Access to Polycyclic Sulfonyl Indolines via Fe(II)-Catalyzed or UV-Driven Formal [2 + 2 + 1] Cyclization Reactions of N-((1H-indol-3-yl)methyl)propiolamides with NaHSO3

Article abstract of DOI:10.1021/acs.orglett.9b00573

A variety of structurally novel polycyclic sulfonyl indolines have been synthesized via FeCl₂-catalyzed or UV-driven intramolecular formal [2 + 2 + 1] dearomatizing cyclization reactions of N-(1H-indol-3-yl)methyl)propiolamides with NaHSO₃ in an aqueous medium. The reactions involve the formation of one C-C bond and two C-S bonds in a single step.

Full text of DOI:10.1021/acs.orglett.9b00573

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Access to Polycyclic Sulfonyl Indolines via Fe(II)-Catalyzed or UV-
Driven Formal [2 + 2 + 1] Cyclization Reactions of N‑((1H-indol-3-
yl)methyl)propiolamides with NaHSO3
Lin Lu,† Chenguang Luo,‡ Hui Peng,† Huanfeng Jiang,† Ming Lei,‡,* and Biaolin Yin*,†
†Key Laboratory of Functional Molecular Engineering of Guangdong Province, School of Chemistry and Chemical Engineering,
South China University of Technology, Guangzhou 510640, People’s Republic of China
‡State Key Laboratory of Chemical Resource Engineering, College of Science, Beijing University of Chemical Technology, Beijing
100029, China
S
* Supporting Information
ABSTRACT: A variety of structurally novel polycyclic sulfonyl
indolines have been synthesized via FeCl2-catalyzed or UV-driven
intramolecular formal [2 + 2 + 1] dearomatizing cyclization reactions
of N-(1H-indol-3-yl)methyl)propiolamides with NaHSO3 in an
aqueous medium. The reactions involve the formation of one C−C
bond and two C−S bonds in a single step.
olycyclic indoline moieties are found in numerous
pharmaceuticals and biologically active natural products,
carbon monoxide or a variety of other one-atom units.3 Such
cyclizations have recently been exploited as key steps in the
syntheses of many natural products.4 However, although the
use of cyclizations involving masked alkenes, such as that of an
indole, to access biologically interesting bicyclic indolines via
dearomatization5 would be highly desirable, no such
cyclizations have yet been reported, to our knowledge. In
addition, despite the electronic similarity between carbon
monoxide and sulfur dioxide,6 a sulfonylative variant, which
would provide biologically7 and photoelectrically valuable8
cyclic sulfone products, has also not been achieved. Herein, as
part of our ongoing work on the synthesis of polyheterocycles
via dearomatization of heteroaromatic rings,9 we herein wish to
report a formal intramolecular sulfonyl [2 + 2 + 1] cyclization
accompanying the dearomatization of the indole ring to access
polycyclic sulfonyl Indolines 1 (Scheme 1).
P
such as perophoramidine, (−)-aspidospermidine, and
(−)-kopsinine (Scheme 1).1 Thus, the development of
Scheme 1. Bioactive Cyclic Sulfones and Polycyclic Sulfonyl
Indolines
To access sulfonyl indolines 1, we decided to introduce SO2,
in the form of a surrogate (designated [SO2]), into N-(1H-
indol-3-yl)methyl)propiolamides 2. This approach has the
advantages of atom economy and step economy, and it avoids
the need to use SO2, which is hazardous. Introduction of SO2
by means of a surrogate to form sulfones has been explored by
Willis, Wu, Jiang, and others.10 However, most of the reported
protocols involve three-component reactions that form acyclic
sulfones and require preactivation of the substrates to produce
free radicals or organic metal species prior to insertion of SO2
(method a in Scheme 2). To our knowledge, two-component
insertion of SO2 into enynes (or masked enynes) without
substrate preactivation to form cyclic sulfones is still unknown
by far.
