An economic, simple and convenient synthesis of 2-aryl/heteroaryl/styryl/alkylbenzothiazoles using SiO2-HNO3

Article abstract of DOI:10.1007/s11164-013-1529-x

The present work was undertaken to develop an economic method for the synthesis of 2-aryl/heteroaryl/styryl/alkylbenzothiazoles mediated by SiO₂-HNO₃. In this report, we have demonstrated the catalytic potential of SiO₂-HN

Full text of DOI:10.1007/s11164-013-1529-x

Res Chem Intermed
DOI 10.1007/s11164-013-1529-x
An economic, simple and convenient synthesis
of 2-aryl/heteroaryl/styryl/alkylbenzothiazoles using
SiO2–HNO3
•
Parvin Kumar Rimpy Bhatia Dinesh Kumar
•
•
•
•
•
Ramesh C. Kamboj Suresh Kumar Raj Kamal
Ramesh Kumar
Received: 22 August 2013 / Accepted: 30 December 2013
Ó Springer Science+Business Media Dordrecht 2014
Abstract The present work was undertaken to develop an economic method for the
synthesis of 2-aryl/heteroaryl/styryl/alkylbenzothiazoles mediated by SiO2–HNO3. In
this report, we have demonstrated the catalytic potential of SiO2–HNO3 for the oxidative
condensation of 2-aminothiophenol and aldehydes. Instant reaction at room temperature
under solvent-free condition, high substrate to catalyst ratio 50:1 (by weight), practical
application by large-scale synthesis, and excellent yields are the main advantages of the
present protocol.
Keywords 2-Aminothiophenol Á Aldehydes Á 2-Aryl/heteroaryl/styryl/
alkylbenzothiazoles Á Nitric acid Á Silica gel
Introduction
2-Aryl/styryl/alkylbenzothiazoles are an important class of heterocyclic compounds
due to their wide range of applications [1–9] in medicinal, industrial, and
agricultural chemistry, material chemistry, and nonlinear optics. For this reason,
synthesis of this heterocyclic moiety has attracted much attention and many
methods [10–34] have been developed (Scheme 1). Out of these, the most popular
approach involves the direct condensation of 2-aminothiophenol with aryl
Electronic supplementary material The online version of this article (doi:10.1007/s11164-013-1529-
x) contains supplementary material, which is available to authorized users.
P. Kumar (&) Á R. Bhatia Á D. Kumar Á R. C. Kamboj (&) Á S. Kumar Á R. Kamal Á R. Kumar
Department of Chemistry, Kurukshetra University, Kurukshetra 136119, Haryana, India
e-mail: parvinchem@kuk.ac.in
R. C. Kamboj
e-mail: rckamboj@rediffmail.com
123

P. Kumar et al.
aldehydes under oxidative conditions [11–19]. However, the utility and practical
applicability of the above methods suffer from certain disadvantages such as
expensive catalysts [27, 31, 32], prolonged reaction times [17, 21, 31, 32], poor
yields [33], by-products [28], high reaction temperature [17, 30, 32], tedious
procedure [26, 31, 32], and usage of toxic organic solvents [22, 29, 31, 34].
Recently, green protocols have been extensively studied [35–38], which would
provide a more convenient and efficient technique for the production of the
benzothiazole skeleton.
Over recent years, supported reagents [39, 40] have occupied an important place
in the realm of synthetic organic chemistry, mainly because of the advantages over
unsupported reagents such as cleaner reactions, easier work-up, high yield, milder
conditions, reduced reaction times, and above all environmentally benign.
Nitric acid supported on a silica gel (SiO2–HNO3) catalyst is an inexpensive and
easily available oxidizing reagent that possesses high efficiency in organic
transformations [41, 42]. With our continuing interest in the development of new
methodologies [43] and in a quest to design an economic method, here, we selected
the SiO2–HNO3 as a catalyst for oxidative cyclocondensation of 2-aminothiophenol
with various aldehydes for the synthesis of 2-aryl/heteroaryl/styryl/alkylbenzothia-
zoles under solvent-free conditions.
H
r
A
N
NH2
S
r
A
H X
C
2
NH2
X
+
+
r
A
-
r
H A
2S SC
2
H
C
=
r
=
r
l B I
C
, ,
X
l B
C
,
X
H
,
H
S
H
S
NH2
H
S
N
C
-
30 32
H
O
33
+
r
A
29
H
S
3
4
+
28
N
S
H
10
N
r
A
r
A
S
N
S
27
-
11 19
X
6
NH2
-
20 25
=
r
l B
,
e
M
S
X
H
,
C
,
26
NH2
r
H
C O
+
A
r
B
r
+
A
NH2
H
S
r
B
H
S
r
R A
(
X
CO
)
+
H
S
=
X
H
R' l
O C
,
O
,
Scheme 1 Strategies for synthesis of 2-arylbenzothiazoles
123

