Synthesis and?antimicrobial studies of?a?new series of?2-{4-[2-(5-ethylpyridin-2-yl)ethoxy]phenyl}-5-substituted-1,3,4-oxadiazoles

Article abstract of DOI:10.1016/j.ejmech.2006.03.002

A series of novel 2-{4-[2-(5-ethylpyridin-2-yl)ethoxy]phenyl}-5-substituted-1,3,4-oxadiazoles were synthesized by the oxidative cyclisation of hydrazones derived from 4-[2-(5-ethylpyridin-2-yl)ethoxy]benzaldehyde and aroylhydrazines using chloramine-T as oxidant. IR, NMR and elemental analysis characterized the newly synthesized compounds. The synthesized compounds were evaluated for their antimicrobial activity and were compared with standard drugs. The compounds demonstrated potent to weak antimicrobial activity. Out of the compounds studied, compounds 8c and 8d showed significant inhibition. Compounds 8b, 8f, 8k and 8e showed moderate activity. The minimum inhibitory concentration of the compounds was in the range of 8-26 μg ml^-1 against bacteria and 8-24 μg ml^-1 against fungi. The title compounds represent a novel class of potent antimicrobial agents.

Full text of DOI:10.1016/j.ejmech.2006.03.002

European Journal of Medicinal Chemistry 41 (2006) 841–846
http://france.elsevier.com/direct/ejmech
Elsevier SASOriginal article
Synthesis and antimicrobial studies of a new series
of 2-{4-[2-(5-ethylpyridin-2-yl)ethoxy]phenyl}-
5-substituted-1,3,4-oxadiazoles
S.L. Gaonkar *, K.M.L. Rai *, B. Prabhuswamy
Department of Studies in Chemistry, University of Mysore, Manasagangotri, Mysore 570 006, India
Received in revised form 4 March 2006; accepted 6 March 2006
Available online 17 April 2006
Abstract
A series of novel 2-{4-[2-(5-ethylpyridin-2-yl)ethoxy]phenyl}-5-substituted-1,3,4-oxadiazoles were synthesized by the oxidative cyclisation
of hydrazones derived from 4-[2-(5-ethylpyridin-2-yl)ethoxy]benzaldehyde and aroylhydrazines using chloramine-T as oxidant. IR, NMR and
elemental analysis characterized the newly synthesized compounds. The synthesized compounds were evaluated for their antimicrobial activity
and were compared with standard drugs. The compounds demonstrated potent to weak antimicrobial activity. Out of the compounds studied,
compounds 8c and 8d showed significant inhibition. Compounds 8b, 8f, 8k and 8e showed moderate activity. The minimum inhibitory concen-
tration of the compounds was in the range of 8–26 μg ml–1 against bacteria and 8–24 μg ml–1 against fungi. The title compounds represent a
novel class of potent antimicrobial agents.
© 2006 Elsevier SAS. All rights reserved.
Keywords: Synthesis; 1,3,4-oxadiazoles; Antimicrobial activity; Chloramine-T
1. Introduction
activity of 1,3,4-oxadiazoles, and pyridine derivatives
prompted us to synthesize and evaluate antimicrobial activity
of novel 2-{4-[2-(5-ethylpyridin-2-yl)ethoxy]phenyl}-5-substi-
tuted-1,3,4-oxadiazoles. In our laboratory Rai et al. [22] re-
ported the use of chloramine-T as an efficient reagent for the
synthesis of 2-amino-5-substituted-1,3,4-oxadiazoles with
good yield and purity by the oxidative cyclization of semicar-
bazones derived from aromatic aldehydes. We have synthe-
sized 2,5-substituted-1,3,4-oxadiazoles using chloramine-T by
the oxidative cyclization of hydrazones derived from aromatic
aldehyde and aroylhydrazines.
1,3,4-Oxadiazoles are an important class of heterocyclic
compounds with broad spectrum of biological activities. Sub-
stituted 1,3,4-oxadiazoles have revealed antibacterial [1–3],
antimycobacterial [4], antifungal [5,6], anti-inflammatory [7,
8], analgesic [9], anticonvulsant [10,11], antihypoglycemic
[12], and insecticidal properties [13]. Compounds possessing
oxadiazole moiety show anticancer [14] and tyrosinase inhibi-
tory activity [15]. Oxadiazoles find use as fluorescent white-
ners [16] and also act as muscle relaxants [17].
