Isatin N,N′-Cyclic Azomethine Imine 1,3-Dipole and Abnormal [3 + 2]-Cycloaddition with Maleimide in the Presence of 1,4-Diazabicyclo[2.2.2]octane

Article abstract of DOI:10.1021/acs.orglett.6b03815

A new isatin N,N′-cyclic azomethine imine synthon was devised, and an unexpected abnormal [3 + 2]-cycloaddition with maleimide catalyzed by 1,4-diazabicyclo[2.2.2]octane (DABCO) has been disclosed. A variety of tricyclic spiropyrrolidine oxindoles bearing a dinitrogen heterocycle and succinimide scaffold were obtained in excellent yields (up to 96%) and diastereoselectivities (up to 97:3) under mild conditions.

Full text of DOI:10.1021/acs.orglett.6b03815

Letter
pubs.acs.org/OrgLett
Isatin N,N′‑Cyclic Azomethine Imine 1,3-Dipole and Abnormal [3 + 2]-
Cycloaddition with Maleimide in the Presence of 1,4-
Diazabicyclo[2.2.2]octane
Xiao Wang,†,‡ Peng Yang,†,‡ Yong Zhang,†,‡ Chao-Zhe Tang,†,‡ Fang Tian,† Lin Peng,†
and Li-Xin Wang*,†
†Key Laboratory of Asymmetric Synthesis and Chirotechnology of Sichuan Province, Chengdu Institute of Organic Chemistry,
Chinese Academy of Sciences, Chengdu 610041, China
‡University of Chinese Academy of Sciences, Beijing 100049, China
S
* Supporting Information
ABSTRACT: A new isatin N,N′-cyclic azomethine imine
synthon was devised, and an unexpected abnormal [3 + 2]-
cycloaddition with maleimide catalyzed by 1,4-diazabicyclo-
[2.2.2]octane (DABCO) has been disclosed. A variety of
tricyclic spiropyrrolidine oxindoles bearing a dinitrogen hetero-
cycle and succinimide scaffold were obtained in excellent yields
(up to 96%) and diastereoselectivities (up to 97:3) under mild
conditions.
dipolarophiles have been successfully developed.7,8 However,
1,3-Dipolar cycloaddition (1,3-DC) has been widely studied as
the reported N,N′-cyclic azomethine imine 1,3-dipoles were
mainly prepared by the condensation of pyrazolidin-3-one with
aldehydes. Different sorts of azomethine imines for diversity of
reactions and compounds are still underdeveloped and highly
desirable. To the best of our knowledge, isatin N,N′-cyclic
azomethine imine, devised as a new 1,3-dipole and a useful
synthon in this work, has not been reported yet.
On the other hand, spirooxindole scaffolds, especially spiro
heterocyclic oxindoles, have become the focus for many
synthetic chemists recently, for their frequent occurrence in a
wide range of natural products and pharmacological entities
with diverse bioactive properties.10 Accordingly, numerous
elegant synthetic approaches have been developed for the
construction of those structural skeletons over the past years.11
Although spiro heterocyclic oxindoles fused with a five- or six-
membered ring system at the C3-position have been well
investigated, there are still limited reports on the synthesis of
spiro heterocyclic oxindoles through N,N′-cyclic azomethine
imines.12 In 2013, Wang et al. disclosed the first asymmetric
1,3-dipolar cycloaddition of methyleneindolinones and azome-
thine imines catalyzed by a chiral bis-phosphoric acid bearing
triple axial chirality.12a Later, they also developed a base-
catalyzed formal [3 + 3]-annulation of azomethine imines with
3-isothiocyanatooxindoles.12b In 2014, Shi and co-workers
one of the most powerful approaches for the construction of
carbon−carbon and heterocycles from simple building blocks.1
Until now, several 1,3-dipoles, such as azomethine ylides,2
nitrones,3 and carbonyl ylides,4 have been studied. Among
them, N,N′-cyclic azomethine imines, used as readily accessible
and efficient 1,3-dipoles,5−8 have been increasingly employed in
cycloadditions for the synthesis of pyrazolones and the related
dinitrogen-fused heterocyclic derivatives with significant bio-
logical activities9 (Figure 1). Reviewing recent advances in this
area, asymmetric metal-catalyzed and organocatalytic cyclo-
additions of N,N′-cyclic azomethine imines with a variety of
Figure 1. Selected examples of bioactive molecules with pyrazolone
skeleton.
