Synthesis of Cyclopenta[b]indoles via a Formal [3+2] Cyclization of N-Sulfonyl-1,2,3-triazoles and Indoles

Article abstract of DOI:10.1002/adsc.202000624

Annulation of benzoxy-tethered N-sulfonyl-1,2,3-triazoles and indoles has been developed in this paper, providing an efficient and convenient access to valuable cyclopenta[b]indoles in moderate to good yields. α,β-Unsaturated imine, which generated in situ from denitrogenation and 1,2-OBz migration of triazole, provided three carbons for the formal [3+2] cyclization reaction for the first time. (Figure presented.).

Full text of DOI:10.1002/adsc.202000624

Title: Synthesis of Cyclopenta[b]indoles via a Formal [3 + 2] Cyclization
of N-Sulfonyl-1,2,3-triazoles and Indoles
Authors: shengguo duan, Wan Zhang, Yuntong Hu, Zefeng Xu, and
Chuan-Ying Li
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.202000624
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10.1002/adsc.202000624
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Synthesis of Cyclopenta[b]indoles via a Formal [3 + 2]
Cyclization of N-Sulfonyl-1,2,3-triazoles and Indoles
Shengguo Duan,a Wan Zhang,a Yuntong Hu,a Ze-Feng Xu,a and Chuan-Ying Lia,*
a
Department of Chemistry, Zhejiang Sci-Tech University, Xiasha West Higher Education District, Hangzhou, 310018,
China
E-mail: licy@zstu.edu.cn
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Abstract. Annulation of benzoxy-tethered N-sulfonyl-
N-Sulfonyl-1,2,3-triazole, which is easily prepared
1,2,3-triazoles and indoles has been developed in this from the copper(I)-catalyzed “Click” reaction,[5] has
paper, providing an efficient and convenient access to
valuable cyclopenta[b]indoles in moderate to good yields.
α,β-Unsaturated imine, which generated in situ from
denitrogenation and 1,2-OBz migration of triazole,
provided three carbons for the formal [3+2] cyclization
reaction for the first time.
received much attention in the last decade because its
denitrogenative ring-opening reactions have emerged
as versatile methods for the construction of various N-
containing compounds, especially heterocycles.[6,7] In
general, α-imino rhodium carbene derived from
triazole could undergo many types of reactions, such
as cyclopropanation, X-H (X = C, N, O etc.) insertion,
ylide formation (Scheme 1). Additionally, the 1,2-
migration reaction[8] also occurs frequently when a
suitable substituent (such as SPh, OAc) attaches on
the α-carbon of the rhodium carbene, leading to the
formation of a highly reactive α,β-unsaturated imine
(Scheme 1). In 2015, Tang, Shi and co-workers
reported a rhodium(II)-catalyzed 1,2-sulfur migration
of S-tethered N-sulfonyl-1,2,3-triazole, delivering
sulfur substituted α,β-unsaturated imine.[9] Shortly
afterwards, Anbarasan[10] and our group[7] reported
some new reactions using similar 1,2-migration
strategy.
Keywords: Cyclopenta[b]indole; [3 + 2] Cyclization;
Carbenes; Triazole; Rhodium
Indole scaffolds are privileged substructures owing to
their occurrence in numerous pharmaceuticals,
agrochemicals and materials.[1] Among the indole
derivatives, cyclopenta[b]indoles are of great
significance because of their prevalence in a large
number of natural products and bioactive compounds
(Fig. 1).[2] For example, fischerindole L, which shows
cytotoxicity against HCl-H460 cell lines, was isolated
from Fischerella muscicola;[2a] yuehchukene, which
possesses anti-fertility and estrogenic activities, was
isolated from Murraya paniculata;[2c] and MK-0524
(also named Laropiprant), which developed by Merck,
was a good DP1 receptor inhibitor.[2f] Thus, the
development of convenient, efficient and general
methods to synthesize cyclopenta[b]indoles is of high
interest.[3,4]
Scheme 1. Background.
It is worth noting that the α,β-unsaturated imine
played as aza-[4C] synthon in the cyclization reaction
in all previous works. As shown in Scheme 2,
zwitterionic intermediate A1 could be produced via
Figure 1. Representative compounds.
