A Synthesis of Multifunctionalized Indoles from [3 + 2] Annulation of 2-Bromocyclopropenes with Anilines

Article abstract of DOI:10.1021/acs.orglett.9b01276

A new regioselective method for the synthesis of multifunctionalized indoles from [3 + 2] annulation of 2-bromocyclopropenes with anilines has been developed. By employing a nickel complex as a catalyst, 27 examples of indole products were obtained in goo

Full text of DOI:10.1021/acs.orglett.9b01276

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
A Synthesis of Multifunctionalized Indoles from [3 + 2] Annulation
of 2‑Bromocyclopropenes with Anilines
Zhu Cao,†,¶ Jian-Bo Zhu,‡,¶ Lijia Wang,*,§ Saihu Liao,*,† and Yong Tang‡
†College of Chemistry, Fuzhou University, 2 Xueyuan Road, Fuzhou, Fujian 350108, China
‡State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
University of Chinese Academy of Sciences, 345 Lingling Road, Shanghai, 200032, China
§School of Chemistry and Molecular Engineering, East China Normal University, 500 Dongchuan Road, Shanghai, 200241, China
S
* Supporting Information
ABSTRACT: A new regioselective method for the synthesis of
multifunctionalized indoles from [3 + 2] annulation of 2-
bromocyclopropenes with anilines has been developed. By employing
a nickel complex as a catalyst, 27 examples of indole products were
obtained in good yields with excellent regioselectivity. Synthetic utility
of the resulting product was demonstrated in a concise synthesis of
biologically active compound Paullone.
he indole unit is one of the most important structural
components that is present in abundant natural products
synthesis, remarkable developments in the Bischler indole
synthesis have been achieved involving N−H insertion reaction
from diazo compounds with anilines,9a as well as the Ru(0) or
Zn(II) catalyzed Bischler indole synthesis of propagylic alchols
with anilines.9b In 1991, Larock and co-workers developed a
palladium catalyzed regioselective formation of 2,3-disubsti-
tuted indoles by employing internal alkynes with 2-iodoani-
line.10 In a Larock indole synthesis, the aryl group of the
aniline prefers to attach to the less bulky end (RS) of the triple
bond while the nitrogen moiety is more liable to assemble on
the sterically more hindered end (RL) (eq a, Scheme 1). Since
T
and drug molecules.1 2,3-Disubstituted indoles are very
important synthetic intermediates that could be further
transformed to various biologically active compounds, such
as Paullone, Kenpaullone, and Cilansetron (Figure 1).
Scheme 1. Synthesis of 2,3-Disubstituted Indoles with
Anilines and Alkynes or Cyclopropenes
Figure 1. Indole containing biologically active compounds.
Paullone, Kenpaullone, and their analogues are potent
inhibitors of the cyclin-dependent kinases (CDKs) and
glycogen synthase kinase-3β (GSK-3β), which have become
promising agents for the treatment of neurodegenerative and
proliferative disorders.2 Cilansetron is a 5HT-3 antagonist,
considered as a promising drug to treat irritable bowel
syndrome (IBS).3
The synthesis of multifunctionalized indole and their
derivatives arouses intense interest from chemists.4 New
generations of more efficient and more practical indole
synthesis methods continue to emerge.5 For example, the
Fischer indole synthesis, developed in 1883, has proven to be
one of the most powerful routes.6 Recently, instead of using
carcinogenic hydrazines, by employing more readily available
imines and enamines, the cross-dehydrogenative coupling
strategy was developed for the synthesis of indoles by
palladium catalysis.7 Another example is the classic Bischler
indole synthesis, which was first reported in 1892.8 In modern
the 2,3-disubstituted indole is a key structural motif of various
biologically active natural and unnatural molecules, the
synthetic methods that provide the 2,3-disubstituted indoles
with an alternative regioselectivity is still highly in demand.
Cyclopropenes, as the smallest unsaturated cyclic molecule,
exhibit a unique chemical property.11 The vinylic carbon atoms
of cyclopropene are sp1.19 hybridized,11d indicating that the
cyclopropane sometimes could serve as an alkyne equivalent.
