Microwave synthesis of new biologically important 1,4-dihydropyridines containing benzothiazole moiety

Article abstract of DOI:10.1135/cccc2009051

2,3,5,6-Tetrasubstituted-4-aryl-1-(6-ethoxybenzothiazol-2-yl)-1, 4-dihydropyridines were synthesized by the reaction of 2-amino-6- ethoxybenzothiazole, an aromatic aldehyde and an active methylene compound in methanol by conventional or microwave irradiat

Full text of DOI:10.1135/cccc2009051

Microwave Synthesis
275
MICROWAVE SYNTHESIS OF NEW BIOLOGICALLY IMPORTANT
1,4-DIHYDROPYRIDINES CONTAINING BENZOTHIAZOLE MOIETY
b
c
MITHLESHa1, Pawan K. PAREEKa2, Hemraj CHIPPA , RAVIKANT and
a3,
Krishan G. OJHA
*
a
Department of Pure and Applied Chemistry, Maharshi Dayanand Saraswati University,
Ajmer, India; e-mail: singhmithlesh@rocketmail,com, pareekpawan@gmail.com,
1
2
3
kgojha@rediffmail.com
b
c
Department of Microbiology, Maharshi Dayanand Saraswati University, Ajmer, India;
e-mail: hrchhipa8@gmail.com
Hygia Institute of Pharmaceutical Education and Research, Lucknow, India;
e-mail: drravikant78@gmail.com
Received April 14, 2009
Accepted October 28, 2009
Published online March 8, 2010
2,3,5,6-Tetrasubstituted-4-aryl-1-(6-ethoxybenzothiazol-2-yl)-1,4-dihydropyridines were syn-
thesized by the reaction of 2-amino-6-ethoxybenzothiazole, an aromatic aldehyde and an
active methylene compound in methanol by conventional or microwave irradiation method
(solvent-free or with solid support). All compounds were tested for antibacterial and
antifungal activities and the results were compared with standard drugs. Their acaricidal and
antifeedant activities were also tested.
Keywords: Benzothiazole; 1,4-Dihydropyridine; Microwave synthesis; Antimicrobial activity;
Acaricidal activity; Antifeedant activity.
Dihydropyridine chemistry is of interest from the view point of research on
heterocyclic compounds and also from a biological point of view1. Hantzsch
1,4-dihydropyridines (1,4-DHPs), a class of model compounds of NADH
coenzyme, have been extensively studied in view of their biological impor-
tance in NADH redox processes2. Since the early 1980’s the presence of the
DHP ring in the structure of 1,4-DHP derivatives has been regarded as a pre-
requisite for introduction of calcium (Ca2+) channel3. So, 1,4-DHPs are ex-
cellent starting synthons for development of antitubercular agents4–6. As a
result, newly synthesized 1,4-DHPs possess different pharmacological activ-
ities such as anticancer7, antidiabetic8, antianginal9, bronchodilating10,
neurotropic11, acaricidal, insecticidal, bactericidal and herbicidal12,13. They
are extensively used in the treatment of angina pectoris, hypertension and
arrhythmia14 and of some cardiovascular disorder. Several new 1,4-DHP de-
Collect. Czech. Chem. Commun. 2010, Vol. 75, No. 3, pp. 275–287
© 2010 Institute of Organic Chemistry and Biochemistry
doi:10.1135/cccc2009051

276
Mithlesh, Pareek, Chippa, Ravikant, Ojha:
rivatives have been prepared and pharmacologically evaluated in order to
find drugs with better pharmacological properties15. So, the pharmacology
of 1,4-DHP derivatives is on the eve of a novel boom.
Similarly, benzothiazole rings are present in various marine or terrestrial
natural compounds, which have useful biological activities16–19. In addition
benzothiazoles are a class of therapeutic compounds that exert a broad
spectrum of biological activities such as antimicrobial20,21, anticancer22,23
,
antifungal24, antihelmintic25, antileishmanial26, anticonvulsant27 anti-
inflammatory28 and insecticidal29–31 activities. 2-(4-Aminophenyl)benzo-
thiazoles32,33 comprise novel mechanistic antitumor agents. Based on the
above evidence revealing that both 1,4-DHP and benzothiazole moieties
show enhanced biological effects, we have designed a synthetic strategy
that leads to both moieties in the same molecule. The inclusion of these
two rings in the same molecule may enhance the bioactivity of the com-
pound. 2-Amino-6-ethoxybenzothiazole was treated with active methylene
compounds (ethyl acetoacetate, diethyl malonate, acetylacetone) and
aromatic aldehydes (benzaldehyde, 4-hydroxybenzaldehyde, 4-(dimethyl-
amino)benzaldehyde, 4-methoxybenzaldehyde) on steam bath for 2–3 h
and refluxed in methanol for 10–15 h. The above described method appar-
ently suffers from disadvantages such as prolonged refluxing, use of volatile
organic solvents, low to moderate yields, cumbersome work-up, pollution
of the environment and lack of selectivity in the presence of other func-
tional groups.