1
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10.1002/adsc.202000624
Advanced Synthesis & Catalysis
nucleophilic attack to the α,β-unsaturated imine. The reaction in CHCl3 gave higher yield of 4aa (entry 8).
following intramolecular cyclization would give N- More acids similar with CF3CO2H were tested (entries
heterocycle which contains all the aza-[4C] on the 14–18), and the yield could be increased to 63% when
ring. We envisioned that if protonation is faster than CCl3CO2H was used (entry 14). Further screening of
the cyclization after the formation of intermediate A1, the reaction temperature showed that the yield of 4aa
enamine B1 might be obtained, which can isomerize could be improved to 67% when the reaction
to imine C1. The cyclization at this stage would lead performed at 100 °C (entry 19).
to the formation of D1. Remarkably, only three
carbons of the α,β-unsaturated imine are involved in Table 1. Optimization of the Reaction Conditions.[a]
the ring, which is a totally new reaction model in
triazole-related chemistry. Herein we report a Rh(II)-
catalyzed formal [3 + 2] reaction of N-sulfonyl-1,2,3-
triazoles
and
indoles,
through
which
cyclopenta[b]indoles could be constructed in a
convenient one pot procedure.
entry
acid
solvent
yield (%)[b]
1
2
3
4
5
6
7
8
CF3CO2H
CH3CO2H
TsOH·H2O
Camphorsulfonic acid
BF3·Et2O
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
CHCl3
37
0[c]
0[d] (23)[e]
30[e]
0[c,e]
0[d,e]
0[c,e]
46
Cu(OTf)2
FeCl3
CF3CO2H
9
CF3CO2H
CCl4
39
10
11
12
13
14
15
16
17
18
CF3CO2H
CF3CO2H
CF3CO2H
CF3CO2H
CCl3CO2H
CBr3CO2H
CHF2CO2H
CHCl2CO2H
CH2ClCO2H
CCl3CO2H
1,1,2-TCE
Toluene
PhCl
PhCF3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
22
28
25
29
63
56
51
52
0[c]
67
Scheme 2. Reaction design.
Recently, we reported a Rh(II)-catalyzed [4+2]
reaction of N-sulfonyl-1,2,3-triazoles and indoles,
which the intermediate α,β-unsaturated imine served
as an aza-[4C] synthon in the cyclization reaction.[7d]
In the process of substrate scope investigation, we
found that N-methyl indole (1a) could not deliver the
[4+2] cyclization product under the optimal reaction
conditions of that reaction. The Michael addition
product, which is similar to intermediate B1 in
Scheme 2, of indole and α,β-unsaturated imine was
obtained. This result aroused our great interest and
promoted us to further study this reaction. Fortunately,
after simple condition screening, up to 97% yield of
3aa could be achieved (Eq. 1).[11]
19[f]
CHCl3
[a]
General reaction conditions: 3aa (0.2 mmol), acid (1
equiv), solvent (2 mL) in sealed tube at 90 °C for 4 hours.
[b]
1
Determined by H NMR with 1,3,5-trimethoxylbenzene
as the internal standard.
[c] No reaction.
[d] Decomposed.
[e] 0.2 equiv of acid was used.
[f]
Reaction was performed at 100 °C.
With the optimal conditions in hand, we tried one
pot reaction to synthesize 4aa with the starting
materials 1a and 2a. To our surprise, 4aa could be
isolated in up to 77% yield (Eq. 2).
Encouraged by this result, then we focused on the
realization of intramolecular cyclization. To our
delight, the tricyclic compound 4aa was obtained in
37% yield when 3aa was treated with 1 equivalent of
CF3CO2H in 1,2-DCE at 90 °C in sealed tube for 4
hours (Table 1, entry 1).[12] Then various acids
(entries 2–7) and solvents (entries 8–13) were
screened to improve the reaction efficiency.
Experiment results showed that most Bronsted acids
and Lewis acids we used were less effective, and
Although the ester moiety does not stay in the final
product, its leaving ability would affect the formation
of cyclopenta[b]indoles. Hence, the influence of
2
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10.1002/adsc.202000624
Advanced Synthesis & Catalysis
different esters was tested (Table 2). While methoxy the reactions, providing related products 4ka-na in
group on the aromatic ring led to 4aa in poor yield, a moderate yields. In contrast, substrates with other
trifluoromethyl group had marginal effect on the final groups such as tert-butoxycarbonyl and aromatic ring
yield (entries 1–2). The steric effect of ester was not (phenyl,
4-methoxyphenyl
and
3,4,5-
predominant (entry 3). Triazole 2l and 2m gave the trimethoxyphenyl all failed to obtain the desired
product in 61% and 15% yield respectively, which products. Actually, the reaction did not work in the
may attribute to the more acid-sensitive ester group.
first step, the corresponding indole was recovered.)
did not afford the expected products. The electron
density of indole has a significant effect on the
success of this reaction.
Table 2. The Influence of Different Esters.
entry
1
R
yield (%)
51
2
3
80
78
4
5
61
15
Next, we explored the scope of this transformation
with various N-sulfonyl-1,2,3-triazoles. As
Scheme 3. Substrate scope of N-sulfonyl-1,2,3-triazoles.
summarized in Scheme 3, a series of sulfonyl
protecting groups could be well tolerated in this
Rh(II)-catalyzed formal [3+2] annulation. The
substituents on the benzene ring of the arylsulfonyl
group could either be electron-donating group (4ad,
4ae), electron-withdrawing group (4ac) or hydrogen
(4ab), and the resulting yields were 61–70%. It is
worth noting that substrate with sterically hindered
group 2,4,6-triisopropylbenzenesulfonyl gave the
product (4ae) in 54% yield. Moreover, other
protecting groups such as β-naphthalenesulfonyl and
methylsulfonyl were all applicable to this
transformation (4ag, 78% yield; 4ah, 57% yield).
