Discovery of novel antitubercular 1,5-dimethyl-2-phenyl-4-([5-(arylamino)- 1,3,4-oxadiazol-2-yl]methylamino)-1,2-dihydro-3H-pyrazol-3-one analogues

Article abstract of DOI:10.1016/j.bmcl.2011.12.014

In search of potential therapeutics for tuberculosis, we describe herewith the synthesis, characterization and antimycobacterial activity of 1,5-dimethyl-2-phenyl-4-([5-(arylamino)-1,3,4-oxadiazol-2-yl]methylamino)-1, 2-dihydro-3H-pyrazol-3-one analogues.

Full text of DOI:10.1016/j.bmcl.2011.12.014

Bioorganic & Medicinal Chemistry Letters 22 (2012) 969–972
Contents lists available at SciVerse ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Discovery of novel antitubercular 1,5-dimethyl-2-phenyl-4-([5-(arylamino)-1,
3,4-oxadiazol-2-yl]methylamino)-1,2-dihydro-3H-pyrazol-3-one analogues
Mohamed Jawed Ahsan a,b, , Jeyabalan Govinda Samy a, Chandra Bhushan Jain c, Kunduri Rajeswar Dutt b,
⇑
Habibullah Khalilullah a, Md. Shivli Nomani a
a New Drug Discovery Research, Department of Pharmaceutical Chemistry, Alwar Pharmacy College, Alwar, Rajasthan 301 030, India
b Department of Pharmaceutical Sciences, National Institute of Medical Sciences University, Jaipur, Rajasthan 303 121, India
c Academic, Rajasthan University of Health Sciences, Jaipur, Rajasthan 302 022, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 June 2011
Revised 16 November 2011
Accepted 3 December 2011
Available online 8 December 2011
In search of potential therapeutics for tuberculosis, we describe herewith the synthesis, characterization
and antimycobacterial activity of 1,5-dimethyl-2-phenyl-4-([5-(arylamino)-1,3,4-oxadiazol-2-yl]methyl-
amino)-1,2-dihydro-3H-pyrazol-3-one analogues. Among the synthesized compounds, 4-[(5-[(4-fluor-
ophenylamino]-1,3,4-oxadiazol-2-yl)methylamino]-1,2-dihydro-1,5-dimethyl-2-phenylpyrazol-3-one (4a)
was found to be the most promising compound active against Mycobacterium tuberculosis H37Rv and
isoniazid resistant M. tuberculosis with minimum inhibitory concentrations, 0.78 and 3.12
respectively, free from any cytotoxicity (>62.5 g/mL).
lg/mL,
Keywords:
Oxadiazoles
Antitubercular agents
Molecular properties prediction
Lipinski-rule of five
l
Ó 2011 Elsevier Ltd. All rights reserved.
The pathogenic agent of tuberculosis (TB), Mycobacterium
tuberculosis (MTB) is responsible for death of 2–3 million people
annually and for global financial toll of ꢀ12 billion dollar each
year.1 A recent threat to TB is the development of drug resistant
strains. The drug resistant TB is classified into two categories such
as multidrug resistant tuberculosis (MDR-TB) that is resistant to
isoniazid and rifampicin and extensively drug resistant tuberculo-
sis (XDR-TB) that is resistant either to quinolones or to any one of
kanamycin, capreomycin or amikacin. There are estimated 1.3 mil-
lion multi/extensively drug resistant TB (M/XDR-TB) cases will
need to be treated between 2010 and 2015.2 In spite of major ad-
vances that have been made in the drug discovery process, no new
drugs have been introduced since the discovery of rifampin.3 As a
result, there is an urgent need to develop novel, faster acting che-
motherapeutic agents with lower toxicity. Consequently, today, it
is clear that vigorous and widespread research on drug design will
be necessary to make new advances.
