Graphene oxide (GO): An efficient carbocatalyst for the benign synthesis of functionalized 1,4-benzothiazines

Article abstract of DOI:10.1016/j.tetlet.2017.01.068

Graphene oxide (GO) has been found to be highly efficient and recyclable carbocatalyst for the benign construction of heterocyclic molecule 1,4-benzothiazine from 2-aminothiophenol and 1,3-dicarbonyl compound. A vast range of highly functionalized 1,4-ben

Full text of DOI:10.1016/j.tetlet.2017.01.068

Accepted Manuscript
Graphene Oxide (GO): An Efficient Carbocatalyst for the Benign Synthesis of
Functionalized 1,4-Benzothiazines
Suchandra Bhattacharya, Pranab Ghosh, Baudeb Basu
PII:
S0040-4039(17)30099-0
DOI:
http://dx.doi.org/10.1016/j.tetlet.2017.01.068
TETL 48571
Reference:
Tetrahedron Letters
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20 January 2017
22 January 2017
Please cite this article as: Bhattacharya, S., Ghosh, P., Basu, B., Graphene Oxide (GO): An Efficient Carbocatalyst
for the Benign Synthesis of Functionalized 1,4-Benzothiazines, Tetrahedron Letters (2017), doi: http://dx.doi.org/
10.1016/j.tetlet.2017.01.068
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Graphene Oxide (GO): An Efficient
Carbocatalyst for the Benign Synthesis of
Functionalized 1,4‒Benzothiazines
Leave this area blank for abstract info.
Suchandra Bhattacharya, Pranab Ghosh, Basudeb Basu*
COOH
COOH
HOOC
HOOC
OH
COOH
O
O
O
O
HO
COOH
COOH
OH
O
HO
H+
O
O
O
H
N
R1
O
R
NH2
SH
O
O
R
COOH COOH
+
R1
R2
R2
25 - 80 o
C
S
R
= H, Cl, CF3
metal-free, solvent-free, 8 h
R1 = CH3, CH2C(CH3)2CH2, Ph
R2 = CH3, Ph, OCH2CH3
75-88 %
Yield

1
Tetrahedron Letters
jo u r n a l h o m e p a g e : w w w . e ls e v ie r . c o m
Graphene Oxide (GO): An Efficient Carbocatalyst for the Benign Synthesis of
Functionalized 1,4‒Benzothiazines
Suchandra Bhattacharya, Pranab Ghosh, Baudeb Basu*
Department of Chemistry, North Bengal University, Darjeeling 734013, India
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
Graphene oxide (GO) has been found to be highly efficient and recyclable carbocatalyst for the
benign construction of heterocyclic molecule 1,4‒benzothiazine from 2‒aminothiophenol and
1,3‒dicarbonyl compound. A vast range of highly functionalized 1,4‒benzothiazine derivatives
has been synthesized constituting a general and sustainable protocol. It is proposed that the large
surface area of GO nanosheet along with the presence of acidic and oxidative groups present on
the edges, basal plane and intercalated layers catalyze the oxidative cyclization effectively.
Available online
Keywords:
2009 Elsevier Ltd. All rights reserved.
Graphene Oxide
1,4‒Benzothiazine
Carbocatalyst
2‒Aminothiophenol
1,3‒Dicarbonyl compound
another synthetic approach, microwave‒assisted synthesis of
1,4‒benzothiazine has been reported.17 The author observed that
the reactants mixed in a mortar at room temperature can produce
the same product in lower yield. The scheme 1 is presented with
some protocols showing various drawbacks and limitations. As a
result, there is demand for greener and selective new method for
its synthesis applicable to broader range substrates.
Introduction
Heterocyclic compounds are ubiquitous in nature,1 and
synthesis of functionalized heterocyclic molecules has remained
always attractive target because of their vast applications in
pharmaceutical industries.2,3 Functionalized 1,4‒benzothiazines,
containing benzene fused with a six‒membered thiazine ring,
represent an important class of heterocyclic compounds, which
exhibit vast array of biological properties such as antimicrobial,4,5
F
COOH
N
H
N
R1
N
antifungal,4
15‒lypoxygenase
inhibitor,4
anti‒HIV,4
H3C
C2H5
OCH3
S
neuroleptics,4 anti‒rheumatic,4 calcium antagonist,4 antioxidant,4
cardiovascular,4 antimalarial,4 anthelmintic4 etc. A few important
drug molecules,4,8 bearing 1,4‒benzothiazine scaffold are shown
in Figure 1. Apart from versatile pharmaceutical applications, the
1,4‒benzothiazine is a subunit of many natural pigments,
dyestuffs and also present in one type of melanin, pheomelanin.6,7
S
CH3
S
Antibacterial
CH3
O
Cardiovascular activity
Antioxidant
HO
H
N
NHC(CH3)3
NH2
O
S
OH
HN
S
S
1,4-Benzothiazine
O
N
H
HOOC
DHBT-1 a Putative Nigral Endotoxin of
Relevance to Parkinson's desease
Because of versatile applications of this heterocyclic scaffold,
a vast array of synthetic methods has been developed. The most
straightforward approach for its synthesis is the direct
condensation‒cyclization between 2‒aminobenzenethiol and
1,3‒dicarbonyl compound in the presence of various reagents /
catalysts. Among the myriads, some notable examples are: the
use of DMSO,9-11 H2O2,12 Baker’s yeast,13 hydrazine hydrate,14
etc. Most conditions however suffer from one or more constraints
such as use of strongly basic media, strong oxidizing agent, high
temperature as well as formation of disulfide as by‒product,14,15
Anti-hypertensive
O
H
N
N
N
N
CH3
N
N
S
NHSO2N(CH3)2
O
O
O
S
Anti-inflammatory
N
O
N
CH3
N
Antifungal
Figure 1. Representative examples of biologically active
molecules containing 1,4‒benzothiazine scaffold
or decomposition of the intermediate to benzothiazole.16 In
———
* Corresponding author. Tel.: +91-353-2776-381; fax: +91-353-2699-001; e-mail: basu_nbu@hotmail.com

