Methanesulfonic acid/SiO2 as an efficient combination for the synthesis of 2-substituted aromatic and aliphatic benzothiazoles from carboxylic acids

Article abstract of DOI:10.1080/00397910802431214

Methanesulfonic acid/SiO2 (1 mL/0.3 g) was found to be as an expeditious mixture in the synthesis of 2-substituted aromatic and aliphatic benzothiazoles at 140° using carboxylic acids. After a simple workup, benzothiazoles were obtained in good yields. Simplicity, use of widely available and diverse carboxylic acids, and easy handling of the reaction conditions are among the benefits of the method. Copyright Taylor & Francis Group, LLC.

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Methanesulfonic Acid/SiO as
2
an Efficient Combination for
the Synthesis of 2-Substituted
Aromatic and Aliphatic
Benzothiazoles from Carboxylic
Acids
a
b c
Hashem Sharghi & Omid Asemani
a
Department of Chemistry, Shiraz University, Shiraz,
Iran
b
Department of Medicinal Chemistry, Faculty of
Pharmacy, Shiraz University of Medical Sciences,
Shiraz, Iran
c
Medicinal and Natural Products Chemistry,
Research Center, Shiraz University of Medical
Sciences, Shiraz, Iran
Version of record first published: 29 Jan 2009.
To cite this article: Hashem Sharghi & Omid Asemani (2009): Methanesulfonic Acid/
SiO as an Efficient Combination for the Synthesis of 2-Substituted Aromatic and
2
Aliphatic Benzothiazoles from Carboxylic Acids, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 39:5,
860-867
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1
Synthetic Communications , 39: 860–867, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910802431214
Methanesulfonic Acid/SiO as an Efficient
2
Combination for the Synthesis of 2-Substituted
Aromatic and Aliphatic Benzothiazoles from
Carboxylic Acids
1
2,3
Hashem Sharghi and Omid Asemani
Department of Chemistry, Shiraz University, Shiraz, Iran
1
2
Department of Medicinal Chemistry, Faculty of Pharmacy,
Shiraz University of Medical Sciences, Shiraz, Iran
3
Medicinal and Natural Products Chemistry, Research Center, Shiraz
University of Medical Sciences, Shiraz, Iran
2
Abstract: Methanesulfonic acid=SiO (1 mL=0.3 g) was found to be as an expeditious
mixture in the synthesis of 2-substituted aromatic and aliphatic benzothiazoles at
ꢀ
140 C using carboxylic acids. After a simple workup, benzothiazoles were obtained
in good yields. Simplicity, use of widely available and diverse carboxylic acids, and
easy handling of the reaction conditions are among the benefits of the method.
Keywords: 2-Alkylbenzothiazoles, 2-arylbenzothiazoles, methanesulfonic acid,
silica gel
INTRODUCTION
Benzo-fused heterocyclic systems such as benzimidazole, benzoxazole, and
benzothiazole derivatives have received special attention in the past few years
because of their ever-increasing and undeniable roles in medicinal and
organic fields of chemistry. Attempts for expanding synthetic routes to 2-
substituted benzothiazoles, especially from the time of introduction of 2-
[1]
(4-aminophenyl) benzothiazoles as antineoplastic agents in in vitro studies,
Received June 25, 2008.
Address correspondence to Hashem Sharghi, Department of Chemistry,
Shiraz University, Shiraz 71454, Iran. E-mail: shashem@chem.susc.ac.ir
8
60

Aromatic and Aliphatic Benzothiazoles
861
have increased. Ever since, the synthesis of 2-arylbenzothiazole scaffolds has
[2]
been a greater challenge. In many compounds with antimicrobial, antitu-
[
1,3]
mor,
antagonists]
-Arylbenzothiazoles are mainly synthesized by the condensation of
and antagonistic [like leukotriene D (LTD ) and orexin receptor
4 4
[4,5]
activities, the nucleus is shown as a key element.
2
[6]
2-aminothiophenols with substituted aromatic aldehydes and carboxylic
acids or their derivatives in polyphosphoric acid (PPA), polyphosphate
[7]
[
8]
[9]
ester,
Potassium ferricyanide cyclization of thiobenzanilids, ligand-accelerated
or phosphorus pentoxide=methanesulfonic acid (PPMA).
[10]
[11]
copper-catalyzed cyclization of ortho-halobenzanilides,
palladium-
catalyzed reaction of aryl halides with 2-aminothiophenol in the presence
[12]
of carbon monoxide, ceric ammonium nitrate–mediated reaction of thio-
phenoles and aromatic nitriles, and intramolecular cyclization of thiofor-
manilides using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) in
[13]
[14]
CH Cl at room temperature are among other reported methods in the lit-
2
2
erature. Some protocols have been employed for the preparation of 2-aryl-
benzothiazoles. Nevertheless, most of these methods bear limitations such
as (a) high thermal conditions, (b) long reaction times, (c) use of toxic metal-
lic compounds that result in waste streams, (d) cumbersome working proce-
dures, and (e) more than one step in the synthesis process. Thus, establishing
new methodologies from two points are important: first, in terms of
operational simplicity, economic viability, and reusability, and second, for
discovering new application(s) of previously or newly introduced catalytic
or noncatalytic reaction conditions. Therefore, ongoing research still
continues to expand synthetic routes to the synthesis of this key core.
RESULTS AND DISCUSSION
In continuation of our work to bring in and develop new synthetic meth-
[
15]
odologies,
we report herein a simple, one-pot, easy protocol for the
synthesis of 2-substituted benzothiazoles using 2-aminothiophenol and
different carboxylic acids in a newly introduced and optimized medium,
ꢀ
methanesulfonic acid=silica gel (1 mL=0.3 g), at 140 C (Scheme 1).
Scheme 1. Introducing the new methanesulfonic acid=silica gel (1mL=0.3 g) med-
ium in the synthesis of aliphatic and aromatic benzothiazoles from carboxylic acids.