Received: December 22, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b03815
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Organic Letters
Letter
a
reported a novel one-pot tandem reaction of vinylcyclopro-
panes, 3-diazooxindoles, and maleimides in the presence of
[Pd2(dba)3CHCl3].12c Afterward, Feng disclosed an asymmet-
ric [3 + 2]-cycloaddition of methyleneindolinones with
azomethine imines catalyzed by a N,N′-dioxide−Mg(OTf)2
complex.12d In 2016, Zhao’s group reported a 1,3-diplar
cycloadditions of azomethine imines with iminooxindoles in
the presence of CuI.12e
In view of the importance of pyrazolones and spirocyclic
oxindoles and based on our continuing interest in the
construction of spirooxindoles,13 we devised a new 1,3-dipole
based on isatin (shown as 1 in Scheme 1) and hope to use it as
Table 1. Optimization of the Reaction Conditions
b
temp
time
(h)
yield
c
entry
base
Et3N
solvent
(°C)
(%)
dr
1
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CH2Cl2
DCE
25
45
6
2
90
95
97
88
98
56
84
98
94
98
97
99
99
82
86
87:13
89:11
85:15
93:7
2
Et3N
3
Et3N
65
1
4
DABCO
DIPEA
DMAP
K2CO3
45
3
Scheme 1. 1,3-Dipolar Cycloaddition of N,N′-Cyclic
5
45
10
5
81:19
86:14
90:10
86:14
81:19
90:10
83:17
72:28
44:56
93:7
Azomethine Imines with Maleimides
6
45
7
45
4
8
DABCO
DABCO
DABCO
DABCO
DABCO
DABCO
DABCO
DABCO
reflux
45
1
9
2
10
11
12
13
toluene
THF
45
20
2
45
MeCN
DMF
45
2
45
1
d
14
CHCl3
CHCl3
45
10
10
d,e
15
45
93:7
a
Unless otherwise specified, all reactions were carried out with 1a (0.1
mmol), 2a (0.15 mmol), and base (0.1 mmol) in 1 mL of solvent.
b
c
d
Yield of the diastereoisomeric mixture. Determined by HPLC. 20
e
mol % loading of catalyst. 0.2 mmol of 2a was used.
a new synthon for the construction of spirooxindoles with
potential biological and pharmaceutical activities. During the
development of a new [3 + 2]-cycloaddition, we started to
investigate the model reaction of 1a with maleimide 2a. The
reaction proceeded smoothly under mild conditions. Surpris-
ingly, when we fully analyzed the spectra of the product after
obtaining its single-crystal structure, we found that this reaction
proceeded via an unexpected C−N−C [3 + 2]-cycloaddition,
which was different from the previous report (Scheme 1).14
This abnormal observation prompted us to further study the
reaction under various conditions (Table 1). Our studies were
initiated in CHCl3 with 1 equiv of Et3N at room temperature
(25 °C). In the same reaction mode, the new and unexpected
[3 + 2]-cycloaddition proceeded smoothly to provide the
product in almost quantitative yield and good diastereoselec-
tivity (Table 1, entry 1). After reaction temperature screenings,
45 °C was found to be optimal (Table 1, entry 2). Encouraged
by those promising results, other bases were screened. 1,4-
Diazabicyclo[2.2.2]octane (DABCO) gave better diastereose-
lectivity compared with DIPEA, DMAP, and K2CO3 (Table 1,
entries 4−7) and was selected as the optimal base. Considering
the lower solubility of the new 1,3-dipole in most organic
solvents, solvent effects were then studied (Table 1, entries 8−
13). Polarities of solvents slightly affected the yields but
obviously changed the diastereoselectivities. CHCl3 gave the
best results and was selected as the optimal solvent (Table 1,
entry 4). The diastereoselectivities significantly dropped in
CH3CN and DMF (Table 1, entries 12 and 13). In particular,
in toluene, the reaction became sluggish (Table 1, entry 10).
When the base loading was reduced to 20 mol %, almost the
same results were obtained (Table 1, entry 14). Higher loading
of maleimide gave a better yield (Table 1, entry 15).
Considering all of the active parameters, the optimal reaction
conditions were established as 0.1 mmol of 1a and 0.2 mmol of
2a with 20 mol % of DABCO as catalyst in 1 mL of CHCl3 at
45 °C (Table 1, entry 15).
Under the optimized reaction conditions, the generality for
different substrates was further investigated (Scheme 2).
