A Highly Efficient Copper(I)-Catalyzed Cascade Reaction of o-Alkenylphenyl Isothiocyanates with Isocyanides Leading to 5H-Benzo[d]imidazo[5,1-b][1,3]thiazines

Article abstract of DOI:10.1002/ejoc.201500800

The highly efficient copper(I)-catalyzed cascade reaction of (E)-3-(2-isothiocyanatophenyl)acrylates and (E)-3-(2-isothiocyanatophenyl)acrylonitriles with isocyanides was explored. The method provides an expedient route for the synthesis of 5H-benzo[d]imidazo[5,1-b][1,3]thiazines in good to excellent yields by using CuCl (10 mol-%) as the catalyst and K₃PO₄ (2.0 equiv.) as the base in THF at 70 C. The reaction involves [3+2] cycloaddition and subsequent intramolecular C-S bond formation.

Full text of DOI:10.1002/ejoc.201500800

FULL PAPER
DOI: 10.1002/ejoc.201500800
A Highly Efficient Copper(I)-Catalyzed Cascade Reaction of o-Alkenylphenyl
Isothiocyanates with Isocyanides Leading to 5H-Benzo[d]imidazo[5,1-b]-
[1,3]thiazines
Wenyan Hao,*[a] Jian Huang,[a] Shanshan Jie,[a] and Mingzhong Cai*[a]
Keywords: Domino reactions / Isothiocyanates / Isocyanides / Copper / Nitrogen heterocycles / Sulfur heterocycles
The highly efficient copper(I)-catalyzed cascade reaction of
(E)-3-(2-isothiocyanatophenyl)acrylates and (E)-3-(2-iso-
thiocyanatophenyl)acrylonitriles with isocyanides was ex-
plored. The method provides an expedient route for the syn-
thesis of 5H-benzo[d]imidazo[5,1-b][1,3]thiazines in good to
excellent yields by using CuCl (10 mol-%) as the catalyst and
K3PO4 (2.0 equiv.) as the base in THF at 70 °C. The reaction
involves [3+2] cycloaddition and subsequent intramolecular
C–S bond formation.
Introduction
Nitrogen- and sulfur-containing heterocycles are omni-
present substructures in a large number of biologically
active natural products and pharmaceuticals.[1] Compounds
containing the imidazobenzothiazine core structure have at-
tracted considerable attention in materials, medicinal, and
pharmaceutical sciences owing to their important biological
activities and structural advantages. For example, imid-
azo[2,1-c][1,4]benzothiazine-1-acetamide exhibits affinity
for the omega modulatory sites of GABAA receptors (Fig-
ure 1, 1).[2] 4H-Imidazo[2,1-c][1,4]benzothiazine derivatives
display potential anti-inflammatory properties and are used
as analgesic drugs.[3] Methyl 2-carbamate derivatives of im-
idazobenzothiazines have been reported to possess antihel-
minthic activity.[4] Besides, substituted 5H-benzo[d]imid-
azo[2,1-b][1,3]thiazine (Figure 1, 2) possesses antimicrobial
activity,[5] and polymer-containing motif 3 was found to be
a silver halide color photographic photosensitive material.[6]
Therefore, the synthesis of libraries of such compounds
should be interesting.
However, only a few practical methods have been devel-
oped for the synthesis of these imidazobenzothiazines.[7]
Gauthier and Duceppe explored a three-step condensation
reaction of 2-aminobenzhydrol with thiourea for the con-
struction of 5H-benzo[d]imidazo[2,1-b][1,3]thiazines.[7c] Bao
and co-workers disclosed the efficient synthesis of 5H-
benzo[d]imidazo[2,1-b][1,3]thiazines from readily available
2-mercaptoimidazoles (thiouracils) and bromobenzyl brom-
Figure 1. Examples of bioactive imidazobenzothiazine derivatives.
ides.[7g] Although these synthetic routes work, there are
many inherent drawbacks such as narrow substrate scope,
the use of strong bases, and tedious multistep manipula-
tions. Therefore, the development of concise and efficient
methods for the synthesis of these heterocyclic skeletons is
still in high demand. In continuation of our endeavors to
construct biologically relevant, heterocyclic, small mol-
ecules through tandem synthetic strategies,[8] we herein wish
to introduce our recent efforts for the generation of 5H-
benzo[d]imidazo[5,1-b][1,3]thiazines through a copper(I)-
catalyzed cascade reaction of o-alkenylphenyl isothiocyan-
ates with isocyanides.
