Syntheses of spirocyclic oxindole-butenolides by using three-component cycloadditions of isocyanides, allenoates, and isatins

Article abstract of DOI:10.1002/chem.201100977

Spiro workflow! Efficient syntheses of spirocyclic oxindole-butenolides from readily available isocyanides, allenoates, and isatins are disclosed (see scheme; R¹=alkyl, aryl; R²=halide, nitro, methoxy; PG=protecting group). This protocol also allows the insertion of carbon monoxide into organic molecules without the aid of a transition-metal catalyst after the hydrolysis process. Copyright

Full text of DOI:10.1002/chem.201100977

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DOI: 10.1002/chem.201100977
Syntheses of Spirocyclic Oxindole-Butenolides by Using Three-Component
Cycloadditions of Isocyanides, Allenoates, and Isatins
Jian Li,*[a] Yuejin Liu,[a] Chunju Li,[a] and Xueshun Jia*[a, b]
Dedicated to Professor Yongmin Zhang on the occasion of his 80th birthday
Spirocyclic oxindoles have been coined priviledged targets
due to their common occurence in natural products and clin-
ical pharmaceuticals.[1,2] The key structural characteristic of
these compounds is the spiro-ring fusion at the 3-position of
the oxindole, which gives increasing molecular complexity
and significant biological activities, including insecticidal, an-
titumor, anthelmintic, and antibacterial properties.[3] For ex-
ample, the most notable oxindole, paraherquamide A,[4] iso-
lated from cultures of Pennicillium paraherquei, has dis-
played potent antiparasitic activity and antinematodal prop-
erties.[5]
isocyanides, and isatins to synthesize spirocyclic oxindole-
butenolide derivatives.
The phosphine-catalyzed [3+2] cycloaddition of alle-
noates with electron-deficient species such as olefins and
imines has been fully investigated, thereby providing new
pathways to functionalized five-membered carbo- and het-
erocycles.[8,9] Following the pioneering efforts of Lu and
Zhang,[10] further applications of this annulation strategy to
the total syntheses of natural products and the development
of their asymmetric variants have also been well-estab-
lished.[11,12] Generally, in situ formation of a zwitterionic in-
termediate, stemmed from the nucleophilic addition be-
tween allenoate and phosphine, was believed to be crucial
to the success of the above cycloaddition approach. Accord-
ingly, we envisaged that a novel multicomponent reaction[13]
based on the above [3+2] annulation could be accessed if
isocyanide[14] was employed as a nucleophile instead of
phosphine. According to our assumption, a mixture of isocy-
anide and allenoate would essentially lead to the generation
of analogous reactive intermediate, which will experience
further cyclization to form carbonyl-containing rings when
isatin was used as trapping agent (Scheme 1). To our knowl-
edge, cycloaddition examples based on the nucleophilic
attack of isocyanide towards allenoates have yet to be re-
ported.
Consequently, development of new strategies to construct
this core framework has attracted enormous attention from
organic chemists.[6] Although considerable progress has been
described towards their syntheses, methods to efficiently
construct this core structure in a chemo-, regio- and stereo-
controlled manner is still in high demand. Recently, we have
focused our attention on the syntheses of carbocycles and
potentially bioactive heterocycles.[7]
As a continuation, herein we wish to report an efficient
three-component [2+2+1] cycloaddition from allenoates,
[a] Dr. J. Li, Y. Liu, Dr. C. Li, Prof. Dr. X. Jia
Department of Chemistry, Shanghai University
99 Shangda Road, Shanghai, 200444 (P.R. China)
Fax : (+86)21-66132408
Scheme 1. Representative phosphine-catalyzed [3+2] annulation and
suggested [2+2+1] cycloaddition.
E-mail: lijian@shu.edu.cn
xsjia@mail.shu.edu.cn
[b] Prof. Dr. X. Jia
To probe the feasibility of the proposed transformation,
we selected 1,1,3,3-tetramethylbutyl isocyanide (1a), ethyl
2,3-butadienoate 2a, and isatin 3a as model substrates
(Table 1, entry 1). Pleasingly, the [2+2+1] cycloaddition
State Key Laboratory of Applied Organic Chemistry
Lanzhou University, Lanzhou, 730000 (P. R. China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201100977.
Chem. Eur. J. 2011, 17, 7409 – 7413
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7409

