Rapid microwave-assisted solution phase synthesis of 6,8-disubstituted 2-phenyl-3-(substituted- benzothiazol-2-yl)-4-[3H]-quinazolinones as novel anticonvulsants

Article abstract of DOI:10.1080/10426500801957766

A fast and highly efficient microwave accelerated solution phase procedure for the synthesis of a series of 2-phenyl-3-(benzothiazol-2-yl)-4[3H]- quinazolinones, substituted in the benzothiazole ring, is developed. The title compounds were characterized by elemental analyses, IR, 1H NMR, and EI-MS data. The anticonvulsant activity of all the new compounds (3a-m and 4a-m) was evaluated against Maximum Electroshock (MES) induced seizures and against subcutaneous pentylenetetrazole (PTZ) induced seizures model in mice. The neurotoxicity was assessed using the Rotorod procedure. All the compounds tested were administered intraperitoneally at a various dose levels ranging from 7-200 mg/Kg body weight and the median toxic dose (TD50) and the protection index (PI) values were determined. In general compounds 3a-m were found to be more potent compared to compounds 4a-m. Among the compound tested, the compound 3e in the 2-phenyl-3-(benzothiazole-2-yl)-4[3H]-quinazolinone series and 4l in the 6,8-dibromo-2-phenyl-3-(benzothiazole-2-yl)-4[3H]-quinazolinone series were found to be the most potent. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/10426500801957766

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Phosphorus, Sulfur, and Silicon
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Rapid Microwave-Assisted
Solution Phase Synthesis of
6,8-Disubstituted 2-Phenyl-3-
(substituted- benzothiazol-2-
yl)-4-[3H]-quinazolinones as
Novel Anticonvulsants
Sachin S. Laddha a & Satyendra P. Bhatnagar b
a Division of Medicinal Chemistry , R. C. Patel
College of Pharmacy, Karvand Naka, Shirpur, Dist. ,
Dhule, India
b Department of Pharmaceutical Sciences , Birla
Institute of Technology, Mesra , Ranchi, India
Published online: 12 Aug 2008.
To cite this article: Sachin S. Laddha & Satyendra P. Bhatnagar (2008) Rapid
Microwave-Assisted Solution Phase Synthesis of 6,8-Disubstituted 2-Phenyl-3-
(substituted- benzothiazol-2-yl)-4-[3H]-quinazolinones as Novel Anticonvulsants,
Phosphorus, Sulfur, and Silicon and the Related Elements, 183:9, 2262-2273, DOI:
10.1080/10426500801957766
To link to this article: http://dx.doi.org/10.1080/10426500801957766
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Phosphorus, Sulfur, and Silicon, 183:2262–2273, 2008
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500801957766
Rapid Microwave-Assisted Solution Phase Synthesis
of 6,8-Disubstituted 2-Phenyl-3-(substituted-
benzothiazol-2-yl)-4-[3H]-quinazolinones as Novel
Anticonvulsants
Sachin S. Laddha1 and Satyendra P. Bhatnagar2
1Division of Medicinal Chemistry, R. C. Patel College of Pharmacy,
Karvand Naka, Shirpur, Dist. Dhule, India
2Department of Pharmaceutical Sciences, Birla Institute of Technology,
Mesra, Ranchi, India
A fast and highly efficient microwave accelerated solution phase procedure for the
synthesis of a series of 2-phenyl-3-(benzothiazol-2-yl)-4[3H]-quinazolinones, substi-
tuted in the benzothiazole ring, is developed. The title compounds were characterized
1
by elemental analyses, IR, H NMR, and EI-MS data. The anticonvulsant activ-
ity of all the new compounds (3a-m and 4a-m) was evaluated against Maximum
Electroshock (MES) induced seizures and against subcutaneous pentylenetetrazole
(PTZ) induced seizures model in mice. The neurotoxicity was assessed using the Ro-
torod procedure. All the compounds tested were administered intraperitoneally at
a various dose levels ranging from 7-200 mg/Kg body weight and the median toxic
dose (TD ) and the protection index (PI) values were determined. In general com-
50
pounds 3a-m were found to be more potent compared to compounds 4a-m. Among
the compound tested, the compound 3e in the 2-phenyl-3-(benzothiazole-2-yl)-4[3H]-
quinazolinone series and 4l in the 6,8-dibromo-2-phenyl-3-(benzothiazole-2-yl)-
4[3H]-quinazolinone series were found to be the most potent.
Keywords 2-Phenyl-3-(benzothiazol-2-yl)-4[3H]-quinazolinone; 4-[3H]-quinazolinone;
anticonvulsant; microwave irradiation; neurotoxicity
Received 14 December 2007; accepted 28 January 2008.
Parts of this work have been presented at the International Symposium on Drug
Discovery and Process Research (DDPR-2003) Kolhapur, India, January 23–25 2003,
P107.
One of the authors (SSL) is thankful to Ms. Sweety Laddha for her encourage-
ment during the project. The authors wish to thank CDRI Lucknow, India for record-
ing spectra and Principal, R. C. Patel College of Pharmacy, Shirpur, India, for providing
facilities.
Address correspondence to Sachin S. Laddha, Department of Pharmaceutical Sci-
ences, Birla Institute of Technology, Mesra, Ranchi, India. E-mail: pinkumanno@
rediffmail.com
2262

