Gold(i)-catalyzed pathway-switchable tandem cycloisomerizations to indolizino[8,7-: B] indole and indolo[2,3-a] quinolizine derivatives

Article abstract of DOI:10.1039/c9cc05667j

Experimental and theoretical explorations were performed on the pathways of the cascade cycloisomerizations of tryptamine-N-ethynylpropiolamide substrates. The methodology provided a common strategy to access either indolizino[8,7-b]indoles or indolo[2,3-

Full text of DOI:10.1039/c9cc05667j

Showcasing research from the laboratory of Professor Bin Lin
and Professor Yongxiang Liu, Key Laboratory of Structure-Based
Drug Design and Discovery, Shenyang Pharmaceutical University,
Ministry of Education, Shenyang, China. Image designed by
Professor Yongxiang Liu and illustrated by Kaitong Zhuang.
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Gold(I)-catalyzed pathway-switchable tandem
cycloisomerizations to indolizino[8,7-b]indole and
indolo[2,3-a]quinolizine derivatives
A novel synthetic method was developed to provide a
common strategy to access either indolizino[8,7-b]indoles or
indolo[2,3-a]-quinolizines in a switchable fashion via cascade
cycloisomerizations of tryptamine-N-ethynylpropiolamide
substrates. DFT calculations were performed to offer
theoretical insights into the experimental observations.
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Gold(I)-catalyzed pathway-switchable tandem
cycloisomerizations to indolizino[8,7-b]indole and
indolo[2,3-a]quinolizine derivatives†
Cite this: Chem. Commun., 2019,
55, 14418
Received 23rd July 2019,
Accepted 18th October 2019
Chengjun Liu,abc Zenghui Sun,abc Fukai Xie,abc Guoduan Liang,abc Lu Yang,ac
ac
abc
Yaqiao Li,d Maosheng Cheng,ac Bin Lin
*
and Yongxiang Liu
*
DOI: 10.1039/c9cc05667j
rsc.li/chemcomm
Experimental and theoretical explorations were performed on the
pathways of the cascade cycloisomerizations of tryptamine-N-
ethynylpropiolamide substrates. The methodology provided a common
strategy to access either indolizino[8,7-b]indoles or indolo[2,3-a]-
quinolizines in a switchable fashion.
Tetrahydro-b-carboline exists as a core skeleton in a variety of
natural indole alkaloids. Among them, indolizino[8,7-b]indole
and indolo[2,3-a]quinolizine are two of the representative
structures (Fig. 1). Due to their diverse biological activities,1
indole alkaloids containing these two scaffolds have drawn
much attention from both academia and industry.2 Target-
oriented syntheses of these natural products have been carried
out and synthetic methodologies for these two scaffolds have
been developed in the few past decades.3 So far, N-acyl-iminium
Fig. 1 Natural products containing indolizino[8,7-b]indole and indolo[2,3-a]-
quinolizine scaffolds.
ion or iminium ion cyclizations are the most popular strategies relative stability.5 In particular, tryptamine-derived ynamide
in constructing these two scaffolds.3c,g,h These strategies are represents a unique building block to prepare indoline scaffolds in
primarily aimed at synthesizing the scaffold of either indolizino- the syntheses of polycyclic indole alkaloids and derivatives.6
[8,7-b]indole or indolo[2,3-a]quinolizine. Only a few precedents Previously, two tandem cyclizations based on the same type of
on the development of a common strategy are applicable for tryptamine-derived ynamide substrates were developed in our
both of these two scaffolds.4 Therefore, developing a method to laboratory to synthesize 1H-pyrrolo[2,3-d]carbazole and spiro[indoline-
synthesize both scaffolds in a pathway-switchable fashion is 3,30-pyrrolidin]-2-one skeletons in a pathway-switchable fashion.6d,e
highly desired.
