The chemistry of mercapto- and thione- substituted 1,2,4-triazoles
CHAPTER-I INTRODUCTIONshodhganga.inflibnet.ac.in/bitstream/10603/72670/3... · 2018. 7. 8. ·...
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Chapter-I 1 1.1. General introduction to heterocyclic compounds Heterocyclic compounds constitute the largest and most varied family of organic compounds, and can have a variety of structure. These structures can be acyclic or cyclic. The cyclic systems containing only carbon atoms are called as carbocyclic and if at least one atom other than carbon forms a part of the ring system then it is designated as a heterocyclic compound [1, 2]. Though a number of heteroatoms are known to be part of the heterocyclic rings, the most common heteroatoms are nitrogen, oxygen or sulphur. Enormous number of heterocyclic compounds is known and this number is increasing rapidly. Heterocyclic compounds may be classified into aliphatic and aromatic. The aliphatic heterocyclics are the cyclic analogues of amines, ethers, thioethers, amides, etc., and their properties are particularly influenced by the presence of strain in the ring. The aromatic heterocyclic compounds, in contrast, are those which have heteroatoms in the ring and behave in a manner similar to benzene in some of their properties. These compounds follow the Huckels rule which states that cyclic conjucted and planar systems having (4n+2) π electron are aromatic. Several nitrogen and sulfur containing heterocyclic compounds namely triazoles [3], thiazoles [4], oxadiazoles [5], pyrimidines and isoxazoles [6] display important biological activities. Hence the syntheses of new compounds incorporating these moieties with known pharmacological properties have gained impetus in recent years. The piperazine nucleus is capable of binding to multiple receptors with high affinity and therefore piperazine has been classified as a privileged structure [7]. Various compounds such as alkaloids, antibiotics, essential amino acids, the vitamins, hemoglobin, the hormones, a large number of synthetic drugs and dyes contain heterocyclic ring systems. Knowledge of heterocyclic chemistry is useful in biosynthesis and in drug metabolism as well. There are a large number of synthetic heterocyclic compounds with additional important applications, and many are valuable intermediates in synthesis. Some heterocyclic compounds are shown below. CHAPTER-I INTRODUCTION
Transcript of CHAPTER-I INTRODUCTIONshodhganga.inflibnet.ac.in/bitstream/10603/72670/3... · 2018. 7. 8. ·...
Chapter-I
1
1.1. General introduction to heterocyclic compounds Heterocyclic compounds constitute the largest and most varied family of
organic compounds, and can have a variety of structure. These structures can be acyclic
or cyclic. The cyclic systems containing only carbon atoms are called as carbocyclic
and if at least one atom other than carbon forms a part of the ring system then it is
designated as a heterocyclic compound [1, 2]. Though a number of heteroatoms are
known to be part of the heterocyclic rings, the most common heteroatoms are nitrogen,
oxygen or sulphur. Enormous number of heterocyclic compounds is known and this
number is increasing rapidly. Heterocyclic compounds may be classified into aliphatic
and aromatic. The aliphatic heterocyclics are the cyclic analogues of amines, ethers,
thioethers, amides, etc., and their properties are particularly influenced by the presence
of strain in the ring. The aromatic heterocyclic compounds, in contrast, are those which
have heteroatoms in the ring and behave in a manner similar to benzene in some of
their properties. These compounds follow the Huckels rule which states that cyclic
conjucted and planar systems having (4n+2) π electron are aromatic.
Several nitrogen and sulfur containing heterocyclic compounds namely
triazoles [3], thiazoles [4], oxadiazoles [5], pyrimidines and isoxazoles [6] display
important biological activities. Hence the syntheses of new compounds incorporating
these moieties with known pharmacological properties have gained impetus in recent
years. The piperazine nucleus is capable of binding to multiple receptors with high
affinity and therefore piperazine has been classified as a privileged structure [7].
Various compounds such as alkaloids, antibiotics, essential amino acids, the vitamins,
hemoglobin, the hormones, a large number of synthetic drugs and dyes contain
heterocyclic ring systems. Knowledge of heterocyclic chemistry is useful in
biosynthesis and in drug metabolism as well. There are a large number of synthetic
heterocyclic compounds with additional important applications, and many are valuable
intermediates in synthesis. Some heterocyclic compounds are shown below.
CHAPTER-I INTRODUCTION
Chapter-I
2
In medicinal chemistry they are commonly used as templates to design biologically
active agents [8, 9]. Heterocyclic chemistry is a vast and expanding area of chemistry
because of the obvious applications of compounds derived from heterocyclic rings in
pharmacy, medicine, agriculture and other fields. Moreover, in the past 20 years the drug-
discovery process has undergone extraordinary changes and high-throughput biological
screening of potential drug candidates has led to an ever increasing demand for novel drug-
like compounds [10]. The pharmacological activity of drugs depends mainly on interaction
with their biological targets, which have a complex three-dimensional structure, and
molecular recognition is guided by the nature of the intermolecular interactions.
Compounds with heterocyclic ring are inextricably woven into the most basic
biochemical processes of life. From thousands of compounds thus synthesized, only one or
two may reach to therapeutically useful form. Though, in the early stages of development,
an accurate evaluation is not achieved, a low cost screening which adequately appraises the
potency of a compound is of great economical importance. Most biologically active
compounds are heterocyclic organic compounds which have a ring structure containing
atoms such as sulfur, oxygen or nitrogen in addition to carbon, as part of the ring. There
are large number of synthetic heterocyclic compounds with important practical
applications, as dyestuffs, copolymers, solvents, photographic sensitizers and developers,
as antioxidants and vulcanization accelerators in the rubber industry, and many are
valuable intermediates in synthesis. There are also a vast number of pharmacologically
active heterocyclic compounds, many of which are in regular clinical use. Some of these
are natural products, for example antibiotics such as penicillin and cephalosporin, alkaloids
such as vinblastine, ellipticine, morphine and reserpine.
Chapter-I
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1.1.1. Introduction to triazoles
The search for new agent is one of the most challenging tasks to the medicinal
chemist. The synthesis of high nitrogen containing heterocyclic systems has been
increasing over the past decade because of their utility in various applications such as
propellants, explosives, pyrotechnics and especially chemotherapy. In recent years, the
chemistry of triazoles and their fused heterocyclic derivatives has received considerable
attention owing to their synthetic and effective biological importance. The derivatization of
triazole is considered to be based on the phenomenon of bioisosterism in which
replacement of oxygen of oxadiazole nucleus with nitrogen atom yields triazole analogue.
There are two possible isomers of triazole (1, 2) depending on the position of nitrogen
atom in the ring and are numbered as shown below.
1, 2, 3-Triazole 1, 2, 4-Triazole
Out of the two triazoles, 1,2,4-triazole have drawn great attention to medicinal
chemists from two decades due to its wide variety of activity [11], low toxicity, good
pharmacokinetic and pharmacodynamics profiles. 1,2,4-Triazole derivatives usually exist
in solid form. 3,4,5-Substituted 1,2,4-triazole derivatives melt with thermolysis at high
temperature when heated at 316 °C for 30 minutes [12]. 1,2,4-Triazole derivatives are
readily soluble in polar solvents and only slightly soluble in nonpolar solvents, however,
the solubility in non-polar solvents can be increased by substitution on the nitrogen atom.
Due to annular prototropic tautomerism, 1,2,4-triazoles without substituents on the ring
nitrogen atoms, a priori can exist in three forms. Analysis of the crystal data [13] showed
that with rare exceptions [14], unsymmetrical 3,5-disubstituited 1,2,4-triazoles exists in the
solid state as tautomer bearing an electron substituent at position 3 and an electron-donor
substituent at position 5. The precise experimental data on the annular tautomerism in
1,2,4-triazoles in solution are limited [15]. Tautomeric form with the hydrogen atom at N-1
adjacent to carbon atom bearing relatively less electronegative group was found to
predominate in the solution.
Chapter-I
4
N
NH
R1
N
R2 N
N
R1 R2
HN
NH
NN
R1 R2
Among the substituted 1,2,4-triazoles, 3-mercapto-1,2,4-triazoles exists in two
tautomeric forms (3, 4), because the labile hydrogen may be attached either to the nitrogen
or to the sulfur atom. It exhibits thione-thiol tautomeric forms shown below.
