Pyrazine associated novel 1,2,4-triazolo thiadazines: Synthesis, characterization and as antibacterial agents

Article abstract of DOI:10.14233/ajchem.2016.20011

5-(5-Methyl-pyrazin-2-yl)-[1,3,4]-oxadiazole-2-thiol (4) has been synthesized form the starting material 5-methyl-2-pyrazinecarboxylic acid (1) on esterification with ethanol and condensation with hydrazine hydrate followed by cyclization with carbon disu

Full text of DOI:10.14233/ajchem.2016.20011

Asian Journal of Chemistry; Vol. 28, No. 11 (2016), 2453-2456
A
SIAN
J
OURNAL OF HEMISTRY
C
http://dx.doi.org/10.14233/ajchem.2016.20011
Pyrazine Associated Novel 1,2,4-Triazolo Thiadazines:
Synthesis, Characterization and as Antibacterial Agents
*
CHANDRA SEKHARA REDDI MADDIREDDI , UMA DEVI PARIMI, SREENIVASA REDDY BHIMI REDDY and SATYANARAYANA REDDY MANDAPATI
Department of Chemistry, GITAM Institute of Science, GITAM University, Gandhi Nagar, Rushikonda, Visakhapatnam-530 045, India
*Corresponding author: E-mail: csreddimelpl@gmail.com
Received: 4 April 2016;
Accepted: 30 June 2016;
Published online: 10 August 2016;
AJC-18016
5-(5-Methyl-pyrazin-2-yl)-[1,3,4]-oxadiazole-2-thiol (4) has been synthesized form the starting material 5-methyl-2-pyrazinecarboxylic
acid (1) on esterification with ethanol and condensation with hydrazine hydrate followed by cyclization with carbon disulfide. Compound
4 further converted into the next intermediate, 4-amino-5-(5-methyl-pyrazin-2-yl)-4H-[1,2,4]-triazole-2-thiol (5) when reacts with hydrazine
hydrate. Finally, the target compounds, 3-(5-methyl-pyrazin-2-yl)-6-aryl-7H-[1,2,4]-triazole-[3,4-b][1,3,4]-thiadazine (6a-g) have been
synthesized successfully from the condensation reaction performed using compound 5 and a variety of phenacyl bromides. The chemical
structures of all the newly synthesized intermediates and products were confirmed by IR, 1H NMR, mass spectral studies and elemental
analysis. Additionally all the target compounds have been screened for antibacterial activity.
Keywords: 1,2,4-Triazolo thiadiazines, Pyrazine, Antibacterial activity.
acid (1) (0.01 mol) in absolute ethyl alcohol (10 mL), conc.
H2SO4 (2 mL) was added. The mixture was refluxed for 4 h
with constant stirring. After completion of the reaction (moni-
tored by the TLC), the mixture was poured into ice-cold water.
Crude product was collected by filtration, washed with 10 %
NaHCO3 solution, dried and recrystallized from ethyl alcohol
to get pure 5-methyl-2-pyrazine carboxylic acid ethyl ester (2).
Preparation of 5-methyl-2-pyrazine carboxylic acid
hydrazide (3): A mixture of 5-methyl-2-pyrazinecarboxylic
acid ethyl ester (2) (0.01 mol) and hydrazine hydrate (0.02
mol) in ethyl alcohol (20 mL) was refluxed for 3 h with
consistent stirring. After realization of the reaction (examined
by the TLC), the reaction mixture is cooled to room temperature
and filtered. The crude product was recrystallized from ethanol to
offer pure 5-methyl-2-pyrazine carboxylic acid hydrazide (3).
Preparation of 5-(5-methyl-pyrazin-2-yl)-[1,3,4]-
oxadiazole-2-thiol (4): A mixture of 5-methyl-2-pyrazine
carboxylic acid hydrazide (3) (0.01 mol), potassium hydroxide
(0.02 mol) and carbon disulfide (0.01 mol) in ethanol (25 mL)
was heated under reflux with steady stirring for 9 h. After
completion of the reaction (scanned by the TLC), the solvent
was distilled in vacuo, the residual mass was poured over
crushed ice and neutralized the alkaline solution with 10 %
hydrochloric acid. The precipitated crude product was filtered,
washed with water, dried and recrystallized from ethanol to
achieve pure 5-(5-methyl-pyrazin-2-yl)-[1,3,4]-oxadiazole-2-
thiol (4).
