50 Letters in Organic Chemistry, 2018, Vol. 15, No. 1
Ahsan et al.
the development of synthetic compounds with comparatively
higher selectivity towards cancerous cells and relatively safe.
and sodium cyanate and in the subsequent step, 2,6-dimethyl
phenyl semicarbazide (3a) was obtained by refluxing an
equimolar mixture of intermediated, 2a and hydrazine hydrate
for 24 h in ethanol as per the reported method [12]. For the
synthesis of series two oxadiazoles (4h-n,) the initial step in-
volved the synthesis of ethyl[(2,6-dimethylphenyl)amino]-
acetate (2b) by stirring a mixture of 2,6-xylidine (1) and eth-
ylbromoacetate in ethanol and sodium acetate (trihydrate) for
6 h at 80°C, and in the subsequent step, an equimolar mix-
ture of intermediate, 2b and hydrazine hydrate was refluxed
for 22 h in ethanol to afford the synthesis of 2-[(2,6-
dimethylphenyl)amino]acetohydrazide (3b) [13]. The final
compounds, N-(2,6-dimethylphenyl)-5-aryl-1,3,4-oxadiazol-
2-amine analogues (4a-g)/N-(2,6-dimethylphenyl)[5-aryl-
1,3,4-oxadiazol-2-yl]methyl amine analogues (4h-n) were
synthesized by refluxing, an equimolar amount of 2,6-
dimethyl phenyl semicarbazide (3a)/ 2-[(2,6-dimethylphenyl)-
amino]acetohydrazide (3b) and aromatic aldehyde in wa-
ter/ethanol (1: 2 v/v) system for 10-12 h with the addition of
20% mol solution of NaHSO3 [14]. The progress of the reac-
tion was monitored throughout by thin layer chromatography
(TLC) using eluent n-hexane/ethylacetate/formic acid (5:4:1),
benzene/acetone (8:2) and the spots were visualized in iodine
vapour. All these compounds were obtained with satisfactory
yields ranging between 58% and 86% after crystallization
with ethanol and having sufficient purity as confirmed by
microanalysis (elemental analysis). The title compounds (4a-n)
were further characterized and confirmed by FT-IR, NMR
(1H NMR and 13C NMR) and mass spectral data. The ele-
mental analyses (microanalysis) confirmed the purity of the
compounds. The FT-IR spectra of the compounds, oxadia-
zole stretching (C-O-C), C=N stretching and NH stretching
was observed ranging between 1247-1259 cm-1, 1509-1517
cm-1 and 3202-3219 cm-1, respectively, while the phenolic
(Ar-OH) stretching was observed ranging between 3412 and
3419 cm-1 bands. The nature and number of protons of the
title compounds (4a-n) were verified and confirmed by pro-
ton nuclear magnetic resonance (1H NMR) based on their
chemical shifts, multiplicities (singlet, doublet, and multiplet
etc.), and coupling constants (J values in Hz) in DMSO-d6
using tetramethyl silane (TMS) as the standard. The 1H
NMR spectra of the compounds showed a singlet at δ 2.12-
2.19, 3.71-3.85, and 4.09-4.42 ppm, corresponding to the
methyl group (CH3), methoxy group (OCH3) and methylene
(-CH2-) linkage, respectively. The aromatic protons (ArH)
were observed as singlet/doublet/ multiplet at δ 6.27-7.81
ppm. The peaks of aromatic NH (ArNH) and phenolic OH
(ArOH) were observed as a singlet at δ 7.63-8.57 and 10.08-
11.02 ppm respectively. The nature of carbon atoms was
verified and characterized using 13C NMR and the peak of
the molecular ion peaks (M+), and isotopomeric ion (M+2)+
were prominent in the mass spectra.
The oxadiazoles are good bioisosteres of amide and ester,
participate with the receptor through hydrogen bonding and
increase the biological profile to a large extent [3]. Five
membered 1,3,4-oxadizole analogues are rich in potential
activities [4]. Oxadiazole analogues are reported as antican-
cer [5], antitubercular [6], antimicrobial [7], anti-HIV [8],
anti-inflammatory [9] agents and many more. We reported
preparation, characterization and cytotoxic screening of the
fourteen new oxadiazoles. The oxadiazoles linked with the
aryl core of IMC-038525 (tubulin polymerization inhibitor;
IC50 = 0.39 0.06 µM) and NSC 777948 (cytotoxic agent
with mean GP = 62.61, at 10 µM drug concentration) were
taken into the consideration to design the oxadiazoles with
(4h-n) and without (4a-g) methylene (-CH2-) linkage (Fig. 1)
as cytotoxic agents [10,11]. The cytotoxic evaluation of eight
compounds was tested on panels of nine different human
cancer cell lines (60 NCI cancer cell lines). Today breast
cancer has drawn much attention among the major causes of
cancer related death in female globally, so breast cancer cell
lines (MCF-7 and MDA-MB-231) were tested for cytotoxic-
ity of the remaining five compounds.
Core skeleton of IMC-038525
Incorporation of methylene (-CH2-)
linker to alter biological profile
H
N
N N
O
N N
N
O
H
4h-m
O
R2
NSC 777948
O
N N
O
N N
N
O
N
H
R2
O
H
NH
4a-f
N
IMC-038525
Fig. (1). Design of title compounds (4a-n) based on the structure
IMC-038525 and NSC 777948 [10, 11].
2. RESULTS AND DISCUSSION
2.1. Chemistry
2.2. Cytotoxicity Evaluation
Methods for the synthesis of N-(2,6-dimethylphenyl)-5-
aryl-1,3,4-oxadiazol-2-amine (4a–g) and N-{[5-aryl-1,3,
4-oxadiazol-2-yl]methyl}-2,6-dimethylaniline (4h-n) are
summarized in Scheme 1 and 2, respectively. Both the se-
ries of oxadizoles were synthesized following two different
route starting from 2,6-xylidine (1). For the synthesis of se-
ries one oxadiazoles (4a-g), the initial step involved in the
synthesis of 2,6-dimethyl phenyl urea (2a) from 2,6-xylidine
Eight oxadiazole analogues (4a-e, 4g, 4l, and 4n) were
evaluated for their cytotoxicity studies on nearly 60 cancer
cell lines in one dose assay (10 µM concentration) as per the
reported method [15-18]. The cytotoxicity of the tested com-
pounds was calculated as growth percent (GP) and percent
growth inhibition (GI) at single high dose at 10 µM drug
concentrations. The average GP ranged between 71.97 and
Ahsan, Mohamed Jawed