Synthesis of 2-aryl/heteroaryl/styryl/alkylbenzothiazoles
Experimental
Chemicals used
All chemicals were commercial products. High purity grade o-aminothiophenol
(Acros) and aldehydes (Sigma-Aldrich, Himedia, Spectrochem, SDFCL) were used.
Preparation of catalyst
In a conical flask, 10 ml conc. HNO3 was added to 5 g silica gel (Acme’s;
100–200 mesh size) and stirred for 10 min. Filtering produced a white solid which
was kept in an oven for 10 min at 120 °C to activate the catalyst.
Acidity of reagents The neutralization titration gave us the acidity of 0.080352 g
(NaOH)/1 g SiO2–HNO3.
General procedure for the synthesis of 2-aryl/heteroaryl/styryl/
alkylbenzothiazoles 3
To a mixture of 2-aminothiophenol 1 (2.0 mmol) and aryl/heteroaryl/styryl/alkyl
aldehyde 2 (2.0 mmol) in a vial was added SiO2–HNO3 (2 wt% to the aldehyde)
and shaken at room temperature. Instantly, an exothermic reaction occurred with
completion of the reaction (TLC) and the reaction mixture became a yellowish-
orange solid that was directly chromatographed over a silica gel column to afford
pure 2-aryl/heteroaryl/styryl/alkylbenzothiazole 3 (Table 3). The spectral data of
some selected compounds are given below:
1
2-(Phenyl)benzothiazole, 3a: H NMR (300 MHz, CDCl3): d (ppm) 8.09–8.15
(m, 3H), 7.93 (d, 8.1 Hz, 1H), 7.38–7.54 (m, 5H)
1
2-(p-Tolyl)benzothiazole, 3b: H NMR (300 MHz, CDCl3): d (ppm) 8.08 (d,
J = 8.4 Hz, 1H), 8.01 (d, J = 8.1 Hz, 2H), 7.91 (d, J = 8.1 Hz, 1H), 7.47–7.53 (m,
1H), 7.36–7.41 (m, 1H), 7.32 (d, J = 8.1 Hz, 2H), 2.45 (s, 3H).
1
2-(2-Methoxyphenyl)benzothiazole, 3d: H NMR (300 MHz, CDCl3): d (ppm)
8.04–8.07 (m, 3H), 7.90 (d, J = 8.1 Hz, 1H), 7.34–7.52 (m, 2H), 7.027 (d, 8.7 Hz,
2H), 3.9 (s, 3H).
1
2-(2-Hydroxyphenyl)benzothiazole, 3h: H NMR (500 MHz, CDCl3): d (ppm)
7.98 (d, J = 8.0 Hz, 1H), 7.90 (d, J = 8.0 Hz, 1H), 7.69 (d, J = 8.0 Hz, 1H),
7.49–7.52 (m, 1H), 7.36–7.42 (m, 2H), 7.10 (d, J = 8.5 Hz, 1H), 6.94–6.97 (m,
1H).
1
2-(4-Fluorophenyl)benzothiazole, 3i: H NMR (300 MHz, CDCl3): d (ppm)
8.07–8.17 (m, 3H), 7.92 (d, 8.4 Hz, 1H), 7.38–7.55 (m, 2H), 7.17–7.24 (m, 2H).
2-(2,4-Difluorophenyl)benzothiazole, 3j: 1H NMR (500 MHz, CDCl3): d (ppm)
8.41–8.49 (m, 1H), 8.10 (d, J = 8.0 Hz, 1H), 7.94 (d, J = 8.0 Hz, 1H), 7.51–7.54
(m, 1H), 7.40–7.43 (m, 1H), 6.97–7.07 (m, 2H); 13C NMR (101 MHz CDCl3,): d
(ppm) 165.47 (d, J = 10 Hz), 162.01–163.00 (m), 159.46–160.17 (m), 152.39 (s),
135.42 (d, J = 8 Hz), 131.05-131.18 (m), 126.37 (s), 125.34 (s), 123.19 (s), 121.46
(s), 112.28–112.49 (m), 104.65 (t, J = 25 Hz).
123