In this study, we further applied the strategy to synthesize
Recently, ynamides as versatile building blocks have shown indolizino[8,7-b]indole and indolo[2,3-a]quinolizine scaffolds
extraordinary potential in constructing complex structures from a common tryptamine-N-ethenylpropiolamide intermediate
due to their predictable regioselectivity, high reactivity and via gold-catalyzed respective 6-endo-dig and 5-exo-dig cycloiso-
merizations.
a Key Laboratory of Structure-Based Drug Design and Discovery
(Shenyang Pharmaceutical University), Ministry of Education,
Shenyang 110016, P. R. China. E-mail: randybinlin@gmail.com
b Wuya College of Innovation, Shenyang Pharmaceutical University,
Shenyang 110016, P. R. China. E-mail: yongxiang.liu@syphu.edu.cn
c Institute of Drug Research in Medicine Capital of China, Benxi, 117000,
P. R. China
d College of Chemistry, Jilin University, Changchun, 130012, P. R. China
† Electronic supplementary information (ESI) available: Experimental proce-
dures, condition screening, compound characterization data, NMR spectra and
computational details. X-ray single crystal diffraction data of 2k and 3t. CCDC
1880231 and 1880232. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c9cc05667j
A number of precedent studies on the cyclization of N-alkenyl
(aryl) alkynylamides in different manners have been reported as
demonstrated in Scheme 1.7 For example, Tanaka and coworkers
developed an in situ formation of N-alkenyl alkynylamides/gold-
catalyzed 5-exo-dig cycloisomerization/cyclopropyl gold carbene
formation and rearrangement strategy to generate pyridinones.7b
Vadola and coworkers reported a gold(I)-catalyzed 6-endo-dig
annulation of N-aryl alkynamides to yield 2-quinolinones.7g
Different from these two studies, van der Eycken and coworkers
described a silver-nanoparticle-catalyzed 5-exo-dig cyclization of
the substrates with terminal alkyne to give 3-spiroindolenines.
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Table 1 Scope of cyclization of 1a–1q on condition 1 (C1)a
Yield
[2 + 3] (%) [2 : 3]
Ratio
Entry Substrate R1
R2
R3
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1f
1g
1h
1i
1j
1k
1l
1m
1n
1o
1p
1q
Cl
H
H
H
H
H
H
H
H
H
H
F
Bn
Bn
Me
c-Pr
87
93
76
73
83
74
86
90
84
1.1 : 1
1.1 : 1
1.4 : 1
5.6 : 1
8.2 : 1
6.4 : 1
5.1 : 1
6.5 : 1
8.9 : 1
5.5 : 1
100 : 0
100 : 0
100 : 0
100 : 0
100 : 0
100 : 0
0 : 100
PMB c-Pr
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
n-Pr
t-Bu
Ph
p-FC6H4
p-ClC6H4
p-BrC6H4
9
10
11
12
13
14
15
16
17
p-CH3C6H4 78
TBDPS
TBDPS
TBDPS
TBDPS
TIPS
TIPS
c-Pr
63
72
68
59
78
75
15
Cl
OMe Bn
F
H
H
Bn
Bn
Boc
a
The reaction was performed in THF at the catalysis of 8 mol%
JohnPhosAu(MeCN)SbF6 at room temperature for 15 min.
substitutions on the alkyne of propiolamide favored the products
2f–2j via the 5-exo-dig process regardless of electron-rich or deficient
groups on the benzene rings (Table 1, entries 7–10). To our delight,
Scheme 1 Cyclization of N-alkenyl alkynylamides.