As a useful C1 building block, isocyanides have been
widely used in different branches of organic, inorganic, co-
ordination, polymeric, combinatorial, and medicinal chem-
istry.[9] Apart from the traditional multicomponent reac-
tions,[10] isocyanide-based cycloaddition reactions occupy
an important place in the field of synthetic applications and
reaction diversity, which makes them very attractive objects
for investigation.[11,12] Recently, we reported a tandem reac-
tion of o-alkynylphenyl isothiocyanates with isocyanides to
[a] College of Chemistry & Chemical Engineering, Jiangxi Normal
University,
Nanchang 330022, People’s Republic of China
E-mail: wenyanhao@jxnu.edu.cn
mzcai@jxnu.edu.cn
http://chem.jxnu.edu.cn/
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201500800.
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Scheme 1. Our strategy (working hypothesis).
obtain 5H-benzo[d]imidazo[5,1-b][1,3]thiazine derivatives Table 1. Preparation of o-alkenylphenyl isothiocyanates 1.[a]
(Scheme 1A).[8c] On the other hand, (E)-3-(2-isothiocyan-
Entry R1
R2
R3
Product
Yield[b] [%]
atophenyl)acrylates are versatile organic synthons for the
synthesis of various N,S-containing heterocycles, which was
reported by Kobayashi and co-workers.[13] Given that there
are both a Michael acceptor and a N=C=S moiety in the
structure of (E)-3-(2-isothiocyanatophenyl)acrylates, we
conceived that they might be good partners in reactions
with 2-isocyanoacetate. 5H-Benzo[d]imidazo[5,1-b][1,3]-
thiazines would be obtained if the reaction of (E)-3-(2-iso-
thiocyanatophenyl)acrylates with 2-isocyanoacetate pro-
ceeded smoothly as expected (Scheme 1B). To validate the
feasibility of this hypothesis, we started to explore the pos-
sibility of this transformation.
1
2
H
H
H
H
H
H
H
H
H
H
H
Cl
CH3
H
H
H
H
H
COOMe
COOEt
COOnBu
COOiBu
COOtBu
COOMe
COOMe
COOMe
COOMe
COOMe
COOMe
CN
1a
1b
1c
1d
1e
1f
1g
1h
1i
1j
1k
1l
1m
1n
1o
1p
97
90
95
98
90
95
90
90
85
80
75
96
92
94
95
83
3
4
H
H
5
6
H
F
7
8
9
10
11
12
13
14
15
16
Cl
Br
CH3
H
H
H
F
CN
CN
CN
CN
Cl
Br
CH3
Results and Discussion
[a] CAUTION: Thiophosgene is toxic. [b] Yield of isolated product.
Starting o-alkenylphenyl isothiocyanates 1 were easily
obtained from 3-(2-aminophenyl)acrylates or 3-(2-ami-
nophenyl)acrylonitriles, which were prepared from com-
mercially available o-iodoaniline with acrylates or acrylo-
nitrile, by Heck coupling according to a literature pro-
cedure[14] involving reaction with thiophosgene in good
yields (Scheme 2); the results are summarized in Table 1.
isolated in 82% yield (Table 2, entry 1). The structure of 3a
was further confirmed by X-ray diffraction analysis (see the
Supporting Information). Other copper catalysts were scre-
ened, and CuCl was found to be the best choice (Table 2,
entries 1–4). A blank experiment indicated that the copper
catalyst was necessary, because desired product 3a was ob-
tained in only 40% yield in the absence of a copper catalyst
(Table 2, entry 5). We next examined the effect of base on
the reaction. The yield was improved to 94% if K3PO4 was
employed,
whereas
1,8-diazabicyclo[5.4.0]undec-7-ene
(DBU) and 1,4-diazabicyclo[2.2.2]octane (DABCO) failed
to promote the reaction; other bases such as Na2CO3,
NaHCO3, and tBuOK were substantially less effective
(Table 2, entries 6–11). The reaction did not proceed in the
Scheme 2. Preparation of o-alkenylphenyl isothiocyanates 1 from
3-(2-aminophenyl)acrylates or 3-(2-aminophenyl)acrylonitriles.
