One-Pot Copper-Catalyzed Three-Component Reaction of Sulfonyl Azides, Alkynes, and Allylamines to Access 2,3-Dihydro-1 H-imi-dazo[1,2-a]indoles

Article abstract of DOI:10.1055/s-0037-1610739

A copper-catalyzed multicomponent reaction of sulfonyl azides, alkynes, and allylamines affording 2,3-dihydro-1 H-imidazo-[1,2-a]indoles in moderate yields is reported. Four C-N bonds are constructed- by way of azide-alkyne cycloaddition (CuAAC) and double Ullmann-type coupling reactions in a one-pot process.

Full text of DOI:10.1055/s-0037-1610739

SYNTHESIS
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© Georg Thieme Verlag Stuttgart · New York
2020, 52, A–H
paper
A
en
H. Jin et al.
Paper
Synthesis
One-Pot Copper-Catalyzed Three-Component Reaction of Sulfonyl
Azides, Alkynes, and Allylamines To Access 2,3-Dihydro-1H-
imidazo[1,2-a]indoles
Hongwei Jin
Daohong Liu
Bingwei Zhou*
Yunkui Liu*
R2SO2N3
SO2R2
CuI, Et3N
R1
+
N
R1
then additional
CuI, ligand, base
Br
N
Br
H2N
20 examples (up to 77% yield)
via
College of Chemical Engineering, Zhejiang University of
Technology, Hangzhou 310014, China
zhoubw@zjut.edu.cn
Ts
[Cu]
C
R1
R1
N
SO2R2
Br
N
ykuiliu@zjut.edu.cn
HN
Br
Br
♦
♦
♦
One-pot process
Easily available copper catalyst
♦
Four C–N bonds being constructed High step-economy
Received: 02.09.2019
Accepted after revision: 26.11.2019
Published online: 05.02.2020
substrates, and only a narrow scope of 2,3-dihydro-1H-im-
idazo[1,2-a]indole derivatives has been presented till now.
Consequently, it is still highly desirable to extend the meth-
ods for the efficient synthesis of imidazo[1,2-a]indoles.
DOI: 10.1055/s-0037-1610739; Art ID: ss-2019-h0500-op
Abstract A copper-catalyzed multicomponent reaction of sulfonyl
azides, alkynes, and allylamines affording 2,3-dihydro-1H-imidazo-
[1,2-a]indoles in moderate yields is reported. Four C–N bonds are
constructed by way of azide-alkyne cycloaddition (CuAAC) and double
Ullmann-type coupling reactions in a one-pot process.
R
R
a)
NH
PhBr, 180 °C
R = CHO, COMe
N
N
N3
R3
R3
Key words multicomponent reaction, copper catalysis, azide-alkyne
cycloaddition, Ullmann coupling reaction, 2,3-dihydro-1H-imidazo[1,2-
a]indoles
Br
Br
CuI, ligand
K2CO3
CO2R2
R1
R
R
b)
N
NH
N
toluene
120 °C
H
N
O
CO2R2
O
R1
R3
R3
R1
Indole-fused heterocycles are widely found as core
structures in many natural products and biologically active
molecules.1 Among them, 2,3-dihydro-1H-imidazo[1,2-a]-
indoles are viewed as promising building blocks to access
more complex cyclic compounds such as fumiquinazoline2
and asperlicin.3 Over the past several decades, some syn-
thetic methods have been developed to construct the par-
ent 2,3-dihydro-1H-imidazo[1,2-a]indole. For example,
Cruz-Almanza group reported an intramolecular 1,3-dipo-
lar cycloaddition of 1--azidoalkylindoles under high reac-
tion temperatures to give 2,3-dihydro-1H-imidazo[1,2-a]-
indoles (Scheme 1a).4 Later on, Lautens group disclosed a
copper-catalyzed tandem intramolecular amidation of
gem-dibromovinylanilines affording imidazoindolones in
good yields (Scheme 1b).5 Lin and Yan et al. reported the
synthesis of polycyclic indoles from the condensation of
heterocyclic ketene aminals (HKAs) and 1,4-benzoquinones
(Scheme 1c).6 More recently, Ghorai et al. described a diver-
gent pathway to access isomeric imidazoindoles from gem-
dibromovinylanilines and 2-bromoindoles, respectively, via
sequential ring-opening of aziridines and C–N coupling re-
actions (Scheme 1d).7 Despite a considerable achievement,
the existing methods, however, require a tedious design for
O
COR4
NH
R2
R1
O
NH
HO
AcOH
c)
+
R4
N
1,4-dioxane
reflux
R2
H
N
O
1) NaH, DMF
2) CuI, ligand, DMF, 120 °C
Ts
R
N
N
N
Br
Ts
N
N
H
d)
1) LiClO4, MeCN
2) CuI, ligand, toluene, 120 °C
R
Ts
N
Br
R
Br
NH2
R1
R2SO2N3
R1
SO2R2
CuI, Et3N
2
+
N
e)
then additional
CuI, ligand, base
Br
N
Br
H2N
[Cu]
1
4
3a
Ts
N
via
R1
R1
3a
Br
C
SO2R2
N
HN
Br
Br
Easily available copper catalyst
Four C–N bonds being constructed High step-economy
♦
♦
♦
One-pot process
♦
Scheme 1 Synthesis of 2,3-dihydro-1H-imidazo[1,2-a]indoles
© 2020. Thieme. All rights reserved. Synthesis 2020, 52, A–H
Georg Thieme Verlag KG, Rüdigerstraße 14, 70469 Stuttgart, Germany

B
H. Jin et al.
Paper
Synthesis
Multicomponent reactions (MCRs) enable the incorpo-
ration of three or more simple substrates into a complex
molecule via a one-pot cascade procedure, thus exhibiting
excellent step-economy, low cost, and high efficiency.8 In
recent years, copper-catalyzed multicomponent reactions
involving azide-alkyne cycloaddition (CuAAC)9 have re-
ceived much attention since the pioneering work of
Chang10 in 2005. A reactive ketenimine intermediate11 is
formed in situ, which is able to undergo various nucleophil-
ic addition12–16 and cycloaddition17–20 reactions leading to
the formation of heteroatom-containing acyclic and cyclic
compounds, respectively. Of note, the products generating
from copper-catalyzed multicomponent reactions may pro-
ceed further transformations such as Ullmann-type cou-
pling reaction,21 which provides a strategy to synthesize
heterocyclic compounds in a one-pot manner.22 As part of
our continued interest in copper-catalyzed reactions,23 we
herein report a copper-catalyzed three-component reac-
tion of sulfonyl azides, alkynes, and allylamines involving a
CuAAC reaction followed by two consecutive Ullmann-type
C–N coupling reactions. This way, 2,3-dihydro-1H-imid-
azo[1,2-a]indoles can be accessed in moderate yields with
an easy operation.