862
H. Sharghi and O. Asemani
Although neither methanesulfonic acid nor silica gel alone led to the
product formation, their combination was efficient for the condensation
reaction of 2-aminothiophenol and benzoic acid as a model reaction. The
method is simple, isolation of products from the reaction mixture is con-
venient, and yields are high. The protocol was compatible with both
aromatic and aliphatic carboxylic acids as shown in Table 1.
In a typical procedure, 2-aminothiophenol (1 mmol) and a carboxylic
acid derivative (1.2 mmol) were mixed in a medium of methanesulfonic
acid (1 mL) containing silica gel (0.3 g), and the mixture was kept at
ꢀ
1
40 C with stirring for the specified time (Table 1). After workup, the
corresponding benzothiazoles were obtained in reasonable yields.
To investigate the general scope of the reaction, we studied the
condensation reaction of electronically divergent aromatic and aliphatic
carboxylic acids with 2-aminothiophenol. In all cases, the reaction was
carried out within 2–12 h. The reaction condition was rather incompatible
Table 1. Condensation reaction of 2-aminothiophnol with different carboxylic
ꢀ
acids under optimized reaction conditions, MeSO
3
H=SiO
Melting points
Observed Reported Reference
2
(1 mL=0.3 g) at 140 C
Time Yields
a
Entry
Carboxylic acid
C H COOH
(h)
(%)
1
2
3
4
5
6
7
8
9
2
6
4
4
5
7
3
9
6
87
81
75
80
86
74
91
70
83
72
86
89
89
92
92
35
11
53
113
82
122
131
85
54
227
95
112–114
82
120–121
127–128
85
53–54
227
—
16
17
16
17
17
17
6
5
2-ClC
4-CH
2-HOC H COOH
6
H
4
COOH
3
OC COOH
6
H
4
6
4
4-CH
2-CH
3
C
C
6
H
H
4
COOH
COOH
3
6
4
4-HOC H COOH
6
17
c
4
3-ClC
4-BrC
6
H
4
COOH
—
—
—
c
c
6
H
4
COOH
2,6-diFC H COOH
133
73
—
—
10
11
12
13
14
15
16
17
18
12
5
5
4
2
2.5
10
7
10
6
3
3-HOC
6
H
4
COOH
COOH
169
b
161–163
—
—
16
c
C
CH
6
H
5
CH
2
—
—
—
b
c
c
c
3
COOH
—
—
—
b
CH CH COOH
3
—
—
—
—
2
b
ClCH
2-NO
2
COOH
2
C
6
H
4
COOH
3-NO C H COOH
138
183
230
135–136
181–12
230–231
17
16
17
2
6
4
4-NO
2
C
6
H
4
COOH
a
b
c
Isolated yields.
The products froze solid and then were weighed.
Structural data are cited in the article.