INTRODUCTION
The development of drug resistance towards the clinically
used antibacterial agents has increased the demand for the
design and synthesis of new chemical scaffolds possessing
antimicrobial activity. 1,2,4-Triazole derivatives have consis-
tently attracted scientific and practical interest because of their
widely varying chemical properties, synthetic versatility and
pharmacological activities, such as antibacterial [1], antifungal
[2], antitubercular [3], analgesic [4], anti-inflammatory [5],
anticancer [6], anticonvulsant [7], antiviral, [8], insecticidal
[9] and antidepressant [10] properties.
EXPERIMENTAL
All the reagents and solvents were used as purchased from
Sigma Aldrich without further purification. Melting points
were determined on a Fisher-Johns melting point apparatus
and are uncorrected. Crude products were purified by column
chromatography on silica gel of 60-120 mesh. IR spectra were
obtained on a PerkinElmer BX series FT-IR 5000 spectrometer
using KBr pellet. 1H NMR spectra were recorded on a Varian
300 MHz spectrometer. The chemical shifts were reported as
ppm down field using TMS as an internal standard. Mass
spectra were recorded on aVG-Micromass 7070H spectrometer
operating at 70 eV.
Preparation of 5-methyl-2-pyrazine carboxylic acid ethyl
ester (2): To the solution of 5-methyl-2-pyrazine carboxylic

2454 Maddireddi et al.
Asian J. Chem.
Preparation of 4-amino-5-(5-methyl-pyrazin-2-yl)-4H-
[1,2,4]-triazole-2-thiol (5): To a warm solution of 5-(5-methyl-
pyrazin-2-yl)-[1,3,4]-oxadiazole-2-thiol (4) (0.01 mol) in
ethanol (15 mL), 80 % hydrazine hydrate (0.02 mol) was
added drop wise and the reaction mixture was heated under
reflux for 5 h with constant stirring. After fulfillment of the
reaction (check by the TLC), the solvent was distilled off in
vacuo, cooled and the crystals separated were filtered, washed
with cold ethanol and recrystallized from chloroform to give
pure 4-amino-5-(5-methyl-pyrazin-2-yl)-4H-[1,2,4]-triazole-
2-thiol (5).
Preparation of 3-(5-methyl-pyrazin-2-yl)-6-aryl-7H-
[1,2,4]-triazole-[3,4-b][1,3,4]-thiadazine (6a-g): A mixture
of 4-amino-5-(5-methyl-pyrazin-2-yl)-4H-[1,2,4]-triazole-2-
thiol (5) (0.01 mol) and corresponding phenacyl bromide (0.01
mol) in absolute ethanol (15 mL) was refluxed for 6-7 h with
constant stirring. After completion of the reaction (checked
by the TLC), the reaction mixture was concentrated and cooled
to room temperature and the remaining solvent was removed
under reduced pressure, then diethyl ether (10 mL) was added
and the reaction mixture was left at 0 °C for overnight. The
precipitated solid was filtered off; the crude product thus
obtained was purified by column chromatography on silica
gel with hexane-ethyl acetate as eluent to afford pure 3-(5-
methyl-pyrazin-2-yl)-6-aryl-7H-[1,2,4]-triazole-[3,4-
b][1,3,4]-thiadazine (6a-g).
2928 (C-H, CH3), 2848 (S-H), 1635 (C=N), 1575 (C=C,
pyrazine); 1H NMR (300 MHz, CDCl3) δ 1.45 (s, 2H, NH2),
2.66 (s, 3H, CH3), 7.33 (s, 1H, pyrazine-H), 7.59 (s, 1H, pyrazine-
H), 10.21 (s, 1H, S-H); MS m/z 208 (M+); Elemental analysis:
Calculated for C7H8N6S: C-40.37, H-3.87, N-40.36, S-15.40.
