QSAR modeling of synthesized 3-(1,3-benzothiazol-2-yl) 2-phenyl quinazolin-4(3H)-ones as potent antibacterial agents

Article abstract of DOI:10.1007/s00044-011-9626-0

Present communication elicits the designing and synthesis of 3-(1,3-benzothiazol-2-yl) 2-phenyl quinazolin-4(3H)-ones as potential antibacterial agents. A number of substituted 2-amino benzothiazoles, 2-amino-5-[(E)-phenyl diazenyl] benzoic acid, and 2-phenyl-4H benzo[d] [1,3] oxazin-4-one were synthesized as the precursor substrates. The compounds were synthesized in excellent yields and the structures were corroborated on the basis of IR, ¹H NMR, Mass, and elemental analysis data. These compounds were screened in vitro for their antibacterial activity against a representative panel of Gram positive and Gram negative bacteria and models were generated through quantitative structure-activity relationship (QSAR).The activity contributions due to structural and substituent effects were determined using sequential regression procedure. The antimicrobial assay data show that the synthesized compounds are found to manifest profound antimicrobial activity. Springer Science+Business Media, LLC 2011.

Full text of DOI:10.1007/s00044-011-9626-0

MEDICINAL
CHEMISTRY
RESEARCH
Med Chem Res (2012) 21:1136–1148
DOI 10.1007/s00044-011-9626-0
ORIGINAL RESEARCH
QSAR modeling of synthesized 3-(1,3-benzothiazol-2-yl) 2-phenyl
quinazolin-4(3H)-ones as potent antibacterial agents
•
Pratibha Sharma Ashok Kumar Prerna Kumari
•
•
•
Jitendra Singh M. P. Kaushik
Received: 30 April 2010 / Accepted: 10 March 2011 / Published online: 27 March 2011
Ó Springer Science+Business Media, LLC 2011
Abstract Present communication elicits the designing
and synthesis of 3-(1,3-benzothiazol-2-yl) 2-phenyl qui-
nazolin-4(3H)-ones as potential antibacterial agents. A
number of substituted 2-amino benzothiazoles, 2-amino-
5-[(E)-phenyl diazenyl] benzoic acid, and 2-phenyl-4H
benzo[d] [1,3] oxazin-4-one were synthesized as the pre-
cursor substrates. The compounds were synthesized in
excellent yields and the structures were corroborated on the
Introduction
Quinazoline-4(3H)-one ring system has been consistently
regarded as a promising privileged structural icon owing to
its pharmacodynamic versatility in many of its synthetic
derivatives as well as in several naturally occurring alka-
loids isolated from animals, families of plant kingdoms,
and micro-organisms (Witt and Bergman, 2003; Wong and
Ganesan, 2000; Micheal, 2001). A systematic perusal of
literature reveals that quinazoline derivatives encompass a
broad spectrum of pharmaceutical activity profile viz,
antitumor, sedative, analgesic, antidiabetic, antibacterial,
antifungal, and antiinflammatory (Dandia and Singh, 2005;
Grover and Kini, 2006; Reddy et al., 2003; Kumar et al.,
2005; Bertelli et al., 2000; Malecki et al., 2004). The
quinazoline nucleus is the scaffold of many antitumor
drugs mainly acting as inhibitors of tyrosine kinase
receptors (TKR). Over expression of these receptors has
been observed in a number of cancers (e.g., breast, ovarian,
colon, and prostate). Quinazoline derivatives have a ther-
apeutic potential as an antiinvasive agents with potential
activity in early and advanced solid tumors, metastatic
bone disease and leukemia (Ashcroft et al., 2003). Some of
the known quinazoline derivatives are reported to exhibit
remarkable anticancer activity. More specifically trime-
trexate (TMQ) and piritrexim (PTX) have been considered as
new generation potent lipophilic DHFR inhibitors (Sielecki
et al., 2001). In particular, quinazoline-4-one alkaloids such
as asperlicin C, possessing cholecystokinin antagonist
properties, and benzomalvins, which are neurokinin receptor
antagonists, as well as other similar molecules has gained
significant importance. These alkaloids are often biosyn-
thetically derived from anthranilic acid and chiral amino
acids, and as a result contain a chiral center in the R-position
of the 2-substituent. Thus, in view of the broad horizon of
1
basis of IR, H NMR, Mass, and elemental analysis data.
These compounds were screened in vitro for their anti-
bacterial activity against a representative panel of Gram
positive and Gram negative bacteria and models were
generated through quantitative structure–activity relation-
ship (QSAR).The activity contributions due to structural
and substituent effects were determined using sequential
regression procedure. The antimicrobial assay data show
that the synthesized compounds are found to manifest
profound antimicrobial activity.
Keywords Quinazoline-4(3H)-ones ꢀ Benzothiazole ꢀ
Quantitative structure–activity relationship ꢀ
Antibacterial activity
P. Sharma (&) ꢀ A. Kumar ꢀ P. Kumari ꢀ J. Singh
School of Chemical Sciences, Devi Ahilya University,
Takshashila Campus, Khandwa Road, Indore 452001, MP, India
e-mail: drpratibhasharma@yahoo.com
M. P. Kaushik
Defense Research and Development Establishment,
Jhansi Road, Gwalior 474 002, MP, India
123

