A Highly Regio- and Diastereoselective Four-Component Reaction to Construct Polycyclic Bispiroindolines from 2-Isocyanoethylindoles and Isocyanates

Article abstract of DOI:10.1021/acs.orglett.8b03019

A one-pot multicomponent domino reaction between 2-isocyanoethylindoles and isocyanates for the diastereoselective construction of polycyclic bispiroindolines was developed. Fused polycyclic bispiroindolines containing two contiguous spiral atoms were afforded in moderate to good yields with excellent regio- and diastereoselectivities through a four-component Ugi-type reaction (U-4CR) under mild conditions.

Full text of DOI:10.1021/acs.orglett.8b03019

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
A Highly Regio- and Diastereoselective Four-Component Reaction
to Construct Polycyclic Bispiroindolines from
2
‑Isocyanoethylindoles and Isocyanates
Longhai Li, Jiaxin Liu, and Min Shi*
State Key Laboratory of Organometallic Chemistry, Center for Excellence in Molecular Synthesis, University of Chinese Academy of
Sciences, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, China
*
S Supporting Information
ABSTRACT: A one-pot multicomponent domino reaction
between 2-isocyanoethylindoles and isocyanates for the
diastereoselective construction of polycyclic bispiroindolines
was developed. Fused polycyclic bispiroindolines containing
two contiguous spiral atoms were afforded in moderate to
good yields with excellent regio- and diastereoselectivities
through a four-component Ugi-type reaction (U-4CR) under
mild conditions.
olycyclic spiroindolines are privileged complex molecular
skeletons that often exist in both natural products and
chemo- and enantioselective synthetic methods have been
established for the dearomatization of 2-isocyanoethylindoles to
afford spiroindolines. In this aspect, Feng and co-workers have
made significant contributions to the catalytic asymmetric
transformation of 2-isocyanoethylindoles into polycyclic spi-
roindolines in the presence of N,N′-dioxide ligands, thus
affording a series of enantiomerically enriched polycyclic
P
pharmaceutical molecules with pronounced biological activ-
1
,2
1a,2b,c
ities.
For example, strychnine,
communesin A-
are character-
2
a,d,e,g,i,l−n,n
2h,j,m,o
H,
and (+)-perophoramindine
istic members of the alkaloid family that continue to capture the
imagination of organic chemists (Figure 1). Strychnine is a
7d,e
spiroindolines in good yields and high ee values.
the chemoselectivity on the construction of spiroindolines from
-isocyanoethylindoles still faces some challenges. For example,
However,
2
a Bischler−Napieralski reaction may take place as a side
reaction, and 2-isocyanoethylindoles can only perform as
8
normal isocyanides to react with the other reagents, making
the polycyclic spiroindolines ultimately difficult to achieve.
Moreover, the substrate scopes in the cascade cyclization of 2-
isocyanoethylindoles to construct polycyclic spiroindolines are
still limited. As shown in Scheme 1, two-component domino
cyclization of 2-isocyanoethylindoles and electron-deficient
Figure 1. Structures of polycyclic spiroindoline alkaloids.
notorious poison, causing postsynaptic inhibition in the spinal
3
cord where it antagonizes the transmitter glycine. Commune-
olefins to give spiroindolines has been disclosed by Ji and
sins demonstrate significant cytotoxicity and insecticidal
7c−e
Feng’s groups (Scheme 1a);
Ji’s group and Ruijter’s group
4
activity. (+)-Perophoramidine possesses modest cytotoxicity
also independently reported a three-component cyclization of 2-
isocyanoethylindoles with aldehydes and malononitriles as well
against the HCT 116 human colon carcinoma cell line (IC =
5
0
6
0 μM). Notable features of communesin or (+)-perophor-
as 2-isocyanoethylindoles with primary amines and aldehydes
amindine are the two contiguous spiral atoms at C7/8 and the
presence of the bisaminal or bisamidine moieties.