P. Kumar et al.
Table 1 Screening conditionsa
NH2
N
S
conditions
Cl
Cl
CHO
+
SH
1
2m
3m
Entry
Catalyst
Time (min)
Yield (%)a
1
Catalyst-free
HNO3 (conc.)
SiO2–HNO3
SiO2 [39]
540
180
2
14
76
87
12
78
74
79
75
83
74
71
53
78
72
80
67
77
2
3
4
180
5
5
SiO2/MW [19]
6
Co(No3)2Á6H2O[15]
Bi(NO3)2 [12, 16]
Ni(NO3)2Á6H2O[15]
H2O2/CAN [34]
I2 [13, 36]
60
7
120
180
30
8
9
10
11
12
13
14
15
16
17
60
NBS [16]
300
300
180
180
240
300
108
CuCl2 [16]
PTSA [16]
H3PO4 [16]
HClO4 [44]
H2SO4 (conc.) [16]
SiO2–H2SO4 [37, 38]
Reaction conditions: 1 (2 mmol), 2 m (2 mmol), catalyst (10 wt% to aldehyde 2m)
a
Isolated yield
2-(2,5-Difluorophenyl)benzothiazole, 3k: 1H NMR (500 MHz, CDCl3): d
(ppm) 8.11–8.16 (m, 2H), 7.94 (d, J = 8.0 Hz, 1H), 7.51–7.54 (m, 1H),
7.41–7.44 (m, 1H), 7.12–7.22 (m, 2H); 13C NMR (101 MHz CDCl3,): d (ppm)
159.66–160.04 (m), 157.56–157.83 (m), 155.32 (d, J = 4 Hz), 152.26 (s), 137.81
(d, J = 7 Hz), 126.49 (s), 125.62 (s), 123.44 (s), 121.52 (s), 118.48–118.81 (m),
117.45–117.78 (m), 115.51–115.81 (m).
1
2-(4-Chlorophenyl)benzothiazole, 3m: H NMR (300 MHz, CDCl3): d (ppm)
8.04–8.14 (m, 3H), 7.93 (d, J = 7.8 Hz, 1H), 7.48–7.55 (m, 3H), 7.39–7.45 (m, 1H).
1
2-(4-Nitrophenyl)benzothiazole, 3o: H NMR (300 MHz, CDCl3,): d (ppm)
8.38 (d, J = 9.0 Hz, 2H), 8.29 (d, J = 9.0 Hz, 2H), 8.16 (d, J = 8.1 Hz, 1H), 7.98
(d, J = 7.8 Hz, 1H), 7.55–7.60 (m, 1H), 7.46–7.51 (m, 1H).
Results and discussion
We began our study by the examination of various catalysts for the current reaction
using 2-aminothiophenol 1 and 4-chlorobenzaldehyde (2m) in a 1:1 ratio as
123

