Baker's yeast catalyzed synthesis of 1,4-benzothiazines, performed under ultrasonication

Article abstract of DOI:10.1016/j.molcatb.2010.09.018

An efficient and simple one pot method has been developed for the synthesis of 1,4-benzothiazines by allowing the condensation of 2-aminobenzenethiols and 1,3-dicarbonyls using cheaper biocatalyst, baker's yeast. The role of ultrasonication in the rate expediting of the condensation has been discussed.

Full text of DOI:10.1016/j.molcatb.2010.09.018

Journal of Molecular Catalysis B: Enzymatic 68 (2011) 94–97
Contents lists available at ScienceDirect
Journal of Molecular Catalysis B: Enzymatic
journal homepage: www.elsevier.com/locate/molcatb
Baker’s yeast catalyzed synthesis of 1,4-benzothiazines, performed under
ultrasonication
Umesh R. Pratap, Dhanaji V. Jawale, Balaji S. Londhe, Ramrao A. Mane∗
Department of Chemistry, Dr. Babasaheb Ambedkar Marathwada University, Aurangabad 431 004, MS, India
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient and simple one pot method has been developed for the synthesis of 1,4-benzothiazines
by allowing the condensation of 2-aminobenzenethiols and 1,3-dicarbonyls using cheaper biocatalyst,
baker’s yeast. The role of ultrasonication in the rate expediting of the condensation has been discussed.
© 2010 Elsevier B.V. All rights reserved.
Received 16 May 2010
Received in revised form
23 September 2010
Accepted 29 September 2010
Available online 7 October 2010
Keywords:
Baker’s yeast, 1,4-Benzothiazine
Ultrasonication
Cyclocondensation
2-Aminothiophenol
1. Introduction
fore, the design of new, concise and efficient synthetic route for
this important class of compounds using easily accessible reagents
1,4-Benzothiazines have attracted great deal of interest as a
synthetic target because of their broad spectrum of biological activ-
ities [1]. 1,4-Benzothiazines have been reported to exhibit wide
pharmacological activities such as antagonists [2], anticancer [3],
vasorelaxant [4], antidiabetic [5], antihypertensive [6] and antimi-
crobial [7]. 1,4-Benzothiazines have also been used as dyestuff in
industry [8].
Because of their diverse biological significance synthetic
chemists have developed numerous routes for the syntheses of
1,4-benzothiazine derivatives. One of the most widely employed
methods for the preparation of 1,4-benzothiazines is the oxidative
cyclocondensation of 2-aminobenzenethiols with 1,3-dicarbonyl
compounds using DMSO/microwave irradiation/H2O2–NaOH [9].
This route has two steps; (i) step first includes the oxidation
of 2-aminobenzenethiols to respective disulfides and (ii) the
cyclocondensation of the disulfides with 1,3-dicarbonyls to the
corresponding 1,4-benzothiazines. Another method includes the
separate condensations of 2-aminobenzenethiols with alkynes [10]
and ␣-haloketones or ␣-haloesters [11].
and catalysts is highly desired.
The use of biocatalysts in organic synthesis is very promising.
Biocatalysts show remarkable chemo, regio and stereoselectivities.
In most of the transformations carried using biocatalysts, no side
reaction products are generated. Therefore, critical processes are
found to become simple [12]. Among the various possible biocata-
lysts baker’s yeast (Saccharomyces cerevisiae) has emerged as a one
of the frequently employed microorganisms in whole cell form due
to its high bioavailability and easy handling. It is found to display
its catalytical behavior even at mild conditions in aqueous as well
as in organic media.
Baker’s yeast is having ability to catalyze different types of
organic transformations [13,14]. Literature survey reveals that
there are some reports on the baker’s yeast catalyzed cyclocon-
densations leading to 2-furyl benzothiazoles [15], isoxazolines
[16], polyhydroquinolines [17], benzotraizole oxides [18], 1,4-
dihydropyridines [19], and 3,4-dihydropyrimidine-2-(1H)-ones
[20].
Considering the demerits of the classical synthesis of 1,4-
benzothiazines and to explore the use of biocatalysts in the
cyclocondensations, here we thought to develop an efficient bio-
catalytical route for the cyclocondensation. As a part of our ongoing
research program on biocatalysis [21] and the synthesis of 1,4-
benzothiazine [22], here we have explored the use of baker’s yeast
as a biocatalyst for the synthesis of 1,4-benzothiazines.
These methods require the use of toxic organic solvents,
organic/inorganic bases and explosive/corrosive oxidants. There-
∗
Corresponding author. Tel.: +91 240 240311; fax: +91 240 2400291.
E-mail address: manera@indiatimes.com (R.A. Mane).
1381-1177/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcatb.2010.09.018