In this paper, we describe the synthesis and antimicrobial
activity of a series of novel 2-{4-[2-(5-ethylpyridin-2-yl)
Pioglitazone is a well known pharmaceutically active com-
pound used as insulin sensitizing agent in the treatment of dia-
betes [18]. 4-[2-(5-ethylpyridin-2-yl)ethoxy]benzaldehyde is a
main active metabolite, which is one of the key intermediate of
pioglitazone. The biological activity of pyridine derivatives is
well established [19–21]. This broad spectrum of biological
ethoxy]phenyl}-5-substituted-1,3,4-oxadiazole
derivatives
bearing 6-membered pyridine and 5-membered oxadiazole
moiety.
2. Chemistry
* Corresponding author. Tel.: +91 821 251 5110; fax: +91 821 242 1263.
E-mail addresses: gaonkarslg@rediffmail.com (S.L. Gaonkar),
kmlrai@yahoo.com (K.M.L. Rai).
Aromatic acid 1 was esterified with ethanol using sulfuric
acid as catalyst and the resulting ester 2 was refluxed with hy-
0223-5234/$ - see front matter © 2006 Elsevier SAS. All rights reserved.
doi:10.1016/j.ejmech.2006.03.002

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S.L. Gaonkar et al. / European Journal of Medicinal Chemistry 41 (2006) 841–846
adiazoles 8(a–k) in good purity and yield. IR, 1H NMR and
elemental analyses characterized all the synthesized 1,3,4-oxa-
diazoles. The IR spectrum of oxadiazoles 8(a–k) showed the
absence of amide carbonyl frequency in the region
1760–1650 cm–1 and the NH frequency in the region
3400–3200 cm–1 and showed a new peak at 1630–1640 cm–1
due to C=N frequency. 1H NMR showed peaks due to aromatic
protons and other substituents in the expected region.
3.2. Antimicrobial activity
All synthesized oxadiazole derivatives were evaluated for
antimicrobial activity by disc diffusion and microdilution
method against the various strains. Streptomycin and tetracy-
cline were used as standard drugs against bacteria and nystatin
was used against fungi. In all the determinations tests were
performed in triplicate and the results were taken as a mean
of at least three determinations. Among the series of oxadia-
zoles synthesized, compounds 8c, 8d shown significant inhibi-
tion. Compounds 8b, 8f, 8k and 8e shown moderate activity.
The significant inhibition shown by 8c and 8d may be due to
the presence of two chloro groups present on benzene ring at
tached to C5 of oxadiazole moiety. The moderate activity of
8b, 8f and 8e might be due to chloro group, nitro group and
methoxy group respectively at para position of benzene ring.
The moderate activity of 8k may be due to 4-pyridinyl ring
attached C5 of oxadiazole moiety. Remaining compounds were
not active against any of the strains tested. (Tables 1–4).
Scheme 1. Where R = (a) phenyl, (b) 4-chlorophenyl, (c) 2,3-dichlorophenyl,
(d) 2,4-dichlorophenyl, (e) 4-methoxyphenyl, (f) 4-nitrophenyl, (g) 2-
nitrophenyl, (h) 4-tolyl, (i) 2-tolyl, (j) 3-pyridinyl, (k) 4-pyridinyl.
drazine hydrate in ethanol to give aroylhydrazine 3(a–k). Ar-
oylhydrazine 3 was condensed with 4-[2-(5-ethylpyridyl)
ethoxy]benzaldehyde 6 to gave aroyl hydrazone 7(a–k) which
was oxidatively cyclized in presence of oxidant chloramine-T
to gave 2-{4-[2-(5-ethylpyridin-2-yl)ethoxy]phenyl}-5-substi-
tuted-1,3,4-oxadiazoles derivatives 8(a–k) in good quality and
yield. The aldehyde 6 was synthesized by treating 2-(5-ethyl-
pyridin-2-yl)ethanol 4 with triethylamine and methanesulfonyl
chloride to get an ester 5 which was refluxed with 4-hydroxy-
benzaldehyde and NaOH flakes to get 4-[2-(5-ethyl-pyridin-
2yl)ethoxy]benzaldehyde 6 as a thick oil. The synthesis and
X-ray crystallographic studies of 4 have been reported [23]
(Scheme 1).