Med Chem Res (2012) 21:1136–1148
1137
pharmacological potential associated with 2,3-disubstituted
3H-quinazoline-4-ones, it has been realized that this het-
erocyclic architecture represents one of the most indispens-
able groups of heterocycles. A number of such derivatives
corroborates as the important constituents in the drug design
studies vis a vis to synthesize certain drug compounds.
Likewise, benzothiazole is also one of the most potent
heterocyclic scaffold possessing a wide range of applica-
tions in bio-organic and medicinal chemistry armamen-
prepared by condensing together pertinent aniline deriva-
tives and ammonium thiocyanate. Furthermore, as per the
reaction conditions of Hugerschoff’s synthesis the con-
struction of the benzothiazole system involves oxidative
cyclization of arylthioureas with bromine. It is known for
the Hugerschoff reaction that excess bromine usually pro-
duces the perbromide (as the byproduct) of the amino-
benzothiazole, which can then brominate the benzene ring
under prolonged reaction conditions. Therefore, as an
alternative approach, to accomplish the synthesis of
2-aminobenzothiazole in a facile way, the reaction was
conducted with 1 molar equivalent of liquid bromine in
chloroform at room temperature to provide a series of
substituted 2-amino benzothiazoles (4) in 60–75% yields.
As per the synthetic strategy depicted for route A,
anthranilic acid reacts with diazonium salt to form
2-amino-5-[(E)-phenyl diazenyl] benzoic acid (5),which in
turn gives 2-amino-5-[(E)-phenyl diazenyl] benzoyl chlo-
ride(6) by treating with PCl5. Compound (6) reacts with
substituted 2-amino benzothiazole to form compound (7)
which was further benzoylated to yield 8. Enolization of
Compound (8) gives compound 9 and its subsequent
cyclization ultimately yielded the product 10, after elimi-
nating a water molecule. Furthermore, treatment of com-
pound (10) with sodium dithionite provided compound (11)
after reduction of the aryl azo moiety and upon deamina-
tion, compound (12) has been obtained.
tarium. Moreover, 2-aminobenzothiazoles governs
a
plethora of applications in drug discovery and development
for the treatment of diabetes (Suter and Zutter, 1967),
epilepsy (Hays et al., 1994; Jimonet et al., 1999; He et al.,
2002), inflammation (Sawhney et al., 1978), amyotro-
phic lateral sclerosis (Bensimon et al., 1994), analgesia
(Foscolos et al., 1977), tuberculosis (Shirke et al., 1990),
and viral infections (Paget et al., 1969).
Hence, buoyed from these findings and in view of the
significant biological potential alluded by these heterocy-
cles, it was thought worthwhile to synthesize their amal-
gamated product and to explore their antibacterial
potential. Considering this rationale we have designed
and synthesized a new series of quinazolin-4(3H) ones
fused with benzothiazoles followed by their screening
as potential antibacterial agents. We have reported herein
the reaction of substituted 2-amino benzothiazole with
2-amino-5-[(E)-phenyl diazenyl] benzoyl chloride (Route
A) and 2-phenyl-4H benzo[d] [1,3] oxazin-4-one (Route B)
in order to obtain 3-(1,3-benzothiazol-2-yl) 2-phenyl qui-
nazolin-4(3H)-one as the target compounds.
In an alternate manner, according to the route B, esteri-
fication of anthranilic acid (13) was carried out to obtain
ethyl anthranilate (14). Compound (14) reacts with benzoic
anhydride to form the cyclic product 2-phenyl-4H
benzo[d] [1,3] oxazin-4-one (15) as the desired target. An
overview of synthetic routes is depicted in Scheme 1.
Chemistry
In view of the high biological activity profile of quinazoline
compounds, we have developed a new and more efficient
synthesis of this class of compounds. The classical synthetic
approach to quinazolines viz; the Niementowski quinazoline
reaction involves the formation of the intermediate 3(H)-4-
quinazolinone (derived from anthranilic acid) followed by
the formation of 4-chloroquinazoline and subsequent
nucleophilic aromatic substitution at the pyrimidine ring
(Orfi et al., 2004). Keeping this in view, we have developed a
modified strategy to prepare some new 3-(1,3-benzothiazol-
2-yl) 2-phenyl quinazolin-4(3H)-ones considering two dif-
ferent reaction routes. Route A delineates the reaction of
substituted benzothiazoles with 2-amino-5-[(E)-phenyl
diazenyl] benzoyl chloride whereas Route B undertakes the
reaction of substituted benzothiazoles with 2-phenyl-
4H benzo[d] [1,3] oxazin-4-one (prepared from ethyl
anthranilate and benzoic anhydride).
Antibacterial activity and QSAR
The wide range of activity profile of quinazolin-4(3H)-ones
prompted us to develop efficacious insight for its antibacte-
rial activity. All the synthesized compounds were screened
for their in vitro antibacterial activity against Gram positive
(B. subtilis and S. aureus) and Gram negative (E. coli and K.
pneumoniae) bacteria. It was considered of interest to
investigate the influence of substituents on the anticipated
biological activity. All the synthesized compounds showed a
remarkable effect on the bacteriocidal/bacteriostatic potency
of the synthesized quinazolin-4(3H)-ones as has been
revealed from the data presented in Table 1.
Quantitative structure–activity relationship (QSAR)
models correlate molecular chemical structure to biological
activity. Appropriate changes in the chemical structure
altering the electron distribution within a molecule, are
directly responsible for the activity of the molecule
In order to obtain the substituted 2-aminobenzothiazoles
(4) as the precursor substrates, aryl thioureas (2) were
123

1138
Med Chem Res (2012) 21:1136–1148
NH
S
N
NH2.HCl
N
S
Br2/CHCl3
Liq. NH3
NH4SCN
NH2
R
R
R
NH2.HBr
R
NH2
S
Stirring
0 - 5 0C
Reflux, 4h
(2)
(4)
(3)
(1)
Route A
N
N
COOH
NH2
PCl5
N
N
R
S
COCl
NH2
O
(4)
NH
N
N
dry pyridine,
N
(6)
(5)
NH
2
(7)
C6H5COCl
dry pyridine,
Entry
12a
R
0- 5 0
C
H
R
S
12b
12c
5-Cl
O
5-OH
6-OH
6-Br
NH
N
N
12d
12e
12f
N
NHCOC
H
5
6
(8)
5-Br
12g
12h
12i
5-CH
3
6-CH
4-OH
7-OH
3
R
S
O
NH
OH
N
12j
N
N
12k
12l
5-OCH
4-OCH
3
N
3
C
H
5
6
(9)
4-CH
12m
12n
3
3
7-CH
4-Br
7-Br
R
O
S
12o
12p
N
N
N
N
N
C
H
5
(10)
6
Na2S2O4
Route B
R
O
S
O
H N
2
N
N
OH
N
C
H
5
6
(11)
NH
2
(13)
(i) NaNO2.H2SO4
(ii) C2H5OH, Heat
H2SO4
C2H5OH
O
O
R
O
S
(4)
Pyridine, reflux 6hrs
O
O
(C6H5CO)2O
N
N
N
NH
2
N
C
H
5
6
(14)
(12)
(15)
Scheme 1 Overview of synthetic strategy for the synthesis of 3-(1,3-benzothiazol-2-yl) 2-phenyl quinazolin-4(3H)-ones (12a–p)
123

Med Chem Res (2012) 21:1136–1148
1139
analysis of chemical structure. A wide variety of molecular
descriptors are available and descriptor selection is an
integral process in QSAR modeling (Sharma et al., 2004).
QSAR methodology, developed by Hansch (Sharma
et al., 2009) provided a foundation to establish a correla-
tion between physicochemical properties and elicited bio-
logical activity using multivariable regression. Regression
analysis models the activities of molecules through an
equation constructed using a linear combination of physi-
cochemical properties. The coefficient for each variable in
the equation can, consequently, be examined to determine
the extent to which each property contributes towards the
activity of the molecule.
Table 1 In vitro antimicrobial activities tested against Gram positive
and Gram negative bacteria
Entry
R
B. subtilis S. aureus E. coli K. pneumoniae
pIC50
pIC50
pIC50
pIC50
12a
12b
12c
12d
12e
12f
12g
12h
12i
–H
-2.528
-1.602
-3.116
-2.905
-1.204
-1.342
-1.740
-1.431
-2.846
-2.446
-2.602
-1.398
-3.146
-2.908
-1.301
-0.301
-1.845
-1.699
-2.903
-2.778
-2.531
-2.477
-1.903
-2.380
-1.079
-0.845
-2.638 -2.643
-1.699 -1.778
-3.161 -3.193
-2.982 -3.052
-1.311 -1.342
-0.477 -0.602
-2.301 -2.114
-2.041 -2.097
-3.053 -3.057
-2.869 -2.908
-2.785 -2.839
-2.677 -2.719
-2.146 -2.100
-2.398 -2.380
-1.079 -1.146
-1.255 -1.415
5-Cl
5-OH
6-OH
6-Br
5-Br
5-CH3
6-CH3
4-OH
7-OH
Prompted by these findings, the QSAR analysis of syn-
thesized quinazoline-4(3H)-one series has been performed
based on the assumption of linear additive contributions of
the different physicochemical properties. In order to deduce
the correlation of observed activity, in terms of pIC50 of
reported compounds with different structural parameters, a
systematic QSAR investigation has been carried out using
the model proposed by Hansch and coworkers. The activity
data, pIC50 represents the concentration of compounds that
inhibited visible growth. The same are further expressed as
pIC50 on molar basis and used as dependent variables to get
linear relationship in QSAR model. The calculated param-
eters used in the present study include log of octanol–water
partition coefficient (LOG P), molar refractivity (MR) and
Connolly accessible area (CAA), Connolly molecular area
(CMA), dipole–dipole energy (DDE), Van der Waals vol-
ume (VDW) and non-Van der Waals volume (NVDW). The
above-mentioned parameters were calculated by using
12j
12k
12l
5-OCH3 -2.358
4-OCH3 -2.215
12m 4-CH3
-2.009
-2.301
-1.000
-0.301
12n
12o
12p
7-CH3
4-Br
7-Br
(Hansch and Fujita, 1964; Free and Wilson, 1964; Kubinyi,
1993). This fundamental ideology can be used to elucidate
a quantitative description of changes in biological activity
arising due to the presence of the functional groups within
a molecule. QSAR modeling requires three main features: a
data set of molecules, appropriate descriptors and an effi-
cient statistical method for capturing correlation. Descrip-
tors are characteristic properties of molecules, often
represented as numerical values, which facilitate the
Table 2 Values of selected descriptors calculated for 3-(1,3-benzothiazol-2-yl) 2-phenyl quinazolin-4(3H)-ones (12a–p)
Compda
HOMO
LUMO
CAA
CMA
CSEV
LOG P
DDE
MR
VDW
NVDW
–H
-9.0261
-9.0952
-8.7861
-8.9855
-9.2298
-9.1714
-8.909
-0.6171
-0.7946
-0.6936
-0.609
576.858
600.026
584.507
584.872
608.137
608.213
607.41
313.905
328.387
318.877
318.894
333.543
333.592
332.46
272.093
286.135
276.378
276.219
291.829
291.817
288.63
5.9682
6.5264
5.5787
5.5787
6.7971
6.7971
6.4533
6.4533
5.5787
5.5787
5.8418
5.8418
6.4533
6.4533
6.7971
6.7971
-6.4505
-5.4122
-6.3366
-6.277
102.307
107.111
104.001
104.001
109.929
109.929
107.348
107.348
104.001
104.001
108.77
12.8305
13.4327
11.5711
11.6351
13.9503
13.8909
13.585
-5.9131
-6.0882
-6.3986
-6.3874
-6.1557
-6.1545
-6.126
-6.1145
-6.5624
-6.488
-2.4225
-2.7914
-6.45
5-Cl
5-OH
6-OH
6-Br
-0.8948
-0.8368
-0.591
-5.3819
-5.4688
-6.668
5-Br
5-CH3
6-CH3
4-OH
7-OH
5-OCH3
4-OCH3
4-CH3
7-CH3
4-Br
-8.991
-0.6044
-0.6169
-0.6577
-0.6438
-0.576
607.298
582.948
583.537
620.972
619.316
596.47
332.479
318.974
318.519
339.965
340.054
332.7
288.683
276.789
276.441
294.733
295.137
291.69
-6.6811
-5.9735
-5.4508
-6.3259
-5.7843
-6.921
13.584
-8.7776
-8.7179
-8.7458
-8.7209
-8.895
19.8508
11.6381
15.5769
19.5692
13.571
108.77
-0.601
107.348
107.348
109.929
109.929
-8.7468
-9.1257
-9.1452
-0.8009
-0.8476
-0.8139
598.56
329.491
333.481
332.345
288.84
-8.1377
-4.3452
-3.4214
13.584
-4.86
600.123
604.514
294.487
291.958
13.7398
14.128
-6.5328
-6.247
7-Br
a
Substituent numbering is done as per their location in 2-amino benzothiazole nucleus
123

