Synthesis and pharmacological investigation of novel 1-substituted-4- phenyl-1,2,4-triazolo[4,3-a]quinazolin-5(4H)-ones as a new class of H 1-antihistaminic agents

Article abstract of DOI:10.1016/j.bmcl.2005.02.016

A series of novel 1-substituted-4-phenyl-1,2,4-triazolo[4,3-a]quinazolin- 5(4H)-ones 7 were synthesized by the cyclization of 2-hydrazino-3- phenylquinazolin-4(3H)-one 6 with various one carbon donors. The starting material 2-hydrazino-3-phenylquinazolin-4(3H)-one 6, was synthesized from aniline 1 by a novel innovative route. When tested for their in vivo H ₁-antihistaminic activity on conscious guinea pigs all the test compounds protected the animals from histamine induced bronchospasm significantly, whereas the compound 1-methyl-4-phenyl-1,2,4-triazolo[4,3-a] quinazolin-5(4H)-one 7b (percentage protection 70.7%) was found to be equipotent with the reference standard chlorpheniramine maleate (percentage protection 71%). These compounds show negligible sedation (～5%) when compared to the reference standard (26%). Hence they could serve as prototype molecules for future development.

Full text of DOI:10.1016/j.bmcl.2005.02.016

Bioorganic & Medicinal Chemistry Letters 15 (2005) 1877–1880
Synthesis and pharmacological investigation of novel
1-substituted-4-phenyl-1,2,4-triazolo[4,3-a]quinazolin-5(4H)-ones
as a new class of H1-antihistaminic agents
Veerachamy Alagarsamy,a,* Rajani Giridharb and Mangai Ram Yadavb
aMedicinal Chemistry Laboratory, Periyar College of Pharmaceutical Sciences for Girls, Trichy 620 021, Tamilnadu, India
bDrug Design and Development Laboratory, Department of Pharmacy, Faculty of Technology and Engineering,
The MS University of Baroda, Baroda 390 001, India
Received 8 January 2005; revised 1 February 2005; accepted 4 February 2005
Abstract—A series of novel 1-substituted-4-phenyl-1,2,4-triazolo[4,3-a]quinazolin-5(4H)-ones 7 were synthesized by the cyclization
of 2-hydrazino-3-phenylquinazolin-4(3H)-one 6 with various one carbon donors. The starting material 2-hydrazino-3-phenylquinaz-
olin-4(3H)-one 6, was synthesized from aniline 1 by a novel innovative route. When tested for their in vivo H1-antihistaminic activity
on conscious guinea pigs all the test compounds protected the animals from histamine induced bronchospasm significantly, whereas
the compound 1-methyl-4-phenyl-1,2,4-triazolo[4,3-a]quinazolin-5(4H)-one 7b (percentage protection 70.7%) was found to be equi-
potent with the reference standard chlorpheniramine maleate (percentage protection 71%). These compounds show negligible seda-
tion (ꢀ5%) when compared to the reference standard (26%). Hence they could serve as prototype molecules for future development.
Ó 2005 Elsevier Ltd. All rights reserved.
The prevalence of asthma and other allergic diseases is
increasing1–3 providing a rapidly expanding market for
antiallergic drugs. The first generation antihistamines
penetrate the blood–brain barrier and also possess anti-
cholinergic properties and this has led to the develop-
ment of a second generation4 of H1-antagonists such
as terfenadine, cetirizine and astemizole. A common fea-
ture of first generation compounds includes two aryl or
heteroaryl rings linked to an aliphatic tertiary amine via
the side chain5 (e.g., Diphenhydramine and Phenir-
amine), the second generation compounds (terfenadine
and cetirizine) also contain many of the structural fea-
tures of first generation compounds. The real break-
through of non-sedative antihistamines came in the
early 80s of 20th century with the discovery of modern
antihistamines which were found to exhibit potent anti-
histaminic activity without a sleep-inducing effect.6 Con-
densed heterocycles containing new generation of H1-
antihistamines (e.g., Loratadine, Azelastine and Flaz-
elastine) that do not possess the above mentioned phar-
macophore for H1-antihistamines gave way for the
discovery of many novel antihistamines (temelastine7
and mangostin8). A literature survey reveals excellent
antihistaminic activity in quinazolines and condensed
quinazolines.9,10 In view of these facts and to continue
our efforts11,12 in the search of quinazoline-derived po-
tent antihistamines with least sedation, in the present
study we aimed to synthesize a series of 1,2,4-triazolo-
[4,3-a]quinazolin-5(4H)-ones containing phenyl substi-
tution at position 4 and alkyl substitution at position
1. In spite of a few reported methods available for the
synthesis of 1,2,4-triazolo[4,3-a] quinazolines,13,14 the
title compounds, we aimed to synthesize these com-
pounds by a novel innovative route (Scheme 1).
Melting points were determined in open capillary tubes
on a Thomas Hoover apparatus and are uncorrected. IR
spectra were recorded in KBr on a Shimadzu FT-IR,
8300 spectrometer (cmÀ1), mass spectra on a MASPEC
msw 9629 mass spectrometer at 70 eV and NMR spectra
on a Varian 300 MHz spectrometer, using tetramethylsi-
lane as internal standard. Elemental analyses were per-
formed on Carlo Erba 1108.
Keywords: Quinazoline; Triazole; Antihistamine; Pyrimidine; Triazolo-
quinazoline.
*
The key intermediate 2-thioxo-3-phenylquinazolin-4(3H)-
one was prepared by adding carbondisulfide and sodium
Corresponding author. Tel.: +91 431 2459911; fax: +91 431
2456677; e-mail: samy_veera@yahoo.com
0960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.02.016

