Synthesis and microbiological activity of some novel 5-benzamido- and 5- phenylacetamido- substituted 2-phenylbenzoxazole derivatives

Article abstract of DOI:10.1016/S0014-827X(00)00059-8

The synthesis and microbiological activity of a new series of 5- benzamido- and 5-phenylacetamidosubstituted-2-phenylbenzoxazole derivatives (1-26) were described. The in vitro microbiological activity of the compounds was determined against Gram-positive, Gram-negative bacteria and the yeast Candida albicans in comparison with standard drugs. Microbiological results indicated that the synthesized compounds possessed a broad spectrum of activity against the tested microorganisms. The compounds 1, 21, 25 showed higher activity than tetracycline and streptomycin against Pseudomonas aeruginosa. (C) 2000 Elsevier Science S.A.

Full text of DOI:10.1016/S0014-827X(00)00059-8

www.elsevier.nl/locate/farmac
Il Farmaco 55 (2000) 397–405
Synthesis and microbiological activity of some novel 5-benzamido-
and 5-phenylacetamido- substituted 2-phenylbenzoxazole
derivatives
a,
a
a
b
8
:
Esin Akı S¸ener *, Ozlem Temiz Arpacı , Ismail Yalc¸ın , Nurten Altanlar
a Department of Pharmaceutical Chemistry, Faculty of Pharmacy, Ankara Uni6ersity, 06100 Tandog˘an, Ankara, Turkey
b Department of Microbiology, Faculty of Pharmacy, Ankara Uni6ersity, 06100 Tandog˘an, Ankara, Turkey
Received 20 December 1999; accepted 15 May 2000
Abstract
The synthesis and microbiological activity of a new series of 5-benzamido- and 5-phenylacetamidosubstituted-2-phenylbenzox-
azole derivatives (1–26) were described. The in vitro microbiological activity of the compounds was determined against
Gram-positive, Gram-negative bacteria and the yeast Candida albicans in comparison with standard drugs. Microbiological results
indicated that the synthesized compounds possessed a broad spectrum of activity against the tested microorganisms. The
compounds 1, 21, 25 showed higher activity than tetracycline and streptomycin against Pseudomonas aeruginosa. © 2000 Elsevier
Science S.A. All rights reserved.
Keywords: Benzoxazole; Synthesis; Antimicrobial activity
T-lymphoid cell culture has been selected for clini-
cal evaluation as an antiviral agent [11–13]. A benzox-
azole derivative, 3-(4,7-dichlorobenzoxazol-2-ylmethyl-
amino)-5-ethyl-6-methyl pyridin-2(1H)-one (L-697,661),
was also observed as a specific non-nucleoside reverse
transcriptase inhibitor for the human immunodeficiency
virus HIV-1 type and combined therapy with zido-
vudine and L-697, 661 achieved a marked decrease of
viremia in some primary HIV infected patients
[7,14,15].
1. Introduction
Benzoxazoles and related heterocycles such as benz-
imidazoles have been extensively studied for their po-
tential chemotherapeutic activity with lower toxicities in
man [1–9].
A series of 5-formyl-, 5-(aminocarbonyl)-, or 5- and
6-nitro derivatives of 2-(4-metoxyphenyl)benzoxazoles
were synthesized as topoisomerase 1 inhibitors [10].
Furthermore, the compound, 3-[(benzoxazol-2-yl)ethyl]-
5-ethyl-6-methylpyridin-2-(1H)-on (L-696,299), which
was found to be a highly selective antagonist of the
reverse transcriptase enzyme and inhibited the spread
of HIV-1 III-b infection by \95% in MT4 human
Moreover,
a new series of 2-(4-aminophenyl)-
benzothiazoles and their benzoxazole analogues were
found as potent inhibitors against human breast cancer
cell lines in vitro and in vivo [16].
In the last few years, we reported the synthesis and
the antimicrobial activity of 2,5- or 2,6-disubstituted
benzoxazole derivatives as given in general Formulae
I–III [8,9,17] which showed significant in vitro anti-
microbial activity against some Gram-positive, Gram-
negative bacteria and the fungus, Candida albicans.
* Corresponding author. Tel.: +90-312-2126805; fax: +90-312-
2131081.
E-mail address: sener@pharmacy.ankara.edu.tr (E.A. S¸ener).