NH
NH
R
N
S NH
N
R SH
N
3 4
1, 2, 4-Triazole derivatives undergo Mannich reaction (Mannich reaction is a 3-
component condensation reaction involving an active hydrogen containing formaldehyde
and a secondary amine). The amino methylation of aromatic substrates by the Mannich
reaction is of considerable importance for the synthesis and modification of biologically
active compounds [16]. Amino group at position 4 of 1,2,4-triazole derivative undergoes
reaction with 4-methoxybenzaldehyde [17] and eliminate water molecule with the
formation of Schiff base. Compounds having Schiff base structure may exist as E/Z
geometrical isomers about the –N=CH- double bond. Compounds containing imine bond
are present in higher percentage in dimethyl-d6 sulfoxide solution in the form of
geometrical E isomer about –N=CH- double bond. The Z isomer can be stabilized in less
polar solvents by an intramolecular hydrogen bond. It is known that 1,2,4-triazole and 4,5-
dihydro-1H triazol-5-one rings have weak acidic properties and so some 1,2,4-triazole and
4, 5-dihydro-1H derivatives were titrated potentiometrically with tetrabutylammonium
hydroxide in non-aqueous solvents such as acetonitrile, isopropyl alcohol and N, N-
dimethylformamide, and the half-neutralization potential values and the corresponding pKa
values of the compounds were determined [18].
Chapter-I
5
Triazoles are potential bioactive agents due to their wide spectrum of therapeutic
importance. Drug molecules having 1,2,4-triazole nucleus with good activity are listed
under,
O
O
N
NN
ClCl
Azaconazole
Chapter-I
6
OOHO
HO OH
N NN
H2NO
Rifaviran
Literature survey reveals that various 1,2,4-triazole derivatives display wide
spectrum of biological activities. 3-Amino-1,2,4-triazole was the first 1,2,4-triazole to be
manufactured on large scale from aminoguanidine formate [19]. A number of biological
activity such as antimicrobial [20], anti-inflammatory [21], anticonvulsant [22], anticancer
[23], antimycobacterial [24], antioxidant [25] and antimalarial [26] have been associated
with N-substituted triazoles attached with different heterocyclic nuclei. It has been noticed
continuously over the years that interesting biological activities were associated with
triazole derivatives.
Jubie et al. [27] have synthesized some novel ciprofloxacin analogues as
antimicrobial agents. Ciprofloxacin have been incorporated to the new series of Schiff
bases of 1,2,4-triazole via Mannich reaction. The new compounds have been evaluated in
vitro for their antimicrobial activity against B. subtilis, K. pneumoniae, and P. aeruginosa
at 10 µg/ml concentration. All the compounds showed in vitro gram positive and gram
negative activity and generally comparable or superior to that of reference ciprofloxacin. Bijul lakshman et al. [28] synthesized twenty eight derivatives of 4-amino-5-substituted
aryl-3-mercapto-1,2,4-triazoles and these compounds have been tested in vitro against
Rhizoctonia solani, Sclerotium rolfasii, Fusarium oxysporum, Pythium aphanidermatum,
Puccinia reconditeand Bipolaris sorokiana
Veena Vani Ktla et al. [29] synthesized novel 1,2,4-triazole derivatives by
cyclization of 1-(2-(3-chloro-4-methyl-2-oxo-2H-chromen-7-yloxy)-4-substituted phenyl
thiosemicarbazide in alkaline medium. The newely synthesized compounds were
characterized by 1H NMR, IR and Mass spectral data. Further, all the compounds were
screened for their antibacterial activity against Bacillus substilis, Escherichia coli and
Pseudomonas aeruginosa, and antifungal activity against Aspergillus Niger and
Chapter-I
7
Aspergillus flaves using flucinzole as standard drug. The screening results revealed that,
most of the compounds were found to exhibit significant antimicrobial activity. Tomasz
Plech et al. [30] described a fast and efficient method for the synthesis of some 1,4-
disubstituted thiosemicarbazide derivatives. The reaction of 3-chlorobenzoic acid
hydrazide with various aryl isothiocyanates gave thiosemicarbazide derivatives in good
yield. The cyclization of compounds in the presence of 2 % NaOH resulted in the
formation of compounds containing 1,2,4-triazole ring. A series of new Mannich bases
related to the structure 1,2,4-triazole has also been synthesized. An attempt was made to
clarify the influence of nature/position of substituents on antibacterial activity of
compounds described. A variety of novel s-triazoles and their Mannich bases were
prepared as well. Some derivatives showed promising antimicrobial activity, especially
against Gram-postive bacteria. Monika et al. [31] synthesized derivatives of the
9-substituted-3-aryl-5H,13aH-quinolino[3,2-f][1,2,4]triazolo[4,3-b][1,2,4]triazepines from
the 5-aryl-3,4-diamino-1,2,4-triazols and 2-chloro-3-formyl quinolones using catalytic
amount of p-TsOH and N,N-dimethylformamide as an energy transfer medium using
microwave, and the synthesized compounds were screened for antifungal activity against
Aspergillus flaves, Aspergillus niger, Rhizopus species and Pencillum notatum species by
paper disc technique against 500 and 1000 µg/ml concentrations.
Aniket et al. [32] synthesized Schiff’s bases of 5-mercapto-3-(3-pyridyl)-4H-1,2,4-
triazole-4-yl-thiosemicarbazide by microwave assisted method. The synthesized
compounds have been evaluated in vitro for their antibacterial, antifungal and
anticonvulsant activities. Mali et al. [33] synthesized 5-(N-substituted
carboxamidomethylthio)-3-(3'-pyridyl)-1,2,4-triazole and their antifungal activity was
carried out against C. albicans and A. niger at the concentrations of 50 and 100 µg/mL
using Fluconazole as the standard, and in vitro antitubercular activity was done at 50
µg/mL against Mycobacterium tuberculosis H37 Rv. A series of new coumarin based
1,2,4-triazoles were synthesized and evaluated for antimicrobial activitiy in vitro against
Gram-positive bacteria (Staphylococcus aureus, MRSA, Bacillus substilis and micrococcus
luteus), and Gram-negative bacteria (Escheichia coli, Proteus vulgaris, Salmonella typi
and Shigella dysenteriae) as well as fungi (Candida albicans, Sacchoromyces cerevisiae
and Aspergillus fumigatus) by two fold serial dilution tehniques [34].
Siddiqui et al. [35] synthesized some 4-[{1-(aryl)methylidene}-amino]-3-(4-
pyridyl)-5-mercapto-4H-1,2,4-triazole starting from isonicotinic acid hydrazide, potassium
Chapter-I
8
hydroxide and carbon disulphide, and screened for analgesic and antipyretic activities.
Analgesic activity evaluated by tail-flick method in rats at a dose of 25 mg/kg and
antipyretic activity was evaluated using Brewer’s yeast-induced pyrexia in rats. Fever was
induced by subcutaneously administered 20 ml/kg of 20 % aqueous suspension of
Brewer’s yeast in normal saline, below the nape of the neck and rectal temperature was
recorded with a clinical thermometer. Aspirin (300 mg/kg) was used as a standard drug for
comparing the antipyretic action of compounds. Singha et al. [36] synthesized a series of
potential bioactive 4-amino-5-mercapto-3-aryl-1,2,4-triazoles according the literature
methods. The synthesized compounds were characterized by spectroscopy and evaluated
their anticancer activity aganist EAC (Ehrlich Ascites Carcinoma). Compounds were given
at a dose of 25 mg/kg body weight intraperitoneally. Groups were found to reduce tumer
volume, viable cell count and increase the tumer weight (%) inhibition, ascites cells (%)
inhibition and non-viable cell count and increase in life span. All the compounds exhibited
the significant anticancer activity compared to control and some of the compounds were
found to be most potent.