Incresing growing energy demands, economic and environmental issue
over the recent decade has compelled the synthetic chemist to add a new
twist to an old theme34. The microwave-induced organic reaction enhance-
ment (MORE) chemistry can be termed eco-chemistry because it is easy, ef-
fective, economic, eco-friendly and is believed to be a step towards green
chemistry. We now report a novel environmental approach using a facile
microwave synthesis of the title compounds carried out by the solvent-free
method, and various solid supports like silica gel, basic alumina, neutral
alumina and acidic alumina. Hence, with a view to further assess the phar-
macological profile of benzothiazole and 1,4-DHPs, and benefits associated
with microwave synthesis, it was thought worthwhile to synthesize some
new compounds by incorporating the 2-amino-6-ethoxybenzothiazole and
1,4-DHP moieties in a single molecule. The present work deals with the syn-
thesis of 2,3,5,6-tetrasubstituted-4-aryl-1-(6-ethoxybenzothiazol-2-yl)-1,4-
dihydropyridines followed by antimicrobial susceptibility test (AST) against
Lactobacillus sp., Pseudomonas aeruginosa, Staphylococcus aureus, Micrococcus
leutius, Kocuria rosea, Aspergillus niger, Aspergillus candidus using standard
Collect. Czech. Chem. Commun. 2010, Vol. 75, No. 3, pp. 275–287

Microwave Synthesis
277
methods and comparison with standard drugs. All the synthesized com-
pounds were also screened for their acaricidal and antifeedant activities.
RESULTS AND DISCUSSIONS
2-Amino-6-ethoxybenzothiazole (1) was synthesized by the reported meth-
ods35,36. 2,3,5,6-Tetrasubstituted-4-aryl-1-(6-ethoxybenzothiazol-2-yl)-1,4-
dihydropyridines 4 were synthesized by the reaction of equimolar quanti-
3
3
R
1
a
b
c
H
OH
OMe
1
2
2
R
R
d
NH
2
a
b
c
Me COOEt
EOt COOEt
Me COOH
1
2
3
4
R
R
R
a
b
c
d
e
f
g
h
i
Me COOEt NMe
OEt COOEt NMe
Me COOEt OMe
Me COOEt OH
OEt COOEt OH
2
2
OEt COOEt
Me COMe OMe
Me COOEt
Me COMe OH
Me COMe
H
H
j
H
k
l
Me COMe NMe
OEt COOEt OMe
2
SCHEME 1
Synthesis of 2,3,5,6-tetrasubstituted-4-aryl-1-(6-ethoxybenzothiazol-2-yl)-1,4-dihydropyridines 4
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278
Mithlesh, Pareek, Chippa, Ravikant, Ojha:
ties of 1 and aromatic aldehyde 3 with 2 equivalents of active methylene
compound 2. The mixture was heated without solvent on steam bath for
2–3 h. After elimination of water, methanol (25 ml) was added and the re-
action mixture was refluxed for 10–15 h. In view of a long reaction time,
moderate yields (Table I), tedious work-up and requirement for large quan-
tities of solvent, a more versatile yet simplified procedure was designed. The
procedure consists of reacting 2-amino-6-ethoxybenzothiazole, an aromatic
aldehyde and an active methylene compound reacted without using any
solvent with or without solid support using microwave synthesis37. The
strategy worked well affording the desired product in improved yields and
in a significantly lower reaction time (Tables I and II). In microwave-
promoted reactions solid support like silica gels, basic alumina, neutral alu-
mina and acid alumina have also been used. It was found that acid alumina
is the best solid support for the present purpose. The synthesized com-
pounds were characterized by their analytical and IR, 1H NMR and mass
spectral data.
TABLE I
Synthesis of 2,3,5,6-tetrasubstituted-4-aryl-1-(6-ethoxybenzothiazol-2-yl)-1,4-dihydro-
pyridines 4
Conventional
heating
Microwave
heating
M.p.
°C
Compd.
