Synthesis and antimicrobial activities of some novel 1,3,4-oxadiazoles carrying alkylthio and alkylsulphonyl phenoxy moieties

Article abstract of DOI:10.1080/10426500701542981

A series of new 2-[(4-alkylthio/alkylsulfonyl phenoxy) methyl]-5- substituted-1,3,4-oxadiazoles 4a-y, 5a-h) have been synthesized from 4-alkylthio phenol (1a-b) through a multi-step reaction sequence. Compounds 1a-b reacted with ethyl chloroacetate to giv

Full text of DOI:10.1080/10426500701542981

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Synthesis and Antimicrobial
Activities of Some novel 1,3,4-
Oxadiazoles Carrying Alkylthio
and Alkylsulphonyl phenoxy
Moieties
T. Karabasanagouda a , Airody Vasudeva Adhikari b
N. Suchetha Shetty c
a Strides Research and Specialty Chemicals Ltd. ,
New Mangalore, India
&
b Department of Chemistry , National Institute of
Technology Karnataka , Surathkal, Srinivasnagar,
India
c Department of Biochemistry , Justice K.S. Hegde
Medical Academy , Deralakatte, India
Published online: 03 Dec 2007.
To cite this article: T. Karabasanagouda , Airody Vasudeva Adhikari & N.
Suchetha Shetty (2007) Synthesis and Antimicrobial Activities of Some novel
1,3,4-Oxadiazoles Carrying Alkylthio and Alkylsulphonyl phenoxy Moieties,
Phosphorus, Sulfur, and Silicon and the Related Elements, 182:12, 2925-2941, DOI:
10.1080/10426500701542981
To link to this article: http://dx.doi.org/10.1080/10426500701542981
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Phosphorus, Sulfur, and Silicon, 182:2925–2941, 2007
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500701542981
Synthesis and Antimicrobial Activities of Some novel
1,3,4-Oxadiazoles Carrying Alkylthio and Alkylsulphonyl
phenoxy Moieties
T. Karabasanagouda
Strides Research and Specialty Chemicals Ltd., New Mangalore, India
Airody Vasudeva Adhikari
Department of Chemistry, National Institute of Technology Karnataka,
Surathkal, Srinivasnagar, India
N. Suchetha Shetty
Department of Biochemistry, Justice K.S. Hegde Medical Academy,
Deralakatte, India
A series of new 2-[(4-alkylthio/alkylsulfonyl phenoxy) methyl]-5-substituted-1,3,4-
oxadiazoles (4a–y, 5a–h) have been synthesized from 4-alkylthio phenol (1a–
b) through a multi-step reaction sequence. Compounds 1a–b reacted with ethyl
chloroacetate to give ethyl [4-(alkylthio) phenoxy] acetate (2a–b) which on oxidation
yielded ethyl [4-(alkylsulfonyl)phenoxy]acetate (2c–d). They were readily converted
to the corresponding hydrazides 3a–d, and then cyclized to the title compounds
2-{[4-(alkylthio/alkylsulfonyl) phenoxy] methyl}-5-aryl-1,3,4-oxadiazoles (4a–y) by
condensing with aromatic carboxylic acids. Further, 3a–b, on condensation with
carbon disulphide afforded the title compounds 2-{[4-(alkylthio) phenoxy]methyl}-
1,3,4-oxadiazole-5-thiols(5a–b), which on alkylation yielded the title compounds
2-[(4-alkylthio phenoxy) methyl]-1,3,4-oxadiazole-5-alkyl thiols (5c–f), while with
chloroacetic acid gave the target compounds 2-[(4-alkylthio phenoxy) methyl]-1,3,4-
oxadiazole-5-thioacetic acid (5g–h). The structures of new compounds 4a–5h were
1
13
established on the basis of their elemental analysis and IR, H NMR, C NMR &
MASS spectral data. All the title compounds were subjected to in vitro antibacterial
Received March 24, 2007; accepted June 5, 2007.
We are grateful to the Heads of Chemistry Department, National Institute of Tech-
nology Karnataka, Surathkal, and Strides Research & Specialty Chemicals Ltd., New
Mangalore, for providing laboratory facilities. The authors are also thankful to Prof. A.
Srikrishna, IISc Bangalore and the Head, SAIF, CDRI for providing 1H NMR, 13C NMR,
and Mass spectral facilities.
Address correspondence to Airody Vasudeva Adhikari, Department of Chemistry, Na-
tional Institute of Technology Karnataka, Surathkal, Srinivasnagar, PIN 575025, India.
E-mail: avchem@nitk.ac.in
2925