Received: April 12, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01276
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
With continuing effort and interest in the field of both indole
derivatives12 and small ring compounds,13 in this study, we
have developed an unexpected synthesis of 2,3-disubstituted
indoles from [3 + 2] annulation of donor−acceptor cyclo-
propenes14,15 with anilines, which displays different regiose-
lectivity, compared with the Larock indole synthesis, for the
larger diester moiety to be installed at the 3-position and the
smaller aryl group to be placed at the 2-position of indole (eq
b, Scheme 1). In the presence of nickel(II) as the catalyst,
various multifunctionalized indoles were furnished in up to
86% yield with excellent regioselectivities. Herein, we report
the preliminary results.
in 16% yield of 3a (entry 9 vs 5). To our delight, with 20 mol
% of Ni(ClO4)2·6H2O, the reactivity of the [3 + 2] annulation
was increased to 70%, and the reaction time was shortened to
11 h (Table 1, entry 10).
Under the optimized reaction conditions, various aniline
derivatives were reacted with cyclopropenes bearing different
ester groups, affording good to high yields with excellent
regioselectivites. As shown in Scheme 2, N-Bn-anilines
a
Scheme 2. Substrate Scope of Aniline Derivatives
Initially, we employed a bipyridine−Lewis acid complex as
the catalyst to start our investigation of the [3 + 2] annulation
of N-Bn-4-chloroanliline (1a) and dimethyl 2-bromo-3-
phenylcyclopropene-1,1-dicarboxylate (2a).16 As shown in
Table 1, by using 10 mol % of Ni(ClO4)2·6H2O as a Lewis
a
Table 1. Optimization of the Reaction Conditions
b
entry
Lewis acid
L
t (h)
rr
yield (%)
1
2
3
4
5
6
7
8
9
Ni(ClO4)2·6H2O
Ni(ClO4)2·6H2O
Ni(ClO4)2·6H2O
Ni(ClO4)2·6H2O
Ni(ClO4)2·6H2O
Cu(OTf)2
FeCl3
Ga(OTf)3
Ni(OTf)2
Ni(ClO4)2·6H2O
L1
L2
L3
L4
L5
L5
L5
L5
L5
L5
45
45
45
24
23
25
24
25
25
11
6/1
5/1
5/1
23
25
23
43
50
9
a
Conditions in entry 10, Table 1; Isolated yields of 3; With >20/1 rr
5/1
b
in all cases unless noted. With 11/1 rr. rr = regiomeric ratio.
>20/1
>20/1
3/1
>20/1
>20/1
>20/1
13
13
16
containing different substituents, such as Cl-, Me-, MeO-,
tBu- at the para-position, could reacted smoothly, leading to
the corresponding multifunctionalized indoles 3a−e in 65−
77% yields. N-Me-anilines were also suitable substrate,
delivering the N-Me-indole derivatives 3f−j in 61−70% yields,
while anilines with a strong electron-withdrawing substituent
such as CF3 and NO2 could also afford the desired products
but with lower yields (3f−g). Multisubstituted anilines such as
3,5-dimethoxyanilines and 3,4,5-trimethoxyanilines were tol-
erated in the current catalyst system. In both cases, the N-Me-
3,5-dimethoxyindoles 3l and the N-Me-3,4,5-trimethoxyin-
doles 3n were obtained in 70−81% yields. Furthermore,
cyclopropenes bearing different substituents at both para- and
ortho- positions of aryl groups (3o−q) also worked well,
affording the corresponding indoles in 70−76% yields.
The structure of 3a was determined by the X-ray diffraction
of the single crystal of the reaction product.17 As shown in
Figure 2, the phenyl group from the cyclopropene was installed
at the 2-position of the indole product, while the diester
moiety was located at the 3-position.
