Carbon tetrabromide mediated oxidative cyclocondensation of ketones and thioureas: An easy access to 2-aminothiazoles

Article abstract of DOI:10.1016/j.tetlet.2015.08.064

A simple, mild, and efficient one-pot method for the synthesis of substituted 2-aminothiazoles has been reported. The reaction involves the formation of sulfenyl bromide as an umpolung intermediate of nucleophilic sulfur, which is responsible for C-S bond formation leading to oxidative cyclization of ketones and thioureas to furnish the desired products. Carbon tetrabromide was used as a convenient and mild brominating reagent under basic condition at room temperature to give 2-aminothiazoles in good to excellent yields.

Full text of DOI:10.1016/j.tetlet.2015.08.064

Accepted Manuscript
Carbon tetrabromide mediated oxidative cyclocondensation of ketones and thi-
oureas: an easy access to 2-aminothiazoles
Twinkle Keshari, Ritu Kapoorr, Lal Dhar S. Yadav
PII:
S0040-4039(15)30018-6
DOI:
http://dx.doi.org/10.1016/j.tetlet.2015.08.064
TETL 46648
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
2 July 2015
20 August 2015
22 August 2015
Please cite this article as: Keshari, T., Kapoorr, R., Yadav, L.D.S., Carbon tetrabromide mediated oxidative
cyclocondensation of ketones and thioureas: an easy access to 2-aminothiazoles, Tetrahedron Letters (2015), doi:
http://dx.doi.org/10.1016/j.tetlet.2015.08.064
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Graphical Abstract
Carbon tetrabromide mediated oxidative
cyclocondensation of ketones and thioureas: an easy
access to 2-aminothiazoles
Leave this area blank for abstract info.
Twinkle Keshari, Ritu Kapoorr, Lal Dhar S. Yadav*
R2
R3
O
C
CBr4, Et3N
CH3CN, rt, 2-6 h
S
N
R2
H2N
N
N
R
R3
S
R1
23 examples
67-94% yields