2926
T. Karabasanagouda et al.
testing against four pathogenic strains and antifungal screening against three
fungi. Preliminary results indicate some of them exhibited promising activities and
they deserve more consideration as potential antimicrobials.
Keywords 2-{[4-(Alkylthio/alkylsulfonyl) phenoxy] methyl}-5-aryl-1,3,4-oxadiazoles;
2-{[4-(alkylthio) phenoxy]methyl}-1,3,4-oxadiazole-5-thiols; 5-{[4-(alkylthio) phenoxy]
methyl}-1,3,4-oxadiazole-2-substituted thiols; antibacterial & antifungal screening
INTRODUCTION
Various 1,3,4-oxadiazole derivatives have been reported to possess di-
verse types of biological properties such as antibacterial,1,2 antifungal,3
anti-inflammatory,4 antiviral,5 anticonvulsant, antitubercular,6 and
antipyretic activities.7 A thorough literature survey reveals that pres-
ence of 4-alkylthiophenoxy and 4-alkylsulphonyl phenoxy moieties is
an important structural feature of wide variety of synthetic drugs.8−10
It has been established that introduction of 4-methylmercapto phenyl
group and 4-methylsulphonyl phenyl groups to different heterocycles
have yielded many biologically active compounds endowed with wide
spectrum of pharmacological and antimicrobial activities.11−12 Encour-
aged by the above reports, we planned to synthesize new 5-substituted
1,3,4-oxadiazoles containing 4-alkylthio/sulphonyl phenoxy group at
position 2 of it, aiming at investigation of new heterocycles of enhanced
pharmacological activities. The present study describes the synthe-
sis of hitherto unreported 2-{[4-(alkylthio or alkylsulfonyl) phenoxy]
methyl}-5-aryl-1,3,4-oxadiazoles and 2-{[4-(alkylthio or alkylsulfonyl)
phenoxy] methyl}-1,3,4-oxadiazole-5-substituted thiols (4a–y and 5a–
h) and evaluation of their antibacterial and antifungal activities.
RESULTS AND DISCUSSION
The reaction sequences employed for the synthesis of title compounds
is shown in Figures 1 and 2. The key intermediate, ethyl [4-(alkylthio)
phenoxy] acetate (2a–b), was prepared by treating ethyl chloroacetate
with 4-(alkylthio) phenol (1a–b) in boiling dry acetone in presence
of potassium carbonate. The compounds, ethyl [4-(alkylsulfonyl
phenoxy] acetate (2c–d) were obtained by the oxidation of ethyl
[4-(alkylthio) phenoxy] acetate (2a–b) with 30% hydrogen peroxide
in acetic acid. These esters (2a–d) were conveniently converted to
2-[4-(alkylthio/alkyl sulphonyl phenoxy] acetohydrazides (3a–d) by
refluxing with hydrazine hydrate in methanol. Further condensa-
tion of (3a–d) with various aromatic carboxylic acids in presence of
phosphorous oxychloride afforded the title compounds 2-{[4-(alkylthio

Antimicrobial Activities of Oxadiazoles
2927
FIGURE 1 Where R1 = CH3, C2H5.
or alkylsulfonyl) phenoxy] methyl}-5-aryl-1,3,4-oxadiazoles (4a–y).
Prolonged reaction leads to the formation of by-products instead of
the desired 4a–y. The acetohydrazides (3a,b), on refluxing with CS2
in methanolic KOH afforded the 2-{[4-(alkylthio or alkylsulfonyl)
phenoxy] methyl}-1,3,4-oxadiazole-5-thiols (5a,b) which on treatment
with dimethylsulphate or diethylsulphate in alkaline medium gave the

2928
T. Karabasanagouda et al.
FIGURE 2 Where R1 = CH3, C2H5 and R2 = H, CH3, C2H5, CH2COOH.
2-[(4-alkylthio phenoxy) methyl]-1,3,4-oxadiazole-5-alkyl thiols (5c–f).
Reactions of 5a,b with chloroacetic acid in presence of alkali yielded the
2-[(4-alkylthio phenoxy) methyl]-1,3,4-oxadiazole-5-thioacetic acids
(5g–h) in good yield.
The structural assignments of the compounds were based on their el-
emental analysis and spectral (IR, 1H NMR 13C NMR and MASS) data.
The characterization data of all the new compounds were summarized
in Tables I and II.
The formation of 2-[4-(methylthio) phenoxy] acetohydrazide (3a)
from ethyl [4-(methylthio) phenoxy] acetate (2a) was confirmed by its
IR, 1H NMR spectral and elemental analysis. IR spectrum of it showed
absorption band at 3310, 3224, 3045, 1680, 1544 cm−1 due to NH2,
NH, SCH3, C O, and C C groups respectively, while 1H NMR showed
sharp singlets at δ 2.40 and 4.46, which correspond to SCH3 and
CH2 protons respectively. It appeared two doublets at δ 6.94 and 7.24
indicating the presence of four aromatic protons. Further cyclization
of 3a to 2-{[4-(methylthio) phenoxy] methyl}-5-phenyl-1,3,4-oxadiazole
(4a) was confirmed by IR spectrum which showed peaks at 3009, 1697,
1494, and 1242 cm−1 indicating the presence of SCH3, C N, phenyl

