Structure-activity relationships of tyrosinase inhibitory combinatorial library of 2,5-disubstituted-1,3,4-oxadiazole analogues

Article abstract of DOI:10.1016/j.bmc.2005.03.012

Here the tyrosinase inhibition studies of library of 2,5-disubstituted-1,3, 4-oxadiazoles have been reported and their structure-activity relationship (SAR) also have been discussed. The library of the oxadiazoles was synthesized under the microwave irradiation and was structures of these were characterized by different spectral techniques. From this study it could be concluded that for a better inhibition of tyrosinase, electronegative substitution is essential as most probably the active site of the enzyme contain some hydrophobic site and position is also very important for the inhibition purposes due to the conformational space. The electronegativity of the compounds is somewhat proportional to the inhibitory activity. The compound 3e (3′-[5-(4′- bromophenyl)-1,3,4-oxadiazol-2-yl]pyridine) exhibited most potent (IC ₅₀ = 2.18 μM) inhibition against the enzyme tyrosinase which is more potent than the standard potent inhibitor l-mimosine (IC₅₀ = 3.68 μM). This molecule can be the best candidate as a lead compound for further development of drug for the treatments of several skin disorders.

Full text of DOI:10.1016/j.bmc.2005.03.012

Bioorganic & Medicinal Chemistry 13 (2005) 3385–3395
Structure–activity relationships of tyrosinase
inhibitory combinatorial library of
2,5-disubstituted-1,3,4-oxadiazole analogues
b,c
Mahmud Tareq Hassan Khan,a,c, Muhammad Iqbal Choudhary,
*
Khalid Mohammed Khan,b Mubeen Ranib and Atta-ur-Rahmanb
aPharmacology Research Laboratory, Faculty of Pharmaceutical Sciences, University of Science and Technology,
Chittagong 4000, Bangladesh
bH.E.J. Research Institute of Chemistry, University of Karachi, Karachi 75270, Pakistan
cDr. Panjwani Center for Molecular Medicine and Drug Development, University of Karachi, Karachi 75270, Pakistan
Received 20 January 2005; revised 2 March 2005; accepted 2 March 2005
Available online 30 March 2005
Abstract—Here the tyrosinase inhibition studies of library of 2,5-disubstituted-1,3,4-oxadiazoles have been reported and their struc-
ture–activity relationship (SAR) also have been discussed. The library of the oxadiazoles was synthesized under the microwave irra-
diation and was structures of these were characterized by different spectral techniques. From this study it could be concluded that for
a better inhibition of tyrosinase, electronegative substitution is essential as most probably the active site of the enzyme contain some
hydrophobic site and position is also very important for the inhibition purposes due to the conformational space. The electroneg-
ativity of the compounds is somewhat proportional to the inhibitory activity. The compound 3e (30-[5-(40-bromophenyl)-1,3,4-oxa-
diazol-2-yl]pyridine) exhibited most potent (IC50 = 2.18 lM) inhibition against the enzyme tyrosinase which is more potent than the
standard potent inhibitor L-mimosine (IC50 = 3.68 lM). This molecule can be the best candidate as a lead compound for further
development of drug for the treatments of several skin disorders.
ꢀ 2005 Elsevier Ltd. All rights reserved.
1. Introduction
insect and adhesion of marine organisms.1 In insects,
several functions of this enzyme have been reported in
the generation of o-diphenols and quinones for pigmen-
tation, wound healing, parasite encapsulation, and sclero-
tization and the enzyme is an alternative target site for
the control of insect pests. In food industry, tyrosinase
is responsible for the enzymatic browning reactions in
damaged fruits during post-harvest handling and pro-
cessing. Control of enzymatic browning during process-
ing is important in fruit pulp manufacturing. In
addition, tyrosinase inhibitors are becoming important
constituents of cosmetic products that relate to hyper-
pigmentation. Therefore, there is a concerted effort to
search for naturally occurring tyrosinase inhibitors from
plant, because plants constitute a rich source of bioac-
tive chemicals and many of them are largely free from
harmful adverse effects.2
Tyrosinase (E.C. 1.14.18.1), also known as polyphenol
oxidase (PPO), is a multifunctional copper-containing
enzyme, widely distributed in plants and animals. It
catalyses the o-hydroxylation of monophenols and also
the oxidation of o-diphenols to o-quinones. Tyrosinase
is known to be a key enzyme for melanin biosynthesis
in plants and animals. Therefore, tyrosinase inhibitors
should be clinically useful for the treatment of some der-
matological disorders associated with melanin hyperpig-
mentation and also important in cosmetics for whitening
and depigmentation after sunburn. In addition, tyrosi-
nase is known to be involved in the molting process of
Keywords: Tyrosinase inhibitor; 2,5-Disubstituted-1,3,4-oxadiazole
library; Melanin; Vitiligo; Hyperpigmentation; Depigmentation.
*
Corresponding author at present address: Department of Bioche-
mistry and Molecular Biology, Center for Biotechnology, University of
Ferrara, Via L. Borsari 46, FE 44100 Italy. Tel.: +39 0335 1553899;
fax: +39 532 424505; e-mail addresses: khnmmd@unife.it;
mthkhan2002@yahoo.com
In recent years numbers of potent tyrosinase inhibitors
have been reported from our and other groups. Very re-
cently, we have reported two long chain esters, methyl
2b(2S)-hydroxyl-7(E)-tritriacontenoate and methyl
0968-0896/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmc.2005.03.012

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M. T. H. Khan et al. / Bioorg. Med. Chem. 13 (2005) 3385–3395
2b(2S)-O-b-D-galactopyranosyl-7(E)-tetratriaconteno-
ate, showing strong to moderate inhibitory activities
against tyrosinase.3 In another paper we have reported
that, (+)-androst-4-ene-3,17-dione and its five metabolic
analogues having steroidal skeletons, namely androsta-
1,4-diene-3,17-dione, 17b-hydroxyandrosta-1,4-dien-3-one,
11a-hydroxyandrost-4-ene-3,17-dione, 11a,17b-dihydr-
oxyandrost-4-en-3-one and 15a-hydroxyandrosta-1,4-
dien-17-one, exhibited moderate inhibitory activities
against the enzyme.4 Ahmad et al. in 2004 reported
that, a new coumarinolignoid 80-epi-cleomiscosin A to-
gether with the new glycoside 8-O-b-D-glucopyranosyl-
6-hydroxy-2-methyl-4H-1-benzopyrane-4-one, exhibited
strong inhibition against the enzyme tyrosinase, when
compared to the standard tyrosinase inhibitors kojic
acid and L-mimosine. The new coumarinolignoid exhib-
ited two times more potency than that of the standard
potent inhibitor L-mimosine.5 Recently, Karbassi et al.
reported the inhibition kinetics of two new synthetic
bi-pyridine molecules, [1,40]bipiperidinyl-10-yl-naph-
than-2-yl-methanone (I) and [1,40]bipiperidinyl-10-yl-4-
methylphenyl-methane (II) of the catecholase activity
of mushroom tyrosinase. The kinetics studies indicated
that these are uncompetitive inhibitors and the values
of the Ki are 5.87 and 1.31 lM for I and II, respectively,
which showed high potency. Fluorescent studies con-
firmed the uncompetitive type of inhibition for these
two inhibitors. They also suggested that, the inhibition
mechanism presumably coming from the presence of a
particular hydrophobic site which can accommodate
these inhibitors. This site could be formed due to a prob-
able conformational change that was induced by bind-
ing of substrate with the enzyme.6
O
O
N3
2
4 N
5
(ii)
(i) or
1
O
R
NHNH2 R'
OH
2a-2m
R
'
R
1
3a-3m
4a-4m
3. R = 3-Pyridinyl, 4. R = 2-MeOC6H4
Scheme 1. Reagents: (i) POCl3; (ii) POCl3, Al2O3.