4. Conclusion
3. Results and discussion
In conclusion 2-{4-[2-(5-ethylpyridin-2-yl)ethoxy]phenyl}-
5-substituted-1,3,4-oxadiazoles derivatives were synthesized
and their antimicrobial activities have been evaluated. Com-
pounds 8c and 8d demonstrated potent inhibition against all
the strains tested. Further research in this area is under progress
in our laboratory.
3.1. Chemistry
Chloramine-T was used as an efficient reagent for the oxi-
dative cyclization of aroyl hydrazone 7(a–k) to gave 1,3,4-ox-
Table 1
Minimal inhibitory concentration (MIC) in μg ml–1 of synthesized compounds against tested bacterial strains by microdilution method
Compound
Minimal inhibitory concentration (MIC) in μg ml–1
Bacillus substilis
Escherichia coli
Pseudomonas fluorescens Xanthomonas campestris Xanthomonas oryzae
pvs.
8a
8b
8c
8d
8e
8f
8g
8h
26
18
14
15
20
19
20
26
25
24
20
19
–
28
14
11
12
15
14
18
24
24
19
14
13
–
25
12
10
11
14
13
16
20
21
19
14
12
–
28
09
08
09
11
10
15
23
22
16
10
–
23
12
11
11
15
13
18
28
27
21
15
–
8i
8j
8k
Streptomycin
Tetracycline
09
13

S.L. Gaonkar et al. / European Journal of Medicinal Chemistry 41 (2006) 841–846
843
Table 2
Minimal inhibitory concentration (MIC) in μg ml–1 of synthesized compounds against tested fungal strains by microdilution method
Compound
Minimal inhibitory concentration (MIC) in μg ml–1
Aspergillus niger
Aspergillus flavus Fusarium oxysporum
Trichoderma species
Fusarium monaliforme Penicillum species
8a
8b
8c
8d
8e
8f
8g
8h
8i
21
16
12
13
17
16
16
22
23
18
16
15
23
15
12
12
16
14
17
23
22
19
17
15
20
11
08
10
12
12
13
19
21
16
11
11
21
12
11
11
14
12
12
21
24
16
13
12
19
10
08
08
10
10
10
22
19
13
10
9
22
11
08
09
12
11
12
24
22
15
11
10
8j
8k
Nystatin
Table 3
Inhibitory zone (diameter) mm of synthesized compounds against tested bacterial strains by disk diffusion method
Compound
Bacillus substilis
06 mm
14 mm
17 mm
16 mm
10 mm
12 mm
07 mm
05 mm
Escherichia coli Pseudomonas fluorescens
Xanthomonas campestris pvs. Xanthomonas oryzae
8a
8b
8c
8d
8e
8f
8g
8h
8i
06 mm
13 mm
18 mm
17 mm
12 mm
13 mm
08 mm
06 mm
04 mm
09 mm
12 mm
14 mm
–
07 mm
16 mm
21 mm
19 mm
14 mm
15 mm
07 mm
07 mm
06 mm
11 mm
16 mm
17 mm
–
05 mm
13 mm
16 mm
15 mm
10 mm
10 mm
06 mm
05 mm
05 mm
07 mm
11 mm
–
05 mm
11 mm
15 mm
14 mm
09 mm
11 mm
07 mm
04 mm
06 mm
06 mm
10 mm
–
04 mm
09 mm
12 mm
8j
8k
Streptomycin 13 mm
Tetracycline
–
12 mm
12 mm
Streptomycin sulfate (25 μg per disc); Tetracycline (25 μg per disc) were used as positive reference standard antibiotic discs, Synthesized compounds (25 μg per
disc).