operationally simple, step-economical reactions that generate
structurally novel polycyclic indolines from readily available
starting materials (such as indoles) via dearomatization would
be highly desirable.2
One of the most common methods for the construction of
complex polycyclic molecules involves [2 + 2 + 1] cyclization
reactions of unfunctionalized or functionalized 1,n-enynes with
Received: February 14, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b00573
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
was optimal. When (NH4)2S2O8 was used as the persulfate, the
yield of 1a decreased to 60% (entry 8 in Table 1), whereas the
use of Na2S2O8 increased the yield to 86% (entry 9 in Table
1). Lowering the amount of Na2S2O8 to 1 equiv, the yields
decreased dramatically (entry 10 in Table 1). Screening of a
variety of additives (KBr, LiBr, KCl, NaCl, and LiClO4,
corresponding to entries 11−15 in Table 1) revealed that KCl,
NaCl, and LiClO4 gave only slightly lower yields than LiCl, but
KBr and LiBr gave very low yields, or only trace amounts, of
1a. In the absence of FeCl2, the yield of 1a sharply decreased
(to 15%; see entry 16 in Table 1). In addition, when NaHSO3
or Na2S2O8 was absent, none of the desired product formed
and most of the starting 2a was recovered (see entries 17 and
18 in Table 1). The addition of LiCl accelerated the reaction;
in the absence of LiCl, decomposition of either the starting
material or the product was observed (entry 19 in Table 1).13
To explore the substrate scope of the reaction, we subjected
N-(1H-indol-3-yl)methyl)propiolamides 2 bearing a variety of
R1−R4 groups to the optimized reaction conditions and found
that the nature of these R groups clearly influenced the
reaction outcome (Scheme 3). Specifically, for R2 = R3 = H
and R4 = n-Pr, when R1 was an unsubstituted phenyl group or a
Scheme 2. Formation of Sulfones by Introduction of SO2
Surrogates
At the outset, we chose N-((1-methyl-1H-indol-3-yl)-
methyl)-3-phenyl-N-propylpropiolamide (2a) as the model
substrate and K2S2O8, which generates sulfate radicals in the
presence of an iron catalyst11 or light,12 which serves as the
initiator (Table 1). Fortunately, reaction of 2a in the presence
a
Table 1. Optimization of Reaction Conditions
Scheme 3. Substrates Scope (Reaction Conditions Are
Described in Footnotes)
b
entry
[Fe]
FeCl2
FeCl2
[SO2]
persulfate
additive
1a (%)
1
2
3
4
5
6
7
8
9
NaHSO3
Na2SO3
Na2S2O5
NaHSO3
NaHSO3
NaHSO3
NaHSO3
NaHSO3
NaHSO3
NaHSO3
NaHSO3
NaHSO3
NaHSO3
NaHSO3
NaHSO3
NaHSO3
−
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
(NH4)2S2O8
Na2S2O8
Na2S2O8
Na2S2O8
Na2S2O8
Na2S2O8
Na2S2O8
Na2S2O8
Na2S2O8
Na2S2O8
−
LiCl
LiCl
LiCl
LiCl
LiCl
LiCl
LiCl
LiCl
LiCl
LiCl
KBr
LiBr
KCl
NaCl
LiClO4
LiCl
LiCl
LiCl
−
81
trace
50
51
39
65
76
60
86
trace
30
trace
67
81
78
15
ND
ND
51
FeCl2
Fe(acac)3
Fe(acac)2
Fe(OAc)2
Fe(OTf)2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
−
FeCl2
FeCl2
FeCl2
c
10
11
12
13
14
15
16
17
18
19
d
d
NaHSO3
NaHSO3
Na2S2O8
a
Reaction conditions: unless otherwise noted, 2a (0.2 mmol), [SO2]
(2.5 equiv), catalyst (0.1 equiv), persulfate (2.0 equiv), additive (2.0
equiv), CH3CN/H2O = 3:1 (v/v, 2 mL), room temperature, N2
b
c
atmosphere, 6 h. Isolated yields are provided. Na2S2O8 (1 equiv).
d
Not detected.
of FeCl2 (0.1 equiv), NaHSO3 (2.5 equiv), K2S2O8 (2.0 equiv),
and LiCl (2.0 equiv) in CH3CN/H2O at room temperature for
6 h gave desired product 1a in 81% yield (entry 1 in Table 1).
The structure of 1a was verified by single-crystal X-ray analysis
(see the Supporting Information (SI)). Screening of other SO2
surrogates, Na2SO3 and Na2S2O5, did not improve the yield
(see entries 2 and 3 in Table 1). We examined several iron
catalysts, specifically, Fe(acac)3, Fe(acac)2, Fe(OAc)2, and
Fe(OTf)2 (entries 4−7 in Table 1), and we found that FeCl2
a
Condition A: 2 (0.2 mmol), NaHSO3 (2.5 equiv), FeCl2 (0.1 equiv),
Na2S2O8 (2.0 equiv), LiCl (2.0 equiv), CH3CN/H2O = 3/1 (v/v, 2
mL), room temperature, N2 atmosphere, 6 h. Isolated yields are
provided. LiClO4 (2.0 equiv) instead of LiCl (2.0 equiv). Condition
B: 2 (0.2 mmol), NaHSO3 (2.5 equiv), Na2S2O8 (2.0 equiv), LiCl
(2.0 equiv), CH3CN/H2O = 3/1 (v/v, 2 mL), irradiation with 254
nm UV light at 16 W, room temperature, N2 atmosphere, 6 h. Isolated
yields are provided.
b
c
B
DOI: 10.1021/acs.orglett.9b00573
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
phenyl group substituted with an electron-donating group
(Me, MeO, or t-Bu), the desired products were obtained in
good to excellent yields (86%−94%, 1a−1g); moderately
electron-withdrawing groups (F and Cl) also gave good yields
(75%−88%, 1h−1j). However, a strongly electron-withdraw-
ing group (4-CF3) gave desired product 1k in only 65% yield.