Initially, we selected the three-component reaction of 2-
bromophenylacetylene (1a), sulfonyl azide 2a and 2-bro-
moprop-2-en-1-amine (3a) as the model reaction. The
three components were first treated with CuI as a catalyst,
triethylamine (Et3N) as a base in THF at room temperature
for 1 hour. Then another portion of CuI, dimethylethylene-
diamine (L1), and K2CO3 were sequentially added to the re-
action mixture and the reaction temperature was elevated
to 80 °C for 6 hours. To our surprise, 2,3-dihydro-1H-imid-
azo[1,2-a]indole 4a was isolated in 35% yield (Table 1, entry
1). Solvents were then screened among which DMSO im-
proved the yield of 4a to 66% (entries 2–5). Removal of Et3N
in the first step resulted in a sharp decrease in the yield of
4a (entry 6). Other metal salts were examined and CuI (30
mol%) remained the most efficient one (entries 7–14 vs. 5).
A control experiment revealed that copper catalyst played
an indispensable role in this reaction (entry 15). Finally, a
range of ligands and bases were, respectively, evaluated in
the second step of this reaction. It was found that ligand L1
was the most suitable ligand and K2CO3 was the most suit-
able base (entries 16–23 vs 5).
Table 1 Screening of the Reaction Parametersa
TsN3
2a
cat., Et3N
solvent
Ts
N
+
Br
N
then additional cat.
ligand, base
Br
H2N
1a
3a
4a
Entry Solvent
Cat. (mol%)b
Ligand
Base
K2CO3
Yield (%)
1
2
THF
CuI (10/20)
CuI (10/20)
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L2
L3
L4
L5
L1
L1
L1
L1
35
40
27
54
66
16c
16
60
44
0
MeCN
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
3
1,4-dioxane CuI (10/20)
4
DMF
CuI (10/20)
CuI (10/20)
CuI (10/20)
CuCl (10/20)
CuBr (10/20)
CuSCN (10/20)
FeCl2 (10/20)
NiCl2 (10/20)
CuI (10/0)
5
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
0
38
51
65
0
CuI (10/10)
CuI (10/30)
d
–
CuI (10/20)
CuI (10/20)
CuI (10/20)
CuI (10/20)
CuI (10/20)
CuI (10/20)
CuI (10/20)
CuI (10/20)
23
56
18
48
26
55
0
e
–
Na2CO3
K3PO4
t-BuONa
0
a Reaction conditions: 1) CuI, Et3N (1.0 equiv), 1a (0.5 mmol), 2a (0.6
mmol), 3a (0.5 mmol), DMSO (3 mL) at r.t. for 1 h; 2) CuI, ligand (0.3
mmol), K2CO3 (2 equiv) at 80 °C for 6 h.
b Catalyst loading in the 1st/2nd step is shown in parentheses.
c No Et3N was used.
d No catalyst was used.
e No base.
sition of phenyl ring were compatible in this transforma-
tion, providing the corresponding products in moderate
yields (→ 4a–e, Scheme 2). Sterically bulky sulfonyl azides,
thiophene-2-sulfonyl azide, and N,N-dimethylamino-sub-
stituted sulfonyl azide were also viable substrates (→ 4f–i,
Scheme 2). In addition, aliphatic sulfonyl azides such as
methanesulfonyl azide and butane-1-sulfonyl azide per-
formed efficiently in the reaction affording the desired
products in 77% and 55% yield, respectively (→ 4j, 4k,
Scheme 2). In addition, different 2-halogen-substituted
phenylacetylene were subjected to the optimal conditions.
It was found that 1-bromo-2-ethnylbenzene exhibited a
higher reactivity compared to chloro- and iodo-substituted
ones (→ 4a, Scheme 2). Aromatic alkynes possessing either
With the optimal reaction conditions in hand, we first
evaluated the substrate scope of sulfonyl azides (Scheme 2).
A range of electron-varied functional groups (R) at para-po-
H
N
COOH
N
N
NH HN
O
O
N
N
L1
L2
L3
L4
L5
Figure 1 Ligands evaluated in this study: L1 = N,N′-dimethylethylene-
diamine; L2 = N,N,N′,N′-tetramethylethylenediamine; L3 = 1,10-
phenanthroline; L4 = ethylene glycol dimethyl ether; L5 = L-proline.
© 2020. Thieme. All rights reserved. Synthesis 2020, 52, A–H

C
H. Jin et al.
Paper
Synthesis
electron-donating or -withdrawing groups reacted smooth-
ly with p-toluenesulfonyl azide (→ 4l–s, Scheme 2). Note-
worthy that heteroaromatic alkyne was tolerated under
these reaction conditions although it provided the corre-
sponding product in a relatively low yield (→ 4t, Scheme 2).
CuI and Et3N in DMSO at room temperature for 1 hour. As a
result, product 5a was isolated in 91% yield, the structure of
which was confirmed by NMR and MS analysis (Scheme
4a). Second, the double C–N coupling reaction of 5a was ex-
amined under indicated reaction conditions affording the
desired product 4a in 67% yield (Scheme 4b). These results
suggested that 5a might be a key intermediate in this three-
component reaction.