R1
R2
R3
time, h yield, %
time, s yield, %
4a
4b
4c
4d
4e
4f
Me
OEt
Me
Me
OEt
OEt
Me
Me
Me
Me
Me
OEt
COOEt
COOEt
COOEt
COOEt
COOEt
COOEt
COMe
COOEt
COMe
COMe
COMe
COOEt
NMe2
NMe2
OMe
OH
155
170
90
20.0
20.0
20.0
20.0
20.0
20.0
20.0
20.0
20.0
20.0
20.0
20.0
53
55
53
56
58
60
60
50
57
56
52
56
180
190
180
200
170
190
180
180
130
180
170
160
65
70
72
70
72
75
75
65
67
68
68
67
130
125
136
134
98
OH
H
4g
4h
4i
OMe
H
OH
110
158
98
4j
H
4k
4l
NMe2
OMe
152
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Microwave Synthesis
279
EXPERIMENTAL
Reagent grade chemicals were used without further purification. The chemicals and solvents
were used as received. All the melting points were measured in open capillaries and are un-
corrected. The purity of the synthesized compounds was checked by thin layer chromatogra-
phy. IR spectra were scanned at FT IR Perkin–Elmer (Spectrum RX1) spectrophotometer (nmax
in cm–1) using KBr discs. 1H NMR was recorded in CDCl3 with tetramethylsilane (TMS) as
the internal standard at 300 MHz on a Bruker DRTX-300 spectrophotometer. The chemical
shifts are reported in ppm (d-scale), coupling constants, J in Hz. Fast atom bombardment
mass spectra (FABMS) were recorded at room temperature on a Jeol SX-102/DA-6000 mass
spectrophotometer/data system using argon/xenon (6 kV, 10 mA) as the FAB gas. The ac-
celerating potential was 10 kV. Microwave (MW) synthesis was carried out in a Q-pro-M
Modified Microwave system. Elemental analysis of compounds was performed on a Carlo
Erba-1108 elemental analyzer.
TABLE II
Microwave synthesis of 2,3,5,6-tetrasubstituted-4-aryl-1-(6-ethoxybenzothiazol-2-yl)-1,4-di-
hydropyridines (4a–4l) – the effect of solid supported
Alumina
Silica
Compd.
basic
neutral
acid
time, s yield, %
time, s yield, %
time, s yield, %
time, s yield, %
4a
4b
4c
4d
4e
4f
200
210
200
220
190
200
230
210
210
220
200
190
60
65
67
66
67
70
70
59
62
63
60
63
240
270
250
280
255
240
240
260
270
250
250
250
53
58
50
55
57
58
58
53
54
52
48
50
100
120
90
72
79
78
79
80
85
87
78
79
76
76
74
60
50
40
70
60
60
80
60
90
60
60
60
82
91
93
92
93
95
98
97
92
90
90
89
100
100
100
120
100
120
100
100
100
4g
4h
4i
4j
4k
4l
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280
Mithlesh, Pareek, Chippa, Ravikant, Ojha:
Synthesis of 2,3,5,6-Tetrasubstituted-4-aryl-1-(6-ethoxybenzothiazol-2-yl)-
1,4-dihydropyridines. General Procedure
A. Conventional method. A mixture of 2-amino-6-ethoxybenzothiazole (1; 0.1 mol), an aro-
matic aldehyde 3 (0.1 mol) and an active methylene compound 2 (0.2 mol) was heated
without solvent on steam bath for 2–3 h. After evaporations of water, methanol (25 ml) was
added and the reaction mixture was refluxed for 10–15 h. Then the reaction mixture was
poured into ice water, the insoluble portion was separated, extracted with diethyl ether
(50 ml) and dried over anhydrous magnesium sulfate, and the residue after evaporation was
recrystallized from methanol (Scheme 1, Table I).
B. Microwave method. A mixture of 2-amino-6-ethoxybenzothiazole (1; 0.01 mol), an aro-
matic aldehyde 3 (0.01 mol) and an active methylene compound 2 (0.02 mol) was placed
in a microwave oven, microwave-irradiated with a low power and monitored by TLC in
benzene–DMF (7:3). The reaction mixture was cooled to room temperature, extracted with
diethyl ether (10 ml) and the extract was dried over anhydrous magnesium sulfate. The
crude product was recrystallized from methanol (Table I).
