Synthesis and antimicrobial activity of new 2-[p-substituted-benzyl]-5- [substituted-carbonyl-amino]benzoxazoles

Article abstract of DOI:10.1002/ardp.200300851

A series of 23 new 2-[p-substituted-benzyl]-5-[p-substituted-phenyl/benzyl- carbonylamino]benzoxazole derivatives has been synthesized by reacting 5-amino-2-[p-substituted-benzyl]benzoxazoles with the appropriate carboxylic acid chlorides. The structures of the synthesized compounds were confirmed by IR and ¹H-NMR spectral data. Antimicrobial activities of the compounds were investigated using the twofold serial dilution technique against two gram-positive and two gram-negative bacteria and three Candida species in comparison with standard drugs. Microbiological results indicated that the newly synthesized 2-[p-substituted-benzyl]-5-[p-substituted-phenyl/benzyl- carbonylamino]benzoxazole derivatives (3-25) possessed a broad spectrum of activity, showing MIC values of 6.25-200 μg/mL against the gram-positive and gram-negative microorganisms tested. Moreover, they showed significant antifungal activity with MIC values of 3.12-100 μg/mL against the Candida species tested. Especially, with a MIC value of 3.12 μg/mL, 2-benzyl-5-[p-bromobenzyl-carbonylamino]benzoxazole 9 displayed the same activity against C. glabrata as the standard drug myconazol.

Full text of DOI:10.1002/ardp.200300851

402 Aki-Sener et al.
Arch. Pharm. Pharm. Med. Chem. 2004, 337, 402−410
Ilkay Yildiz-Orena,
Betul Tekiner-Gulbasa,
Ismail Yalcina,
Synthesis and Antimicrobial Activity of New
2-[p-Substituted-benzyl]-5-[substituted-carbonyl-
amino]benzoxazoles
Ozlem Temiz-Arpacia,
Esin Akı-Senera,
Nurten Altanlarb
A series of 23 new 2-[p-substituted-benzyl]-5-[p-substituted-phenyl/benzyl-car-
bonylamino]benzoxazole derivatives has been synthesized by reacting 5-amino-
2-[p-substituted-benzyl]benzoxazoles with the appropriate carboxylic acid chlo-
rides. The structures of the synthesized compounds were confirmed by IR and
1H-NMR spectral data. Antimicrobial activities of the compounds were investi-
gated using the twofold serial dilution technique against two gram-positive and
two gram-negative bacteria and three Candida species in comparison with
standard drugs. Microbiological results indicated that the newly synthesized
2-[p-substituted-benzyl]-5-[p-substituted-phenyl/benzyl-carbonylamino]-
benzoxazole derivatives (3؊25) possessed a broad spectrum of activity, show-
ing MIC values of 6.25Ϫ200 µg/mL against the gram-positive and gram-negative
microorganisms tested. Moreover, they showed significant antifungal activity
with MIC values of 3.12Ϫ100 µg/mL against the Candida species tested. Es-
pecially, with a MIC value of 3.12 µg/mL, 2-benzyl-5-[p-bromobenzyl-carbonyl-
amino]benzoxazole 9 displayed the same activity against C. glabrata as the
standard drug myconazol.
a Ankara University, Faculty of
Pharmacy, Department of
Pharmaceutical Chemistry,
Tandogan, Ankara, Turkey
b Ankara University, Faculty of
Pharmacy, Department of
Microbiology, Tandogan,
Ankara, Turkey
Keywords: 2-Benzylbenzoxazole; Benzylcarbonylamino-benzoxazole; Phenyl-
carbonylamino-benzoxazole; Antibacterial and antifungal activity
Received: August 14, 2003; Accepted: March 12, 2004 [FP851]
DOI 10.1002/ardp.200300851
Introduction
crease of viremia in some primary HIV-infected
patients [6]. Moreover, substituted pyrimido[1,6-a]-
The number of cases of multidrug-resistant bacterial
infections is increasing at an alarming rate. Although
clinicians now rely on vancomycin as the antibiotic for
serious infections resistant to traditional agents [1],
there is still need for new classes of antibacterial
agents.
benzimidazoles were synthesized as a new class of
potent DNA gyrase inhibitors; however, their antibac-
terial activity was inferior to the quinolone-type anti-
bacterial agents such as norfloxacin or fleroxacin [7].
Recently, a new series of benzothiazoles have been
synthesized as antitumor agents that showed potent
inhibitory activity against human breast cancer cell
lines in vitro and in vivo [8]. Among them, lysyl-amide
of 2-(4-amino-3-methylphenyl)-5-fluorobenzothiazole
has been selected for phase 1 clinical evaluation [17].
Substituted benzoxazole derivatives and their anal-
ogues such as benzimidazoles and benzothiazoles
have been the aim of many researchers for many
years, because they constitute an important class of
heterocyclic compounds with antitumor, antiviral and
antibiotic activities [2Ϫ16].