Generally, most isatin N,N′-cyclic azomethine imines 1 and
maleimides 2 were well tolerated. For most N-aryl and aliphatic
maleimides 2, good yields and diastereoselectivities were
smoothly obtained. The position of substituents on the aryl
ring delivered significant influence on the yields and
diastereoselectivities. The ortho-substituted maleimide 2b gave
71% yield with 48:36:16 dr (3ab) (Scheme 2), and 2c delivered
the product in 72% yield and 53:25:22 dr (3ac) (Scheme 2),
probably due to the axial chirality.15 Differently, only two
diastereoisomers were detected when meta- and para-
substituted maleimides were used (3ad−an) (Scheme 2).
The meta-substituted maleimides gave better diastereoselectiv-
ities compared with the para-substituted counterparts (3ad−ai
vs 3aj−an) (Scheme 2). The electronic characteristics of the
substituents were also studied, while only a slight impact on
yields and diastereoselectivities was observed, regardless of the
substituted positions (3ab−an) (Scheme 2). Disappointedly,
the aliphatic maleimides 2o and 2p gave only moderate yields
and diastereoselectivities. The scope of 1 was also studied
(3ba−ia) (Scheme 2). N-Allyl 1c afforded the product with
excellent results (94%, dr = 92:8), while N-H 1b cannot work
well, and only a trace amount of product was detected. The
substitutions, whether electron-withdrawing or -donating on
the aromatic ring in N,N′-cyclic azomethine imine, gave
excellent yields (92−96%) and diastereoselectivities (dr =
91:9−95:5). However, when R4 was changed to a methyl group
instead of a hydrogen atom (1i), the reaction did not occur
anymore, and no other reaction was observed. The absolute
configuration of the reactant 1g and the relative configuration
B
DOI: 10.1021/acs.orglett.6b03815
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Letter
a c
Scheme 2. Generality of Substrate Scope −
Figure 2. Representative X-ray crystallographic structure of reactant
1g and cycloadduct 3ef.
Scheme 3. Proposed Reaction Pathway
Scheme 4. Catalytic Enantioselective Variant
a
Unless otherwise specified, all reactions were carried out with 1 (0.1
mmol), 2 (0.2 mmol), and DABCO (0.02 mmol, 20 mol %) in 1 mL
b
of CHCl3 at 45 °C. Yield of the diastereoisomeric mixture.
c
d
Determined by HPLC. Determined by 1H NMR of the crude
e
reaction mixture. No reaction.
cycloaddition with maleimides catalyzed by DABCO has been
disclosed. A variety of tricyclic spiropyrrolidine oxindoles
bearing dinitrogen heterocycles and succinimide scaffolds were
obtained in excellent yields (up to 96%) and diastereoselectiv-
ities (up to 97:3) under mild conditions, and a series of N,N′-
cyclic azomethine imines and maleimides are compatible with
this protocol. Further studies on the potential of this reaction
and catalytic applications of the new isatin N,N′-cyclic
azomethine imine 1,3-dipole with other substrates are currently
underway in our laboratory.
of the product 3ef were determined by X-ray crystallographic
analysis (Figure 2).16
To understand the unexpected [3 + 2]-cycloaddition, we
proposed a plausible catalytic mechanism as illustrated in
Scheme 3. The reaction may be initiated by the resonance of 1a
to form I. After tautomerism in the presence of a base, the
delocalized and more stable intermediate II was formed.
Intermediate II reacted with maleimide 2a to give the final
product via [3 + 2]-cycloaddition.
To achieve an asymmetric version of the present reaction, a
series of organocatalysts, such as cinchonas, bifunctional
cinchona thioureas, primary amines, and chiral phase-transfer
catalysts, were screened. Unfortunately, no positive results were
obtained. Only in the presence of phase-transfer catalyst 4 and
K2CO3 was a good yield (80%) with excellent diastereose-
lectivity but poor enantioselectivity (ee = 24%) obtained
(Scheme 4).
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.6b03815.
■
S
X-ray crystal details for 1g (CIF)
X-ray crystal details for 3ef (CIF)
Experimental procedures and detailed characterization
data of all compounds (PDF)
In summary, a new isatin N,N′-cyclic azomethine imine
synthon was devised and an unexpected abnormal [3 + 2]-
C
DOI: 10.1021/acs.orglett.6b03815
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: wlxioc@cioc.ac.cn.
ORCID
Li-Xin Wang: 0000-0003-0830-7421
Notes
The authors declare no competing financial interest.
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