Microwave-assisted synthesis of novel 1,3,4-thiadiazolylsubstituted 1,2,4-triazines as potential antitubercular agents

Article abstract of DOI:10.1006/bioo.1998.1108

A series of triazines have been synthesized starting from 5-alkyl- 1,3,4-thiadiazole-2-thioles (1a-d). On reaction with ethyl bromoacetate in the presence of anhydrous K₂CO₃ under microwave irradiation (MWI), these yielded corresponding esters (2a-d) which on hydrazinolysis under MWI produced (5-alkyl-1,3,4-thiadiazol-2-yl sulfanyl) acetohydrazides (3a-d). The reaction of 3a-d with ω-bromoacetophenone under MWI yielded 6-aryl-3-[(5- alkyl-1,3,4-thiadiazol-2-yl sulfanyl)methyl]-1,2,4-triazines (4a-h). All the synthesized triazines showed in vitro antitubercular activity.

Full text of DOI:10.1006/bioo.1998.1108

BIOORGANIC CHEMISTRY 26, 289–294 (1998)
ARTICLE NO. BH981108
Microwave-Assisted Synthesis of Novel 1,3,4-Thiadiazolyl-
Substituted 1,2,4-Triazines as Potential Antitubercular Agents
Mazaahir Kidwai,*,1 Rajesh Kumar,* Anil Srivastava,† and H. P. Gupta†
*Department of Chemistry, University of Delhi, Delhi 110007, India; and †Department of
Microbiology, Central Drug Research Institute, Lucknow 226001, India
Received June 29, 1998
A series of triazines have been synthesized starting from 5-alkyl-1,3,4-thiadiazole-2-thioles
(1a–d). On reaction with ethyl bromoacetate in the presence of anhydrous K2CO3 under
microwave irradiation (MWI), these yielded corresponding esters (2a–d) which on
hydrazinolysis under MWI produced (5-alkyl-1,3,4-thiadiazol-2-yl sulfanyl) acetohydrazides
(3a–d). The reaction of 3a–d with Ͷ-bromoacetophenone under MWI yielded 6-aryl-3-[(5-
alkyl-1,3,4-thiadiazol-2-yl sulfanyl)methyl]-1,2,4-triazines (4a–h). All the synthesized triazines
showed in vitro antitubercular activity.
1998 Academic Press
INTRODUCTION
1,2,4-Triazines have been associated with diverse pharmacological activities such
as antihypertension and inhibition of platelets (1) and antileukemic (2) and anti-
inflammatory (3) activities. Thiadiazoles are also well known for their antibacterial
activity (4).
There is considerable interest in the rapid synthesis of a variety of heterocyclic
compounds under MWI (5). In continuation of our work on microwave-assisted
synthesis (6–8) of pharmacologically active heterocycles (9, 10) we thought that it
was worthwhile to develop a rapid synthesis of title compounds and to screen the
products for antitubercular activity. The synthesis entails the union of two biologi-
cally active nuclei, viz. triazine and thiadiazole.
EXPERIMENTAL
The melting points were determined on an electrothermal apparatus and are
1
uncorrected. IR spectra (␯max in cmϪ ) were recorded on a Perkin–Elmer Spectro-
photometer Model 599 using KBr. 1H NMR spectra were recorded on an FT NMR
Hitachi R-600 instrument using CDCl3 ϩ DMSO-d6 as solvent and Me4Si as internal
standard (chemical shifts in ͳ ppm). Microwave irradiations were carried out in a
Padmini Essentia oven, Model Brownie at 2450 MHz, and a power of 700 W was
used for each sequence of irradiation.
1 To whom correspondence should be addressed.
289
0045-2068/98 $25.00
Copyright 1998 by Academic Press
All rights of reproduction in any form reserved.