Found: C-40.14, H-3.82, N-40.28, S-15.35.
3-(5-Methyl-pyrazin-2-yl)-6-phenyl-7H-[1,2,4]-
triazole-[3,4-b][1,3,4]-thiadazine (6a): Yellow solid; Yield
77 %; m.p.: 120-122 °C; IR (KBr, νmax, cm–1): 3036 (C-H, Ar),
2974 (C-H, CH3), 1634 (C=N), 1623 (C=C, Ar), 1285 (C-S);
1H NMR (300 MHz, CDCl3) δ2.42 (s, 3H, CH3), 3.84 (s, 2H,
CH2-S), 7.19 (s, 1H, pyrazine-H),), 7.35-7.54 (m, 5H, Ar-H),
7.52 (s, 1H, pyrazine-H); MS m/z 308 (M+); Elemental analysis:
Calculated for C15H12N6S: C-58.43, H-3.92, N-27.25, S-10.40.
Found: C-58.12, H-3.90, N-27.18, S-10.34.
3-(5-Methyl-pyrazin-2-yl)-6-p-tolyl-7H-[1,2,4]-
triazole-[3,4-b][1,3,4]-thiadazine (6b): Orange solid; Yield
71 %; m.p.: 139-141 °C; IR (KBr, νmax, cm–1): 3038 (C-H, Ar),
2968 (C-H, CH3), 1642 (C=N), 1624 (C=C), 1280 (C-S); 1H
NMR (300 MHz, CDCl3) δ 2.39 (s, 3H, CH3), 2.44 (s, 3H,
CH3), 3.95 (s, 2H, CH2-S), 7.28 (s, 1H, pyrazine-H), 7.49 (s,
1H, pyrazine-H), 7.51 (d, 2H, J = 7.0 Hz, Ar-H), 7.65 (d, 2H,
J = 7.0 Hz, Ar-H); MS m/z 322 (M+); Elemental analysis:
Calculated for C16H14N6S: C-59.61, H-4.38, N-26.07, S-9.95.
Found: C-59.18, H-4.35, N-25.99, S-9.94.
6-(2-Methoxy-phenyl)-3-(5-methyl-pyrazin-2-yl)-7H-
[1,2,4]-triazole-[3,4-b][1,3,4]-thiadazine (6c): Yellow solid;
Yield 70 %; m.p.: 158-160 °C; IR (KBr, νmax, cm–1): 3051 (C-
H, Ar), 2964 (C-H, CH3), 1648 (C=N), 1623 (C=C, Ar),1270
(C-S), 1148 (C-O);1H NMR (300 MHz, CDCl3) δ2.71 (s, 3H,
CH3), 3.54 (s, 3H, O-CH3), 3.87 (s, 2H, S-CH2), 7.32 (s, 1H,
pyrazine-H), 7.34-7.59 (m, 4H, Ar-H), 7.52 (s, 1H, pyrazine-
H); MS m/z 338 (M+); Elemental analysis: Calculated for
C16H14N6OS: C-56.79, H-4.17, N-24.84, S-9.48. Found: C-
56.48, H-4.15, N-24.48, S-9.40.
Physical and spectral data
5-Methyl-2-pyrazine carboxylic acid ethyl ester (2):
White solid;Yield 74 %; m.p.: 132-134 °C; IR (KBr, νmax, cm–1):
3024 (C-H, pyrazine), 2945 (C-H, CH3), 1695 (C=O), 1640
(C=N) 1560 (C=C, pyrazine), 1145 (C-O);1H NMR (300 MHz,
CDCl3) δ 1.29 (t, 3H, J = 5.4 Hz, CH3), 2.42 (s, 3H, CH3),
4.12 (q, 2H, J = 5.4 Hz, CH2), 7.12 (s, 1H, pyrazine-H), 7.45
(s, 1H, pyrazine-H); MS m/z 166 (M+); Elemental analysis:
Calculated for C8H10N2O2: C-57.82, H-6.07, N-16.86. Found:
C-57.25, H-6.00, N-16.74.