A derivative of camptothecin has been prepared
wherein the A-ring has been fused to an oxazole ring
by Peel et al. [18]. This compound (Figure 1) was
found to be significantly more potent as inhibitor of
topoisomerase I than camptothecin.
A benzoxazole derivative, 3-(4,7-dichlorobenzoxazole-
2-yl-methylamino)-5-ethyl-6-methylpyridin-2(1H)-one
(L-697,661) was identified as a specific non-nucleo-
side reverse transcriptase inhibitor for the human im-
munodeficiency virus HIV-1 type, and its use in com-
bined therapy with zidovudine achieved a marked de-
Recently, we reported the synthesis and antimicrobial
activity of 2-[p-substituted-phenyl]-5-[substituted-aryl-
carbonylamino]benzoxazole derivatives (Figure 2)
[19, 20].
Correspondence: Esin Aki-Sener, Ankara University,
Faculty of Pharmacy, Department of Pharmaceutical
Chemistry, TR-06100 Tandogan, Ankara, Turkey. Phone:
+90 312 223-69-40, Fax: +90 312 223-69-40, e-mail:
sener@pharmacy.ankara.edu.tr
In this study, a series of 2-[p-substituted-benzyl]-5-
[p-substituted-phenyl/benzyl-carbonylamino]benz-
oxazole derivatives (3Ϫ25) (Figure 3) has been syn-
 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Arch. Pharm. Pharm. Med. Chem. 2004, 337, 402−410
Synthesis and Activity of New Substituted Benzoxazoles 403
rivatives (3Ϫ25) has been synthesized by using a two-
step procedure as shown in Scheme 1 [19Ϫ22]. All of
the derivatives (3Ϫ25) were supported by spectral
1
data. The IR and H-NMR spectra are in agreement
with the proposed structures. Physical and spectral
data of the compounds are reported in Table 1.
In order to determine the antimicrobial activity of the
synthesized compounds (3Ϫ25), two gram-positive
bacteria, two gram-negative bacteria and three Can-
dida species were screened using the twofold serial
dilution technique [23, 24]. All the biological results of
the compounds are given in Table 2. The combined
data reported that the newly synthesized compounds
showing MIC values between 200Ϫ3.12 µg/mL were
able to inhibit the in vitro growth of the microorgan-
isms screened.
Figure 1.
In this study, our goal was to investigate the role of the
second position of the benzoxazole ring for antimi-
crobial activity. Therefore, we put p-substituted-benzyl
instead of p-substituted-phenyl [19, 20] at position 2 of
benzoxazole. Additionally, we examined the effect of
the 5-carbonylamino substitutents of the benzoxazole
ring system on antimicrobial activity. Consequently, the
newly presented compounds 3Ϫ25 were compared
with previously prepared compounds 26Ϫ64 [19, 20]
with regard to their antibacterial and antifungal activity
(Table 2).
Figure 2.
thesized as the target compounds, in order to examine
their microbiological activity, and that of the previously
synthesized
2-[p-substituted-phenyl]-5-[substituted-
According to Table 2, all of the new compounds 3Ϫ25
showed lower antibacterial activity against the screen-
ed gram-positive bacteria S. aureus or B. subtilis
(MIC values between 200Ϫ12.5 µg/mL) than the
control drugs. With a MIC value of 12.5 µg/mL against
S. aureus, only compound 21 was found to be more
active than the others.
aryl-carbonylamino]benzoxazole derivatives [19, 20],
against various gram-positive and gram-negative bac-
teria and diverse fungi in comparison with several con-
trol drugs, including structure-activity relationship
(SAR) studies.
Chemistry
Furthermore, the antibacterial activity of compounds
3Ϫ25 against the gram-negative bacteria E. coli and
P. aeruginosa results in MIC values between 25 and
100 µg/mL. While none of the compounds showed
lower activity than the standard drugs against E. coli,
some of the compounds (5Ϫ10, 13, 16, 21Ϫ25) were
found to be more active against P. aeruginosa (with
MIC values of 25 µg/mL) than the standard drugs
tetracycline and streptomycin.
5-Amino-2-[p-substituted-benzyl]benzoxazoles (1, 2)
were obtained by heating p-substituted phenylacetic
acids with 2,4-diaminophenol in PPA (polyphosphoric
acid) [19, 20].
Compounds (3Ϫ25) were obtained from 5-amino-2-[p-
substituted-benzyl]benzoxazoles with p-substituted
benzoic acid or p-substituted phenylacetic acid chlo-
rides obtained by treating appropriate carboxylic acids
with thionyl chloride [21, 22] as given in Scheme 1.
Compounds 3Ϫ25 were also tested against C. al-
bicans, C. krusei and C. glabrata for their antimycotic
activity, and most of the compounds showed signifi-
cant antimycotic activity displaying MIC values be-
tween 3.12 and 100 µg/mL. Compound 9, having a
MIC value of 3.12 µg/mL against C. glabrata, was
more active than the other compounds tested. More-
over, compound 9 showed the same activity as the
control drug myconazole. Compounds 13 and 23 were
Compounds 3؊25 are new, and their structures were
supported by spectral data (Table 1).