1
Tetrahedron Letters
Carbon tetrabromide mediated oxidative cyclocondensation of ketones and thioureas: an easy access
to 2-aminothiazoles
Twinkle Keshari, Ritu Kapoorr, Lal Dhar S. Yadav*
Green Synthesis Lab, Department of Chemistry, University of Allahabad, Allahabad 211002, India
 Corresponding author. Tel.: +91 532 2500652; fax: +91 532 2460533; E-mail address: ldsyadav@hotmail.com (L.D.S. Yadav)
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
A simple, mild and efficient one-pot method for the synthesis of substituted 2-aminothiazoles has
been reported. The reaction involves the formation of sulfenyl bromide as an umpolung
intermediate of nucleophilic sulfur, which is responsible for C-S bond formation leading to
oxidative cyclization of ketones and thioureas to furnish the desired products. Carbon tetrabromide
was used as a convenient and mild brominating reagent under basic condition at room temperature
to give 2-aminothiazoles in good to excellent yields.
Available online
Keywords:
2009 Elsevier Ltd. All rights reserved.
Aminothiazoles
Carbon tetrabromide
Ketones
Thioureas
Umpolung
The thiazole ring constitutes
a broad spectrum of
in our protocol. Thus, keeping the above points in view and our
ongoing efforts for devising practical synthesis of bioactive
heterocyclic compounds,27 we envisaged a novel, efficient
protocol for the synthesis of 2-aminothiazoles from simple and
readily available ketones and thioureas in the presence of a base,
using CBr4 as a promoter in a one-pot procedure (Scheme 1).
compounds useful in the field of agriculture and medicinal
chemistry.1,2 Moreover, aminothiazole heterocycles have been
explored as building blocks in organic synthesis and material
science.3 Aminothiazoles are also a privileged structural motif in
numerous biologically active molecules.4 They have been
employed for the preparation of different important drugs
required for the treatment of tuberculosis,5a inflammations,5a,6,7
allergies,8 hypertension,9 schizophrenia,10 bacterial and HIV
infections5a,11-13 and human lymphatic filarial parasite (Fig. 1).14
In addition, aminothiazoles are also known to be ligands of
estrogen receptors as well as of adenosine receptor antagonists.15
Analogs of aminothiazole have also agricultural importance for
being used as fungicides.16
S
S
CF3
R
O
O
N
NH
NH
Anti-HIV
N
N-(2-phenethyl)-N'-C2-thiazolyl)thiourea5d
Br
Antitubercular5c
H2N
S
2-aminothiazole-4-yl)methyl-2
-(3-trifluoromethyl)phenylacetate
In view of widespread biological applications and
physiochemical properties of 2-aminothiazole and its derivatives,
several synthetic protocols have been developed for their
synthesis. The most popular protocol involves cyclocondensation
of halo-carbonyl compounds with thioureas or thioamides called
Hantzsch thiazole synthesis.17 Other reported methods include
synthesis using various catalysts such as silica chloride,18
Cl
F
S
N
N
H
Cl
Anti-inflammatory
2-substituted-4-(2,4-dichloro-5-fluorophenyl)thiazole5e
iodine,19
1,3-di-n-butylimidazolium
tetrafluoroborate,20
Figure 1. Some drugs containing 2-aminothiazole moiety.
cyclodextrin, ammonium 1,2-molybdophosphate,21,22 ionic
liquid23 and NaICl2.24 Recently, Wang and coworkers obtained 2-
aminothiazoles by KI/NH4NO3 catalysed aerobic oxidative
cyclization of ketones and thioureas in ionic liquid.23
Furthermore, Telvekar et. al. have also developed a synthetic
method for 2-aminothiazoles using aqueous NaICl2.24
In order to investigate our envisaged protocol, we commenced
with the study of a model reaction by stirring a reaction mixture
of ketone 1a (1.0 mmol), thiourea 2a (1.0 mmol), CBr4 (1.0
mmol) and triethylamine (1 equiv) as a base in CH3CN (3 mL) at
rt for 2 h. To our delight, 2-aminothiazole 3a was obtained in an
excellent yield of 89% (Table 1, entry 1). Encouraged by this
result, we focused our effort to know the minimum time required
for completion of the reaction and it was found to be two hours.
With this gratifying result in our hand, we proceeded to ensure
the necessity of the reaction parameters. A series of control
experiments were performed, which revealed that there was no
Carbon tetrabromide is a commercially available and cheap
reagent, which has found various applications in organic
transformations.25 One of the most impressive and current
applications of CBr4 is its use as an efficient reagent for the
umpolung activity of sulfur species.26
Inspired by these
results, we attributed CBr4 as an efficient thiophilic mediator