(a–m) in the presence of phosphorous oxychloride to af-
ford 2,5-disubstituted-1,3,4-oxadiazoles 3 (a–m) and 4
(a–m) (Scheme 1). To establish the general validity of
our newly developed method, several selected one-pot
microwave-assisted syntheses were carried out. The
reaction was found to proceed smoothly under micro-
wave irradiation within 6–16 min whereas under reflux
conditions in 4–10 h (shown in Tables 1 and 2). The
products were isolated by simple cold aqueous work-
up followed by either solvent extraction or precipitation
and were finally purified by column chromatography
wherever necessary to afford pure 2,5-disubstituted-
1,3,4-oxadiazole. This method appeared to be the rapid
and economical with wide range of applications.7
2.2. Tyrosinase inhibition studies
In the present studies, two types of 26 derivatives of the
oxadiazole basic skeleton have been studied to explain
their inhibition patterns and structure–activity relation-
ships (SAR) against the enzyme tyrosinase, which is a
multifunctional copper-containing enzyme, widely dis-
tributed in plants and animals and catalyses the o-
hydroxylation of monophenols and also the oxidation
of o-diphenols to o-quinones.1
The microwave radiation endows with an unconven-
tional to the usual heating as it employs the capability
of liquids or solids to convert electromagnetic energy
into heat. The exploitation of microwave radiations
has commenced numerous innovative perceptions in
chemistry, while the absorption and transmission of
the energy is completely different from the conventional
mode of heating. This technology has been applied to a
number of useful research and development processes
such as polymer technology, organic synthesis, applica-
tion to waste treatment; drug release/targeting; ceramic
and alkane decomposition.7
In one type of compounds, substitutions were changing
at different positions of the phenyl ring at C-5 while
keeping the pyridine ring constant at C-2. In another
type of compounds, substitutions were changing at dif-
ferent positions of the phenyl ring while keeping the
o-methoxy phenyl ring constant at C-2 position.
In a previous report it was found that 3-hydroxypyr-
idine-4-ones is showing inhibition against tyrosinase.8
This was established that alkyl substitution at position
2 in the aromatic ring minimizes the interaction with
tyrosinase. Several phenolic compounds have been re-
ported to have potent tyrosinase inhibitory activity.9–11
Here in this paper, we have discussed the tyrosinase
inhibitory activities of a library of 26 analogues of 2,5-
disubstituted-1,3,4-oxadiazoles, which were synthesized
using microwave-assisted combinatorial synthetic ap-
proach and finally their structure–activity relationships
(SAR) also have been discussed.
Compound 3a exhibited potent tyrosinase inhibition
and the IC50 value is 5.15 lM, where the IC50 value of
reference tyrosinase inhibitor kojic acids (KA) is
16.67 lM. This compound was totally unsubstituted.
When C-200 position was substituted with –NO2 group
2. Results and discussion
2.1. General chemistry
the resulting 3b was showing highly potent (IC50 =
3.18 lM) inhibition against tyrosinase, when compared
with highly potent reference tyrosinase inhibitor
L-mimosine (LM) (IC50 = 3.68 lM). Due to the substitu-
tion of this –NO2 group the resulting compound exhib-
ited potent inhibition. But when the same phenyl ring
was found to have bromine atom at C-200 (3c,
IC50 = 5.23 lM) and C-300 (3d, IC50 = 6.04 lM)
The detailed chemistry and the synthetic parts of the
compounds have been reported recently and discussed
elsewhere.7 Briefly, a number of commercially available
hydrazides were treated with different carboxylic acids 2

M. T. H. Khan et al. / Bioorg. Med. Chem. 13 (2005) 3385–3395
3387
Table 1. Comparison between microwave-assisted and conventional method of synthesis of 2,5-disubstituted-1,3,4-oxadiazole 3 (a–m) in terms of
time and yield
Sr.
R0
Structures
Microwave
Time (min)
Conventional
Time (h) Yield (%)
Yield (%)
N
N
3a
C6H5
12
92
6
81
O
N
N
NO2
N
N
3b
o-NO2C6H4
9
96
5
86
O
Br
N
N
3c
3d
3e
3f
o-BrC6H4
m-BrC6H4
p-BrC6H4
3-Pyridinyl
CH2Cl
12
12
12
12
7
92
87
85
89
87
6
6
6
9
5
76
71
68
75
78
O
O
O
N
N
N
N
N
Br
N
N
Br
N
N
O
N
N
N
N
3g
CH2Cl
O
N
N
N
N
N
3h
CHCl2
7
6
85
91
4
4
77
76
CHCl2
O
N
N
CCl3
O
3i
CCl3
N
N
3j
p-CH3C6H4
13
81
7
69
O
N
CH3
(continued on next page)

3388
M. T. H. Khan et al. / Bioorg. Med. Chem. 13 (2005) 3385–3395
Table 1 (continued)
Sr.
R0
Structures
Microwave
Time (min) Yield (%)
Conventional
Time (h) Yield (%)
N
N
O
3k
3,4,5-Trimethoxybenzoyl
15
79
9
63
N
H3CO
OCH3
OCH3
N
N
3l
1-C10H7
12
12
83
81
9
8
69
72
O
N
N
N
3m
2-C10H7
O
N
Table 2. Comparison between microwave-assisted and conventional method of synthesis of 2,5-disubstituted-1,3,4-oxadiazole 4 (a–m) in terms of
time and yield
Sr.
R0
Structures
Microwave
Time (min) Yield (%)
Conventional
Time (h) Yield (%)
OCH3
N
N
N
N
4a
C6H5
12
89
6
5
78
80
O
O
OCH3
OCH3
NO2
4b
4c
4d
4e
4f
o-NO2C6H4
o-BrC6H4
m-BrC6H4
p-BrC6H4
3-Pyridinyl
9
95
90
86
85
87
Br
N
N
12
12
12
12
6
6
6
9
73
69
66
74
O
OCH3
OCH3
N
N
N
Br
O
N
O
Br
OCH3
N
N
O
N

M. T. H. Khan et al. / Bioorg. Med. Chem. 13 (2005) 3385–3395
3389
Table 2 (continued)
Sr.
R0
Structures
Microwave
Time (min) Yield (%)
Conventional
Time (h) Yield (%)
OCH3
N
N
7
86
5
75
4g
CH2Cl
CH2Cl
CHCl2
CCl3
O
OCH3
OCH3
N
N
4h
4i
4j
CHCl2
7
6
83
88
79
4
4
7
70
73
67
O
N
N
CCl3
O
OCH3
OCH3
N
N
p-CH3C6H4
13
O
O
CH3
N
N
4k
3,4,5-Trimethoxybenzoyl
15
76
9
60
H3CO
OCH3
OCH3
OCH3
OCH3
N
N
N
4l
1-C10H7
12
12
79
76
9
8
67
69
O
O
N
4m
2-C10H7
positions, the activities were decreased although the po-
tency was much better than the KA. Again when the
bromination was done at C-400 position, the resulting
compound 3e exhibited highest potency against the en-
zyme tyrosinase and IC50 value is 2.18 lM, which is
1.96 times more potent than the LM. We believe that
40-bromophenyl group also has some extra effects on
the tyrosinase inhibition. Recently Wang et al. reported
that the 4-halobenzoic acids (4-fluorobenzoic acid, 4-
chlorobenzoic acid and 4-bromobenzoic acid) can
strongly inhibit both monophenolase activity and diphe-
nolase activity of the enzyme, and the inhibition displays
a reversible course, where the inhibition of 4-bromoben-
zoic acid is more potent than the other two. The kinetic
analyses exhibited that the inhibition mechanism of all
three 4-halobenzoic acids is noncompetitive inhibition
to the diphenolase activity.12
When pyridine was attached at C-5 position, the result-
ing compound 3f was showing highly potent
(IC50 = 3.29 lM) inhibition against tyrosinase, even bet-
ter than the LM.
When chloromethyl group was present at the C-5 posi-
tion, the resulting compound 3g was showing highly po-
tent (IC50 = 4.18 lM) inhibition against tyrosinase, if
compared with the KA, which is 3.99 times more
potent. At the same position one more chlorine was
attached, the resulting compound 3h exhibited little
more potency (IC50 = 4.01 lM) than the previous one.