To explore the feasibility of the hypothesis and to de- absence of a base (Table 2, entry 12), which indicates that
velop a new synthetic method for the target derivatives, we the combination of a copper catalyst and a base is indis-
used methyl (E)-3-(2-isothiocyanatophenyl)acrylate (1a) pensable to afford the target product in high yield. The sol-
and ethyl 2-isocyanoacetate (2a) as model substrates to op- vent of the reaction was also screened. No improved yield
timize the conditions; the results are summarized in Table 2. was observed upon using CH3CN, toluene, DMF, DMSO,
Initially, the reaction was performed in the presence of CuI and dioxane as solvents (Table 2, entries 13–17). Finally, we
(10 mol-%) and Cs2CO3 (2.0 equiv.) in anhydrous THF at examined the effect of temperature on the reaction and
70 °C. To our delight, desired product 3a was successfully found that 70 °C gave the best result (Table 2, entries 6 and
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Fused Heterocycles by CuI-Catalyzed Cascade Reaction
18–20). Therefore, the optimized conditions include the use ylbenzene (2c) could act as partners under the optimized
of a combination of CuCl (10 mol-%) as the catalyst and conditions. Corresponding products 3l–o were obtained in
K3PO4 (2.0 equiv.) as the base in THF at 70 °C (Table 2, moderate to good yields.
entry 6).
The structure of 5H-benzo[d]imidazo[5,1-b][1,3]thiazine
was corroborated by X-ray diffraction analysis of the crys-
tal structure of 3a, the ORTEP diagram of which is dis-
played in Figure 2.
Table 2. Initial studies for the tandem reaction of methyl (E)-3-
(2-isothiocyanatophenyl)acrylate (1a) with ethyl 2-isocyanoacetate
(2a).[a]
Entry Copper
catalyst
Base
Solvent
Temp.
[°C]
Yield[b]
[%]
1
2
3
4
5
6
7
8
CuI
Cs2CO3
Cs2CO3
Cs2CO3
THF
THF
THF
70
70
70
70
70
70
70
70
70
70
70
70
80
80
80
80
80
r.t.
40
60
82
85
91
CuBr
CuCl
CuO
–
Cs2CO3[c] THF
complex
40
Cs2CO3
K3PO4
THF
THF
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
94
57
67
n.r.
trace
37
n.r.
35
80
55
trace
82
87
90
92
NaHCO3 THF
Na2CO3 THF
DBU
DABCO THF
tBuOK
–
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
9
THF
Figure 2. Single-crystal X-ray diffraction structure of 3a; the ther-
mal ellipsoids are at the 30% probability level.
10
11
12
13
14
15
16
17
18
19
20
THF
THF
CH3CN
toluene
DMF
DMSO
dioxane
THF
Encouraged by the above results, we next investigated the
reaction of (E)-3-(2-isothiocyanatophenyl)acrylonitriles
with isocyanides. As shown in Table 4, the reactions pro-
ceeded smoothly under the optimized conditions to afford
desired products 3p–v in moderate to high yields. For exam-
ple, the reaction of (E)-3-(2-isothiocyanatophenyl)acrylo-
nitrile with ethyl 2-isocyanoacetate afforded desired product
3p in 90% yield. Upon changing R2 to CO2Me or tosyl
(Ts), the reaction also occurred smoothly to generate corre-
sponding products 3q and 3r in yields of 84 and 61%,
THF
THF
[a] Reaction conditions: 1a (0.2 mmol), 2a (0.4 mmol), base
(0.4 mmol), and copper catalyst (10 mol-%) in solvent (2 mL) for
3 h. [b] Yield of isolated product based on 1a; n.r.: no reaction.
[c] K3PO4 (0.4 mmol) was used instead of Cs2CO3 for 12 h, which
provided 5H-benzo[d]imidazo[5,1-b][1,3]thiazine in 55% yield.
respectively.
(E)-3-(2-Isothiocyanatophenyl)acrylonitriles
with various substituents attached to the benzene ring were
With the optimal conditions established, we next ex- also tested. For instance, the reactions of (E)-3-(2-isothio-
plored the substrate scope with different (E)-3-(2-isothio- cyanatophenyl)acrylonitriles bearing fluoro, chloro, bromo,
cyanatophenyl)acrylates and isocyanides. The results are and methyl substituents in the benzene ring, regardless of
summarized in Table 3. The reactions of substrates 1 bear- their electronic properties, gave desired products 3s–v in
ing different ester groups such as CO2Me, CO2Et, CO2nBu, good yields.