Oxadiazoles are important classes of compounds which have
long attracted attention, owing to their remarkable biological and
pharmacological properties, such as antitubercular, antibacterial,
antiviral, anti-inflammatory, antifungal and insecticidal activi-
ties.4–12 The azole group of heterocyclic compounds possess signif-
icant pharmacokinetic property, lipophilicity that influence the
ability of drug to reach the target by transmembrane diffusion
and show promising activity against resistant TB by inhibiting
the biosynthesis of lipids.13,14 Most of the antitubercular drugs
(isoniazid, pyrazinamid, etc.) are inhibitors of mycobacterial cell
wall synthesis by inhibiting fatty acid synthetase. In the present
investigation it has been tried to design and synthesized such no-
vel compounds which include the advantage of both pyrazole and
oxadiazole nucleus in the single molecule. All the title compounds
(4a–p) were subjected to molecular properties prediction by Mol-
inspiration and MolSoft (MolSoft, 2007) software in order to filter
the drugs for synthesis and biological screening and to reduce
enormous wastage of expensive chemicals and precious time. Ear-
lier we have reported the antimycobacterial activity of diketones
and pyrazoline.15–17
A computational study for prediction of ADME properties of all
molecules was performed and is presented in Table 1. Topological
polar surface area (TPSA), that is, surface belonging to polar atoms,
is a descriptor that was shown to correlate well with passive
molecular transport through membranes and, therefore, allows
prediction of transport properties of drugs in the intestines and
blood–brain barrier crossing.18 TPSA and volume are inversely pro-
portional to %ABS. TPSA was used to calculate the percentage of
absorption (%ABS) according to the equation: %ABS = 109 0.345 Â
TPSA, as reported.19 From all these parameters, it can be observed
that all the title compounds exhibited a great %ABS ranging from
66.57% to 84.30%. Good intestinal absorption, reduced molecular
flexibility (measured by the number of rotatable bonds), low polar
surface area or total hydrogen bond count (sum of donors and
⇑
Corresponding author. Mobile: +91 9694087786; fax: +91 144 5121120.
E-mail address: jawedpharma@gmail.com (M.J. Ahsan).
0960-894X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2011.12.014

970
M. J. Ahsan et al. / Bioorg. Med. Chem. Lett. 22 (2012) 969–972
Table 1
Pharmacokinetic properties important for good oral bioavailability of the title compounds (4a–p)
Compound
%ABS
Volume (A3)
TPSA (A2)
NROTB
HBA
HBD
LogP
65
MW
Lipinski’s violations
Rule
4a
4b
4c
4d
4e
4f
4g
4h
4i
4j
4k
4l
4m
4n
4o
4p
—
—
—
—
6
6
6
6
6
6
7
6
7
8
6
6
8
6
7
6
<10
4
4
4
4
4
4
6
4
5
5
4
4
6
5
7
4
<5
2
2
2
2
2
2
2
2
2
2
2
2
2
3
4
2
<500
61
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
84.03
84.03
83.59
84.30
84.03
84.30
72.51
84.03
81.14
81.29
84.30
84.26
78.66
77.64
66.57
84.03
377.22
380.50
395.57
386.94
392.97
409.87
309.29
392.24
403.15
421.71
412.31
410.87
434.71
384.26
415.96
371.30
72.35
72.35
73.64
71.59
72.35
71.59
105.75
72.35
80.74
80.32
71.59
71.71
87.94
90.90
122.98
72.35
2.18
394.16
410.13
454.08
410.13
428.12
424.14
421.15
390.18
403.15
421.71
404.20
404.20
436.19
392.16
455.14
376.16
2.74
2.88
2.86
3.01
3.18
1.96
2.52
2.11
2.59
3.00
2.26
2.12
1.53
0.77
2.06
%ABS, percentage of absorption; TPSA, topological polar surface area; NROTB, number of rotatable bonds; MW, molecular weight; LogP, logarithm of compound partition
coefficient between n-octanol and water; HBA, number of hydrogen bond donors; HBD, number of hydrogen bond acceptors.