Finally when all chlorine atoms are attached, the result-
ing 3i exhibited more potency (IC50 = 3.98 lM) than the
previous two compounds. These compounds were prov-
ing that, for the better inhibition of tyrosinase, electro-
negativity is necessary and the tyrosinase inhibition is

3390
M. T. H. Khan et al. / Bioorg. Med. Chem. 13 (2005) 3385–3395
Table 3. Tyrosinase inhibitory activities of the series 3 (a–m) as
compared with the standard inhibitors
directly proportional to the electronegativity of
substituent(s).
Compounds
IC50(Mean SEMa) (lM)
Again when the compound has methyl group at C-400
position, the resulting compound 3j was showing less
potency with IC50 = 10.40 lM, as compared to the
unsubstituted one 3a and with reference inhibitor KA.
When the compound was substituted with 100-naphthyl
group at C-5 position, the resulting compound 3l was
showing potent (IC50 = 3.23 lM) inhibition, if com-
pared with LM.
3a
3b
5.15 0.02574
3.18 0.01774
5.23 0.023
6.04 0.00081
2.18 0.01774
3.29 0.02956
4.18 0.00293
4.01 0.0271
3.98 0.003112
10.40 0.01207
NDc
3c
3d
3e
3f
3g
3h
3i
3j
3k
In the second series of the library of compounds, substi-
tutions have been done at C-5 position of the oxadiazole
ring, while keeping the methoxy group at C-20. When
bromine was present at C-300 position, the resulting com-
pound 4d was showing potent inhibition (IC50 = 7.18 lM)
when compared with KA. When bromine was present at
C-400 as in the case of compound 4e, then the potency of
the compound decreased (IC50 = 7.82 lM). Again it has
been noted that when bromine is present at C-200 rather
than at C-300, as in the case of compound 4c, the potency
has been found to increase (IC50 = 5.16 lM) and exhibit
better inhibition than the 4d and 4e. When the bromine
was totally replaced by phenyl ring, as in the case of
compound 4a, then the potency of the compound
decreased (IC50 = 8.71 lM). It means that, the presence
and the position of the –Br is essential for tyrosinase
inhibition, which is also supporting the findings of
Wang et al.12
3l
3m
3.23 0.01936
NDc
Kojic acidb
L-Mimosineb
16.67 0.5190
3.68 0.02234
a SEM: Standard error of the mean.
b Standard inhibitors (KA and LM) of the enzyme tyrosinase.
c Not done.
Table 4. Tyrosinase inhibitory activities of the series 4 (a–m), as
compared with the standard inhibitors
Compounds
IC50(Mean SEMa) (lM)
4a
4b
8.71 0.022
NDc
4c
4d
5.16 0.02371
7.18 0.513
7.82 0.563
NDc
4e
4f
4g
NDc
When the tricholoromethyl group is present at C-5 posi-
tion 4i of oxadiazole ring the compound showed better
inhibition (IC50 = 6.21 lM), when compared with the
compound 4h (IC50 = 7.28 lM), where one –Cl was re-
placed with proton. The compound 4j, containing
methyl group at aromatic ring, exhibited excellent
potency (IC50 = 6.45 lM). The compound 4m, having
naphthyl group have IC50 = 7.81 lM.
4h
4i
7.28 0.0694
6.21 0.00371
6.43 0.1003
NDc
4j
4k
4l
4m
NDc
7.81 0.563
16.67 0.5190
3.68 0.02234
Kojic acidb
L-Mimosineb
a SEM: Standard error of the mean.
b Standard inhibitors (KA and LM) of the enzyme tyrosinase.
c Not done.
All these studies are presented in tabular and graphical
forms in Tables 3 and 4 and also graphically in Figures
1 and 2.
The inhibition mechanisms of the inhibitors most likely
appearing from the presence of a particular hydrophobic
site, which can accommodate these inhibitors which
could be formed due to a probable conformational
change that was induced by binding of substrate with
the enzyme.6 Unfortunately, the crystal structure of
the enzyme mushroom tyrosinase has not yet been
published. So we are unable to explain the probable
binding interactions between the inhibitors and the pro-
tein in this stage, by taking the three-dimensional struc-
ture of the protein by molecular docking experiments.
We are going to report the modeling of the enzyme
through homology modeling approaches13 and after val-
idating properly of the model, especially solving the
folding related problems we will further study the dock-
ing interactions.
molecular modeling, docking and 3D-QSAR (three-
dimensional quantitative structure–activity relation-
ships), like CoMFA, CoMSIA, Golpe, etc., studies of
the same and similar tyrosinase inhibitors.
It can be concluded from this whole study that for the
better inhibition of enzyme tyrosinase, electronegative
changeover is crucial and the location of the group is
also imperative in support of the inhibition. If the elec-
tronegativity is increased the inhibition also increases.
The compound 3e (30-[5-(40-bromophenyl)-1,3,4-oxa-
diazol-2-yl]pyridine) exhibited most potent inhibition
against the enzyme tyrosinase, while compared with
both of the reference inhibitors.
The molecule 3e preserves the best candidature as a lead
molecule for further development of drug for the treat-
ments of several skin disorders.
In the near future we will communicate about the
calculations of QSAR–QSPR molecular descriptors,

M. T. H. Khan et al. / Bioorg. Med. Chem. 13 (2005) 3385–3395
3391
First the compounds were screened for the o-dipheno-
lase inhibitory activity of tyrosinase using L-DOPA as
substrate. All the active inhibitors from the preliminary
screening were subjected for IC50 studies. Briefly, all the
compounds were dissolved in DMSO and finally the sol-
vent mixture was 2.5%. Mushroom tyrosinase (30 units,
28 nM) was first preincubated with the compounds, in
50 nM Na–phosphate buffer (pH 6.8) for 10 min at
25 ꢂC. Then the L-DOPA (0.5 mM) was added to the
reaction mixture and the enzyme reaction was moni-
tored by measuring the change in absorbance at
475 nm (at 37 ꢂC) of the formation of the DOPA
chrome for 10 min.
Figure 1. Graphical presentation of the comparative IC50 values of the
series 3 (a–m) against the enzyme tyrosinase.
The percent inhibition of the enzyme and IC50 values of
the active compounds were calculated using a program
developed with JAVA and MACRO EXCEL 2000 (Micro-
20
ꢁ
16.67
soft Corp., USA) for this purpose. The following equa-
tion has been followed:
15
10
5
8.71
Percent inhibition ¼ ½ABSBlank ꢀ ABSSample=ABSBlank
ꢂ 100
ꢁ
7.82
7.81
7.28
7.18
6.43
6.21
5.16
3.68
Here the ABSBlank and ABSSample are the absorbances
for the blank and samples, respectively. All the studies
have been done atleast in triplicates and the results here
represents the Mean SEM. (standard error of the
mean). All the regents, enzyme, substrate and reference
compounds, were purchased from Sigma Chem. Co.,
MO, USA.
0
4a
4c
4d
4e
4h
4i
4j
4m
KA
LM
Figure 2. Graphical presentation of the comparative IC50 values of the
series 4 (a–m) against the enzyme tyrosinase.
3. Materials and methods
3.1. General experimental
3.3. Spectral data of the compounds
3.3.1. 30-(5-Phenyl-1,3,4-oxadiazol-2-yl)pyridine (3a).
The ultraviolet spectra were measured in chloroform on
a Lambda 5 UV/vis spectrophotometer (Perkin–Elmer).