Table 4
Inhibitory zone (diameter) mm of synthesized compounds against tested fungal strains by disk diffusion method
Compound
8a
8b
8c
8d
8e
8f
8g
8h
8i
8j
Aspergillus niger
04 mm
07 mm
12 mm
11 mm
07 mm
07 mm
05 mm
04 mm
03 mm
05 mm
07 mm
08
Aspergillus flavus Fusarium oxysporum
Trichoderma species
04 mm
15 mm
21 mm
19 mm
13 mm
14 mm
10 mm
10 mm
09 mm
10 mm
14 mm
16
Fusarium monaliforme Penicillum species
05 mm
09 mm
13 mm
12mm
08 mm
09 mm
06 mm
06 mm
05 mm
07 mm
09 mm
10
05 mm
12 mm
17 mm
15 mm
12 mm
13 mm
10 mm
09 mm
08 mm
10 mm
12 mm
14
03 mm
10 mm
14 mm
12 mm
10 mm
10 mm
08 mm
08 mm
07mm
08 mm
10 mm
12
02 mm
09 mm
13 mm
11 mm
08 mm
08 mm
06mm
06 mm
05 mm
06 mm
08 mm
10
8k
Nystatin
Nystatin (25 μg per disc) was used as positive reference standard antifungal disc, Synthesized compounds (25 μg per disc).
5. Experimental section
meter. Elemental analyses were determined on Vario-EL instru-
ment. Thin layer chromatography (TLC) was done on pre-
coated silica gel G plates using chloroform-acetone as eluent.
5.1. Chemistry
5.1.1. General procedure for the synthesis of aroyl hydrazines
3(a–k)
Melting points were determined on Thomas Hoover melting
point apparatus and are uncorrected. H NMR (300 MHz) spec-
1
tra were measured on a Bruker AM FT spectrometer using
CDCl3 using tetramethylsilane (TMS) as internal standard. The
chemical shifts are expressed in δ and following abbreviations
were used. s = singlet, d = doublet, t = triplet and m = multiplet.
Infrared (IR) spectra were measured on Shimadzu 8300 spectro-
A solution of respective aromatic acid 1 (1 g), ethanol
(10 ml), and catalytic amount of conc. H2SO4 were refluxed
for 3 h. The reaction mixture was cooled and the solid formed
was filtered to get ester 2, which was refluxed with 98% hy-
drazine hydrate (2 ml) in ethanol (10 ml) for 2 h. After com-

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S.L. Gaonkar et al. / European Journal of Medicinal Chemistry 41 (2006) 841–846
pletion of the reaction by TLC (toluene/ethyl acetate/DEA,
7.5:2.5:1) the reaction mixture was cooled and the solid formed
was filtered and washed with chilled ethanol (1 ml) to get cor-
responding aroyl hydrazines 3(a–k).
(d, J = 8.8 Hz, 2H), 7.20–7.60 (m, 9H), 8.41 (s, 1H). IR
(KBr pellets cm–1) n 3070, 2955, 2835, 1635, 1460, 1204.
Anal. CHN: calcd C 74.37, H 5.70, N 11.31, found C 74.46,
H 5.76, N 11.27.
The same procedure was used in all cases.
5.1.2. Procedure for the synthesis of 4-[2-(5-ethylpyridyl)-
ethoxy]benzaldehyde 6
5.1.3.2. Synthesis of 2-(2-{4-[5-(4-chlorophenyl) [1,3,4]oxa-
diazol-2-yl]phenoxy}ethyl)-5-ethylpyridine 8b. Obtained from
7b (1.0 g, 2.46 mmol) and chloramine-T.3H2O (0.83 g,
2.95 mmol) as a white crystalline solid (0.78 g, 79%), m.p.
165–167 °C. 1H NMR (CDCl3, 300 MHz): δ 1.27 (t,
J = 7.6 Hz, 3H), 2.65 (q, J = 7.6 Hz, 2H), 3.28 (t, J = 6.3 Hz,
2H), 4.36 (t, J = 7.0 Hz, 2H), 6.93 (d, J = 8.8 Hz, 2H), 7.20–
7.65 (m, 8H), 8.44 (s, 1H). IR (KBr pellets cm–1) n 3070,
2952, 2835, 1632, 1584, 1478, 1446, 1210. Anal. CHN: calcd
C 68.06, H 4.97, N 10.35, found C 68.01, H 5.02, N 10.31.