3-(Benzothiazol-2-yl)-4-[3H]-quinazolinones
2263
INTRODUCTION
The development of novel agents, particularly of compounds effective
against complex seizures, remains a major focus of antiepileptic drug
research.1 Literature survey reveals that 2-aminobenzothiazole deriva-
tives possess potent anticonvulsant activity.2,3 In 1985, Riluzole (6-
(trifluromethoxy)-2-benzothiazolamine) (Scheme 1) was reported as a
potent anticonvulsant agent that functions by action on the voltage-
dependent sodium channels.3–5 Similarly the sedative hypnotic (neuro-
toxic) properties of another pharmacophore, the 4-[3H]-quinazolinones
are well documented.6–18 The prototype in this class of com-
pound is methaqualone (2-methyl-3-ortho-tolyl-4-[3H]-quinazolinone)
(Scheme 1).19 In spite of the fact that according to the literature thou-
sands of quinazolinones and 2-aminobenzothiazole related compounds
have been synthesized and tested for a possible central nervous system
depressant and anticonvulsant activity, no attempt has been made to
incorporate the benzothiazole moiety and the quinazoline nucleus in a
single molecular framework.
SCHEME 1
We previously reported20 a conventional method for the syn-
thesis of 6,8-disubstituted 2-phenyl-3-(benzothiazol-2-yl)-4-[3H]-
quinazolinones having further substituents in the benzothiazole ring.
The synthesis involves the condensation of 2-aminobenzothiazole with
2-phenyl-4H-benzo[d][1,3] oxazine-4-one in pyridine. This procedure
usually needs high temperatures and requires lengthy and tedious
conditions. Microwave irradiation is known to allow a striking re-
duction of reaction times and good yields compared to the purely
thermal procedures. In this paper, we report the benefits associated
with this new methodology and the established standard experimental
conditions. Following the strategy previously reported by us20 for the
synthesis of 6,8-disubstituted 2-phenyl-3-(benzothiazol-2-yl)-4-[3H]-
quinazolinones further substituented in the benzothiazole ring, it
involves long heating (for several hours) of the 2-aminobenzothiazole
with 2-phenyl-4H- benzo[d][1,3]oxazine-4-one in pyridine at reflux
temperature. Here we report, the synthesis of 6,8-disubstituted

2264
S. S. Laddha and S. P. Bhatnagar
SCHEME 2
2-phenyl-3-(benzothiazol-2-yl)-4-[3H]-quinazolinones 3a-m and 4a-m,
which was accomplished under microwave irradiation with the aim to
develop an original and environmentally friendly procedure. A further
scope of our investigations was to evaluate the anticonvulsant activity
of the compounds 3a-m and 4a-m.
RESULTS AND DISCUSSION
Chemistry
Synthesis of the compounds 3a-m and 4a-m was carried out as de-
picted in Scheme 2. The starting 2-aminobenzothiazoles21 (1a-m),
2-phenyl-4H-benzo[d][1,3]oxazine-4-one22 (2a) and 6,8-dibromo-2-
phenyl-4H-benzo[d][1,3]oxazine-4-one22 (2b) were prepared according
to known procedures from commercially available substituted anilines,
antharanilic acid, and 3,5-dibromo anthranilic acid. Mixtures of the