0014-827X/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0014-827X(00)00059-8

398
E.A. S¸ener et al. / Il Farmaco 55 (2000) 397–405
2. Chemistry
The synthesis of the compounds (1–26) was per-
formed by reacting 5-amino-2-phenyl- or 5-amino-2-(p-
ethylphenyl)benzoxazoles with appropriate carboxylic
acid chlorides obtained by treating carboxylic acids
with thionyl chloride [18,19].
5-Amino-2-phenyl- or 5-amino-2-(p-ethylphenyl)-
benzoxazoles were obtained by heating benzoic acids or
p-ethylbenzoic acids with 2,4-diaminophenol in PPA
(polyphosphoric acid) as the cyclodehydration reagent
in a one step procedure. All of these syntheses are
shown at Scheme 1
The compounds 1–26 were prepared as new prod-
ucts. The structures of 1–26 were supported by spectral
1
data. The IR, H-NMR and Mass spectra are in agree-
ment with the proposed structures. Physical and spec-
tral data of the compounds are reported in Table 1.
3. Experimental
3.1. Chemistry
Kieselgel HF254 chromatoplates (0.3 mm) were used
for TLC and the solvent systems were chloroform–
methanol–ether (20:0.5:4) for 1–6, 8, 11–22, chloro-
form–methanol (20:0.5) for 7, 9–10, 23–24. All the
melting points were taken on a Buchi SMP 20 capillary
apparatus and are uncorrected. IR spectra were
In this study, a series of novel 5-benzamido-, 5-
phenylacetamido-2-phenyl-benzoxazoles (1–26) have
been synthesized in order to examine the microbiologi-
cal activitiy against different Gram-positive, Gram-neg-
ative bacteria and the yeast C. albicans in comparison
with several control drugs (Formula IV).
1
recorded by Pye Unicam SP-1025 with KBr discs. H-
NMR spectra were obtained with a Bruker GMBH-
DPX 400 MHz spectrometer in d-chloroform or
d-chloroform–d6-dimethylsulfoxide and tetramethylsi-
lane (TMS) was used as an internal standard. Mass
analysis were carried out with a Fisions Instruments
VG Platform II mass spectrometer using electron
ionization.
3.2. General procedure for the synthesis of
5-amino-2-phenylbenzoxazole and
5-amino-2-(p-ethylphenyl)benzoxazole
5-Amino-2-phenylbenzoxazole and 5-amino-2-(p-
ethylphenyl)benzoxazole were synthesized by heating
0.01 mol 2,4-diaminophenol.2 HCl with 0.01 mol ben-
zoic acid and p-ethylbenzoic acid in 24 g polyphospho-
ric acid and stirring for 2.5 h. At the end of the reaction
Scheme 1.

Table 1
Physical properties and spectral data of the compounds 1–26
Comp.
no.
A
Y
–
R
H
H
M.P. (°C) Yield (%)
Emperical
formula
IR (cm−1
)
1H-NMR l ppm (J=Hz)
Mass m/e (%X)
1
2
Phenyl
188–189
205–206
63.69
75.30
C20H14O2N2
3340, 3060–3100, 1660,
1530–1565, 1480, 1020–1290,
690–960
3320, 3050–3090, 1650,