acceptors), are important predictors of good oral bioavailabil-
ity.20,21 Membrane permeability and bioavailability is always asso-
ciated with some basic molecular descriptors such as logP
(partition coefficient), molecular weight (MW), or hydrogen bond
acceptors and donors counts in a molecule. In the present investiga-
tion, the number of rotatable bonds (NROTB) and Lipinski’s rule of
five were calculated for the compounds (4a–p).22 The ‘rule of 5’ is
based on a distribution of calculated properties among several
thousand drugs and some drugs will lie outside the parameter cut-
offs in the rule. Interestingly, only a small number of therapeutic
categories account for most of the United States Adopted Names
(USAN) drugs with properties falling outside these parameter cut-
offs. The orally active therapeutic classes outside the ‘rule of 5’
are: antibiotics, antifungals, vitamins and cardiac glycosides. Lipin-
ski suggest that these few therapeutic classes contain orally active
drugs that violate the ‘rule of 5’ because members of these classes
have structural features that allow the drugs to act as substrates
for naturally occurring transporters. When the ‘rule of 5’ is modified
to exclude these few drug categories only a very few exceptions can
be found. Among the NCEs between 1990 and 1993 falling outside
the double cutoffs in ‘the rule of 5’ were nine non-orally active
drugs and the only orally active compounds outside the double cut-
offs were seven antibiotics. Fungicides, protoazocides and antisep-
tics also fall outside the rule. Among the 41 USAN drugs with
molecular weight (MW) >500 and MLogP >4.15, there were nine
drugs in this class. Vitamins are another orally active class drug
with parameter values outside the double cutoffs and close to 100
vitamins fell into this category. Cardiac glycosides, an orally active
drug class also fall outside the parameter limits of the rule of 5.
Among 90 USANs with high MW and low MLogP there were two
cardiac glycosides. The Lipinski’s ‘rule of 5’ states that: poor absorp-
tion or permeation is more likely when, there are more than 5
H-bond donors (expressed as the sum of OHs and NHs), the MW
is over 500, the LogP is over 5 (or MLogP is over 4.15) and there
are more than 10 H-bond acceptors (expressed as the sum of Ns
and Os). Compound classes that are substrates for biological
transporters are exceptions to the rule. This rule is widely used as
a filter for drug-like properties. Furthermore, none of the com-
pounds (4a–p) violated Lipinski’s parameters, making them poten-
tially promising agents for antitubercular therapy. Number of
rotatable bonds is important for conformational changes of mole-
cules under study and ultimately for the binding with receptors
or channels. It is revealed that for passing oral bioavailability crite-
ria, number of rotatable bond should be 610.21 The compounds in
this series (4a–p) in general possess high number of rotatable
bonds (6–8) and therefore, exhibit large conformational flexibility.
The reaction sequence leading to the synthesis of tittle com-
pounds, viz. 1,5-dimethyl-2-phenyl-4-([5-(arylamino)-1,3,4-oxa-
diazol-2-yl]methylamino)-1,2-dihydro-3H-pyrazol-3-one (4a–p)
is summarised in Scheme 1. The 4-amino-1,5-dimethyl-2-phenyl-1,
NH
NH
2
N
N
COOH
N
Dry Acetone
K CO
N
ClCH COOH
2
O
+
O
2
3
3
2
1
POCl
ArNHCONHNH
3
2
N
NH
N
NH
N
O
R
N
O
4a-p
Scheme 1. Protocol for the synthesis of oxadiazoles.

M. J. Ahsan et al. / Bioorg. Med. Chem. Lett. 22 (2012) 969–972
971
Table 2
2-dihydro-3H-pyrazol-3-one 1 and chloroacetic acid 2 in dry
Antimycobacterial activity of the synthesized compounds
acetone and potassium carbonate was refluxed for 8 h to obtain
[(1,5-dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazol-4-yl)ami-
no]acetic acid 3.23 The [(1,5-dimethyl-3-oxo-2-phenyl-2,3-dihy-
dro-1H-pyrazol-4-yl)amino]acetic acid 3 was then refluxed with
appropriate semicarbazides in 5 ml phosphorus oxychloride to
furnish the titled compounds 4a–p. The substituted phenyl semi-
carbazides were synthesized as per the reported method.24,25 Thin
layer chromatography (TLC) was run throughout the reactions to
monitor the reaction using eluants, toluene/ethyl acetate/formic
acid (5:4:1) and petroleum ether/toluene/ethyl acetate (5:4:1).