R2SO2N3
2
SO2R2
CuI, Et3N
DMSO
N
+
R1
N
Br
R1
X
then additional CuI
L1, K2CO3
H2N
3a
(X = Br)
4
Ts
N
TsN3 2a
1
CuI, Et3N
a)
+
Br
Mes
R
Br
O
DMSO, r.t., 1 h
4a: 66% from 1a (R = Me, X = Br)
16% from 1b (R = Me, X = Cl)
60% from 1c (R = Me, X = I)
4b: R = H, 51%
HN
Br
S
Br
1a
H2N
O
O
N
S
5a: 91%
3a
N
O
N
N
4c: R = OMe, 53%
4d: R = F, 39%
4e: R = Br, 36%
Ts
N
Ts
N
CuI, L1, K2CO3
DMSO, 80 °C, 6 h
4f: 49%
b)
Br
N
HN
Br
5a
O
O
O
N
4a: 67%
S
S
S
O
S
N
N
N
O
N
O
N
N
Scheme 4 Mechanistic experiments
4g: 57%
4h: 44%
4i: 57%
O
O
S
On the basis of previous literature clues10,21 and mecha-
nistic experiments, a possible reaction mechanism is pro-
posed in Scheme 5. First, a copper-catalyzed azide-alkyne
cycloaddition reaction (CuAAC) takes place to generate in-
termediate A, which then transforms to ketenimine B by
the extrusion of N2. Nucleophilic addition of 2-bromoprop-
2-en-1-amine (3a) to intermediate B affords carboxamidine
5a and/or its tautomer 5a′. Finally, a consecutive copper-
catalyzed C–N coupling reactions proceeds to provide 2,3-
dihydro-1H-imidazo[1,2-a]indole 4a.
S
Ts
N
N
O
O
N
N
N
N
R
4j: 77%
4k: 55%
4l: R = Me, 49%
4m: R = i-Pr, 61%
R
Ts
S
N
Ts
4n: R = OMe, 59%
4o: R = F, 54%
N
N
N
4p: R = Cl, 55%
4q: R = CF3, 72%
4r: R = F, 60%
4s: R = Cl, 48%
4t: 24%
Scheme 2 Substrate scope of azides and alkynes. Reagents and condi-
tions: 1) CuI (10 mol%), Et3N (1.0 equiv), 1 (0.5 mmol), 2 (0.6 mmol), 3a
(0.5 mmol), DMSO (3 mL) at r.t. for 1 h; 2) CuI (20 mol%), L1 (0.3
mmol), K2CO3 (2 equiv) at 80 ° for 6 h. Mes = 2,4,6-Trimethylphenyl.
N
N
[Cu]
Ts
N
CuI
Ts
C
N
TsN3
+
Et3N
– N2
[Cu]
Br
Br
In addition, we chose phenylacetylene as an alkyne
component to react with sulfonyl azides 2a and 2-bromo-
prop-2-en-1-amine (3a) under the standard conditions
(Scheme 3). To our surprise, the imidazole derivative 4u′
was isolated in 56% yield rather than our expected product
4u. We also tried the reaction of 2-bromophenylacetylene
(1a) and sulfonyl azide 2a with allylamine generated in situ
from allylamine hydrochloride. However, no desired prod-
uct was detected under the optimal reaction conditions.
Br
1a
2a
A
B
3a,Et3NH
double
C–N
coupling
Ts
Ts
Ts
NH
N
N
Br
Br
N
HN
Br
5a'
HN
Br
5a
4a
Scheme 5 Possible reaction mechanism
Ph
Ph
standard
conditions
Ph
In conclusion, a multicomponent reaction of azides,
alkynes, and allylamine is successfully developed. This
transformation undergoes sequential copper-catalyzed
azide-alkyne cycloaddition (CuAAC) followed by double
copper-catalyzed C–N coupling reactions, providing 2,3-di-
Br
Ts
Ts
+
+
N
N
H2N
N
N
TsN3
2a
3a
4u': 56%
4u: ND
Scheme 3 The reaction of phenylacetylene with 2a and 3a. ND: Not
detected.
hydro-1H-imidazo[1,2-a]indoles in
a one-pot manner.
From the synthetic point of view, this protocol thus offers
an efficient way to access N-containing heterocycles with
an easy operation and high step-economy. Further explora-
tion of copper catalysis in the application of multicompo-
nent reaction is still underway in our laboratory.
To clarify the possible reaction mechanism, we per-
formed stepwise reactions (Scheme 4). First, the reaction of
2-bromophenylacetylene (1a), sulfonyl azide 2a, and 2-bromo-
prop-2-en-1-amine 3a was carried out in the presence of
© 2020. Thieme. All rights reserved. Synthesis 2020, 52, A–H

D
H. Jin et al.
Paper
Synthesis
13C NMR (125 MHz, CDCl3):  = 164.11, 142.63, 141.25, 131.79,
130.73, 129.73, 128.11, 120.72, 120.66, 120.24, 114.23, 108.55, 95.59,
83.69, 55.60, 46.48.
HRMS (ESI): m/z calcd for C18H17N2O3S+ [M + H]+: 341.0954; found:
341.0958.
Unless otherwise noted, all reactions were conducted in 25 mL flame-
dried Schlenk tube under positive N2 atmosphere. 1H NMR spectra
were recorded on 500 MHz Bruker AVANCE III NMR spectrometer.
Chemical shifts () are reported as parts per million (ppm) downfield
from TMS. Coupling constants (J) are reported in hertz (Hz). High-res-
olution mass spectra were recorded on a DECAX-60000 LCQ Deca XP
spectrometer. Melting points were measured on X4 melting point ap-
paratus and are uncorrected. Flash column chromatography was per-
formed on silica gel (100–200 mesh) with the indicated solvent mix-
tures. DMSO (extra dry, H2O content: below 50 ppm) was commer-
cially available and used as received.
1-[(4-Fluorophenyl)sulfonyl]-2-methylene-2,3-dihydro-1H-imid-
azo[1,2-a]indole (4d)
Yield: 64 mg (39%); yellow solid; mp 147–151 °C.
1H NMR (500 MHz, CDCl3):  = 8.02 (d, J = 8.5 Hz, 2 H), 7.69 (d, J = 8.5
Hz, 2 H), 7.57–7.54 (m, 1 H), 7.12–7.09 (m, 2 H), 7.06–7.03 (m, 1 H),
6.37 (s, 1 H), 5.72 (dt, J1 = 2.5 Hz, J2 = 2.5 Hz, 1 H), 4.85 (dt, J1 = 2.5 Hz,
J2 = 2.5 Hz, 1 H), 4.63 (dd, J1 = 2.5 Hz, J2 = 2.5 Hz, 2 H).