C. Solid-supported microwave synthesis. A mixture of 2-amino-6-ethoxybenzothiazole (1;
0.01 mol), an aromatic aldehyde 3 (0.01 mol), an active methylene compound 2 (0.02 mol),
alumina (acid/basic/neutral) and silica gel was mixed thoroughly in a mortar. Then the mix-
ture was placed in a flask which was irradiated in microwave oven for 30-s intervals at pre-
determined times. On completion of the reaction as monitored by TLC in benzene–DMF
(7:3), the reaction mixture was cooled to room temperature and extracted with diethyl ether
(10 ml). The extract dried over anhydrous magnesium sulfate gave pure product which was
recrystallized from methanol (Table II).
4-[4-(Dimethylamino)phenyl]-1-(6-ethoxybenzothiazol-2-yl)-3,5-bis(ethoxycarbonyl)-
2,6-dimethyl-1,4-dihydropyridine (4a). 1H NMR: 1.29 t, 6 H, J = 7.2 (DHP CH2CH3); 1.33 t,
3 H, J = 7.2 (benzothiazole OCH2CH3); 2.22 s, 6 H (DHP CH3); 2.86 s, 6 H (N(CH3)2); 3.97 q,
2 H, J = 7.1 (benzothiazole OCH2CH3); 4.18 q, 4 H, J = 7.2 (DHP CH2); 4.90 s, 1 H (DHP);
6.46–8.11 m, 7 H (Ar-H). IR: 1063, 1108, 1150, 1452, 1532, 1586, 1628, 1664, 2996. MS,
m/z: calculated 549.68, found 549.67. For C30H35N3O5S calculated: 65.55% C, 6.42% H,
7.64% N, 5.83% S; found: 65.40% C, 6.41% H, 7.60% N, 5.84% S.
4-[4-(Dimethylamino)phenyl]-2,6-diethoxy-1-(6-ethoxybenzothiazol-2-yl)-3,5-bis(ethoxy-
carbonyl)-1,4-dihydropyridine (4b). 1H NMR: 1.21 t, 6 H, J = 7.2 (DHP OCH2CH3); 1.29 t, 6 H,
J = 7.2 (COOCH2CH3); 1.33 t, 3 H, J = 7.1 (benzothiazole OCH2CH3); 2.81 s, 6 H (N(CH3)2);
3.97 q, 2 H, J = 7.0 (benzothiazole OCH2); 4.01 q, 4 H, J = 7.2 (DHP OCH2); 4.16 q, 4 H, J =
7.1 (COOCH2CH3); 4.93 s, 1 H (DHP); 6.46–8.11 m, 7 H (Ar-H). IR: 1062, 1105, 1151, 1450,
1530, 1583, 1622, 1652, 2995. MS, m/z: calculated 609.74, found 609.72. For C32H39N3O7S
calculated: 63.04% C, 6.45% H, 6.89% N, 5.26% S; found: 63.03% C, 6.42% H, 6.87% N,
5.25% S.
1-(6-Ethoxybenzothiazol-2-yl)-3,5-bis(ethoxycarbonyl)-4-(4-methoxyphenyl)-2,6-dimethyl-
1,4-dihydropyridine (4c). 1H NMR: 1.28 t, 6 H, J = 7.2 (DHP CH2CH3); 1.33 t, 3 H, J = 7.0
(benzothiazole OCH2CH3); 2.28 s, 6 H (DHP CH3); 3.73 s, 3 H (Ar-OCH3); 3.97 q, 2 H, J =
7.1 (benzothiazole OCH2CH3); 4.18 q, 4 H, J = 7.2 (DHP CH2CH3); 4.91 s, 1 H (DHP);
6.46–8.11 m, 7 H (Ar-H). IR: 1054, 1225, 1450, 1535, 1590, 1628, 1655. MS, m/z: calculated
536.64, found 536.62. For C29H32N2O6S calculated: 64.91% C, 6.01% H, 5.22% N, 5.97% S;
found: 64.87% C, 6.00% H, 5.20% N, 5.94% S.
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Microwave Synthesis
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1-(6-Ethoxybenzothiazol-2-yl)-3,5-bis(ethoxycarbonyl)-4-(4-hydroxyphenyl)-2,6-dimethyl-
1,4-dihydropyridine (4d). 1H NMR: 1.31 t, 6 H, J = 7.0 (DHP CH3); 1.33 t, 3 H, J = 7.2
(benzothiazole OCH2CH3); 2.14 s, 6 H (DHP CH3); 3.97 q, 2 H, J = 7.1 (benzothiazole
OCH2CH3); 4.19 q, 4 H, J = 6.9 (DHP CH2); 4.94 s, 1 H (DHP); 5.01 s, 1 H (OH); 6.48–8.14 m,
7 H (Ar-H). IR: 1056, 1136, 1227, 1451, 1536, 1627, 1662, 1694, 2973, 3650. MS, m/z: calcu-
lated 522.16, found 522.15. For C28H30N2O6S calculated: 64.35% C, 5.79% H, 5.36% N,
6.13% S; found: 64.34% C, 5.78% H, 5.34% N, 6.14% S.