U.R. Pratap et al. / Journal of Molecular Catalysis B: Enzymatic 68 (2011) 94–97
95
2. Experimental
2.1. General
Melting points were determined by open capillary method and
are uncorrected. Progress of the reaction was monitored by thin
layer chromatography on MERKs silica plates. 1H and 13C NMR
spectra were recorded on Bruker DRX-300 (300 MHz FT NMR) using
TMS as internal standard. Mass spectral data were determined by
JEOL AccuTOF DART mass spectrometer. Baker’s yeast was obtained
from local market. All chemicals used were reagent grade and used
without further purification.
Scheme 1.
Initially when the reaction was run in water at room tempera-
ture (rt) it was found that the cyclocondensation did not occurred
and the intermediate disulfide (4) (Table 1, entry 1) was formed
even after prolonged stirring (40 h). When the model reaction was
performed in the solvent like dichloromethane, the starting materi-
als were recovered (Table 1, entry 2). When the cyclocondensation
was carried in ethanol, the noticeable yield of 1,4-benzothiazine
was observed after 20 h of stirring (Table 1, entry 3). Inspired by this,
we next investigated the effect of methanol on the yield and time of
the reaction. Here reaction time was found to be decreased to 10 h
and the yield was increased to 78% (Table 1, entry 4). After having
these results we decided to carry the model reaction by using ultra-
sonic irradiation, as ultrasonication is one of the most widely used
laboratory methods for the disruption of cells of baker’s yeast for
the fast release of enzymes [23].
Ultrasound assisted (US) reaction of 2-aminobenzenethiol and
acetyl acetone in ethanol, carried at room temperature gave 68%
yield of the product within 6 h (Table 1, entry 5). Same reaction
when carried in methanol the reaction time has been decreased
by 3 h and the yield was found to be increased to 82% (Table 1,
entry 6). In view of these observations we have selected methanol
as the reaction medium for baker’s yeast catalyzed synthesis of 1,4-
benzothiazines.
Subsequently the other substituted 2-aminobenzenethiols and
1,3-dicarbonyl compounds were subjected under the optimized
reaction conditions to obtain the respective 1,4-benzothiazines.
The results are recorded in Table 2 (Scheme 2). From these results it
seems that the baker’s yeast accepts broad array of substrate combi-
nations. One of the substrates 1,3-diphenyl 1,3-propanedione failed
to react with 2-aminobenzenethiol. This indicates that baker’s
yeast does not accept the substrate 1,3-diphenyl 1,3-propanedione
(Table 2, entry 10). The chemical reactivity of 1,3-diphenyl 1,3-
dicarbonyls has been well explored in presence of other catalysts
and obtained respective 1,4-benzothiazines. However, under the
optimized reaction conditions 1,3-diphenyl 1,3-dicarbonyl did not
undergo cyclocondensation.
2.2. General experimental procedure
To the stirred solution of 2-aminobenzenethiol (10 mmol) in
methanol (25 mL), active dry baker’s yeast (2 g) and ␤-dicarbonyl