Results and discussion
A series of 23 new 2-[p-substituted-benzyl]-5-[substi-
tuted-phenyl/benzyl-carbonylamino]-benzoxazole de-
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404 Aki-Sener et al.
Arch. Pharm. Pharm. Med. Chem. 2004, 337, 402−410
benzamido having fluorine or ethyl groups at the para
position of 2-phenylbenzoxazole (compounds 52Ϫ56)
generally causes an increase in the activity against S.
aureus resulting in MIC values of 25 µg/mL. Hetero-
cyclic rings such as 2-furyl and 2-thienyl attached at
5-carbonylamino of 2-(p-dimethylamino-phenyl)benz-
oxazole also increase the activity against S. aureus
(compounds 63, 64). With MIC values of 25 µg/mL,
most of the compounds were found to be significantly
active against B. subtilis.
Furthermore, the antibacterial activity of compounds
3Ϫ64 showed lower activity (MIC values of 25Ϫ200
µg/mL) than the standard drugs against E. coli. Com-
pounds 5Ϫ10, 13, 16, 21Ϫ26, 46, 50, and 54 were
found to be more active against P. aeruginosa (MIC
values of 25 µg/mL) than the standard drugs tetracyc-
line and streptomycin. Most of the 2-benzylbenz-
oxazole derivatives showed significant activity against
P. aeruginosa.
Figure 3.
found to be more active than the others with a MIC
value of 6.25 µg/mL against C. krusei.
Table 2 indicated that substitution at position 5 of the
benzoxazole ring with 4-chloro / fluoro / methoxy-
Scheme 1.
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Arch. Pharm. Pharm. Med. Chem. 2004, 337, 402−410
Synthesis and Activity of New Substituted Benzoxazoles 405
Table 1. Physical, preparation and spectral data of the synthesized compounds 3Ϫ25.
Comp.
No:
Y
R
R1
m.p.
(°C)
Yield
(%)
Empirical
formula
IR
(cm؊1
1H-NMR
δppm (J = Hz)
)
3
Ϫ
Ϫ
H
H
136
54
C21H16O2N2
3295, 1573Ϫ1529, 3032,
1491Ϫ1470, 2940, 1295Ϫ1025,
1647, 1619,
4.00Ϫ4.10 (2H, s), 7.25Ϫ8.05 (14H, m)
961Ϫ691
4
H
H
C2H5
143
29.74 C23H20O2N2
3376, 3030, 2960Ϫ2870, 1653,
1622, 1570Ϫ1507, 1481,
1289Ϫ1017, 966Ϫ668
1.20Ϫ1.30 (3H, t, J = 7.57), 2.70Ϫ2.80
(2H, q, J = 7.61), 4.25Ϫ4.30 (2H, s),
7.20Ϫ7.42 (7H, m), 7.42Ϫ7.50 (1H, dd,
J7,6 = 8.74), 7.55Ϫ7.65 (1H, dd, J6,7
8.74, J6,4 = 1.82), 7.78Ϫ7.85 (2H, d,
=
J2Љ,3Љ = J6Љ,5Љ = 8.60), 7.85Ϫ7.90 (1H, s),
7.92Ϫ8.00 (1H, d, J4,6 = 1.70)
5
Ϫ
NO2
205Ϫ206 57.57 C21H15O4N3