1510–1560, 1480, 1050–1230,
690–960
7.50–8.04 (m, 10H), 8.05 (s,
1H), 8.20–8.30 (m, 2H)
314 (21.43), 105 (100)
332 (33.11)6.29), 210 (40.39),
4-Fluorophenyl
–
C20H13O2N2F
7.13–7.21 (t, 2H, J=8.59),
7.50–7.80 (m, 5H), 8.06–8.12 123 (74.17), 69 (100)
(dd, 2H, J=8.59), 8.20–8.30
(m, 3H), 10.10 (s, 1H)
3
4-Bromophenyl
–
H
210–211
66.30
C20H13O2N2Br
3300, 3060–3080, 1650,
1530–1560, 1480, 1040–1220,
690–970
7.52–7.80 (m, 5H), 7.65–7.70 394 (22.86)– 392 (22.04), 76
(dd, 1H, J=8.46), 7.95–8.00 (100)
(dd, 1H, J=8.47), 8.22–8.27
(m, 2H), 8.29–8.32 (d, 1H,
J=1.75), 10.30 (s,1H)
/
4
5
4-Chlorophenyl
–
–
H
H
213–214
201
57.38
72.67
C
20H13O2N2Cl
3380, 3000–3030, 1660,
1530–1560, 1485, 1050–1220,
700–940
7.48–7.54 (dd, 2H, J=7.57), 350 (12.80)–348 (34.65), 139
7.55–7.80 (m, 5H), 8.00–8.07 (100)
(dd, 2H, J=7.54), 8.20–8.30
(m, 3H), 10.25 (s, 1H)
4-Methoxyphenyl
C
21H16O3N2
3320, 3060–3100, 2860–2920,
1650, 1500–1530, 1430,
1030–1260, 700–920
3.90 (s, 3H), 6.78–7.24 (dd,
2H, J=7.07), 7.50–7.80
(m, 5H), 7.98–8.04 (dd, 2H,
J=6.99), 8.20–8.24 (m, 2H),
8.27–8.30 (d, 1H, J=1.93),
10.00 (s,1H)
345 (32.77), 135 (100)
(
397
405
6
7
4-Methylphenyl
4-Ethylphenyl
–
–
H
H
185–186
174–175
76.22
87.72
C
C
21H16O2N2
3400, 3050–3100, 2850–2920,
1660, 1530–1565, 1480,
1020–1290, 700–930
3.15 (s, 3H), 7.25–7.32 (dd,
2H, J=7.84), 7.55–7.82
(m, 5H), 7.88–7.95 (dd, 2H,
J=7.85), 8.20–8.26 (m, 2H),
8.30 (s, 1H), 10.05 (s, 1H)
1.22–1.30 (t, 3H, J=7.40),
2.67–2.76 (q, 2H, J=7.45),
7.28–7.34 (dd, 2H, J=7.71),
7.50–7.79 (m, 5H), 7.91–7.96
(dd, 2H, J=7.85), 8.20–8.30
(m, 2H), 8.30 (s, 1H), 10.00
(s, 1H)
329 (2.70), 119 (97.35), 104
(97.35), 91 (97.35), 77 (97.35)
22H18O2N2
3340, 3100–3060, 2860–2940,
1650, 1530–1560, 1480,
1025–1250, 690–920
343 (11.58), 133 (100)
8
4-Nitrophenyl
–
H
254–256
73.82
C
20H13O4N3
3300, 3060, 1660, 1520–1550,
1480, 1020–1260, 700–960
7.50–7.70 (m, 5H), 7.95 (s,
1H), 8.10–8.20 (dd, 2H,
359 (7.15), 76 (100)
J=8.47), 8.28–8.34 (m, 2H),
8.37–8.48 (dd, 2H, J=8.45),

Table 1 (Continued)
Comp.
no.
A
Y
–
R
H
M.P. (°C) Yield (%)
Emperical
formula
IR (cm−1
)
1H-NMR l ppm (J=Hz)
Mass m/e (%X)
9
4-tert-Butyl phenyl
208
81.08
58.48
C24H22O2N2
3340, 3010–3040, 2860–2960,
1670, 1540–1570, 1485,
1030–1280, 710–970
0.90 (s, 9H), 7.05–7.14 (dd,
2H, J=8.05), 7.15–7.40 (m,
5H), 7.55–7.60 (dd, 2H,
370 (27.15), 161 (100)
J=8.12), 7.80–7.90 (m, 2H),
7.90 (s, 1H), 9.70 (s, 1H)
1.24–1.32 (t, 3H, J=7.60),
2.70–2.78 (q, 2H, J=7.60),
7.35–7.40 (dd, 2H, J=7.93),
7.47–7.50 (dd, 2H, J=7.38),
7.50–8.02 (m, 5H), 8.12–8.16
(d, 2H, J=7.88), 8.26 (s, 1H),
10.10 (s, 1H)
1.25–1.35 (t, 3H, J=6.37),
2.40 (s, 3H), 2.70–2.78 (q, 2H,
J=7.45), 7.24–7.30 (dd, 2H,