The spots were located under iodine vapour/UV light. The yields
of the title compounds were ranging from 68% to 91% after recrys-
tallization with absolute ethanol. The structure assignments to the
new compounds were based on their elemental analysis and spec-
tral (FT-IR, 1H NMR and mass) data. In general, the FT-IR spectra of
the compounds afforded absorption at 1531–1567 cmÀ1 due to
C@N group, band at 1153–1174 cmÀ1 due to stretching of oxadiaz-
ole ring. In the Nuclear Magnetic Resonance spectra (1H NMR) the
signals of the respective protons of the synthesized title compounds
were verified on the basis of their chemical shift, multiplicities and
coupling constants in DMSO-d6. The spectra showed a singlet at d
1.91–2.23 ppm indicates the presence of CH3 group (pyrazolone
ring), a singlet at d2.27–2.39 ppm indicates the Ar-CH3 group, while
a singlet at d 2.49–2.61 ppm corresponds to N–CH3 group. 1H NMR
showed a doublet at d 3.93–4.07 ppm indicates the CH2, while a
singlet at d 4.9–5.2 ppm indicates the NH. A multiplet at d 6.38–
7.39 ppm was observed for aromatic protons and a singlet at d
7.39–8.42 ppm corresponds to Ar-NH. The elemental analysis
results were within 0.4% of the theoretical values.
N
NH
N
NH
N
O
R
N
O
4a-p
Compound
R
MIC (
l
g/mL)
MTBb
3.12
MTBa
4a
4b
4c
4d
4e
4f
4-Fluoro–
4-Chloro–
4-Bromo–
2-Chloro–
3-Chloro-4-fluoro–
3-Chloro-2-methyl–
4-Nitro–
0.78
6.25
6.25
10
6.25
10
3.12
12.5
10
12.5
>12.5
10
>12.5
6.25
>12.5
>12.5
NT
>12.5
>12.5
NT
12.5
12.5
>12.5
12.5
4g
4h
4i
4j
4k
4l
4m
4n
4o
4-Methyl–
4-Methoxy–
4-Ethoxy–
2,4-Dimethyl–
2,6-Dimethyl–
2,4-Dimethoxy–
4-Hydroxy–
4-Sulphamyl–
H
12.5
>12.5
12.5
10
>12.5
10
3.12
10
0.78
4p
Isoniazid
—
NT, Not tested.
a
Mycobacterium tuberculosis H37Rv.
INHR-TB.
b
All the synthesized compounds (4a–p) were tested for their
in vitro antimycobacterial activity against MTB and INHR-MTB by
agar dilution method using double dilution technique recom-
mended by the National Committee for Clinical Laboratory Stan-
dards.26 The MTB and INHR-MTB clinical isolates were obtained
from Tuberculosis Research Centre, Alwar, India. The MIC was
defined as the minimum concentration of compound required to
inhibit 90% of bacterial growth and MIC of the compounds were re-
derivatives discovered in this study may provide valuable thera-
peutic intervention for the treatment of tubercular disease and
promising lead for further optimization.
Acknowledgments
Authors are thankful to the Dr. V.K. Agrawal, Dr. (Mrs.) Manju
Agrawal and Mr. S.P. Garg, Alwar Pharmacy College, Alwar, Rajas-
than, India for providing research facilities. The authors also wish
to express their gratitude to Dr. (Mrs.) Shobha Tomar and Dr. K.P.
Singh, National Institute of Medical Sciences, Jaipur, India for their
guidance.
ported in Table
comparison.
2 with standard drug isoniazid (INH) for
Among the 16 synthesized compounds, six compounds were
found to be active with minimum inhibitory concentration of
0.78–6.25
lg/mL against MTB. The compound 4a was found to be
active against MTB at MICs of 0.78 and 3.12
lg/mL against MTB
and INHR-MTB, respectively. When compared with INH, the com-
pound 4a was found to be equipotent against MTB while 4-folds
more active against INHR-MTB. In the title compounds (4a–p),
the substitution on N-aryl group influenced the antitubercular
activity. The electron withdrawing groups such as 4-flourophenyl,
4-nitrophenyl, 4-sulphamyl produced more inhibitory and 4-chlo-
rophenyl and 4-bromophenyl produced moderate inhibitory activ-
ity while the electron releasing groups such as 4-methylphenyl,
4-methoxyphenyl, 4-ethoxyphenyl, 2,4-dimethylphenyl, 2,6-di-
methylphenyl, 2,4-dimethoxyphenyl showed less inhibitory activ-
ity. The compounds 4b, 4c, 4e, 4g and 4o showed moderate to good
Supplementary data
Supplementary data (experimental procedures and the com-
plete characterization data of the synthesized compounds) associ-
ated with this article can be found, in the online version, at
doi:10.1016/j.bmcl.2011.12.014.
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