2,6-Diethoxy-1-(6-ethoxybenzothiazol-2-yl)-3,5-bis(ethoxycarbonyl)-4-(4-hydroxyphenyl)-
1,4-dihydropyridine (4e). 1H NMR: 1.24 t, 6 H (DHP OCH2CH3); 1.29 t, 6 H, J = 7.2 (DHP
COOCH2CH3); 1.33 t, 3 H, J = 7.1 (benzothiazole OCH2CH3); 3.97 q, 2 H, J = 7.1
(benzothiazole OCH2CH3); 4.02 q, 4 H (DHP OCH2); 4.19 q, 4 H, J = 7.2 (DHP COOCH2);
4.90 s, 1 H (DHP); 5.01 s, 1 H (OH); 6.47–8.12 m, 7 H (Ar-H). IR: 1055, 1137, 1228, 1452,
1538, 1593, 1594, 1624, 1650, 1689, 2975, 3575. MS, m/z: calculated 582.67, found 582.65.
For C30H34N2O8S calculated: 61.84% C, 5.88% H, 4.81% N, 5.50% S; found: 61.82% C,
5.86% H, 4.80% N, 5.49% S.
2,6-Diethoxy-1-(6-ethoxybenzothiazol-2-yl)-3,5-bis(ethoxycarbonyl)-4-phenyl-1,4-dihydro-
pyridine (4f). 1H NMR: 1.20 t, 6 H, J = 7.2 (DHP OCH2CH3); 1.26 t, 6 H, J = 7.1 (DHP
COOCH2CH3); 1.33 t, 3 H, J = 7.0 (benzothiazole OCH2CH3); 3.97 q, 2 H, J = 6.9
(benzothiazole OCH2CH3); 4.00 q, 4 H, J = 7.2 (DHP OCH2); 4.17 q, 4 H, J = 7.1 (DHP
COOCH2); 4.94 s, 1 H (DHP); 6.47–8.12 m, 8 H (Ar-H). IR: 1058, 1210, 1380, 1458, 1546,
1597, 1628, 1670, 2975. MS, m/z: calculated 566.67, found 566.65. For C30H34N2O7S calcu-
lated: 63.59% C, 6.05% H, 4.94% N, 5.66% S; found: 63.58% C, 6.04% H, 4.92% N, 5.65% S.
3,5-Diacetyl-1-(6-ethoxybenzothiazol-2-yl)-4-(4-methoxyphenyl)-2,6-dimethyl-1,4-dihydro-
pyridine (4g). 1H NMR: 1.33 t, 3 H, J = 7.1 (benzothiazole OCH2CH3); 2.18 s, 6 H (DHP CH3);
2.30 s, 6 H (DHP COCH3); 3.73 s, 3 H (Ar-OCH3); 3.97 q, 2 H, J = 7.0 (benzothiazole
OCH2CH3); 4.96 s, 1 H (DHP); 6.47–8.12 m, 7 H (Ar-H). IR: 1053, 1226, 1450, 1535, 1596,
1622, 1662. MS, m/z: calculated 476.59, found 476.54. For C27H28N2O4S calculated: 68.05% C,
5.92% H, 5.88% N, 6.73% S; found: 68.04% C, 5.91% H, 5.87% N, 6.72% S.
1-(6-Ethoxybenzothiazol-2-yl)-3,5-bis(ethoxycarbonyl)-2,6-dimethyl-4-phenyl-1,4-dihydro-
pyridine (4h). 1H NMR: 1.26 t, 6 H, J = 7.2 (DHP CH3); 1.33 t, 3 H, J = 7.1 (benzothiazole
OCH2CH3); 2.33 s, 6 H (DHP CH3); 3.97 q, 2 H, J = 7.1 (benzothiazole OCH2CH3); 4.15 q,
4 H (DHP CH2); 4.95 s, 1 H (DHP); 6.46–8.10 m, 8 H (Ar-H). IR: 1058, 1209, 1378, 1450,
1545, 1595, 1628, 1668, 2973. MS, m/z: calculated 506.62, found 506.60. For C28H30N2O5S
calculated: 66.38% C, 5.97% H, 5.43% N, 6.83% S; found: 66.37% C, 5.96% H, 5.52% N,
6.82% S.