2929

2930

Antimicrobial Activities of Oxadiazoles
2931
TABLE II Characterization Data of Compounds 5a–h
Elemental analysis
% found (cal)
Mol.
formula
Mol. M.P. Yield
mass (◦C) (%)
Compd. Ar
R1
C
H N
5a
5b
5c
5d
5e
5f
SH
CH3 C10H10N2O2S2 254.33 102 69 47.20 3.98 11.04
(47.22) (3.96) (11.07)
SH
C2H5 C11H12N2O2S2 268.30 99
CH3 C11H12N2O2S2 268.35 70
C2H5 C12H14N2O2S2 282.38 50
CH3 C12H14N2O2S2 282.38 46
C2H5 C13H16N2O2S2 296.41 35
69 49.26 4.54 10.46
(49.23) (4.51) (10.44)
73 49.20 4.50 10.46
(49.23) (4.51) (10.44)
SCH3
SCH3
SC2H5
SC2H5
73 51.06 5.03
9.94
(51.04) (5.00) (9.92)
70 51.03 5.01
9.93
(51.04) (5.00) (9.92)
65 52.65 5.43
9.43
(52.68) (5.44) (9.45)
5g
5h
SCH2COOH CH3 C12H12N2O4S2 312.36 72
SCH2COOH C2H5 C13H14N2O4S2 326.39 60
75 47.15 4.89
(46.14) (3.87) (8.97)
70 47.87 4.34
8.95
8.56
(47.84) (4.32) (8.58)
1
and C O C groups respectively. In its H NMR spectrum, peaks due
to SCH3 and CH2 protons appeared at δ 2.45 and 5.30, respectively.
Peaks due to two phenyl groups appeared at δ 6.80, 7.28, and 8.00 as
doublets and at 7.50 as multiplet. Further, FAB Mass Spectrum showed
molecular ion peak at m/z 317 (100%) and 316 which is in agreement
with the molecular formula [C16H14N2O2S NH4]+1 and C16H14N2O2S
NH4 13C NMR spectrum showed signals at δ 17.32 due to SCH3 while
peaks at δ 60.14, 115.41, 129.08, 129.52, 129.61, 131.02, 132.06, 132.50,
155.89, 162.19, and 164.97 are due to, OCH2, C2 & C6 of phenoxy group,
C2 & C6 of 6-phenyl moiety, C3 & C5 of phenoxy moiety, C4 of phenoxy
moiety, C4 of 6-phenyl moiety, C3 & C5 of 6-phenyl moiety, C1 of 6-phenyl
moiety, C1 of phenoxy moiety, C5 of oxadiazole and C2 of oxadiazole
moiety respectively, The peaks due to quaternary carbon atoms of the
structure disappeared on DEPT experimentation
The buildup of 5b from 3b is evidenced by its IR, 1H NMR, 13C NMR
and Mass spectral data. IR spectrum of 5b indicates the presence of
SC2H5, C N, phenyl and C O C groups due to peaks at 3089, 1641,
1599, and 1253 cm−1, respectively. 1H NMR spectrum showed the pres-
ence of SCH2CH3 at δ 1.26 as triplet δ 2.85 as quartet. The singlet at
δ 5.00 shows presence of OCH2 and also two doublets at δ 6.88 and δ
7.32 are due to four aromatic protons. 13C-NMR spectrum of 5b showed
signals at δ 29.16 and 14.49 due to SCH2CH3, while peaks at δ 59.91,

2932
T. Karabasanagouda et al.
TABLE III Antibacterial Activity of the Prepared Compounds
MIC in µg/ml and zone of inhibition (mm)
Compd.
E. coli
K .pneumoniae
P.aeruginosa
S. aureus
4a
4b
4c
4d
4e
4f
12.5 (11–15)
12.5 (11–15)
6.25 (16–20)
12.5 (11–15)
12.5 (11–15)
25 (<10)
12.5 (11–15)
12.5 (11–15)
12.5 (11–15)
12.5 (11–15)
12.5 (11–15)
25 (<10)
6.25 (16–20)
25 (<10)
12.5 (11–15)
25 (<10)
12.5 (11–15)
25 (<10)
25 (<10)
12.5 (11–15)
12.5 (11–15)
25 (<10)
25 (<10)
25 (<10)
4g
4h
12.5 (11–15)
25 (<10)
25 (<10)
25 (<10)
12.5 (11–15)
25 (<10)
6.25 (16–20)
25 (<10)
4i
4j
4k
12.5 (11–15)
12.5 (11–15)
25 (<10)
12.5 (11–15)
25 (<10)
25 (<10)
25 (<10)
25 (<10)
25 (<10)
6.25 (16–20)
25 (<10)
25 (<10)
4l
4m
4n
25 (<10)
25 (<10)
25 (<10)
25 (<10)
25 (<10)
25 (<10)
12.5 (11–15)
25 (<10)
25 (<10)
25 (<10)
12.5 (11–15)
25 (<10)
4o
4p
12.5 (11–15)
25 (<10)
12.5 (11–15)
25 (<10)
12.5 (11–15)
25 (<10)
25 (<10)
25 (<10)
4q
4r
4s
4t
4u
4v
4w
4x
4y
5a
5b
5c
5d
5e
5f
5g
12.5 (11–15)
6.25 (16–20)
6.25 (16—20)
12.5 (11–15)
6.25 (16–20)
12.5 (11–15)
6.25 (16–20)
6.25 (16–20)
6.25 (16–20)
6.25 (16–20)
6.25 (16–20)
12.5 (11–15)
6.25 (16–20)
25 (<10)
12.5 (11–15)
12.5 (11–15)
12.5 (11–15)
12.5 (11–15)
6.25 (16–20)
25 (<10)
12.5 (11–15)
12.5(11–15)
6.25 (16–20)
6.25 (16–20)
12.5 (11–15)
12.5 (11–15)
6.25 (16–20)
12.5 (11–15)
12.5 (11–15)
25 (<10)
25 (<10)
25 (<10)
6.25 (16–20)
12.5 (11–15)
12.5 (11–15)
12.5 (11–15)
12.5 (11–15)
6.25 (16–20)
6.25 (16–20)
6.25 (16–20)
6.25 (16–20)
6.25 (16–20)
12.5 (11–15)
6.25 (16–20)
12.5 (11–15)
25 (<10)
6.25 (16–20)
12.5 (11–15)
12.5 (11–15)
6.25 (16–20)
12.5(11–15)
6.25 (16–20)
6.25 (16–20)
12.5 (11–15)
6.25 (16–20)
6.25 (16–20)
6.25 (16–20)
6.25 (16–20)
25 (<10)
12.5 (11–15)
12.5 (11–15)
25 (<10)
12.5 (11–15)
25 (<10)
12.5 (11–15)
1.56 (22–30)
25 (<10)
25 (<10)
6.25 (25–33)
5h
Standard
25 (<10)
6.25 (23–27)
6.25 (30–40)
(ciprofloxacin)
The MIC values were evaluated at concentration range, 1.56–25 µg/ml. The figures
in the table show the MIC values and the corresponding zone of inhibition (in mm).
115.49, 129.38, 132.39, 156.04, 159.41, and 178.76 are due to OCH2,
C3 & C5 of phenoxy group, C4 of phenoxy moiety, C2 & C6 of phenoxy
moiety, C1 of phenoxy moiety, C3 of oxadiazole and C5 of oxadiazole, re-
spectively. The peaks due to quaternary carbon atoms disappeared on
DEPT experimentation. Further, FAB MS of 5b showed the molecular