However, the substrates with alkyl substituted alkyne afforded a the substrates with extremely bulky t-butyldiphenylsilyl (TBDPS) or
mixture of 5-exo-dig and 6-endo-dig cyclization products.7f These triisopropylsilyl (TIPS) substituted alkynes of propiolamide gave 5-exo-
examples demonstrated that the selectivity of these ring-closing dig cyclization products 2k–2p exclusively (Table 1, entries 11–16).
reactions was governed by several factors such as the types of
Next, the protective groups on the indole nitrogen were
substrates and the substitutions of alkyne. In this study, we investigated. It was observed that when the electron-donating
showed that steric effects and electronic effects can be fine-tuned in functional groups (Bn and PMB) were switched to the electron-
the cycloisomerization of tryptamine-based N-alkenyl alkynylamide. withdrawing group t-butyloxycarbonyl (Boc), the 6-endo-dig
Thus a common pathway-switchable strategy for the syntheses of cyclization product 2l was obtained exclusively, although in a
both indolizino[8,7-b]indole and indolo[2,3-a]quinolizine scaffolds relatively low yield (Table 1, entry 17). The result indicated that
was developed based on a gold(I)-catalyzed cascade cyclization.
the electron density on the indole nitrogen could determine the
Our initial research commenced with the preparation of a pathway of the second cyclization. Efforts were therefore made
model substrate followed by condition optimizations (see the to improve the yields of the 6-endo-dig cyclization products. In
ESI†). A thorough screening of catalysts, additives, solvents and the end, the optimal conditions were obtained as to perform
catalyst loadings led to the determination of the optimal the reaction under the catalysis of [tris(2,4-di-tert-butylphenyl)
conditions as to perform the reaction in THF with the catalysis phosphite]gold chloride9 and AgOTf in THF at room tempera-
8
of 8 mol% JohnPhosAu(MeCN)SbF6 at room temperature for ture for 1.5 h (C2, see the ESI†).
15 min (C1, see the ESI†). With the optimal conditions in hand,
We further examined the influence of the substitutions of
the regioselectivity of the second cyclization was first investigated alkyne of propiolamide moieties on the regioselectivity by utilizing
by changing the substitutions on the propiolamide moieties. It N-Boc substituted substrates under the optimized reaction con-
was observed that the substrates with the least bulky functional ditions (C2). It was found that when the substrates had c-Pr and
groups such as methyl (Me) and cyclopropyl (c-Pr) substitutions Me substitution, 6-endo-dig cyclization products were obtained
on the alkynes of propiolamide afforded the 5-exo-dig and 6-endo- exclusively (Table 2, entries 1–7). When n-Pr was used, the
dig cyclization products with a ratio of up to 1.4 :1 (Table 1, 6-endo-dig cyclization products were also the major products
entries 1–3). For the substrate with more bulky n-propyl (n-Pr) (Table 2, entries 8 and 9). For the substrate with more bulky
substituted alkyne, the ratio of the two isomers reached up to phenyl substitution, the ratio of 5-exo-dig cyclization increased
5.6 : 1 (Table 1, entry 4). The substrate with very bulky t-butyl dramatically (Table 2, entry 10). Moreover, when the very bulky
(t-Bu) substitution provided the products 2e and 3e with a ratio t-Bu group was introduced to the alkyne of propiolamide, the
of 8.2 : 1 (Table 1, entry 5). The substrates with phenyl (Ph) regioselectivity was reversed leading to a 4.3 : 1 ratio of 5-exo-dig
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Table 2 Scope of cyclization of 1q–1z, and 1aa on condition 2 (C2)a
Yield
[2 + 3] (%)
Ratio
[2 : 3]
Entry
Substrate
R1
R2
R3
1
2
3
4
5
6
7
8
1q
1r
1s
1t
1u
1v
1w
1x
1y
1z
1aa
H
H
H
H
Me
F
Cl
H
H
H
H
H
c-Pr
c-Pr
Me
Me
Me
Me
Me
n-Pr
n-Pr
Ph
70
68
66
72
63
60
71
87
80
91
74
0 : 100
0 : 100
0 : 100
0 : 100
0 : 100
0 : 100
0 : 100
1 : 6.3
1 : 5.7
1 : 1.8
4.3 : 1
Me
Me
H
H
H
H
H
Me
H
H
9
10
11
t-Bu
a
The reaction was performed under the catalysis of [tris(2,4-di-tert-
butylphenyl)phosphite]gold chloride and AgOTf in THF at room tem-
perature for 1.5 h.