3308 , 3067, 2900Ϫ2840, 1648,
1620, 1601Ϫ1536, 1484Ϫ1430,
1295Ϫ1012, 968Ϫ693
4.20Ϫ4.40 (2H, s), 7.25Ϫ7.45 (5H, m),
7.45Ϫ7.55 (1H, d, J7,6 = 8.78), 7.65Ϫ7.75
(1H, d, J6,7 = 8.37), 8.00Ϫ8.05 (1H, s),
8.05Ϫ8.15 (2H, d, J2Љ,3Љ = J6Љ,5Љ = 8.45),
8.25Ϫ8.40 (3H, m)
6
7
Ϫ
Ϫ
H
H
C(CH3)3 144Ϫ145 37.25 C25H24O2N2
3407, 3026, 2958Ϫ2868, 1662,
1620, 1566Ϫ1535, 1481,
1268Ϫ1027, 964Ϫ693
1.30Ϫ1.40 (9H, s), 4.20Ϫ4.30 (2H, s),
7.20Ϫ7.90 (12H, m), 7.90Ϫ8.05 (1H, s)
Br
197Ϫ198 11.23 C21H15O2N2Br
3271, 3085, 2930Ϫ2850, 1672,
1625, 1588Ϫ1558, 1485,
1297Ϫ1010, 970Ϫ695
4.30Ϫ4.50 (2H, s), 7.40Ϫ7.55 (5H, m),
7.55Ϫ7.65 (1H, d, J7,6 = 8.75), 7.65Ϫ7.75
(1H, dd, J6,7 = 8.77, J6,4 = 1.9), 7.75Ϫ7.85
(2H, d, J3Љ,2Љ = J5Љ,6Љ = 8.46), 7.85Ϫ7.95
(2H, d, J6Љ,5Љ = J2Љ,3Љ = 8.47), 8.00Ϫ8.05
(1H, s), 8.05Ϫ8.15 (1H, d, J4,6 = 1.84)
8
9
Ϫ
H
H
F
153Ϫ154 27.81
C
21H15O2N2F
3360, 3068, 2950Ϫ2900, 1648,
1624, 1602Ϫ1540, 1455Ϫ1423,
1269Ϫ1027, 960Ϫ695
4.00Ϫ4.10 (2H, s), 6.90Ϫ7.75 (12H, m),
7.75Ϫ7.85 (1H, s)
CH2
Br
181Ϫ182 53.25 C22H17O2N2Br
140Ϫ141 34.56 C22H17O2N2F
3263, 3062, 2960Ϫ2890, 1652,
1620, 1574Ϫ1542, 1482Ϫ1454,
1271Ϫ1013, 967Ϫ675
3.70Љ380 (2H, s), 420Ϫ4.30 (2H, s),
7.20Ϫ725 (2H, d, J3Љ,2Љ = J5Љ,6Љ = 8.22),
7.25Ϫ7.40 (8H, m), 7.45Ϫ7.60 (2H, d,
J2Љ,3Љ = J6Љ,5Љ = 8.14), 7.75Ϫ7.85 (1H, s)
10
11
12
13
14
CH2
CH2
CH2
CH2
Ϫ
H
H
H
H
Cl
F
3256, 3073, 2924, 1649, 1622,
1573Ϫ1511, 1482Ϫ1455,
1279Ϫ1017, 977Ϫ695
3.70Ϫ3.80 (2H, s), 4.30Ϫ4.40 (2H, s),
7.00Ϫ7.50 (12H ,m), 7.80Ϫ7.90 (1H, s)
H
123
12.19 C22H18O2N2
3255, 3061Ϫ3027, 2930, 1649,
1620, 1573Ϫ1540, 1480Ϫ1454,
1259Ϫ1029, 975Ϫ671
3.70Ϫ3.80 (2H, s), 4.20Ϫ4.30 (2H, s),
7.20Ϫ7.50 (13H, m), 7.75Ϫ7.85 (1H, s)
Cl
CH3
H
171Ϫ172 38.60 C22H17O2N2Cl
138Ϫ139 29.37 C23H20O2N2
3264, 3063, 2960, 1652, 1620,
1575Ϫ1542, 1482Ϫ1455,
1271Ϫ1017, 980Ϫ695
3.70Ϫ3.80 (2H, s), 4.20Ϫ4.30 (2H, s),
7.30Ϫ7.45 (12H, m), 7.70Ϫ7.80 (1H, s)
3270, 3050, 2920, 1615,
1567Ϫ1538, 1483Ϫ1455,
1267Ϫ1029, 1662, 972Ϫ694
2.30Ϫ2.40 (3H, s), 3.70Ϫ3.80 (2H, s),
4.20Ϫ4.30 (2H, s), 7.10Ϫ7.40 (12H, m),
7.80 (1H, s)
160
44.99 C21H15O2N2Cl
3296, 3058, 2930, 1648, 1619,
1574Ϫ1533, 1482, 1299Ϫ1015,
967Ϫ692
4.50Ϫ4.70 (1H, s), 7.25Ϫ8.05 (13H, m)
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406 Aki-Sener et al.
Arch. Pharm. Pharm. Med. Chem. 2004, 337, 402−410
Table 1. (continued).
Comp.
No:
Y
R
R1
m.p.