Synthesis of 2-aryl/heteroaryl/styryl/alkylbenzothiazoles
Fig. 1 Graphical representation of screening conditions
Table 2 Optimization of
reaction conditions with
different ratios of the SiO2–
HNO3 catalyst for the synthesis
of 2-(4-
Entry
Aldehyde
2 m:SiO2–HNO3
weight ratio
Reaction
condition
Yield
(%)a
1
2
3
4
5
6
7
8
9
50:1
50:2
50:3
50:5
50:10
50:20
50:1
50:1
50:1
rt/solvent-free
rt/solvent-free
rt/solvent-free
rt/solvent-free
rt/solvent-free
rt/solvent-free
rt/MeOH/10 min
rt/CH3CN/15 min
rt/CH2Cl2/25 min
98
97
97
95
71
63
89
88
81
chlorophenyl)benzothiazole 3m
a
Product was isolated by
column chromatography
substrate (Table 1). In an initial experiment, using traditional conditions, equimolar
amounts of 2-aminothiophenol 1 and 4-chlorobenzaldehyde (2m) were kept at room
temperature with occasional shaking which resulted in a sluggish reaction. After
9 h, only 14 % of product 2-(4-chlorophenyl)benzothiazole (3m), was obtained
(Table 1, entry 1). To improve the product yield and to optimize the reaction
conditions, nitric acid was used in catalytic amount (10 mol%). Under parallel
reaction conditions, the speed of the reaction was accelerated, even though in a still
unsatisfactory yield (76 %; Table 1, entry 2). We subsequently changed the
123

P. Kumar et al.
Fig. 2 a Reaction after mixing of 2-aminothiophenol and salicylaldehyde. b Reaction after adding a
pinch of SiO2–HNO3. c Reaction with excess of SiO2–HNO3
Fig. 3 a Reaction after mixing of 2-aminothiophenol and p-chlorobenzaldehyde. b Reaction after adding
a pinch of SiO2–HNO3. c Reaction after adding excess of SiO2–HNO3
reagents from concentrated nitric acid to silica-supported nitric acid (SiO2–HNO3,
10 wt%), and a significant improvement was observed in the yield (87 %) after just
2 min (Table 1, entry 3). In order to determine the efficiency of SiO2–HNO3 as a
catalyst, the same reaction was carried out with other commercially available
catalysts [12, 13, 15, 16, 34, 36] (Table 1, entries 6–16) and SiO2 alone [14, 32]
(entries 4 and 5). The results are summarized in Table 1 which revealed that these
systems gave lower yields of the product, even with high catalytic loadings, and, in
the presence of SiO2 alone, a very poor yield was obtained after 180 min at room
temperature (12 %, entry 4). Graphical representations of yield and reaction times
with different reagents are shown in Fig. 1.
A TGA experiment showed that the moisture contained in the freshly prepared
SiO2–HNO3 was 58 %. Therefore, the catalyst SiO2–HNO3 was activated at 120 °C
for 10 min, and we investigated the reaction using the same substrates (1 and 2m) in
123

Synthesis of 2-aryl/heteroaryl/styryl/alkylbenzothiazoles
Fig. 4 a Reaction after mixing of 2-aminothiophenol and p-chlorobenzaldehyde in methanol. b Reaction
after adding a pinch of SiO2–HNO3. c Reaction after adding excess of SiO2–HNO3
the presence of different ratios of the activated SiO2–HNO3 catalyst under the same
reaction conditions (Table 2). We found, to our surprise, that the activated SiO2–
HNO3 catalyst dramatically enhanced the reaction rate and reactions could be
accomplished just by shaking the reaction partners at room temperature to furnish
the desired product (3m) (Figs. 2b, 3b) in excellent yields (95–98 %; Table 2,
entries 1–4). It was important to note that, in excess amounts of the catalyst, the
reaction mixture became a red-colored paste (Figs. 2c, 3c) and resulted (3m) in
lower yields (71 %, entry 5; and 63 %, entry 6). Thus, a ratio of 50:1::alde-
hyde:SiO2–HNO3 (2 wt% of SiO2–HNO3 to aldehyde, entry 1) was found to be
optimum for this reaction and no side product was observed. Finally, using the
optimum amount of catalyst, this reaction was also studied in solution phase
conditions. Although, good yields were obtained in solution phase conditions, they
required a longer reaction time (Table 2, entries 7–9) (Fig. 4b) than the reaction
time in solvent-free condition (entry 1).
Using the optimal reaction conditions (SiO2–HNO3 catalyst 2 wt% to the
aldehyde, solvent-less, room temperature), the applicability of the activated SiO2–
HNO3 catalyst was then explored for the preparation of a series of 2-aryl/heteroaryl/
styryl/alkylbenzothiazoles, 3(a–w) by the reactions of 2-aminothiophenol 1 with
various substituted aryl/heteroaryl/styryl/alkyl aldehydes, 2(a–w) (Table 3). The
results demonstrated that the SiO2–HNO3 catalyst was found to be compatible with
both electron-donating and electron-withdrawing groups on the aromatic ring of the
aldehydes, and no significant substituent effect was observed on the yields of the
products. The structures of all synthesized benzothiazoles were supported by their
1
IR, H NMR and melting point.
In order to demonstrate the practical application of this protocol on a preparative
scale, we carried out some reactions of 2-aminothiophenol (1) with 4-methylbenz-
aldehyde (2b), 2-hydroxybenzaldehyde (2h), 4-chlorobenzaldehyde (2m) and
4-nitrobenzaldehyde (2o) separately, on a grams scale (20 mmol). As expected,
123

