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mixture was then irradiated with microwaves for 2 min at 720
Watt. DMF was removed using rotary flash evaporator and to
the residue H2O (10 ml) was added. The obtained product
was collected by filtration and dried. The crude product was
recrystallized from ethanol to give the desired compound
1.02 g in 57% yield as a yellow solid. mp 225—226 °C.
1H-NMR (CDCl3) d: 2.28 (6H, s, N(CH3)2), 3.74 (2H, s,
CH2–N(CH3)2), 7.12—8.34 (7H, m, ArH), 9.86 (1H, br s,
CONH), 11.47 (1H, br s, OH). IR (KBr) (cmꢀ1): 3406, 3217,
1670, 1545. MS m/z: 356 (Mꢃ). Anal. Calcd for
C18H17ClN4O2: C, 60.67; H, 4.77; N, 15.73. Found: C, 60.93;
H, 4.56; N, 16.03. Compounds (3b—n) were prepared by the
same methodology. Physical data of compounds (3a—n) are
listed in Table 1. To make the compounds soluble in water
(for pharmacological evaluation), the free bases were con-
verted into the corresponding hydrochloride salts as de-
scribed by Blackburn et al.9)
Fig. 1. Synthesis of 3-Chloroquinoxaline-2-carboxamides
Fig. 2. Pharmacophore of 5-HT3 Receptor Antagonists
PHARMACOLOGY
Evaluation of 5-HT3 Antagonism in the LMMP of
Guinea Pig Ileum10) Experimentation on animals was ap-
proved by the Institutional Animal Ethics Committee of the
Birla Institute of Technology & Science, Pilani, India. (Pro-
tocol No. IAEC/RES/6, dated 21.04.03). Male Dunkin Hart-
ley guinea pigs (250—300 g; Hissar Agricultural University,
Hissar, Haryana, India) were sacrificed by cervical disloca-
tion. The abdomen was cut open and a length of ileum was
excised about 2 cm from the ileo-caecal junction. The
LMMP, 3—4 cm in length was prepared and mounted as de-
scribed by Paton and Zar.11) The tissue was equilibrated for
30 min under a resting tension of 500 mg and constant aera-
tion in a 40 ml organ bath containing Tyrode solution main-
tained at ca. 37 °C. Non-cumulative concentrations of 2-
methyl-5-HT (Tocris, U.K.) were added with a 15 min dosing
cycle (to prevent desensitization) and left in contact with the
tissue until the maximal contraction had developed. To study
the antagonist effect of the test compounds on the response
evoked by 2-methy-5-HT, the compounds were added to the
organ bath and left in contact with the tissue for at least
10 min prior to the addition of 2-methyl-5-HT. The contrac-
tions were recorded using a T-305 Force transducer coupled
to a Student’s physiograph (Bio Devices, Ambala, India). An-
tagonism was expressed in the form of pA2 values, which
were graphically determined.12) The pA2 values of the test
compounds were compared with the standard antagonist On-
dansetron (Natco Pharma, Hyderabad, India).
Fig. 3. Possible Interaction of 2-Methyl-5-HT and 3g with the 5-HT3 Re-
ceptor
oxygen to basic nitrogen of mannich derivatives and het-
eroaromatic ring to basic nitrogen. The molecules identified
for synthesis complies with the pharmacophoric model.6)
The 3-chloroquinoxaline-2-carboxamides were prepared
by the condensation of 3-chloro-2-quinoxaloylchloride (1)
with Mannich derivatives of p-aminophenol (2) in microwave
environment (Fig. 1). This method was optimized on the
basis of the conventional method in which the title com-
pounds were prepared by refluxing the mixture of 3-chloro-2-
quinoxaloylchloride, appropriate Mannich derivates of p-
aminophenol, DMF and triethylamine for about 6 h. 3-
Chloro-2-quinoxaloylchloride was prepared by the oxidation
of 2-chloro-3-methylquinoxaline by sodium dichromate and
sulfuric acid mixture,13) followed by chlorination with thionyl
chloride. 2-Chloro-3-methylquinoxaline was prepared ac-
cording to the method of Krishnan et al.7) by the reaction of
o-phenylene diamine and pyruvic acid to yield 3-
methylquinoxalin-2-ol, followed by chlorination with POCl3.
Mannich derivaties of p-aminophenol were prepared8) by the
reaction of paracetamol, dialkylamine and formaldehyde fol-
lowed by acid hydrolysis, which is a modified method of
Burckhalter et al.14) and Stout et al.15) These compounds
were evaluated for their 5-HT3 antagonistic activity in the
LMMP preparation from guinea pig ileum. The 5-HT3 antag-
onism of the 3-chloroquinoxaline-2-carboxamides is repre-
sented as pA2 and is shown in Table 1. From the fourteen
compounds tested, compound 3g showed higher antagonism
(pA2 6.4) than other compounds but lesser than standard an-
tagonist Ondansetron (pA2 6.9). Other compounds (3a—f,
3h—n) exhibited mild to moderate antagonist activity. It has
RESULTS AND DISCUSSION
The title compounds, 3-chloroquinoxaline-2-carboxam-
ides, were designed according to the pharmacophoric re-
quirements (proposed by Hibert et al.6)) for 5-HT3 receptor
antagonists. The pharmacophore consists of three compo-
nents (Fig. 2): an aromatic\heteroaromatic ring, a carbonyl-
containing linking moiety, and a basic center in a specific
spatial arrangement. In the proposed series, the least energy
conformation of the molecules were generated by Tripos-
Alchemy 2000 software (Tripos Associates Inc., St. Louis,
U.S.A.) and the pharmacophoric distances were measured
from the heteroaromatic ring to carbonyl oxygen, carbonyl
Mahesh, Radhakrishnan