A solution of 2-(5-ethyl-pyridin-2-yl)ethanol 4 (15.1 g,
0.1 mol) in toluene (100 ml) and triethylamine (20.2 g,
0.2 mol) were treated with methanesulfonyl chloride (11.4 g,
0.1 mol) at 0–5 °C. The reaction mixture was stirred at r.t. for
1 h. After completion of the reaction the precipitate was filtered
off and the filtrate was washed first with water (50 ml) then 5%
sodium bicarbonate (50 ml) and finally with water (50 ml) and
dried (anhy. Na2SO4). It was then concentrated under reduced
pressure to give 5 as oil, which was refluxed with 4-hydroxy-
benzaldehyde (12.2g, 0.1 mol) and NaOH flakes (5.0 g, 0.12
mol) in isopropyl alcohol (300 ml) for 6 h. After completion of
the reaction the solvent was removed under vacuum. The resi-
due was dissolved in ethyl acetate (300 ml), washed with 5%
aq. NaOH (100 ml), water (100 ml) and dried (anhy. Na2SO4).
The solvent was removed under reduced pressure to gave oily
residue which was purified by column chromatography using
ethyl acetate-n-hexane (3:7) as an eluent to gave 4-[2-(5-ethyl-
5.1.3.3. Synthesis of 2-(2-{4-[5-(2,3-dichlorophenyl) [1,3,4]ox-
adiazol-2-yl]-phenoxy} ethyl)-5-ethylpyridine 8c. Obtained
from 7c (1.0 g, 2.27 mmol) and chloramine-T.3H2O (0.76 g,
2.72 mmol) as a white crystalline solid (0.77 g, 78%), m.p.
172–175 °C. 1H NMR (CDCl3, 300 MHz): δ 1.27 (t,
J = 7.6 Hz, 3H), 2.65 (q, J = 7.6 Hz, 2H), 3.28 (t, J = 6.3 Hz,
2H), 4.36 (t, J = 7.0 Hz, 2H), 6.93 (d, J = 8.8 Hz, 2H), 7.15–
7.60 (m, 7H), 8.41 (s, 1H). IR (KBr pellets cm–1) n 3072,
2954, 2835, 1630, 1590, 1478, 1446, 1215. Anal. CHN: calcd
C 62.74, H 4.35, N 9.54, found C 62.79, H 4.40, N 9.49.
1
pyridyl)-ethoxy]benzaldehyde 6 as yellow oil (18 g, 71%). H
NMR (CDCl3, 300 MHz): δ 1.29 (t, J = 7.6 Hz, 3H), 2.68 (q,
J = 7.6 Hz, 2H), 3.29 (t, J = 6.3 Hz, 2H), 4.40 (t, J = 7.0 Hz,
2H), 7.0 (d, J = 8.8 Hz, 2H), 7.21 (d, J = 8.2 Hz, 1H), 7.50–
7.75 (m, 3H), 8.39 (s, 1H), 9.85 (s, 1H). IR (KBr pellets cm–1)
n 3060, 2950, 2835, 1732, 1635, 1430, 1204. Anal. CHN:
calcd C 75.27, H 6.71, N 5.49, found C 75.19, H 6.79, N 5.46.
5.1.3.4. Synthesis of 2-(2-{4-[5-(2,4-dichlorophenyl) [1,3,4]ox-
adiazol-2-yl]phenoxy} ethyl)-5-ethylpyridine 8d. Obtained
from 7d (1.0 g, 2.27 mmol) and chloramine-T.3H2O (0.76 g,
2.72 mmol) as a white crystalline solid (0.78 g, 80%), m.p.
178–180 °C. 1H NMR (CDCl3, 300 MHz): δ 1.27 (t,
J = 7.6 Hz, 3H), 2.65 (q, J = 7.6 Hz, 2H), 3.28 (t, J = 6.3 Hz,
2H), 4.36 (t, J = 7.0 Hz, 2H), 6.95 (d, J = 8.8 Hz, 2H), 7.20–
7.60 (m, 7H), 8.44 (s, 1H). IR (KBr pellets cm–1) n 3070,
2956, 2835, 1632, 1582, 1476, 1444, 1215. Anal. CHN: calcd
C 62.74, H 4.35, N 9.54, found C 62.71, H 4.42, N 9.56.