1140
Med Chem Res (2012) 21:1136–1148
error (SPRESS). These statistical data were used to evaluate
the obtained QSAR models. The best-derived QSAR model
was used to predict activity of the untested compounds and
to suggest structural feature(s) which should be modified in
order to improve activity. The predictive powers of the
equations were validated by the leave one out (LOO) cross
validation method. In the equations described below, n is
the number of data points and r is the correlation coefficient.
%EV, F, SEE, PRESS, SSY, r2cv, rb2sp, SPRESS are percentage
of explained variance, ratio between the variances of
observed and calculated activities, standard error of esti-
mate, predicted residual sum of squares, total sum of
squares, cross-validated R2, Bootstrapping r2, and standard
deviation error of prediction, respectively (Sharma et al.,
2005).
Fig. 1 PM3 optimized geometry of more active 3-(1,3-benzothiazol-
2-yl) 2-phenyl quinazolin-4(3H)-ones
Chem3D 6.0 software as depicted in Table 2. Further,
HOMO and LUMO energies were calculated by semi
empirical PM3 (Stewart, 1989) method using MOPAC 6.0
package (Stewart, 1990) implemented in Chem3D 6.0
package. PM3 optimized geometry of more active 3-(1,3-
benzothiazol-2-yl) 2-phenyl quinazolin-4(3H)-ones is
shown in Fig. 1.
QSAR equations for antibacterial profile against
B. subtilis
pIC50 ¼ ½ꢁ8:462ðꢂ2:605Þꢃ þ LOGP½1:311ðꢂ 0:332Þꢃ
þ DDE½0:273ðꢂ 0:166Þꢃ
ð1Þ
Correlation studies were performed for various descrip-
tors, and intercorrelated parameters were discarded
depending upon their individual correlation with biological
activities as indicated in Table 3. The best fit between pIC50
values and these explaining parameters were obtained
through multiple regression analysis (MRA) employing the
method of least square using Valstat (Valstat). The most
commonly used statistical method, multiple linear regres-
sion (MLR) analysis, was applied to create QSAR models
and to obtain statistical data values, i.e., correlation coef-
ficient (r), standard deviation (s), F test, cross-validated
correlation coefficient (q2), and sum of square standard
Contribution of parameters to model
is LOGP:DDE::5.08325:1
n = 12, r = 0.972, r2 = 0.944, variance = 0.051, S =
0.225, F = 76.35, PRESS = 0.457, SSY = 8.21, PRESS/
SSY = 0.056, r2cv = 0.944,
% EV = 93.878, SEE =
0.195, Bootstrapping r2 = 0.944, Bootstrapping S = 0.031,
Q2 = 0.887, SPRESS = 0.320, SDEP = 0.277.
Equation 1 explains up to 93.88% of the variation of
the activities. From the Eq. 1 of the proposed QSAR
model it is revealed that the LOG P and DDE are the main
governing factors to control the observed activities of
Table 3 Correlation matrix
HOMO LUMO CMA CSEV LOGP DDE
MR
VDW NVDW CAA
BS
SA
EC
KP
HOMO
LUMO
CMA
CSEV
LOGP
DDE
1.000
0.420
0.130
0.229
0.740
0.540
0.430
0.287
1.000
0.350
0.399
0.523
0.155
0.456
0.629
0.081
0.342
0.464
0.465
0.496
0.490
1.000
0.980 1.000
0.562 0.675
0.136 0.208
0.910 0.956
0.361 0.304
0.557 0.441
0.967 0.910
0.579 0.673
0.548 0.644
0.493 0.605
0.503 0.615
1.000
0.306
0.772
0.129
0.206
0.470
0.894
0.923
0.916
0.934
1.000
0.392 1.000
MR
VDW
0.070 0.221 1.000
0.212 0.284 0.487
0.122 0.855 0.371
0.624 0.797 0.031
0.523 0.785 0.066
0.522 0.764 0.103
0.478 0.764 0.105
NVDW 0.467
1.000
0.640
0.158
0.215
0.245
0.256
CAA
BS
0.103
0.723
0.795
0.820
0.811
1.000
0.520 1.000
0.488 0.915 1.000
0.422 0.880 0.985 1.000
0.428 0.877 0.984 0.996 1.000
SA
EC
KP
BS, B. subtilis; SA S. aureus; EC, E. coli; KP, K. pneumoniae
123

