Synthesis and antimicrobial activity of 2H-pyrimido[2,1-b]benzothiazol-2- ones

Article abstract of DOI:10.1007/s11164-010-0211-9

4-Phenyl-2H-pyrimido[2,1-b]benzothiazol-2-ones have been synthesized in quantitative yields by the reaction of 2-aminobenzothiazoles with alkynoic acid. The antimicrobial activity of the synthesized compounds was tested against bacterial species, Bacillus coagulans, Bacillus subtilis, Staphylococcus aureus, and Pseudomonas aeruginosa. The synthesized compounds have shown significant activity against microorganisms which can be correlated with the fused heterocyclic systems.

Full text of DOI:10.1007/s11164-010-0211-9

Res Chem Intermed (2010) 36:985–993
DOI 10.1007/s11164-010-0211-9
Synthesis and antimicrobial activity
of 2H-pyrimido[2,1-b]benzothiazol-2-ones
•
Praveen Kumar Sharma M. Kumar Vimal Mohan
•
Received: 7 June 2010 / Accepted: 14 October 2010 / Published online: 17 November 2010
Ó Springer Science+Business Media B.V. 2010
Abstract 4-Phenyl-2H-pyrimido[2,1-b]benzothiazol-2-ones have been synthe-
sized in quantitative yields by the reaction of 2-aminobenzothiazoles with alkynoic
acid. The antimicrobial activity of the synthesized compounds was tested against
bacterial species, Bacillus coagulans, Bacillus subtilis, Staphylococcus aureus, and
Pseudomonas aeruginosa. The synthesized compounds have shown significant
activity against microorganisms which can be correlated with the fused heterocyclic
systems.
Keywords 2-Aminobenzothiazoles Á 4-Phenyl-2H-pyrimido[2,1-b]benzothiazol-
2-ones Á 2-Phenyl-4H-pyrimido[2,1-b]benzothiazole-4-ones Á Antimicrobial activity
Introduction
The synthesis of heterocyclic systems is of continuing interest because most of the
compounds with biological activity are derived from the heterocyclic structures,
which appear frequently in natural products. The heterocyclic structures in
particular form the basis of many pharmaceuticals, agrochemicals, and veterinary
products [1, 2]. In recent years, polyheterocycles with fused biodynamic hetero-
systems have received increasing attention due to their diverse range of potential
biological activities, and considerable efforts have been undertaken to exploit
synthetic routes to these compounds [3, 4]. Thiazole and pyrimidine heterosystems
are considered to be the typical privileged structures in pharmaceutical research
as both the heterocyclic systems are the active core of various bioactive molecules
P. K. Sharma (&) Á M. Kumar
Department of Chemistry, University of Rajasthan, Jaipur 302055, India
e-mail: pk_pandit1982@yahoo.com
V. Mohan
Mahatma Gandhi Institute of Applied Science, JECRC Campus, Jaipur, India
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[5–10]. Pyrimidobenzothiazoles have been reported to exhibit a wide spectrum of
pharmacological activities such as antiviral, antitumor, anti-inflammatory, analge-
sic, anticonvulsant, muscle relaxant, sedative etc. [11–14]. Pyrimido[2,1-b]benzo-
thiazoles have also been reported as GABA receptor binding agents [15, 16]. In
view of the diverse rang of biological activities of pyrimidobenzothiazoles and our
continuing interest in the design and synthesis of nitrogen and sulphur-containing
bioactive heterocycles [17–20], it was considered interesting to synthesize hitherto
unknown pyrimido[2,1-b]benzothiazoles in order to evaluate their antimicrobial
activities. The synthesis of pyrimido[2,1-b]benzothiazoles has already been reported
in the literature by annulation protocols involving two steps (via amidation of
2-aminobenzothiazoles) [14] or by multicomponent reactions [21–26], but in the
present work we have synthesized pyrimido[2,1-b]benzothiazoles by a simple and
convenient regioselective one-pot synthesis by the reaction of 2-aminobenzothia-
zole with alkynoic acid. All the synthesized pyrimido[2,1-b]benzothiazole-2-ones
are new compounds and their structures have been characterized by IR,
1H NMR,13C NMR, and mass spectral data.