2-Methylene-1-tosyl-2,3-dihydro-1H-imidazo[1,2-a]indole (4a);
Typical Procedure
1
13C NMR (125 MHz, CDCl3):  = 166.11 (d, JC,F = 255.5 Hz), 142.48,
In a 10 mL flame-dried Schlenk tube, anhyd DMSO (3 mL), 2-bro-
mophenylacetylene (1a; 90 mg, 0.5 mmol), TsN3 (2a; 118 mg, 0.6
mmol), 2-bromoallylamine (3a; 68 mg, 0.5 mmol), and CuI (9.5 mg,
0.05 mmol) were added sequentially under N2. After stirring for 5
min, Et3N (50 mg, 0.5 mmol) was added and the mixture was stirred
at r.t. for 1 h. CuI (19 mg, 0.1 mmol), K2CO3 (138 mg, 1 mmol), and
dimethylethylenediamine (26 mg, 0.3 mmol) were added to the reac-
tion mixture. The tube was sealed and stirred at 80 °C for 6 h. After
completion, the reaction mixture was diluted with EtOAc (10 mL) and
washed with sat. aq NH4Cl (5 mL). The combined organic phases were
concentrated and purified by silica gel column chromatography
(PE/EtOAc 15:1 to 7:1) to provide the product 4a; yield: 107 mg
(66%); white solid; mp 140–142 °C.
1H NMR (500 MHz, CDCl3):  = 7.76 (d, J = 8.5 Hz, 2 H), 7.55–7.52 (m, 1
H), 7.19 (d, J = 8.0 Hz, 2 H), 7.10–7.05 (m, 2 H), 7.03–7.00 (m, 1 H),
6.33 (s, 1 H), 5.66 (dt, J1 = 2.5 Hz, J2 = 2.5 Hz, 1 H), 4.78 (dt, J1 = 2.5 Hz,
J2 = 2.5 Hz, 1 H), 4.58 (dd, J1 = 2.5 Hz, J2 = 2.5 Hz, 2 H), 2.35 (s, 3 H).
13C NMR (125 MHz, CDCl3):  = 145.23, 142.43, 141.01, 133.42,
131.64, 130.62, 129.62, 127.42, 120.61 (overlap), 120.17, 108.53,
95.60, 83.52, 46.35, 21.54.
4
3
140.74, 132.51 (d, JC,F = 3.3 Hz), 131.66, 130.78, 130.30 (d, JC,F = 9.5
2
Hz), 120.94, 120.84, 120.42, 116.43 (d, JC,F = 22.6 Hz), 108.65, 96.06,
83.92, 46.44.
HRMS (ESI): m/z calcd for C17H14FN2O2S+ [M + H]+: 329.0755; found:
329.0765.
1-[(4-Bromophenyl)sulfonyl]-2-methylene-2,3-dihydro-1H-imid-
azo[1,2-a]indole (4e)
Yield: 70 mg (36%); yellow brown; mp 168–170 °C.
1H NMR (500 MHz, CDCl3):  = 7.75–7.72 (m, 2 H), 7.57–7.54 (m, 3 H),
7.12–7.08 (m, 2 H), 7.06–7.03 (m, 1 H), 6.34 (s, 1 H), 5.68 (dt, J1 = 2.5
Hz, J2 = 2.5 Hz, 1 H), 4.83 (dt, J1 = 2.5 Hz, J2 = 2.5 Hz, 1 H), 4.62 (dd, J1 =
2.5 Hz, J2 = 2.5 Hz, 2 H).
13C NMR (125 MHz, CDCl3):  = 142.40, 140.58, 135.29, 132.41,
131.61, 130.77, 129.55, 128.87, 121.00, 120.87, 120.45, 108.68, 96.21,
83.98, 46.44.
HRMS (ESI): m/z calcd for C17H14BrN2O2S+ [M + H]+: 388.9954,
390.9939; found: 388.9948, 390.9928.
HRMS (ESI): m/z calcd for C18H17N2O2S+ [M + H]+: 325.1005; found:
325.1015.
1-(Mesitylsulfonyl)-2-methylene-2,3-dihydro-1H-imidazo[1,2-
a]indole (4f)
Yield: 86 mg (49%); yellow solid; mp 142–144 °C.
2-Methylene-1-(phenylsulfonyl)-2,3-dihydro-1H-imidazo[1,2-
a]indole (4b)
1H NMR (500 MHz, CDCl3):  = 7.48–7.46 (m, 1 H), 7.09–7.04 (m, 3 H),
6.99 (s, 2 H), 5.89 (s, 1 H), 4.82 (dt, J1 = 2.5 Hz, J2 = 2.5 Hz, 1 H), 4.78
(dd, J1 = 2.5 Hz, J2 = 2.5 Hz, 2 H), 4.59 (dt, J1 = 2.5 Hz, J2 = 2.5 Hz, 1 H),
2.61 (s, 6 H), 2.31 (s, 3 H).
13C NMR (125 MHz, CDCl3):  = 143.96, 142.40, 141.38, 140.74,
132.53, 132.30, 131.97, 130.56, 120.53, 120.46, 120.17, 108.40, 92.95,
82.21, 46.81, 22.82, 21.06.
Yield: 79 mg (51%); white solid; mp 153–156 °C.
1H NMR (500 MHz, CDCl3):  = 7.89 (dd, J1 = 8.8 Hz, J2 = 1.3 Hz, 2 H),
7.59–7.54 (m, 2 H), 7.44–7.41 (m, 2 H), 7.10–7.06 (m, 2 H), 7.04–7.02
(m, 1 H), 6.35 (d, J = 0.5 Hz, 1 H), 5.69 (dt, J1 = 2.5 Hz, J2 = 2.5 Hz, 1 H),
4.80 (dt, J1 = 2.5 Hz, J2 = 2.5 Hz, 1 H), 4.60 (dd, J1 = 2.5 Hz, J2 = 2.5 Hz, 2
H).
13C NMR (125 MHz, CDCl3):  = 142.45, 140.98, 136.54, 134.12,
131.75, 130.74, 129.07, 127.50, 120.81, 120.78, 120.32, 108.58, 95.84,
83.77, 46.46.
HRMS (ESI): m/z calcd for C20H21N2O2S+ [M + H]+: 353.1318; found:
353.1335.
2-Methylene-1-(naphthalen-2-ylsulfonyl)-2,3-dihydro-1H-imid-
azo[1,2-a]indole (4g)
HRMS (ESI): m/z calcd for C17H15N2O2S+ [M + H]+: 311.0849; found:
311.0840.
Yield: 102 mg (57%); yellow solid; mp 170–172 °C.