Multicomponent reactions involving cascade processes
provide powerful strategies for the total synthesis of natural
7a,b
(
Scheme 1b). However, in all of these cases, only one spiral
atom was incorporated in the present products. Herein, we
report a novel one-pot, catalyst-free, four-component Ugi-type
reaction (U-4CR) to construct polycyclic bispiroindolines
bearing two spiral atoms and having bispirocyclic motifs from
simple benzsulfonyl isocyanates and readily available 2-
isocyanoethylindoles under mild conditions (Scheme 1c, this
work).
5
,6
products and synthetically useful building blocks. Among
them, isocyanide-based multicomponent reactions are partic-
5
b,c
ularly important. Recently, 2-isocyanoethylindoles, bearing a
nucleophilic isocyanide at the C3-position of the indoles, have
been used as precursors for the rapid construction of
7
spiroindolines. Because the chemoselective synthesis of
spiroindolines and pyrroles have been achieved by using a 2-
Received: September 20, 2018
7b,c
isocyanoethylindole-based domino reaction,
a number of
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b03019
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 1. Strategies of Constructing Polycyclic
Spiroindolines from 2-Isocyanoethylindoles
Table 1. Optimization of the Reaction Conditions
entry x equiv
solvent
DCM
DCM
DCM
DCM
DCM
THF
MTBE
EA
toluene
DCM
DCM
DCM
DCM
temp (°C) time (h)
yield (%)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1.0
1.2
1.5
2.0
2.5
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
−65∼rt
−65∼rt
−65∼rt
−65∼rt
−65∼rt
−65∼rt
−65∼rt
−65∼rt
−65∼rt
−78
12
12
12
12
12
12
12
12
12
1
61
64
67
76
76
n.p.
38
23
58
71
72
71
74
76
70
0
1
2
3
4
5
−78
2
−78
3
−78
5
DCM
−78
−78
−78
−78
12
12
12
12
DCM (c = 0.05)
DCM (c = 0.2)
DCM (c = 0.4)
b
During our ongoing investigation on the multicomponent
reactions of 2-isocyanoethylindoles under catalyst-free con-
ditions, we assumed that 2-isocyanoethylindole 1a would react
with isocyanate, producing 1,3-dipolar intermediate I and then
undergo an intramolecular [3 + 2] dearomative cyclization to
give polycyclic spiroindoline II (Scheme 2). Unlike previous
16
17
82 (80)
76
a
Reactions were carried out using 1a (0.4 mmol) and 2a (x equiv).
1
Yields were determined by H NMR spectroscopic data of the crude
reaction mixture using benzyl methyl ether as an internal standard.
b
Isolated yield in parentheses.
Scheme 3. Substrate Scope for 2-Isocyanoethylindoles with
TsNCO 2a
Scheme 2. Our Working Hypothesis of the Dearomative
Cyclization of 2-Isocyanoethylindoles with Isocyanates
a
works, because of the effect of an adjacent carbonyl group,
spiroindoline II as a much more reactive imide that could
undergo an Ugi-type reaction with another 1a and isocyanate 2a
to give a polycyclic bispirocyclic framework under the same
conditions. Indeed, we found that, in the reaction of 1a with 2a,
the polycyclic bispirocyclic product 3aa derived from a four-
component reaction was obtained in 61% yield in dichloro-
methane (DCM) from −65 °C to room temperature within 12 h
(
Table 1, entry 1). Then, the optimization of the reaction
conditions was performed, and the results are shown in Table 1.
Examination of the employed amount of 2a revealed that the use
of 2.0 equiv of 2a afforded 3aa in 76% yield (entries 1−5).
Subsequently, other solvents such as tetrahydrofuran, methyl
tert-butyl ether, ethyl acetate, and toluene were also tested, but
none of them could give 3aa in better yields (entries 6−9).
Furthermore, changing the reaction time and carrying out the
reaction at −78 °C did not significantly improve the yield of 3aa
(
entries 10−14). The influence of the reaction mixture’s
concentration on the reaction outcome has also been examined,
and we pleasingly found that using 1a in DCM (c = 0.2) gave 3aa
in 80% isolated yield (entries 15−17).
