acyclic b,g-Unsaturated Amides with Isatylidene Malono-
nitriles: Asymmetric Construction of Spirocyclic Oxindoles
Tian-Ze Li,a Jin Xie,a Yu Jiang,a Feng Sha,a,* and Xin-Yan Wua,*
a
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and
Technology, 130 Meilong Road, Shanghai 200237, Peopleꢀs Republic of China
E-mail: shaf@ecust.edu.cn or xinyanwu@ecust.edu.cn
Received: May 12, 2015; Revised: August 10, 2015; Published online: November 12, 2015
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500469.
Abstract: The first direct and enantioselective vinyl-
ogous Michael/cyclization cascade reaction between
acyclic b,g-unsaturated amides and isatylidene ma-
lononitriles has been developed. Optically active
spirocyclic oxindoles have been obtained in good-
to-excellent yields (84–96%) and enantioselectivity
(79–97% ee) in the presence of 2 mol%
(DHQD)2PYR [2,5-diphenyl-4,6-bis(9-O-dihydro-
quinyl)pyrimidine].
ploited to build these sophisticated scaffolds.[9] We
have recently described the direct enantioselective vi-
nylogous aldol/cyclization cascade reaction of allyl
pyrazole amides with isatins, providing a new protocol
for the asymmetric construction of spirocyclic oxin-
dole-dihydropyranones.[10] With our continuing inter-
est, we considered the possibility of using isatylidene-
malononitriles as vinylogous Michael acceptors, prob-
ably facilitating the construction of structurally novel
spirocyclic oxindoles. As part of an ongoing program
towards the asymmetric synthesis of chiral 3,3-disub-
stituted-2-oxindoles,[11] herein, we show the first enan-
tioselective direct vinylogous Michael reaction of allyl
amides with isatylidenemalononitriles. With a subse-
quent cyclization, new spirooxindoles skeletons have
been constructed.
Keywords: cascade reaction; cyclization; enantiose-
lective organocatalysis; spirocyclic oxindoles; vinyl-
ogous Michael reaction
Initially, the reaction of allyl pyrazole amide 1a
The vinylogous Michael reaction, that is the Michael with N-benzylisatylidenemalononitriles 2a was used
addition of a vinylogous enolate-type nucleophile to as a model to screen for a suitable chiral organocata-
a Michael acceptor, is a powerful tool for enantiose- lyst (Figure 1). The reactions were carried out in tolu-
lective carbon-carbon bond formation at the g-posi- ene with 10 mol% catalyst at 258C, and the results
tion of the donor.[1] In the last decade, significant are summarized in Table 1. Takemoto catalyst C1 was
progress has been made in the organocatalytic enan- determined to be an efficient catalyst for the vinylo-
tioselective vinylogous Michael reaction, with 2-sily- gous aldol/cyclization cascade reaction of allyl pyra-
loxyfurans,[2] g-butenolides,[3] g-butyrolactams,[4] 1,1- zole amides with isatins,[10] but produced racemic spi-
dicyanoalkenes[5] and 3-alkylideneoxindoles[6] as fre- rooxindoles (entry 1). Quinine (C2) afforded the de-
quently used Michael donors. However, acyclic nucle- sired product 3aa in 82% yield with 25% ee (entry 2).
ophiles have been hardly employed, probably due to Cinchona alkaloid-derived bifunctional catalysts C3–
the poor reactivity and the difficulty of regioselectivi- C5 were determined to exhibit inferior enantioselec-
ty control (g addition versus a addition). Recently, tivities (entries 3–5). To our delight, commercially
unsaturated aldehydes and ketones have been used as available bis-Cinchona alkaloids showed improved
nucleophiles in the enantioselective vinylogous Mi- enantioselectivities (entries 6–9). Among them,
chael reaction via dienamine catalysis.[7] Nevertheless, (DHQD)2PYR turned out to be the optimum catalyst
the use of inherently less reactive acyclic unsaturated for the reaction, providing the corresponding product
carboxylic acid derivatives as a nucleophile in the 3aa in 85% yield and 60% ee (entry 9).
direct vinylogous Michael has never been reported.[8]
The effect of solvent was then investigated using
Spirooxindoles are privileged structural motifs in 10 mol% of (DHQD)2PYR as chiral catalyst
many biologically active natural and pharmaceutical (Table 2). It turned out the formal cycloaddition reac-
compounds. A diverse range of tactics has been ex- tion proceeded well in all the solvents examined. Rel-
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Tian-Ze Li et al.