P. Kumar et al.
Table 3 Synthesis of 2-aryl/heteroaryl/styryl/alkylbenzothiazoles 3(a–w) using the SiO2–HNO3 catalyst
NH2
SH
N
S
SiO2-HNO3
+
R
CHO
R
rt/solvent-free
1
2(a-w)
3(a-w)
Entry Compound
R
Yield (%) Observed mp (°C) Literature mp (°C)
Aa
Bb
1
3a
3b
3c
3d
3e
3f
C6H5
86
86
90
89
88
85
86
95
94
87
83
91
95
91
95
89
90
90
89
88
–
93 112–114
112–113 [20]
85–86 [20]
119–120 [20]
102–103 [14]
142–143 [14]
228–229 [20]
161–163 [22]
131 [22]
2
4-CH3C6H4
91 84–86
96 118–120
93 100–102
92 138–140
93 226–228
91 160–162
98 128–130
98 102–104
94 92–94
3
4-OCH3C6H4
2-OCH3C6H4
4
5
3,4-(OCH3)2C6H3
4-OHC6H4
3-OHC6H4
2-OHC6H4
4-FC6H4
6
7
3g
3h
3i
8
9
101–102 [20]
– [34]
10
11
12
13
14
15
16
17
18
19
20
21
22
23
3j
2,4-F2C6H3
2,5-F2C6H3
4-CF3C6H4
4-ClC6H4
3k
3l
90 100–102
97 160–162
98 114–116
93 128–130
98 230–232
92 124–126
92 124–126
91 130–132
91 100–102
91 98–100
89c 92–94
87c 70–72
90c Oil
–
159–160 [26]
114–115 [20]
131–132 [20]
230–231 [20]
124–125 [37, 38]
123–124 [37, 38]
133–134 [37, 38]
102–103 [20]
99–100 [20]
– [17]
3m
3n
3o
3p
3q
3r
3s
4-BrC6H4
4-NO2C6H4
4-Pyridyl
3-Pyridyl
2-Pyridyl
2-Furyl
3t
2-Thiophenyl
2-Styryl
3u
3v
3w
2-(1-Phenylprop-1-en- 2yl)
2-Ethyl
–
–
–
[19, 45]
Reaction conditions: 2-aminothiophenol (2 mmol), aldehyde (2 mmol), SiO2–HNO3 catalyst (2 wt% to
aldehyde)
a
Products were isolated by recrystallization using methanol
b
Products were isolated by column chromatography
c
Products were not isolated by recrystallization
the reactions proceeded smoothly to provide target compounds in high yield as
obtained in the similar reaction on a milligrams scale (entries 2, 8, 13, and 15,
respectively; Table 3), which expressed the practical utility of this method.
A possible mechanism was proposed for the synthesis of benzothiazoles as
outlined in Scheme 2. It involves the initial condensation of the amino group
123