Med Chem Res (2012) 21:1136–1148
1141
synthesized compounds. In terms of the % contribution of
these physiochemical factors to the antibacterial profile of
the compounds, LOGP has 83.6% contribution to the model,
whereas the contribution of DDE term is only 16.4%.
PRESS (predicted residual sum of squares) is a cross vali-
dation parameter whose value less than SSY (sum of the
squares of response value) points out that the model predicts
better than chance and can be considered statistically
important. Further, to be a reasonable QSAR model PRESS/
SSY ratio should be smaller than 0.4 and its value smaller
than 0.11 indicates an excellent model. In our proposed
model this ratio is 0.056, indicating that the proposed QSAR
model is better model. Moreover, the predictive power of all
above models has been proved by employing the leave-one-
out (LOO) method. The value of cross-validated correlation
coefficient (r2cv) and bootstrapping r2 are further supporting
the predictive power of these explaining models. Also, the
whole data set was divided into two groups; one is training
set containing 12 compounds and another is test set con-
taining 4 compounds. The predicted activities for the test set
(i.e., compounds 12b, 12f, 12k, 12n) for bacteria B. subtilis
are -1.046, -1.385, -2.226, and -2.532, respectively. The
value of r2pred for this bacterium for the test set is found to be
0.778. Furthermore, the studies were extended to propose
the QSAR behavior of the synthesized compounds against
S. aureus, E. coli, and K. pneumoniae. The result for another
Gram positive bacteria S. aureus is presented in Eq. 2.
Contribution of parameters to model is
LOGP:DDE::6.8079:1
n = 12 r = 0.988, r2 = 0.976, variance = 0.016, S =
0.125, F = 186.495, PRESS = 0.141, SSY = 6.005,
PRESS/SSY = 0.023, rc2v = 0.976, % EV = 97.408, SEE =
0.141, Bootstrapping r2 = 0.983, Bootstrapping S = 0.015,
Q2 = 0.937, SPRESS = 0.205, SDEP = 0.178.
The predicted activities for the test set for bacteria E.
coli are -1.455, -1.711, -2.372, and -2.769, respec-
tively. The value of r2pred for this bacterium for the test set is
found to be 0.687.
QSAR equations for antibacterial profile
against K. pneumoniae
pIC50 ¼ ½ꢁ9:304ðꢂ1:287Þꢃ þ LOGP½1:254ðꢂ 0:164Þꢃ
þ DDE½0:126ðꢂ 0:082Þꢃ
ð4Þ
Contribution of parameters to model
is LOGP:DDE::10.5484:1
n = 12, r = 0.990, r2 = 0.980, variance = 0.012, S =
0.111, F = 227.019, PRESS = 0.112, SSY = 5.744, PRESS/
SSY = 0.019, r2cv = 0.980, % EV = 97.861, SEE = 0.112
Bootstrapping r2 = 0.985, Bootstrapping S = 0.013, Q2 =
0.937, SPRESS = 0.199, SDEP = 0.173.
The predicted activities for the test set for bacteria K.
pneumoniae are -1.469, -1.801, -2.237, and -2.775,
respectively. The value of r2pred for this bacterium for the
test set is found to be 0.724.
QSAR equations for antibacterial profile against
S. aureus
The F value obtained in Eqs. 1–4 is found statistically
significant at 99% level. Similarly, cross-validated and
leave-one-out (LOO) predicted activities explain variation
of activities with physiochemical descriptors LOGP and
DDE. In all above equations, (except Eq. 1) the value of
r2 [ 0.976 and rc2v [ 0.976, indicating significance of pro-
posed models. For Eq. 1 the value of r2 and rc2v is in the range
to explain the significance of model up to considerable
extent. Experimentally observed, calculated, and LOO pre-
dicted activities are shown in Table 4. The calculated and
predicted activities of the synthesized compounds were in
accordance with the observed activities as shown in Fig. 2.
pIC50 ¼ ½ꢁ9:459ðꢂ1:201Þꢃ þ LOGP½1:340ðꢂ 0:153Þꢃ
þ DDE½0:157ðꢂ 0:077Þꢃ
ð2Þ
Contribution of parameters to model is
LOGP:DDE::9.01955:1
n = 12, r = 0.993, r2 = 0.986, variance = 0.011, S =
0.104, F = 308.102, PRESS = 0.097, SSY = 6.759, PRESS/
SSY = 0.014, r2cv = 0.985,
% EV = 98.416, SEE =
0.090, Bootstrapping r2 = 0.987, Bootstrapping S = 0.007,
Q2 = 0.976, SPRESS = 0.134, SDEP = 0.116.
Equation 2 explains up to 98.42% of the variation of the
activities of the compounds against S. aureus. The predicted
activities for the test set for bacteria S. aureus are -1.212,
-1.566, -2.093, and -2.627, respectively. The value of
r2pred for this bacterium for the test set is found to be 0.703.
Results and discussion
Preliminary experiments were carried out to determine the
antibacterial activities of titled compounds 12a–p in vitro
against (i) Gram positive bacteria:(B. subtilis and S. aur-
eus) and (ii) Gram negative bacteria: (E. coli and
K. pneumoniae). Nutrient agar media was employed for
bacterial growth and inocula containing approximately
QSAR equations for antibacterial profile against E. coli
pIC50 ¼ ½ꢁ8:634ðꢂ1:449Þꢃ þ LOGP½1:207ðꢂ 0:185Þꢃ
þ DDE½0:188ðꢂ 0:092Þꢃ
ð3Þ
123

1142
Med Chem Res (2012) 21:1136–1148
Table 4 Experimentally calculated and LOO predicted activities of training set compounds against Gram positive and Gram negative bacteria
Entry
R
B. subtilis
S. aureus
E. coli
K. pneumoniae
pIC50
pIC50
pIC50
pIC50
Calculated LOO predicted Calculated LOO predicted Calculated LOO predicted Calculated LOO predicted
12a
12c
12d
12e
12g
12h
12i
–H
-2.40082
-2.88031
-2.86402
-1.02199
-1.82432
-1.8279
-2.38356
-2.82282
-2.85404
-0.96975
-1.85078
-1.9545
-2.47719
-2.98119
-2.97181
-1.19814
-1.86139
-1.86346
-2.92401
-2.84169
-2.54165
-1.90124
-1.03487
-0.88938
-2.46025
-2.94099
-2.98735
-1.16862
-1.86654
-1.91591
-2.92946
-2.86499
-2.55089
-1.90047
-1.01504
-0.94894
-2.64076
-3.08961
-3.0784
-2.64113
-3.0722
-2.63245
-3.10654
-3.09903
-1.4583
-2.05152
-2.05317
-3.06078
-2.99491
-2.707
-2.63101
-3.08545
-3.11049
-1.49168
-2.03191
-2.03919
-3.06177
-3.02671
-2.70529
-2.07619
-1.40927
-0.93776
5-Cl
5-OH
6-OH
6-Br
5-Br
-3.10189
-1.47585
-2.03161
-2.11673
-3.01313
-2.94287
-2.66685
-2.14244
-1.3184
-1.43908
-2.09595
-2.09842
-3.02135
-2.92308
-2.66812
-2.14352
-1.24419
-1.07052
5-CH3 -2.78104
6-CH3 -2.63814
-2.76418
-2.70845
-2.40861
-1.84331
-0.62111
-0.73431
12j
12l
4-OH
7-OH
-2.3844
12m
12o
12p
-1.89349
-2.0834
-1.32765
-1.21123
H-4Br -0.73857
H-7Br -0.48601
-0.82293
107 CFUs/ml of bacteria were prepared from broth culture
in log phase. Bacterial plate was incubated at 37°C for
24 h. The biological activities (pIC50s), the lowest con-
centration (lg/ml) of the test compound that resulted no
visible growth on the plates were recorded. DMSO was
used as the solvent control to ensure that solvent had no
bad effect on bacterial growth.
melting point apparatus. All the reported melting points
are uncorrected. 1H NMR spectra were recorded at
300 MHz with a Bruker advance DRX 300 instrument
using TMS as an internal stranded. IR spectra were run on
a Perkin Elmer model 377-spectrophotometer using KBr
pellets. Analytical thin layer chromatography was per-
formed using E. Merck Silica gel 0.50 mm plate (Merck
No. 5700).
Interpretation of QSAR model reveals that only, partition
coefficient and dipole–dipole energy exhibited (r2 [ 0.91) a
good correlation with biological activity. Further, a close
inspection of screening results reveals that the substitution
in phenyl ring of 2-aminobenzathiazole nucleus exerted
significant influence on the antibacterial armament. It is
observed from the data that the compounds 12b, 12e, 12f,
12o, and 12p possessing electron-attracting group, i.e., Br
and Cl are found remarkably more active. However, pres-
ence of electron releasing groups decreases the activity of
the synthesized compounds. The fairly high positive coef-
ficient of log P suggests that increase in the hydrophobicity
of the molecule increases the antibacterial activity. This is
in good agreement with our experimental finding. More-
over, it is evident from proposed QSAR studies that the rise
in dipole–dipole energy (DDE) increases the antibacterial
profile of synthesized compounds. LOG P and DDE
parameters were the main governing physicochemical fac-
tors for the displayed antimicrobial activities. Such a QSAR
evaluation would open up future perspectives to use these
compounds as new lead compounds in clinical trials.
Preparation of 1,3-benzothiazol-2-amine (4): general
procedure
A mixture of pertinently substituted aniline (0.1 M), conc.
HCl (9.0 ml) and water (25 ml) were taken in a 250 cc
round bottomed flask and contents were refluxed for half an
hour. The solution was cooled at room temperature and
ammonium thiocyanate (7.6 g, 0.1 M) was added. The
reaction mixture was then refluxed for 4 h till two layers
were separated out. The solution was poured into crushed
ice and was filtered, dried, and crystallized from ethanol to
yield the crystals of aryl thiourea (2).
In a 500 ml two-necked R.B. flask equipped with
mechanical stirrer, appropriately substituted aryl thiourea
(2) (0.1 M) was dissolved in chloroform (100 ml). Now
bromine (0.1 M) in chloroform (50 ml) was taken in a
dropping funnel and 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 3–4 h and
the contents of the flask were refluxed on water bath till
the evolution of hydrogen bromide vapors ceased. The
resulting solid material was treated with aqueous sulfur
dioxide solution and filtered. The filtrate was neutralized
with aqueous ammonia solution and the precipitate
Experimental
All the chemicals used were of AR grade purity. Melting
points were taken in open capillary tubes using an electric
123