Antimicrobial Activities of Oxadiazoles
2933
ion peak at m/z 269 [M+H]+ and 268 [M]+ which is in agreement with
their molecular formula, C11H12N2O2S2.
EXPERIMENTAL
Melting points were determined in open capillaries and uncorrected
[melting point apparatus: SERWELL Instruments INC, India]. Purity
of the compounds was checked by thin-layer chromatography (TLC) on
a silica coated aluminum sheet (silica gel 60F254) using chloroform and
methanol (9:1, v/v). IR spectra were recorded on NICOLET AVATAR
1
330-FTIR Spectrometer. H and 13C NMR spectra recorded on a Var-
ian 300 MHz NMR spectrometer using TMS as an internal standard.
Chemical shifts are reported in ppm (δ) and signals are described as
singlet (s), doublet (d), triplet (t), quartet (q), broad (br,), and multiplet
(m). The FAB mass spectra were recorded on a JEOL SX 102/DA-6000
spectrophotometer/Data system using Argon/Xenon (6KV, 10 mA) FAB
gas, at 70 eV. Elemental analysis was carried out using Flash EA 1112
Series, CHNSO Analyzer (Thermo). Solvents and reagents were pur-
chased from the commercial venders in the appropriate grade and were
used without purification.
General Procedure for the Preparation of Ethyl [4-(Thioalkyl)
phenoxy]acetate (2a–b)
4-Alkylthio phenol, 1a–b, (1a: 1.4 g, 1b: 1.54 g, 10 mmol) was dissolved
in 50 mL of dry acetone and mixed with (1.6 g, 12 mmol) of anhydrous
potassium carbonate. This was treated with (1.47 g, 12 mmol) of ethyl
chloroacetate slowly with shaking over 10 min, and the mixture was
heated under reflux for 3 h. When the reaction was complete (monitored
by TLC), the reaction mixture was filtered and the filtrate was distilled
under 10-mm pressure. The fraction collected at appropriate boiling
point (140–144◦C for 2a and 130–135◦C for 2b) was directly used for
the next step without further purification.
Procedure for the Preparation of Ethyl [4-Alkyl sulphonyl
phenoxy] acetate (2c–d)
The compound 2a–b (2a: 2.26 g, 2b: 2.4 g 10 mmol), dissolved in 50 ml
of glacial acetic acid was added slowly with (2.2 g, 20 mmol) of 30–33%
hydrogen peroxide over the period of 1 h, below 70◦C. It was further kept
at 70–75◦C with stirring for 3 h, cooled to ambient temperature, and
quenched to 100 ml ice-cold water. The compound 2c–d was extracted
with ethyl acetate and after giving water wash, the solvent was removed