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KIDWAI ET AL.
SCHEME 1
5-Alkyl-1,3,4-thiadiazole-2-thioles (1a–d)
These compounds were prepared according to the literature (7).
Ethyl(5-alkyl-1,3,4-thiadiazol-2yl Sulfanyl) Acetates (2a–d)
To a solution of 1a–d (0.01 mol) in DMF (5 ml) in an Erlenmeyer flask, anhydrous
K2CO3 (0.5 g) and ethyl bromoacetate (0.01 mol) were added. The reaction mixture
was subjected to MWI for a period of 2–3 min at 2450 MHz. The inorganic salt
was filtered in hot water and the filtrate was cooled, diluted with an excess of ice-
cold water (100 ml), and extracted with CH2Cl2 . The organic layer was washed with
water (2 ϫ 50 ml), dried over Na2SO4 (anhyd.) and evaporated to get the products.

SYNTHESIS OF 1,2,4-TRIAZINES
291
TABLE 1
Physical and Analytical Data of Compounds (2a–d, 3a–d, 4a–h)
Analysis found (calcd) (%)
mp
Yield
Compound
(ЊC)
(%)
C
H
N
2a7
2b
2c
2d
3a7
3b
3c
3d
4a
4b
4c
4d
4e
4f
—
—
—
—
86
83
82
83
85
79
78
77
84
76
74
75
86
79
77
76
38.62(38.53)
51.59(51.66)
54.58(54.54)
57.06(56.98)
29.45(29.41)
45.86(45.83)
49.31(49.37)
52.38(52.33)
51.88(51.83)
59.12(59.22)
61.11(61.02)
62.51(62.59)
46.59(46.50)
54.38(54.35)
56.24(56.31)
58.11(58.04)
4.51(4.58)
7.24(7.28)
7.81(7.88)
8.31(8.38)
3.96(3.92)
6.98(6.94)
7.51(7.59)
8.18(8.14)
3.61(3.65)
6.05(5.97)
6.61(6.54)
7.08(7.03)
2.91(2.98)
5.25(5.24)
5.74(5.81)
6.79(6.31)
12.84(12.84)
9.24 (9.27)
8.56 (8.48)
7.86 (7.82)
27.42(27.42)
19.12(19.44)
17.78(17.72)
16.21(16.28)
23.21(23.26)
18.08(18.18)
16.91(16.95)
15.95(15.87)
20.82(20.86)
16.61(16.69)
15.71(15.64)
14.61(14.72)
126
117–118
105–117
95–96
178–179
161–163
154–155
143–144
173–174
158–159
151–152
138–139
4g
4h
(5-Alkyl-1,3,4-thiadiazol-2yl Sulfanyl) Acetohydrazides (3a–d)
A solution of 2a–d (0.01 mol) and hydrazine hydrate (99%, 0.01 mol) in absolute
ethanol was subjected to MWI for 1.5–2.0 min. The reaction mixture was cooled,
and the solid was separated, filtered, dried, and recrystallized with ethanol.
6-Aryl-3-[(5-alkyl-1,3,4-thiadiazol-2yl sulfanyl)methyl]-1,2,4-triazine (4a–d)
A mixture of 3a–d (0.02 mol) and phenacyl bromide (12)/4-chlorophenacyl bro-
mide (0.01 mol) in DMF (5 ml) was subjected to MWI for 5–6 min. The reaction
mixture was cooled and poured over crushed ice; the solid was separated, filtered,
washed with water, and recrystallized with a DMF–EtOH mixture.
BIOLOGICAL ACTIVITY
The compounds were screened at 100 and 50 Ȑg/ml concentrations for their
in vitro antitubercular activity by twofold serial dilutions against Mycobacterium
tuberculosis ATCC 27294. The desired concentrations of each compound were
prepared aseptically with the help of their respective solvents. Sterilized Mid-
dlebrook’s 7H10 (Difco) medium was dispensed in tubes and OADL (oleic albumin
dextrose catalase) was added to each medium tube. Aseptically, 0.1 ml of each
concentration of compound was added into the medium.
Fresh growth of M. tuberculosis ATCC 27294 was harvested, homogenized, and