3-(Benzothiazol-2-yl)-4-[3H]-quinazolinones
2265
2-amino-benzothaizole 1 and 2-phenyl-4H-benzo[d][1,3]oxazine-4-one
2a or its dibromo derivative 2b in dry pyridine were irradiated in a
scientific microwave oven at reflux temperature (power input: 210 W).
The products were isolated by pouring the reaction solution in a beaker
containing crushed ice and 5 mL of conc. HCl. The solid separated
(3 or 4) was filtered, dried, and recrystallized from an appropriate sol-
vent. The reaction mechanism for the synthesis of compounds 3 and 4
has been reported.20 The proposed work-up is fast, easy and clean, and
presents the advantage of less time and better yields than the conven-
tional method. The transformation proceeded very cleanly and without
any traces of side products. The results are summarized in Table I show
that in the most cases quantitative conversions were achieved after 10–
20 min irradiation.
All of the synthesized compounds were characterized by their physi-
cal, analytical and spectroscopic data. The IR, EI MS, and 1H NMR data
of all the synthesized compounds were in conformity with the structures
assigned.
Pharmacology
The anticonvulsant activity of the compounds 3a-m and 4a-m was eval-
uated against Maximum Electroshock (MES) induced seizures and sub-
cutaneous pentylenetetrazole (PTZ) induced seizures in mice.23 Using
the Rotorod procedure the neurotoxicity of all test compounds was as-
sessed in mice. All the tested compounds were administered intraperi-
toneally at various dose levels ranging from 7–200 mg/kg body weight
and the medium effective dose (ED50), the medium toxic dose (TD50)
and the protection index (PI) values were determined. Suspensions of
the tested compounds in polyethylene glycol were administered to mice
0.5 h or 4 h before evaluation of their activity. The results of anticon-
vulsant activity and neurotoxicity are presented in Tables II and III.
The Anti MES and Anti sc PTZ (ED50 values in Tables II and III) indi-
cated significant anticonvulsant activity for the tested compounds 3a-m
and 4a-m. However, they were found to be less potent when compared
with the reference standard phenytoin (ED50: 6.48 and 7.1 at t = 0.5
and 4 h in MES model). The different substituents at the aromatic ring
exert a significant influence on the biological activity by modulating the
lipophilicity and thereby facilitating penetration across the blood-brain
barrier. The presence of electron withdrawing groups (halogen and ni-
tro) at the aromatic ring in general decreases the potency of the tested
compounds compared to compounds having electron-donating groups.
This is because of the decresed lipophilicity, which in turn inhibits per-
meability across biological membrane. Further it has been found that

2266

2267

2268

2269

2270
S. S. Laddha and S. P. Bhatnagar
the ED50 and TD50 values of the tested compounds increase significantly
at t = 4 h, compared to t = 0.5 h, in contrast to the reference compound,
indicating that the tested compounds were metabolized with time in the
biological environment. This trend was found to be more pronounced in
compounds 3g, 3j, 3k, and 4a-m (having electron withdrawing groups)
compared to compounds 3a-f, 3h, 3i, 3l-m (having electron donating
groups). To confirm this phenomenon in vivo, it will be necessary to
carry out a kinetic study in an animal model. Based upon the results, it
will also be necessary to optimize the lead compound by substituting a
series of electron donating groups at the aromatic ring and selectively
modifying the quinazoline nucleus. The protection index (PI) values
are found to be more significant for determining the relation between
lipophilicity and toxicity. Tables II and III show that PI values >3 were
found for more potent compounds in contrast to less lipophilic com-
pounds. Thus, as the lipophilicity increases, so does the toxicity and
therefore also the protection index (PI). Among the compounds tested,
2-phenyl-3-(4-methyl-benzothiazol-2-yl)-4(3H)-quinazolinone (3e) was
found to be most potent: ED50 = 7.1 and 12.8 in MES model and 10
and 14 in sc PTZ model at t = 0.5 h and 4 h, respectively, and TD50
=
37.8 and 43 at t = 0.5 h and 4 h, respectively, with protection index
(PI) 5.3.
Further studies are in progress to optimize this lead compound and
fully characterize its mode of action.
EXPERIMENTAL
General
The microwave-irradiated reactions were performed in RAGA’s scien-
tific microwave oven. Melting points were determined in open capillar-
ies using a Thermonik precision melting point and boiling point appa-
ratus, Model C-PMB-2 (Mumbai, India), and are uncorrected.
The purity of the compounds was routinely checked by TLC using
silica gel-G and the spots were exposed to iodine vapour. IR spectra
were recorded using KBr pellets on a Perkin-Elmer 337 spectropho-
1
tometer. H NMR spectra were recorded with a Bruker WM 400 spec-
trometer at 360 MHz using TMS as internal standard. Mass spec-
tra (EI-MS) were recorded on a Jeol D-300 spectrometer. Elemental
analyses were carried out with Heraeus Carlo Erba 1180 CHN ana-
lyzer. All the chemicals were purchased from Aldrich Company Ltd.
Dorset (UK).