2
Tetrahedron Letters
Different bases such as Et3N, DABCO, DBU and iPr2NEt were
tested of which triethylamine proved to be the best (Table 1,
entry 1 vs 12-14). CH3CN was found the most suitable solvent
among CH3CN, THF, DMF, CH2Cl2 and C2H5OH (Table 1, entry
1 vs 4-7).
Recent reports
NH2
S
O
S
aq. NaICl2 (0.5 equiv)
N
C
H2N
NH2
R
reflux, THF, 12 h
ref. 24
1.
R
With the established reaction conditions in hand, scope of the
reaction was investigated for the synthesis of 2-aminothiazoles as
depicted in Table 2. The generality of protocol was evaluated
across a wide variety of ketones and it was found that
phenylethanones with an electron-donating substituent were more
efficient as compared to those with an electron- withdrawing
substituent on the phenyl ring (Table 2, entries 2-6 vs 7-9).
Moreover, heterocyclic and aliphatic ketones were also well
compatible with the present reaction conditions (Table 2, entries
11-14).
O2 balloon, KI (20 mol %)
NH4NO3 (20 mol %)
H2SO4 (1 equiv)
R2
N
R3
S
R1
R2
N
2.
O
H2N
S
N
R1
[Bmim]OTf/H2O (4:1)
40 oC, 4 h
R3
ref. 23
Present work
R2
N
R3
S
O
CBr4 (1 equiv)
R2
R3
N
Further studies on various thioureas were carried out for the
cyclization reaction with ketones and it was noted that alkyl
thioureas like 1-butylthiourea and 1-benzylthiourea were
successful to give target compounds (Table 3, entries 1 and 2).
Aryl thioureas containing an electron-donating or an electron-
withdrawing substituent on the phenyl ring were also suitable for
the present method. However, an electron-withdrawing
substituent slightly decreased the yield (Table 3, entries 5 and 6).
N, N-Disubstituted thioureas such as morpholine-4-
carbothioamide, piperidine-1-carbothioamide and 1-methyl-1-
phenylthiourea also underwent efficient cyclocondensation to
afford 2-aminothiazoles in excellent yields (Table 3, entries 7-9).
When phenylethanone was reacted with formamidine disulfide 4
(a dimer of thiourea), the desired product 3 was not obtained
under the optimal reaction conditions (Scheme 2). This ruled out
the possibility of formation of the dimer of thiourea and
subsequent nucleophilic attack of enol form of ketone to give the
product 3a28
C
H2N
N
R1
S
CH3CN, Et3N, rt, 2-6 h
R1
Scheme 1. Transition metal free synthesis of 2-aminothiazoles.
product formation in the absence of a bromine source or a base
(Table 1, entry 1 vs 2 and 3), which signifies the necessity of a
base and a reaction mediator.
Table 1
Optimization of reaction conditionsa
NH2
N
S
bromine source
S
O
solvent, base, rt,
time
H2N
NH2
1a
2a
3a
Entry
Bromine
source
(equiv)
CBr4 (1.0)
CBr4 (1.0)
-
CBr4 (1.0)
CBr4 (1.0)
CBr4 (1.0)
CBr4 (1.0)
CBr4 (0.5)
CBr4 (1.5)
TBABd (1.0)
NBS (1.0)
CBr4 (1.0)
CBr4 (1.0)
CBr4 (1.0)
Base
Time Yieldb
Solvent
(h)
(%)
NH
NH2
O
standard conditions
H2N
S
S
1
2
3
4
5
6
7
8
Et3N
-
CH3CN
CH3CN
CH3CN
THF
2
6
6
6
6
6
6
2
2
6
6
6
6
6
89
n.r.c
n.r.
20
30
55
15
68
89
30
35
18
20
30
No product
NH
3a
4
1a
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
DMF
Scheme 2. Reaction with the dimer of thiourea.
CH2Cl2
C2H5OH
CH3CN
CH3CN
CH3CN
CH3CN
On the basis of the above observations and the literature
reports,26,28,29,30 a plausible mechanism for the formation of 2-
aminothiazoles 3 is depicted in Scheme 3. Nucleophilic attack of
the sulfur of thiourea (1′) on CBr4 results in the formation of
sulfur-centred electrophilic species 4.29,30 Then, enol form of 2’of
the ketone 2 attacks the electrophilic sulfur to give 5, which
undergoes cyclocondensation to give the desired product 3.
Bromoform was detected as a by-product in the reaction, which
also supports the proposed mechanism. It is noteworthy that CBr4
promoted formation of 4 with the umpolung reactivity of
nucleophilic sulfur is the key step in the present
heterocyclization.
9
10
11
12
13
14
DABCO CH3CN
DBU
iPr2NEt
CH3CN
CH3CN
a Reaction conditions: 1a (1.0 mmol), 2a (1.0 mmol), base (1.0 equiv),
bromine source (1.0 mmol) in a solvent (3 mL) were stirred at rt.
b Isolated yield of the product 3a,
c n.r. = no reaction.
d TBAB = tetrabutylammonium bromide
In summary, we have developed a new and convenient route
for the synthesis of 2-aminothiozoles from ketones and thioureas
in a one-pot procedure. The reaction proceeds through carbon
tetrabromide promoted in situ generation of electrophilic sulfenyl
bromide, which has an umpolung activity of the nucleophilic
sulfur. The method is metal-free, highly efficient and works in
operationally simple manner at room temperature. Thus, it offers
a superior alternative to the existing methods for the synthesis of
2-aminothiazoles.
Next, we optimized the reaction conditions with respect to the
bromine source and base and the results are summarized in Table
1. Among CBr4, TBAB and NBS, CBr4 was found to act as the
best bromine source in terms of the reaction time and yield
(Table 1, entry 1 vs 10 and 11). The yield of 3a was considerably
reduced when the loading of CBr4 was decreased from 1 equiv to
0.50 equiv (Table 1, entry 8). Moreover, the use of 1.5 equiv of
CBr4 instead of 1 equiv did not affect the yield (Table 1, entry 9).