Scheme 2 The proposed mechanism.
and 6-endo-dig cyclization (Table 2, entry 11). Finally, both
structures of the 5-exo-dig cyclization product 2k and 6-endo-dig
cyclization product 3t were confirmed by X-ray crystallographic
analysis (Fig. 2).
functional group on the propiolamide was fixed as c-Pr. The
less bulky c-Pr and the more bulky TBDPS functional groups on
the propiolamide moieties were considered for the steric effects
for the N-Bn substrates. The energy barrier was 4.6 kcal molꢀ1
for the transition state of the N-Boc substrate (1q-6-ts) com-
pared to 6.8 kcal molꢀ1 for the transition state of the N-Bn
substrate (1q-5-ts). The energy barrier for the 6-endo-dig path-
way was 2.2 kcal molꢀ1 lower than that of the 5-exo-dig pathway
and the reaction proceeded by following the latter to yield
indolo[2,3-a]quinolizine products. This was consistent with
the experimental observations, explaining the electronic
effects. As for the steric effects, it was found that the transition
state energy barrier was 9.0 kcal molꢀ1 lower for the 5-exo-dig
pathway (1k-5-ts) than the 6-endo-dig pathway (1k-6-ts) for the
TBDPS substrate, explaining why the 6-endo-dig product was
favored. Such a significantly large energy barrier difference was
Based on precedent studies5,6 and control experimental results
(see the ESI†), a plausible mechanism was proposed for the gold(I)-
catalyzed ynamide-based pathway-switchable tandem cyclizations
to the syntheses of indolizino[8,7-b]indole and indolo[2,3-a]quino-
lizine derivatives as depicted in Scheme 2. A selective activation of
alkyne in ynamide 1k (1q) promoted the formation of a gold(I)–
ynamide complex 1k-1 (1q-1), which underwent the first cyclization
to furnish a cyclopropyl gold carbene intermediate 1k-2 (1q-2). An
instantaneous rearrangement occurred to give intermediate 1k-3
(1q-3), followed by an aromatization and protodeauration to
form the intermediate 1k-4 (1q-4). The substrates with N-Bn
substituted indole and bulky substitutions on the alkyne of
propiolamide preferred a 5-exo-dig pathway leading to the formation
of indolizino[8,7-b]indole derivatives (pathway a). The substrates
with N-Boc substituted indole and less bulky substitutions on the
alkyne of propiolamide favored 6-endo-dig cyclization yielding
indolo[2,3-a]quinolizine derivatives (pathway b).
In order to gain insight into the regioselectivity of the
cyclizations, density functional theory (DFT) calculations were
performed by employing the M11-L method10 to calculate the
energy profiles for the competing pathways. The results are
shown in Fig. 3. Bn and Boc functional groups on the nitrogen
of indole were considered for the electronic effects when the
Fig. 3 The energy profiles of the proposed mechanism.
Fig. 2 ORTEP diagrams of 2k (a) and 3t (b). The ellipsoid probability is 50%.
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syntheses of both indolizino[8,7-b]indole and indolo[2,3-a]-
quinolizine scaffolds has been developed based on the cascade
6-exo-dig/5-exo-dig and 6-exo-dig/6-endo-dig cyclosiomerization of
tryptamine N-ethynylpropiolamide substrates. The substitutions
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and indolo[2,3-a]quinolizine derivatives in a switchable fashion.
This project was supported by the Natural Science Founda-
tion of Liaoning Province of China (Grants 20170540855) and
the Foundation of Liaoning Province Education Administration
of China (Grants 2017LQN08). We are indebted to the program
for the innovative research team of the Ministry of Education
and the program for the Liaoning innovative research team in
university, Liaoning Revitalization Talents Program, Liaoning
BaiQianWan Talents Program and the Career Development
Support Plan for Young and Middle-aged Teachers in Shenyang
Pharmaceutical University.
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There are no conflicts to declare.
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