(°C)
Yield
(%)
Empirical
formula
IR
(cm؊1
1H-NMR
δppm (J = Hz)
)
15
Ϫ
Cl C2H5
172Ϫ173 28.20 C23H19O2N2Cl
3271, 3080, 2968Ϫ2931, 1623,
1572, 1481, 1298Ϫ1015, 1671,
969Ϫ697
1.00Ϫ1.10 (3H, t, J= 7.60), 2.50Ϫ2.60
(2H, q, J = 7.60), 4.00Ϫ4.10 (2H, s),
7.05Ϫ7.20 (6H, m), 7.20Ϫ7.30 (1H, d,
J7,6 = 8.75), 7.35Ϫ7.45 (1H, dd, J6,7
8.77, J6,4 = 1.99), 7.55Ϫ7.65 (2H, d,
=
J2Љ,3Љ = J6Љ,5Љ = 8.18), 7.70Ϫ7.80 (2H, m,
J4,6 = 2.7)
16
17
18
Ϫ
Ϫ
Ϫ
Cl NO2
205Ϫ207 50.86 C21H14O4N3Cl
3316, 3079, 2850, 1651, 1618,
1603Ϫ1529, 1481, 1290Ϫ1015,
973Ϫ683
4.00Ϫ4.20 (2H, s), 7.00Ϫ7.50 (7H, m),
7.75Ϫ7.85 (1H, s), 7.85Ϫ8.20 (4H, m)
Cl C(CH3)3 145
21.54 C25H23O2N2Cl
3296, 2950Ϫ2850, 2964, 1638,
1615, 1572Ϫ1528, 1480,
1267Ϫ1015, 968Ϫ689
1.00Ϫ1.40 (9H, s), 4.00Ϫ4.20 (2H, s),
7.00Ϫ7.90 (12H, m)
Cl Br
197
41.26 C21H14O2N2ClBr 3270, 3082, 2980Ϫ2900, 1673,
1624, 1588Ϫ1558, 1482,
4.00Ϫ4.10 (2H, s), 7.10Ϫ7.20 (4H, m),
7.20Ϫ7.30 (1H, d, J7,6 = 8.75), 7.35Ϫ7.40
(1H, dd, J6,7 = 8.74), 7.40Ϫ7.50 (2H, d,
J3Љ,2Љ = J5Љ,6Љ = 8.53), 7.50Ϫ7.65 (2H, d,
J2Љ,3Љ = J6Љ,5Љ = 8.43), 7.70 (1H, s),
7.75Ϫ7.80 (1H, s)
1275Ϫ1012, 970Ϫ683
19
Ϫ
Cl
F
173Ϫ174 20.78 C21H14O2N2ClF
3397, 3073, 2930, 1660, 1619,
1602Ϫ1550, 1485, 1286Ϫ1015,
972Ϫ692
4.00Ϫ4.10 (2H, s), 6.90Ϫ7.00 (2H, q),
7.10Ϫ7.20 (4H, q), 7.20Ϫ7.30 (1H, d,
J7,6 = 8.77), 7.35Ϫ7.45 (1H, dd, J6,7
8.76, J6,4 = 1.87), 7.65Ϫ7.90 (4H, q)
=
20
21
22
23
24
CH2 Cl Br
178
55.73 C22H16O2N2ClBr 3264, 3054, 2980Ϫ2920, 1662,
1618, 1590Ϫ1536, 1487,
3.80Ϫ3.95 (2H, s), 4.30Ϫ4.45 (2H, s),
7.25Ϫ7.75 (11H, m), 7.90Ϫ8.00 (1H, s)
1266Ϫ1013, 972Ϫ678
CH2 Cl
CH2 Cl
F
139Ϫ140 51.72 C22H16O2N2ClF
3261, 3096, 2920, 1666, 1623,
1569Ϫ1509, 1482, 1278Ϫ1014,
982Ϫ694
3.70Ϫ3.85 (2H, s), 4.20Ϫ4.30 (2H, s),
7.00Ϫ7.50 (11H, m), 7.80Ϫ7.90 (1H, s)
H
164
8.15 C22H17O2N2Cl
3265, 3071, 2950Ϫ2900, 1682,
1621, 1572, 1478, 1297Ϫ1015,
973Ϫ694
3.60Ϫ3.80 (2H, s), 4.10Ϫ4.20 (2H, s),
7.00Ϫ7.50 (12H, m), 7.70Ϫ7.80 (1H, d,
J4,6 = 1.67)
CH2 Cl Cl
167Ϫ168 13.12 C22H16O2N2Cl2
232Ϫ233 15.05 C22H16O4N3Cl
3262, 3055, 2360, 1663, 1618,
1596Ϫ1539, 1482, 1267Ϫ1015,
973Ϫ674
3.80Ϫ3.95 (2H, s), 4.35Ϫ4.45 (2H, s),
7.30Ϫ7.60 (11H, m), 7.95Ϫ8.00 (1H, d,
J4,6 = 1.63)
CH2 Cl NO2
3265Ϫ3219, 3073, 2980Ϫ2860,
1687, 1624, 1567Ϫ1509, 1482,
1255Ϫ1015, 982Ϫ728
3.40Ϫ3.60 (2H, s), 3.90Ϫ4.00 (2H, s),
7.00Ϫ7.06 (2H, m), 7.06Ϫ7.12 (1H, d,
J7,6 = 8.79), 7.20Ϫ7.25 (1H, dd, J6,7 = 8.8,
J6,4 = 2.07), 7.25Ϫ7.35 (2H, d, J6Љ,5Љ
J2Љ,3Љ = 8.72), 7.35Ϫ7.45 (2H, s),
=
7.70Ϫ7.75 (1H, d, J4,6 = 1.96), 7.85Ϫ7.95
(2H, dd, J3Љ,2Љ = J5Љ,6Љ = 8.76, J3Љ,5Љ = J5Љ,3Љ
1.89), 9.80Ϫ10.00 (1H, s)
=
25
CH2 Cl CH3
169Ϫ171 32.49 C23H19O2N2Cl
3249, 3026, 2920Ϫ2890, 1644,
1617, 1571Ϫ1535, 1480,
1268Ϫ1015, 971Ϫ686
2.50Ϫ2.65 (3H, s), 3.80Ϫ3.90 (2H, s),
4.35Ϫ4.50 (2H, s), 7.30Ϫ7.55 (11H, m),
7.90Ϫ8.00 (1H, d, J4,6 = 1.71)
On the other side, with MIC values of 12.5Ϫ50 µg/mL,
all the compounds showed notable activity against C.
albicans, except for 32 and 34.