J=7.88), 7.30–7.40 (dd, 2H,
J=8.06), 7.48–7.78 (m,2H),
7.87–7.95 (dd, 2H, J=7.96),
8.11–8.17 (dd, 2H, J=8.07),
8.24 (s,1H), 9.85 (s,1H)
1.25–1.34 (m, 6H), 2.68–2.76
(m, 4H), 7.27–7.31 (dd, 2H,
J=8.20), 7.31–7.36 (dd, 2H,
J=8.27), 7.50–7.78 (m, 2H),
7.83–7.89 (dd, 2H, J=8.19),
8.11–8.17 (dd, 2H, J=8.49),
8.24–8.26 (d, 1H, J=1.88),
9.90 (s,1H)
4.00 (s, 3H), 7.08–7.04 (d,1H, 345 (11.30), 135 (100)
J=8.33), 7.13–7.18 (t, 1H,
J=7.66), 7.50–7.60 (m, 5H),
7.70–8.40 (m, 5H), 10.00
10
Phenyl
–
–
Ethyl
Ethyl
209–210
C22H18O2N2
3300, 3060–3100, 2860–2940,
1640, 1550–1580, 1480,
1060–1290, 700–970
342 (19.09), 105 (100)
356 (24.33), 119 (100)
11
12
13
4-Methylphenyl
190–191
56.18
54.05
87.21
C23H20O2N2
3300, 3080–3100, 2860–2960,
1650, 1550–1580, 1480,
1030–1280, 700–970
/
4-Ethylphenyl
–
–
Ethyl
188
C24H22O2N2
3320, 3020–3080, 2860–2960,
1650, 1540–1580, 1480,
1020–1280, 690–960
371 (11.98), 133 (100)
)
397
405
2-Methoxyphenyl
H
165–166
C
21H16O3N2
3380, 3020–3080, 2880–2980,
1670, 1550–1580, 1485,
1030–1250, 680–970
(s, 1H)
14
15
2-Chlorophenyl
–
–
H
H
191–192
129–130
57.47
53.48
C
20H13O2N2Cl
22H18O4N2
3300, 3060–3100, 1670,
1540–1580, 1485, 1020–1270,
690–980
3380, 3080–3100, 2850–2960,
1660, 1540–1570, 1480,
1020–1240, 640–950
7.30–7.70 (m, 7H), 7.80–8.20
(m, 3H), 8.25–8.30 (dd, 2H,
J=6.51, J=1.47)
3.90, 4.10 (s, 3H), 6.55
(s, 1H), 6.60–6.70 (d, 1H,
J=8.06), 7.50–7.70 (m, 5H),
8.10 (s, 1H), 8.20–8.40 (m,
3H), 9.80 (s, 1H)
350 (12.59), 348 (32.59), 139
(100)
2,4-Dimethoxy phenyl
C
374 (17.17), 165 (100)

Table 1 (Continued)
Comp.
no.
A
Y
–
R
H
M.P. (°C) Yield (%)
Emperical
formula
IR (cm−1
)
1H-NMR l ppm (J=Hz)
Mass m/e (%X)
16
2,4-Dimethyl phenyl
168–169
73.09
C22H18O2N2
3300, 3080–3120, 2860–2920,
1660, 1500–1565, 1440,
1030–1260, 690–970
2.36, 2.50 (2s, 3H), 7.00–7.10
(m, 2H), 7.40–7.45 (d, 1H,
J=7.63), 7.50–7.80 (m, 5H),
8.00 (s, 1H), 8.20–8.30
(m, 2H)
342 (8.84), 133 (100)
17
18
Phenyl
CH2
CH2
H
H
170–171
209–210
60.98
49.14
C21H16O2N2
3320, 3020–3080, 2940–2960,
1660, 1540–1560, 1480,
1030–1280, 690–980
3300, 3090, 2860–2930, 1670,
1530–1560, 1480, 1000–1270,
700–970
3.65 (s, 2H), 7.20–7.60 (m,
10H), 8.10 (s, 1H), 8.18–8.25
(m, 2H), 9.95 (s, 1H)
3.70 (s, 2H), 7.30–7.35 (dd,
2H, J=8.29), 7.42–7.47 (dd,
2H, J=8.30), 7.48–7.58 (m,
5H), 8.12–8.14 (d, 1H,
J=1.56), 8.19–8.23 (m, 2H),
10.00 (s, 1H)
329 (14.70), 91 (100)
4-Bromophenyl
C21H15O2N2Br
408 (13.10), 406 (12.46), 89
(100)
19
4-Chlorophenyl
CH2
H
211–212
55.17
C21H15O2N2Cl
3300, 3060–3100, 2860–2920,
1660, 1560, 1480, 1050–1230,
700–970
3.70 (s, 2H), 7.28–7.34 (dd,
2H, J=8.36), 7.34–7.40 (dd,
2H, J=8.33), 7.50–7.60 (m,
5H), 8.11–8.14 (d, 1H,