Figure 1. Structures of the chiral organocatalysts.
atively less polar solvents such as toluene, CH2Cl2, on the allyl amide had a significant impact on the re-
ether, THF, and ethyl acetate gave better results than activity and caused a slight change in the enantiose-
polar solvents such as CH3CN and DMF (entries 1– lectivity. On the other hand the sterically hindered
7). Aromatic solvents are more suited for the reaction allyl 3,5-dimethylpyrazole amide 1b retarded the reac-
and the use of o-xylene achieved the best enantiose- tion. Allyl 1H-benzotriazole amide 1c smoothly un-
lectivity (entries 8–11). Lower reaction temperatures derwent the cascade reaction and provided the corre-
provided better enantioselectivities (entries 12–14 vs. sponding spirooxindole in 79% ee with 90% yield.
entry 8). The catalyst loading could be reduced to Next, the effect of the substituent groups at the N-
1 mol% without deteriorating the reaction yield and atom of the isatylidenemalononitriles was investigated
enantioselectivity, while the reaction time had to be using 1c as the nucleophile. N-PMB-substituted isaty-
increased (entry 16). In the presence of 2 mol% lidenemalononitrile was tolerated in this cascade reac-
(DHQD)2PYR, the increase of substrate concentra- tion and gave 3cc in 89% yield and 77% ee. The spi-
tion to 0.3M resulted in a decrease of enantioselectiv- rooxindoles were obtained in higher ee values when
ity (entry 17 vs. 15). In contrast, a decrease of the sub- the hindrance of the N-substituent was increased (en-
strate concentration could not facilitate the enantiose- tries 3–6). The enantioselectivity of the product was
lectivity, but caused an obviously decrease in the reac- increased to 90% ee when using N-benzhydrylisatyli-
tion rate (entry 18 vs. entry 15).
denemalononitrile as the substrate (entry 6). Yet, it
To further improve the enantioselectivity of this should be noted that an isatylidene malononitrile with
cascade reaction, different allyl amides and N-substi- the even higher sterically hindered trityl group was
tuted isatylidene malononitriles were investigated unreactive under the typical reaction condition
(Table 3). A nitrogen-containing heterocyclic group (entry 7). According to the optical rotation value re-
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Enantioselective Vinylogous Michael/Cyclization Cascade Reaction
Table 1. Screening of the chiral organocatalysts for the vinyl-
Table 2. Optimization of the reaction conditions[a]
ogous Michael/cyclization cascade reaction[a]
Entry Solvent
Temp.
[oC]
Time
[h]
Yield
[%][b]
ee
Entry Catalyst
Time [h] Yield [%][b] ee [%][c]
[%][c]
1
2
3
4
5
6
7
8
9
C1
C2
C3
C4
1
0.5
2
12
12
1
4
2
1
85
82
79
73
75
76
75
80
85
0
25
9
26
14
38
54
-44
60
1
2
3
4
5
6
7
8
DMF
25
25
25
25
25
25
25
25
0.25
0.1
1
1
1
2
1
1
1
1
1
3
14
24
60
120
48
96
64
70
84
81
84
84
85
89
86
87
91
91
87
90
87
85
90
90
20
22
52
57
50
42
60
62
60
60
58
69
77
78
77
77
73
78
CH3CN
EtOAc
THF
C5
Et2O
(DHQD)2AQN
(DHQD)2PHAL
(DHQ)2PHAL
(DHQD)2PYR
CH2Cl2
toluene
o-xylene
m-xylene 25
p-xylene 25
mesitylene 25
o-xylene
o-xylene
toluene
9
10
11
12
13
14
[a]
The reactions were performed with 0.02 mmol organoca-
talyst, 0.2 mmol N-benzylisatylidenemalononitrile 2a and
0.3 mmol allyl pyrazole amide 1a in 1 mL toluene at
258C.