2
Tetrahedron Letters
Carbonaceous nanomaterials are important in various fields
from the reaction of 2‒aminobenzenethiol with 1,3‒diketone in
the presence of catalytic p‒TsOH,21 (Scheme 1). Under this
situation, it would be interesting to explore the reaction between
2‒aminobenzenethiol and 1,3‒diketone in the presence of GO as
the catalyst. We report herein our studies, which constitute a
robust protocol for the GO‒catalyzed selective synthesis of
1,4‒benzothiazines with diverse functional groups of
pharmaceutical interests (Scheme 1). Our studies also establish
clearly that there is profound effect of the catalyst GO in
governing the course of the reaction and the product. In
consonance with the demand for developing new reactions using
metal‒free sustainable catalysts like graphene oxide (GO), the
present protocol expands its diverse catalytic functions leading to
like nanotechnology, tailoring new composites, electrochemistry,
sensor, catalysis etc. Graphene oxide (GO) has been considered
as a promising carbocatalyst since the seminal paper published
by Bielawaski in 2010.18 Since then, a considerable variety of
organic reactions have been reported to be catalyzed efficiently
by GO. Single or few layers of GO with large surface area and
oxygenated functional groups like carboxyl groups on the edges,
hydroxyl or epoxy on the basal plane did show acidic and
oxidative properties in the catalytic process. We explored first
time the catalytic function of GO towards the synthesis of small
heterocyclic molecules, 3‒sulphenyl imdazo[1,2‒a]pyridine, an
important class of biologically active pharmacophore, using a
combination of GO‒NaI under multi‒component manner.19
Recently, GO‒catalyzed synthesis of benzothiazoles has been
reported starting from 2‒aminobenzenethiol and aryl aldehyde.20
On the other hand, the same product benzothiazole was obtained
the formation of
a
different class of heterocycles
1,4‒benzothiazine and not the benzothiazole. Further
recyclability and post‒reaction characterization of the catalyst
have also been made in this study.
Other Methods
O
O
DMSO or EtOH-reflux
or Baker's yeast
R1CHO, GO,
Methanol 60 oC
R1
R2
H
N
R1
O
NH2NH2 H2O,
100 oC
H2O2 / NaOH, or
NH2
SH
N
R
O
O
R2
R
R1
R
S
This Work
GO, RT
R1
R2
S
R = H, Cl, CF3
TsOH H2O; solvent-free or
Acetonitrile; RT or 80 oC
Solvent-free, Metal-free
R1 = CH3, CH2C(CH3)2CH2, Ph
R2 = CH3, Ph, OCH2CH3
O
O
R1
R2
Scheme 1. Different processes of 1,4‒benzothiazine synthesis
Result and Discussion
We began our investigations using
a
mixture of
Encouraged by this finding, we examined the loading of the
catalyst GO that is required for the conversion. Thus, reducing
the quantity of GO by 50% i.e. loading of 25 mg also afforded
excellent conversion (entry 7). However, further decrease of GO
loading resulted in lower yield (entry 8) and without the catalyst
GO, the reaction did not proceed at all (entry 9). Formation of no
product in the absence of GO at our hand in contrast to previous
observation by Dandia et al.,17 which gave 1,4‒benzothiazine in
poor yield, may be due to the use of a mortar‒pastel that gave
intimate mixing in addition to generation of some heat. Scaling
up the reaction using 10 mmol each of the starting components
using much lower loading of the catalyst GO also afforded
significant conversion to the desired product (entry 10).
2‒aminothiophenol and acetylacetone, as the model reaction.
Firstly, we tried the reaction in different solvents like acetonitrile,
DMF, toluene, ethanol and water. Loading of the catalyst GO
was started at 50 mg per mmol of 2‒aminothiophenol. The
results are presented in Table 1. It was observed that except DMF
(entry 2), other solvents afforded the desired product 3a in
varying yields (entries 1‒5). Poor yield of 3a in the reaction
carried out in water could be due to the low solubility of
acetylacetone (entry 5). In general, different solvents except
DMF produced the desired product in the range of 56‒71% yield.
However, while performing the reaction under complete
solvent‒free condition and stirring the neat mixture at room
temperature, excellent yield of 3a was achieved. (entry 6).
H
N
R1
Graphene oxide
(GO)
NH2
O
O
R
R
R2
R1
R2
S
SH
Neat mixture, 8h,
Room temperature
(1)
(2)
(3)
(75-88 %)
O
R = H, Cl, CF3
R1 = CH3, CH2C(CH3)2CH2, Ph
R2 = CH3, Ph, OCH2CH3

