Enantioselective synthesis of C3 substituted benzofuroindolines via catalytic asymmetric [3u202f+u202f2] cyclization of 3-substituted indoles with p-benzoquinones

Article abstract of DOI:10.1016/j.tetlet.2021.153233

An efficient catalytic asymmetric [3 + 2] cyclization of 3-substituted indoles with p-benzoquinones has been realized using a binol-chiral phosphoric acid. A large variety of C3 substituted benzofuroindolines compounds were achieved in moderate to high yields (up to 91% yield) and excellent enantioselectivities (up to 94% ee).

Full text of DOI:10.1016/j.tetlet.2021.153233

Tetrahedron Letters xxx (xxxx) xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Enantioselective synthesis of C3 substituted benzofuroindolines via
catalytic asymmetric [3 + 2] cyclization of 3-substituted indoles with
p-benzoquinones
a,
Zhen-yu Lu a,1, Wei-qiao Lan a,1, Fang-yi Liu a, Jun-yi Wang a, Xiao-mei Zhang b, , Li-hua Liao
⇑
⇑
a College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang 330022, PR China
b Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient catalytic asymmetric [3 + 2] cyclization of 3-substituted indoles with p-benzoquinones has
been realized using a binol-chiral phosphoric acid. A large variety of C3 substituted benzofuroindolines
compounds were achieved in moderate to high yields (up to 91% yield) and excellent enantioselectivities
(up to 94% ee).
Received 3 April 2021
Revised 21 May 2021
Accepted 8 June 2021
Available online xxxx
Ó 2021 Elsevier Ltd. All rights reserved.
Keywords:
Enantioselectivity
Asymmetric synthesis
Indole
p-Benzoquinone
Chiral phosphoric acid
Benzofuroindolines skeletons are part of the key structural core
of fused indolines, which constitute the scaffolds of many impor-
tant natural alkaloids [1–3], such as the very potent anticancer
agents diazonamides [2] and azonazine [3], due to their moderate
antitumor activity (Fig. 1). Consequently, constructing a benzo-
furoindoline core, especially in a catalytic asymmetric fashion,
has become a fascinating-goal in the synthetic chemistry commu-
nity. Although there are several elegant examples of organocat-
alytic cyclization for the construction of benzofuroindolines,
there are few catalytic asymmetric methods for the synthesis of
benzofuroindolines [4]. MacMillan [5b], Zhang [5l], Tang [5o],
and Zhong [5p] independently reported methods for the prepara-
tion of benzofuroindolines through enantioselective cycloaddition
reactions. As a result, exploring alternative strategies for the highly
enantioselective construction of benzofuroindoline skeletons with
diverse substitution and fusion patterns is completely desirable.
Quinones are extraordinary flexible starting materials of elec-
tron-deficient particles with multiple sites and modes, and they
can be potentially used for reactions with electron-rich unsatu-
rated species. Exploring the selectivity of these reactive molecules
is an interesting strategy. Yadav and co-workers described a strat-
egy involving the conjugate addition of indoles for p-benzoquinone
to the synthesis of 3-indolylquinones by InBr3 [6a], Bi(OTf)3 [6b], or
microwave irradiation [6c]. Nayak [6d], Li [6e], Wang [6f], and Song
[6g] independently reported a direct coupling approach with 2-
substituted indoles and 1,4-benzoquinone to construct 3-indolyl-
quinones compounds by CuBr2, iodine reagents and Fe(OH)3,
Recently, Shi and co-workers accomplished an elegant cascade
1,4-addition/alcohol elimination reaction involving 2,3-disubsti-
tuted indoles and quinone imine ketals, the 3-indolyquinone
derivatives were generated in high yields and excellent
stereoselectivities [6h]. Zhong and co-workers developed an enan-
tioselective cyclization reaction with p-benzoquinone and 2,3-dis-
ubstituted indoles to construct benzofuroindolines bearing two
vicinal tetrasubstituted carbon stereogenic centers by utilizing a
chiral phosphoric acid catalyst [6l]. In Zhong’s work, 3-methyl
indole and p-benzoquinones underwent a 1,4-addition reaction
to form 2-indolylquinone derivatives, using chiral binol-derived
phosphoric acid and DCE as the solvent. Howerer, in our study,
when CH3CN was employed as the solvent in such reactions, a
cyclization reaction occurred instead of the 1,4-addition reaction.
We reasoned that the nature of the solvent has a direact impact
on the reaction outcome. It is possible that the highly polar solvent
CH3CN is conducive to the cyclization reaction, while the less polar
solvent DCE is conducive to the 1,4-addition reaction. We con-
ducted experimental studies based on this interesting strategy of
controlling the chemoselectivity by switching the solvent, and
⇑
Corresponding authors.
E-mail addresses: xmzhang@cioc.ac.cn (X.-m. Zhang), liaolihua@jxnu.edu.cn
(L.-h. Liao).
1
These authors contributed equally to this work.
https://doi.org/10.1016/j.tetlet.2021.153233
0040-4039/Ó 2021 Elsevier Ltd. All rights reserved.
Please cite this article as: Zhen-yu Lu, Wei-qiao Lan, Fang-yi Liu et al., Enantioselective synthesis of C3 substituted benzofuroindolines via catalytic asym-
metric [3 + 2] cyclization of 3-substituted indoles with p-benzoquinones, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2021.153233