1H NMR (500 MHz, CDCl3):  = 8.47 (s, 1 H), 7.93 (d, J = 7.9 Hz, 1 H),
7.84–7.81 (m, 3 H), 7.63–7.55 (m, 3 H), 7.10–7.04 (m, 2 H), 7.00–6.98
(m, 1 H), 6.42 (d, J = 0.5 Hz, 1 H), 5.73 (dt, J1 = 2.5 Hz, J2 = 2.5 Hz, 1 H),
4.78 (dt, J1 = 2.5 Hz, J2 = 2.5 Hz, 1 H), 4.57 (dd, J1 = 2.5 Hz, J2 = 2.5 Hz, 2
H).
13C NMR (125 MHz, CDCl3):  = 142.46, 141.08, 135.50, 133.48,
131.75, 131.72, 130.73, 129.57, 129.50 (overlap), 129.30, 127.88,
127.68, 122.09, 120.77, 120.74, 120.29, 108.57, 95.72, 83.69, 46.44.
1-[(4-Methoxyphenyl)sulfonyl]-2-methylene-2,3-dihydro-1H-im-
idazo[1,2-a]indole (4c)
Yield: 90 mg (53%); yellow solid; mp 143–146 °C.
1H NMR (500 MHz, CDCl3):  = 7.83–7.80 (m, 2 H), 7.56–7.52 (m, 1 H),
7.10–7.05 (m, 2 H), 7.04–7.02 (m, 1 H), 6.87–6.84 (m, 2 H), 6.33 (d, J =
0.5 Hz, 1 H), 5.66 (dt, J1 = 2.5 Hz, J2 = 2.5 Hz, 1 H), 4.78 (dt, J1 = 2.5 Hz,
J2 = 2.5 Hz, 1 H), 4.60 (dd, J1 = 2.5 Hz, J2 = 2.5 Hz, 2 H), 3.79 (s, 3 H).
© 2020. Thieme. All rights reserved. Synthesis 2020, 52, A–H

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Synthesis
HRMS (ESI): m/z calcd for C21H17N2O2S+ [M + H]+: 361.1005; found:
361.1020.
1H NMR (500 MHz, CDCl3):  = 7.75 (d, J = 8.3 Hz, 2 H), 7.42 (d, J = 8.0
Hz, 1 H), 7.19 (d, J = 8.1 Hz, 2 H), 6.91 (d, J = 8.1 Hz, 1 H), 6.83 (s, 1 H),
6.28 (s, 1 H), 5.66 (dt, J1 = 2.5 Hz, J2 = 2.5 Hz, 1 H), 4.77 (dt, J1 = 2.5 Hz,
J2 = 2.5 Hz, 1 H), 4.54 (dd, J1 = 2.5 Hz, J2 = 2.5 Hz, 2 H), 2.40 (s, 3 H),
2.35 (s, 3 H).
2-Methylene-1-(thiophen-2-ylsulfonyl)-2,3-dihydro-1H-imid-
azo[1,2-a]indole (4h)
13C NMR (125 MHz, CDCl3):  = 145.15, 142.64, 140.55, 133.59,
131.08, 130.59, 129.63, 129.41, 127.52, 121.76, 120.40, 108.77, 95.69,
83.54, 46.33, 21.60, 21.59.
HRMS (ESI): m/z calcd for C19H19N2O2S+ [M + H]+: 339.1162; found:
339.1167.
Yield: 70 mg (44%); yellow solid; mp 125–128 °C.
1H NMR (500 MHz, CDCl3):  = 7.67 (dd, J1 = 3.8 Hz, J2 = 1.3 Hz, 1 H),
7.58–7.54 (m, 1 H), 7.52 (dd, J1 = 5 Hz, J2 = 1.5 Hz 1 H), 7.11–7.08 (m, 2
H), 7.06–7.04 (m, 1 H), 7.01 (dd, J1 = 4.9 Hz, J2 = 3.9 Hz, 1 H), 6.34 (s, 1
H), 5.72 (dt, J1 = 2.5 Hz, J2 = 2.5 Hz, 1 H), 4.89 (dt, J1 = 2.5 Hz, J2 = 2.5 Hz,
1 H), 4.63 (dd, J = 2.5 Hz, 2 H).
13C NMR (125 MHz, CDCl3):  = 142.37, 140.35, 135.11, 133.58,
133.42, 131.40, 130.83, 127.35, 120.92, 120.90, 120.34, 108.65, 96.87,
84.51, 46.42.
6-Isopropyl-2-methylene-1-tosyl-2,3-dihydro-1H-imidazo[1,2-
a]indole (4m)
Yield: 111 mg (61%); brown oil.
+
HRMS (ESI): m/z calcd for C15H13N2O2S2 [M + H]+: 317.0413; found:
317.0428.
1H NMR (500 MHz, CDCl3):  = 7.76 (d, J = 8.3 Hz, 2 H), 7.45 (d, J = 8.2
Hz, 1 H), 7.18 (d, J = 8.2 Hz, 2 H), 6.98 (dd, J1 = 8.2 Hz, J2 = 1.4 Hz, 1 H),
6.88 (s, 1 H), 6.28 (s, 1 H), 5.65 (dt, J1 = 2.5 Hz, J2 = 2.5 Hz, 1 H), 4.76
(dt, J1 = 2.5 Hz, J2 = 2.5 Hz, 1 H), 4.58 (dd, J1 = 2.5 Hz, J2 = 2.5 Hz, 2 H),
2.95 (m, 1 H), 2.34 (s, 3 H), 1.26 (d, J = 6.9 Hz, 6 H).
N,N-Dimethyl-2-methylene-2,3-dihydro-1H-imidazo[1,2-a]in-
dole-1-sulfonamide (4i)
13C NMR (125 MHz, CDCl3):  = 145.13, 142.62, 142.07, 140.69,
133.57, 130.97, 129.77, 129.62, 127.50, 120.47, 119.39, 105.95, 95.43,
83.40, 46.37, 34.26, 24.48, 21.56.
Yield: 79 mg (57%); yellow solid; mp 150–152 °C.
1H NMR (500 MHz, CDCl3):  = 7.41–7.38 (m, 1 H), 7.03–6.98 (m, 3 H),
5.96 (s, 1 H), 5.29 (dt, J1 = 2.5 Hz, J2 = 2.5 Hz, 1 H), 4.76 (dd, J1 = 2.5 Hz,
J2 = 2.5 Hz, 2 H), 4.63 (dt, J1 = 2.5 Hz, J2 = 2.5 Hz, 1 H), 2.88 (s, 6 H).