3,5-Diacetyl-1-(6-ethoxybenzothiazol-2-yl)-4-(4-hydroxyphenyl)-2,6-dimethyl-1,4-dihydro-
pyridine (4i). 1H NMR: 1.33 t, 3 H, J = 7.2 (benzothiazole OCH2CH3); 2.21 s, 6 H (DHP CH3);
2.31 s, 6 H (DHP COCH3); 3.97 q, 2 H, J = 7.1 (benzothiazole OCH2CH3 ); 4.96 s, 1 H (DHP);
5.00 s, 1 H (OH); 6.48–8.14 m, 7 H (Ar-H). IR: 1054, 1137, 1207, 1450, 1535, 1590, 1628,
1660, 1691, 2975, 3573. MS, m/z: calculated 462.56, found 462.52. For C26H26N2O4S calcu-
lated: 67.51% C, 5.67% H, 6.06% N, 6.93% S; found: 66.50% C, 5.60% H, 6.04% N, 6.91% S.
3,5-Diacetyl-1-(6-ethoxybenzothiazol-2-yl)-2,6-dimethyl-4-phenyl-1,4-dihydropyridine (4j).
1H NMR: 1.33 t, 3 H (benzothiazole OCH2CH3); 2.16 s, 6 H (DHP CH3); 2.30 s, 6 H (DHP
COCH3); 3.97 q, 2 H (benzothiazole OCH2CH3); 4.96 s, 1 H (DHP); 6.47–8.12 m, 8 H (Ar-H).
IR: 1056, 1209, 1375, 1455, 1543, 1594, 1628, 1667, 2974. MS, m/z: calculated 446.56, found
446.52. For C26H26N2O3S calculated: 69.93% C, 5.87% H, 6.27% N, 7.18% S; found: 69.90% C,
5.80% H, 6.24% N, 7.15% S.
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Mithlesh, Pareek, Chippa, Ravikant, Ojha:
3,5-Diacetyl-4-[4-(dimethylamino)phenyl]-1-(6-ethoxybenzothiazol-2-yl)-2,6-dimethyl-1,4-di-
hydropyridine (4k). 1H NMR: 1.33 t, 3 H, J = 7.1 (benzothiazole OCH2CH3); 2.28 s, 6 H (DHP
CH3); 2.30 s, 6 H (DHP COCH3); 2.85 s, 6 H (N(CH3)2); 3.97 q, 2 H, J = 7.1 (benzothiazole
OCH2CH3); 4.93 s, 1 H (DHP); 6.47–8.12 m, 7 H (Ar-H). IR: 1063, 1103, 1150, 1451, 1528,
1582, 1622, 1656, 2994. MS, m/z: calculated 489.63, found 489.60. For C28H31N3O3S calcu-
lated: 68.69% C, 6.38% H, 8.58% N, 6.55% S; found: 68.60% C, 6.32% H, 8.54% N, 6.52% S.
2,6-Diethoxy-1-(6-ethoxybenzothiazol-2-yl)-3,5-bis(ethoxycarbonyl)-4-(4-methoxyphenyl)-1,4-di-
hydropyridine (4l). 1H NMR: 1.22 t, 6 H, J = 7.2 (DHP OCH2CH3); 1.30 t, 6 H, J = 7.2 (DHP
COOCH2CH3); 1.33 t, 3 H, J = 7.0 (benzothiazole OCH2CH3); 3.73 s, 3 H (Ar-OCH3); 3.97 q,
2 H, J = 6.9 (benzothiazole OCH2CH3 ); 4.00 q, 4 H, J = 6.9 (DHP OCH2); 4.19 q, 4 H, J = 7.1
(DHP COOCH2); 4.95 s, 1 H (DHP); 6.47–8.12 m, 7 H (Ar-H). IR: 1053, 1136, 1224, 1450,
1534, 1590, 1622, 1653, 1701, 2973. MS, m/z: calculated 596.70, found 596.65. For
C
31H36N2O8S: 62.40% C, 6.08% H, 4.69% N, 5.37% S; found: 62.35% C, 6.02% H, 4.65% N,
5.32% S.
Antimicrobial Activity
All the synthesized compounds were tested for their antibacterial activity against Lacto-
bacillus sp., Pseudomonas aeruginosa, Staphylococcus aureus, Micrococcus leutius and Kocuria
rosea as well as antifungal activity against Aspergillus niger and Aspergillus candidus using the
paper disc method. Muller–Hinton agar (Hi-Media Pvt. Ltd. Mumbai, India) was used to for
culturing of the test bacteria and potato dextrose agar was used to culture fungi. The micro-
bial cultures were grown at 37 °C for 8 h and then appropriately diluted with sterile 0.8%
saline solution. The concentration of test drugs was kept 200 mg/ml in DMF. Standard drugs
novobiocine, gentamycin, kanamycin and amikacin (for antibacterial tests), and ampicilline
(for antifungal tests) were used for comparison. The antimicrobial activity was evaluated by
measuring the growth zone inhibition around the disc of the tested organism (Table III).