(10 mmol) were added. Then the reaction mixture was sonicated at
20 kHz for 3 h at 25–30 ◦C. The progress of the reaction was mon-
itored by thin layer chromatography by using ethyl acetate: pet
ether (2:8) as eluent. After completion of the reaction the reaction
mass was filtered through the bed of celite (1 g). From the filtrate,
the solvent methanol was removed under reduced pressure and the
crude products isolated were crystallized from hot ethanol (Table 2,
entries 1–10).
2.2.1. 1-(3-Methyl-4H-benzo[b][1,4]thiazine-2-yl)ethanone (3a)
1H NMR (300 MHz, CDCl3): ı = 2.33 (s, 3H), 2.42 (3H, s, CH3), 5.91
(s, 1H, NH), 6.98–8.21 (m, 4H).
13C NMR (75 MHz, CDCl3): ı = 190.68, 153.41, 139.36, 127.44,
126.36, 124.99, 120.49, 115.43, 98.15, 30.24, 21.41.
ESI DARTMS: calculated for C11H11NOS + 1: 206.0561; found:
206 0611.
2.2.2. 1-(3,7-Dimethyl-4H-benzo[b][1,4]thiazin-2-yl)ethanone
(3d)
1H NMR (300 MHz, CDCl3): ı = 1.96 (s, 3H), 2.20 (s, 3H), 2.45 (s,
3H), 5.91 (s, 1H, NH), 7.01 (t, 1H, J = 4.0 and 8.0 Hz, 1H), 6.37 (d,
J = 8.0 Hz, 1H), 6.75 (d, J = 4.0 Hz, 1H).
13C NMR (75 MHz, CDCl3): ı = 22.49, 28.96, 29.92, 110.12,
114.61, 127.33, 129.37, 133.49, 135.13, 136.17, 153.54 and 194.13.
ESI DARTMS: calculated for C12H13NOS + 1: 220.0717; found:
220.1154.
2.2.3. Ethyl
3,7-dimethyl-4H-benzo[b][1,4]thiazine-2-carboxylate (3e)
1H NMR (300 MHz, CDCl3): ı = 1.19 (t, 3H), 2.08 (s, 3H), 2.38 (s,
3H), 4.13 (q, 3H), 6.15 (S, 1H, NH), 7.15 (d, J = 8.0 Hz, 1H), 7.24 (t, 1H,
J = 4.0 and 8.0 Hz, 1H), 7.63 (d, J = 4.0 Hz, 1H).
ESI DARTMS: calculated for C13H15NO2S + 1: 250.0823; found:
250.0796.
To investigate the role of baker’s yeast in cyclocondensation the
model reaction was run in absence of baker’s yeast, no formation
Table 1
Effect of solvent on the reaction of 2-aminobenzenethiol and acetyl acetone.a
Entry
Solvent
Reaction
Time (h)
Yield (%)b
conditions
3. Results and discussion
1
2
3
4
5
6
7
8
Water
Dichloromethane
Ethanol
Methanol
Ethanol
Methanol
Methanol
Methanol
rt
rt
rt
rt
US
US
US
US
<40
<40
20
10
6
90c
–
Here, we describe very simple and one pot protocol for the
synthesis of 1,4-benzothiazines. This involves the oxidative cyclo-
condensation with 2-aminobenzenethiols with 1,3-dicarbonyl
compounds, expedited by baker’s yeast as a whole cell biocatalyst
at ambient temperature.
Our investigations started with an optimization study of model
reaction by allowing cyclocondensation of 2-aminobenzenethiol
(1a) and acetyl acetone (2a) in presence of baker’s yeast (Scheme 1).
To see the effect of reaction medium on the rate and yield of the
reaction we carried model reaction in various solvents like water,
dichloromethane, ethanol and methanol under stirring at room
temperature (rt).
61
78
68
82
n.d.
n.d.
3
4 days
10
n.d. = not detected.
a
Reaction conditions: 2-aminobenzenethiol (8 mmol), acetyl acetone (8 mmol),
baker’s yeast (2 g) in 25 mL solvent.
b
Isolated yields.
c
Formation of disulfide (4). Formation of disulfide was confirmed by the com-
parison of the M.P. and spectral studies with disulfide prepared by earlier method
[27].