HRMS (ESI): m/z calcd for C21H23N2O2S+ [M + H]+: 367.1475; found:
367.1492.
13C NMR (125 MHz, CDCl3):  = 142.92, 142.50, 131.89, 130.65,
120.47, 120.46, 120.24, 108.40, 93.02, 81.76, 46.80, 39.07.
HRMS (ESI): m/z calcd for C13H16N3O2S+ [M + H]+: 278.0958; found:
278.0951.
6-Methoxy-2-methylene-1-tosyl-2,3-dihydro-1H-imidazo[1,2-
a]indole (4n)
Yield: 104 mg (59%); orange solid; mp 138–141 °C.
1H NMR (500 MHz, CDCl3):  = 7.75 (d, J = 8.4 Hz, 2 H), 7.40 (d, J = 8.6
Hz, 1 H), 7.20 (d, J = 8.1 Hz, 2 H), 6.74 (dd, J1 = 8.6 Hz, J2 = 2.3 Hz, 1 H),
6.54 (d, J = 2.2 Hz, 1 H), 6.25 (d, J = 0.7 Hz, 1 H), 5.66 (dt, J1 = 2.5 Hz, J2 =
2.5 Hz, 1 H), 4.77 (dt, J1 = 2.5 Hz, J2 = 2.5 Hz, 1 H), 4.53 (dd, J1 = 2.5 Hz,
J2 = 2.5 Hz, 2 H), 3.80 (s, 3 H), 2.35 (s, 3 H).
13C NMR (125 MHz, CDCl3):  = 155.54, 145.15, 142.56, 140.06,
133.57, 131.37, 129.63, 127.54, 125.67, 121.26, 109.10, 95.73, 93.43,
83.47, 55.79, 46.34, 21.61.
2-Methylene-1-(methylsulfonyl)-2,3-dihydro-1H-imidazo[1,2-
a]indole (4j)
Yield: 95 mg (77%); white solid; mp 137–139 °C.
1H NMR (500 MHz, CDCl3):  = 7.55–7.54 (m, 1 H), 7.16–7.11 (m, 3 H),
6.16 (s, 1 H), 5.58 (dt, J1 = 2.5 Hz, J2 = 2.5 Hz, 1 H), 4.90 (dd, J1 = 2.5 Hz,
J2 = 2.5 Hz, 2 H), 4.86 (dt, J1 = 2.5 Hz, J2 = 2.5 Hz, 1 H), 3.07 (s, 3 H).
13C NMR (125 MHz, CDCl3):  = 142.84, 140.82, 131.65, 130.87,
120.97, 120.78, 120.48, 108.64, 94.58, 82.75, 46.75, 35.42.
HRMS (ESI): m/z calcd for C19H19N2O3S+ [M + H]+: 355.1111; found:
355.1118.
HRMS (ESI): m/z calcd for C12H13N2O2S+ [M + H]+: 249.0692; found:
249.0700.
6-Fluoro-2-methylene-1-tosyl-2,3-dihydro-1H-imidazo[1,2-a]in-
dole (4o)
1-(Butylsulfonyl)-2-methylene-2,3-dihydro-1H-imidazo[1,2-a]in-
dole (4k)
Yield: 92 mg (54%); yellow solid; mp 149–152 °C.
Yield: 80 mg (55%); white solid; mp 107–109 °C.
1H NMR (500 MHz, CDCl3):  = 7.74 (d, J = 8.4 Hz, 2 H), 7.40 (dd, J1 =
8.8 Hz, J2 = 5.3 Hz, 1 H), 7.19 (d, J = 8.2 Hz, 2 H), 6.83–6.79 (m, 1 H),
6.69 (dd, J1 = 9.2 Hz, J2 = 2.2 Hz, 1 H), 6.27 (s, 1 H), 5.65 (dt, J1 = 2.5 Hz,
J2 = 2.5 Hz, 1 H), 4.78 (dt, J1 = 2.5 Hz, J2 = 2.5 Hz, 1 H), 4.52 (dd, J1 = 2.5
Hz, J2 = 2.5 Hz, 2 H), 2.34 (s, 3 H).
1H NMR (500 MHz, CDCl3):  = 7.50–7.48 (m, 1 H), 7.11–7.07 (m, 3 H),
6.08 (s, 1 H), 5.48 (dt, J1 = 2.5 Hz, J2 = 2.5 Hz, 1 H), 4.83 (dd, J1 = 2.5 Hz,
J2 = 2.5 Hz, 2 H), 4.76 (dt, J1 = 2.5 Hz, J2 = 2.5 Hz, 1 H), 3.24 (t, J = 8 Hz, 2
H), 1.84–1.78 (m, 2 H), 1.44–1.37 (m, 2 H), 0.88 (t, J = 7.4 Hz, 3 H).
13C NMR (125 MHz, CDCl3):  = 142.80, 141.25, 131.72, 130.75,
120.71, 120.61, 120.34, 108.56, 93.85, 82.15, 49.38, 46.72, 24.68,
21.41, 13.35.
1
13C NMR (125 MHz, CDCl3):  = 158.86 (d, JC,F =235.8Hz), 145.33,
4
3
142.12, 141.22 (d, JC,F = 3.2 Hz), 133.43, 130.48 (d, JC,F = 12.4 Hz),
129.66, 127.92, 127.44, 121.15 (d, 3JC,F = 9.7 Hz), 108.34 (d, 2JC,F = 23.8
Hz), 95.86, 95.59 (d, 2JC,F = 26.8 Hz), 83.44, 46.34, 21.54.
HRMS (ESI): m/z calcd for C15H19N2O2S+ [M + H]+: 291:1162; found:
291.1170.
HRMS (ESI): m/z calcd for C18H16FN2O2S+ [M + H]+: 343.0911; found:
343.0923.
6-Methyl-2-methylene-1-tosyl-2,3-dihydro-1H-imidazo[1,2-a]in-
dole (4l)
6-Chloro-2-methylene-1-tosyl-2,3-dihydro-1H-imidazo[1,2-a]in-
Yield: 83 mg 49%, pale yellow solid; mp 143–145 °C.
dole (4p)
Yield: 98 mg (55%); yellow solid; mp 162–164 °C.