IR spectra (KBr discs or MeOH) were recorded on a
Bruker FT-IR IFS48 spectrophotometer. EI mass spec-
tra data were recorded with various MAT 711 (70 eV)
Yield 92%; mp 112–114 ꢂC; Rf = 0.34 (ethyl acetate–ace-
tone, 9:1); UV (methanol): kmax (loge) 256 (2.32) nmꢀ1
;
IR (KBr) mmax: 3073 (C–H), 1667 (C@N), 1557 (C@C),
1287 (C–O), 832, 659 (C–Br); 1H NMR (400 MHz,
DMSO-d6): d 9.21 (d, 1H, J = 1.1 Hz, H-20), 9.13 (dd,
1
spectrophotometers and data are tabulated as m/z H
1H, J6 ,5 = 4.9 Hz, H-60), 8.81 (br d, J4 ,5 = 8.3 Hz,
0 0 0 0
NMR and 13C NMR spectra were recorded in CDCl3
and DMSO-d6 using Bruker AC400 (500 and
400 MHz) spectrophotometer, respectively. Splitting
patterns are as follows: s, singlet; d, doublet; dd, double
doublets; t, triplet; m, multiplet. Chemical shifts are re-
ported in d (ppm) and coupling constants are given in
hertz. The progress of all reactions was monitored by
TLC, which was performed on 2.0 · 5.0 cm aluminum
sheets precoated with silica gel 60F254 to a thickness of
0.25 mm (Merck). The chromatograms were visualized
under ultraviolet light (254–366 nm) or iodine vapours.
1H, H-40), 8.59 (dd, 1H, J5 ,6 = 4.9, J5 ,4 = 8.3 Hz, H-
0
0
0
0
50), 7.63 (dd, 2H, J2 ,3 /6 ,5 = 7.8, J2 ,4 /6 ,4 = 2.3 Hz,
H-200/H-600), 7.50 (t, 1H, J = 7.8 Hz, H-400), 7.38 (t, 2H,
J = 7.8, H-300/500); EI MS (m/z): 223 (M+, 21), 145 (31),
106 (100), 77 (79), 78 (65), 68 (35), 51 (72). Anal. Calcd
for C13H9N3O: C, 9.95; H, 4.06; N, 18.82; O, 7.17.
Found: C, 69.86; H, 3.97; N, 18.73; O, 7.08.
00 00 00 00
00 00 00 00
3.3.2. 30-[5-(200-Nitrophenyl)-1,3,4-oxadiazol-2-yl]pyridine
(3b). Yield 96%; mp 89 ꢂC; Rf = 0.40 (ethyl acetate–ace-
tone, 8:2); UV (methanol): kmax (loge) 283 (3.00) nmꢀ1
;
IR (KBr) mmax: 1563 (C@C), 3065 (C–H), 1667 (C@N),
1283 (C–O), 1545 (NO2), 831, 651; 1H NMR
(500 MHz, DMSO-d6): d 9.23 (d, 1H, J = 1.2 Hz, H-
20), 9.16 (dd, 1H, J = 4.9 Hz, H-60), 8.92 (dd, 1H,
J = 8.2, J = 1.7 Hz, H-300), 8.87 (t, 1H, J = 7.8, H-500),
8.76 (br d, 1H, J = 8.3 Hz, H-40), 8.60 (dd, 1H,
J = 4.9, 8.3 Hz, H-50), 8.13 (dd, 1H, J = 7.8, 1.8 Hz,
H-600), 7.85 (t, 1H, J = 7.8 Hz, H-400); EI MS (m/z): 268
(M+, 6), 223 (9), 147 (29), 106 (100), 93 (43), 78 (75),
68 (39), 51 (84). Anal. Calcd for C13H8N4O3: C, 58.21;
H, 3.01; N, 20.89; O, 17.89. Found: C, 58.29; H, 3.08;
N, 20.98; O, 17.97.
The hydrazides used as intermediates for the title com-
pounds were synthesized and characterized satisfacto-
rily. Phosphorous oxychloride and all the carboxylic
acids is commercially available (Flulka Aldrich).
3.2. Tyrosinase inhibition assay
Tyrosinase inhibition assays were performed in 96-well
microplate format using SpectraMaxꢁ 340 (Molecular
Devices, CA, USA) microplate reader according to the
developed method earlier described by Hearing.14

3392
M. T. H. Khan et al. / Bioorg. Med. Chem. 13 (2005) 3385–3395
3.3.3. 30-[5-(200-Bromophenyl)-1,3,4-oxadiazol-2-yl]pyr-
idine (3c). Yield 92%; mp 102–114 ꢂC; Rf = 0.38 (ethyl
acetate–acetone, 8:2); UV (methanol): kmax (loge) 261
(2.28) nmꢀ1; IR (KBr) mmax: 3069 (C–H), 1665 (C@N),
1561 (C@C), 1289 (C–O), 647 (C–Br); 1H NMR
(500 MHz, DMSO-d6): d 9.21 (d, 1H, J = 1.1 Hz, H-
20), 9.12 (br d, 1H, J = 4.8 Hz, H-60), 8.59 (br d, 1H,
J = 8.3 Hz, H-40), 8.91 (dd, J = 7.9, 2.1 Hz, H-300), 7.86
(dd, 1H, J = 4.8, 8.3 Hz, H-50), 7.58 (t, 1H, J = 7.9 Hz,
H-400), 7.49 (dd, 1H, J = 7.9, 1.9 Hz, H-600), 7.28 (t, 1H,
J = 7.9 Hz, H-500); EI MS (m/z): 303 (M+1, 48), 301
(41), 223 (33), 157 (29), 106 (69), 78 (100), 68 (49), 51
(52). Anal. Calcd for C13H8N4O3: C, 58.21; H, 3.01;
N, 20.89; O, 17.89. Found: C, 58.29; H, 3.08; N,
20.98; O, 17.97.
tate–acetone, 9:1); UV (methanol): kmax (loge) 207
(2.41) nmꢀ1; IR (KBr) mmax: 3059 (C–H), 2993 (C–H),
1667 (C@N), 1561 (C@C), 1285 (C–O), 741 (C–Cl),
1
610; H NMR (500 MHz, DMSO-d6): d 9.19 (d, 1H,
J = 1.1 Hz, H-20), 9.11 (dd, 1H, J = 4.9, 1.7 Hz, H-60),
8.89 (br d, 1H, J = 8.3 Hz, H-40), 8.67 (dd, 1H,
J = 4.9, 8.3 Hz, H-50), 3.21 (s, 2H, H-100); EI MS (m/z):
197 (M+2, 4), 195 (M+, 13), 159 (29), 146 (35), 78
(100), 68 (33), 51 (78). Anal. Calcd for C8H6ClN3O: C,
49.12; H, 3.09; Cl, 18.12; N, 21.48; O, 8.18. Found: C,
49.01; H, 3.20; Cl, 18.19; N, 21.41; O, 8.19.
3.3.8.
30-[5-(Dichloromethyl)-1,3,4-oxadiazol-2-yl]pyr-
idine (3h). Yield 85%; mp 145–147 ꢂC; Rf = 0.34 (ethyl
acetate–acetone, 9:1); UV (methanol): kmax (loge) 209
(2.42) nmꢀ1; IR (KBr) mmax: 3061 (C–H), 2998 (C–H),
1664 (C@N), 1564 (C@C), 1281 (C–O), 757 (C–Cl),
3.3.4. 30-[5-(300-Bromophenyl)-1,3,4-oxadiazol-2-yl]pyr-
idine (3d). Yield 87%; mp 127–130 ꢂC; Rf = 0.31 (ethyl
acetate–acetone, 8:2); UV (methanol): kmax (loge) 263
(2.29) nmꢀ1; IR (KBr) mmax : 3069 (ArC–H), 1664
1
612; H NMR (500 MHz, DMSO-d6): d 9.20 (d, 1H,
J = 1.1 Hz, H-20), 9.12 (br d, 1H, J = 4.9 Hz, H-60),
8.87 (br d, 1H, J = 8.3 Hz, H-40), 8.69 (dd, 1H,
J = 8.3, 4.9 Hz, H-50), 3.39 (s, 1H, H-100); EI MS (m/z):
234 (M+4, 6), 232 (M+2, 19), 230 (M+, 29), 194 (66),
159 (29), 146 (63), 106 (73), 78 (100), 51 (85). Anal.
Calcd for C8H5Cl2N3O: C, 41.77; H, 2.19; Cl, 30.82;
N, 18.27; O, 6.95. Found: C, 41.86; H, 2.10; Cl, 30.77;
N, 18.32; O, 6.94.