Med Chem Res (2012) 21:1136–1148
1143
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
-3.5
0.0
(a)
(b)
Test Set
Test Set
Training Set
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
-3.5
Training Set
r2 = 0.986
r2 = 0.943
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
Calculated
Calculated
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
-3.5
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
-3.5
(c)
(e)
(g)
(d)
Test Set
Test Set
Training Set
Training Set
r2 = 0.976
r2 = 0.980
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
Calculated
Calculated
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
-3.5
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
-3.5
(f)
r=2 0.89
r2= 0.974
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
Predicted
Predicted
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
-3.5
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
-3.5
(h)
r2 =0.941
r2= 0.971
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
Predicted
Calculated
123

1144
Med Chem Res (2012) 21:1136–1148
Fig. 2 a–d Plots of observed vs. calculated activities for test and
training set of 3-(1,3-benzothiazol-2-yl) 2-phenyl quinazolin-4(3H)-
ones (12a–p) against B. subtilis, S. aureus, E. coli, and K.
pneumoniae, respectively. e–h Plots of observed vs. LOO-predicted
antibacterial activities of 3-(1,3-benzothiazol-2-yl) 2-phenyl quinaz-
olin-4(3H)-ones (for training set) against B. subtilis, S. aureus, E. coli,
and K. pneumoniae, respectively
b
6-Bromo-1,3-benzothiazol-2-amine(4e)
Yield 73%; mp. 134–135°C; Anal Calcd for C7H5BrN2S:
C, 36.20; H, 2.20; N, 12.23%; Found: C, 55.97; H, 3.96; N,
18.55%; IR (m, cm-1): 3254 (–NH), 3048 (C–H, sp2), 1625,
1493, 1404 (C C, ring str.), 1558 (C=C/C=N), 607 (C–Br);
1HNMR (d ppm): 5.10 (s, 2H, –NH2), 7.83–8.46 (m, 3H,
Ar–H); FAB-Mass: 230 (M??1).
obtained was filtered and washed with water and crys-
tallized from ethanol. Thus, colored crystals of 1,3-ben-
zothiazol-2-amine (4) were obtained in good yields
(60–75%).
5-Bromo-1,3-benzothiazol-2-amine(4f)
Yield 67%; mp. 144–145°C; Anal Calcd for C7H5BrN2S:
C, 36.20; H, 2.20; N, 12.23%; Found: C, 36.16; H, 2.17; N,
12.28%; IR (m, cm-1): 3250 (–NH), 3046 (C–H, sp2), 1622,
1498, 1398 (C C, ring str.), 1562 (C=C/C=N), 609 (C–Br);
1HNMR (d ppm): 5.12 (s, 2H, –NH2), 7.92–8.46 (m, 3H,
Ar–H); FAB-Mass: 230 (M??1).
1,3-Benzothiazol-2-amine (4a)
Yield 75%; mp. 112–113°C; Anal Calcd for C7H6N2S: C,
55.94; H, 4.00; N, 18.61%; Found: C, 55.97; H, 3.96; N,
18.55%; IR (m, cm-1): 3253 (–NH), 3052 (C–H, sp2), 1605,
1495, 1395 (C C, ring str.), 1575 (C=C/C=N); HNMR (d
ppm): 5.0 (s, 2H, –NH2), 7.52–8.20 (m, 4H, Ar–H); FAB-
Mass: 151 (M??1).
1
5-Methyl-1,3-benzothiazol-2-amine(4g)
Yield 65%; mp. 132.5–133.5°C; Anal Calcd for C8H8N2S:
C, 58.51; H, 4.91; N, 17.06%; Found: C, 58.49; H, 4.88; N,
17.01%; IR (m, cm-1):3259 (–NH), 3058 (C–H, sp2),
2872(C–H, sp3),1608, 1502, 1392 (C C, ring str.), 1568
(C=C/C=N); 1HNMR (d ppm): 2.46 (s, 3H, –CH3), 5.08 (s,
2H, –NH2), 7.68–8.20 (m, 3H, Ar–H); FAB-Mass: 165
(M??1).
Compounds (4a–o) were synthesized by taking different
substituted anilines at 4, 5, 6 and 7 positions. The spectral
analyses of synthesized compounds are as follows:
5-Chloro-1,3-benzothiazol-2-amine(4b)
Yield 62%; mp. 136–137°C; Anal Calcd for C7H5ClN2S:
C, 45.53; H, 2.73; N, 15.17%; Found: C, 55.97; H, 3.96; N,
18.55%; IR (m, cm-1): 3260 (–NH), 3058 (C–H, sp2), 1608,
1490, 1392 (C C, ring str.), 1578 (C=C/C=N), 550 (C–Cl);
1HNMR (d ppm): 5.10 (s, 2H, –NH2), 7.66–8.38 (m, 3H,
Ar–H); FAB-Mass: 185 (M??1).
6-Methyl-1,3-benzothiazol-2-amine(4h)
Yield 61%; mp. 138.5–139.5°C; Anal Calcd for C8H8N2S:
C, 58.51; H, 4.91; N, 17.06; Found: C, 58.42; H, 4.89; N,
16.98; IR (m, cm-1):3257 (–NH), 3055 (C–H, sp2),
2878(C–H, sp3),1605, 1500, 1390 (C C, ring str.), 1574
(C=C/C=N); 1HNMR (d ppm): 2.42 (s, 3H, –CH3), 5.12 (s,
2H, –NH2), 7.98–8.36 (m, 3H, Ar–H); FAB-Mass: 165
(M??1).
5-Hydroxy-1,3-benzothiazol-2-amine(4c)
Yield 62%; mp. 120–122°C; Anal Calcd for C7H6N2OS:
C, 50.59; H, 3.64; N, 16.86%; Found: C, 50.47; H, 3.63;
N, 16.43%; IR (m, cm-1): 3362(–OH), 3256 (–NH), 3050
(C–H, sp2), 1602, 1498, 1402 (C C, ring str.), 1568 (C=C/
6-Hydroxy-1,3-benzothiazol-2-amine(4i)
1
C=N); HNMR (d ppm): 5.08 (s, 2H, –NH2), 7.18–8.38
(m, 3H, Ar–H), 10.52 (s, 1H, –OH); FAB-Mass: 167
Yield 68%; mp. 119–120°C; Anal Calcd for C7H6N2OS: C,
50.59; H, 3.64; N, 16.86%; Found: C, 50.57; H, 3.96; N,
16.79%; IR (m, cm-1): 3350(–OH), 3255 (–NH), 3054
(C–H, sp2), 1600, 1495, 1405 (C C, ring str.), 1565
(C=C/C=N); 1HNMR (d ppm): 5.01 (s, 2H, –NH2),
7.12–8.18 (m, 3H, Ar–H); 10.45 (s, 1H, –OH), FAB-Mass:
167 (M??1).
(M??1).
6-Hydroxy-1,3-benzothiazol-2-amine(4d)
Yield 66%; mp. 117–118°C; Anal Calcd for C7H6N2OS: C,
50.59; H, 3.64; N, 16.86%;O, 9.63; Found: C, 50.61; H,
3.65; N, 16.59%; IR (m, cm-1): 3355(–OH), 3253 (–NH),
3054 (C–H, sp2), 1600, 1495, 1405 (C C, ring str.), 1565
(C=C/C=N); 1HNMR (d ppm): 5.07 (s, 2H, –NH2),
7.14–8.56 (m, 3H, Ar–H), 10.22 (s, 1H, –OH); FAB-Mass:
167 (M??1).
7-Hydroxy-1,3-benzothiazol-2-amine(4j)
Yield 69%; mp. 124–125°C; Anal Calcd for C7H6N2OS: C,
50.59; H, 3.64; N, 16.86%; Found: C, 50.47; H, 3.62; N,
16.81%; IR (m, cm-1): 3354(–OH), 3253 (–NH), 3052
123