6-(4-Methoxy-phenyl)-3-(5-methyl-pyrazin-2-yl)-7H-
[1,2,4]-triazole-[3,4-b][1,3,4]-thiadazine (6d): Orange solid;
Yield 73 %; m.p.: 114-116 °C; IR (KBr, νmax, cm–1): 3048 (C-
H, Ar), 2968 (C-H, CH3), 1660 (C=N), 1625 (C=C, Ar), 1275
(C-S), 1158 (C-O);1H NMR (300 MHz, CDCl3) δ2.61 (s, 3H,
CH3), 3.51 (s, 2H, S-CH2), 3.74 (s, 3H, O-CH3), 7.25 (s, 1H,
pyrazine-H), 7.39 (d, 2H, J = 7.8 Hz, Ar-H), 7.49 (s, 1H,
pyrazine-H), 7.68 (d, 2H, J = 7.8 Hz, Ar-H); MS m/z 338
(M+); Elemental analysis: Calculated for C16H14N6OS: C-56.79,
H-4.17, N-24.84, S-9.48. Found: C-56.48, H-4.15, N-24.48,
S-9.40.
5-Methyl-2-pyrazinecarboxylic acid hydrazide (3):
Yellow solid; Yield 72 %; m.p.: 142-144 °C; IR (KBr, νmax
,
cm–1): 3240 (N-H), 3035 (C-H, pyrazine), 2954 (C-H, CH3),
1640 (C=O), 1630 (C=N), 1568 (C=C, pyrazine); 1H NMR
(300 MHz, CDCl3) δ 2.59 (s, 3H, CH3), 5.28 (s, 2H, NH2),
7.21 (s, 1H, pyrazine-H), 7.45 (s, 1H, pyrazine-H), 7.58 (s,
1H, NH); MS m/z 152 (M+); Elemental analysis: Calculated
for C6H8N4O: C-47.36, H-5.30, N-36.82. Found: C-47.02, H-
5.27, N-36.65.
5-(5-Methyl-pyrazin-2-yl)-[1,3,4]-oxadiazole-2-thiol
(4): Pale yellow solid;Yield 76 %; m.p.: 145-147 °C; IR (KBr,
6-(4-Chloro-phenyl)-3-(5-methyl-pyrazin-2-yl)-7H-
[1,2,4]-triazole-[3,4-b][1,3,4]-thiadazine (6e): Brown solid;
Yield 70 %; m.p.: 130-132 °C; IR (KBr, νmax, cm–1): 3048 (C-
H, Ar), 2974 (C-H, CH3), 1634 (C=N), 1610 (C=C, Ar), 1275
(C-S); 1H NMR (300 MHz, CDCl3) δ 2.68 (s, 3H, CH3), 3.81
(s, 2H, S-CH2), 7.32 (s, 1H, pyrazine-H), 7.42 (d, 2H, J = 7.4
Hz, Ar-H), 7.51 (s, 1H, pyrazine-H), 7.74 (d, 2H, J = 7.4 Hz,
Ar-H); MS m/z 342 (M+); Elemental analysis: Calculated for
C15H11N6SCl: C-52.55, H-3.23, Cl-10.34, N-24.52, S-9.35.
Found: C-52.12, H-3.21, Cl-10.28, N-24.47, S-9.32.
ν
max, cm–1): 3041 (C-H, pyrazine), 2936 (C-H, CH3), 2836 (S-
H), 1642 (C=N), 1610 (C=C, pyrazine), 1128 (C-O);1H NMR
(300 MHz, CDCl3) δ2.39 (s, 3H, CH3), 7.32 (s, 1H, pyrazine-
H), 7.52 (s, 1H, pyrazine-H), 10.85 (s, 1H, S-H); MS m/z 194
(M+); Elemental analysis: Calculated for C7H6N4OS: C-43.29,
H-3.11, N-28.85, S-16.51. Found: C-43.06, H-3.07, N-28.68,
S-16.46.