3
Scheme 2. GO‒catalyzed synthesis of functionalized 1,4‒benzothiazines at room temperature and solvent‒free conditions
Table 1. Optimization of reaction conditions for GO‒catalyzed synthesis of 1,4‒benzothiazine from 2‒aminothiophenol and
acetylacetonea
Entry
GO (mg)
50
Solvent
CH3CN
Temp (oC) / time (h)
RT / 8
(3a) Yield (%)b
1
71
2
3
50
50
DMF
RT / 8
RT / 8
No product
63
Toluene
4
50
50
Ethanol
Water
Neat
RT / 8
RT / 24
RT / 8
68
56
90
88
39
NR
82
5
6
50
7
25
Neat
Neat
RT / 8
RT / 20
RT / 24
RT / 12
8
10
9c
None
100
Neat
10d
Neat
a Reactants (1mmol each)
bIsolated yield after purification by column chromatography on silica gel.
c No GO was added.
d Reactants are used (10 mmol each)
With this optimized condition, as in entry 7, i.e., carrying out
the reaction simply by mixing 2‒aminothiophenol (1 mmol) and
acetylacetone (1 mmol) with the catalyst GO (25 mg) and stirring
for 8 h at room temperature, we then explored the generality of
the reaction with regard to varying both reacting partners. First,
we varied the dicarbonyl compounds such as acyclic
1,3‒diketone (Table 2, entries 1, 2, 5) , cyclic diketone (entry 3),
b‒ketoester (entry 4), dimethyl malonate (entry 7), and ethyl
cyanoacetate (entry 8). Again, we changed the 2‒arylaminothiol
with substituents like ‒Cl, ‒CF3 groups (entries 9‒17). It can be
seen from Table 2 that 1,3‒diketo compounds worked well
giving high yields of the desired products except for 1,3‒di
t‒butyl ketone (entry 6). There was no reaction in this case,
presumably due to the steric hindrance rendered by two bulky
substituents (t‒butyl) and preventing the approach at the other
reacting partner. On the other hand, cyclic diketone like
dimedone reacted efficiently yielding the desired product in 80%
isolated yield (entry 3). On some occasions, the reaction worked
at higher temperatures (60‒80 oC). In the case of using dibenzoyl
methane (entry 5), the presence of two phenyl rings considerably
reduce the electrophilicity of the carbonyl groups and hence the
reactivity is lowered thereby affording relatively poor yield of the
product (entry 5). While the reaction was found to be smooth
with ethyl acetoacetate (b‒keto ester) (table 2, entry 4), similar
reaction with dimethyl malonate (diester) was not successful
even after 24 h at 80 oC (table 2, entry 7). This reactivity
difference can be attributed to the more active keto‒carbonyl
group in ethyl acetoacetate as compared to both ester‒carbonyl
functions in dimethyl malonate. Further extension of the protocol
was performed with substituted 2‒aminothiophenols derivatives.
Both 1,3‒diketones or b‒ketoester reacted with substituted
2‒aminothiophenols in the similar manner producing
corresponding functionalized derivatives of 1,4‒benzothiazines
in good yields (entries 9‒17). It is therefore clear that the
presence of strong electron withdrawing group like ‒CF3 does not
have
significant
effect
towards
the
reaction.