The SAR of the synthesized compounds revealed that
compounds possessing p-bromo / chloro / methyl-phen-
yl-acetamido groups at position 5 of the fused hetero-
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Arch. Pharm. Pharm. Med. Chem. 2004, 337, 402−410
Synthesis and Activity of New Substituted Benzoxazoles 407
Table 2. Antimicrobial activity results (MIC, µg/mL) of the newly and previously [19, 20] synthesized compounds
as well as the standard drugs.
Microorganisms*
gram-positive
gram-negative
fungus
Comp. Ref.
No.
A
X
Y
Sa
Bs
Ec Pa
Ca
Ck
Cg
3
4
5
6
7
Phenyl
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
CH2
CH2
CH2
CH2
CH2
Ϫ
H
H
H
H
H
H
H
H
200
50
50
200
50
50
25
50
25
100
25
200
50
100
25
50
50
50
12.5
50
50
50
50
50
25
50
50
25
25
25
25
100
25
50
50
50
50
50
25
25
100
25
25
50
25
50
100
50
50
50
100 100 50
50 50 25
100 25 12.5 12.5
25 25 50
50 25 25
50 25 12.5 50
25 25 25
25 25 50
25 50 12.5 12.5
50 50 25
25 25 25
100 100 50
50 50 25
25 25 50
50 50 50
50 50 50
12.5
25
25
4-Ethylphenyl
4-Nitrophenyl
4-tert-Butylphenyl
4-Bromophenyl
4-Fluorophenyl
4-Bromophenyl
4-Fluorophenyl
Phenyl
12.5
12.5
25
25
25
3.12
12.5
25
25
25
8
9
12.5
12.5
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26*
27*
28
29
30
31
32
33
34
35
36
37
38
39
40
41
H
H
H
4-Chlorophenyl
4-Methylphenyl
Phenyl
25
25
6.25 12.5
25
25
25
50
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
H
H
H
H
H
H
H
H
H
25
25
25
50
100
4-Ethylphenyl
4-Nitrophenyl
4-tert-Butylphenyl
4-Bromophenyl
4-Fluorophenyl
4-Bromophenyl
4-Fluorophenyl
Phenyl
Ϫ
Ϫ
Ϫ
Ϫ
100
Ϫ
25 50 12.5 12.5
50 50 12.5 25
50 25 50
50 25 25
50 25 50
12.5
CH2
CH2
CH2
CH2
CH2
CH2
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
50
50
25
25
25
4-Chlorophenyl
4-Nitrophenyl
4-Methylphenyl
6.25 12.5
50 25 12.5 25
50 25 12.5 12.5
25
12.5
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
100
50
100
50
50
50
[19] Phenyl
50 25 25
200 200 50
50 50 50
50 50 50
50 50 50
50 50 25
100 50 100
100 50 25
100 100 100
50 50 50
50 50 50
50 50 50
50 50 25
50 50 25
50 100 25
50 100 25
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
[19] 4-Fluorophenyl
[19] 4-Bromophenyl
[19] 4-Chlorophenyl
[19] 4-Methoxyphenyl
[19] 4-Methylphenyl
[19] 4-Ethylphenyl
[19] 4-Nitrophenyl
[19] 4-tert-Butylphenyl
[19] Phenyl
[19] 4-Methylphenyl
[19] 4-Ethylphenyl
[19] 2-Methoxyphenyl
[19] 2-Chlorophenyl
50
50
50
100
100
100
200
100
50
100
50
25
50
200
25
C2H5 50
C2H5 100
C2H5 50
H
H
H
H
25
50
100
50
Ϫ
Ϫ
Ϫ
Ϫ
[19] 2,4-Dimethoxyphenyl Ϫ
[19] 2,4-Dimethylphenyl
Ϫ
 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

408 Aki-Sener et al.
Arch. Pharm. Pharm. Med. Chem. 2004, 337, 402−410
Table 2. (continued).
Comp. Ref.
No.