1
(C@N), 1565 (C@C), 1289 (C–O), 829, 653 (C–Br); H
NMR (500 MHz, DMSO-d6):
d
9.22 (d, 1H,
J = 1.1 Hz, H-20), 9.11 (br d, 1H, J = 4.9 Hz, H-60),
8.84 (br d, 1H, J = 8.3 Hz, H-40), 8.58 (dd, 1H,
J = 4.9, 8.3 Hz, H-50), 7.78 (d, 1H, J = 1.7 Hz, H-200),
7.68 (dd, J = 8.3, 1.7 Hz, H-400), 7.46 (dd, 1H, J = 7.8,
J = 1.7 Hz, H-600), 7.41 (dd, 1H, J = 8.3, J = 7.8 Hz, H-
500); EI MS (m/z): 303 (M+2, 43), 301 (M+, 48), 223
(33), 157 (27), 106 (59), 78 (100), 77 (69), 68 (53), 51
(78). Anal. Calcd for C13H8BrN3O: C, 51.68; H, 2.67;
Br, 26.45; N, 13.91; O, 5.30. Found: C, 51.61; H, 2.74;
Br, 26.39; N, 13.97; O, 5.31.
3.3.9. 30-[5-(Trichloromethyl)-1,3,4-oxadiazol-2-yl]pyr-
idine (3i). Yield 91%; mp 127–129 ꢂC; Rf = 0.34 (ethyl
acetate–acetone, 9:1); UV (methanol): kmax (loge) 207
(2.41) nmꢀ1; IR (KBr) mmax: 1562 (C@C), 3059 (C–H),
1
1663 (C@N), 1283 (C–O), 781 (C–Cl), 613; H NMR
(500 MHz, DMSO-d6): d 9.23 (d, 1H, J = 1.1 Hz, H-
20), 9.12 (br d, 1H, J = 4.9 Hz, H-60), 8.86 (br d, 1H,
J = 8.3 Hz, H-40), 8.63 (dd, 1H, J = 4.9, 8.3 Hz, H-50);
EI MS (m/z): 270 (M+6, 1), 268 (M+4, 6), 266 (M+2,
18), 264 (M+, 13), 158 (29), 118.5 (39), 106 (43), 78
(100), 68 (49), 51 (41). Anal. Calcd for C8H4Cl3N3O:
C, 36.33; H, 1.52; Cl, 40.21; N, 15.89; O, 6.05. Found:
C, 36.45; H, 1.40; Cl, 40.14; N, 15.96; O, 6.06.
3.3.5. 30-[5-(400-Bromophenyl)-1,3,4-oxadiazol-2-yl]pyr-
idine (3e). Yield 85%; mp 199–102 ꢂC; Rf = 0.37 (ethyl
acetate–acetone, 8:2); UV (methanol): kmax (loge) 262
(2.30) nmꢀ1; IR (KBr) mmax: 3067 (C–H), 1662 (C@N),
1
1568 (C@C), 1283 (C–O), 821, 656 (C–Br); H NMR
(400 MHz, CDCl3): d 9.20 (d, 1H, J = 1.1 Hz, H-20),
9.10 (dd, 1H, J = 4.9, 1.6 Hz, H-60), 8.83 (br d, 1H,
J = 8.3 Hz, H-40), 8.59 (dd, 1H, J = 4.9, 8.3 Hz, H-50),
7.76 (dd, 2H, J = 8.4, 1.2 Hz, H-300/500), 7.57 (dd, 2H,
J = 8.4, 1.3 Hz, H-200/600); EIMS (m/z): 303 (M+2, 48),
301 (M+, 50), 223 (39), 157 (31), 106 (76), 78 (100), 68
(43), 51 (59). Anal. Calcd for C13H8BrN3O: C, 51.68;
H, 2.67; Br, 26.45; N, 13.91; O, 5.30. Found: C, 51.58;
H, 2.77; Br, 26.37; N, 13.99; O, 5.31.
3.3.10. 30-[5-(400-Methylphenyl)-1,3,4-oxadiazol-2-yl]pyr-
idine (3j). Yield 81%; mp 102–104 ꢂC; Rf = 0.43 (ethyl
acetate–acetone, 9:1); UV (methanol): kmax (loge) 206
(2.41) nmꢀ1; IR (KBr) mmax: 3063 (C–H), 2912 (C–H),
1
1663 (C@N), 1561 (C@C), 1283 (C–O), 608; H NMR
(500 MHz, DMSO-d6): d 9.14 (d, 1H, J = 1.1 Hz, H-
20), 9.04 (br d, 1H, J = 4.9 Hz, H-60), 8.78 (br d, 1H,
J = 8.3 Hz, H-40), 8.54 (dd, 1H, J = 4.9, 8.3 Hz, H-50),
7.49 (br d, 2H, J = 7.8 Hz, H-200/600), 7.32 (d, 2H,
J = 7.8 Hz, H-300/500); EI MS (m/z): 237 (M+, 13), 222
(28), 146 (58), 106 (43), 78 (100), 77 (62), 68 (47). Anal.
Calcd for C14H11N3O: C, 70.87; H, 4.67; N, 17.71; O,
6.74. Found: C, 70.79; H, 4.75; N, 17.63; O, 6.69.
3.3.6. 30-[5-(300-Pyridinyl)-1,3,4-oxadiazol-2-yl]pyridine
(3f). Yield 89%; mp 124–126 ꢂC; Rf = 0.43 (ethyl ace-
tate–acetone, 8:2); UV (methanol): kmax (loge) 208
(2.41) nmꢀ1; IR (KBr) mmax: 3061 (C–H), 1659 (C@N),
1563 (C@C), 1286 (C–O), 831, 625; 1H NMR
(500 MHz, DMSO-d6): d 9.22 (d, 2H, J = 1.1 Hz, H-20/
200), 9.11 (br d, 2H, J = 4.9 Hz, H-60/600), 8.84 (br d,
2H, J = 8.3 Hz, H-40/400), 8.63 (dd, 1H, J = 4.9, 8.3 Hz,
H-50/500); EI MS (m/z): 303 (M+, 12), 146 (9), 106
(100), 78 (80), 51 (66). Anal. Calcd for C12H8N4O: C,
64.28; H, 3.60; N, 24.99; O, 7.14. Found: C, 64.19; H,
3.69; N, 24.88; O, 7.27.
3.3.11. 30-[5-(300,400,500-Trimethoxyphenyl)-1,3,4-oxadiaz-
ol-2-yl]pyridine (3k). Yield 79%; mp 137–140 ꢂC;
Rf = 0.39 (ethyl acetate); UV (methanol): kmax (loge)
283 (3.17) nmꢀ1; IR (KBr) mmax: 3059 (C–H), 1665
(C@N), 1562 (C@C), 1284 (C–O), 1198 (Ph–O–Me),
1
619; H NMR (400 MHz, MeOD-d4): d 9.20 (d, 1H,
3.3.7. 30-[5-(Chloromethyl)-1,3,4-oxadiazol-2-yl]pyridine
(3g). Yield 87%; mp 112–114 ꢂC; Rf = 0.34 (ethyl ace-
J = 1.1 Hz, H-20), 9.13 (br d, 1H, J = 4.9 Hz, H-60),
8.87 (br d, 1H, J = 8.3 Hz, H-40), 8.65 (dd, 1H,

M. T. H. Khan et al. / Bioorg. Med. Chem. 13 (2005) 3385–3395
3393
J = 4.9, 8.3 Hz, H-50), 7.49 (s, 2H, H-200/600), 3.93 (s, 6H,
–OCH3), 3.88 (s, 3H, OCH3); EI MS (m/z): 313 (M+, 9),
282 (11), 251 (13), 220 (18), 146 (37), 106 (73), 78 (100),
77 (53), 68 (38), 51 (79). Anal. Calcd for C16H15N3O4: C,
61.34; H, 4.38; N, 13.41; O, 20.43. Found: C, 61.27; H,
4.45; N, 13.34; O, 20.51.