4
Tetrahedron
TABLE 2. Synthesis of diversely functionalized 1,4‒benzothiazinea
Entry
2‒Aminothiophenol
1,3‒diketo
Temp
Time
(h)
Product (3)
Yieldb
(%)
compound
(oC)
1
O
O
NH2
H
RT
RT
60
8
8
8
88
84
80
81
75
N
CH3
CH3
H3C
CH3
SH
S
O
3a
2
3c
4
O
O
NH2
SH
H
N
CH3
Ph
H3C
Ph
S
O
3b
NH2
SH
CH3
CH3
H
N
CH3
CH3
O
S
O
O
3c
O
O
NH2
SH
H
RT
80
8
N
CH3
H3C
OCH2CH3
OCH2CH3
S
O
3d
5d
24
O
O
NH2
SH
H
N
Ph
Ph
O
Ph
Ph
S
3e
6d
7d
8d
9
80
80
80
24
24
24
8
No reaction
No reaction
No reaction
─
─
─
O
O
O
NH2
t-Bu
t-Bu
O
SH
NH2
H3CO
NC
OCH3
SH
OCH2CH3
NH2
O
SH
Cl
Cl
NH2
O
O
H
N
RT
RT
82
83
Cl
CH3
CH3
H3C
CH3
SH
S
O
3f
10
8
NH2
SH
O
O
H
N
Cl
CH3
Ph
H3C
Ph
S
O

5
3g
11c
Cl
NH2
SH
CH3
CH3
H
CH3
CH3
O
60
8
8
76
Cl
N
S
O
O
3h
12
Cl
Cl
NH2
SH
H
O
O
Cl
N
CH3
RT
81
79
H3C
OCH2CH3
OCH2CH3
S
O
3i
13d
NH2
SH
O
O
H
80
24
Cl
N
Ph
Ph
O
Ph
Ph
S
3j
14d
15d
16d
17c
80
80
80
24
24
24
No reaction
No reaction
No reaction
─
─
─
Cl
Cl
Cl
NH2
O
O
O
t-Bu
t-Bu
SH
NH2
O
H3CO
NC
OCH3
SH
NH2
OCH2CH3
O
SH
O
O
H
F3C
NH2
F3C
N
CH3
60
8
84
.
HCl
H3C
OCH2CH3
SH
OCH2CH3
S
O
3k
a2‒Aminothiophenol (1 mmol), 1,3‒diketo compound (1 mmol), GO (25 mg) were mixed and stirred at room temperature
bIsolated yield after purification through column chromatography on silica gel.
cReaction was carried out at 60 oC
dReaction was carried out at 80 oC for 24 h
Towards the plausible mechanism of the reaction, most
literature reports using other catalysts or reagents suggest the
formation disulfide from 2‒aminothiophenol, which actually acts
as the electrophile. Although GO is known to catalyze thiols to
disulfide via oxidative dimerization,22 we did not notice the
formation of any disulfide in our reaction conditions (checked tlc
at different intervals). The preparation of GO following
Hummers method is most common,23 though it may contain some
S‒containing oxygenated groups depending on the washing
procedure.24 Recent method for the preparation of GO by Tour et
al.,25 using H3PO4 however rule out such possibility and we
preferred to prepare pristine GO following this method (see SI).
Now, we propose that the condensation between amino (‒NH2)
and keto (C=O) groups occurred to form an imine 4 (Scheme 3),
and then the tautomer 5 undergoes oxidative cyclization in the
presence of GO to produce 1,4‒benzothiazine 3. The proposition
is in good agreement with the lower reactivity of some diketo
components (entries 5, 6, 13, 14) towards the corresponding
imine formation. The initial attack of ‒NH2 at the carbonyl center
to form the imine is difficult in diphenylketone due to lower
electrophilicity, while steric hindrance by bulky t‒butyl groups
did stop initial formation 3. Similarly, in the case of dimethyl
malonate, a diester, there is no chance for the formation of imine
4 and thus no reaction was realized (entries 7 and 15). Since there
is no disulfide formed during the reaction, possibility of other
routes as proposed in some reports may be ruled out.