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KIDWAI ET AL.
TABLE 2
Spectral Data of Compounds (2a–d, 3a–d, 4a–h)
Compound
IR
1H NMR
2a7
2b
1740(CuO)
1720(CuO)
1.3(t, 3H, CH2CH3), 2.7(s, 3H, CH3), 3.9(q, 2H, CH2CH3), 4.6(s,
2H, SCH2)
0.9(t, 3H, 7Ј-CH3), 1.23(m, 8H, 4 ϫ CH2), 1.31(t, 3H, CH2CH3),
1.6(m, 2H, 2Ј-CH2), 2.36(t, 2H, 1Ј-CH2), 3.9(q, 2H, CH2CH3),
4.6(s, 2H, SCH2).
2c
1715(CuO)
1715(CuO)
0.89(t, 3H, 9Ј-CH3), 1.25(m, 12H, 6 ϫ CH2), 1.3(t, 3H, CH2CH3),
1.61(m, 2H, 2Ј-CH2), 2.34(t, 2H, 1Ј-CH2), 3.8(q, 2H, CH2CH3),
4.6(s, 2H, SCH2).
0.88(t, 3H, 11Ј-CH3), 1.25(m, 16H, 8 ϫ CH2), 1.3(t, 3H, CH2CH3),
1.60(m, 2H, 2Ј-CH2), 2.35(t, 2H, 1Ј-CH2), 3.8(q, 2H, CH2CH3),
4.6 (s, 2H, SCH2).
2.7(s, 3H, CH3), 3.7(brs, 2H, NH2), 4.7(s, 2H, SCH2), 8.8(brs, 1H,
NH)
0.81(t, 3H, 7Ј-CH3), 1.28(m, 8H, 4 ϫ CH2), 1.63(m, 2H, 2Ј-CH2),
2.36(t, 2H, 1Ј-CH2), 3.7(brs, 2H, NH2), 4.7(s, 2H, SCH2),
8.7(brs, 1H, NH).
2d
3a7
3b
3320–3260(NH2)
1640(CONH)
3310–3260(NH2)
1630(CONH)
3c
3320–3260(NH2)
1635(CONH)
0.89(t, 3H, 9Ј-CH3), 1.26(m, 12H, 6 ϫ CH2), 1.62(m, 2H, 2Ј-CH2),
2.34(t, 2H, 1Ј-CH2), 3.7(brs, 2H, NH2); 4.7(s, 2H, SCH2),
8.7(brs, 1H, NH).
3d
3310–3260(NH2)
1630(CONH)
0.88(t, 3H, 11Ј-CH3), 1.25(m, 16H, 8 ϫ CH2), 1.6(m, 2H, 2Ј-CH2),
2.34(t, 2H, 1Ј-CH2), 3.65(brs, 2H, NH2), 4.7(s, 2H, SCH2),
8.7(brs, 1H, NH).
4a
4b
1595(CuN)
1585(CuN)
2.7(s, 3H, CH3), 4.7(s, 2H, SCH2), 5.2(s, 1H, 5-CH), 7.6–7.8(m,
5H, Ar-H).
0.8(t, 3H, 7Ј-CH3), 1.23(m, 8H, 4 ϫ CH2), 1.62(m, 2H, 2Ј-CH2),
2.36(t, 2H, 1Ј-CH2), 4.6(s, 2H, SCH2), 5.1(s, 1H, 5-CH),
7.5–7.7(m, 5H, Ar-H).
4c
1580(CuN)
1580(CuN)
0.87(t, 3H, 9Ј-CH3), 1.25(m, 12H, 6 ϫ CH2), 1.6(m, 2H, 2Ј-CH2),
2.35(t, 2H, 1Ј-CH2), 4.6(s, 2H, SCH2), 5.1(s, 1H, 5-CH),
7.5–7.8(m, 5H, Ar-H).
0.86(t, 3H, 11-CH3), 1.24(m, 16H, 8 ϫ CH2), 1.6(m, 2H, 2Ј-CH2),
2.34(t, 2H, 1Ј-CH2), 4.62(s, 2H, SCH2), 5.15(s, 1H, 5-CH),
7.5–7.8(m, 5H, Ar-H).
4d
4e
4f
1600(CuN)
1590(CuN)
2.7(s, 3H, CH3), 4.7(s, 2H, SCH2), 5.25(s, 1H, 5-CH), 7.8–8.0(m,
4H, Ar-H).
0.91(t, 3H, 7Ј-CH3), 1.24(m, 8H, 4 ϫ CH2), 1.61(m, 2H, 2Ј-CH2),
2.36(t, 2H, 1Ј-CH2), 4.65(s, 2H, SCH2), 5.2(s, 1H, 5-CH),
7.8–8.0(m, 4H, Ar-H).
4g
4h
1585(CuN)
1585(CuN)
0.89(t, 3H, 9Ј-CH3), 1.25(m, 12H, 6 ϫ CH2), 1.61(m, 2H, 2Ј-CH2),
2.34(t, 2H, 1Ј-CH2), 4.6(s, 2H, SCH2), 5.2(s, 1H, 5-CH),
7.7–7.9(m, 4H, Ar-H).
0.86(t, 3H, 11Ј-CH3), 1.24(m, 16H, 8 ϫ CH2), 1.61(m, 2H, 2Ј-
CH2), 2.34(t, 2H, 1Ј-CH2), 4.61(s, 2H, SCH2), 5.2(s, 1H, 5-CH),
7.75–7.9(m, 4H, Ar-H).