3-(Benzothiazol-2-yl)-4-[3H]-quinazolinones
2271
Synthesis of 2-Aminobenzothiazoles (1a-m)
Compounds 1a-m were synthesized starting from aniline and sub-
stituted anilines using known methods.21 They were obtained in pure
form after recrystallization from ethanol.
Synthesis of 2-Phenyl-4H-benzo[d][1,3]oxazine-4-ones (2a,b)
Compounds 2a,b were synthesized starting from anthranilic acid
and 3,5-dibromoanthranilic acid using known methods.22 They were
recrystallized from ethanol.
Synthesis of 6,8-Disubstituted 2-Phenyl-3-(benzothiazol-2-yl)-
quinazolin-4-ones (3a-m and 4a-m)
A solution of the 2-phenyl-4H-benzo[d][1,3]oxazine-4-one (0.005 mol)
in 15 mL of dry pyridine and the corresponding 2-aminobenzothiazole
(0.01 mol) was irradiated at reflux temperature (power input: 210 W)
until completion of the reaction (TLC, 10 min). After cooling to ambient
temperature the reaction mixture was transferred in a beaker contain-
ing 100 g of crushed ice and 5 mL of conc. HCl. The solid separated
(3 or 4) was filtered, dried and recrystallized from glacial acetic acid.
The physical data of 3a-m and 4a-m prepared by this procedure are
listed in Table I. The spectral data of compounds 3a-m and 4a-m are
all in accordance with the literature.20
Pharmacology
Albino mice of either sex weighing between 20-25 g, obtained from Na-
tional Center for Laboratory Animal Sciences, Hyderabad, India, were
used in the present study. Animals were housed in wire-mesh cages
under the laboratory conditions (23 2◦C), 12 h light. Animals were
allowed to acclimatize with free access to food and water for a 24 h
period before testing. During the course of the experiment, the general
behavior of the animal was normal. All the experimental protocols were
approved by the institutional animal ethical committee and the experi-
ments were conducted in accordance with the standard guidelines. The
animals were divided into three groups (control, standard and test) and
each group consisted of six animals. The homogenous suspension of the
tested compounds (3a-m and 4a-m) and the standard drug (phenytoin)
was prepared in polyethylene glycol and distilled water (1:9/mL). All
the test compounds were administered intraperitoneally (ip) at a dose
in the range of 7–200 mg/kg body weight 30 min prior to the start of
the experiments. The maximal Electroshock Seizures (MES) were in-
duced by electroconvulsometer (Techno Instruments, Lucknow), using

2272
S. S. Laddha and S. P. Bhatnagar
a technique described earlier.23 The animals were subjected to elec-
troshock (60 mA/0.2 s) via the transauricular electrodes. Further the
compounds were evaluated against subcutaneous pentylenetetrazole
(PTZ) model in mice. The anticonvulsant effect was assessed by record-
ing the Tonic Hind-limb Extension (THE) at various dose levels at
t = 0.5 and 4 h. Absence of seizure component like hind limb tonic
extension with drug treatment was considered to be evidence of protec-
tion. Medium effective dose (ED50) was calculated for each compound
and is presented in Tables II and III.
Acute neurotoxicity of all the test compounds was assessed in mice
using the method described by Dunham and Miya.24 Briefly, group of
animals (mice) were trained to balance on a rotating rod (3 cm diameter
and 6 rpm speed) and they were allowed three attempts to remain on the
rotating rod for 20 s. Such trained animals were treated with the tested
compounds at a various dose levels between 30–200 mg/kg body weight.
by intraperitoneal administration. The tested compounds were consid-
ered to be neurotoxic at a particular dose level if the trained animal
showed lack of Rolling Roller Performance. Each of the trained animals
was tested in this manner at 30 min and 4 h after the drug adminis-
tration, and the neurotoxic effect was recorded in terms of TD50. Based
upon these results PI values were calculated as shown in Tables II
and III.
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