Med Chem Res (2012) 21:1136–1148
1145
(C–H, sp2), 1600, 1495, 1405 (C C, ring str.), 1565 (C=C/
C=N); 1HNMR (d ppm): 5.02 (s, 2H, –NH2), 7.15–8.25 (m,
3H, Ar–H), 10.32 (s, 1H, –OH); FAB-Mass: 167 (M??1).
7-Bromo-1,3-benzothiazol-2-amine(4p)
Yield 75%; mp. 148–149°C; Anal Calcd for C7H5BrN2S:
C, 36.20; H, 2.20; N, 12.23%; Found: C, 55.97; H, 3.96; N,
18.55%; IR (m, cm-1): 3250 (–NH), 3052 (C–H, sp2), 1622,
1495, 1402 (C C, ring str.), 1557 (C=C/C=N), 607 (C–Br);
1HNMR (d ppm): 5.07 (s, 2H, –NH2), 7.74–8.44 (m, 3H,
Ar–H); FAB-Mass: 230 (M??1).
5-Methoxy-1,3-benzothiazol-2-amine(4k)
Yield 73%; mp. 146–147°C; Anal Calcd for C8H8N2OS: C,
53.51; H, 4.47; N, 15.54%; Found: C, 53.42; H, 4.44; N,
15.45%; IR (m, cm-1): 3255 (–NH), 3045 (C–H, sp2),
2870(C–H, sp3),1615, 1490, 1395 (C C, ring str.), 1575
(C=C/C=N), 2826 (OCH3); 1HNMR (d ppm): 3.73 (s, 3H, –
CH3), 5.14 (s, 2H, –NH2), 7.26–8.12 (m, 3H, Ar–H); FAB-
Mass: 181 (M??1).
Synthesis of 3-(1,3-benzothiazol-2-yl) 2-phenyl quinazolin-
4(3H)-ones via 2-amino-5-[(E)-phenyl diazenyl] benzoic
acid as precursors (Route A)
5.0 g (4.9 ml, 0.054 mol) of aniline was dissolved in a
mixture of 14.0 ml HCl and 14.0 ml of water in a 250 ml
round bottom flask. The flask was immersed in a bath of
crushed ice and salt mixture was cooled until the temper-
ature of the stirred solution was fallen below 5°C. Further,
3.47 g (0.05 mol) of sodium nitrite was dissolved in
17.0 ml of water and the solution was chilled by immersion
in the ice bath. The sodium nitrite solution was added in
small volumes to the cold aniline hydrochloride solution
through shaking. The last 5% of sodium nitrite solution was
added more slowly after shaking for 3–4 min. Meanwhile
7.5 g (0.05 mol) of anthranilic acid was dissolved in
75.0 ml of 2 M-hydrochloric acid in a 600 ml beaker, the
reaction mixture was cooled and to this benzenediazonium
chloride solution was added rapidly and with vigorous
stirring. Then sodium acetate solution (25.0 g of the tri-
hydrate in 12.5 ml of water) was added slowly and with
stirring until precipitation of the dyestuff was complete,
stirring was continued for 1 h. The solution was heated and
filtered through the heated funnel. 50.0 g of sodium chlo-
ride was added to the filtrate and was heated on the steam
bath until the precipitated dyestuff appeared crystalline.
The yield of 2-amino-5-[(E)-phenyl diazenyl] benzoic acid
(6) was 71%.
4-Methoxy-1,3-benzothiazol-2-amine(4l)
Yield 70%; mp. 149–150°C; Anal Calcd for C8H8N2OS: C,
53.51; H, 4.47; N, 15.54%; Found: C, 53.48; H, 4.39; N,
15.43%; IR (m, cm-1): 3250 (–NH), 3048 (C–H, sp2), 2854
(C–H, sp3),1612, 1490, 1395 (C C, ring str.), 1578 (C=C/
1
C=N); HNMR (d ppm): 3.83 (s, 3H, –CH3), 5.10 (s, 2H,
–NH2), 7.28–7.88 (m, 3H, Ar–H); FAB-Mass: 181 (M??1).
4-Methyl-1,3-benzothiazol-2-amine(4m)
Yield 60%; mp. 136.5–137.5°C; Anal Calcd for C8H8N2S:
C, 58.51; H, 4.91; N, 17.06%; Found: C, 58.48; H, 4.90; N,
16.99%; IR (m, cm-1): 3257 (–NH), 3055 (C–H, sp2),
2864(C–H, sp3),1605, 1500, 1390 (C C, ring str.), 1570
(C=C/C=N); 1HNMR (d ppm): 2.52 (s, 3H, –CH3), 5.06 (s,
2H, –NH2), 7.65–8.22 (m, 3H, Ar–H); FAB-Mass: 165
(M??1).
7-Methyl-1,3-benzothiazol-2-amine(4n)
Yield 63%; mp. 142.5–143.5°C; Anal Calcd for C8H8N2S:
C, 58.51; H, 4.91; N, 17.06%; Found: C, 58.42; H, 4.87; N,
17.03; IR (m, cm-1):3257 (–NH), 3052 (C–H, sp2),
2860(C–H, sp3),1605, 1500, 1390 (C C, ring str.),
0.05 mol of phosphorous pentachloride was added in
12.05 g (0.05 mol) of 2-amino-5-[(E)-phenyl diazenyl]
benzoic acid to give compound 6. Further equimolar
quantities of compound 6 (0.05 mol) and 2-aminobenzo-
thiazole were taken in 12.0 ml of pyridine and were
refluxed for 4 h to give compound 7 which was further
benzoylated using (.05 mol) benzoyl chloride followed by
dehydration in dry pyridine at 0–5°C for 3 h to give
compound 10. Compound 10 was treated with 0.114
molar aqueous solution of sodium dithionite with con-
tinuous stirring which discharges the color and resulted
the finely divided precipitate of compound 11. (0.03 mol)
compound 11 was dissolved by heating on water bath
with 60 ml of rectified spirit and 15 ml of benzene in a
200-ml two-necked flask fitted with reflux condenser, the
1
1572(C=C/C=N); HNMR (d ppm): 2.45 (s, 3H, –CH3),
5.09 (s, 2H, –NH2), 7.65–8.24 (m, 3H, Ar–H); FAB-Mass:
165 (M??1).
4-Bromo-1,3-benzothiazol-2-amine(4o)
Yield 72%; mp. 134–135°C; Anal Calcd for C7H5BrN2S:
C, 36.20; H, 2.20; N, 12.23%; Found: C, 36.18; H, 2.16; N,
12.15%; IR (m, cm-1): 3252 (–NH), 3048 (C–H, sp2), 1620,
1495, 1402 (C C, ring str.), 1555 (C=C/C=N), 608 (C–Br);
1HNMR (d ppm): 5.10 (s, 2H, –NH2), 7.80–8.34 (m, 3H,
Ar–H); FAB-Mass: 230 (M??1).
123