J = 8.4 Hz, H-40), 7.15 (d, 1H, J = 8.4 Hz, H-30), 7.00
(t, J = 7.6 Hz, H-50), 3.88 (s, 3H, –OCH3); EI MS (m/
z): 297 (M+, 8), 251 (44), 266 (32), 122 (56), 107 (23),
68 (39), 77 (100). Anal. Calcd for C15H11N3O4: C,
60.61; H, 3.73; N, 14.14; O, 21.53. Found: C, 60.67;
H, 3.67; N, 14.21; O, 21.46.
3.3.12. 30-[5-(100-Naphthyl)-1,3,4-oxadiazol-2-yl]pyridine
(3l). Yield 83%; mp 125–127 ꢂC; Rf = 0.34 (ethyl ace-
tate–acetone, 8:2); UV (methanol): kmax (loge) 289
3.3.16.
2-(2-Bromophenyl)-5-(2-methoxyphenyl)-1,3,4-
oxadiazole (4c). Yield 90%; UV (methanol): kmax (loge)
273 (3.17) nmꢀ1; IR (KBr) mmax: 1568 (aromatic C@C),
1662 (C@N), 1267 (C–O), 619, 601; 1H NMR
(3.17) nmꢀ1 1H NMR (500 MHz, DMSO-d6): d 9.22
;
(d, 1H, J = 1.1 Hz, H-20), 9.15 (br d, 1H, J = 4.9 Hz,
H-60), 8.89 (br d, 1H, J = 8.3 Hz, H-40), 8.60 (dd, 1H,
J = 4.9, 8.3 Hz, H-50), 8.46 (dd, 1H, J = 8.4, 1.3 Hz,
H-800), 8.38 (dd, 1H, J = 8.4, 1.4 Hz, H-400), 8.29 (dd,
1H, J = 8.0, 1.3 Hz, H-200), 8.19 (t, 1H, J = 8.0 Hz, H-
300), 7.68 (t, 1H, J = 8.4 Hz, H-700), 7.60 (dd, 1H,
J = 7.8, 1.4 Hz), 7.51 (t, 1H, J = 8.4 Hz, H-600); EI MS
(m/z): 273 (M+, 57), 146 (47), 106 (77), 78 (100), 77
(63), 68 (49), 51 (67); IR (KBr) mmax: 3063 (C–H), 1561
(C@C), 1661 (C@N), 1281 (C–O), 637, 609. Anal. Calcd
for C19H14N2O2: C, 74.71; H, 4.06; N, 15.38; O, 5.85.
Found: C, 74.65; H, 4.12; N, 15.31; O, 5.92.
(400 MHz, DMSO-d6): d 8.03 (dd, 1H, J = 8.1,
J = 1.1 Hz, H-60), 7.87 (br d, J = 7.9 Hz, H-300), 7.60 (t,
1H, J = 7.9 Hz, H-400), 7.92 (dd, 1H, J = 7.6, 1.5 Hz,
H-60), 7.34 (t, 1H, J = 7.9 Hz, H-500), 7.44 (br t, 1H,
J = 8.4 Hz, H-40), 7.15 (d, 1H, J = 8.4 Hz, H-30), 7.00
(t, J = 7.6 Hz, H-50), 3.88 (s, 3H, –OCH3); EI MS (m/
z): 330 (M+, 18), 332 (M+2, 16), 300 (39), 251 (100),
136 (59), 107 (62), 81 (6), 79 (17), 77 (71), 68 (46). Anal.
Calcd for C15H11N2O2Br: C, 54.40; H, 3.35; Br, 24.13;
N, 8.46; O, 9.66. Found: C, 54.45; H, 3.30; Br, 24.23;
N, 8.38; O, 9.63.
3.3.17.
oxadiazole (4d). Yield 86%; Rf = 0.34 (ethyl acetate–ace-
tone, 9:1); UV (methanol): kmax (loge) 273 (3.17) nmꢀ1
2-(3-Bromophenyl)-5-(2-methoxyphenyl)-1,3,4-
3.3.13. 30-[5-(200-Naphthyl)-1,3,4-oxadiazol-2-yl]pyridine
(3m). Yield 81%; mp 166–169 ꢂC; Rf = 0.34 (ethyl ace-
tate–acetone, 9:1); UV (methanol): kmax (loge) 288
(3.17) nmꢀ1; IR (KBr) mmax: 1567 (arom. C@C), 3062
(C–H), 1662 (C@N), 628, 603; 1H NMR (500 MHz,
DMSO-d6): d 9.25 (d, 1H, J = 1.1 Hz, H-20), 9.16 (br
d, 1H, J = 4.9, 1.3 Hz, H-60), 8.71 (br d, 1H,
J = 8.3 Hz, H-40), 8.49 (dd, 1H, J = 4.9, 8.3 Hz, H-50),
8.32 (br s, 1H, H-100), 8.13 (d, 1H, J = 8.1 Hz, H-400),
8.03 (br d, J = 8.1, H-300), 7.82 (dd, 1H, J = 7.8,
1.6 Hz, H-500), 7.76 (dd, 1H, J = 7.9, 1.6 Hz, H-800),
7.54 (t, J = 7.9 Hz, H-700), 7.45 (t, J = 7.9 Hz, H-600); EI
MS (m/z): 273 (M+, 57), 146 (47), 106 (76), 78 (100),
77 (63), 68 (49), 51 (67). Anal. Calcd for C19H14N2O2:
C, 74.71; H, 4.06; N, 15.38; O, 5.85. Found: C, 74.65;
H, 4.12; N, 15.31; O, 5.92.
;
IR (KBr) mmax: 1564 (arom. C@C), 1659 (C@N), 1285
(C–O), 621, 609; 1H NMR (400 MHz, DMSO-d6): d
8.20 (d, 1H, J = 1.7 Hz, H-200), 7.91 (dd, 1H, J = 7.6,
1.5 Hz, H-60), 7.89 (dd, 1H, J = 8.2 Hz, H-400), 7.79
(dd, 1H, J = 7.8, 1.7 Hz, H-600), 7.55 (dd, 1H, J = 8.2,
7.8 Hz, H-500), 7.44 (br t, 1H, J = 8.4 Hz, H-40), 7.11
(d, 1H, J = 8.4 Hz, H-30), 7.00 (t, J = 7.6 Hz, H-50),
3.88 (s, 3H, –OCH3); EI MS (m/z): 330 (M+, 20), 332
(M+2, 19), 300 (48), 251 (100), 136 (27), 107 (69), 81
(8), 79 (6), 77 (76), 68 (31). Anal. Calcd for
C17H11N3O: C, 54.40; H, 3.35; Br, 24.13; N, 8.46; O,
9.66. Found: C, 54.44; H, 3.31; Br, 24.19; N, 8.40; O,
9.66; O, 9.66.
3.3.18.
2-(4-Bromophenyl)-5-(2-methoxyphenyl)-1,3,4-
3.3.14. Methyl 2-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl
ether (4a). Yield 89%; UV (methanol): kmax (loge) 290
(3.19) nmꢀ1; IR (KBr) mmax: 1568 (arom. C@C), 1662
(C@N), 1267 (C–O), 619, 601; 1H NMR (400 MHz,
DMSO-d6): d 8.19 (overlapped, 2H, H-200/H-600), 7.91
(dd, 1H, J = 7.6, 1.5 Hz, H-60), 7.49 (overlapped, 3H,
H-300,400,500), 7.00 (t, J = 7.6 Hz, H-50), 7.41 (br t, 1H,
J = 8.4 Hz, H-40), 7.11 (d, 1H, J = 8.4 Hz, H-30), 3.88
(s, 3H, –OCH3); EI MS (m/z): 252 (M+, 8), 175 (136),
135 (47), 107 (100), 77 (81), 68 (29). Anal. Calcd for
C15H12N2O2: C, 71.42; H, 4.79, N, 11.10; O, 12.68.