Results and discussion
4-Phenyl-2H-pyrimido[2,1-b]benzothiazole-2-ones (5a–5f) have been synthesized
by the reaction of 2-aminobenzothiazoles (3a–3f) with phenylpropiolic acid in
butanol. The proposed reaction mechanism has been given in Scheme 1. The
structures of the synthesized heterocycles have been confirmed by their spectral
characteristics. The IR spectra of 4-phenyl-2H-pyrimido[2,1-b]benzothiazol-2-ones
(5a–5f) have shown characteristic absorption bands at 1,590–1,605 cm-1 (C=N),
1,630–1,650 cm-1 (C=O) and 1,530–1,575 cm-1 (C=C). The absence of absorp-
tions bands corresponding to asymmetric and symmetric stretching vibrations of
–NH2 group and the appearance of absorption band in the region 1,630–1,650 cm-1
corresponding to C=O group suggest that the heterocyclization has occurred with
the formation of 4-phenyl-2H-pyrimido[2,1-b]benzothiazol-2-ones. The structure
has also been supported by one proton singlet at d 6.13–6.31 ppm due to C3–H and a
doublet at d 6.04–6.64 ppm in compounds 5b, 5e, and 5f due to aromatic proton
1
(C6–H) in the HNMR spectra of the synthesized compounds. The spectral data
exclude the formation of the isomeric 2-phenyl-4H-pyrimido[2,1-b]benzothiazol-
4-ones (6) (Scheme 2). The reaction of 2-aminobenzothiazoles (1) with phenylpro-
piolic acid is considered to involve Michael-type addition and is rationalized as
shown (Scheme 1). The formation of an isomeric product 6 due to attack of 2-amino
group of 2-aminobenzothiazole on b-carbon has been excluded on the basis of the
fact that no deshielding effect was observed on proton (C6–H) due to anisotropic
1
effect of the carbonyl group, and hence the H NMR spectra of the compounds
exhibit a sharp singlet at d 6.13–6.31 ppm due to C3–H while doublet appears in the
8-substituted pyrimido[2,1-b]benzothiazole-2-ones at d 6.04–6.64 due to protons at
(C6–H). The shielding effect has been observed for C6–H in the compounds 5b,
5e, and 5f due to diamagnetic effect of phenyl group attached at C-4. 13C NMR
spectra of the synthesized pyrimido[2,1-b]benzothiazole-2-ones showed distinct
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Antimicrobial activity of 2H-pyrimido[2,1-b]benzothiazol-2-ones
987
resonance signals in agreement with the structures assigned to the synthesized
compounds. In the mass spectrum of compound 5b, the molecular ion peak at
309(M?)(100%) as base peak, corresponding to the molecular weight, also supports
that the heterocyclization of 2-aminobenzothiazoles with alkynoic acid has occurred
according to the proposed reaction mechanism with the formation of pyrimido[2,1-
b]benzothiazole-2-ones included.
2-Aminobenzothiazoles (3a–3f) required for the synthesis of 4-phenyl-
2H-pyrimido[2,1-b] benzothiazol-2-ones have been synthesized from substituted
anilines by thiazolylation [27, 28] involving the sequence of the reactions given in
(Scheme 1). Thiazolylation of thioureas has also been reported by using organic
ammonium tribromide (OATB) as an alternative electrophilic bromine source in
place of bromine-chloroform [29].
R
R
R
NH4CNS
HCl
S
N
(i) Br2/CHCl3
NH2
R1
NH2
R1
NH
C
S
NH2
R1
R2
(ii) liq. NH3
R2
R2
1a-f
2a-f
3a-f
1a R=H, R1=Cl, R2=Cl
1b R=-OCH3, R1=H R2=H
1c R=CH3, R1=H, R2=CH3
1d R=H, R1=H, R2=CH3
1e R=Cl, R1=H, R2=H
1-butanol
Ph
COOH
1f R=-NO2, R1=H, R2=H
R
S
R
S
NH2
NH
O
N
C
R1
N
O
R1
C
C
C
R2
OH
C
C
R2
OH
H
4a-f
R
S
N
R
S
H
N
-H2O
N
R1
O
O
N
R1
R2
OH
R2
5a-f
Scheme 1 Mechanistic presentation of synthesis of 4-phenyl-2H-pyrimido[2,1-b]benzothiazol-2-ones
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P. K. Sharma et al.
R
R
S
N
S
C6H5
COOH
NH2
N
R1
N
O
R1
4
R2
R2
1
6
Scheme 2 Synthesis of 2-phenyl-4H-pyrimido[2,1-b]benzothiazol-4-ones
Antimicrobial activity
The antimicrobial activities of new compounds (5a–5f), were investigated against a
variety of microorganisms, Bacillus coagulans, Bacillus subtilis, Staphylococcus
aureus, and Pseudomonas aeruginosa, by using Disk diffusion method [30, 31].
Sample preparation
Sterilized filter paper discs (5 mm in diameter) were wetted with 10 ll each of a
solution of the tested compound (25, 50, 75, and 100 lg/ml of the compound in
ethanol). The discs were then allowed to dry and placed on the surface of agar plates
seeded with the test organism.