A series of [1,2,4]triazolo[1,5-a]pyridines were synthesized and characterized by
Wang et al [37]. The in vitro antiproleferative activity was evaluated by MTT against three
human cancer cell lines, HCT-116, U-87 MG and MCF-7. The SAR of target compounds
was preliminarily dicussed. Some of the compounds with potent antiproleferative activity
were tested for their effects on the AKT and p-AKT473. The anticancer effect was
evaluated in mice bearing sarcoma S-180 model. The results suggest that the title
compounds are potent anticancer agents. Ying-Chao Duan et al. [38] synthesized a series
of novel 1,2,3-triazolethiosemicarbazide hybrids and their antiproleferative activity was
evaluated against four human cancer lines. The results showed that number of hybrids
exhibited potent activity in selected human cancer cell lines. Among them few compounds
showed broad spectrum anticancer activity with IC50 values ranging from 0.76 to 20.84
mm. Evidences of cell cycle arrest and apoptosis induction were obtained for the most
effective compounds.
Olcay Bekircan et al. [39] synthesized a series of 4-arylideneamino-4H-1,2,4-
triazoles and 4-(1-aryl)ethylidene-4H-1,2,4-triazoles by the treatment of 4-amino-1,2,4-
triazole with certain aldehydes and ketones. Compounds have been reduced with NaBH4 to
yield corresponding 4-arylmethylamino-4H-1,2,4-triazoles and 4-(1-aryl)ethylamino-4H-1,
2, 4-triazoles. The chalcones were reacted with thiourea in the presence of KOH in ethanol,
Chapter-I
9
which led to the formation of dihydropyrimidine derivatives as a beneficial antimicrobial,
anticonvulsant and anticancer agents by Khanage et al [40]. All the synthesised
compounds were screened for their in vitro antimicrobial activity by agar well method and
their anticonvulsant activity by the MES model. Anticancer activity of two newely
synthesised heterocycles was evaluated against 60 cell lines of different human tumer at a
single dose of 10-5 M.
Number of articles were found for the anticonvulsant potential of 1,2,4-triazole
where substitution on 2,3,5 positions were done. Recently, anticonvulsant activity of
clubbed thiazolidinone-barbituric acid and thiazolidinone-triazole derivatives has been
reported by Shiradkar et al [41]. 3-(2-Chloroacetyl)-2-arylimino-5-[(Z)-arylmethylidene]-
1,3-thiazolan-4-ones on treatment with 5-(1-phenoxyethyl)-4H-1,2,4-triazole-3-thiol in
identical conditions provided a set of bulkier derivatives which have also shown the
anticonvulsant potential. Pradeep Goyal et al. [42] synthesized some new derivatives of 3-
substituted-4H-1,2,4-triazoles. All the synthesized compounds were evaluated for anti-
inflammatory activity and acute toxicity. Most of the compounds showed potent and
significant results compared to standard ibuprofen.
Krzysztof Sztanke et al. [43] reported the synthesis of ethyl 1-(7-phenyl-2H-3, 5, 6,
7-tetrahydroimidazo [2,1-c] [1,2,4]triazol-3-yl)formate. The influence of ethyl 1-(7-
phenyl-2H-3,5,6,7-tetrahydro-imidazo[2,1-c][1,2,4]triazol-3-yl)formaten on human
adenovirus 5 (Ad-5) and human enterovirus (Echo-9) replication has been investigated.
The activity against the selected DNA (Ad-5) and RNA (Echo-9) viruses and the
cytotoxicity towards normal GMK (Green Monkey Kidney) cells were also determined.
1,2,4-Triazoles were used as analytical reagents for the determination of boron [44],
antimony [45] and cobalt [46]. Other triazoles find many synthetic uses as halogenating
agents [47] or as activating polymeric reagents [48]. It has been noticed that, modification
on triazole moiety displayed valuable biological activities, and can be utilized as potent
therapeutic agents in future. Thus, the quest to explore many more modifications on
triazole moiety needs to be continued for the use of mankind.
1.1.2. Introduction to oxadiazoles
Oxadiazole is a heterocyclic compound containing an oxygen atom and two nitrogen
atoms in a five-membered ring. It is derived from furan by substitution of two methylene
Chapter-I
10
groups (=CH) with two pyridine type nitrogens (-N=). There are four known isomers, 1,3,4-
oxadiazole (5), 1,2,4-oxadiazole (6), 1,2,3-oxadiazole (7) and 1,2,5-oxadiazole (8). However,
1,3,4-oxadiazole and 1,2,4-oxadiazole are better known, and more widely studied because of
their many important chemical and biological properties, but 1,2,3-isomer is unstable and
reverts to the diazoketone tautomer.
NN
O1
2
34
5
N
ON
1
2
34
5
N
ON
1
2
34
5 NO
N
1
2
34
5
5 6 7 8
1,3,4-oxadiazole is thermally stable aromatic molecule [50], and it has become an
important construction motif for the development of new drugs. Oxadiazole rings have been
introduced into drug discovery programs for several different purposes. In some cases, they
have been used as an essential part of the pharmacophore, favorably contributing to ligand
binding. In other cases, oxadiazole moieties have been shown to act as a flat, aromatic linker
to place substituent’s in the appropriate orientation, as well as modulating the molecular
properties by positioning them in the periphery of the molecules. Oxadiazoles display
interesting hydrogen bond acceptor properties, and it will be shown that the regioisomers
display significantly different hydrogen bonding potentials. Compounds containing 1,3,4-
oxadiazole cores have a broad spectrum of biological activity including antibacterial,
antifungal, analgesic, anti-inflammatory, antiviral, anticancer, antihypertensive,
anticonvulsant and anti-diabetic properties. During the last decades, synthesis and
transformations of five member heterocyclic compounds has received considerable attention
and importance due to their remarkable and wide variety of applications. 2,5-Disubstituted-
1,3,4-oxadiazole and its derivatives constitute an important family of heterocyclic
compounds. Due to their remarkable unique properties, 1,3,4-oxadiazole and its derivatives
have been frequently employed in drug synthesis, various commercial and industrial
applications. The 1,3,4-oxadiazole ring carrying substitution in an appropriate position and
substituent with a nucleophilic center are excellent precursors for further synthesis of
heterocyclic compounds. For example, ring rearrangements for the synthesis of five and six
membered heterocycles. 5-Substituted-2-mercapto-1,3,4-oxadiazoles are interesting and
Chapter-I
11
important class of compounds. They have general formula (9) (R = alkyl or aryl group).
These compounds are known to exist in tautomeric thiol (9) and thione (10) forms.
NN
OR SH
NHN
OR S
9 10
In recent past, extensive study of 1,3,4-oxadiazole derivatives show diverse biological
activities. Substituted 1,3,4-oxadiazoles and 5-substituted-2-mercapto-1,3,4-oxadiazoles are
of considerable pharmaceutical interest. They display remarkable biological activities. For
instance, 2-amino-1,3,4-oxadiazoles have been reported as muscle relaxants [51], analgesic
[52-58], anti-inflammatory [59-62], anticonvulsive [63-68] and diuretic properties. Whereas
derivatives of 2-mercapto-1,3,4-oxadiazoles have been reported to exhibit antimicrobial [69-
70] and anticancer [71] activities. As part of interest in heterocyclics derived from Schiff
bases that have been explored for developing pharmaceutically important molecules, 2-
azetidinones, 4-thiazolidinones and fused thiazolidinones have played a pivotal role in
medicinal chemistry. Moreover, they have been studied extensively because of their ready
accessibility, diverse reactivity and broad spectrum of biological activity.
Some examples of compounds containing the 1,3,4-oxadiazole unit currently used in
clinical medicine are: Raltegravir (11), antiretroviral drug and zibotentan (12), are anticancer
agents. Vadrin a leprostatic drug, eudromil a hypnotic drug, nesapidil (13) used in anti-
arrhythmic therapy, furamizole (14), a nitrofuran derivative has strong antibacterial activity,
tiodazosin (15), an antihypertensive drug and ataluren (16) used for the treatment of cystic
fibrosis.
N
N
O
OHHN
O
NN
OHN
O
F
11
NN
O
N
SNH
N
N
O
O
O
12
Chapter-I
12
ON
N
O
OHN
N
O
N N
O NH2
O
O
O2N
13 14
NO
NS
NN
N
NO
O
H2N
O
15
O
N
NOH
O
F
16
A wide variety of 1,3,4-oxadiazole derivatives have been synthesized and evaluated
their broad spectrum of biological activity. Farshori et al. [72] synthesized 5-
alkenyl/hydroxyalkenyl-2-phenylamine-1,3,4-oxadiazoles and tested for in vitro
antimicrobial activities by disc diffusion method. Among the synthesized compounds, few
compounds were found to be more active against the tested strain and were compared with
griseofulvin as standard drug. 5-(3,4,5-Trimethoxyphenyl)-2-sulfonyl-1,3,4-oxadiazole
derivatives were synthesized and tested for their antifungal activity against fungal strain
[73]. Among the tested compounds, few compounds exhibiting promising antifungal
activity even better than that of the commercial fungicide hymexazol.