4-Amino-5-(5-methyl-pyrazin-2-yl)-4H-[1,2,4]-
triazole-2-thiol (5): Orange solid; Yield 71 %; m.p.: 155-157
°C; IR (KBr, νmax, cm–1): 3365 (N-H, NH2), 3038 (C-H, pyrazine),
6-(4-Bromo-phenyl)-3-(5-methyl-pyrazin-2-yl)-7H-
[1,2,4]-triazole-[3,4-b][1,3,4]-thiadazine (6f): Pale yellow

Vol. 28, No. 11 (2016)
Studies of Pyrazine Associated Novel 1,2,4-Triazolo Thiadazines 2455
solid; Yield 74 %; m.p.: 150-152 °C; IR (KBr, νmax, cm–1):
3032 (C-H, Ar), 2971 (C-H, CH3), 1631 (C=N), 1578 (C=C,
Ar), 1265 (C-S); 1H NMR (300 MHz, CDCl3) δ 2.50 (s, 3H,
CH3), 3.92 (s, 2H, CH2-S), 7.19 (s, 1H, pyrazine-H), 7.39 (s,
1H, pyrazine-H), 7.42 (d, 2H, J = 7.4 Hz, Ar-H), 7.59 (d, 2H,
J = 7.4 Hz, Ar-H); MS m/z 387 (M+); Elemental analysis:
Calculated for C15H11N6SBr: C-46.52, H-2.86, Br-20.63, N-
21.70, S-8.28. Found: C-46.02, H-2.83, Br-20.54, N-21.51,
S-8.24.
6-(4-Nitro-phenyl)-3-(5-methyl-pyrazin-2-yl)-7H-
[1,2,4]-triazole-[3,4-b][1,3,4]-thiadazine (6g): Brown solid;
Yield 73 %; m.p.: 118-120 °C; IR (KBr, νmax, cm–1): 3028 (C-
H, Ar), 2965 (C-H, CH3), 1630 (C=N), 1584 (C=C, Ar), 1248
(C-S); 1H NMR (300 MHz, CDCl3) δ 2.65 (s, 3H, CH3), 3.74
(s, 2H, CH2-S), 7.32 (s, 1H, pyrazine-H), 7.39 (d, 2H, J = 7.0
Hz, Ar-H), 7.54 (s, 1H, pyrazine-H), 7.56 (d, 2H, J = 7.0 Hz,
Ar-H); MS m/z 353 (M+); Elemental analysis: Calculated for
C15H11N7O2S: C-50.99, H-3.14, N-27.75, S-9.07. Found: C-
50.02, H-3.12, N-27.58, S-9.01.
N
N
NH2
NH
N
N
CS2, KOH, EtOH
reflux, 9 h
SH
N
3
N
4
O
O
Scheme-III
Then this intermediate 4 is converted into final interme-
diate, 4-amino-5-(5-methyl-pyrazin-2-yl)-4H-[1,2,4]-triazole-
2-thiol (5) in excellent yield when reacts with hydrazine
hydrate in ethanol at reflux for 5 h with consistent stirring
(Scheme-IV).
N
N
N
N
N
N
NH2 NH2, EtOH
reflux, 5 h
SH
SH
N
5
N
N
4
O
NH2
Scheme-IV
Finally, the target compounds, 3-(5-methyl-pyrazin-2-yl)-
6-aryl-7H-[1,2,4]-triazole-[3,4-b][1,3,4]-thiadazine (6a-g)
have been synthesized in good to excellent yields successfully
from the condensation reaction performed between compound
5 and a variety of phenacylbromides in ethanol under reflux
for 6-7 h on uniform stirring (Scheme-V). The chemical struc-
tures of all the newly synthesized intermediates and products
RESULTS AND DISCUSSION
The biological and medicinal characteristics of triazole
derivatives stimulated us to synthesize some new biologically
active heterocycles, we integrated thiadiazine and triazole
moieties into pyrazine molecule, since these systems possess
well documented antimicrobial activity. Accordingly, in this
paper, we reported the synthesis of 3-(5-methyl-pyrazin-2-yl)-
6-aryl-7H-[1,2,4]-triazole-[3,4-b][1,3,4]-thiadazine (6a-g).