With the optimized reaction conditions in hand (Table 1,
entry 16), we next surveyed the substrate scope of this reaction,
and the results are summarized in Scheme 3. Various substituted
N-methyl-2-isocyanoethylindoles 1, regardless of whether they
have electron-poor or -rich substituents on the aromatic rings,
a
Reaction conditions: 1 (0.4 mmol), 2a (0.8 mmol), DCM (2.0 mL),
78 °C, 12 h, under Ar; yields of isolated products are given.
−
B
DOI: 10.1021/acs.orglett.8b03019
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Organic Letters
Letter
were all compatible, affording the corresponding products 3ba−
Scheme 5. Scale-up Synthesis and Hydrolysis Experiment
3
la in moderate to good yields ranging from 41 to 90%. It is
worth mentioning that using 1b, 1c, and 1d as substrates, in
which the substituent was introduced at the 4-position of indole,
afforded desired products 3ba, 3ca, and 3da in 41−85% yields,
suggesting that the steric effect may have some impact on the
reaction proceeding because the sterically more bulky substrate
1
b having a methyl group at the 4-position gave corresponding
product 3ba in lower yield. For substrate 1m, in which N1 and
C7 were connected with an aliphatic chain, the reaction
proceeded smoothly, giving desired product 3ma in 54% yield.
The use of the 7-aza-indole derivative as substrate afforded a
trace of 3na. Extending the aliphatic carbon chain of isonitrile
from two to three carbon atoms did not give the corresponding
product 3oa. For N-benzyl, allyl, and phenyl group-substituted
indoles, the desired products 3pa, 3qa, and 3ra were obtained in
6
0−90% yields, respectively. However, for N-Boc- and N-Ts-
substituted substrates, desired products 3sa and 3ta were not
formed under identical conditions, perhaps due to the electronic
effect. Delightfully, N-H indole derivatives were also tolerated,
furnishing the products 3ua, 3va, and 3wa in 46−84% yields.
The structure of 3wa has been unambiguously confirmed by X-
ray diffraction, and its CIF data are presented in the Supporting
Information (SI). The ORTEP drawing is shown in Scheme 3.
Next, we investigated the scope with respect to a series of
isocyanate derivatives 2 (Scheme 4). All these reactions
corresponding product 4 in 92% yield. The structure of 4 was
unambiguously determined by X-ray diffraction and its CIF data
are presented in the SI.
On the basis of previous reports and the obtained results
described above, a plausible mechanism for this four-component
cascade cyclization reaction of 2-isocyanoethylindoles and
isocyanates is depicted in Scheme 6. First, p-toluenesulfonyl
Scheme 4. Substrate Scope for N-Methyl-2-
Isocyanoethylindole 1a with Isocyanate Derivatives
a
Scheme 6. Proposed Reaction Mechanism
isocyanate 2a accepts the nucleophilic attack of N-methyl-2-
isocyanoethylindole 1a to produce 1,3-dipolar intermediate I,
which subsequently undergoes an intramolecular [3 + 2]
dearomative cyclization to give polycyclic spiroindoline II via
intermediate I′. Then, spiroindoline II reacts with another 1a
and 2a again through a synergistic [2 + 2 + 1] ring-closure
process to give 3aa. The regioselective insertion of C−N and C−
O bonds of TsNCO in the formation of 3aa may be due to the
existence of a stepwise process and a concerted pathway during
the reaction proceeding. The concerted process took place faster
than the stepwise process once spiroindoline II was produced,
which may be a reasonable explanation for why polycyclic
spiroindoline II can not be separated even by controlling the
reaction parameters or switching the reaction sequences.