J=1.39), 8.18–8.22 (m, 2H),
10.05 (s, 1H)
362 (21.85), 105 (100)
/
20
21
4-Nitrophenyl
CH2
CH2
H
H
210
169
63.00
54.40
C
C
21H15O4N3
24H22O3N2
3300, 3080–3100, 2860–2920,
1650, 1520–1550, 1470,
1020–1270, 690–970
3300, 3040–3100, 2860–2980,
1660, 1510–1560, 1470,
1020–1240, 690–970
3.40 (s, 2H), 7.00–7.30 (m,
7H), 7.60–7.80 (m, 5H), 9.70
(s, 1H)
373 (2.13), 89 (100)
2
4-Propyloxyphenyl
0.98 (t, 3H, J=7.00),
387 (10.56), 107 (100)
397
1.70–1.80 (m, 2H), 3.65
(s, 2H), 3.85–3.95 (t, 2H,
J=5.79, J=7.07), 6.86–6.82
(dd, 2H, J=8.56), 7.24–7.30
(dd, 2H, J=9.08), 7.45–7.60
(m, 5H), 8.10–8.12 (d, 1H,
J=1.49), 8.17–8.27 (m, 2H),
9.80 (s, 1H)
405
22
23
Phenyl
CH2
CH2
Ethyl
Ethyl
164–165
70.22
57.47
C
23H20O2N2
3300, 3040–3100, 2860–2960,
1655, 1520–1560, 1480,
1060–1280, 700–970
1.22–1.32 (t, 3H, J=8.14),
2.70–2.80 (q, 2H, J=7.40),
3.70 (s, 2H), 7.20–7.40
(m, 7H), 7.47–7.60 (m, 2H),
8.08–8.15 (m, 3H), 10.00
(s, 1H)
1.25–1.32 (t, 3H, J=7.60),
2.70–2.80 (q, 2H, J=7.55),
3.70 (s, 2H), 7.28–7.56
(m, 8H), 8.08–8.15 (m, 3H),
10.05 (s, 1H)
357 (42.35), 356 (100), 91
(100), 51 (100)
4-Bromophenyl
202
C
23H19O2N2Br
3300, 3020–3100, 2840–2960,
1650, 1520–1560, 1480,
1010–1270, 710–980
436 (16.59), 434 (15.63), 89
(100)

Table 1 (Continued)
Comp.
no.
A
Y
R
M.P. (°C) Yield (%)
Emperical
formula
IR (cm−1
)
1H-NMR l ppm (J=Hz)
Mass m/e (%X)
24
4-Chlorophenyl
CH2
Ethyl
194–195
76.82
C23H19O2N2Cl
3300, 3040–3100, 2840–2960,
1650, 1530–1580, 1490,
1020–1270, 710–970
1.22–1.32 (t, 3H, J=7.60),
2.70–2.78 (q, 2H, J=07.559,
3.65 (s, 2H), 7.28–7.57 (m,
8H), 8.08–8.14 (m, 3H), 10.00
(s, 1H)
392 (23.45), 390 (60.31), 238
(100)
/
25
26
2-Chlorophenyl
CH2
–
H
H
206
55.17
37.43
C
21H15O2N2Cl
3340, 3100, 2920–2990, 1680,
1550–1580, 1490, 1045–1290,
710–950
3.85 (s, 2H), 7.20–7.62 (m,
9H), 8.10–8.12 (d, 1H,
J=1.58), 8.20–8.24 (m, 2H),
9.80 (s, 1H)
3.85 (s, 6H), 6.60–6.63 (t, 1H, 375 (14.19), 165 (100)
J=2.21), 7.16–7.20 (dd, 2H,
J=2.25, 2.04), 7.52–7.80 (m,
5H), 8.20–8.25 (m, 2H),
364 (19.12), 362 (48.28), 125
(100)
0
3,5-Dimethoxyphenyl
149–150
C
22H18O4N2
3480, 3060–3100, 2840–2940,
1650, 1550–1570, 1480,
1040–1240, 690–970
397
405
8.28–8.30 (d, 1H, J=1.80),
10.05 (s, 1H)

E.A. S¸ener et al. / Il Farmaco 55 (2000) 397–405
403
period, the residue was poured into ice–water mixture
and neutralized with excess of 10% NaOH solution
extracted with benzene, the benzene solution was dried
over anhydrous sodium sulphate and evaporated under
diminished pressure. The residue was boiled with 200
mg charcoal in ethanol and filtered. After the evapora-
tion of solvent in vacuo, the crude product was ob-
tained and recrystallized.