Medium inoculation and cultivation condition
Each plate contained 15 ml of the agar medium, previously seeded with 0.2 ml of an
18-h-old broth culture of each organism. The inoculated plates were incubated at
37°C for 24 h with the test discs in place and the inhibition zones were measured in
mm. Discs impregnated with ethanol were used as controls. The standard drug used
for reference was chloramphenicol.
Conclusions
The data presented in Table 1 indicate that the synthesized compounds are
active against four bacterial species, B. coagulans, B. subtilis, S. aureus, and
P. aeruginosa. The graphical representation of their activities against four bacterial
species have also been presented. The compound 5a shows moderate activity
against B. subtilis and P. aeruginosa, while significant activity is shown against the
bacteria S. aureus. The compound 5a shows weak activity against B. coagulans. The
compound 5b shows moderate activity against B. subtillis and S. aureus, but
significant activity is shown against B. coagulans and P. aeruginosa. The compound
5c shows moderate activity against B.subtilis whereas weak activities are shown
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P. K. Sharma et al.
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Antimicrobial activity of 2H-pyrimido[2,1-b]benzothiazol-2-ones
991
against B. coagulans, P. aeruginosa, and S. aureus. The compound 5d shows
moderate activity against S. aureus, while significant activities are shown against
B. coagulans, B. subtilis. The compound 5d has shown weak activity against
P. aeruginosa. The compound 5e shows moderate activity against B. coagulans and
P. aeruginosa but weak activities are shown against B. subtilis and S. aureus. The
compound 5f shows moderate activity against P. aeruginosa while significant
activities are shown against B. coagulans and S. aureus. The compound 5f has
shown weak activity against B. subtillis. The standard drug used for the reference
was chloramphenicol (100 lg/ml). The antimicrobial activity of these synthesized
compounds can be correlated with their structures. It has been observed that the
fusion of two biodynamic heterosystems: benzothiazole and pyrimidine, results in
the formation of a new heterocyclic scaffold with significant bioactivity.
Melting points of the synthesized compounds were determined on an electric
melting point apparatus and are uncorrected. The IR spectra were recorded on a
Shimadzu 470 spectrometer in KBr discs. The 1H NMR and 13C NMR spectra were
recorded on a JEOL-300 MHz spectrophotometer using TMS as an internal
standard. The chemical shifts are expressed as d ppm. Mass spectrum was recorded
on a Jeol SX-102/Da-600 mass spectrometer using argon/xenon (6 kV, 10 mA) as
the FAB gas.
Synthesis of substituted 2-aminobenzothiazoles
Substituted 2-aminobenzothiazoles have been prepared from substituted anilines in
two steps:
(i) Preparation of substituted phenylthioureas (2a–2f): A mixture of substituted
aniline (0.1 mol), conc. HCl (9 ml) and water (25 ml) were taken in R.B. flask
and refluxed for half an hour. The solution of aniline hydrochloride obtained
was then allowed to cool to room temperature and then ammonium thiocynate
(0.1 mol) was added. The reaction mixture was then refluxed for 4 h until two
layers were separated out. The solution was poured into crushed ice and was
filtered, dried, and crystallized from ethanol.
(ii) Preparation of substituted 2-aminobenzothiazoles (3a–3f): Substituted phen-
ylthiourea (0.1 mol) and chloroform (100 ml) were taken in a two-necked
R.B. flask (500 ml) equipped with a mechanical stirrer. A solution of
bromine (0.1 mol) in chloroform (100 ml) was added drop-wise with stirring
to the reaction mixture by maintaining the temperature below 5°C. After the
complete addition of bromine, the stirring was continued for a period of 4 h
and the contents of the flask were refluxed on a water bath until the
evolution of HBr ceased. Chloroform was removed by filtration and the
resulting solid material was treated with aqueous sulphur dioxide solution
and then filtered. The filtrate was neutralized by aqueous ammonia solution
and the precipitate obtained was filtered, washed with water and crystallized
from ethanol.
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Synthesis of 2H-pyrimido[2,1-b]benzothiazol-2-ones (5a–5f)
The phenylpropiolic acid (20 mmol) and the appropriate 2-aminobenzothiazole
(3a–3f) (10 mmol) were heated to reflux in 1-butanol (50 ml) for 48 h. Evapora-
tion of solvent under reduced pressure gave the crude product, which was
crystallized from hexane/ethyl acetate to give 2H-pyrimido [2,1-b] benzothiazol-2-
ones (5a–5f).