The scope of indole 1 was also investigated under
the optimized conditions (Scheme 4). Indoles bearing
a methyl group at the 4-, 5-, 6- or 7-position were
well tolerated, and the resulting cyclization products
were produced in 68−79% yields (4ba-ea). Products
containing valuable halogen atoms were also
synthesized conveniently (4fa-ia, 40−52% yields). 5-
Methoxy substituted indole could also work in this
transformation, leading to the formation of 4ja
smoothly in 68% yield. At last, the effect of the
protecting group of indole nitrogen was examined.
Electron-donating substituents such as allyl, benzyl,
chloroethyl and bromopropyl group were tolerated in
Scheme 4. Substrate scope of indoles.
Moreover, we tested 1,2-dimethyl-1H-indole 1t
under the optimal conditions (Eq. 3). To our surprise,
the 2-methyl group participated in the cyclization
reaction, and two tetrahydrocarbazole isomers were
obtained in 44% yield.
3
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10.1002/adsc.202000624
Advanced Synthesis & Catalysis
Further derivatization of 4 has been performed as
shown in Scheme 5. Imine 4aa could be converted
into the corresponding ketone 6 under hydrolysis
conditions. The corresponding amine 7 could be
obtained in 91% yield when 4aa was reduced with
LiAlH4. An allyl group could be introduced into the
α-position of the imine easily. Moreover, reduction of
imine 4na with diisobutyl aluminium hydride
followed by cyclization of the amine under base
condition afforded the tetracyclic compound 9 in high
yield.
Scheme 6. Proposed mechanism.
In summary, we have developed a rhodium(II)-
catalyzed formal [3 + 2] annulation of benzoxy-
tethered N-sulfonyl-1,2,3-triazoles and indoles,
providing rapid and convenient access to valuable
cyclopenta[b]indoles. Significantly, only three
carbons of the α,β-unsaturated imine were involved in
the final ring system, which is a novel reaction model
in triazole-related chemistry.
Experimental Section
General Procedure for the Synthesis of 4.
To a 10 mL sealed tube charged with 1 (0.2 mmol, 1.0
equiv), 2 (0.2 mmol, 1.0 equiv) in anhydrous CHCl3 (2 mL)
under N2 was added Rh2(OAc)4 (0.004 mmol, 0.02 equiv).
The reaction mixture was stirred for 4 hrs at 100 °C. Then
CCl3CO2H (0.2 mmol, 1.0 equiv) was added to the mixture
under N2 after cooling to room temperature. The mixture
was stirred at 100 °C for 4 hrs again. The reaction mixture
filtered through a short plug of silica gel after cooling to
room temperature. The residue was purified by silica gel
column (PE:EA = 8:1 to 6:1 to 4:1) to give corresponding
compound 4.
Scheme 5. Derivatization of 4aa and 4na.
The mechanism of this reaction was proposed in
Scheme 6 on the basis of present results and pervious
reports.[7d] The key intermediate α,β-unsaturated
imine C could be obtained easily from 1,2-OBz
migration of the α-imino rhodium carbene A, which
generated from denitrogenation of triazole 2 in the
presence of rhodium(II) catalyst. Then the Michael
addition of indole 1 to α,β-unsaturated imine and
Acknowledgements
The work was supported by the National Natural Science
Foundation of China (21871235, 21901230, 21801224), the
Natural
Science
Foundation
of
Zhejiang
Province
following intramolecular proton transfer produced (LQ18B020009), the Fundamental Research Funds of Zhejiang
Sci-Tech University (2019Y003) and Science Foundation of
compound 3, which could be isolated by silica gel
column. In the presence of strong acid, the enamine
Zhejiang Sci-Tech University (ZSTU) under Grant No. 18062301-
Y.
part of compound 3 could isomerize to imine E. An
acid promoted Friedel-Crafts reaction followed by
elimination of BzOH would lead to the formation of
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5
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Advanced Synthesis & Catalysis
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Advanced Synthesis & Catalysis
COMMUNICATION
Synthesis of Cyclopenta[b]indoles via a Formal
[3 + 2] Cyclization of N-Sulfonyl-1,2,3-triazoles
and Indoles
Adv. Synth. Catal. 2020, Volume, Page – Page
Shengguo Duan, Wan Zhang, Yuntong Hu, Ze-
Feng Xu, and Chuan-Ying Li*
7
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