3
Table 2
Scope of ketonesa
S
NH2
S
R1
O
CBr4 (1 equiv)
N
H2N
NH2
CH3CN, Et3N, rt
R1
1
3a-3n
2a
Entry Ketone 1
Product 3
Time Yield
(h)
Entry
8
Ketone 1
Product 3
Time
(h)
Yield
(%)b,c
(%)b,c
NH2
S
O
NH2
S
O
N
N
1
2
3
2
89
4
6
3
80
67
84
Br
1a
1h
Br
3h
3a
NH2
O
NH2
O
N
N
S
S
3
2
91
94
9
O2N
1b
1i
O2N
3i
3b
NH2
S
O
O
NH2
S
N
N
10
O
1c
1j
O
3j
3c
NH2
O
NH2
S
O
O
N
O
N
S
4
5
2
93
92
11
12
3
85
O
1k
O
1d
3d
3k
3l
O
NH2
S
NH2
S
O
N
N
2
4
3
3
86
85
1l
1e
3e
NH2
O
O
NH2
N
N
S
S
6
7
90
75
13
14
1m
Ph
3m
1f
Ph
3f
NH2
S
O
NH2
Cl
O
N
N
S
Cl
4
82
1n
6
3n
F
F
1g
3g
a For the experimental procedure, see Ref. 31
.b Isolated yield of the products 3.
c All compounds are known and were characterized by comparison of their 1H NMR, 13C NMR and MS spectra with those reported in
theliterature.23,24
Table 3
Scope of thioureasa
R1
N
R2
S
O
R1
CBr4 (1 equiv)
N
H2N
N
S
CH3CN, Et3N, rt
R2
1a
2
3a-3aj

4
Entry
Tetrahedron Letters
Thiourea 2
Product 3
Time Yield Entry
Thiourea 2
Product 3
Time Yield
(h)
(h)
(%)b,c
(%)b,c
F
HN
S
n-Bu
S
HN
S
N
S
F
1
2
3
90
6
7
6
75
N
H2N
NH
n-Bu
H2N
N
H
2b
2g
3ag
3ab
S
S
Ph
HN
S
3
3
88
93
2
2
93
92
H2N
N
H2N
NH
N
S
N
2c
N
Ph
2h
3ac
3ah
S
S
O
HN
3
8
9
H2N
N
H2N
N
N
S
H
N
O
S
N
2d
2i
3ad
3ai
Ph
S
S
O
O
N
HN
Ph
4
5
2
92
3
93
H2N
N
H2N
N
H
N
N
S
S
2j
2e
3ae
3aj
S
Br
Br
HN
S
4
82
H2N
N
H
N
2f
3af
a For the experimental procedure, see Ref. 31.
b Isolated yield of the products 3.
c All compounds are known and were characterized by comparison of their 1H NMR, 13C NMR and MS spectra with those reported in the
literature.23,24
R1
R1
CH2
OH
S
Br
S
ref. 26
O
H2N
NH
4
H2N
NH
Acknowledgments
2'
5
CHBr3
CBr4
ref. 26
We sincerely thank the SAIF, Punjab University, Chandigarh,
for providing spectra. T.K. is grateful to the CSIR, New Delhi,
for the award of a Senior Research Fellowship (File No.
09/001(0370)/2012-EMR-1). R.K. is grateful to the DST, New
Delhi for a SERB Young Scientist Award (Registration No: CS-
271/2014).
O
HO
R1
N
R1
CH3
SH
2
NH2
S
6
H2N
NH
1'
H2O
Supplementary data
O
R1
R1
CH3
S
N
Supplementary data associated with this article can be found
in the online version, at doi-----
2
NH2
H2N
NH2
S
Et3N, CH3CN
1
3
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