Table 1. Three-component [2+2+1] cycloaddition of isocyanide 1, ethyl
2,3-butadienoate (2a), and isatin 3a.[a]
proceeded cleanly upon heating all of the substrates togeth-
er in toluene. Encouraged by this result, a series of aliphatic
and aromatic isocyanides were successively employed to
react with allenoate 2a and isatin 3a. As shown in Table 1,
the present reactions proceeded smoothly in all cases and all
1
compounds 4 were characterized by IR, H NMR, 13C NMR,
and HRMS spectroscopic analyses.[15] Moreover, the struc-
tures of compounds 4 were unambiguously confirmed by
single-crystal X-ray analysis.[16] Notably, aromatic isocya-
nides containing an electron-withdrawing group at the aro-
matic ring usually gave low yields together with long reac-
tion times, which may contribute to their low reactivity
(Table 1, entries 7 and 8). The 1H NMR spectra of 4 also
Entry
Product
t
Yield
[%][b]
4
[h]
1
4a
4b
4c
18
18
18
83
72
68
ꢀ
showed that the rotation around the COOCH2 CH3 bond
was apparently hindered because two protons of CH2
became diastereotopic (each proton of CH2 appeared as a
quartet of doublets).
To explore the scope and limitation of this process, vari-
ous substituted isatins 3 having electron-neutral (Table 2,
entry 1), electron-deficient (Table 2, entries 2–7), and elec-
tron-rich substituents (Table 2, entries 8 and 9) on the aryl
ring were used to give adducts 4. During our investigation,
isatins with electron-donating and -withdrawing groups at
position 5, 6, and 7 all performed well to experience the an-
nulation in satisfactory yields (Table 2, entries 2–5 and en-
tries 8 and 9). However, the presence of substituents at posi-
tion 4 of isatin 3 proved to have obvious steric hindrance
and low yields were observed in such cases (Table 2, en-
tries 6 and 7). On the other hand, halide, nitro, and methoxy
group substitution on the aromatic ring were all tolerated,
2
3
4
4d
48
62
Table 2. Three-component [2+2+1] cycloaddition of isocyanide 1a,
ethyl 2,3-butadienoate (2a) and substituted isatin 3.[a]
5
6
7
8
4e
4 f
4g
4h
48
48
72
72
53
64
38
39
Entry
R2
PG[b]
Product
4
Yield
[%][c]
1
2
3
4
5
6
7
8
H
Me
Me
Me
Me
Me
Me
Me
Me
Me
Ph
4a
4i
4j
4k
4l
4m
4n
4o
4p
4q
4r
4s
4t
83
78
89
84
67
52
47
71
79
81
88
77
91
5-chloro
5-fluoro
5-nitro
6-bromo
4-chloro
4,6-dichloro
5-methoxy
5,7-dimethyl
H
9
10
11
12
13
H
H
H
Bn[d]
Ac[e]
Boc[f]
[a] Reaction conditions: 1,1,3,3-tetramethylbutyl isocyanide (1a,
0.5 mmol), ethyl 2,3-butadienoate (2a, 0.6 mmol), and substituted isatin 3
(0.5 mmol) in toluene (5 mL), at 1008C for 18 h in the air. [b] PG=pro-
tecting group. [c] Yields of product after silica gel chromatography.
[d] Bn=benzyl. [e] Ac=acetyl. [f] Boc=tert-butyloxycarbonyl.
[a] All reactions were carried out with isocyanide 1 (0.5 mmol), ethyl 2,3-
butadienoate (2a, 0.6 mmol), and isatin 3a (0.5 mmol) in toluene (5 mL)
and heated at 1008C in the air. [b] Yields of product after silica gel chro-
matography.
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Syntheses of Spirocyclic Oxindole-Butenolides
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which were potentially useful for further functionalization.
Experiments with different protecting groups at the nitrogen
atom of isatins were also conducted. To our delight, isatin
with different protecting groups underwent the correspond-
ing three-component cycloaddition readily to give products
4 in moderate to good yields (Table 2, entries 10–13).
The mechanism of this cycloaddition reaction has not
been unequivocally established, but one reasonable possibil-
ity is proposed in Scheme 2. The beginning of the present