The syntheses of the target compounds 6a-g commenced from
commercially available 5-methyl-2-pyrazine carboxylic acid
(1). The initial intermediate, 5-methyl-2-pyrazine carboxylic
acid ethyl ester (2), has been prepared in excellent yield by
boiling of a mixture of compound 1 and absolute ethyl alcohol
in the presence of catalytic amount of conc. H2SO4 for 4 h with
constant stirring (Scheme-I).
1
were confirmed by IR, H NMR, mass spectral studies and
elemental analysis. Antibacterial activity of the final com-
pounds has been evaluated and all the compounds have shown
significant inhibition of bacterial growth.
N
N
N
N
N
N
RCOCH2Br
SH
N
6a-g
N
N
S
N
5
N
EtOH, reflux, 6-7 h
NH2
N
N
R
H SO ,
EtOH
2
4
6a-g, (a) R = H, (b) R = 4-CH3, (c) R = 2-OCH3,
(d) R = 4-OCH3, (e) R = 4-Cl, (f) R = 4-Br, (g) R = 4-NO2
OH
O
reflux, 4 h
N
1
N
2
Scheme-V
O
O
Scheme-I
Antibacterial activity: The newly synthesized compounds,
3-(5-methyl-pyrazin-2-yl)-6-aryl-7H-[1,2,4]-triazole-[3,4-
b][1,3,4]-thiadazine (6a-g) were used to evaluate for their in
vitro antibacterial activity against four representative bacterial
strains such as Bacillus subtilis, Staphylococcus aureus, Salmo-
nella typhimurium and Escherichia coli with broth dilution
method [11] using roxythromycin as standard drug. The assay
was performed in duplicates and the mean diameters of the
zone of inhibition in mm were recorded. The results of the in
vitro antibacterial activity of the target compounds are reported
in Table-1. Compound 6g with para nitro derivative was perfor-
med highest activity against B. subtilis with zone of inhibition
20 mm. The same 6g was exhibited prominent antibacterial
activity towards S. aureus with zone of inhibition 21 mm.
Compound 6f with para bromo group showed eminent activity
against S. typhimurium with zone of inhibition 23 mm. Same
compound 6f executed significant activity towards E. coli with
Then the intermediate 2 is turned into next intermediate,
5-methyl-2-pyrazine carboxylic acid hydrazide (3) when reacts
with hydrazine hydrate in ethyl alcohol at reflux for 3 h with
uniform stirring (Scheme-II).
N
N
NH2
NH
NH NH ,
EtOH
2
2
O
N
2
N
3
reflux, 3 h
O
O
Scheme-II
Further intermediate, 5-(5-methyl-pyrazin-2-yl)-[1,3,4]-
oxadiazole-2-thiol (4) has been achieved in good yield from the
reaction occurred between compound 3 and carbon disulfide
in presence of potassium hydroxide in ethanol at reflux for
9 h on steady stirring followed by acidification (Scheme-III).

2456 Maddireddi et al.
Asian J. Chem.
TABLE-1
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Entry
B. subtilis S. aureus S. typhimurium E. coli
6a
6b
6c
6d
6e
6f
6g
06
12
18
16
17
11
20
26
07
13
16
14
19
21
21
25
09
14
15
13
20
23
18
28
10
09
17
15
20
21
17
27
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21 mm. The remaining compounds of this series disclosed
moderate to good antibacterial activity. The outstanding
properties of this new class of antibacterial substances deserve
further investigation in order to clarify the mode of action at
molecular level, responsible for the activity observed. More
extensive study is also warranted to determine additional
physico-chemical and biological parameters to have a deeper
insight into structure-activity relationship (SAR) and to
optimize the effectiveness of this series of molecules.