6
Tetrahedron
R1
R2
N
R1
O
NH
O
O
NH2
SH
R
R
R
R2
+
R1
R2
O
SH
S
H
H
4
5
2
1
Oxidative
cyclization
GO
H2N
H
N
R1
O
R
S
GO
S
R
R
R2
2
S
NH2
3
Scheme 3. Plausible mechanism for the formation of 3 via GO‒catalyzed oxidative cyclization of 5
Furthermore, we tested the recycling of the catalyst GO in the
model reaction between 2‒aminothiophenol (2 mmol) and
acetylacetone (2 mmol) in the presence of GO (50 mg). After
performing the first run, the reaction mixture was taken in ethyl
acetate (5 mL) and centrifuged at 5000 rpm. The supernatent
liquid containing the product was transferred and this process
was repeated for two times. The catalyst was then washed with
ethyl acetate and acetone followed by drying to afford the
free‒flowing dark GO for further use. Recovery of the catalyst
was slightly lower than the quantity used in the first run, which
may be due to the loss of some oxygenated functional groups
present on GO as well as for manual process. However, the
recovered catalyst used for subsequent five runs with almost
equal efficiency and no significant drop in the conversion (Figure
2) and exhibited similar FT‒IR data (Figure 3). It confirms that
the recovered GO, though might lose some oxygenated groups, is
equally effective for the reaction with residual functional groups.
Figure 3. Comparative FT‒IR of GO, after 1st, 2nd, 3rd. 4th run
In summary, graphene oxide (GO) has been found to act as a
good carbocatalyst in the reaction between 2‒aminothiophenol
o
and 1,3‒dicarbonyl compounds at 25‒80 C under solvent‒free
conditions leading to the formation of 1,4‒benzothiazines in
good to excellent yields. Functionally substituted substrates also
work efficiently and no byproducts like disulfide or
benzothiazole are formed. The catalyst is recyclable for five runs
tested. Towards the effort to harness catalytic activity of
sustainable carbon material like GO in diverse organic reactions,
the present protocol is not only an addition but also an example
for catalytic applications in the synthesis of small heterocyclic
molecules of biological importance and until now, this
exploration is very limited.26
Figure 2. Recyclability of GO in the oxidative cyclization between
2‒aminothiophenol and acetyleacetone
References
Acknowledgments
1.
2.
Arora, P.; Arora, V.; Lamba, H. S.; Wadhwa, D. Int. J. Pharm.
Res. Sci. 2012, 3, 2947‒2955.
Financial support under the SERB project (No.
EMR/2015/000549) is gratefully acknowledged. SB is thankful
to CSIR, India for awarding JRF‒NET fellowship.
Fan, W.‒Q.; Katritzky, A. R. In Comprehensive Heterocyclic
Chemistry II; Kartitzky, A. R.; Rees, C. W., Seriven, E. F. V.,
Eds.; Elsevier Science Oxford, U. K, 1996; Vol. 4, pp 1‒126.
Wamhoff, H. H. In Comprehensive Heterocyclic Chemistry;
Katritzky, A. R.; Rees, C. W.; Eds.; Pergamon: Oxford, 1984;
Vol. 5, pp 669‒732.
3.
Supplementary Material
Supplementary data associated with this article can be found
in the online version, at
4.
5.
Maheshwari, M.; Goyal, A. Asian J. Pharm. Clin. Res. 2015, 8,
41‒46.
Sonawane, E. A.; Pawar, A. Y.; Nagle, S. P.; Mahulikar, P. P.;
Dhananjay H. Chin. J. Chem. 2009, 27, 2049‒2054.