3.4.1. Antibacterial assay
The culture were obtained in Mueller–Hinton broth
(Difco) for all the bacteria after 24 h of incubation at
3791°C. Testing was carried out in Mueller–Hinton
Broth at pH 7.4 and the two fold serial dilution tec-
nique was applied. The final inocolum size was 105
CFU/ml. A set of tubes containing only inoculated
broth was kept as controls. After incubation for 24 h at
3791°C, the last tube with no growth of microorgan-
ism was recorded to represent MIC expressed in mg/ml.
3.3. General procedure for amide deri6ati6es (1–26)
3.4.2. Antimycotic assay
Appropriate carboxylic acid (0.5 mmol) and thionyl
chloride (1.5 ml) were refluxed in benzene (5 ml) at
80°C for 3 h. Excess thionyl chloride was then removed
in vacuo. The residue was dissolved in ether (10 ml)
and solution added during 1 h to a stirred, ice-cooled
mixture of 5-amino-2-phenylbenzoxazole or 5-amino-
2-(p-ethylphenyl)benzoxazole (0.5 mmol), sodiumbi-
carbonate (0.5 mmol), diethylether (10 ml) and water
(10 ml). The mixture was kept stirred overnight at room
temperature and filtered. The precipitate was washed
with water, 2 N HCl and water, respectively and finally
with ether to give 1–26. The product was recrystallized
from ethanol–water mixture and needles are dried in
vacuo.
The yeast C. albicans was maintained in Sabouraud
dextrose broth (Difco) after incubation for 24 h of
incubation at 2591°C. Testing was performed in
Sabouraud dextrose broth at pH 7.4 and the two fold
serial dilution tecnique was applied. The final inocolum
size was 104 CFU/ml. A set of tubes containing only
inoculated broth was kept as controls. After incubation
for 48 h at 2591°C, the last tube with no growth of
microorganism was recorded to represent MIC ex-
pressed in mg/ml
4. Results and discussion
The chemical, physical and spectral data of the syn-
thesized compounds 1–26 are reported in Table 1. The
antimicrobial activity of the compounds was investi-
gated against two Gram-positive, three Gram-negative
bacteria strains and the yeast C. albicans using two fold
serial dilution technique in comparison to control drugs
such as ampicillin, amoxycillin, tetracycline, strepto-
mycin, clotrimazole, haloprogine and the results re-
ported at Table 2.
The synthesized compounds showed some antibacte-
rial activity against the Gram-positive bacteria such as
S. aureus and S. faecalis possesing MIC values between
50 and 100 mg/ml, except the compound 13 having a
MIC value of 25 mg/ml.
Furthermore, the antibacterial activity of the com-
pounds 1–26 against E. coli as Gram-negative bacteria
revealed lower potencies than the compared control
drugs. However, the activity against the Gram-negative
enterobacter P. aeruginosa, which is effective in nosoco-
mial infections and often resistant to antibiotic therapy,
the compounds 1, 21, and 25 indicated significant activ-
ity, with a MIC value of 25 mg/ml, being more potent
than the compared control drugs tetracycline and
streptomycin.
The compounds 1–26 were also tested against C.
albicans for their antimycotic activity and most of the
compounds indicated significant antimycotic activity
performing MIC values between 25 and 50 mg/ml ex-
cept the compounds 7 and 9. However, antimycotic
potencies of the compared control drugs clotrimazole
3.4. Microbiology
For the antibacterial and antimycotic assays, the
compounds 1–26 were dissolved in absolute ethanol
(0.8 mg/ml). Further dilutions of the compounds and
standard drugs in the test medium were prepared at the
required quantities of 400, 200, 100, 50, 25, 12.5, 6.25,
3.12, 1.56, 0.78 mg/ml concentrations. The minimum
inhibitory concentrations (MIC) were determined using
the method of two fold serial dilution technique [20–
22]. In order to ensure that the solvent per se had no
effect on bacterial growth, a control test was also
performed containing inoculated broth supplemented
with only ethanol at the same dilutions used in our
experiments and found inactive in culture medium.