Antifeedant Activity
The antifeedant activity of the compounds was measured by the leaf dip method38,39, using
fourth instars larvae of Spodoptera litura. The leaf discs of ca. 25 cm2 area were prepared and
dipped for 30 s in various concentrations of the test compounds. They were air-dried to
evaporate the excess of acetone and the leaf discs offered for feeding. The insects were al-
lowed to feed for 24 h. Then the uneaten leaf area was measured with a leaf area meter. The
difference between leaf area provided and the leaf area uneaten is taken as the amount of
the consumed leaf area. The feeding inhibition was calculated and used for calculation of
effective concentration (EC50/LD50) using a maximum probability programmer (MPP) 3.01.
The results of antifeedant activity are summarized in Table IV.
Acaricidal Activity
The acaricidal activity of the compounds was measured by the leaf dip method38,39. Mul-
berry leaf discs (5 cm2 diameter) were dipped in different concentrations of compounds for
30 s. The leaf discs were dried to remove excess of acetone and placed on wet cotton in Petri
dish. The adult female mites were released on the treated leaf discs and the mortality data
were recorded after 48 h. Mites released on leaf treated only with acetone and Tween 20
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283
emulsifier served as control. The mortality data were used for calculation of LC50/LD50 using
a programmer MPP 3.01. The results of acaricidal activity are summarized in Table V.
Statistical Analysis
Statistical analysis of the experimental data was performed using ‘Probit analysis’ to find out
the LC50/LD50 values, regression, chi-square and variance40. The data were analyzed by com-
pletely randomized, one-way analysis of variance (ANOVA) and the means were separated
using Duncan’s multiple range test41 (DMRT).
TABLE III
Antimicrobial activity of 2,3,5,6-tetrasubstituted-4-aryl-1-(6-ethoxybenzothiazol-2-yl)-1,4-di-
hydropyridines (4a–4l)
Antibacterial activity (inhibition zone, mm)
Antifugal activity
Compd.
B1
15
B2
B3
B4
B5
F1
F2
20.0
19.0
26.0
18.0
17.0
21.0
4a
4b
16
10
23.0
17.0
13.0
11.0
18.0
16.0
12.0
11.0
17.0
16.0
18.0
16.0
13.0
17.0
15.0
4.0
28.0
22.0
16.0
15.0
22.0
19.0
24.0
17.0
17.0
25.0
16.0
7.0
20.0
15.0
12.0
10.0
15.0
14.0
16.0
13.0
13.0
17.0
12.0
5.0
20.0
15.0
13.0
10.0
15.0
12.0
16.0
10.0
14.0
17.0
11.0
6.0
22.0
16.0
12.0
15.0
17.0
18.0
17.0
14.0
18.0
20.0
17.0
4.0
4c
4d
7.0
4e
6.0
10.0
9.0
4f
1.08
4g
15.0
19.0
17.0
14.0
22.0
12.0
6.0
4h
13.0
8.0
4i
4j
8.0
4k
14.0
7.0
4l
Benzothiazole
Novobiocin
Gentamycin
Kanamycin
Amikacin
Ampicilin
5.0
7.0
20.0
24.0
11.0
25.0
15.0
22.0
18.0
18.0
35.0
35.0
25.
22.0
24.0
22.0
24.0
–
–
19.0
7.0
–
–
–
–
16.0
32.0
–
–
–
–
–
–
–
30.0
0.0
32.0
0.0
Blank
0.0
0.0
0.0
0.0
0.0
B1, Lactobacillus sp.; B2, Pseudomonas aeruginosa; B3, Staphylococcus aureus; B4, Micrococcus
leutius; B5, Kocuria rosea; F1, Aspergillus candidus; F2, Aspergillus niger.