Mishra et al. [74] synthesized a series of oxadiazole derivatives and tested for their
antimicrobial activity by cup and plate method. Among the tested compounds few showed
promising antibacterial activity against Gram +ve bacteria and Gram –ve bacteria when
compared to standard drugs ofloxacin and levofloxacin. A series of novel unsymmetrical
2,5-disubstituted 1,3,4-oxadiazoles [75] have been synthesized and the final compounds
were tested for their antibacterial and antifungal activities. Among the tested compounds,
few showed maximum antibacterial activity and were compared with ciprofloxacin as
standard drug. Arora et al. [76] synthesized a series of 2,5-disubstituted oxadiazole based
Chapter-I
13
chalcone derivatives and their antimicrobial activity by cup-plate method and compared
with vancomycin as standard drug. Some compounds showed promising result and were
considered for further studies. A series of 5-(1-benzoylamino-2-(substituted phenyl)vinyl)-
2-amino-1,3,4-oxadiazole derivatives were synthesized by Rajitha et al [77]. Antimicrobial
activity of these compounds was tested by using filter paper disc. These compounds
showed moderate activity.
A series of novel 1,3,4 oxadiazole derivatives were synthesized by Vipul et al. [78],
and screened for antimicrobial activity against Staphylococcus aureus, Bacillus subtilis,
Pseudomonas aeruginosa and Escherichia coli. Biological results indicated that the
synthesized compounds showed a broad spectrum activity against tested microorganisms.
Patel et al. [79] synthesized 2-benzylsulfanyl-nicotinic acid based 1,3,4-oxadiazoles and
these were showed promising activity against Escherichia coli compared to ampicillin. A
series of novel 2-[5-(substituted phenyl)-[1,3,4] oxadiazol-2-yl]-benzoxazoles have been
synthesized [80], and their antimicrobial activity has been evaluated against various
bacteria and fungi, and almost all compounds showed good activity against bacteria and
fungi.
Afshin Zarghi et al. [81] reported a new series of 2-substituted-5-[2-(2-
halobenzyloxy)phenyl]-1,3,4-oxadiazoles. The anticonvulsant activity of all the
synthesized compounds was evaluated against maximal electroshock induced seizures
(MES) and subcutaneous pentylenetetrazole (sc PTZ) induced seizure models in mice. The
size and nature of groups at C-2 position of 1,3,4-oxadiazole ring are very important on
anticonvulsant activity in both PTZ and MES models. In addition, the size of electron
withdrawing substituent at ortho position of benzyloxy moiety is also important for their
anticonvulsant effects. A series of isoninicotinic acid hydrazide incorporated derivatives of
1,3,4-oxadiazoles has been synthesized by Sadaf Jamal Gilani et al [82]. 1,3,4-Oxadiazole
ring systems were active in MES test at a dose of 300 mg/kg indicative of their ability to
protect the seizure spread. The compounds showed protection at a dose of 100 mg/kg after
0.5 h. These compounds also showed protection after 4h but at a higher dose of 300 mg/kg.
A new series of 2-substituted-5-{2-[(2-halobenzyl)thio)phenyl}-1,3,4-oxadiazoles
was synthesized by Afshin Zarghi et al [83]. The designed compounds contain the main
essential pharmacophore for binding to the benzodiazepine receptors. The structure-
activity relationship study of these compounds indicated that the introduction of an amino
Chapter-I
14
group at position 2 of 1,3,4-oxadiazole ring and a fluoro substituent at the ortho position of
the benzylthio moiety had the best anticonvulsant activity. Anticonvulsant effects of active
compounds were antagonized by flumazenil, a benzodiazepine antagonist, which
establishes the involvement of benzodiazepine receptors in these effects.
A series of novel N1-{5-[(naphthalene-2-yloxy)methyl]-1,3,4-oxadiazol-2-yl}-N4-
[1-(4-substitutedphenyl)(phenyl)methanone]-semicarbazones were designed and
synthesized on the basis of semicarbazone based pharmacophoric model to meet the
structural requirements necessary for anticonvulsant activity by Harish Rajak et al [84].
The anticonvulsant activity of the compounds was investigated using maximal
electroshock seizure (MES), subcutaneous pentylenetrtrazole (scPTZ) and subcutaneous
strychnine (scSTY) models. A novel semicarbazone based 2,5-disubstituted 1,3,4-
oxadiazoles possessing four vital structural features hydrophobic aryl ring system,
hydrogen binding domain, electron donor moiety and distal aryl ring required for
anticonvulsant activity are disclosed. The aryl semicarbazones have been found to
possess anticonvulsant activity through GABA mediation.
A novel series of 3-[5-(4-substituted) phenyl-1,3,4-oxadiazole-2yl]-2
phenylquinazoline-4(3H)-ones [85] have been synthesized and screened for their
anticonvulsant and neurotoxic activities. After i.p. injection to mice at doses of 30, 100,
and 300 mg/kg body weight and they were examined in the maximal electroshock induced
seizures (MES) and subcutaneous pentylenetetrazole (scPTZ) induced seizure models in
mice. The compounds showed protection at a dose of 30,100 and 300 mg/kg after 0.5 h and
4.0 h. Two novel series of (Z)-N-[5-(4-substituted)phenyl-1,3,4-oxadiazol-2-yl]-2-[(Z)-3,7-
dimethylocta-2,6-dien-1-ylidene]hydrazinecarboxamide and 5-(4-substituted)phenyl-N-
[(Z)-3,7-dimethylocta-2,6-dien-1-ylidene]-1,3,4-oxadiazol-2-amine [86] have been
synthesized and screened for their anticonvulsant and neurotoxic activities. After i.p.
injection to mice at doses of 30,100 and 300 mg/kg body weight, 2,5-disubstituted-1,3,4-
oxadiazole analogues were examined in the maximal electroshock induced seizures (MES)
and subcutaneousmetrazole (ScMET) induced seizure models in mice. Amongst all the
compounds, some compounds exhibited activity at 100 mg/kg body weight at 0.5 and 4 h
in ScMET method. The neurotoxicity was assessed by rotorod method and some of the
compounds were found to be safe at maximum administered dose.
Chapter-I
15
Tabatabai et al [87] synthesized 2-(2-phenoxy)phenyl-1,3,4-oxadiazole derivatives.
Anticonvulsant activity of the synthesized compounds was determined by
pentylenetetrazole-induced lethal convulsion test and showed that the introduction of an
amino substituent in position 5 of 1,3,4- oxadiazole ring generates considerable effect. The
results are in agreement with SAR of benzodiazepine receptor ligands since the elimination
of electronegative substituent in position 2 of phenoxy ring or position 4 of phenyl ring
reduces the anticonvulsant activity. A series of new 1,2,4-oxadiazole derivatives [88]
containing 3,4-dihydro-2H-chromen-2-amine moiety were synthesized and screened for
their anticonvulsant properties. Few of the compounds exhibited excellent anticonvulsant
activity as compared to the standard drug diazepam. A series of novel 1,3,4-oxadiazole
derivatives of phthalimide [89] were synthesized and evaluated for their anticonvulsant and
neurotoxicity studies. All the compounds were active in MES screen and less neurotoxic
than phenytoin. Compounds having methoxy substitution at para position of the distal aryl
ring emerged as most promising anticonvulsant agent with low neurotoxicity.
A series of new 1,3,4-oxadiazole and 1,2,4-triazole derivatives were synthesized by
Yan et al [90]. The compounds were evaluated for their antiproleferative activity against
K562 (human erythromyeloblastoid leukemia cell line), MDA-MB-231 (human breast
adenocarcinoma cell line), HT29 (human colon adenocarcinoma grade II cell line) and
HepG2 (human breast adenocarcinoma cell line) in vitro. The results showed that 7
compounds displayed inhibitory activities against K562 with the inhibition rate more than
50 %. Especially, few compounds exhibited the most potent activity against K562 with
85% inhibition ratio and could be used as lead compounds to search newer 1,3,4-
oxadiazole derivatives as antiproliferative agents. Luo et al. [91] synthesized a series of
novel 1,3,4-oxadiazole derivatives based on benzisoselenazolane and tested for
antiproleferative activity in vitro against the human cancer cell lines, SSMC-7721 (human
liver cancer cell), MCF-7 (human breast cancer cell) and A549 (human lung cancer cell).