Aromatic and Aliphatic Benzothiazoles
863
with the nitro containing carboxylic acids as shown in Table 1 (entries
13
1
6–18). All products were characterized by infrared (IR), H and
1
C
NMR, and mass spectroscopy. The present method was efficient for both
aliphatic and aromatic carboxylic acids possessing either electron-
donating or electron-withdrawing substituents.
To sum up, we have developed the novel heterogeneous mixture of
methanesulfonic acid and silica gel as an expeditious medium for the
condensation reaction of aromatic and aliphatic carboxylic acids with
2
-aminothiophenol for the synthesis of 2-substituted benzothiazoles.
Yields are high, the process trend is simple, and silica gel is recoverable
for the subsequent uses.
EXPERIMENTAL
Instrumentation, Analysis, and Starting Materials
Progress of reactions was monitored by the using silica-gel PolyGrams
SILG=UV 254 plates. IR spectra were recorded on the Shimadzu FT-
IR 8300 spectra photometer. NMR spectra were recorded on a Bruker
DPX 250-MHz instrument, and mass spectra were measured on a
Shimadzu GC-MS-QP 1000 EX instrument at 70 or 20 ev. Melting points
were determined in open capillary tubes in a B u¨ chi-535 circulating oil
melting-point apparatus. Chemical materials were purchased from Fluka,
Aldrich, and Merck companies.
General Procedure
For each reaction, 2-aminothiophenol (1 mmol) and a carboxylic acid
derivative (1.2 mmol) were blended in a mixture of methanesulfonic acid
and silica gel (1 mL=0.3 g) in a Quickfit experimental tube. The tube was
ꢀ
heated in an oil bath with a temperature of 140 C while stirring mag-
netically. After completion of the reaction, the mixture was cooled, diluted
with ethylacetate, and filtered. The filtrate was washed with a 5% sodium
bicarbonate solution (3 ꢁ 30mL) and 30mL water in the fourth step. The
solution was dried over magnesium sulphate; the solvent was evapora-
ted to give the crude product, which was purified by recrystallization or
using column chromatography employing n-hexane=ethylacetate (10:1) as
eluent.
Unknown compounds or compounds for which incomplete physical
data were reported in the literature were characterized by FT-IR, NMR
1
(
13
H, C), mass, and elemental analysis.

864
H. Sharghi and O. Asemani
Data
2
-(3-Chlorophenyl)-1,3-benzothiazole (8)
ꢀ
White crystals, mp 95 C. IR (KBr): 3049 (w), 1562 (w), 1494 (m), 1425 (s),
ꢂ1
1
234 (m), 762 (s), 729 (s), 675 (m) cm . H NMR (CDCl , 250 MHz):
1
3
d (ppm) 7.18–7.38 (m, 4H), 7.73 (t, 2H, J ¼ 8.65 Hz), 7.92–7.95 (m, 2H).
1
3
C NMR (CDCl , 62.9 MHz): d (ppm) 121.7, 123.4, 125.5, 126.1, 126.5,
3
1
27.3, 128.6, 129.2, 130.2, 130.8, 135.1, 153.9, 166.2. Mass (m=z) (%):
þ
þ
2
for C H ClNS: C, 63.54; H, 3.28. Found: C, 64.01; H, 3.09.
47 (M þ 2, 38.0), 245 (M , 100), 210 (17.5), 108 (56.4). Anal. calcd.
1
3
8
2
-(4-Bromophenyl)-1,3-benzothiazole (9)
ꢀ
White crystals, mp: 133 C. IR (KBr): 1049 (w), 1467 (m), 1427 (m), 1390
ꢂ1
1
m), 1063 (m), 962 (s), 827 (s), 752 (s). 715 (s) cm . H NMR (CDCl ,
(
3
2
50 MHz): d (ppm) 7.21–7.45 (m, 4H), 7.63–7.78 (m, 3H), 7.81 (d, 1H,
13
J ¼ 8.1 Hz). C NMR (CDCl , 62.9 MHz): d (ppm) 121.6, 123.3, 123.7,
3
1
25.4, 126.3, 128.8, 132.2, 132.4, 135.0, 154.0, 166.6. Mass (m=z) (%):
þ
þ
2
C H BrNS: C, 53.81; H, 2.78. Found: C, 53.68; H, 2.84.
92 (M þ 2, 31.7), 290 (M , 47.3), 211 (15.4). Anal. calcd. for
1
3
8
2
-(2,6-Difluorophenyl)-1,3-benzothiazole (10)
ꢀ
White crystals, mp 73 C. IR (KBr): 3067 (w), 1618 (m), 1581 (m), 1466
ꢂ1
1
s), 1240 (m), 1009 (s), 957 (s), 762 (s), 723 (m) cm . H NMR (CDCl ,
(
3
2
50 MHz): d (ppm) 7.03–7.12 (m, 2H), 7.37–7.57 (m, 3H), 7.96 (d, 1H,
1
3
J ¼ 8.1 Hz), 8.19 (d, 1H, J ¼ 7.89 Hz) (ppm).
C NMR (CDCl3,
2.9 MHz): d (ppm) 112.2, 121.3, 123.8, 125.7, 126.3, 131.8, 135.6,
6
þ
153.0, 158.4, 162.5. Mass (m=z) (%): 247 (M , 8.7), 169 (5.2). Anal. calcd.
for C H F NS: C, 63.15; H, 2.85. Found: C, 63.35; H, 2.83.
1
3
7 2
2
-Benzyl-1,3-benzothiazole (12)
Pale yellow liquid. IR (neat): 3059 (w), 3030 (w), 2912 (w), 1504 (m), 1427
ꢂ1
1
(
m), 1113 (m), 756 (m) cm . H NMR (CDCl , 250 MHz): d (ppm) 4.18
3
(
s, 2H), 7.02–7.22 (m, 7H), 7.49 (d, 1H, J ¼ 7.91 Hz), 7.82 (d, 1H,
1
3
J ¼ 8.1 Hz). C NMR (CDCl , 62.9 MHz): d (ppm) 40.6, 121.6, 122.8,
3
1
24.9, 126.0, 127.6, 128.9, 129.0, 135.7, 137.3, 153.4, 171.1. Mass (m=z)
þ
(%): 225 (M , 65.7), 91 (31.1). Anal. calcd. for C H NS: C, 74.63; H,
1
4
11
4
.92. Found: C, 75.05; H, 4.69.