© 2020. Thieme. All rights reserved. Synthesis 2020, 52, A–H

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H. Jin et al.
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Synthesis
1H NMR (500 MHz, DMSO-d6):  = 7.79 (d, J = 8.4 Hz, 2 H), 7.50 (d, J =
8.4 Hz, 1 H), 7.40 (d, J = 8.1 Hz, 2 H), 7.30 (d, J = 1.9 Hz, 1 H), 7.03 (dd,
J1 = 8.4 Hz, J2 = 2.0 Hz, 1 H), 6.25 (d, J = 0.7 Hz, 1 H), 5.53 (dt, J1 = 2.5
Hz, J2 = 2.5 Hz, 1 H), 4.95 (dt, J1 = 2.5 Hz, J2 = 2.0 Hz, 1 H), 4.80 (dd, J1 =
2.5 Hz, J2 = 2.5 Hz, 2 H) 2.34 (s, 3 H).
1H NMR (500 MHz, CDCl3):  = 7.72 (d, J = 8.0 Hz, 2 H), 7.20 (d, J = 8.1
Hz, 2 H), 6.91 (d, J = 5.2 Hz, 1 H), 6.73 (dd, J1 = 5.2 Hz, J2 = 0.4 Hz, 1 H),
6.34 (d, J = 0.5 Hz, 1 H), 5.66 (dt, J1 = 2.5 Hz, J2 = 2.5 Hz, 1 H), 4.78 (dt,
J1 = 2.5 Hz, J2 = 2.5 Hz, 1 H), 4.53 (dd, J1 = 2.5 Hz, J2 = 2.5 Hz, 2 H), 2.36
(s, 3 H).
13C NMR (125 MHz, DMSO-d6):  = 145.79, 142.31, 141.14, 132.57,
130.77, 130.08, 129.71, 127.18, 124.94, 121.43, 120.09, 109.54, 95.36,
82.59, 46.41, 21.06.
13C NMR (125 MHz, CDCl3):  = 145.08, 142.66, 139.37, 133.40,
131.43, 129.61, 127.62, 126.58, 121.16, 109.75, 96.65, 85.35, 47.78,
21.61;
+
HRMS (ESI): m/z calcd for C18H16ClN2O2S+ [M + H]+: 359.0616; found:
HRMS (ESI): m/z calcd for C16H15N2O2S2 [M + H]+: 331.0569; found:
359.0633.
331.0564.
2-Methylene-1-tosyl-6-(trifluoromethyl)-2,3-dihydro-1H-imid-
azo[1,2-a]indole (4q)
2-Benzyl-5-methyl-1-tosyl-1H-imidazole (4u′)
Yield: 91 mg (56%); white gum.
Yield: 141 mg (72%); orange solid; mp 156–158 °C.
1H NMR (500 MHz, CDCl3):  = 7.27–7.23 (m, 7 H), 7.13 (d, J = 8 Hz, 2
1H NMR (500 MHz, CDCl3):  = 7.76 (d, J = 8.4 Hz, 2 H), 7.59 (d, J = 8.3
Hz, 1 H), 7.31 (d, J = 8.5 Hz, 1 H), 7.29 (s, 1 H), 7.22 (d, J = 8.1 Hz, 2 H),
6.38 (d, J = 0.6 Hz, 1 H), 5.70 (dt, J1 = 2.5 Hz, J2 = 2.5 Hz, 1 H), 4.83 (dt,
J1 = 2.5 Hz, J2 = 2.5 Hz, 1 H), 4.65 (dd, J1 = 2.5 Hz, J2 = 2.5 Hz, 2 H), 2.36
(s, 3 H).
H), 6.66 (d, J = 1 Hz, 1 H), 4.43 (s, 2 H), 2.37 (s, 3 H), 2.27 (d, J = 1 Hz, 3
H).
13C NMR (125 MHz, CDCl3):  = 149.17, 145.46, 137.18, 135.76,
129.93, 129.24, 129.15, 128.46, 127.30, 127.05, 126.58, 35.83, 21.57,
12.00.
13C NMR (150 MHz, CDCl3):  = 145.57, 143.43, 141.93, 134.39,
133.55, 129.79, 129.71, 127.49, 126.02 (q, 1JC,F = 269.7 Hz), 122.73 (q,
MS (ESI): m/z for C18H18N2O2SNa+ [M + Na]+ = 349.1.
4
3
2JC,F = 31.8 Hz), 120.74, 117.08 (q, JC,F = 3.6 Hz), 105.97 (q, JC,F = 4.3
Procedure for the Stepwise Reactions
Hz), 96.12, 83.97, 46.57, 21.58.
HRMS (ESI): m/z calcd for C19H16 F3N2O2S+ [M + H]+: 393.0879; found:
393.0891.
N-(2-Bromoallyl)-2-(2-bromophenyl)-N′-tosylacetimidamide (5a)
In a 10 mL flame-dried Schlenk tube, anhyd DMSO (3 mL), 2-bro-
mophenylacetylene (1a; 90 mg, 0.5 mmol), sulfonyl azide (2a; 118
mg, 0.6 mmol), 2-bromoallylamine (3a; 68 mg, 0.5 mmol), and CuI
(9.5 mg, 0.05 mmol) were added sequentially under N2. After stirring
for 5 min, Et3N (50 mg, 0.5 mmol) was added and the mixture was
stirred at r.t. for 1 h. The reaction mixture was extracted with CH2Cl2
and the combined CH2Cl2 layers were washed with saturated NH4Cl
solution. The organic phase was evaporated and the residue was puri-
fied by silica gel column chromatography (EtOAc/PE 1:5) to provide
the product 5a; yield: 221 mg (91%).
7-Fluoro-2-methylene-1-tosyl-2,3-dihydro-1H-imidazo[1,2-a]in-
dole (4r)
Yield: 103 mg (60%); brown solid; mp 147–149 °C.