All of the compounds were tested for their in vitro
growth inhibitory activity against different bacteria and
a fungus C. albicans RSKK 628. Origin of the
bacterial strains are Staphylococcus aureus RSSK 250,
Streptococcus faecalis RSKK 500, Bacillus subtilis
ATCC 6033 as Gram-positive and Escherichia coli
RSKK 256, Pseudomonas aeruginosa RSKK 356 as
Gram-negative bacteria which were maintained at the
microbiology department, faculty of pharmacy, Ankara
University.
Ampicillin, amoxycillin, tetracycline, streptomycin,
clotrimazole and haloprogin were used as control
drugs. The observed data on the antimicrobial activity
of the compounds and control drugs are given Table 2.

404
E.A. S¸ener et al. / Il Farmaco 55 (2000) 397–405
Table 2
The in vitro antimicrobial activity of the compounds 1–26 and the control drugs (MIC in mg/ml)
Comp. no.
A
Y
R
Sa a
50
100
50
50
50
50
100
100
100
50
100
50
25
50
100
50
100
50
Sf b
50
100
50
50
50
50
100
100
100
50
100
50
25
50
50
50
100
50
100
50
50
50
50
50
50
100
Bs c
50
100
50
50
50
50
50
200
100
50
100
50
25
50
200
25
50
50
50
200
200
50
50
50
100
200
Ec d
Pa e
Ca f
25
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
Phenyl
H
H
H
H
H
H
H
H
50
200
50
50
50
25
4-Fluorophenyl
4-Bromophenyl
4-Chlorophenyl
4-Methoxyphenyl
4-Methylphenyl
4-Ethylphenyl
4-Nitrophenyl
4-tert-Butylphenyl
Phenyl
4-Methylphenyl
4-Ethylphenyl
2-Methoxyphenyl
2-Chlorophenyl
2,4-Dimethoxyphenyl
2,4-Dimethylphenyl
Phenyl
4-Bromophenyl
4-Chlorophenyl
4-Nitrophenyl
4-Propyloxyphenyl
Phenyl
4-Bromophenyl
4-Chlorophenyl
2-Chlorophenyl
3,5-Dimethoxyphenyl
200
50
50
50
50
50
25
100
25
100
50
50
50
25
25
25
25
50
25
50
25
25
50
50
50
25
25
50
50
50
50
50
100
100
100
50
50
50
50
50
50
50
100
50
100
50
25
50
50
50
25
50
50
H
100
50
C2H5
C2H5
C2H5
H
H
H
H
H
H
H
H
H
C2H5
C2H5
C2H5
H
50
50
50
50
100
100
100
50
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
100
50
50
50
50
50
50
100
50
50
25
50
50
50
25
50
H
Ampicillin
1.56
1.56
1.56
1.56
1.56
1.56
1.56
12.5
3.12
3.12
1.56
\200
\200
50
Amoxycillin
Tetracycline
Streptomycin
Clotrimazole
Haloprogin
1.56
1.56
3.12
100
50
100
6.2
3.1
a Sa, Staphylococcus aureus
b Sf, Streptococcus faecalis
c Bs, Bacillus subtilis
d Ec, Escherichiae coli
e Pa, Pseudomanas aeruginosa
f Ca, Candida albicans
and haloprogin were found more active than the corre-
sponding compounds, showing MIC values of 6.2 mg/
ml and 3.1 mg/ml, respectively.
As a result of structure and antimicrobial activity
relationships against Gram-positive bacteria substition
at position five of the benzoxazole ring with a 2-
methoxybenzamido moiety for the compounds 1-26
causes an increase in the activity compared to other
substitutions.
tution with 4-florobenzamido at the same position
causes a decrease in the activity.
On the other side, 4-tert-butylbenzamido group at
the fifth position of the benzoxazole nucleus decreases
the potency against C. albicans.
Acknowledgements
If the benzoxazole ring holds a 4-propyloxyphenyac-
etamido or a 2-chlorophenylacetamido group at posi-
tion five, the activity increases against E. coli and P.
aeruginosae as Gram-negative bacteria whereas substi-
We would like to thank the Research Fund of
Ankara University (Grant No. 96-30-00-22) for the
financial support of this research.

E.A. S¸ener et al. / Il Farmaco 55 (2000) 397–405
405
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