All the compounds obtained exhibited antiproliferative activity and showed selective
cytotoxicity against different cancer cells.
Samir et al. [92] explained the synthetic strategies and characterization of some
novel 1,3,4-oxadiazole derivatives carrying different pharmacophores and heterocyclic
rings that are relevant to potential antitumer and cytotoxic activities. The antitumer activity
of the newly synthesized compounds was evaluated according to the protocol of the
National Cancer Institute (NCI) in vitro disease-oriented human cells screening panel
Chapter-I
16
assay. The results revealed that five compounds displayed promising in vitro antitumer
activity in the 4-cell lines assay. Incorporating a thiazole ring to 1,3,4-oxadiazole skeleton
resulted in better antitumor activity than those displayed by the pyrazole and thiophene
ring systems. The results of the anticancer screening revealed that five compounds were
found to exhibit variable degrees of anticancer activity against the four cell lines used.
Pushpan et al. [93] designed and synthesized a novel combinatorial library of S-
substituted-1,3,4-oxadiazole bearing N-methyl-4-(trifluoromethyl) phenyl pyrazole moiety
and tested for in vitro cytotoxic activity by MTT assay. Amongst the tested compounds, few
compounds showed the most promising anticancer activity with IC50 value of 15.54 Mm in
MCF-7 cells compared to doxorubicin as standard drug. Three dimensional quantitative
structure activity relationship (3D QSAR) study by means of partial least square regression
(PLSR) method was performed on a series of 3-(aryl)-N-(aryl)-1,2,4-oxadiazole-5-amines as
antiproliferative agents using molecular design suite by Sanmati et al [94]. This study was
performed with 20 compounds using sphere exclusion (SE) algotitham and manual selection
method used for the division of the data set into training and test set. The molecular field
analysis (MFA) contour plots provided further understanding of the relationship between
structural features of the substituted oxadiazole derivatives and their activities, which should
be applicable to design newer potential antiproliferative agent.
1.1.3. Introduction to pyrimidines
Pyrimidine is a heterocyclic aromatic organic compound similar to benzene and pyridine, containing two nitrogen atoms at positions 1 and 3 of the six-member ring (17). It is isomeric with two other forms of diazine.
Pyrimidine has many properties in common with pyridine, as the number of nitrogen atoms in the ring increases, the ring electrons become less energetic and electrophilic aromatic substitution gets more difficult while nucleophilic aromatic substitution gets easier. Pyrimidines can also be prepared within the laboratory by organic synthesis. One
Chapter-I
17
method is the classic Biginelli reaction. Three nucleobases found in nucleic acids (cytosine, thymine, and uracil) are pyrimidine derivatives [95].
NH
N
NH2
O
Cytosine Thymine Uracil
Pyrimidines and its derivatives are integral part of DNA and RNA, and found to be assosiated with diverse biological activities. The substituted pyrimidines are complex molecules because of natural substituents. Uracil and thyamine may be considered to contain neutral urea unit or acidic imide moiety. Thymine is also referred to as 5-methyluracil. The metabolism of these pyrimidines is unique and important to understand
both biochemical utilization of these compounds and drug metabolism of pyrimidine derivatives. Uracil is converted into a useful uridylic acid needed for the synthesis of RNA. Thymine is metabolized by conjugation via salvage pathway with PRPP to the thymine ribosyl-5-phosphate. This form of thymidylic acid can be utilized in specific RNA molecule. In a similar manner cytosine is conjugated with PRPP to yield cytosine-5-monophosphate or cytidylic acid. Pyrimidine is the most important member of all the
diazines as this ring system occurs widely in living organisms [96-100]. Pyrimidine and its derivatives have gained prominence because of their potential pharmaceutical values. Several pyrimidine derivatives play vital role in many physiological actions. They are among those molecules that make life possible as being some of the building blocks of DNA and RNA.
Pyrimidine is considered to be a resonance hybrid of the charged and uncharged
cannonical structures and its resonance energy has been found to be less than benzene or
pyridine [101-104]. The naturally occurring pyrimidine derivative was first isolated by
Gabrial and Colman in 1870, and its structure was confirmed in 1953 as 5-β-D-gluco-
pyranoside of divicine. Pyrimidines and their derivatives are considered to be important for
drugs and agricultural chemicals. The use of pyrimidines is critical to successful treatment of
various diseases. Many pyrimidine derivatives used for thyroid drugs and leukaemia are
shown below.
Chapter-I
18
The pyrimidine moiety is a versatile lead molecule in pharmaceutical development and has a wide range of biological activities. In the fast few years, the therapeutic interest of pyrimidine derivatives in pharmaceutical and medicinal field has been given a great attention to the medicinal chemist. Literature survey reveals that pyrimidine derivatives are well known to have antimicrobial [105-107], antimalarial [108], anticancer [109-112], anti-
inflammator, analgesic [113] and antimycobacterial activities [114-116]. In recent years, the extensive studies have been focused on pyrimidine derivatives because of their diverse chemical reactivity, accessibility and wide range of biological activities.
An efficient method has been described by Shantaram et al. [117] for the synthesis of 6-(substituted aryl)-4-(3,5-diphenyl-1H-1,2,4-triazol-1-yl-1,6-dihydropyrimidine-2-thiol as beneficial antimicrobial, anticonvulsant and anticancer agents. Compounds were screened for their in vitro antimicrobial activity by agar well method and their anticonvulsant
activity by the MES model. Anticancer activity of the two newly synthesized heterocycles was evaluated at National Cancer Institute (NCI). USA against 60 cell lines of different human tumor at a single dose of 10-5 M. Ten novel dihydropyrimidine analogues have been synthesized, characterized and found to be promising antibacterial and anticonvulsant agents.
Li et al. [118] synthesized a novel series of 7-substituted-[1,2,4]triazole[4,3-f]pyrimidine derivatives as potential anticonvulsant agents. The anticonvulsant activity was evaluated by the maximal electroshock (MES) test, and their neurotoxicities were
Chapter-I
19
evaluated by the rotarod neurotoxicity test. The pharmacological results showed that the
compound 7-(4-chlorophenoxy)-[1,2,4]triazolo[4,3-f]pyrimidine was among the most active agent with median effective dose (ED50) value of 34.7 mg/kg, median toxicity dose (TD50) of 262.9 mg/kg, and providing a protective index value of 7.6. Few compounds also showed oral activity against MES-induced seizures and lower oral neurotoxicity. A series of N-(4,6-substituted diphenylpyrimidine-2-yl)semicarbazones were synthesized by Alam et al [119] and tested for their anticonvulsant activity against the two seizure models,
maximal electroshock seizure (MES) and subcutaneously pentylenetetrazole (scPTZ). All the synthesized compounds possesses the four essential pharmacophoric elements for good anticonvulsat activity. Most of the compounds displayed good anticonvulsant activity with lesser neuroxicity. To assess the unwanted effects of compounds on liver, estimation of enzymes and proteins were carried out.
Jiang et al. [120] synthesized several new 7-substituted-5-phenyl-[1,2,4]triazolo[1,5-a]pyrimidines through incorporating triazole moiety into the pyrimidine ring, which are expected to have the synergistic effects in dealing with the epilepsy. Their anticonvulsant
activities were measured through the Maximal Electroshock Seizure (MES) test. Among the compounds tested, 7-(heptyloxy)-5-phenyl-[1,2,4]triazolo[1,5-a]pyrimidine showed potent anticonvulsant activity which was weaker than carbamazepine, but better than valproate. Carbamazepine and valproate were considered as positive control drugs. Wang et al [121] synthesized a series of 5-alkoxytetrazolo[1,5-c]thieno[2,3-e]pyrimidine derivatives and their anticonvulsant and antidepressant activities were evaluated.