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V. Alagarsamy et al. / Bioorg. Med. Chem. Lett. 15 (2005) 1877–1880
DMSO
S
S
NH2
+
+
NaOH
CS2
C
NH
-
Na+
1
(CH3)2SO4
O
O
S
EtOH
OCH3
NH2
OCH3
C
NH
+
SCH3
N
H
C
S
3
N
H
2
3
a
O
O
N
N
N
NaOH / EtOH
(CH3)2SO4
N
SCH3
S
H
5
4
EtOH
.
NH2NH2 H2O
O
O
N
N
RCOOH / RCOCl
N
N
N
NH
N
NH2
R
7
a
e
6
Scheme 1. Synthetic protocol of the compounds 7a–e.
hydroxide solution simultaneously to a vigorously stir-
red solution of aniline 1 in dimethylsulfoxide over
30 min, stirring was continued for an additional
30 min. Dimethyl sulfate was added to the reaction mix-
ture whilst stirring at 5–10 °C, it was further stirred for
2 h and poured into ice water to get a solid dithiocarba-
mic acid methyl ester 2. The compound 2 and methyl
anthranilate 3 when refluxed in ethanol for 18 h yielded
the desired 2-thioxo-3-substituted quinazolin-4(3H)-one
4 (yield 86%; mp 305–306 °C). The product obtained
was cyclic and not an open chain thiourea 3a. It was
confirmed by its low Rf value, high melting point and
its solubility in sodium hydroxide solution. The IR spec-
tra of these compounds show intense peaks at
3220 cmÀ1 for amino (NH), 1660 cmÀ1 for carbonyl
(C@O) and 1200 cmÀ1 for thioxo (C@S) stretching.
NMR spectrum of 4 showed signals at d 7–9 (m, 9H,
ArH) and 10.5 (s, 1H, NH). Data from the elemental
analyses have been found to be in conformity with the
assigned structure. Furthermore the molecular ion re-
corded in the mass spectrum is also in agreement with
the molecular weight of the compound.
hydroxide solution and methylating with dimethyl sul-
fate whilst stirring at room temperature (yield 88%;
mp 124–126 °C). The IR spectrum of 5 showed disap-
pearance of amino (NH) and thioxo (C@S) stretching
signals of the starting material. It showed a peak for car-
bonyl (C@O) stretching at 1680 cmÀ1. The NMR spec-
trum of compound 5 showed signals at d 2.5 (s, 3H,
SCH3) and 7.0–8.6 (m, 9H, ArH). Data from the ele-
mental analyses and molecular ion recorded in the mass
spectrum further confirmed the assigned structure.
Nucleophilic displacement of methylthio group of 5 with
hydrazine hydrate was carried out using ethanol as sol-
vent to afford 2-hydrazino-3-substituted quinazolin-
4(3H)-one 6 (yield 81%; mp 158–160 °C). The long dura-
tion of reaction (22 h) required might be due to the pres-
ence of bulky aromatic ring at position 3, which might
have reduced the reactivity of quinazoline ring system
at C-2 position. The formation of 6 was confirmed by
the presence of NH and NH2 signals around 3334–
3280 cmÀ1 in the IR spectrum. It also showed a peak
for carbonyl (C@O) at 1680 cmÀ1. The NMR spectrum
of the compound 6 showed signals at d 5.0 (s, 2H,
NHNH2), 7.0–8.1 (m, 9H, ArH) and 8.7 (s, 1H,
NHNH2). Data from the elemental analyses have been
The 2-methylthio-3-substituted quinazolin-4(3H)-one 5
was obtained by dissolving 4 in 2% alcoholic sodium