6,7-Dichloro-4-phenyl-2H-pyrimido[2,1-b]benzothiazol-2-one (5a), yield: 85%,
1
m.p.: 195°C, IR (KBr, cm-1), 1,630 (C=O), 1,595 (C=N), 1,550 (C=C), HNMR
(CDCl3, d ppm), 6.27 (s, 1H, C3–H), 6.76-7.76 (m, 7H, aromatic protons) Anal.
Calcd. for C16H8Cl2N2OS: C, 55.35, H, 2.32, N, 8.07. Found: C, 55.01, H, 2.29,
N, 8.03.
8-Methoxy-4-phenyl-2H-pyrimido[2,1-b]benzothiazol-2-one (5b), yield: 82%,
1
m.p.: 210°C, IR (KBr, cm-1), 1,650 (C=O), 1,600 (C=N), 1,575 (C=C), HNMR
(CDCl3, d ppm), 3.85 (s, 3H, C8–OCH3), 6.04–6.06 (d, 1H, C6–H J6,7 = 9.2 Hz),
6.25 (s, 1H, C3–H), 6.54–7.64 (m, 8H, aromatic protons). 13C NMR (CDCl3, d ppm)
56.02(–OCH3), 105.61, 113.59, 113.88, 117.57, 119.53, 122.48, 126.05, 128.63,
129.35, 129.53, 129.67, 131.17, 132.33, 157.92, 163.0, 167.47 MS: m/z: 309 (M?)
Anal. Calcd. for C17H12N2O2S: C, 66.22, H, 3.92, N, 9.08. Found: C, 66.19, H, 3.90,
N, 9.05.
6,8-Dimethyl-4-phenyl-2H-pyrimido[2,1-b]benzothiazol-2-one (5c), yield: 88%,
1
m.p.: 230°C, IR (KBr, cm-1), 1,650 (C=O), 1,590 (C=N), 1,545 (C=C), HNMR
(CDCl3, d ppm), 2.36 (s, 3H, C6–CH3), 2.64 (s, 3H, C8–CH3), 6.31 (s, 1H, C3–H),
6.76–7.60 (m, 7H, aromatic protons) Anal. Calcd. for C18H14N2OS: C, 70.56,
H, 4.61, N, 9.14. Found: C, 70.52, H, 4.60, N, 9.11.
6-Methyl-4-phenyl-2H-pyrimido[2,1-b]benzothiazol-2-one (5d), yield: 80%,
1
m.p.: 210°C, IR (KBr, cm-1), 1,645 (C=O), 1,605 (C=N), 1,535 (C=C), HNMR
(CDCl3, d ppm), 2.55 (s, 3H, C6–CH3), 6.13 (s, 1H, C3–H), 7.06–7.63 (8H,
m, aromatic protons) Anal. Calcd. for C17H12N2OS: C, 69.84, H, 4.14, N, 9.58.
Found: C, 69.74, H, 4.12, N, 9.51.
8-Chloro-4-phenyl-2H-pyrimido[2,1-b]benzothiazol-2-one (5e), yield: 83%,
1
m.p.: 215°C, IR (KBr, cm-1), 1,630 (C=O), 1,595 (C=N), 1,540 (C=C), HNMR
(CDCl3, d ppm), 6.17 (s, 1H, C3–H), 6.81–7.98 (m, 8H, aromatic protons) Anal.
Calcd. for C16H9ClN2OS: C, 61.44, H, 2.90, N, 8.96. Found: C, 61.41, H, 2.89,
N, 8.91.
8-Nitro-4-phenyl-2H-pyrimido[2,1-b]benzothiazol-2-one (5f), yield: 84%, m.p.:
220°C, IR (KBr, cm-1), 1,640 (C=O), 1,605 (C=N), 1,530 (C=C), 1HNMR (CDCl3,
d ppm), 6.20 (s, 1H, C3–H), 6.61–6.64 (d, 1H, C6–H J6,7 = 9 Hz), 7.41–8.38
(m, 8H, aromatic protons) Anal. Calcd. for C16H9N3O3S: C, 59.44, H, 2.81, N,
13.00. Found: C, 59.41, H, 2.80, N, 12.98.
Acknowledgments The authors are thankful to the head of the Department of Chemistry, University of
Rajasthan, Jaipur (India) for providing the necessary laboratory as well as instrumentation facilities
(IR, 1HNMR and 13C NMR). The director of the RSIC, CDRI, Lucknow (India) is acknowledged for
providing mass spectrometry of the synthesized compound. We express our thanks to UGC, New Delhi,
for providing Meritorious student fellowship to P.K.S.
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