1H NMR (500 MHz, CDCl3):  = 7.76 (d, J = 8.4 Hz, 2 H), 7.22 (d, J = 8.1
Hz, 2 H), 7.19 (dd, J1 = 9.7 Hz, J2 = 2.4 Hz, 1 H), 6.91 (dd, J1 = 8.7 Hz, J2 =
4.3 Hz, 1 H), 6.81–6.77 (m, 1 H), 6.29 (d, J = 0.5 Hz, 1 H), 5.66 (dt, J1=
2.5 Hz, J2 = 2.5 Hz, 1 H), 4.79 (dt, J1 = 2.5 Hz, J2 = 2.5 Hz, 1 H), 4.59 (dd,
J1 = 2.5 Hz, J2 = 2.5 Hz, 2 H), 2.37 (s, 3 H).
1H NMR (500 MHz, CDCl3):  = 7.87 (d, J = 8.5 Hz, 2 H), 7.63 (d, J = 8.0
Hz, 1 H), 7.38–7.34 (m, 2 H), 7.29 (d, J = 8.0 Hz, 2 H), 7.26–7.22 (m, 1
H), 5.68 (dt, J1 =2.5 Hz, J2 = 1.1 Hz, 1 H), 5.48 (d, J = 2.5 Hz, 1 H), 5.43 (s,
1 H), 4.45 (s, 2 H), 4.10 (d, J = 6 Hz, 2 H), 2.42 (s, 3 H).
1
13C NMR (125 MHz, CDCl3):  = 158.33 (d, JC,F = 233.4 Hz), 145.39,
3
142.51, 142.31, 133.54, 132.29 (d, JC,F =10.4 Hz), 129.72, 127.52,
3
2
127.32, 108.99 (d, JC,F = 9.8 Hz), 108.56 (d, JC,F = 9.8 Hz), 106.12 (d,
2JC,F = 24.3 Hz), 95.76, 83.87 (d, 4JC,F = 4.3 Hz), 46.67, 21.62.
13C NMR (125 MHz, CDCl3):  = 164.89, 142.47, 140.25, 133.67,
133.10, 132.40, 130.25, 129.21, 128.52, 126.88, 126.61, 125.48,
119.36, 49.44, 40.19, 21.51.
HRMS (ESI): m/z calcd for C18H16FN2O2S+ [M + H]+: 343.0911; found:
343.0925.
7-Chloro-2-methylene-1-tosyl-2,3-dihydro-1H-imidazo[1,2-a]in-
MS (ESI): m/z for C18H18Br2N2O2SNa+ [M + Na]+ = 507.0, 509.0, 511.0.
dole (4s)
Under N2, 5a (146 mg, 0.3 mmol), CuI (11 mg, 0.06 mmol), K2CO3 (83
mg, 0.6 mmol), dimethylethylenediamine (10 mg, 0.12 mmol), and
DMSO (2 mL) were added to a dried Schlenk tube. The tube was
sealed and stirred at 80 °C for 6 h. After completion of the reaction,
the mixture was extracted with EtOAc (10 mL) and the organic layer
was washed with sat. aq NH4Cl (5 mL). The organic phase was concen-
trated and the residue was purified by silica gel column chromatogra-
phy to provide the product 4a; yield: 65 mg (67%).
Yield: 86 mg (48%); brown solid; mp 172–174 °C.
1H NMR (500 MHz, DMSO-d6):  = 7.78 (d, J = 8.4 Hz, 2 H), 7.55 (d, J =
2.0 Hz, 1 H), 7.40 (d, J = 8.1 Hz, 2 H), 7.18 (d, J = 8.5 Hz, 1 H), 7.04 (dd,
J1 = 8.5 Hz, J2 = 2.1 Hz, 1 H), 6.22 (d, J = 0.5 Hz, 1 H), 5.52 (dt, J1 = 2.5
Hz, J2 = 2.5 Hz, 1 H), 4.94 (dt, J1 = 2.5 Hz, J2 = 2.5 Hz, 1 H), 4.82 (dd, J1 =
2.5 Hz, J2 = 2.5 Hz, 2 H), 2.34 (s, 3 H).
13C NMR (150 MHz, acetone-d6):  = 146.72, 143.97, 143.44, 134.59,
133.82, 130.75, 130.36, 128.40, 126.26, 121.32, 120.52, 111.21, 96.17,
83.77, 47.41, 21.49.
Funding Information
HRMS (ESI): m/z calcd for C18H16ClN2O2S+ [M + H]+: 359.0616; found:
359.0606.
This work was supported by the National Science Foundation of China
(Grant / Award Number: 'Nos. 21772176 and 21372201').
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6-Methylene-7-tosyl-6,7-dihydro-5H-thieno[2′,3′:4,5]pyrrolo[1,2-
a]imidazole (4t)
Yield: 40 mg (24%); brown solid; mp 127–130 °C.
© 2020. Thieme. All rights reserved. Synthesis 2020, 52, A–H

G
H. Jin et al.
Paper
Synthesis
Supporting Information
Grenda, K.; Jimeno, M. J.; Chiara, J. L. Adv. Synth. Catal. 2013, 355,
913. (k) Mahendran, V.; Shanmugam, S. RSC Adv. 2015, 5, 20003.
(l) Choi, W.; Kim, J.; Ryu, T.; Kim, K.-B.; Lee, P. H. Org. Lett. 2015,
17, 3330. (m) Murugavel, G.; Punniyamurthy, T. J. Org. Chem.
2015, 80, 6291.
Supporting information for this article is available online at
https://doi.org/10.1055/s-0037-1610739.
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n
(13) Selected examples for O-containing nucleophiles, see: (a) Cho,
S. H.; Yoo, E. J.; Bae, I.; Chang, S. J. Am. Chem. Soc. 2005, 127,
16046. (b) Cassidy, M. P.; Raushel, J.; Fokin, V. V. Angew. Chem.
Int. Ed. 2006, 45, 3154. (c) Cui, S.; Lin, X.; Wang, Y. Org. Lett.
2006, 8, 4517. (d) Cho, S. H.; Chang, S. Angew. Chem. Int. Ed.
2007, 46, 1897. (e) Yoo, E. J.; Park, S. H.; Lee, S. H.; Chang, S. Org.
Lett. 2009, 11, 1155. (f) Song, W.; Lei, M.; Shen, Y.; Cai, S.; Lu,
W.; Lu, P.; Wang, Y. Adv. Synth. Catal. 2010, 352, 2432. (g) Liu, Y.;
Wang, X.; Xu, J.; Zhang, Q.; Zhao, Y.; Hu, Y. Tetrahedron 2011, 67,
6294. (h) Li, S.; Wu, J. Chem. Commun. 2012, 48, 8973.
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