Synthesis of 2-aryl/heteroaryl/styryl/alkylbenzothiazoles
-
OH
NO3 , SiO2
O
NH2
SH
R
H
R
H
H2O
SiO2.HNO3
0.12663g HNO3/ 1g
of SiO2.HNO3
H
N
R
H
S
SiO2
HNO3
O2 (Air)
N
S
R
Scheme 2 Possible mechanism for the synthesis of 2-aryl/heteroaryl/styryl/alkylbenzothiazoles
(–NH2) on the carboxaldehyde group (C=O), activated by the SiO2–HNO3 catalyst,
to form a Schiff base or benzothiazoline, and subsequent aerial oxidation furnished
the products 3(a–w).
Conclusion
In conclusion, we have developed a mild, economic, rapid and practical method for
the synthesis of 2-aryl/heteroaryl/styryl/alkylbenzothiazoles in excellent yields.
Highly efficient and easy preparation of SiO2–HNO3 catalyst make a valid
contribution to the existing catalysts for the synthesis of benzothiazole derivatives.
Acknowledgment Financial support as a junior research fellowship by the Haryana State Council of
Science and Technology is gratefully acknowledged for accomplishing this work.
References
1. I. Hutchinson, T.D. Bradshaw, M.F.G. Matthews Stevens, A.D. Westwell, Bioorg. Med. Chem. Lett.
13, 471 (2003)
2. S.J. Ji, H.B. Shi, Dyes Pigment 70, 246 (2006)
3. R.G. Caccese, J.F. Di Joseph, J.S. Scotnicki, L.E. Borella, L.M. Adams, Agents Act. 34, 223 (1991)
4. L. Racane, M. Kralj, L. Suman, R. Stojkovic, V.T. Kulenovic, G.K. Zamola, Bioorg. Med. Chem. 18,
1038 (2010)
5. P.J. Palmer, R.B. Trigg, J.V. Warrington, J. Med. Chem. 14, 248 (1971)
6. P. Hrobarik, P. Zahradnik, W.M.F. Fabian, Phys. Chem. Chem. Phys. 6, 495 (2004)
123