Isolated yields.
Determined by chiral HPLC analysis.
0
À20
À35
À20
À20
À20
À20
[b]
[c]
15[d] o-xylene
16[e] o-xylene
17[d,f] o-xylene
18[d,g] o-xylene
[a]
ported in the literature, the absolute configuration of
product 3cc was assigned as the R configuration,[12]
and the absolute configurations of other products
were deduced by analogy.
Having established the optimal reaction parame-
ters, we then examined the scope of the vinylogous
Michael/cyclization cascade reaction (Table 4). Struc-
turally diverse isatylidenemalononitrile derivatives 2
were shown to be well tolerated, and high yields were
obtained in all examined cases. Regarding the enan-
tioselectivity, the introduction of electron-attracting
substituents at the 5-position caused a negative effect,
Unless stated otherwise, the reactions were performed
with 10 mol% (DHQD)2PYR, 0.2 mmol N-benzylisatyli-
denemalononitrile 2a and 0.3 mmol allyl pyrazole amide
1a in 1 mL solvent (0.2M).
Isolated yields.
Determined by chiral HPLC analysis.
2 mol% (DHQD)2PYR were used.
1 mol% (DHQD)2PYR was used.
The amount of solvent was 0.66 mL (0.3M).
The amount of solvent was 2 mL (0.1M).
[b]
[c]
[d]
[e]
[f]
[g]
whereas introduction of electron-donating groups fa- stituent at the phenyl group gave a better enantiose-
vored the reaction. When 5-Cl and 5-Br substituted 2- lectivity than those with an electron-deficient sub-
oxindoles were used as substrate, the reaction was stituent at phenyl group (entries 16 and 17 vs. en-
carried out in a mixed solvent of o-xylene/THF (4:1) tries 13–15). Allyl amides with an aliphatic substituent
due to their poor solubility in o-xylene. The enantio- (R2 =n-Bu) also smoothly produced 3id in 84% yield
selectivities increased obviously when substituents and 85% ee (entry 18). A large scale synthesis using
were introduced to the 6- or 7-position of 2-oxindole 3 mmol N-benzhydrylisatylidenemalononitrile 2d also
(entries 7–12).
provided an excellent yield (94%, 1.4222 g) and enan-
The substrate scope with respect to allyl amides tioselectivity (90% ee, entry 19).
was also examined. To our delight, both electron-
According to the above-mentioned experimental
withdrawing and electron-donating substituents on results and the related literature,[13] a possible transi-
the phenyl moiety were well tolerated, affording the tion state was proposed as shown in Figure 2. The
corresponding spirooxindoles 3 in excellent yields allyl amide is deprotonated by the quinuclidine base
(90–95%). The substrates with an electron-rich sub- of the catalyst to form an enalate, which can be en-
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Table 3. Substrate scope of the enantioselective vinylogous
Table 4. Enantioselective intramolecular vinylogous Mi-
Michael/cyclization cascade reaction[a]
chael/cyclization cascade reaction of various substrates[a]
Entry
1
R
Time [h]
Yield [%][b]
ee [%][c]
Entry R1
R2
Time [d] Yield [%][b] ee [%][[c]
1
2
3
4
5
6
7
1a
1b
1c
1c
1c
1c
1c
Bn
Bn
Bn
Me
PMB
CHPh2
CPh3
60
120
48
72
84
48
48
87 (3aa)
trace
77
n.d.[d]
79
1
H
Ph
Ph
2
88 (3cd)
87 (3ce)
89 (3cf)
94 (3cg)
91 (3ch)
90 (3ci)
90
93
92
87
80
79
92
92
93
93
96
97
84
88
89
92
92
85
90
2
5-Me
4.5
3
2.5
2
90 (3aa)
87 (3cb)
89 (3cc)
88 (3cd)
trace
3
5-MeO Ph
29
77
4
5-F
5-Cl
5-Br
6-Cl
6-Br
7-Me
7-F
7-Cl
7-Br
H
H
H
H
H
H
H
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
5[d]
6[d]
7
90
n.d.[d]
2
3
3
3
2
2
2
2
2
2
2
2
7
96 (3cj)
91 (3ck)
95 (3cl)
93 (3cm)
90 (3cn)
89 (3co)
90 (3dd)
91 (3ed)
94 (3fd)
95 (3gd)
94 (3hd)
84 (3id)
94 (3 cd)
[a]
The reactions were performed with 1 (0.3 mmol), 2
(0.2 mmol) and (DHQD)2PYR (2 mol%) in o-xylene
(1.0 mL) at À208C.