Found: C, 71.40; H, 4.81, N, 11.18; O, 12.60.
oxadiazole (4e). Yield 85%; Rf = 0.31 (ethyl acetate–
acetone, 9:1); UV (methanol): kmax (loge) 275
(3.17) nmꢀ1; IR (KBr) mmax: 1567 (arom. C@C), 1661
(C@N), 1279 (C–O), 626, 611; 1H NMR (400 MHz,
DMSO-d6): d 7.89 (dd, 1H, J = 7.6, 1.5 Hz, H-60), 7.74
(dd, 2H, J = 8.4, J = 1.3 Hz, H-300/500), 7.54 (dd, 2H,
J = 8.4, J = 1.3, H-200/600), 7.42 (br t, 1H, J = 8.4 Hz,
H-40), 7.02 (t, J = 7.6 Hz, H-50), 7.16 (d, 1H,
J = 8.4 Hz, H-30), 3.87 (s, 3H, –OCH3); EI MS (m/z):
330 (M+, 20), 332 (M+2, 19), 300 (48), 251 (100), 136
(27), 107 (69), 81 (8), 79 (6), 77 (76), 68 (31). Anal. Calcd
for C15H11N2O2Br: C, 54.40; H, 3.35; Br, 24.13; N, 8.46.
Found: C, 54.47; H, 3.28; Br, 24.20; N, 8.38; O, 9.66.
3.3.15. 2-(2-Methoxyphenyl)-5-(2-nitrophenyl)-1,3,4-oxa-
diazole (4b). Yield 95%; UV (methanol): kmax (loge) 291
(3.19) nmꢀ1; IR (KBr) mmax: 1563 (arom. C@C), 3069
3.3.19. Methyl 2-[5-(3-pyridinyl)-1,3,4-oxadiazol-2-yl]-
phenyl ether (4f). Yield 87%; UV (methanol): kmax (loge)
273 (3.17) nmꢀ1UV (methanol): kmax (loge) 251
(3.15) nmꢀ1; IR (KBr) mmax: 1571 (arom. C@C), 1661
(C@N), 1269 (C–O), 625, 607; 1H NMR (400 MHz,
DMSO-d6): d 9.21 (d, 1H, J = 0.98 Hz, H-200), 9.11
(dd, 1H, J = 4.9, 1.6 Hz, H-600), 8.87 (dd, 1H, J = 4.9,
1
(C–H), 1658 (C@N), 1265 (C–O), 627, 598; H NMR
(400 MHz, DMSO-d6): d 8.87 (dd, 1H, J = 7.8, 1.7 Hz,
H-300), 8.79 (t, 1H, J = 7.8 Hz, H-500), 8.3 (dd, 1H,
J = 7.8, J = 1.4 Hz, H-600), 8.08 (t, 1H, J = 7.8 Hz, H-
400), 7.98 (dd, 1H, J = 7.6, 1.5 Hz, H-60), 7.44 (br t, 1H,

3394
M. T. H. Khan et al. / Bioorg. Med. Chem. 13 (2005) 3385–3395
8.3 Hz, H-500), 8.54 (br d, 1H, J = 8.3 Hz, H-400), 7.95
(dd, 1H, J = 7.6, 1.5 Hz, H-60), 7.48 (br t, 1H,
J = 8.4 Hz, H-40), 7.16 (d, 1H, J = 8.4 Hz, H-30), 7.05
(t, J = 7.6 Hz, H-50), 3.88 (s, 3H, –OCH3); EI MS (m/
z): 253 (M+, 18), 175 (66), 135 (35), 107 (48), 78 (100),
68 (38). Anal. Calcd for C14H11N3O2: C, 66.40; H,
4.38; N, 16.59; O, 12.63. Found: C, 66.48; H, 4.30; N,
16.62; O, 12.60.
C16H14N2O2: C, 72.16; H, 5.30; N, 10.52; O, 12.02.
Found: C, 72.20; H, 5.26; N, 10.49; O, 12.05.
3.3.24. 2,3-Dimethoxy-5-[5-(2-methoxyphenyl)-1,3,4-oxa-
diazol-2-yl]phenyl methyl ether (4k). Yield 76%; UV
(methanol): kmax (loge) 261 (2.49) nmꢀ1; IR (KBr) mmax
:
1576 (arom. C@C), 1658 (C@N), 1264 (C–O), 629, 597;
1H NMR (400 MHz, DMSO-d6): d 7.88 (dd, 1H,
J = 7.6, J = 1.5 Hz, H-60), 7.41 (br t, 1H, J = 8.4 Hz,
H-40), 7.12 (d, 1H, J = 8.4 Hz, H-30), 6.99 (t,
J = 7.6 Hz, H-50), 6.58 (s, 2H, H-200/600), 3.95 (s, 3H,
OCH3), 3.86 (s, 3H, OCH3), 3.80 (s, 6H, OCH3); FD
MS (m/z): 342 (M+). Anal. Calcd for C18H18N2O5: C,
63.15; H, 5.30; N, 8.18; O, 23.37. Found: C, 63.19; H,
5.26; N, 8.23; O, 23.32.
3.3.20. 2-(Chloromethyl)-5-(2-methoxyphenyl)-1,3,4-oxa-
diazole (4g). Yield 86%; UV (methanol): kmax (loge) 235
(2.46) nmꢀ1; IR (KBr) mmax: 1571 (arom. C@C), 1661
1
(C@N), 1269 (C–O), 764 (C–Cl), 625, 607; H NMR
(400 MHz, DMSO-d6): d 7.92 (dd, 1H, J = 7.6, 1.5 Hz,
H-60), 7.43 (br t, 1H, J = 8.4 Hz, H-40), 7.15 (d, 1H,
J = 8.4 Hz, H-30), 7.04 (t, J = 7.6 Hz, H-50), 3.86 (s,
3H, –OCH3), 3.21 (s, 2H, H-100); EI MS (m/z): 224
(M+, 21), 226 (M+2, 7), 189 (8), 209 (25), 181 (43),
175 (31), 105 (52), 135 (100), 77 (68), 51 (29). Anal.
Calcd for C10H9N2O2Cl: C, 53.47; H, 4.04; Cl, 15.78;
N, 12.47; O, 14.24. Found: C, 53.52; H, 3.99; Cl,
15.76; N, 12.49; O, 14.25.
3.3.25. 30-[5-(100-Naphthyl)-1,3,4-oxadiazol-2-yl]pyridine
(3l). Yield 83%; mp 125–127 ꢂC; Rf = 0.34 (ethyl ace-
tate–acetone, 8:2); UV (methanol): kmax (loge) 289
(3.17) nmꢀ1; 7.44 (br t, 1H, J = 8.4 Hz, H-40), 7.15 (d,
1H, J = 8.4 Hz, H-30), 7.00 (t, J = 7.6 Hz, H-50), 3.88
(s, 3H, –OCH3); 1H NMR (500 MHz, DMSO-d6): d
9.22 (d, 1H, J = 1.1 Hz, H-20), 9.15 (br d, 1H,
J = 4.9 Hz, H-60), 8.89 (br d, 1H, J = 8.3 Hz, H-40),
8.60 (dd, 1H, J = 4.9, 8.3 Hz, H-50), 8.46 (dd, 1H,
J = 8.4, J = 1.3 Hz, H-800), 8.38 (dd, 1H, J = 8.4,
1.4 Hz, H-400), 8.29 (dd, 1H, J = 8.0, 1.3 Hz, H-200),
8.19 (t, 1H, J = 8.0 Hz, H-300), 7.68 (t, 1H, J = 8.4 Hz,
H-700), 7.60 (dd, 1H, J = 7.8, 1.4 Hz), 7.51 (t, 1H,
J = 8.4 Hz, H-600); EI MS (m/z): 273 (M+, 57), 146
(47), 106 (77), 78 (100), 77 (63), 68 (49), 51 (67); IR
(KBr) mmax: 3063 (C–H), 1561 (C@C), 1661 (C@N),
1281 (C–O), 637, 609. Anal. Calcd for C19H14N2O2: C,
74.71; H, 4.06; N, 15.38; O, 5.85. Found: C, 74.65; H,
4.12; N, 15.31; O, 5.92.