5.1.3. General procedure for the synthesis of aroyl hydrazones
7(I–XI)
An equimolar mixture of aroyl hydrazine 3(a–k) and 4-[2-
(5-ethylpyridyl)ethoxy]benzaldehyde 6 were refluxed in etha-
nol (10 vol.) for 3 h. The progress of the reaction was moni-
tored by TLC (toluene/ethyl acetate/DEA, 7.5:2.5:1). After
completion of the reaction, the mass was cooled and the solid
formed was filtered to give aroyl hydrazones 7(a–k), which
were used directly for next reaction.
5.1.3.5. Synthesis of 5-ethyl-2-(2-{4-[5-(4-methoxyphenyl)
[1,3,4]oxadiazol-2-yl]phenoxy} ethyl)pyridine 8e. Obtained
from 7e (1.0 g, 2.48 mmol) and chloramine-T.3H2O (0.84 g,
1
2.97 mmol) as a yellow oil (0.82 g, 83%). H NMR (CDCl3,
5.1.3.1. Synthesis of 5-ethyl-2-{2-[4-(5-phenyl [1,3,4]oxadia-
zol-2-yl)phenoxy]ethyl} pyridine 8a. Typical procedure. A
mixture of aroyl hydrazone 7a (1.0 g, 2.68 mmol) and chlor-
amine-T.3H2O (0.91 g, 3.2 mmol) in ethanol (10 ml) were re-
fluxed under stirring for 3 h. The reaction mass was then con-
centrated under reduced pressure and the residue was extracted
into diethyl ether (2 × 10 ml), washed with 10% NaOH solu-
tion (10 ml), water (10 ml), finally with brine solutions and
dried (anhy. Na2SO4). Ether was evaporated and the resulting
residue was stirred in n-hexane. The solid formed was filtered
and recrystalised from ethanol to gave 8a as a white crystalline
solid (0.82 g, 81%), m.p. 139–141 °C. 1H NMR (CDCl3,
300 MHz): δ 1.27 (t, J = 7.6 Hz, 3H), 2.65 (q, J = 7.6 Hz,
2H), 3.28 (t, J = 6.3 Hz, 2H), 4.36 (t, J = 7.0 Hz, 2H), 6.91
300 MHz): δ 1.29 (t, J = 7.6 Hz, 3H), 2.68 (q, J = 7.6 Hz, 2H),
3.31 (t, J = 6.3 Hz, 2H), 3.79 (s, 3H), 4.29 (t, J = 7.0 Hz, 2H),
6.90–6.95 (m, 4H), 7.20–7.60 (m, 6H), 8.46 (s, 1H). IR (KBr
pellets cm–1) n 3064, 2952, 2833, 1636, 1442, 1247. Anal.
CHN: calcd C 71.80, H 5.77, N 10.47, found C 71.74, H
5.79, N 10.43.
5.1.3.6. Synthesis of 5-ethyl-2-(2-{4-[5-(4-nitrophenyl) [1,3,4]
oxadiazol-2-yl]phenoxy} ethyl)pyridine 8f. Obtained from 7f
(1.0 g, 2.38 mmol) and chloramine-T.3H2O (0.81 g,
2.87 mmol) as a white crystalline solid (0.80 g, 81%), m.p.
176–178 °C. 1H NMR (CDCl3, 300 MHz): δ 1.25 (t,
J = 7.6 Hz, 3H), 2.68 (q, J = 7.6 Hz, 2H), 3.29 (t, J = 6.3 Hz,

S.L. Gaonkar et al. / European Journal of Medicinal Chemistry 41 (2006) 841–846
845
2H), 4.32 (t, J = 7.0 Hz, 2H), 6.91 (d, J = 8.8 Hz, 2H), 7.20–
7.70 (m, 6H), 8.22 (d, J = 8.8 Hz, 2H), 8.45 (s, 1H). IR (KBr
pellets cm–1) n 3090, 2952, 2836, 1636, 1528, 1482, 1342,
1247. Anal. CHN: calcd C 66.34, H 4.84, N 13.45, found C
66.42, H 4.89, N 13.40.