Zhen-yu Lu, Wei-qiao Lan, Fang-yi Liu et al.
Tetrahedron Letters xxx (xxxx) xxx
moderate enantioselectivity (Table 1, entry 1), Under the analo-
gous conditions, CPA1-CPA2 and CPA4-CPA7 were a little lower
enantioselectivity than CPA3 for the reaction. As CPA3 could bring
about product 3a in the highest enantioselectivity of 75% ee with
good yield of 91% (Table 1, entry 3). These data suggest that the
binol structure of chiral phosphoric acid is superior to the spiral
structure of chiral phosphoric acid in both its reactivity and enan-
tioselectivity. Therefore, binol-CPA3 was determined to be the
optimal catalyst, and it was used in the following investigations.
With the optimal reaction conditions determined, the screening
of various parameters was modified. A variety of common organic
solvents were screened based on the optimal reaction conditions
(Table 1, entries 9–11). The results revealed that CH3CN was better
than the other solvents at 25 °C. When the temperature was
reduced to 0 °C, the reaction took 2 days to generate the desired
product with a 91% yield and slightly higher enantioselectivity of
80% (Table 1, entry 12). To our delight, when the temperature
was reduced to À20 °C, the reaction proceeded well, generating
benzofuroindoline 3a in up to a 91% yield and 84% ee (Table 1,
entry 13). When the temperature continued to decrease to
À40 °C, the ee improved slightly to 89% (Table 1, entry 14), but
the reaction time was extended to 7 days. Furthermore, when the
catalyst loading amount was further reduced to 5 mol%, it resulted
in a decrease in both the yield (77% yield) and ee value (70% ee)
(Table 1, entry 15). Thus, the optimized reaction for the 3-methyl
indole (1a) to p-benzoquinone (2a) included the reaction in CH3CN
and use of the 10% of CPA3 at À20 °C.
Fig. 1. Representative benzofuroindoline natural products.
we also hope to show an enantioselective [3 + 2] cyclization of 3-
substituted indoles with p-benzoquinones to construct C3-substi-
tuted benzofuroindolines by bifunctional chiral phosphoric acid
(CPA) [7–9]. We believe that this study will offer an opportunity
for employing p-benzoquinone as a competent electrophile for
diverse catalytic enantioselective transformations by solvent-con-
trolled chemoselective [3 + 2] cyclization reactions, which is com-
plementary to previous work by Zhong et al. [3i] (Scheme 1).
The studies were initiated by evaluating the reaction between
3-methyl indole (1a) and p-benzoquinone (2a) using various chiral
phosphoric acids in acetonitrile at 25 °C. The reaction proceeded
smoothly to give the desired 3a product in a good yield with
Scheme 1. Previously reported reactions between indoles and 1,4-quinones and our designed strategy to access C3 substituted benzofuroindolines.
2