When R1 was Me or H, corresponding products 1l and 1m
were not detected, and a small amount of the substrate
decomposed. Various other substituents (R2) on the phenyl
moiety of the indole ring were also screened. For R1 = Ph, R3 =
H, and R4 = n-Pr, the R2 group could be electron-donating
(Me or MeO) or moderately electron-withdrawing (F, Cl, or
Br), and the yields of the products ranged from 56%−96% (1n,
1q−1x). Unfortunately, when R2 was strongly electron-
withdrawing (CN or NO2), desired products 1o and 1p
were not detected, and most of the substrate was recovered.
For R1 = Ph, R2 = H, and R4 = n-Pr, when R3 was Me, Ph, or a
substituted Ph group, the yields of the corresponding products
(1y−1ac) were lower than the yield of 1a (R3 = H). For R1 =
Ph and R2 = R3 = H, when R4 was Me or Et, the yields of 1ad
and 1ae were good. However, when R4 was Bn, the yield of 1af
was only 17%, indicating that the Bn group was not well-
tolerated, possibly because of overoxidation of the methylene
group.
driven by UV,11,12,14 the exact radical species in this organic
reaction are unclear at the moment. On the other hand,
electron-withdrawing of R2 (2o and 2p, Scheme 3) and N-
protective group of Boc or Ac of the indole ring (2ag and 2ah,
Scheme 4b) did not lead to the corresponding products,
indicating that the reactivity was strongly dependent on the
electron density of indole rings. Thus, the formation of cation
radicals cannot be ruled out. Two possible mechanisms are
proposed for the formation of 1a shown in the Supporting
Information (SI) for preliminary consideration, and the further
mechanism study is underway.
In summary, we have developed a protocol for access to
polycyclic indoles via formal [2 + 2 + 1] cyclization reactions
of N-(1H-indol-3-yl)methyl)propiolamides with NaHSO3
driven by FeCl2 catalysis or UV irradiation in an aqueous
medium at room temperature. These mild reactions use readily
available starting materials, show high step-economy, and
generate structurally complex products. Further studies of this
protocol, including the use of variants of the heteroaromatic
ring and its replacement by other unsaturated moieties, and
exploration of the bioactivities of the structurally novel
products are underway in our laboratory.
ASSOCIATED CONTENT
* Supporting Information
■
S
Encouraged by the above-described results, we explored the
possibility of accomplishing this transformation with UV
irradiation, in the absence of FeCl2. Using 2a as the substrate,
we tested the following irradiation sources: 254 nm UV light
(16 W), 380−385 nm UV light (18 W), blue light (450−470
nm, 18 W), green light (520−530 nm, 18 W), yellow light
(580−585 nm,18 W), reddish orange light (600−610 nm,
18W), and red light (620−630 nm, 18 W), and we obtained 1a
in yields of 72%, 33%, 40%, 38%, 13%, 63%, and 46%,
respectively. That is, UV light with a wavelength of 254 nm
was optimal for this reaction. With irradiation at 254 nm, the
reaction showed a broad substrate scope; most of the tested
substrates 2 gave the desired products in moderate to good
yields, except 2l, 2m, 2o, and 2p (Scheme 3). Notably, the
yields of the UV-driven reactions were not higher than the
yields of the corresponding FeCl2-catalyzed reactions, except in
the cases of products 1y, 1z, 1aa, 1ab, 1ac, 1ae, and 1af.
To gain insight into the reaction mechanism, some control
experiments were conducted. On one hand, when the radical
scavenger 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO, 2
equiv) was added under the standard conditions for the
FeCl2-catalyzed reaction, the formation of 1a was not detected,
indicating that radical step(s) might be involved in this
transformation (Scheme 4a). Furthermore, an aryl group at R1
is essential, probably because it stabilizes the intermediary
radical. Although formation of the HSO3 radical from K2S2O8
is known in the references under the catalysis of Fe salt or
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b00573.
Typical experimental procedures, characterization for all
products, computational results and crystallographic data
for 1a (PDF)
Accession Codes
CCDC 1889285 contains 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 Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: blyin@scut.edu.cn.
ORCID
Huanfeng Jiang: 0000-0002-4355-0294
Ming Lei: 0000-0001-5765-9664
Biaolin Yin: 0000-0002-2547-0231
Notes
The authors declare no competing financial interest.
Scheme 4. Control Experiment
ACKNOWLEDGMENTS
■
This work was supported by grants from the National Program
on Key Research Project (No. 2016YFA0602900), the
National Natural Science Foundation of China (Nos.
21572068 and 21871094), the Science and Technology
Program of Guangzhou, China (No. 201707010057),
Guangdong Natural Science Foundation (No.
2017A030312005), and the Science and Technology Planning
Project of Guangdong Province, China (No.
2017A020216021).
C
DOI: 10.1021/acs.orglett.9b00573
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
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DOI: 10.1021/acs.orglett.9b00573
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