A
X
Y
Sa
Bs
Ec
Pa
Ca
Ck
Cg
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
[19] Phenyl
Ϫ CH2
Ϫ CH2
Ϫ CH2
Ϫ CH2
Ϫ CH2
Ϫ CH2 C2H5
Ϫ CH2 C2H5
Ϫ CH2 C2H5
H
H
H
H
H
100
50
50
50
200
200
50
50
50
100
200
25
25
25
25
25
100
50
25
25
100
25
100
50
100
50
25
50
50
50
25
50
25
25
25
25
25
50
50
25
50
200
100
25
50
100
50
50
50
25
50
50
50
25
50
50
50
25
50
50
100
50
50
50
200
100
50
50
50
25
50
25
25
50
50
50
25
25
25
25
12.5
25
25
50
50
25
25
50
50
12.5
25
Ϫ
Ϫ
Ϫ
[19] 4-Bromophenyl
[19] 4-Chlorophenyl
[19] 4-Nitrophenyl
[19] 4-Propyloxyphenyl
[19] Phenyl
[19] 4-Bromophenyl
[19] 4-Chlorophenyl
[19] 2-Chlorophenyl
50
100
50
50
50
50
50
50
100
25
25
25
25
25
50
50
50
50
50
25
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ CH2
H
H
F
C2H5
C2H5
F
C2H5
F
H
F
H
C2H5
C2H5
[19] 3,5-Dimethoxyphenyl Ϫ Ϫ
[20] 4-Chlorophenyl
[20] 4-Chlorophenyl
[20] 4-Methoxyphenyl
[20] 4-Fluorophenyl
[20] 4-Fluorophenyl
[20] 2-Thienyl
[20] 2-Thienyl
[20] 2-Furyl
[20] 2-Furyl
[20] 2-Thienyl
Ϫ Ϫ
Ϫ Ϫ
Ϫ Ϫ
Ϫ Ϫ
Ϫ Ϫ
Ϫ Ϫ
Ϫ Ϫ
Ϫ Ϫ
Ϫ Ϫ
Ϫ Ϫ
Ϫ Ϫ
Ϫ Ϫ
Ϫ Ϫ
58
59
60
61
62
63
64
Ampicillin
Amoxycillin
Tetracycline
Streptomycin
Ciprofloxacine
Gentamisin
Myconazole
Clotrimazole
Haloprogin
[20] 2-Furyl
[20] 2-Thienyl
[20] 2-Furyl
N(CH3)2 25
N(CH3)2 25
25
50
1.56
1.56
1.56
1.56
1.56
1.56
12.5 >200
3.12 >200
Ϫ
Ϫ
Ϫ
Ϫ
3.12
1.56 100
3.13 0.78
1.56 12.5 12.5
50
3.12 50
3.12
3.12
Ϫ
Ϫ
Ϫ
1.56
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
3.12 1.56 3.12
6.25 Ϫ
3.12 Ϫ
Ϫ
Ϫ
*Sa: Staphylococcus aureus (ATCC 25923); Bs: Bacillus subtilis (ATCC 6633); Ec: Escherichia coli (ATCC
23556); Pa: Pseudomonas auruginosa (ATCC 10145); Ca: Candida albicans; Ck: Candida krusei (ATCC 6258);
Cg: Candida glabrata (isolate)
cyclic system and a chlorine substituent at the para
position of the 2-benzyl moiety of the benzoxazole had
increased antimycotic activity against C. krusei and
C. glabrata.
tivities than 2-phenyl-benzoxazole derivatives against
C. albicans and that they showed notable activities
against Candida species such as C. glabrata and C.
krusei, and these observations could guide us to de-
sign further new lead antifungal compounds.
In conclusion, 2-benzyl-benzoxazole derivatives
showed significant activities against gram-positive and
gram-negative bacteria. However, the alternates of the
substituents attached at 5-carbonylamino of benz-
oxazoles made no important difference for the antimi-
crobial activity. It could be pointed out that 2-benzyl-
benzoxazole derivatives generally indicated better ac-
Acknowledgments
We thank the Research Fund of Ankara University
(Grant No. 2001-08-03-27) for financial support of
this research.
 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Arch. Pharm. Pharm. Med. Chem. 2004, 337, 402−410
Synthesis and Activity of New Substituted Benzoxazoles 409
Microbiology
Experimental
For the antibacterial and antimycotic assays, the compounds
were dissolved in absolute ethanol (0.8 mg/mL). Further di-
lutions of the compounds and standard drugs in the test me-
dium were prepared at the required quantities at concen-
trations of 400, 200, 100, 50, 25, 12.5, 6.25, 3.12, 1.56, and
0.78 µg/mL with Mueller-Hinton broth and Sabouraud dex-
trose broth. The minimum inhibitory concentrations (MIC)
were determined using the twofold serial dilution technique
[23, 24]. A control test with inoculated broth supplemented
with only ethanol at the same dilutions as used in our experi-
ments was also performed, and this ethanol-supplemented
broth was found to be inactive in the culture medium. All the
compounds were tested for their in vitro growth inhibitory ac-
tivity against different bacteria and the yeasts Candida al-
bicans (ATCC 10145), Candida krusei (ATCC 6258), and
Candida glabrate (isolated). Origins of bacterial strains are
Staphylococcus aureus (ATCC 25923) and Bacillus subtilis
(ATCC 6633) as gram-positive and Escherichia coli (ATCC
23556) and Pseudomonas aeruginosa (ATCC 10145) as
gram-negative bacteria. ATCC strains of the microorganisms
used in this study were obtained from the culture collection
of Refik Saydam Health Institution of Health Ministry, Ankara,
and maintained at the Microbiology Department of the Faculty
of Pharmacy of Ankara University.