Aromatic and Aliphatic Benzothiazoles
865
2
-Methyl-1,3-benzothiazole (13)
Pale yellowish liquid. IR (neat): 3059 (w), 2980 (w), 2921 (w), 1527 (m),
ꢂ1 1
1
440 (m), 1238 (m), 756 (m) cm . H NMR (CDCl , 250 MHz): d (ppm)
3
2
.63 (S, 3H), 7.2 (t, 1H, J ¼ 7.1, J ¼ 1.14 Hz), 7.26 (t, 1H, J ¼ 7.1,
1
2
1
1
3
J ¼ 1.18 Hz), 7.81 (d, 1H, J ¼ 8.0 Hz), 7.66 (d, 1H, J ¼ 8.0 Hz).
C
2
NMR (CDCl , 62.9 MHz): d (ppm) 18.9, 120.2, 121.2, 123.6, 124.8,
3
þ
1
34.5, 152.2, 165.7. Mass (m=z) (%): 149 (M , 14.3), 135 (6.3), 73
(46.3). Anal. calcd. for C H NS: C, 64.40; H, 4.73. Found: C, 64.12;
8 7
H, 4.58.
2
-Ethyl-1,3-benzothiazole (14)
Pale yellowish liquid, IR (neat): 3059 (w), 2970 (w), 2931 (w), 1518 (m),
ꢂ1 1
431 (m), 1122 (m), 945 (m), 756 (m) cm . H NMR (CDCl , 250 MHz):
3
1
d (ppm) 1.17 (t, 3H, J ¼ 7.56, J ¼ 2.25 Hz), 2.85 (q, 2H, J ¼ 7.53,
1
2
1
J ¼ 2.30 Hz), 6.99–7.18 (m, 2H), 7.53 (d, 1H, J ¼ 7.95 Hz), 7.72 (d, 1H,
2
1
3
J ¼ 8.0 Hz). C NMR (CDCl , 62.9 MHz): d (ppm) 13.7, 27.7, 121.4,
3
þ
1
6
6
22.4, 124.6, 125.8, 135.0, 153.2, 173.4. Mass (m=z) (%): 163 (M ,
0.6), 148 (13.8), 134 (3.2), 108 (9.6). Anal. calcd. for C H NS: C,
9
9
6.22; H, 5.56. Found: C, 66.18; H, 5.55.
2
-(Chloromethyl)-1,3-benzothiazole (15)
Pale yellowish liquid. IR (neat): 3068 (w), 2999 (w), 2941 (w), 1514 (m),
ꢂ1
1
431 (m), 1263 (m), 1113 (m), 760 (s), 727 (m) cm . H NMR (CDCl ,
1
3
2
50 MHz): d (ppm) 4.88 (s, 2H), 7.30–7.46 (m, 2H), 7.79 (d, 1H,
1
3
J ¼ 7.92 Hz), 7.98 (d, 1H, J ¼ 8.0 Hz). C NMR (CDCl , 62.9 MHz): d
3
(
ppm) 42.1, 121.7, 123.4, 125.4, 127.8, 135.8, 152.8, 166.7. Mass (m=z)
þ
(%): 183 (M , 49), 148 (100), 108 (31.6). Anal. calcd. for C H ClNS:
8
C, 52.32; H, 3.29. Found: C, 52.30; H, 3.36.
6
ACKNOWLEDGMENTS
We gratefully acknowledge the support of this work by the Shiraz Uni-
versity Research Council. This work was also supported by a grant
from Shiraz University of Medical Sciences (No. 86-3745). We are also
grateful to Mr. H. Sajedian Fard and Mr. M. S. Tafvizi for helpful
cooperation.

866
H. Sharghi and O. Asemani
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