3.3.21.
2-(Dichloromethyl)-5-(2-methoxyphenyl)-1,3,4-
oxadiazole (4h). Yield 83%; UV (methanol): kmax (loge)
235 (2.46) nmꢀ1; IR (KBr) mmax: 1571 (arom. C@C),
1661 (C@N), 1269 (C–O), 769 (C–Cl), 625, 607; 1H
NMR (400 MHz, DMSO-d6): d 7.94 (dd, 1H, J = 7.6,
1.5 Hz, H-60), 7.46 (br t, 1H, J = 8.4 Hz, H-40), 7.15
(d, 1H, J = 8.4 Hz, H-30), 7.02 (t, J = 7.6 Hz, H-50),
3.88 (s, 3H, –OCH3), 3.38 (s, 1H, H-100); EI MS (m/z):
262 (M+4, 3), 260 (M+2, 9), 258 (M+, 28), 225 (M+2,
11), 223 (31), 175 (48) 135 (29), 107 (58), 83 (27), 85
(8), 87 (3), 68 (69). Anal. Calcd for C10H8N2O2Cl2: C,
46.36; H, 3.11; Cl, 27.37; N, 10.81; O, 12.35. Found:
C, 46.39; H, 3.08; Cl, 27.43; N, 10.79; O, 12.31.
3.3.26. 2-(2-Methoxyphenyl)-5-(2-naphthyl)-1,3,4-oxa-
diazole (4m). Yield 79%; UV (methanol): kmax (loge)
294 (2.49) nmꢀ1; IR (KBr) mmax: 1568 (arom. C@C),
1662 (C@N), 1267 (C–O), 619, 601; 1H NMR
(400 MHz, DMSO-d6): d 8.29 (br s, 1H, H-100), 8.10 (d,
1H, J = 8.1 Hz, H-400), 8.02 (br d, 1H, J = 8.1 Hz, H-
300), 7.91 (dd, 1H, J = 7.6, 1.5 Hz, H-60), 7.80 (dd, 1H,
J = 8.2, 1.1 Hz, H-500), 7.73 (dd, 1H, J = 8.3, 1.3 Hz,
H-800), 7.50 (t, 1H, J = 8.3 Hz, H-700), 7.41 (t, 1H,
J = 8.0 Hz, H-600), 7.44 (br t, 1H, J = 8.4 Hz, H-40),
7.15 (d, 1H, J = 8.4 Hz, H-30), 7.00 (t, J = 7.6 Hz, H-
50), 3.88 (s, 3H, –OCH3); EI MS (m/z): 302 (M+, 12),
195 (54), 175 (29), 127 (24), 107 (32), 68 (79), 76 (100).
Anal. Calcd for C19H14N2O2: C, 75.48; H, 4.67; N,
9.27; O, 10.58. Found: C, 75.51; H, 4.64; N, 9.21; O,
10.64.
3.3.22. 2-(2-Methoxyphenyl)-5-(trichloromethyl)-1,3,4-
oxadiazole (4i). Yield 88%; UV (methanol): kmax (loge)
232 (2.46) nmꢀ1; IR (KBr) mmax: 1566 (arom. C@C),
1661 (C@N), 1269 (C–O), 751 (C–Cl), 614, 597; 1H
NMR (400 MHz, DMSO-d6): d 7.96 (dd, 1H, J = 7.6,
1.5 Hz, H-60), 7.47 (br t, 1H, J = 8.4 Hz, H-40), 7.16
(d, 1H, J = 8.4 Hz, H-30), 7.01 (t, J = 7.6 Hz, H-50),
3.88 (s, 3H, –OCH3); EI MS (m/z): 299 (M+6, 0.20),
297 (M+4, 6), 295 (M+2, 15), 293 (M+, 18), 174 (68),
135 (69), 107 (100), 68 (51), 118 (16), 120 (M+2, 12),
122 (M+4, 3), 124 (M+6, 0.10). Anal. Calcd for
C10H7N2O2Cl3: C, 40.92; H, 2.40; Cl, 36.23; O, 10.90.
Found: C, 40.88; H, 2.44; Cl, 36.27; O, 10.86.
3.3.23. 2-(2-Methoxyphenyl)-5-(4-methylphenyl)-1,3,4-
oxadiazole (4j). Yield 79%; UV (methanol): kmax (loge)
267 (3.11) nmꢀ1; IR (KBr) mmax: 1573 (arom. C@C),
1663 (C@N), 1276 (C–O), 618, 595; 1H NMR
(400 MHz, DMSO-d6): d 8.12 (br d, 2H, J = 7.8, H-200/
600), 7.90 (dd, 1H, J = 7.6, J = 1.5 Hz, H-60), 8.17 (br d,
2H, J = 8.6 Hz, H-200/H-600), 7.42 (overlapped, 3H, H-
40/300/500), 7.15 (d, 1H, J = 8.4 Hz, H-30), 7.00 (t,
J = 7.6 Hz, H-50), 3.80 (s, 3H, OCH3), 2.1 (s, 3H,
CH3); EI MS (m/z): 266 (M+, 7), 175 (54), 135 (34),
105 (61), 91 (46), 77 (100). Anal. Calcd for
Acknowledgements
One of us (M.T.H.K.) gratefully acknowledges the tra-
vel support from Third World Academy of Sciences
(TWAS), Italy, under the ÔSouth–South FellowshipÕ
scheme. He is also the recipient of the fellowships from
the UNESCO–MCBN (Grant no. 1056); the CIB, Italy
and the AVTL, Italy.

M. T. H. Khan et al. / Bioorg. Med. Chem. 13 (2005) 3385–3395
3395
7. Khan, K. M.; Ullah, Z.; Rani, M.; Perveen, S.; Haider, S.
M.; Choudhary, M. I.; Atta-ur-Rahman; Voelter, W. Lett.
Org. Chem. 2004, 1, 50.
8. Hider, R. C.; Lerch, K. Biochem. J. 1989, 257(1),
289.
9. Kubo, I.; Kinst-Hori, I.; Yokokawa, Y. J. Nat. Prod.
1994, 57(4), 545.
10. Sakuma, K.; Ogawa, M.; Sugibayashi, K.; Yamada, K.;
Yamamoto, K. Arch. Pharm. Res. 1999, 22(4), 335.
11. Yang, F.; Boissy, R. E. Pigment Cell Res. 1999, 12(4),
237.
References and notes
1. Shiino, M.; Watanabe, Y.; Umezawa, K. Bioorg. Med.
Chem. 2001, 9, 1233.
2. Lee, H. S. J. Agric. Food Chem. 2002, 50, 1400.
3. Khan, S. B.; Azhar-Ul-Haq; Afza, N.; Malik, A.; Khan,
M. T. H.; Shah, M. R.; Choudhary, M. I. Chem. Pharm.
Bull. 2005, 53(1), 86.
4. Choudhary, M. I.; Sultan, S.; Khan, M. T. H.; Yasin, A.;
Shaheen, F.; Atta-ur-Rahman. Nat. Prod. Res. 2004,
18(6), 529.
12. Wang, Q.; Shi, Y.; Song, K. K.; Guo, H. Y.; Qiu, L.;
Chen, Q. X. Protein J. 2004, 23(5), 303.
13. Khan, M. T. H.; Choudhary, M. I.; Ather, A.; Atta-ur-
Rahman. Minerva Biotechnol. 2005, in press.
14. Hearing, V. J. In Methods in Enzymology; Academic: New
York, 1987; Vol. 142, pp 154–165.
5. Ahmad, V. U.; Ullah, F.; Hussain, J.; Farooq, U.; Zubair,
M.; Khan, M. T. H.; Choudhary, M. I. Chem. Pharm.
Bull. 2004, 52(12), 1458.
6. Karbassi, F.; Saboury, A. A.; Khan, M. T. H.; Choudh-
ary, M. I.; Saifi, Z. S. J. Enzyme Inhib. Med. Chem. 2004,
19(4), 349.