7
6.
7.
Napolitano, A.; Memoli, S.; Prota, G. J. Org. Chem. 1999, 64,
3009‒3011.
Simon, N.; Peles, D.; Wakamatsu, K.; Ito, S. Pigment Cell
Melanoma Res. 2009, 22, 563–579.
Li, H.; Dryhurst, G. J. Neurochem. 1997, 69, 1530‒1541.
Gupta, R.R.; Kumar, P. J. Fluorine Chem.1986, 31, 19‒24.
17. Dandia, A.; Sarawgi, P.; Hursthouse, M. B.; Bingham, A.
L.; Light, M. E.; Drake, J. E.; Ratnani, R. J. Chem. Res. 2006,
445-448.
18. Dreyer, D. R.; Jia, H.‒P.; Bielawski, C. W. Angew. Chem. Int. Ed.,
2010, 49, 6813‒6816.
19. Kundu, S.; Basu, B. RSC Adv. 2015, 5, 50178‒50185.
20. Dhopte, K. B.; Zambare, R. S.; Patwardhana, A. V.; Nemade, P.
R. RSC Adv. 2016, 6, 8164–8172.
8.
9.
10. Gupta, R. R.; Kumar, R.; Gautam, R. K.; J. Heterocyclic Chem.
1984, 21, 1713‒1714.
11. Srivastav, V.; Gupta, R.; Gupta, R. R. Indian J. Chem. 2000, 39B,
223‒224.
21. Mayo, M. S.; Yu, X.; Zhou, X.; Feng, X.; Yamamoto, Y.; Bao, M.
Org. Lett. 2014, 16, 764−767.
12. Sheibani, H.; Islami, R. M.; Hassanpour, A.; Saidi, K.
Phosphorus, Sulfur, and Silicon, 2008, 183,13–20.
13. Pratap, U. R.; Jawale, D. V.; Londhe, B. S.; Mane, R. A. J. Mol.
Catal. B: Enzym. 2011, 68, 94–97.
22. Dreyer, D. R.; Jia, H.‒P.; Todd, A. D.; Geng, J.; Bielawski, C. W.
Org. Biomol. Chem. 2011, 9, 7292‒7295.
23. Hummers, W. S.; Offemann, R. E. J. Am. Chem. Soc. 1958, 80,
1339.
14. Munde, S. B.; Bondge, S. P.; Bhingolikar, V. E.; Mane, R. A.
Green Chem. 2003, 5, 278–279.
15. Aloui, S.; Forsal, I,; Sfaira, M.; Touhami, M. E.; Taleb, M.; Baba,
M. F.; Daoudi, M. Portugaliae Electrochimica Acta 2009, 27,
599‒613.
24. Dimiev, A. M. In Graphene Oxide: Fundamentals and
Applications; Dimiev, A. M.; Eigler, S. Eds.; Wiley, 2016;
Chapter 2; pp 36–84.
25. Marcano, D. C.; Kosynkin, D. V.; Berlin, J. M.; Sinitskii, A.; Sun,
Z.; Slesarev, A,; Alemany, L. B.; Lu, W.; Tour, J. M. ACS Nano,
2010, 4, 4806–4814.
16. Alyea, E. C.; Malek, A. J. Heterocycl. Chem. 1985, 22,
1325‒1326.
26. Khalili, D. Tetrahedron Lett. 2016, 57, 1721–1723.

Highlights
Graphene Oxide (GO): An Efficient Carbocatalyst for the Benign Synthesis of
Functionalized 1,4‒Benzothiazines
Suchandra Bhattacharya, Pranab Ghosh, Baudeb Basu*
Department of Chemistry, North Bengal University, Darjeeling 734013, India
Ø Synthesis of functionalized 1,4-benzothiazines using GO as recyclable catalyst
Ø Solvent–free and metal–catalyst free reaction is generalized with diverse examples
Ø No byproducts like disulfide or benzothiazole are formed in this protocol
Ø Expands the scope of GO as carbocatalyst in heterocyclic synthesis
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*Corresponding author. Tel.: +91-353-2776-381; fax: +91-353-2699-001; e-mail: basu_nbu@hotmail.com (B. Basu).