P. Kumar et al.
7. M. Zajac, P. Hrobarik, P. Magdolen, P. Foltinova, P. Zahradnik, Tetrahedron 64, 10605 (2008)
8. P. Hrobarik, I. Sigmundova, P. Zahradnik, P. Kasak, V. Arion, E. Franz, K. Clays, J. Phys. Chem. C
114, 22289 (2010)
9. P. Hrobarik, V. Harobarikova, I. Sigmundova, P. Zahradnik, M. Fakis, I. Polyzos, P. Persephonis, J.
Org. Chem. 76, 8726 (2011)
10. T. Itoh, K. Nagata, H. Ishikawa, A. Ohsawa, Heterocycles 63, 2769 (2004). (Sc(OTF)3)
11. K. Bougrin, A. Loupy, M. Soufiaoui, Tetrahedron 54, 8055 (1998). (MnO2/SiO2)
12. S. Rostamizadeh, K.S.A. Housaini, Phosphorus Sulfur Silicon Elem 180, 1321 (2005). (p-TsOH/
graphite)
13. Y. Li, G.Y. Wang, J.Y. Wang, L. Jacquline, Chem. Lett. 35, 460 (2006). (I2/DMF)
14. F. Al-Qalaf, R.R. Mekheimer, K.U. Sadek, Molecules 13, 2908 (2008). (CAN)
15. P.S. Chandrachood, D.R. Garud, T.V. Gadakari, R.C. Torane, N.R. Deshpande, R.V. Kashalkar, Acta
Chim. Slov. 58, 367 (2011). (Co(NO3)2)
16. N. Azizi, A.K. Amiri, R. Baghi, M. Bolourtchian, M.M. Hashemi, Monatsh. Chem. 140, 1471 (2009).
(p-TsOH)
17. A.K. Chakraborti, S. Rudrawar, K.B. Jadhav, G. Kaur, S.V. Chankeshwara, Green Chem. 9, 1335
(2007). (H2O/reflux)
18. N. Parikh, D. Kumar, S.R. Roy, A.K. Chakraborti, J. Chem. Soc. Chem. Commun. 47, 1797 (2011).
(SODSS)
19. M. Kodomari, Y. Tamaru, T. Aoyama, Synth. Commun. 34, 3029 (2004). (SiO2/MW)
20. H. Sharghi, O. Asemani, Synth. Commun. 39, 860 (2009). (MeSO3H/SiO2)
21. H. Mutsushita, S.H. Lee, M. Joung, B. Clapham, K.D. Janda, Tetrahedron Lett. 45, 313 (2004).
(AlMe3)
22. D.L. Boger, J. Org. Chem. 43, 2296 (1978). (P2O5/MeSO3H)
23. A.K. Chakraborti, S. Rurdawar, G. Kaur, L. Sharma, Synletter 9, 1533 (2004). (NMP)
24. A.K. Chakraborti, C. Selvam, G. Kaur, S. Bhagat, Synletter 5, 851 (2004). (MW)
25. S. Rurdawar, A. Kondaskar, A.K. Chakraborti, Synthesis 15, 2521 (2005). (SOCl2)
26. C. Siddappa, V. Kambappa, M. Umashankara, K.S. Rangappa, Tetrahedron Lett. 52, 5474 (2011).
(I2/t-BuOK)
27. D. Alagille, R.M. Baldwin, G.D. Tamagnan, Tetrahedron Lett. 46, 1349 (2005). (Pd(OAc)2, Cs2CO3)
28. R.H. Tale, Org. Lett. 4, 1641 (2002). (CAN)
29. C. Zhu, T. Akiyama, Synletter 16, 2457 (2010). (DMSO)
30. X.J. Mu, J.P. Zou, R.S. Zeng, J.C. Wu, Tetrahedron Lett. 46, 4345 (2005). (Mn(OAc)3)
31. S.D. Bose, M. Idrees, B. Srikant, Synthesis 6, 819 (2007). (DDQ)
32. G. Evindar, R.A. Batey, J. Org. Chem. 71, 1802 (2006). (CuI/1,10-Phen)
33. V.Z. Shirinian, S.Y. Melkova, L.I. Belenkii, M.M. Krayushkin, Russ. Chem. Bull. Int. Ed. 49, 1859
(2000)
34. G.M. Raghavendra, A.B. Ramesha, C.N. Revanna, K.N. Nandeesh, K. Mantelingu, K.S. Rangappa,
Tetrahedron Lett. 52, 5571 (2011). (T3P/DMSO)
35. K. Bahrami, M.M. Khodaei, F. Naali, J. Org. Chem. 73, 6835 (2008). (H2O2/CAN)
36. S.D. Gupta, H.P. Singh, N.S.H.N. Moorthy, Synth. Commun. 37, 4327 (2007). (I2)
37. K.S. Niralwad, B.B. Shingate, M.S. Shingare, Bull. Korean Chem. Soc. 31, 981 (2010). (SiO2-
H2SO4)
38. G.F. Chen, L.Y. Zhang, H.M. Jia, B.H. Chen, J.T. Li, S.X. Wang, G. Wi, Res. Chem. Intermed. 39,
2077 (2013)
39. I. Mohammadpoor-Baltork, M.A. Zolfigol, M. Abdollahi-Alibeik, Tetrahedron Lett. 45, 8687 (2004)
40. D. Azarifar, M.A. Zolfigol, B. Maleki, Synthesis 11, 1744 (2004)
41. M.A. Zolfigol, K. Amani, M. Hajjami, A. Ghorbani-Choghamarani, R. Ayazi-Nasrabadi, S. Jafari,
Catal. Commun. 9, 1739 (2008)
42. A. Ghorbani-Choghamarani, M. Nikoorazm, H. Goudarziafshar, L. Shiri, Z. Chenani, Bull. Korean
Chem. Soc. 30, 972 (2009)
43. P. Kumar, A. Kumar, K. Hussain, Ultrasonics Sonochem. 19, 729 (2012)
44. M. Abdollahi-Alibeik, S. Poorirani, Phosphorus Sulfur Silicon Relat. Elem. 184, 3182 (2009).
(HClO4)
45. X.-L. Yang, C.-M. Xu, S.-M. Lin, J.-X. Chen, J.-C. Ding, H.-Y. Wu, W.-K. Su, J. Braz. Chem. Soc.
21(1), 37 (2010). (CTAB/H2O)
123