In summary, we have developed a novel catalyst-free one-pot
U-4CR between 2-isocyanoethylindoles and isocyanates for the
construction of polycyclic bispiroindolines in moderate to good
yields under mild conditions. A variety of polycyclic bispiroindo-
lines containing two contiguous spiral atoms could be prepared
with a good substrate scope and excellent regio- and
a
Reaction condition: 1a (0.4 mmol), 2 (0.8 mmol), DCM (2.0 mL),
78 °C, 12 h, under Ar; yields of isolated products are given.
−
proceeded smoothly, giving the corresponding products 3ab−
ad in 65−74% yields. The structure of 3ab was also
3
unambiguously determined by X-ray diffraction, and its CIF
data are presented in the SI. The ORTEP drawing is shown in
Scheme 4. It is worth noting that benzoyl-, phenyl-, benzyl-, and
tert-butylisocyanates did not react with 1a under the standard
reaction conditions.
A scale-up synthesis using 3.0 mmol of 1r and further
hydrolysis of 3ra were performed as shown in Scheme 5. The use
of 3.0 mmol of 1r afforded 3ra in 68% yield (905 mg) under the
standard conditions. In addition, the hydrolysis of 3ra with 1.5
equiv of 3.0 M aqueous HCl solution produced the
C
DOI: 10.1021/acs.orglett.8b03019
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
diastereoselectivity. Further studies for extending the applica-
tion of this method and biological evaluation of these complex
polycyclic bispiroindoline compounds are in progress.
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ASSOCIATED CONTENT
Supporting Information
■
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.8b03019.
Experimental procedure and characterization data for all
compounds (PDF)
(5) For reviews on natural product synthesis using multicomponent
reaction strategies, see: (a) de Meijere, A.; von Zezschwitz, P.; Bras
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̈
e, S.
Accession Codes
̈
1
(
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CCDC 1525040, 1583908, and 1874462 contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_request/
cif, or by emailing data_request@ccdc.cam.ac.uk, or by
contacting The Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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2
AUTHOR INFORMATION
Corresponding Author
■
́
́
*E-mail: mshi@mail.sioc.ac.cn.
̈
(
ORCID
́
Min Shi: 0000-0003-0016-5211
Notes
Rodriguez, J. J. Am. Chem. Soc. 2005, 127, 17176. (f) Lu, M.; Zhu, D.;
Lu, Y.; Hou, Y.; Tan, B.; Zhong, G. Angew. Chem., Int. Ed. 2008, 47,
1
9
0187. (g) Santra, S.; Andreana, P. R. Angew. Chem., Int. Ed. 2011, 50,
418. (h) Snyder, S. A.; Breazzano, S. P.; Ross, A. G.; Lin, Y.; Zografos,
The authors declare no competing financial interest.
A. L. J. Am. Chem. Soc. 2009, 131, 1753. (i) Hashimoto, S.; Katoh, S.-i.;
Kato, T.; Urabe, D.; Inoue, M. J. Am. Chem. Soc. 2017, 139, 16420.
ACKNOWLEDGMENTS
■
(
j) Huang, K.-H.; Huang, C.-C.; Isobe, M. J. Org. Chem. 2016, 81, 1571.
7) (a) Saya, J. M.; Oppelaar, B.; Cioc, R. C.; van der Heijden, G.;
Vande Velde, C. M. L.; Orru, R. V. A.; Ruijter, E. Chem. Commun. 2016,
2, 12482. (b) Wang, X.; Wang, S.-Y.; Ji, S.-J. Org. Lett. 2013, 15, 1954.
We are grateful for financial support from the National Basic
Research Program of China (973-2015CB856603), the
Strategic Priority Research Program of the Chinese Academy
of Sciences (no. XDB20000000) and sioczz201808, and the
National Natural Science Foundation of China (nos. 20472096,
(
5
(
(
c) Wang, X.; Wang, S.-Y.; Ji, S.-J. J. Org. Chem. 2014, 79, 8577.
d) Zhao, X.; Liu, X.; Mei, H.; Guo, J.; Lin, L.; Feng, X. Angew. Chem.,
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DOI: 10.1021/acs.orglett.8b03019
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