2934
T. Karabasanagouda et al.
by distillation; the liquid left over was further purified by distillation
under reduced pressure. The fraction collected at appropriate boiling
point (160–164◦C for 2c and 155–159◦C for 2d) at 10-mm pressure was
used directly for the next step.
General Procedure for the Preparation of 2-[4-(Alkylthio)
phenoxy] acetohydrazide (3a–b) and 2-[4-Alkyl sulphonyl
phenoxy] acetohydrazide (3c–d)
A mixture of [4-(alkylthio)/alkyl sulphonyl phenoxy] acetate (2a: 2.26
g, 2b: 2.4 g, 2c: 2.58, 2d: 2.72 g, 10 mmol), hydrazine hydrate (1.0 g,
20 mmol) in 50 ml of ethanol was heated under reflux for 5–6 h. The
reaction mixture was left overnight at room temperature, and the solid
separated was collected by filtration. It was washed with ethanol, dried
and recrystallized from ethanol.
3a. 2-[4-(Methylthio) phenoxy] acetohydrazide
This compound was obtained as white solid in a yield of 90%, m.p.
128–129◦C, IR: 3310 cm−1 (NH2), 3224 cm−1 (NH), 3045 cm−1 (SCH3),
1680 cm−1 (>C O), 1544 cm−1 (C C), 814 cm−1 (SCH3); 1H NMR
(DMSO-d6): δ2.4 (s, 3H, SCH3), δ4.46 (s, 2H, CH2), δ6.94 (d, 2H, C2,
C6-H of 4-methylthio phenoxy moiety J = 8.5 Hz), δ 7.24 (d, 2H, C3,
C5-H of 4-ethylthio phenoxy moiety J = 8.5 Hz); Anal. Calculated for
C9H12N2O2S: C, 50.92; H, 5.70; N, 13.20; found: C, 50.8; H, 5.9; N, 13.1.
3b. 2-[4-(Ethylthio) phenoxy] acetohydrazide
This compound was obtained as white solid in a yield 92%, m.p. 126◦C
(methanol), IR:3310 cm−1 (NH2), 3224 cm−1 (NH), 3041 cm−1 (SCH3),
1680 cm−1 (C O), 1547cm−1 (C C), 813cm−1 (SCH3); Anal. Calculated
for C10H14N2O2S: C, 53.08; H, 6.24; N, 12.38; found: C, 53.12; H, 6.30;
N, 12.45.
3c. 2-[4-Methyl sulphonyl phenoxy] acetohydrazide
This compound was obtained as white solid in a yield 85%, m.p.
190◦C, IR: 3315 cm−1 (NH2), 3276 cm−1 (NH), 3101 cm−1 (SCH3), 1689
cm−1 (C O), 1615 cm−1 (C C), 1496 cm−1 (C C); Anal. Calculated for
C9H12N2O4S: C, 44.25, H, 4.95; N, 11.47; found: C, 44.28; H, 4.99; N,
11.45.
3d. 2-[4-Ethyl sulphonyl phenoxy] acetohydrazide
This compound was obtained as white solid in a yield 85%, mp 176◦C
IR: 3317 cm−1 (NH2), 3276 cm−1 (NH), 3104 cm−1 (SCH3), 1688 cm−1
(C O), 1614 cm−1 (C C), 1496 cm−1 (C C).

Antimicrobial Activities of Oxadiazoles
2935
General Procedure for the Preparation of 2-{[4-Alkyl thio/alkyl
sulphonyl) phenoxy] methyl}-5-phenyl-1,3,4-oxadiazole (4a–y)
A mixture of 2-[4-(alkyl thio/alkyl sulphonyl) phenoxy] acetohydrazide
(3a–d) (1mmol), aromatic acid (1 mmol), and phosphorous oxy chloride
(5 ml) was refluxed for 5 to 10 min. The reaction mixture was gradually
poured onto crushed ice with stirring and neutralized with solid sodium
bicarbonate. The aqueous layer was extracted with ethyl acetate. The
extract was washed with water, dried over anhydrous sodium sulfate,
and finally distilled under reduced pressure to remove the solvent. The
resulting white solid was recrystallized using ethyl acetate.
2-{[4-(Methylthio)phenoxy]methyl}-5-phenyl-1,3,4- oxadiazole
(4a)
IR: 3009 cm−1 (SCH3), 1696 cm−1 (C N), 1494 cm−1 (C C), 1242
(C O C), 1H NMR (CDCl3): δ 2.45 (s, 3H, SCH3), δ 5.30 (s, 2H, OCH2),
δ 6.89 (d, 2H, C2, C6-H of 4-methyl thio phenoxy moiety J = 8.4), δ 7.28
(d, 2H, C3, C5-H of 4-methylthio phenoxy moiety J = 7.2), δ 7.50 (m, C3,
C4,C5-H 5-phenyl moiety J = 8.1), δ 8.00 (d 2H, C2, C6-H of 5-phenyl
moiety J = 7.1), 13C-NMR δ: 17.32, 60.14, 115.41, 129.08, 129.52, 129.6,
131.02, 131.22, 132.5, 155.89, 162.19, 164.97; DEPT: CH and CH3δ:
17.32, 115.41, 129.08, 129.5 131.02, 131.22, CH2δ: 60.14; MS FAB+:
m/z (%) = 317 (100) [M+NH4+H]+, 316(80) [M+NH4 ]+.
2-(4-Methylphenyl)-5-{[4-(methylthio) phenoxy]
methyl}-1,3,4-oxadiazole (4b)
IR: 3043 cm−1 (SCH3), 1736 cm−1 (C N), 1496 cm−1 (C C), 1239
1
(C O C) , H NMR (DMSO-d6): δ 2.50 (s, 3H, SCH3), δ 3.33 (s, 3H,
CH3 of 5-phenyl moiety), δ 5.45 (s, 2H, OCH2), δ 7.00 (d, 2H, C2, C6-H
of 4-methyl thio phenoxy moiety J = 7.2), δ 7.28 (d, 2H, C3, C5-H of 4-
methylthio phenoxy moiety J = 8.1), δ 7.42 (d 2H , C3, C5-H of 5-phenyl
moiety J = 6.9), δ 7.90 (d 2H, C2, C6-H of 5-phenyl moiety J = 8.1); MS
FAB+: m/z (%) = 313 (100%) [M+ 1]+, 312 (60%) M+, 173(30%), 119
(30%), 91 (10%).
2-(2-Methoxyphenyl)-5-{[4-(methylthio) phenoxy]
methyl}-1,3,4-oxadiazole (4f)
IR: 3045 cm−1 (SCH3), 1612 cm−1 (C N), 1493 cm−1 (C C),1243
1
(C O C); H NMR (DMSO-d6): δ 2.50 (s, 3H, SCH3), δ 3.80 (s, 3H,
OCH3 of 5-phenyl moiety), δ 4.65 (s, 2H, OCH2), δ 7.00 (d, 2H, C2, C6-H
of 4-methyl thio phenoxy moiety J = 8.7), δ 7.28 (d 2H , C3, C5-H of 4-
methylthio phenoxy moiety J = 8.5), δ 7.49 (m, 4H, C2, C4, C5, C6-H of