Pharmacological tests showed that, four of the synthesized compounds had weak anticonvulsant activity, while most of the compounds had excellent antidepressant activity. The most active compound was 5-(2,4-dichlorobenzoloxy)tetrazolo[1,5-c]thieno[2,3-e]pyrimidine which decreased the immobility time by 51.61 % at a dose of 100 mg/kg. The results of open-field of this compound indicated that it had no significant effects on the
locomotor activity compared with the control group at the dose assayed in the forced swimming tests. This means that the antidepressant activity detected in the FST for the compound is not the result of central nervous system stimulant properties, and further confirms its antidepressant–like effect.
Free radicals are well known for playing a dual role in our body-delterious as well as beneficial. It includes a metabolic pathway for its generation. Oxidative stress in our body occurs due to excessive generation of free radicals and reduced level of antioxidants, but at low concentration, these radicals help to perform normal physiological function of the
Chapter-I
20
body. Scientific evidence suggests that antioxidants reduce the risk for chronic diseases
including cancer and heart disease. Bano et al. [122] review shows the current tendency in the pyrimidine synthesis and revealed the pyrimidine core to be a very potent moiety which can be a rich source for the synthesis of new compounds having desirable antioxidant activity.
Abu-Hashem et al. [123] synthesized a series of pyrimidine derivatives and were subjected for their antioxidant activity. Out of the synthesized compounds, two compounds manifested potent antioxidant activity by lipid peroxidation assay. The authors concluded that thiazolopyrimidine derivatives incorporated with carbohydrazide, amino, pxadiazol
and other moieties possess pote1ntial antioxidant activities. Gressler et al. [124] synthesized a series of 4-trifluoro-methyl-2-(5-arytl-3-styryl-1H-puazol-1 yl)-pyrimidine derivatives and were screened for their in vitro antioxidant activity. The antioxidant activity was evaluated using the DPPH and HRP/H2O2 chemiluminescence assay methods. The DPPH antioxidant assay measures the hydrogen-donating capacity of the molecules in the sample. On the other hand, the chemiluminescence method is based on the light
emission produced by a chemical reaction. Mahesh et al. [125] carried out the synthesis of pyrimidine derivatives and were investigated for their in vitro antioxidant activity. The results revealed that, some of the tested compounds showed potent antioxidant activity. Among the compounds few have shown moderate activity. The remaining compounds were found to be weakly active.
1.2. Introduction to biological activity
1.2.1. Antibacterial activity
Diseases caused by microbial infection are a serious menace to the health of human
beings, and often have connection to some other diseases whenever the body system gets
debilitated. In order to combat these diseases, a large number of drugs are available in
clinical practice ranging from natural product antibacterial to tailor-made antibacterial
drugs. Developing antimicrobial drugs and maintaining their potency in opposition to
resistance by different classes of microorganisms as well as a broad spectrum of
antimicrobial activity are some of the major concern of research in this area. The health
problem demands to search and synthesize a new class of antimicrobial compounds which
are effective against pathogenic microorganisms, and develop resistance to the antibiotics
used in the current regime [126]. The increasing resistance of human pathogens to current
antimicrobial agents is a serious medical problem. During the 20th century, vaccines for
Chapter-I
21
bacterial toxins and many other common acute viral infections were developed and made
widely available. There are thirty vaccines that are mainly given prophylactically to
prevent or minimize diseases by agents infectious to human. The World Health
Organization (WHO) estimates that sixty percent of hospital-acquired (nosocomial)
infections are drug-resistant [127-129].
Various different classes of antibacterial [130, 131] and antifungal agents [132] have
been discovered. Although, since the discovery of several synthetic and semi-synthetic
antibacterial sulfa drugs, nitrofuranes, penicillins, cephalosporins, tetracyclines, macrolides
and oxazolidinones, and antifungal agents such as fluconazole, ketoconazole and
miconazole, including amphotericin B, there has been much progress in this field. Despite
advances in antibacterial and antifungal therapies, many problems remain to be solved for
most antimicrobial drugs available. The extensive use of antibiotics has led to the
appearance of multi-drug resistant microbial pathogens [133]. This highlights the incessant
need for the development of new classes of antimicrobial agents, and alteration of known
drugs in such a way that would allow them to retain their physiological action, but
reducing their resistance to the pathogen. The design of novel chemotherapeutic agents is
particularly beneficial due to their dissimilar mode of action which can avoid cross
resistance to known drugs.
Antibacterial drug discovery research accompanied by clinical development has
historically been conducted by large pharmaceutical companies. Although the earliest
antibiotics were first identified in academic laboratories such as those of Alexander
Fleming (Penicillium notatum) [134] and Selman Waksman (Streptomyces griseus) [135],
the pharmaceutical companies were responsible for successful strain optimization,
compound scale-up, formulation and clinical development activities that allowed anti-
infective drug research to gain prominence as a viable area for corporate investment.
Microbiologists distinguish two groups of antimicrobial agents used in the treatment
of infectious disease: (a) Antibiotics which are natural substances produced by certain
groups of microorganisms and (b) Chemotherapeutic agents which are chemically
synthesized. A hybrid substance is a semi synthetic antibiotic wherein a molecular version
produced by the microbe is subsequently modified by the chemist to achieve desired
properties. Furthermore, some antimicrobial compounds originally discovered as products
of microorganisms were synthesized entirely by chemical means. They might be referred
Chapter-I
22
to as synthetic antibiotics to distinguish them from the chemotherapeutic agents. The most
important property of an antimicrobial agent, from a host point of view, is its selective
inhibition, i.e., the agent acts in some way that inhibits or kills bacterial pathogens but has
little or no toxic effect on the host. This implies that the biochemical processes in the
bacteria are in some way different from those in the animal cells, and that the advantage of
this difference can be taken in chemotherapy [136]. Minimum inhibitory concentration
(MIC) is the lowest concentration of an antimicrobial that will inhibit the visible growth of
a microorganism after overnight incubation. MIC values can be determined by a number of
standard test procedures.
Antimicrobial susceptibility testing methods are divided into two types based on the
principle applied in each system. They include disc diffusion method (Stokes method and
Kirby-Bauer method) and dilution method (Broth dilution and Agar dilution). The most
commonly employed method is disc diffusion method. Antimicrobial agents are any
chemical or biological agents that either destroy or inhibit the growth of microorganisms.
Some antibacterial agents are:
(a) Ampicillin: Ampicillin is a beta-lactam antibiotic that has been used extensively
to treat bacterial infections since 1961. It demonstrated the activity against Gram-negative
organisms such as H. influenzae, Coliforms and Proteus species. Ampicillin was the first
member of so-called broad spectrum penicillins, subsequently introduced by Beecham.
Ampicillin is part of the aminopenicillin family and is roughly equivalent to its successor,
amoxicillin in terms of spectrum and level of activity.
(b) Ciprofloxacin: Ciprofloxacin is a synthetic chemotherapeutic antibiotic of the
fluoroquinolone drug class. It is a second generation fluoroquinolone antibacterial. It kills
bacteria by interfering with the enzymes that cause DNA to rewind after being copied,
which stops DNA and protein synthesis.
(c) Gentamicin: Gentamicin is an aminoglycoside antibiotic, used to treat many
types of bacterial infections, particularly those caused by Gram-negative bacteria.
However, gentamicin is not used for Neisseria gonorrhoeae, Neisseria meningitidis or
Legionella pneumophila. It was synthesized by Micromonospora, a genus of Gram-
positive bacteria widely present in the environment (water and soil).
Chapter-I
23
(d) Penicillin: Penicillin constitutes one of the most important groups of antibiotics.
These are bactericidal, and act by interfering with the synthesis of bacterial peptidoglycan
cell wall. Penicillin-G and Penicillin-V have a useful antimicrobial spectrum against
Streptococci, Pneumococci, Gonococci, and Meningococci.
(e) Rifampicin: Rifampicin is a bactericidal antibiotic drug of the rifamycin group. It
is active against Gram-positive bacteria as well as many Gram-negative species. It enters
phagocytic cells and kills intercellular microorganisms including the tubercle bacillus.
(f) Sulfadiazine: Sulfadiazine is a sulfonamide antibiotic. It eliminates bacteria that
cause infections by stopping the production of folic acid inside the bacterial cell, and is
commonly used to treat urinary tract infections (UTIs). In combination, sulfadiazine and
pyrimethamine can be used to treat toxoplasmosis, a disease caused by Toxoplasma gondii.