SYNTHESIS OF 1,2,4-TRIAZINES
293
TABLE 3
Antitubercular Activity Data of Compounds (4a–h)
Minimum inhibitory concentrations (MIC)
Compound
(Ȑg/ml)
4a
4b
4c
4d
4e
4f
50
100
100
100
100
100
100
100
4g
4h
suspended in 0.05% saline, and 5 Ȑl containing 5 ϫ 104 AFB (acid fast bacilli) was
inoculated into each tube (with or without compound). All the tubes were incubated
at 37ЊC for 4 to 6 weeks.
RESULTS AND DISCUSSIONS
5-Alkyl-1,3,4-thiadiazole-2-thiole (1a–d) was synthesized according to the litera-
ture (7). Compounds 2a–d were synthesized by treating 1a–d with ethyl bromoace-
tate in the presence of anhydrous K2CO3 under MWI for 2–3 min, whereas conven-
1
tional heating requires 10–12 h (11). In IR absorption at 1715–1740 cmϪ confirms
the presence of the carboxylate function. Hydrazinolysis of 2a–d showed IR absorp-
tion bands at 3260–3320 and 1630–1640 cmϪ1 due to the amine and amide functions.
In the classical approach this reaction requires 3.5–4 h (11) at 100–126ЊC, whereas
the same reaction requires only 2 min under MWI. Hydrazides 3a–d on reaction
with Ͷ-bromoacetophenone (2 : 1) yielded 6-aryl-3-[(5-alkyl-1,3,4-thiadiazol-2-yl
sulfanyl) methyl]-1,2,4-triazines 4a–h as evidenced by the disappearance of bands
at 3260–3320 and 1630–1640 cmϪ1 due to the amine and amide groups, respectively,
and the appearance of a band at 1580–1600 cmϪ1 due to CuN group. This reaction
requires 9–10 h when heated at reflux in ethanol and using microwaves the reaction
is complete in 5–6 min with improved yields. All the new compounds were character-
ized on the basis of spectral and analytical data. The reaction pathway and mecha-
nism for cyclization are depicted in Scheme 1. The analytical and spectral data of
compounds are given in Tables 1 and 2, respectively.
The activity of the compounds is expressed in terms of minimum inhibitory
concentration (MIC) required to inhibit the growth of mycobacteria observed by
visible growth. The antitubercular activity data of the compounds (4a–h) are re-
ported in Table 3. All the compounds are active against M. tuberculosis ATCC
27294. Compound 4a is most active, exhibiting an MIC value of 50 Ȑg/ml. Other
compounds (4b–h) exhibited MIC values of 100 Ȑg/ml.

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