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Mithlesh, Pareek, Chippa, Ravikant, Ojha:
TABLE IV
Antifeedant activity of 2,3,5,6-tetrasubstituted-4-aryl-1-(6-ethoxybenzothiazol-2-yl)-1,4-di-
hydropyridines (4a–4l)
Compound
Fiducial limits
Slope
Chi-square (3)
LC50/LD50 24 h
4a
4b
4c
4d
4e
4f
0.80–3.43
0.65–1.70
0.42–0.84
0.60–1.41
0.85–2.34
0.70–2.43
0.32–0.48
0.34–0.65
0.43–1.06
0.47–0.78
0.29–0.51
0.25–0.56
0.80 0.13
1.02 0.12
1.03 0.14
1.06 0.14
1.07 0.15
0.94 0.14
1.27 0.14
1.00 0.13
0.86 0.13
1.50 0.16
1.01 0.13
0.96 0.13
0.44 (3)
0.66 (3)
0.35 (3)
1.04 (3)
0.78 (3)
0.22 (3)
3.40 (3)
0.68 (3)
1.70 (3)
2.57 (3)
5.35 (3)
0.26 (3)
1.36
0.98
0.58
0.86
1.24
1.14
0.38
0.43
0.63
0.58
0.40
0.39
4g
4h
4i
4j
4k
4l
CONCLUSIONS
Antibacterial activity. 2-Amino-6-ethoxybenzothiazole (1) shows very low
activity against all the bacteria under study. However, 2,3,5,6-tetra-
substituted-4-aryl-1-(6-ethoxybenzothiazol-2-yl)-1,4-dihydropyridine deriv-
atives 4 show enhanced activity comparable to standard drugs. Compounds
4a, 4b and 4k show a good activity against Lactobacillus sp. Compounds 4a,
4b and 4k exhibit a significant activity against Pseudomonas aeruginosa.
Compounds 4a, 4b and 4h exhibit a significant activity against Staphylococ-
cus aureus. Compounds 4a, 4b and 4k exhibit a significant activity against
Micrococcus leutius. Compounds 4b, 4h and 4k exhibit a significant activity
against Kocuria rosea (Table III).
Antifungal activity. 2,3,5,6-Tetrasubstituted-4-aryl-1-(6-ethoxybenzo-
thiazol-2-yl)-1,4-dihydropyridines 4 exhibit higher antifungal activities as
compared to 2-amino-6-ethoxybenzothiazole (1). Compounds 4a, 4b, 4h
and 4k exhibit a good antifungal activity against Aspergillus candidus,
though lower than the standard drug ampicilline. Compounds 4a, 4b, 4g,
4j and 4k exhibit good antifungal activities against Aspergillus niger, but are
less active than the standard drug ampicilline.
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285
Antifeedant activity. It is evident from LC50/LD50 values given in Table IV.
Compound 4g has the lowest LC50/LD50 value, hence possessing a higher
contact toxicity against insect. The decreasing order of contact toxicity of
compounds is 4g > 4l > 4k > 4h > 4c = 4j > 4i > 4d > 4b > 4f > 4e > 4a.
Acaricidal activity. It is evident from LC50/LD50 values given in Table V.
Compound 4e has the lowest LC50/LD50 value, hence possessing a higher
contact toxicity against insect. The decreasing order of contact toxicity of
compounds is 4e > 4i = 4k > 4f > 4b > 4a = 4g = 4j > 4d > 4c > 4l > 4h.
TABLE V
Acaricidal activity of 2,3,5,6-tetrasubstituted-4-aryl-1-(6-ethoxybenzothiazol-2-yl)-1,4-di-
hydropyridines (4a–4l)
Compound
Fiducial limits
Slope
Chi-square (3)
LC50/LD50 24 h
0.18
0.11–0.32
0.78 0.88
4a
4b
4c
4d
4e
4f
1.70 (3)
2.14 (3)
8.28 (3)
7.52 (3)
14.27 (3)
6.12 (3)
6.91 (3)
3.57 (3)
8.43 (3)
1.70 (3)
9.10 (3)
15.89 (3)
0.12–0.24
0.16–0.36
0.14–0.30
0.06–0.10
0.09–0.24
0.12–0.30
0.36–1.88
0.07–0.20
0.12–0.30
0.08–0.17
0.04–0.06
0.88 0.08
0.09 0.09
0.96 0.09
1.24 0.10
0.65 0.07
0.80 0.08
0.64 0.08
0.64 0.06
0.78 0.08
0.81 0.7
0.15
0.23
0.20
0.08
0.13
0.18
0.70
0.11
0.18
0.11
0.40
4g
4h
4i
4j
4k
4l
0.76 0.07
We are thankful to Central Drug Research Institute, Lucknow, for spectral studies. We are also
thankful to Dr. A. Bhatnagar Head, Department of Microbiology, Maharshi Dayanand Saraswati
University, Ajmer, for providing antimicrobial screening facility.
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