Zhen-yu Lu, Wei-qiao Lan, Fang-yi Liu et al.
Tetrahedron Letters xxx (xxxx) xxx
Table 1
Optimization of 3-methyl indole(1a) to p-benzoquinone (2a) by the chiral phosphoric acids.
Entrya
Cat*
Solvent
T(oC)
t (d)
Yield[%]b
Ee[%]c
1
2
3
4
5
6
7
9
10
11
12
13
14
15c
CPA1
CPA2
CPA3
CPA4
CPA5
CPA6
CPA7
CPA3
CPA3
CPA3
CPA3
CPA3
CPA3
CPA3
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
Toluene
Mesitylene
CH2Cl2
CH3CN
CH3CN
CH3CN
CH3CN
25
25
25
25
25
25
25
25
25
25
0
À20
À40
À20
1
1
1
1
1
1
1
4
7
3
2
3
7
7
80
73
91
50
90
77
65
83
60
85
91
91
91
77
57
17
75
13
67
28
49
41
43
60
80
85
89
70
a
Conditions: 1a (0.20 mmol), 2a (0.20 mmol) and 0.01 mmol of the chiral phosphoric acid in 2 mL of CH3CN at room temperature. bIsolated yield based on 1a.
cEnantiomeric excesses were determined by HPLC analysis. c5 mol% CPA3 was used.
After the reaction standard conditions were established (Table 1,
entry 13), the substrate scope was explored by the reactions of var-
ious 3-substituted indoles (1) with various p-benzoquinones (2)
(Table 2). First, a series of 3-substituted indole derivatives were
investigated at the C5 and C6 positions. The study showed that
the electronic nature and the position of substituents on the aro-
matic ring of C5 and C6 had no influence on the enantioselectivity
(3b-3f, 3i, and 3l). The sufficient distance between the C5 and C6
groups from the reaction site may explain why the selectivity
had almost no effect. Under the same conditions, the C4 and C7
groups of 3-substituted indole derivatives, which are both
electron-donating and electron-withdrawing, could react with
p-benzoquinone, leading to a decrease in both the yield and ee val-
ues (3g, 3h, 3j, 3k). The yields of 3g, 3h, 3j, and 3k were presum-
ably lower due to steric hindrance on the reaction site. Meanwhile,
the C3-substituted substrates (1a, 1m) gave the expected product
with a good yield with 84–90% ee. This reaction was also applicable
to 2,3-dimethyl indole 3n and the product was obtained in 88%
yield and 93% ee. For some of the substituted indoles with bulkier
substituents (1o-1q), the products were generated with lower
yields and ee values. When p-benzoquinone was employed with
an orth-methyl substituent or ortho-chlorine substituent, there
were poor outcomes (1r, 1s). When the indole with a 3-phenyl sub-
stituent (1t) was employed, the desired reaction was not observed,
perhaps due to steric hindrance on the reaction site. When the R2
group of the substrate (1u) was a phenyl group, the reaction was
tolerated in 58% yield with 83% ee. In addition, when the p-benzo-
quinone with a fluorine substituent (2v) was employed, resulted in
trace amount of desired product, which may be due to the
decreased electrophilicity of this benzoquinone. Furthermore,
when 2-methyl indole and 2-phenyl indole (1w, 1x) were investi-
gated, a 1,4-addition reaction of benzoquinone occurred instead of
the cyclization reaction (Scheme 2).
We also performed a preparative scale reaction to demonstrate
the practical features of this process. The reaction of 1a and 2a was
carried out on a gram scale with 10% catalyst CPA3 in CH3CN, As
shown in Scheme 3, the reaction proceeded well to afford the cor-
responding product 3a in 85% yield and 70% ee.
To probe the reaction mechanism, 1,3-dimethyl indole (1y)
was investigated to [3 + 2] cyclization reaction under standard
3

Zhen-yu Lu, Wei-qiao Lan, Fang-yi Liu et al.
Tetrahedron Letters xxx (xxxx) xxx
Table 2
Scope of CPA3-catalyzed asymmetric [3 + 2] cyclization of indoles 1 to p-benzoquinone 2.
conditions, the corresponding product 3y was obtained in 76%
yield and 4% ee, thus implying that the NAH group of the 1,3-
dimethyl indole play an significant role in the chirality induc-
2 was activated by the bifunctional phosphoric acid (S-CPA3)
generated hydrogen bonding. In this model, the indole (1n)
attacks the p-benzoquinones from the Si face preferentially as
a result of less steric hindrance, resulting in a temporary inter-
mediate having the formation of the first stereocenter 3S-A,
after a rapid enolization from (3S)-A to phenolic (3S)-B, this
tion through the formation of
a H bond with the CPA3
(Scheme 4). Thus a possible reaction pathway is illustrated in
Scheme 5. It can be presumed that the NH group of 1n and
4

Zhen-yu Lu, Wei-qiao Lan, Fang-yi Liu et al.
Tetrahedron Letters xxx (xxxx) xxx
intermediate further sponataneous cyclization generated (2R,
3S)–3n.
In conclusion, we have developed a catalytic enantioselective
[3 + 2] cyclization of 3-substituted indoles with p-benzoquinones
by chiral phosphoric acid, leading to a chemo-, diastereo- and
enantioselective construction of diverse C3 substituted benzo-
furoindolines with moderate to good yields and with moderate
to excellent enantioselectivity (up to 91% yield, up to 94% ee). A
plausible reaction mechanism was also described. Notably, this
reaction affords an opportunity to employ p-benzoquinone as a
competent electrophile for diverse catalytic enantioselective trans-
formations to construct a more structurally diverse group of
benzofuroindolines.
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Scheme 2. Reactions of benzoquinone with 2-substituted indoles.
Acknowledgements
We thank the National Natural Science Foundation of China
(21662016) and the Natural Science Foundation of Jiangxi Province
(20171BAB213009) and
a Sponsored Program for Cultivating
Youths of Outstanding Ability in Jiangxi Normal University.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.tetlet.2021.153233.
Scheme 3. Reactions on a Gram Scale.
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