CO2iBu, and CO2tBu with ethyl 2-isocyanoacetate (2a) pro-
On the basis of our experimental results and some related
ceeded smoothly to give expected products 3a–e in excellent literature,[15] a possible route for the formation of 5H-
yields. Substituents on the benzene ring of the (E)-3-(2-iso- benzo[d]imidazo[5,1-b][1,3]thiazines 3 through the copper-
thiocyanatophenyl)acrylates did not hamper the reaction catalyzed reaction of 1 with 2-isocyanoacetates 2 is outlined
process. The reactions of fluoro-, chloro-, bromo-, and in Scheme 3. We reason that in the presence of a base, the
methyl-substituted (E)-3-(2-isothiocyanatophenyl)acrylates, formal [3+2] cycloaddition reaction of 2-isocyanoacetate 2
regardless of their electronic properties and substitution po- with the isothiocyanate moiety in compound 1 could occur
sitions, with ethyl 2-isocyanoacetate (2a) all worked well to to produce intermediate A, which undergoes subsequent
afford desired products 3f–k in good to excellent yields. Not isomerization to afford intermediate B. Then, in the pres-
only ethyl 2-isocyanoacetate (2a) but also methyl 2-iso- ence of a copper catalyst, intramolecular nucleophilic cycli-
cyanoacetate (2b) and 1-isocyanomethanesulfonyl-4-meth- zation of intermediate B would occur to give intermediate
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W. Hao, J. Huang, S. Jie, M. Cai
FULL PAPER
Table 3. Synthesis of 5H-benzo[d]imidazo[5,1-b][1,3]thiazines through the copper-promoted tandem reaction of (E)-3-(2-isothiocyanato-
phenyl)acrylates 1 with isocyanides 2.[a]
[a] Reaction conditions: (E)-3-(2-isothiocyanatohenyl)acrylate (1) (0.2 mmol), isocyanide 2 (0.4 mmol), K3PO4 (0.4 mmol), and CuCl
(10 mol-%) in THF (2 mL) at reflux for 3 h.
Table 4. Synthesis of 5H-benzo[d]imidazo[5,1-b][1,3]thiazines through the copper-promoted tandem reaction of (E)-3-(2-isothiocyanato-
phenyl)acrylonitriles 1 with isocyanides 2.[a]
[a] Reaction conditions: (E)-3-(2-Isothiocyanatophenyl)acrylonitrile (1) (0.2 mmol), isocyanide 2 (0.4 mmol), K3PO4 (0.4 mmol), and CuCl
(10 mol-%) in THF (2 mL) at reflux for 3 h.
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Fused Heterocycles by CuI-Catalyzed Cascade Reaction
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Scheme 3. Proposed synthetic route for the generation of 5H-
benzo[d]imidazo[5,1-b][1,3]thiazines through the reaction of (E)-3-
(2-isothiocyanatophenyl)acrylates with 2-isocyanoacetates.
Conclusions
In summary, we developed a highly efficient method for
the synthesis of 5H-benzo[d]imidazo[5,1-b][1,3]thiazines
through the copper(I)-catalyzed cascade reaction of (E)-3-
(2-isothiocyanatophenyl)acrylates and (E)-3-(2-isothiocyan-
atophenyl)acrylonitriles with isocyanides under mild condi-
tions. In this transformation, a [3+2] cycloaddition reaction
mechanism was involved and two new rings were formed.
The tandem addition–cyclization reaction represents a fac-
ile route for the generation of small molecular N,S-hetero-
cycles.
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[11]
Experimental Section
General Procedure for the Synthesis of 5H-Benzo[d]imidazo[5,1-b]-
[1,3]thiazines 3a–v: A mixture of (E)-3-(2-isothiocyanatophenyl)-
acrylate
or
(E)-3-(2-isothiocyanatophenyl)acrylonitrile
(1)
(0.2 mmol), isocyanide 2 (0.4 mmol), CuCl (0.1 equiv.), and K3PO4
(0.4 mmol) in THF (2.0 mL) was stirred at reflux for 3 h. Upon
completion of reaction as indicated by TLC, the mixture was con-
centrated and directly purified by flash column chromatography
(EtOAc/petroleum ether, 1:2) to give desired product 3.
CCDC-1028706 (for 3a) contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgments
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Financial support from the National Natural Science Foundation
of China (NSFC) (grant number 21462021) is gratefully acknowl-
edged.
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Received: June 16, 2015
Published Online: September 23, 2015
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