8
9
10
11
12
13
14
15
16
17
18
19[e]
[b]
[c]
[d]
Isolated yield.
Determined by chiral HPLC.
Not determined.
Ph
4-CF3C6H4
4-ClC6H4
4-FC6H4
4-MeC6H4
3-MeC6H4
n-Bu
Ph
5
[a]
Unless stated otherwise, the reactions were performed
with 1 (0.3 mmol), 2 (0.2 mmol) and (DHQD)2PYR
(2 mol%) in o-xylene (1.0 mL) at À208C.
Isolated yield.
[b]
[c]
[d]
[e]
Determined by chiral HPLC.
Using 1 mL o-xylene/THF (4:1) as solvent.
The reaction was performed on a 3 mmol scale of sub-
strate 2d.
Figure 2. Proposed transition state.
Experimental Section
hancing via the H-bonding interaction and attacks the Typical Procedure
isatylidenemalononitrile from the Re-face. The R-con-
To a solution of (DHQD)2PYR (3.5 mg, 0.004 mmol) and
figurated carbanion (adjacent to the cyano group)
generated from the vinylogous Michael addition then
attacks the carbonyl group, providing the R-product
followed by the elimination of benzotriazole.
In conclusion, we have developed the first enantio-
selective vinylogous Michael/cyclization cascade reac-
tion between allyl amides and isatylidenemalononi-
triles. Good-to-excellent yields (84–96%) and enantio-
selectivities (79–97% ee) were achieved in the pres-
ence of only 2 mol% of (DHQD)2PYR. The method
established here represents a new protocol for the
asymmetric construction of spirocyclic oxindoles.
allyl amide 1c (0.3 mmol, 78.9 mg) in 1 mL of o-xylene was
added isatylidene malononitrile 2d (0.2 mmol, 72.2 mg) at
À208C, and the resulting mixture was stirred at this temper-
ature until the reaction was completed (monitored by TLC).
The solvent was removed under reduced pressure and the
residue was purified by column chromatography (20:1:2
hexane/CH2Cl2/EtOAc) to give the desired product 3cd as
a white solid; yield: 89.1 mg (88%); mp 112.3–112.98C;
[a]2D0: À18.1 (c 0.36, CHCl3). 1H NMR (400 MHz, CDCl3):
d=7.54–7.42 (m, 6H), 7.39–7.29 (m, 10H), 7.16 (t, J=
8.0 Hz, 1H), 7.08 (t, J=7.2 Hz, 1H), 6.99 (s, 1H), 6.84 (s,
1H), 6.56 (d, J=8.0 Hz, 1H), 3.43 (bs, 2H); 13C NMR
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COMMUNICATIONS
Enantioselective Vinylogous Michael/Cyclization Cascade Reaction
(100 MHz, CDCl3): d=177.9, 172.4, 142.3, 137.0, 136.0,
135.8, 131.9, 130.8, 129.2, 128.9, 128.7, 128.5, 128.3, 128.2,
126.4, 125.0, 123.8, 123.8, 120.4, 113.3, 110.0, 109.4, 59.3,
52.0, 48.3, 34.5; IR (KBr): n=3062, 1718, 1606, 1467, 1350,
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1219, 698 cmÀ1
;
HR-MS: m/z=504.1734, calcd. for
C34H22N3O2[MÀH]À: 504.1712; HPLC analysis (Daicel Chir-
alcel OD-H column, n-hexane/EtOH=80:20, 0.9 mLminÀ1,
l=254 nm): tR =7.49 min (major), 8.41 min (minor).
Acknowledgements
We are grateful for the financial support from the National
Natural Science Foundation of China (21102043), the Science
and Technology Commission of Shanghai Municipality
(15ZR1409200), and the Fundamental Research Funds for
the Central Universities.
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