V. Alagarsamy et al. / Bioorg. Med. Chem. Lett. 15 (2005) 1877–1880
Table 1. Physical and pharmacological data of compounds 7a–e
1879
Compd no.
R
Yield, % Mp, °C (recryst solv.a) Mol. formulab Mol. wt.c % Protectiond % CNS
depressiond
7a
7b
–H
–CH3
–CH2CH3
89
87
80
262–265 (C–E)
276–279 (C–E)
190–193 (E)
190–194 (C–E)
258–260 (C–E)
—
C15H10N4O
C16H12N4O
C17H14N4O
C18H16N4O
C16H11N4OCl
—
262
276
290
304
310
—
69.9
70.7
70.0
69.2
68.7
71.0
78.9
4.2
4.8
5.3
7.3
3.7
7c
7d
–(CH2)2CH3 76
7e
Chlorpheniramine maleate
Cetirizine
–CH2Cl
78
—
26
8.5
a Abbreviations for the solvents used are as follows: C = chloroform, E = ethanol.
b Elemental (C, H, N) analysis indicated that the calculated and observed values were within the acceptable limits ( 0.4%).
c Molecular weight determination by mass spectral analysis.
d Values are the means from six separate experiments. SE was less than 10% of the mean. Dose of test compounds, chlorpheniramine maleate and
cetirizine are 10 mg/kg for antihistaminic activity; and 5 mg/kg for sedative-hypnotic activity.
found to be in conformity with the assigned structure.
Furthermore the molecular ion recorded in the mass
spectrum is also in agreement with the molecular weight
of the compound.
7d) lead to decrease in activity (percentage protection
70% and 69.2%, respectively). Replacement of a proton
of the methyl group by a lipophobic group (chloro)
(compound 7e) resulted in a further decrease in activity
(percentage protection 68.7%). The order of activity of
substituents at first position was methyl > ethyl >
unsubstituted > propyl > chloromethyl. Compounds with
a small substituent at C-1 seems to provide optimum
activity. As the test compounds could not be converted
to water soluble form, in vitro evaluation for antihista-
minic activity could not be performed.
The title compounds 7a–e were obtained in fair to good
yields through the cyclization of 6 with a variety of one
carbon donors such as formic acid, acetic acid, propi-
onic acid, butyric acid and chloroacetyl chloride at re-
flux. The formation of cyclic product is indicated by
the disappearance of peaks due to NH and NH2 of the
starting material at 3400–3200 cmÀ1 in IR spectrum of
all the compounds 7a–e. The NMR spectrum of 7a–e
showed the absence of NH and NH2 signals. A multiplet
at 7.0–8.0 integrating for aromatic protons was ob-
served. The molecular ion recorded in the mass spec-
trum is in agreement with the molecular weight of the
compounds. Elemental (C, H, N) analysis indicated that
the calculated and observed values were within the
acceptable limits ( 0.4%). Physical data of the title com-
pounds is represented in Table 1.
As sedation is one of the major side effects associated
with antihistamines, the test compounds were also evalu-
ated for their sedative potentials. It was determined by
measuring the reduction in locomotor activity using
actophotometer.16,17 The test compounds and the refer-
ence standards (chlorpheniramine maleate and cetiri-
zine) were administered orally at a dose of 5 mg/kg in
1% CMC. The percent reduction in locomotor activity
was calculated and shown in Table 1. Student t test
was performed to ascertain the significance of the exhib-
ited activity. The results indicate that all the test com-
pounds were found to exhibit only negligible sedation
(4–7%), whereas the reference standard chlorphenir-
amine maleate showed 26% sedation and cetirizine
showed 8.5%.
The MS University of Baroda, India, Institutional Ani-
mal Ethics committee approved the protocol adopted
for the experimentation of animals. A modification of
the technique of Van Arman et al.15 was adopted to deter-
mine the antihistaminic potential of the synthesized com-
pounds. The test compounds and the reference standards
(chlorpheniramine maleate and cetirizine) were adminis-
tered orally at a dose of 10 mg/kg in 1% CMC. The mean
preconvulsion time of test group animals was compared
with control and is expressed in terms of percentage
protection (Table 1). Student t test was performed to
ascertain the significance of the exhibited activity.
Among the series, 1-methyl-4-phenyl-1,2,4-triazoloqui-
nazolin-5(4H)-one 7b was the most potent with the per-
centage protection of 70.7, which is equipotent with that
of standard chlorpheniramine maleate (percentage pro-
tection 71%) and less potent than cetirizine (percentage
protection 78.9%). Compound 7b showed negligible
sedation (4.8%) compared to chlorpheniramine maleate
(26%) and cetirizine (8.5%), hence it could therefore
serve as a lead molecule for further modification to ob-
tain a clinically useful novel class of non-sedative ant-
ihistamines.
The in vivo antihistaminic activity results indicate that
all test compounds protected the animals from hist-
amine induced bronchospasm significantly. Structural
activity relationship (SAR) studies, indicated that differ-
ent alkyl substituents on the first position of triazoloqui-
nazoline ring exerted varied biological activity.
Compound 7a with no substitution, showed good activ-
ity (percentage protection 69.9%); with increased lipo-
philicity (methyl compound 7b) activity increased
(percentage protection 70.7%). Further increases in lipo-
philicity (i.e., ethyl compound 7c and propyl compound
References and notes
1. Peat, K. Clin. Exp. Allergy 1994, 24, 797.
2. Bummy, P.; Chin, S.; Rona, R. J. Br. Med. J. 1990, 300,
1306.