(1.0 g, 2.67 mmol) and chloramine-T.3H2O (0.90 g,
3.20 mmol) as a yellow oil (0.78 g, 80%). H NMR (CDCl3,
1
300 MHz): δ 1.28 (t, J = 7.6 Hz, 3H), 2.63 (q, J = 7.6 Hz, 2H),
3.28 (t, J = 6.3 Hz, 2H), 4.28 (t, J = 7.0 Hz, 2H), 6.94 (d,
J = 8.8 Hz, 2H), 7.21 (d, J = 8.2 Hz, 1H), 7.41 (d,
J = 8.8 Hz, 2H), 7.62–7.68 (m, 3H), 8.52–8.58 (m, 3H). IR
(KBr pellets cm–1) n 3084, 3030, 2948, 2835, 1635, 1480,
1210.Anal. CHN: calcd C 70.95, H 5.41, N 15.04, found C
71.02, H 5.46, N 14.99.
5.1.3.7. Synthesis of 5-ethyl-2-(2-{4-[5-(2-nitrophenyl) [1,3,4]
oxadiazol-2-yl]phenoxy} ethyl)pyridine 8g. Obtained from 7g
(1.0 g, 2.38 mmol) and chloramine-T.3H2O (0.81 g, 2.87
mmol) as a white crystalline solid (0.79 g, 80%), m.p.
181–184 °C. 1H NMR (CDCl3, 300 MHz): δ 1.24 (t,
J = 7.6 Hz, 3H), 2.62 (q, J = 7.6 Hz, 2H), 3.31 (t, J = 6.3 Hz,
2H), 4.30 (t, J = 7.0 Hz, 2H), 6.93 (d, J = 8.8 Hz, 2H), 7.22 (d,
J = 8.2 Hz, 1H), 7.35 (d, J = 8.8 Hz, 2H), 7.50–7.75 (m, 4H),
8.22 (d, J = 8.6 Hz, 1H), 8.48 (s, 1H). IR (KBr pellets cm–1) n
3092, 2954, 2834, 1636, 1528, 1482, 1345, 1242. Anal. CHN:
calcd C 66.34, H 4.84, N 13.45, found C 66.40, H 4.90, N
13.41.
5.2. Biology
5.2.1. Materials and methods
Five bacteria and six fungal species were used as the anti-
microbial test strains namely: Bacillus substilis, Escherichia
coli, Pseudomonas fluorescens, Xanthomonas campestris pvs.,
Xanthomonas oryzae, Aspergillus niger, Aspergillus flavus, Fu-
sarium oxysporum, Trichoderma species, Fusarium monali-
forme and Penicillum species. The bacterial strains were main-
tained on the LB agar medium and the filamentous fungi were
maintained on Potato dextrose agar (PDA) medium at 28 °C.
The agar disk diffusion method [24] was used to test antimi-
crobial activity using potato dextrose agar medium.
5.1.3.8. Synthesis of 5-ethyl-2-{2-[4-(5-p-tolyl [1,3,4]oxadia-
zol-2-yl)phenoxy]ethyl} pyridine 8h. Obtained from 7h (1.0 g,
2.57 mmol) and chloramine-T.3H2O (0.87 g, 3.10 mmol) as a
1
white crystalline solid (0.77 g, 78%), m.p. 144–147 °C. H
NMR (CDCl3, 300 MHz): δ 1.28 (t, J = 7.6 Hz, 3H), 2.29 (s,
3H), 2.62 (q, J = 7.6 Hz, 2H), 3.32 (t, J = 6.3 Hz, 2H), 4.29 (t,
J = 7.0 Hz, 2H), 6.94 (d, J = 8.8 Hz, 2H), 7.15 (d, J = 8.8 Hz,
2H), 7.21 (d, J = 8.2 Hz, 1H), 7.4–7.65 (m, 5H), 8.48 (s, 1H).
IR (KBr pellets cm–1) n 3072, 2930, 2834, 1636, 1605, 1486,
1242. Anal. CHN: calcd C 74.78, H 6.01, N 10.90, found C
74.82, H 6.09, N 10.81.
The bacteria inoculum was prepared by suspending in 9 ml
of sterile water for colonies from 24 hours culture on LB agar
medium. For the filamentous fungi, the inoculum was prepared
with the spores derived from 5 to 15 days culture on PDA
medium. The mycelia were covered with 10 ml of distilled
water and the conidia were scraped using sterile pipette. The
spores were recovered after filtration on sterile absorbent cot-
ton and were resuspended in sterile distilled water.