2936
T. Karabasanagouda et al.
5-phenyl moiety J = 7.8); MS FAB+: m/z (%) = 347 (100%) [M+NH4+
H]+, 346 (60%) [M+NH4]+, 91 (10%).
2-{[4-(Ethyl thio) phenoxy] methyl}-5-phenyl-1,3,4-oxadiazole
(4j)
IR: 3007 cm−1 (SCH3), 1703 cm−1 (C N), 1494 cm−1 (C C),1245
(C O C); 1H NMR (DMSO-d6): δ 1.20 (t, 3H, CH3 of SCH2CH3), δ 2.86
(q, 2H, CH2 of SCH2 CH3), δ 4.66 (s, 2H, OCH2), δ 6.90 (d, 2H, C2, C6-H
of 4-methyl thio phenoxy moiety J = 8.9), δ 7.32 (d 2H, C3, C5-H of 4-
ethylthio phenoxy moiety J = 8.7), δ 7.50 (m, C3, C4, C5-H of 5-phenyl
moiety J = 7.5), δ 7.90 (d 2H, C2, C6-H of 5-phenyl moiety J = 8.4);
LCMS: m/z (%) = 313 (100%) M+, 154 (10%).
2-{[4-(Methylsulphonyl) phenoxy]methyl}-5-(4-methylphenyl)-
1,3,4-oxadiazole (4q)
IR: 3021 cm−1 (SO2CH3), 1694 cm−1 (C N), 1514 cm−1 (C C), 1253
1
(C O C); H NMR (DMSO-d6): δ 3.10 (s, 3H, SO2CH3), δ 4.90 (s, 2H,
OCH2), δ 7.57 (d, 2H, C2, C6-H of 4-methylsulphonyl phenoxy moiety J
= 6.9). δ 7.66 (m, C2, C3, C4, C5, C6-H of 5-phenyl moiety J=6.0), δ 8.00
(d, 2H, C3, C5-H of 4-methyl sulphonyl phenoxy moiety J = 8.6); MS
FAB+ m/z (%) = 349 (80%) [M+NH4+1]+, 348 (60%) [M+NH4]+, 165
(40%), 115 (40%).
2-{[4-(Ethylsulphonyl) phenoxy] methyl}-5-phenyl-
1,3,4-oxadiazole (4w)
IR: 3006 cm−1 (SO2C2H5), 1703 cm−1 (C N), 1513 cm−1 (C C),1260
(C O C); 1H NMR (DMSO-d6): δ 1.12 (t, 3H, CH3 of SO2C2H5), δ 3.26
(q, 2H, CH2 of SO2C2H5), δ 4.82 (s, 2H, OCH2), δ 7.23 (d, 2H, C2, C6-H
of 4-methylthio phenoxy moiety J = 8.7). δ 7.53 (d, 2H, C3, C5-H of 4-
methyl thio phenoxy moiety J = 7.5), δ 7.87 (m, C2, C3, C4, C5, C6-H of
5-phenyl moiety J = 7.2),
General Procedure for the Preparation of 5-{[4-(Alkylthio)
phenoxy] methyl}-1,3,4-oxadiazole-2-thiol (5a,b)
The mixture of 2-[4-(alkyl thio) phenoxy] acetohydrazide 3a,b (3a: 3.18
g, 3b: 3.39, 15 mmol) in ethanol (25ml) was added to the solution of
potassium hydroxide (1.68 g, 30 mmol) in water (20 ml) at 5–10◦C and
stirred for 10 min.; then CS2 (3.42 g, 45 mmol) was added slowly be-
low 10◦C. Heated the reaction mass to reflux until evolution of H2S
ceased (5–6 h), and the reaction mixture was cooled to room tempera-
ture and poured into ice-cold water. It was then neutralized with diluted