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Med Chem Res (2012) 21:1136–1148
Preparation of 3-(1,3-benzothiazol-2-yl) 2-phenyl
second neck being closed with stopper. 5.30 g (3.5 ml) of
concentrated sulfuric acid was added to the hot solution
of compound 11 via the side neck through graduated
pipette and the liquid was gently swirled, the stopper was
replaced and the solution was heated on the water bath
until the clear solution was boiled. The flask was
removed from the water bath and 3.5 g (0.05 mol) of
powdered sodium nitrite was added in two approximately
equal portions via the side neck; after each addition, the
stopper was replaced and the flask was shaken vigorously;
when the reaction was subsided, the second portion of
sodium nitrite was added. The flask was heated on the
water bath as long as the gas evolved with shaking from
time to time. Further, the solution was cooled for 10 min,
and then the flask was immersed in the ice bath. Fur-
thermore, a mixture of compound 12 and sodium sulfate
was crystallized out. The solution was filtered with suc-
tion on buchner funnel and was washed with a small
quantity of ethanol and then repeatedly with water to
remove all the sodium sulfate. The crude 3-(1,3-ben-
zothiazol-2-yl) 2-phenyl quinazolin-4(3H)-one (7.5 g) was
dissolved in a boiling mixture of 120 ml of glacial acetic
acid and 30 ml of water, the solution was boiled with
2.5 g of activated charcoal and was filtered through a hot
water funnel; the solution was allowed to cool on a
buchner funnel and washed with a small quantity of
chilled rectified spirit to remove the acetic acid. The
crystals were dried in the air upon filter paper to yield the
desired titled compounds.
quinazolin-4(3H)-ones (12a–p)
An equimolar mixture (0.01 mol) of 2-phenyl-
4H benzo[d] [1,3] oxazin-4-one and benzothiazole-2-amine
derivatives was taken, mechanistically stirred and the
mixture was refluxed for 6 h in the presence of 10 ml of
pyridine. Upon cooling, the mixture was poured onto cru-
shed ice. The precipitated solid was collected and crystal-
lized from ethanol to give the desired titled compounds.
Yield 78%, mp. 274–276°C (lit [20] 114°C); Anal Calcd
for C21H13N3OS: C, 70.97; H, 3.69; N, 11.82%; Found: C,
55.97; H, 3.96; N, 18.55%; IR: m 3025 (C–H, Ar–H), 1721
(C=O), 1620 (C=N quinazoline), 1185 (C–O) cm-1; 1HNMR:
d 7.42–8.38 (m, 13H, Ar–H,), FAB-Mass: 356 (M??1).
Compounds (12a–p) were synthesized by condensing
different substituted benzothiazoles with 2-phenyl-4H
benzo[d] [1,3] oxazin-4-one(6) at 4,5,6, and 7 positions. The
spectral analyses data of synthesized compounds are as follows:
3-(5-Chloro-1,3-benzothiazol-2-yl)-2-phenylquinazolin-
4(3H)-one (12b)
Yield 80%, mp. 196–198°C; Anal Calcd for
C21H12ClN3OS: C, 64.70; H, 3.10; N, 10.78%; Found: C,
64.66; H, 3.08; N, 10.74%; IR: m 3042 (C–H, Ar–H), 1719
(C=O), 1625 (C=N), 585 cm-1 (Ar–Cl); 1HNMR: d
7.48–8.34 (m, 12H, Ar–H,), FAB-Mass: 391 (M??1).
3-(5-Hydroxy-1,3-benzothiazol-2-yl)-2-phenylquinazolin-
4(3H)-one (12c)
Synthesis of 3-(1,3-benzothiazol-2-yl) 2-phenyl
quinazolin-4(3H)-ones via 2-phenyl-4H benzo[d] [1,3]
oxazin-4-one as precursors (Route B)
Yield 79%, mp. 276–278°C; Anal Calcd for C21H13N3O2S:
C, 67.91; H, 3.53; N, 11.31%; Found: C, 55.97; H, 3.96; N,
18.55%; IR: m 3363(–OH), 3040 (C–H, Ar–H), 1726 (C=O),
1
1621 (C=N), 1175 (C–O) cm-1; HNMR: d 7.23–8.30 (m,
Preparation of 2-phenyl-4H benzo[d] [1,3] oxazin-4-
one(15)
12H, Ar–H,), 5.54 (bs, 1H, –OH), FAB-Mass: 372 (M??1).
3-(6-Hydroxy-1,3-benzothiazol-2-yl)-2-phenylquinazolin-
4(3H)-one (12d)
The mixture of anthranilic acid (0.246 mol) with 50.0 ml
absolute ethanol and 2.5 ml concentrated sulfuric acid
was refluxed for 4 h, to give ethyl anthranilate. The
mixture of ethyl anthranilate (0.01 mol) with 100 ml
absolute ethanol and benzoic anhydride (0.01 mol) was
refluxed further for 48 h to form compound 15. The
reaction mixture was neutralized with saturated sodium
bicarbonate solution and the pale yellow solid which
separated was filtered, washed with water and recrystal-
lised from ethanol.
Yield 80%, mp. 275–277°C; Anal Calcd for C21H13N3O2S:
C, 67.91; H, 3.53; N, 11.31%; Found: C, 55.97; H, 3.96; N,
18.55%; IR: m 3357(–OH), 3032 (C–H, Ar–H), 1725
(C=O), 1622 (C=N), 1177 (C–O) cm-1 1HNMR: d
;
7.36–8.18 (m, 12H, Ar–H,), 5.40 (bs, 1H, –OH), FAB-
Mass: 372 (M??1).
3-(6-Bromo-1,3-benzothiazol-2-yl)-2-phenylquinazolin-
4(3H)-one (12e)
Yield 83%, mp. 113–115°C (lit [20] 114°C); Anal Calcd
for C14H9NO2: C, 75.33; H, 4.06; N, 6.27%; Found: C,
55.97; H, 3.96; N, 18.55%; IR: m 3030 (C–H, Ar–H), 1725
1
(C=O), 1595 (C=N), 1180 (C–O) cm-1; HNMR: d 7.61
Yield 81%, mp. 212–214°C; Anal Calcd for
C21H12BrN3OS: C, 58.08; H, 2.78; N, 9.68%; Found: C,
(m, 9H, Ar–H,); FAB-Mass: 225 (M??1).
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Med Chem Res (2012) 21:1136–1148
1147
3-(5-Methoxy-1,3-benzothiazol-2-yl)-2-phenylquinazolin-
4(3H)-one (12k)
55.97; H, 3.96; N, 18.55%; IR: m 3040 (C–H, Ar–H),
1723(C=O), 1618 (C=N), 612 cm-1(C–Br); 1HNMR: d
7.52–8.28 (m, 12H, Ar–H,), FAB-Mass: 435 (M??1).
Yield 75%, mp. 188–190°C; Anal Calcd for C22H15N3O2S:
C, 68.55; H, 3.92; N, 10.90%; Found: C, 55.97; H, 3.96; N,
18.55%; IR: m 3035 (C–H, Ar–H), 2867(C–H, sp3), 1722
(C=O), 1621 (C=N); 1HNMR: d 3.85 (s, 3H, –OCH3),
7.29–8.21 (m, 12H, Ar–H,); FAB-Mass: 386 (M??1).
3-(5-Bromo-1,3-benzothiazol-2-yl)-2-phenylquinazolin-
4(3H)-one (12f)
Yield 80%, mp. 208–210°C; Anal Calcd for
C21H12BrN3OS: C, 58.08; H, 2.78; N, 9.68%; Found: C,
55.97; H, 3.96; N, 18.55%; IR: m 3045 (C–H, Ar–H), 1716
(C=O), 1618 (C=N), 613 cm-1(C–Br); 1HNMR: d 7.34–8.48
(m, 12H, Ar–H,), FAB-Mass: 435 (M??1).
3-(4-Methoxy-1,3-benzothiazol-2-yl)-2-phenylquinazolin-
4(3H)-one (12l)
Yield 77%, mp. 214–216°C; Anal Calcd for C22H15N3O2S:
C, 68.55; H, 3.92; N, 10.90%; Found: C, 68.41; H, 3.82; N,
10.53%; IR: m 3038(C–H, Ar–H), 2910(C–H, sp3),1722
(C=O), 1620 (C=N); 1HNMR: d 3.82 (s, 3H, –OCH3),
7.26–8.24 (m, 12H, Ar–H,); FAB-Mass: 386 (M??1).
3-(5-Methyl-1,3-benzothiazol-2-yl)-2-phenylquinazolin-
4(3H)-one (12g)
Yield 71%, mp. 178–180°C; Anal Calcd for C22H15N3OS:
C, 71.52; H, 4.09; N, 11.37%; Found: C, 55.97; H, 3.96; N,
3-(4-Methyl-1,3-benzothiazol-2-yl)-2-phenylquinazolin-
4(3H)-one (12m)
18.55%; IR:
m
3042 (C–H, Ar–H), 2871(C–H,
sp3),1729(C=O), 1626(C=N), 1374 cm-1 (C–H deforma-
1
tion); HNMR: d 2.45 (d, 3H, –CH3), 7.39–8.33 (m, 12H,
Yield 74%, mp. 150–152°C; Anal Calcd for C22H15N3OS: C,
71.52; H, 4.09; N, 11.37%; Found: C, 70.98; H, 3.96; N,
11.25%; IR: m 3045 (C–H, Ar–H), 2869(C–H, sp3),
1722 (C=O), 1628(C=N), 1376 cm-1 (C–H deformation);
1HNMR: d 2.48 (d, 3H, –CH3), 7.36–8.42 (m, 12H, Ar–H,);
FAB-Mass: 370 (M??1).
Ar–H,); FAB-Mass: 370 (M??1).
3-(6-Methyl-1,3-benzothiazol-2-yl)-2-phenylquinazolin-
4(3H)-one (12h)
Yield 72%, mp. 182–184°C; Anal Calcd for C22H15N3OS:
C, 71.52; H, 4.09; N, 11.37%; Found: C, 55.97; H, 3.96; N,
18.55%; IR: m 3043 (C–H, Ar–H), 2866(C–H, sp3),1735
(C=O), 1624 (C=N), 1373 cm-1 (C–H deformation);
1HNMR: d 2.43 (d, 3H, –CH3), 7.39–8.23 (m, 12H, Ar–H,);
FAB-Mass: 370 (M??1).
3-(7-Methyl-1,3-benzothiazol-2-yl)-2-phenylquinazolin-
4(3H)-one (12n)
Yield 69%, mp. 166–168°C; Anal Calcd for C22H15N3OS:
C, 71.52; H, 4.09; N, 11.37%; Found: C, 55.97; H, 3.96; N,
18.55%; IR: m 3042 (C–H, Ar–H), 2853(C–H, sp3),1719
(C=O), 1623 (C=N), 1370 cm-1 (C–H deformation);
1HNMR: d 2.42 (d, 3H, –CH3), 7.36–8.28 (m, 12H,
Ar–H,); FAB-Mass: 370 (M??1).
3-(4-Hydroxy-1,3-benzothiazol-2-yl)-2-phenylquinazolin-
4(3H)-one (12i)
Yield 81%, mp. 278–280°C; Anal Calcd for C21H13N3O2S:
C, 67.91; H, 3.53; N, 11.31%; Found: C, 67.85; H, 3.48; N,
11.09%; IR: m 3371(–OH), 3035 (C–H, Ar–H), 1725
3-(4-Bromo-1,3-benzothiazol-2-yl)-2-phenylquinazolin-
4(3H)-one (12o)
(C=O), 1618 (C=N), 1178 (C–O) cm-1
;
1HNMR: d
7.22–8.12 (m, 12H, Ar–H,), 5.42 (bs, 1H, –OH), FAB-Mass:
372 (M??1).
Yield 83%, mp. 206–208°C; Anal Calcd for
C21H12BrN3OS: C, 58.08; H, 2.78; N, 9.68%; Found: C,
57.91; H, 2.73; N, 9.55%; IR: m 3042 (C–H, Ar–H), 1722
(C=O), 1618 (C=N), 612 cm-1(C–Br); 1HNMR:
d
3-(7-Hydroxy-1,3-benzothiazol-2-yl)-2-phenylquinazolin-
4(3H)-one (12j)
7.35–8.24 (m, 12H, Ar–H,), FAB-Mass: 435 (M??1).
3-(7-Bromo-1,3-benzothiazol-2-yl)-2-phenylquinazolin-
4(3H)-one (12p)
Yield 78%, mp. 268–270°C; Anal Calcd for C21H13N3O2S:
C, 67.91; H, 3.53; N, 11.31%; Found: C, 55.97; H, 3.96; N,
18.55%; IR: m 3356(–OH), 3036 (C–H, Ar–H), 1715
(C=O), 1618 (C=N), 1181 (C–O) cm-1 1HNMR: d
;
Yield 82%, mp. 210–212°C; Anal Calcd for
C21H12BrN3OS: C, 58.08; H, 2.78; N, 9.68%; Found: C,
55.97; H, 3.96; N, 18.55%; IR: m 3038 (C–H, Ar–H), 1724
7.18–8.22 (m, 12H, Ar–H,), 5.42 (bs, 1H, –OH), FAB-
Mass: 372 (M??1).
123