96
U.R. Pratap et al. / Journal of Molecular Catalysis B: Enzymatic 68 (2011) 94–97
Fig. 1. Plausible mechanism for the formation of 1,4-benzothiazines.
Scheme 2.
of product was observed even after 4 days of stirring in methanol
and even 10 h under ultrasonication (Table 1, entries 7 and 8).
Baker’s yeast was inactivatedbyheating at 100 ◦Cinwater for 1 h
and dead cells thus obtained after centrifugation were used in place
of active baker’s yeast for the model reaction. It was found that nei-
ther the disulfide nor the cyclcondensed product was formed. We
also carried the two step synthesis of 1,4-benzothiazine to know
the reaction sequence. 2-Aminothiophenol and activated baker’s
yeast were irradiated under ultrasound in methanol for 30 min. The
reaction mass was filtered and filtrate was found to be a solution
of disulfide (4). In successive step to this disulfide solution acetyl
acetone and active baker’s yeast were added and the reaction mass
was further sonicated for 2.5 h. Then the filtrate on evaporation
gave crude 1,4-benzothiazine (3a).
reaction gave good yield of 1,4-benzothiazine (3a). From this result
it was confirmed that the role baker’s yeast is crucial for catalyz-
ing the cyclocondensation and yeast catalyzed reaction does not
require air for the oxidation.
The
mechanism
of
the
cyclocondensation
of
2-
aminothiophenols and 1,3-dicarbonyls, carried in the presence of
oxidants leading to 1,4-benzothiazines has been already estab-
lished [24] and it is confirmed that the oxidants initially oxidize
2-aminothiophenols to respective disulfides which subsequently
undergo the cyclocondensations with the 1,3-dicarbonyls to give
the desired 1,4-benzothiazines.
Baker’s yeast has variety of enzymes [14,25]. Amongst them
NAD+/FAD dependant oxidoreductases are widely employed in
organic synthesis. The conversion of 2-aminobenzenethiols into
corresponding disulfides would have taken place because of
oxidative co-enzymes, NAD+/FAD, produced by baker’s yeast. The
oxidants might be abstracting hydride ion from one of the thiol
molecules and then the mercapto group of other molecules would
be nucleophilicaly attack on the electron deficient sulfur to form
intermediates, disulfides (4). The yeast enzyme(s) has site of poly-
activations. Probably these sites might be helping for the activation
of 1,3-dicarbonyls by forming 1,3-dicarbonyl: enzyme noncovalent
complexes. This might be helping to enhance localized concen-
tration of the 1,3-dicarbonyls. In view of this, the rate of the
condensation of the disulfides and 1,3-dicarbonyls would have
been increased, resulting into 1,4-benzothiazines (3a–j). A pro-
posed mechanistic path is presented in Fig. 1.
To see the role of baker’s yeast in the oxidative cyclocondensa-
tion model reaction was carried under nitrogen atmosphere. This
Table 2
Baker’s yeast catalyzed synthesis of 1,4-benzothiazines (3a–j) under ultrasonic
irradiation.a
Entry
R
R1
R2
Productc
Yield (%)b
1
2
3
4
5
6
7
8
9
10
H
H
H
CH3
CH3
CH3
Cl
Cl
Cl
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
Ph
CH3
OC2H5
Ph
CH3
OC2H5
Ph
CH3
OC2H5
Ph
3a
3b
3c
3d
3e
3f
3g
3h
3i
82
71
55
80
68
47
66
53
51
n.d.
4. Conclusions
H
Ph
3j
n.d. = not detected.
First time we have demonstrated the use of baker’s yeast as
whole cell biocatalyst to accelerate the oxidative cyclization of 2-
aminibenzenethiols and 1,3-dicarbonyl compounds in an organic
medium, carried for obtaining 1,4-benzothiazines. The role of ultra-
sonication in expedition of the biocatalyzed cyclocondensation has
a
Reaction conditions: 2-aminobenzenethiols (8 mmol), 1,3-dicarbonyls
(8 mmol), baker’s yeast (2 g) in MeOH (25 mL) at 25–30 ◦C for 3 h.
b
Yield refers to isolated products.
Physical constants and spectral data of all compounds are in accordance with
c
earlier observations [26].

U.R. Pratap et al. / Journal of Molecular Catalysis B: Enzymatic 68 (2011) 94–97
97
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Acknowledgements
Authors are thankful to Professor D.B. Ingle for his valuable dis-
cussions. One of the authors U.R.P. is grateful to University Grants
Commission for the award of research fellowship.
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