Antimicrobial Activities of Oxadiazoles
2937
hydrochloric acid. The precipitated solid was filtered, washed with wa-
ter, and dried. The product was recrystallized from ethanol.
5-{[4-(Ethylthio) phenoxy] methyl}-1,3,4-oxadiazole-2-thiol
(5b)
IR: 3007 cm−1 (SCH3), 1703cm−1 (C N), 1492 cm−1 (C C), 1238
1
(C O C); H NMR (CDCl3): δ 1.26 (t, 3H, SCH2CH3), δ 2.85 (q, 2H,
SCH2CH3), δ 5.00 (s, 2H,OCH2), δ 6.87 (d, 2H, C2, C6-H of 4-ethyl thio
phenoxy moiety J = 8.7), δ 7.35 (d, 2H, C3, C5-H of 4-ethylthio phenoxy
moiety J = 8.4); 13C-NMR δ: 14.49, 29.16, 59.91, 115.49, 129.38, 132.39,
156.04, 159.41, 178.76; DEPT: CH and CH3δ: 14.49, 115.49, 132.39
CH2δ: 29.16, 59.91; MS FAB+: m/z (%) = 269(60), [M +1]+, 268[M]+,
107 (30%).
General Procedure for the Preparation of 2-(Alkylthio)-5-
{[4-(alkylthio) phenoxy] methyl}-1,3,4-oxadiazole (5c–f)
A solution of sodium hydroxide (0.8 g, 20 mmol) in water (25 ml) was
cooled to 5–10◦C and slowly added 2-{[4-(alkylthio) phenoxy] methyl}-
1,3,4-oxadiazole-5-thiol 5a,b (5a: 2.54 g, 5b: 2.68 g, 10 mmol) and stirred
the reaction mixture to dissolve completely. To the clear reaction mix-
ture, 12 mmol of dimethyl sulfate (1.512 g) or diethylsulphate (1.848 g)
or chloroacetic acid (1.134 g) was added slowly with stirring; and, the
stirring was continued for 2 h at 50–55◦C. The product was cooled
to room temperature and poured into ice-cold water (100 ml). It was
then neutralized with diluted hydrochloric acid. The precipitated solid
was filtered, washed with water, and dried. It was recrystallized from
ethanol.
2-(Methylthio)-5-{[4-(methylthio) phenoxy]
methyl}-1,3,4-oxadiazole (5c)
IR: 2929 cm−1 (CH3), 1594 cm−1 (C N), 1494 cm−1 (C C), 1231
(C O C), 1H NMR (DMSO-d6); δ 2.45 (s, 6H, SCH3), δ 5.00 (s,
2H,OCH2), δ 6.85 (d, 2H, C2, C6-H of 4-methyl thio phenoxy moiety
J = 8.7), δ 7.35 (d, 2H, C3, C5-H of 4-methylthio phenoxy moiety J =
8.4); LC mass: m/z (%) = 269 (100%) M+, 128.9 (20%),
BIOLOGICAL ACTIVITY
Antibacterial Studies
The newly synthesized compounds were screened for their antibac-
terial activity against Escherichia coli (ATTC-25922), Staphylococcus

2938
T. Karabasanagouda et al.
aureus (ATTC-25923), Pseudomonas aeruginosa (ATCC-27853), and
Klebsiella pneumoniae (recultured) bacterial stains by serial plate
dilution method.13,14 Serial dilutions of the drug in Muller-Hinton
broth were taken in tubes and their pH was adjusted to 5.0 using
phosphate buffer. A standardized suspension of the test bacterium was
inoculated and incubated for 16–18 h at 37◦C. The Minimum Inhibitory
Concentration (MIC) was noted by seeing the lowest concentration of
the drug at which there was no visible growth.
Twenty milliliters of above prepared agar media was poured in to
each petri dishes. Excess of suspension was decanted and plates were
dried by placing in an incubator at 37◦C for 1 h. Using a punch, wells
were made on these seeded agar plates and Minimum Inhibitory Con-
centrations of the test compounds in dimethylsulfoxide (DMSO) were
added into each well. A control was also prepared for the plates in the
same way using solvent DMSO. The Petri dishes were prepared in trip-
licate and maintained at 37◦C for 3–4 days. Antibacterial activity was
determined by measuring the diameter of inhibition zone. Activity of
each compound was compared with Ciprofloxacin as standard.15 Zones
of inhibition were determined for 4a–5h and the results are summa-
rized in Table III.
The investigation of antibacterial screening data revealed that all
the tested compounds 4a–y and 5a–h showed moderate to good inhibi-
tion at 1.56–25 µg/ml in DMSO. The compounds 4r, 4u, 4w, 4x, 4y, 5a,
5b, and 5d showed comparatively good activity against all the bacterial
strains. The good activity is attributed to presence of pharmacologically
active Cl, CH3, OCH3, groups attached to phenyl group in position 5 of
the oxadiazole ring and SH, SCH3 groups attached position 2 of oxadia-
zole ring. The presence of SO2CH3, SO2C2H5, SCH3, and SC2H5 groups
at position 4 of phenoxy group caused good activity against most of the
strains. The compounds 4a, 4b, 4c, 4d, 4e, 4g, 4i, 4o, 4q, 4s, 4t, 4v,
4c, 4e, and 4f exhibited moderate activity compared to that of standard
against all the bacterial strains.
Antifungal Studies
Newly prepared compounds were screened for their antifungal activity
against Aspergilus flavus [NCIM No.524], Aspergilus fumigatus [NCIM
No. 902], Penicillium marneffei [recultured], and Trichophyton menta-
grophytes [recultured] in DMSO by serial plate dilution method.16,17
Sabouraud’s agar media was prepared by dissolving peptone (1 g), D-
glucose (4 g) and agar (2 g) in distilled water (100 ml) and adjusting
the pH to 5.7. Normal saline was used to make a suspension of spore
of fungal strains for lawning. A loopful of particular fungal strain was