Remarkably, tricyclic indoles are also accessible through this
[3 + 2] annulation reaction by using tetrahydroquinolines
instead of anilines (Scheme 3). It is worth mentioning that
tricyclic indoles are found as a core structure in many natural
products and biologically active molecules, such as Cilanse-
d
c
10
70
a
The reactions were carried out under an Ar atmosphere with 1a (0.6
mmol), 2a (0.3 mmol), Lewis acid (10 mol %), L (12 mol %), and 4
Å MS (500 mg) in DCE (3 mL) at 80 °C. NMR yield using 1,3,5-
trimethylbenzene as the internal standard. Isolated yield. With 20
b
c
d
mol % of Ni(ClO4)2·6H2O. rr = regiomeric ratio = 3a/3a′.
acid and bipyridines as the ligand, both L1 and L2 gave the
corresponding product 3a in poor yields with moderate
regioselectivities (23−25% yields, 6/1−5/1 rr, entries 1−2).
With a phenanthroline type ligand L3, the 2,3-disubstituted
indole was obtained in 23% yield with 5/1 rr after 45 h (entry
3).
When an O,N-bidentate ligand L4 was employed, the
reaction was speeded up, providing 3a in 43% yield with
moderate regioselectivity after 24 h (entry 4). Notably, the
commercially available 8-hydroxyquinoline was found to be the
best ligand, leading to 3a in 50% yield with >20/1 rr after 23 h
(entry 5). Different metal salts, such as Cu(OTf)2, FeCl3 and
Ga(OTf)3, in combination of L5 gave poor results (entries 6−
8). Changing the counterion from perchlorate to triflate, the
L5/nickel(II) catalyzed reaction lost efficacy and only resulted
B
DOI: 10.1021/acs.orglett.9b01276
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 4. Scaled-Up Reaction and Applications
Figure 2. Crystal structure of 3a.
Scheme 3. Substrate Scope of Tetrahydroquinoline
a
Derivatives
presence of a nickel complex as the catalyst, a variety of
anilines as well as tetrahydroquinolines were found as
compatible substrates, providing versatile multifunctionalized
indoles (27 examples) in high yields with excellent
regioselectivity. The current method is practical and potentially
synthetically useful, which was demonstrated in a scaled-up
reaction and synthesis of Paullone. Further application of this
reaction in the construction of indole alkaloids is ongoing in
our laboratory.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b01276.
Experimental procedures and characterization data
(PDF)
a
Conditions in entry 10, Table 1; Isolated yields of 5; With >20/1 rr
b
in all cases unless noted. L4 was used with 10 mol % of Ni(ClO4)2·
6H2O, 4i/2b = 1/1.2; with 6.4/1 rr. rr = regiomeric ratio.
Accession Codes
CCDC 1014786 and 1909652 contain the supplementary
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.
tron.3,18 As shown in Scheme 3, various tetrahydroquinolines
with both electron-donating and enlectron-withdrawing groups
substituted at the aryl ring could give the corresponding
tricyclic indoles 5a−g in up to 86% yield with excellent
regioeselectiviy. Benzomorpholine is also a suitable substrate
and gave the corresponding tricyclic indole 5h in moderate
yield. Moreover, the reaction can be extended to a larger cyclic
amine, giving product 5i bearing an eight-numbered ring in
32% yield with 6.4/1 rr.
This method was found to be synthetically promising. As
shown in Scheme 4a, a scaled-up reaction was carried out, and
2.59 g of indole 3s were obtained in 72% yield. The synthetic
utility of this reaction was further demonstrated in a concise
synthesis of Paullone, which was a potent cyclin-dependent
kinase inhibitor and tauprotein kinase inhibitor (Scheme 4b).2a
The product 3q was easily transformed to the monoester 6 by
decarboxylation, which was then subjected to hydrolysis and
amidation to give compound 8. The Paullone was finally
obtained after an intramolecular Ullmann type reaction16 of
compound 8 followed by deprotection of the benzyl group.
In conclusion, we have developed a new method for the
regioselective synthesis of 2,3-functionalized indoles via [3 + 2]
annulation of anilines and 2-bromocyclopropenes. In the
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: ljwang@chem.ecnu.edu.cn.
*E-mail: shliao@fzu.edu.cn.
ORCID
Lijia Wang: 0000-0002-6657-3392
Yong Tang: 0000-0002-5435-9938
Author Contributions
¶(Z.C., J.-B.Z.) These authors contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We are grateful for the financial support from the NSFC (Nos.
21432011, 21772224, and 21602028); CAS (No. QYZDY-
■
C
DOI: 10.1021/acs.orglett.9b01276
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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