(g) Streptocycline: Streptocycline (bacteromycin) is an antibiotic formulation
recommended for effective control of bacterial diseases of plants.
1.2.2. Antifungal activity
The incidence of fungal infections has increased significantly in the past two decades
[137]. The first generation antifungal inhibitors of CYP51 have revolutionized the
treatment of some serious fungal infections. Triazoles have been the leading agents for the
control of fungal diseases of humans and animals for over 20 years [138, 139]. According
to this, azole derivatives are currently the most widely studied class of antifungal agents.
Nystatin is a polyene-macrolide antifungal antibiotic produced by Streptomyces noursei
that was discovered and developed in 1950 [140]. Toxicity problems prevented its use as a
systemic agent, but recently developed liposomal delivery technologies have made it an
attractive candidate for the treatment of severe systemic fungal infections [141]. This has
prompted new investigations of its antifungal properties and spectrum as well as its
physicochemical and pharmacokinetic characteristics [142]. The development of resistance
among pathogens to routinely used pesticides demands that a renewed effort should be
made to seek antimicrobial agents which are effective against pathogenic microbes [143].
Oral candidiasis is a common oral lesion caused by overgrowth of fungal species in the
Candida (genus). Among many species, Candida albicans is the most important
microorganism implicated in fungal infection [144]. Almost all antifungal agents currently
Chapter-I
24
used in human mycoses target the ergosterol biosynthetic pathway, an important
component of fungal membranes.
Fungal infections vary widely with respect to clinical picture and mainly include
superficial mycoses involving infections to skin, hair, mucous membranes and nails.
Fungal infections are being recognized with increasing frequency as an important cause of
both morbidity and mortality. Some antifungal drugs could also help to kill fungi which
infect the human body. Many of the drugs currently available have undesirable side effects
and might be toxic. It is very important to explore additional sources for substances with
potential antifungal activity, which could possibly have different modes of activity or
affect different sites in the fungal cells. In view of wide spread resistant strains of
microorganism, there is an urgent need for the development of new antimicrobial agents to
treat the patients infected with multidrug-resistant bacteria and fungi. Some antifungal
agents are:
(a) Nystatin: Nystatin is a polyene antifungal drug to which many molds and yeast
infections are sensitive, including Candida. It is now widely available for the topical
treatment of localized fungal infections.
(b) Amphotericin B: Amphotericin B is a polyene antifungal drug, often used
intravenously for systemic fungal infections. Two amphotericins, Amphotericin A and
Amphotericin B are known, but only B is used clinically because it is significantly more
active in vivo. Amphotericin A is almost identical to Amphotericin B but has little anti-
fungal activity.
(c) Hamycin: Hamycin is a polyene antimycotic organic compound. It is a heptaene
antifungal compound rather similar in chemical structure to amphotericin B except that it
has an additional aromatic group bonded to the molecule. It is obtained from a strain of
streptomyces bacteria growing in soil i.e., Streptomyces pimprina.
1.2.3. Antioxidant activity
Oxygen, an element indispensable for life, can under certain circumstances,
adversely affect the human body. It is produced by plants during photosynthesis, and is
necessary for aerobic respiration in animals. The oxygen consumption inherent in cell
growth leads to the generation of a series of reactive oxygen species (ROS). Free radical
(FR) is a chemical species or an atom or a molecule that has one or more unpaired
Chapter-I
25
electrons in its valance shell and is capable of existing independently. Free radical contains
an odd number of electrons which makes it unstable, short lived and highly reactive.
Therefore, it reacts quickly with other compounds in order to capture the needed electron
to gain stability. Generally, free radical attacks the nearest stable molecule, stealing its
electron. When the attacked molecule looses its electron, it becomes a free radical itself,
beginning a chain reaction cascade resulting in disruption of a living cell [145, 146].
The most common free radicals are derivatives of oxygen like superoxide free radical
anion (O2·-), hydroxyl free radical (OH·), lipid peroxyl (LO·), lipid alkoxyl (LOO·) and
lipid peroxide (LOOH) as well as non-radical derivatives such as hydrogen peroxide
(H2O2) and singlet oxygen (1O2) which are collectively known as ROS. These free
radicals/reactive oxygen species are produced mainly from two important sources in the
biological system, i.e., cellular metabolism like mitochondrial electron transport chain,
endoplasmic reticulum oxidation, NADPH oxidase, xanthine oxidase, prostaglandin
synthesis, reduced riboflavin, nitric oxide synthetase, reperfusion injury, cytochrome P450,
activated neutrophils and phagocytic cells and environmental sources like drugs,
pesticides, transition metals, tobacco smoke, alcohol, radiations and high temperature.
Owing to the great potentiality of free radicals to react with various compounds by electron
transfer, proton transfer, H-atom abstraction or addition reaction, they are considered
responsible for a series of undesired processes such as aging, material degradation, food
deterioration and many diseases [147].
Free radical and ROS production in the animal cell is inevitable. Normally, there is
an equilibrium between a free radical/reactive oxygen species formation and endogenous
antioxidant defense mechanisms, but if this balance is disturbed, it can produce oxidative
stress [148]. There are two basic categories of antioxidants namely synthetic and natural.
In general, synthetic antioxidants are compounds with phenolic structures of various
degrees of alkyl substitution, whereas natural antioxidants can be phenolic compounds
(tocopherols, flavonoids, and phenolic acids), nitrogen compounds (alkaloids, chlorophyll
derivatives, amino acids, and amines) or carotenoids as well as ascorbic acid [149, 150].
The primary antioxidants comprise essentially sterically hindered phenols and secondary
aromatic amines [151]. These antioxidants act usually both through chain transfer and
chain termination. The first step of the reactive radical’s termination by this type of
antioxidants is hydrogen atom transfer from the antioxidant molecule to the reactive radical
intermediate [152]. Small amounts of antioxidants are added into most synthetic polymers
Chapter-I
26
to prevent or retard oxidation and to increase the service lifetimes of the products [153].
Free radicals and active oxygen species have been related with cardiovascular and
inflammatory diseases, and even with a role in cancer and ageing [154]. Efforts to
counteract the damage caused by these species are gaining acceptance as a basis for novel
therapeutic approaches and the field of preventive medicine is experiencing an upsurge of
interest in medically useful antioxidants [155].
Antioxidants are classified into two broad divisions, depending on whether they are
soluble in water (hydrophilic) or in lipids (hydrophobic). In general, water-soluble
antioxidants react with oxidants in the cell cytosol and the blood plasma while lipid-
soluble antioxidants protect cell membranes from lipid peroxidation [156]. These
compounds may be synthesized in the body or obtained from the diet [157]. The amount of
protection provided by any antioxidant will depend on its concentration, its reactivity
towards the particular reactive oxygen species being considered and the status of the
antioxidants with which it interacts. The relative importance and interactions between these
different antioxidants is a very complex question with the various metabolites and enzyme
systems having synergistic and interdependent effects on one another [158].
Phenolic derivatives are one of the groups of antioxidants that have been studied by
many research groups. A great number of examples have been described in the literature,
such as caffeic acid and its analogues which are known to have antiviral and
antiatherosclerotic properties [159], resveratrol with known anticancer and heart protecting
effects [160] and olive oil phenols, particularly hydroxyl tyrosol which inhibits human
low-density lipoprotein (LDL) oxidation (a critical step in atherosclerosis) [161] inhibits
platelet aggregation [162] and exhibits anti-inflammatory [163] and anticancer properties
[164]. Phenols have been utilized extensively for food preservation.
Various analytical methods have been developed to measure the radical scavenging
activity of antioxidants against free radicals like the 1,1-diphenyl-2-picrylhydrazyl (DPPH)
radical, the superoxide anion radical (O2), the hydroxyl radical (OH) or the peroxyl radical
(ROO). These methods are used to measure the antioxidant activity of food products can
give varying results depending on the specific free radical being used as a reactant. There
are other methods which determine the resistance of lipid or lipid emulsions to oxidation in
the presence of the antioxidant being tested.