1148
Med Chem Res (2012) 21:1136–1148
1
(C=O), 1621 (C=N), 614 cm-1 (C–Br); HNMR: d 7.54–
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of 6-substituted-2-benzothiazolamines and 3-substituted-2-
imino-benzothiazolines. J Med Chem 42:2828
8.32 (m, 12H, Ar–H,), FAB-Mass: 435 (M??1).
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Conclusion
A number of new 3-(1,3-benzothiazol-2-yl) 2-phenyl qui-
nazolin-4(3H)-one compounds were synthesized and
screened for their inherent antibacterial potential. Outcome
of the rigorous QSAR studies reveals that compounds 12b,
12e, 12f, 12o, and 12p are identified as most potent anti-
bacterial agents. In this regard contribution of LOG P and
DDE parameters was found remarkably significant. Hence
in view to cater the needs associated with ever increasing
demand of newer antibacterial agents, exploration of these
findings can envisage these compounds as powerful anti-
bacterial agents.
´
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RA, Keri G (2004) Improved, high yield synthesis of 3H-
´
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benzothiazolyl- and benzoxazolylureas. J Med Chem 12:1016
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4-(40-substituted-2-thiazolylamino)- and 4-(40-substituted-30-alkyl-
DELTA.40-thiazoline-20-imino)phenylacetic acids. Indian J Chem
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Acknowledgments We are grateful to Central Drug Research
Institute, Lucknow and Defence Research Development Establish-
ment, Gwalior, India for providing spectroanalytical facilities.
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