5.1.3.9. Synthesis of 5-ethyl-2-{2-[4-(5-o-tolyl [1,3,4]oxadia-
zol-2-yl)phenoxy]ethyl} pyridine 8i. Obtained from 7i (1.0 g,
2.57 mmol) and chloramine-T.3H2O (0.87 g, 3.10 mmol) as a
The cell density of each inoculum was adjusted with hemo-
cytometer in order to obtain a final concentration of approxi-
mately 104 and 106 CFU ml–1 for the bacteria and filamentous
fungi respectively.
1
white crystalline solid (0.76 g, 77%), m.p. 138–140 °C. H
NMR (CDCl3, 300 MHz): δ 1.26 (t, J = 7.6 Hz, 3H), 2.31 (s,
3H), 2.61 (q, J = 7.6 Hz, 2H), 3.32 (t, J = 6.3 Hz, 2H), 4.29 (t,
J = 7.0 Hz, 2H), 6.91 (d, J = 8.8 Hz, 2H), 7.1–7.4 (m, 7H), 7.6
(d, J = 7.6 Hz, 1H), 8.46 (s, 1H). IR (KBr pellets cm–1) n 3078,
2935, 2832, 1634, 1605, 1484, 1245. Anal. CHN: calcd C
74.78, H 6.01, N 10.90, found C 74.84, H 6.05, N 10.85.
Nystatin (Himedia) was used as positive control for fungi
and streptomycin and tetracycline for bacteria. Each disk con-
tained 25 μg of standard drugs and 25 μg synthesized com-
pounds. Plates were first kept at 4 °C for at least 2 hours to
allow the diffusion of chemicals, and then incubated at 28 °C.
Inhibition zones were measured after 24 hours of incubation
for bacteria and after 48 hours of incubation for fungi.
The microdilution method [25] was used to evaluate the
minimum inhibitory concentration (MIC) of all the synthesized
compounds. The Nutrient liquid medium and Potato dextrose
liquid medium were used as test media. Tests were performed
in 96-well round bottom sterile culture plates. The suspensions
of yeast and filamentous fungi were adjusted in sterile water to
match the density of a 0.5 McFarland Standard. The wells of a
microdilution plate were inoculated with 180 ml of the culture
medium containing a final inoculum of 0.5–2.5 × 103 CFU
ml–1. All the compounds previously solubilised in DMSO were
serially diluted two folds in the liquid medium to give a range
of concentration from 640 to 0.1 μg ml–1. Twenty microliter of
each concentration were added to wells containing culture sus-
pension except the growth control well. The final concentration
5.1.3.10. Synthesis of 5-ethyl-2-{2-[4-(5-pyridin-3-yl [1,3,4]ox-
adiazol-2-yl)phenoxy] ethyl}pyridine 8j. Obtained from 7j
(1.0 g, 2.67 mmol) and chloramine-T.3H2O (0.90 g,
1
3.20 mmol) as a yellow oil (0.77 g, 79%). H NMR (CDCl3,
300 MHz): δ 1.25 (t, J = 7.6 Hz, 3H), 2.61 (q, J = 7.6 Hz, 2H),
3.29 (t, J = 6.3 Hz, 2H), 4.26 (t, J = 7.0 Hz, 2H), 6.95 (d,
J = 8.8 Hz, 2H), 7.22 (d, J = 8.2 Hz, 1H), 7.4–7.65 (m, 4H),
7.95 (d, J = 7.6 Hz, 1H), 8.62–8.88 (m, 3H). IR (KBr pellets
cm–1) n 3082, 3030, 2945, 2835, 1636, 1482, 1212. Anal.
CHN: calcd C 70.95, H 5.41, N 15.04, found C 70.89, H
5.49, N 15.11.
5.1.3.11. Synthesis of 5-ethyl-2-{2-[4-(5-pyridin-4-yl [1,3,4]ox-
adiazol-2-yl)phenoxy] ethyl}pyridine 8k. Obtained from 7k

846
S.L. Gaonkar et al. / European Journal of Medicinal Chemistry 41 (2006) 841–846
ranged from 64 to 0.01 μg ml–1. Plates were incubated at 35 °C
for 48 hours. Fungal growth was assessed at 494 nm by mea-
suring the optical density in each well using an enzyme immu-
noassay multiwell reader (Sigma diagnostic).
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