cloaddition proceeded readily with g-substituted allenoate
2b to produce 4u in 61% yield. To our surprise, structurally
diverse spirocycle derivatives 5 were isolated when a-substi-
tuted allenoate 2c was used. In such runs, the external
double bond of allenoate 2c was utilized to construct the
spirocycles 5a and 5b. These results also advanced the possi-
bility of regioselective synthesis of spirocycles 4 and 5 under
control with different substitution of allenoate 2.
To further demonstrate the utility of this annulation strat-
egy, the hydrolysis of spirocyclic oxindole-butenolides 4 was
also examined (Scheme 4); the acid hydrolysis of the cyclo-
Scheme 2. Proposed mechanism for the formation of 4.
Scheme 4. Hydrolysis of products 4.
procedure involves the formation of zwitterionic species
from isocyanide 1 and allenoate 2a, which exist as a reso-
nance-stabilized form A$B.[17] The species are then trapped
by the reactive carbonyl group of isatin followed by intra-
molecular annulation to form the intermediate C. Further
conversion may proceed through enolization of C to gener-
ate D, which experiences further isomerization process to
yield the more stable product 4.
After having achieved satisfactory experimental data with
ethyl 2,3-butadienoate 2a, we then aimed for the syntheses
of spirocyclic oxindoles with g- and a-substituted allenoate
2b and 2c (Scheme 3). Pleasingly, the above-mentioned cy-
addition adducts proceeded smoothly to give product 6 in
excellent yields. Carbonylation chemistry offers one of the
most straightforward methodologies for introducing carbon-
yl functionalities into organic molecules.[18] The cycloaddi-
tion involving isocyanide followed by a hydrolysis process
achieved the insertion of carbon monoxide without the aid
of transition-metal catalysts. Therefore, the versatility of
present conversion provides new opportunities for their ap-
plications in organic chemistry.
In conclusion, we have described a novel three-compo-
nent [2+2+1] cycloaddition reaction to generate function-
alized spirocyclic oxindole-butenolides[19] from simple and
readily available starting materials in an efficient and atom-
economical manner. This method is also distinguished by its
convenient experimental set up and excellent regioselectivi-
ty. Further experiments, with broader substrate scope and
limitations, are currently underway in our laboratory.
Experimental Section
Typical procedure: Isatin 3 (0.5 mmol) was added to a solution of isocya-
nide 1 (0.5 mmol) and allenoate 2 (0.6 mmol) in toluene (5 mL). The
stirred mixture was heated to 1008C for several hours and the progress
was monitored using TLC detection. After completion of present reac-
tion, the reaction mixture was concentrated under vacuum. The residue
was purified by column chromatography on silica gel (eluent: petroleum
ether/ethyl acetate or cyclohexane/ethyl acetate) to afford the desired
product 4.
Scheme 3. Synthesis of spirocyclic oxindoles with a- and g-substituted al-
lenoate 2, isocyanide 1, and isatin 3. PG=protecting group.
Chem. Eur. J. 2011, 17, 7409 – 7413
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Acknowledgements
We thank the National Natural Science Foundation of China (nos
21002061, 20872087, 20902057), the State Key Laboratory of Applied Or-
ganic Chemistry, Lanzhou University, and the Postgraduate Innovation
Fund of Shanghai University for financial support.
Keywords: allenoate
·
butenolide
·
cycloaddition
·
isocyanide · multicomponent reaction · spiro compounds
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Received: March 30, 2011
Published online: May 20, 2011
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