Chemistry
Silicagel HF254 chromatoplates (0.3 mm) were used for TLC.
The solvent systems were chloroform : methanol (15 : 0.5)
for compounds 3Ϫ25. Melting points were taken on a Buchi
SMP 20 capillary apparatus and are uncorrected. IR spectra
were recorded by FT/IRϪ420 in KBr discs. 1H-NMR spectra
were obtained with a Bruker 80 MHz spectrometer in d6-
chloroform; tetramethylsilan (TMS) was used as an internal
standard. Elemental analyses were carried out with a Perkin
Elmer model 240-C apparatus.
General procedure for the synthesis of 5-amino-2-[p-substi-
tuted-benzyl]benzoxazole (1, 2) [19, 20]
5-amino-2-[p-substituted-benzyl]benzoxazole was synthe-
sized by heating 0.01 mol 2,4-diaminophenol 2HCl with 0.01
mol suitable phenyl acetic acid in 12.5 g polyphosphoric acid
(PPA) and stirring for 1.5Ϫ2.5 h. At the end of the reaction
period, the residue was poured into an ice-water mixture and
neutralized with an excess of 10 M NaOH solution extracted
with benzene. Then, this solution was dried over anhydrous
sodium sulfate and evaporated under diminished pressure.
The residue was boiled with 200 mg charcoal in ethanol and
filtered. After the evaporation of solvent in vacuo, the crude
product was obtained and recrystallized from ethanol.
Ampicillin, amoxycillin, tetracycline, streptomycin, ciprofloks-
azin, gentamycin, myconazole, clotrimazaole, and haloprogin
were used as control drugs. The data on the antimicrobial
activity of the compounds and the control drugs as MIC val-
ues (µg/mL) are given in Table 2.
2-Benzyl-5-aminobenzoxazole
Reaction time: 2.5 h. Reaction temperature: 150Ϫ160°C.
Yield: 66%. MW: 224. M.p.: 82Ϫ83°C. 1H-NMR (CDCl3):
Antibacterial and antifungal assay
4.00Ϫ4.10 (s, 2H, CH2), 6.40Ϫ6.60 (dd, 1H, C-6 H J6,7
=
The cultures were obtained from Mueller-Hinton broth (Difco)
for all the bacterial strains after 24 h of incubation at 37
1°C. Candida albicans, Candida krusei and Candida glabrate
were maintained in Sabouraud dextrose broth (Difco) after
incubation for 24 h at 25 1°C. Testing was carried out in
Mueller-Hinton broth and Sabouraud dextrose broth (Difco)
at pH 7.4, and the twofold serial dilution technique was ap-
plied. The final inoculum size was 105 CFU/mL for the anti-
bacterial assay and 104 CFU/mL for the antifungal assay. A
set of tubes containing only inoculated broth was used as
controls. For the antibacterial assay after incubation for 24 h
at 37 1°C and after incubation for 48 h at 25 1°C for the
antifungal assay, the last tube with no growth of microorgan-
ism and/or yeast was recorded to represent the MIC ex-
pressed in µg/mL. All experiments in the antibacterial and
antifungal assays were replicated twice.
8.56, J6,4 = 2.25), 6.70Ϫ6.80 (d, 1H, CϪ4 H, J4,6 = 2.22),
7.00Ϫ7.05 (d, 1H, C-7 H, J7,6 = 8.58), 7.05Ϫ7.20 (m, 7H, C-
2Ј, C-3Ј, C-4Ј, C-5Ј, C-6Ј, NH). IR (KBr disc): 3385, 1617,
1562, 1486, 1452, 1270, 1190.
2-(p-Chlorobenzyl)-5-aminobenzoxazole
Reaction time: 1.5 h. Reaction temperature: 195Ϫ198°C.
Yield: 100%. MW: 224. M.p.: 85Ϫ87°C. 1H-NMR (CDCl3):
4.10Ϫ4.30 (s, 2H, CH2), 6.70Ϫ6.80 (dd, 1H, C-6 H J6,7
=
8.56, J6,4 = 2.66), 6.95Ϫ7.00 (d, 1H, C-4 H,J4,6 = 2.21),
7.20Ϫ7.25 (d, 1H, C-7 H, J7,6 = 8.6), 7.25Ϫ7.40 (m, 6H, C-
2Ј, C-3Ј, C-5Ј, C-6Ј, NH). IR (KBr disc): 3381, 2210, 1915,
1615, 1563, 1486, 1452, 1298, 1271, 1191.
General procedure for 2-[p-substituted-benzyl]-5-[p-substi-
tuted-phenyl/benzyl-carbonylamino]benzoxazole derivatives
3Ϫ25 [21, 22]
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which were dried in vacuo. The chemical, physical and spec-
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