Antimicrobial Activities of Oxadiazoles
2939
TABLE IV Antifungal Activity of the Prepared Compounds
MIC in µg/ml and zone of inhibition (in mm)
Compd.
P.marneffei
T.mentagrophytes
A.flavus
A.fumigatus
4a
4b
4c
4d
4e
4f
4g
4h
4i
4j
4k
4l
4m
4n
4o
4p
4q
4r
4s
4t
4u
4v
4w
4x
4y
5a
5b
5c
5d
5e
5f
5g
12.5 (11–15)
25 (<10)
12.5 (11–15)
6.25 (16–20)
12.5 (11–15)
25 (<10)
25 (<10)
12.5 (11–15)
12.5 (11–15)
6.25 (16–20)
12.5 (11–15)
12.5 (11–15)
12.5 (11–15)
12.5 (11–15)
6.25 (16–20)
12.5 (11–15)
6.25 (16–20)
6.25 (16–20)
12.5 (11–15)
6.25 (16–20)
6.25 (16–20)
6.25 (16–20)
12.5 (11–15)
6.25 (16–20)
6.25 (16–20)
6.25 (16–20)
12.5 (11–15)
6.25 (16–20)
6.25 (16–20)
12.5 (11–15)
6.25 (16–20)
6.25 (16–20)
6.25 (16–20)
12.5 (11–15)
6.25 (16–20)
12.5 (11–15)
12.5 (11–15)
25 (<10)
25 (<10)
25 (<10)
6.25 (16–20)
12.5 (11–15)
25 (<10)
12.5 (11–15)
12.5 (11–15)
12.5 (11–15)
12.5 (11–15))
6.25 (16–20)
12.5 (11–15)
6.25 (16–20)
12.5 (11–15)
6.25 (16–20)
6.25 (16–20)
6.25 (16–20)
12.5 (11–15)
6.25 (16–20)
6.25 (16–20)
6.25 (16–20)
12.5 (11–15)
6.25 (16–20)
6.25 (16–20)
6.25 (16–20)
6.25 (16–20)
12.5 (11–15)
6.25 (16–20)
12.5 (11–15)
6.25 (16–20)
6.25 (16–20)
6.25 (16–20)
6.25 (16–20)
6.25 (16–20)
6.25 (16–20)
12.5 (11–15)
12.5 (11–15)
25 (<10)
25 (<10)
12.5 (11–15)
6.25 (16–20)
12.5 (11–15)
6.25 (16–20)
6.25 (16–20)
12.5 (11–15)
6.25 (16–20)
6.25 (16–20)
6.25 (16–20)
12.5 (11–15)
6.25 (16–20)
6.25 (16–20)
12.5 (11–15)
6.25 (16–20)
6.25 (16–20)
12.5 (11–15)
6.25 (16–20)
6.25 (16–20)
6.25 (16–20)
6.25 (16–20)
12.5 (11–15)
6.25 (16–20)
6.25 (16–20)
25 (<10)
12.5 (11–15)
6.25 (16–20)
12.5 (11–15)
6.25 (16–20)
12.5 (11–15)
6.25 (16–20)
6.25 (16–20)
12.5 (11–15)
6.25 (16–20)
6.25 (16–20)
6.25 (16–20)
12.5 (11–15)
6.25 (16–20)
6.25 (16–20)
6.25 (16–20)
12.5 (11–15)
6.25 (16–20)
6.25 (16–20)
6.25 (16–20)
6.25 (16–20)
12.5 (11–15)
6.25 (16–20)
12.5 (11–15)
12.5 (11–15)
25 (<10)
12.5 (11–15)
25 (<10)
25 (<10)
12.5 (11–15)
25 (<10)
3.125 (25–30)
5h
(Ciclo-
25 (<10)
3.125 (27–33)
12.5 (11–15)
6.25 (25–30)
6.25 (20–27)
piroxolamine)
The MIC values were evaluated at concentration range, 1.56–25 µg/ml. The figures
in the table show the MIC values and the corresponding zone of inhibition (in mm).
transferred to 3 ml saline to get a suspension of corresponding species.
Twenty mL of agar media was poured in to each petri dish. Excess of
suspension was decanted and plates were dried by placing in an incu-
bator at 37◦C for 1 h. Using a punch, wells were made on these seeded
agar plates Minimum Inhibitory Concentrations of the test compounds

2940
T. Karabasanagouda et al.
in DMSO were added into each well. A control was also prepared for
the plates in the same way using solvent DMSO. The Petri dishes were
prepared in triplicate and maintained at 37◦C for 3–4 days. Antifun-
gal activity was determined by measuring the diameter of inhibition
zone. Activity of each compound was compared with Ciclopiroxolamine
as standard. Zones of inhibition were determined for 4a–5h and the
results are summarized in Table IV.
The results of antifungal screening of tested compounds indicate that
the compounds 4c, 4d, 4h, 4i, 4j, 4k, 4l, 4m, 4n, 4o, 4p, 4q, 4r, 4s, 4t,
4u, 4v, 4w, 4x, 4y, 5a, 5b, 5c, and 5d showed comparatively good ac-
tivity against all the fungal strains. The structure of these compounds
contain biologically active CH3, Cl, OCH3, SH, SCH3 and SCH2COOH
groups attached to phenyl group in position 2 of the oxadiazole. The
compounds 4a, 4b, 4e, 4f, and 4g exhibited moderate activity com-
pared to that of standard. Results of antifungal screening showed that
presence of SCH3, SC2H5, SO2CH3 and SO2C2H5 groups at position 4
of phenoxy group caused increased activity.
CONCLUSION
The research study reports the successful synthesis and antimicrobial
activity of new 1,3,4-oxadiazoles carrying 4-alkylthio and alkylsulfonyl
phenoxy moieties at position 2. The antimicrobial activity study re-
vealed that all the compounds tested showed moderate to good antibac-
terial and antifungal activities against pathogenic strains. Structure
and biological activity relationship of the title compounds showed that
presence of 4-alkylthio and 4-alkylsulfonyl phenoxy groups at position-
2 and biologically active groups like CH3, OCH3, Cl, (CH3)2, SH, SCH3,
SCH2COOH groups at aryl moiety attached to position-5 of title com-
pounds are responsible for increased antimicrobial activity in newly
synthesized title compounds.
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