1880
V. Alagarsamy et al. / Bioorg. Med. Chem. Lett. 15 (2005) 1877–1880
9. Wade, J. J. U.S. Patent 4,528,288; Chem. Abstr. 1986, 104,
3. Naligiami, T.; Itaya, H.; Tominga, T.; Yamaki, H.;
Hisamatsu, S.; Nakagomi, O. Lancet 1994, 343,
121.
4. Simons, F. E. R.; Simons, K. J. N. Engl. J. Med. 1994, 59,
4.
5. Ellis, E. F.; Adkinson, N. F.; Yungingen, J. W.; Busso, W.
W. Histamine H1-Receptor Antagonists; Musby-year book,
Inc.: St. Louis, 1985; p 856.
6. Carr, A. A.; Meyer, D. R. Arzneim.-Forsch./Drug Res.
1982, 32, 157.
5889.
10. West, W. R.; July, W. R. Eur. Patent 34,529; Chem. Abstr.
1982, 96, 20114.
11. Alagarsamy, V.; Venkatesaperumal, R.; Vijayakumar, S.;
Angayarkanni, T.; Pounammal, P.; Senthilganesh, S.;
Kandeeban, S. Pharmazie 2002, 57, 306.
12. Alagarsamy, V. Pharmazie 2004, 59, 753.
13. Kamal, A.; Sattur, S. Indian J. Chem. 1988, 23(B), 1293.
14. Zhohghna, Y. Zaghi 1993, 53, 45.
7. Mc Call, M. K.; Nair, N.; Wong, S.; Townley, R. G.;
Lang, D.; Sweiss, S. J. J. Allergy Clin. Immunol. 1985, 75,
168.
8. Chirungsrilerd, N.; Frukawa, K. I.; Ohta, T.; Nozoe, S.;
Yohazumi, Y. Eur. J. Pharmacol. 1987, 139, 235.
15. Van Arman, G.; Miller, L. M.; Malley, M. P. O. J.
Pharmacol. Exp. Ther. 1960, 90, 133.
16. Dews, B. Br. J. Pharmacol. 1953, 8, 46.
17. Kuhn, W. L.; Van Mannen, E. F. J. Pharmacol. Exp.
Ther. 1961, 134, 60.