Chapter-I
27
DPPH is a well-known radical and a scavenger for other radicals. DPPH has two
major applications in laboratory research. (a) To monitor the chemical reactions involving
radicals and (b) The number of initial radicals can be counted from the change in the
optical absorption at 520 nm or in the electron paramagnetic resonance signal of the
DPPH. The molecule of DPPH (18) is characterized as a stable free radical by virtue of the
delocalization of the electron over the molecule as a whole, so that the molecules do not
dimerise, as would be the case with most other free radicals. The delocalization also gives
rise to the deep violet colour which is characterized by an absorption band in ethanol
solution centered at about 517 nm. When a solution of DPPH is mixed with that of a
substance that can donate a hydrogen atom, then this gives rise to the reduced form (19)
with the loss of this violet colour. Representing the DPPH radical by Z˙ and the donor
molecule by AH, the primary reaction is
Z˙ + AH = ZH + A˙
where ZH is the reduced form and A˙ is free radical produced in this step. The latter radical
will then undergo further reactions which control the overall stoichiometry, i.e., the
number of molecules of DPPH reduced (decolorized) by one molecule of the reductant.
NN
O2N
O2N
NO2
N
HN
O2N
O2N
NO2
18 19
The above reaction is therefore intended to provide the link with the reactions taking
place in an oxidizing system such as the autoxidation of a lipid or other unsaturated
substance and the activity of Z˙ is suppressed by AH. At the present time, the most
commonly used antioxidants are butylated hydroxyanisole (BHA), butylated
hydroxytoluene (BHT), propylgallate, tert-butyl hydroquinone, Vitamin E and Vitamin C.
1.2.4. Anticonvulsant activity
Epilepsy is a major neurological disorder affecting a large section of people both
male and female throughout the world. Currently available drugs for the treatment of
epilepsy are symptomatically effective in only 60-70 % of patients. Every year
Chapter-I
28
approximately 2,50,000 new cases are added to this figure. Epilepsy also poses a
considerable economic burden on society. The direct costs of epilepsy vary significantly
depending on the severity of the disease and the response to treatment. The known
potential causes of epilepsy include brain tumors, infections, traumatic head injuries,
perinatal insults, developmental malformations, cerebrovascular diseases, febrile seizures
and status epilepticus [165]. Many patients have seizures that are resistant to the available
medical therapies. Although 70 – 80 % of epileptics are currently controlled by a variety of
drugs, seizure protection is often accompanied by numerous side effects including
drowsiness, ataxia, gastrointestinal disturbances, gingival hyperplasia, hirsutism and
megaloblastic anemia [166].
Epileptic seizures can be generalized (generalized epileptic seizure), originating in
both hemispheres of the brain simultaneously or partial (focal seizures), originating in one
or more parts of one or both hemispheres, most commonly the temporal lobe. With
generalized seizures, consciousness is always impaired or lost. Consciousness may be
maintained in partial seizures but partial seizures may become generalized seizures in a
process referred to as secondary generalization, at which point consciousness is lost. In
patients, the types of epilepsy or epileptic syndrome are further classified according to
features such as the type of seizure, etiology, age of onset and electroencephalogram.
Epilepsy or epileptic syndromes can be either idiopathic (etiology or cause is unknown)
with a presumed genetic basis or symptomatic (acquired). The pharmacological strategies
for the treatment of epilepsy are aimed at suppressing the initiation or propagation of
seizures rather than the underlying processes that lead to epilepsy. Some epileptic patients
are unresponsive to current antiepileptic drug treatment, and for this reason the major goal
in epilepsy research has been to develop drugs with greater anticonvulsant efficacy and
less toxicity than existing drugs [167]. There is growing evidence that serotonergic
neurotransmission modulates a wide variety of experimentally-induced seizures and is
involved in the enhanced seizure susceptibility observed in some genetically epilepsy-
prone animals [168, 169].
The anticonvulsants are a diverse group of pharmaceuticals used in the treatment of
epileptic seizures. Anticonvulsants are also increasingly being used in the treatment of
bipolar disorder, since many seem to act as mood stabilizers. The goal of an anticonvulsant
is to suppress the rapid and excessive firing of neurons that start a seizure. Failing this, an
effective anticonvulsant would prevent the spread of the seizure within the brain and offer
Chapter-I
29
protection against possible excitotoxic effects that may result in brain damage. Some
studies have site that anticonvulsants themselves are linked to lower IQ in children [170].
Anticonvulsants are more accurately called antiepileptic drugs (AEDs), sometimes referred
to as antiseizure drugs. While an anticonvulsant is a fair description of AEDs, it neglects to
differentiate the difference between convulsions and epilepsy. Convulsive non-epileptic
seizures are quite common and these types of seizures will not have any response to an
antiepileptic drug. In epilepsy, an area of the cortex is typically hyperirritable that can
often be confirmed by completing an electroencephalogram (EEG).
Conventional AEDs such as phenobarbital, primidone, phenytoin, carbamazepine,
ethosuximide and benzodiazepine are widely used, but exhibit an unfavourable side effect
profile and failure to adequately control seizures. These include older ‘first generation’
drugs such as carbamazepine, phenobarbital, valproic acid and newer second generation
drugs such as lamotrigine, vigabatrin, tiagabine, topiramate, gabapentin and levetiracetam
[171]. These newer drugs have proven to be effective in reducing seizure whilst their
therapeutic efficacy is overcome by some undesirable side effects such as headache,
nausea, hepatotoxicity, anorexia, ataxia, drowsiness, gastrointestinal disturbances and
hirsutism [172]. These observations affirm the further scope and need for the development
of newer agents. The long-established AEDs control seizures in 50 % of patients
developing partial seizures and in 60-70 % of those developing generalized seizures [173-
176].
The selection of an antiepileptic drug for treatment is predicated on its efficacy for
the specific type of seizures, tolerability and safety [177, 178]. Therefore, it is essential to
search for newer chemical entities for the treatment of epilepsy. A review on new
structural entities having anticonvulsant activity has recently appeared [179]. The first
strategy is the identification of new targets through better understanding of molecular
mechanisms of epilepsy. Another way is to modify already existing drugs and
formulations. AEDs belong to many different chemical classes of compounds including
hydantoines, iminostilbenes, barbiturates, benzodiazepines, valproate, imides, oxazolidine-
2,3-diones, sulphonamides and miscellaneous agents [180]. The efficacy of AEDs is due to
the main activities which include interaction with ion channels or neurotransmitter systems
[181-183]. Currently available AEDs can be broadly classified into four categories, (1)
those whose main action relates to the inhibition of sustained repetitive firing through
blockage of voltage-dependent sodium channels and consequent inhibition and release of
Chapter-I
30
excitatory neurotransmitters (phenytoin, carbamazepine, oxcarbazepine) (2) those which
enhance GABA-ergic transmission (benzodiazepines, barbiturates, vigabatrin, tiagabine)
(3) those stabilizing thalamic neurons through inhibition of T-type calcium channels
(ethosuximide) and (4) those possessing a combination of the above actions, often coupled
with additional mechanisms (valproic acid, gabapentin, lamotrigine, topiramate,
zonisamide, felbamate). However, this classification has limited value because the majority
of AEDs possess more than one mechanism of action, which may account for their
efficacy, and it is also the fact that some of the clinically used drugs have not been linked
with a specific site of brain, and the exact mechanism of many AEDs remain unknown
[184, 185].
The new AEDs and anticonvulsant agents have been reviewed during last few years
[186]. The chemical diversity and various mechanisms of action of anticonvulsants make it
difficult to find a common way of identifying new drugs. Novel anticonvulsant agents are
discovered through conventional screening and/or structure modification rather than a
mechanism driven design. Therefore, drug identification is usually conducted via in vivo
screening tests on the basis of seizure type rather than etiology. Phenytoin (20) is still one
of the most commonly used antiepileptic drugs. Today, in addition to phenytoin, several
AEDs are widely used in the treatment of the various forms of epilepsy [187]. Some of the
other standard drugs include ethosuximide, valproic acid and various benzodiazepines have
been used for anticonvulsant activity.
HNNH
O
O
20
Many of the compounds presented in the review of the literature have been tested
according to the procedure established by the Antiepileptic Drug Development (ADD)
Program [188]. The anticonvulsant screening project use for its initial screening procedure
includes two major convulsant tests, the maximal electroshock seizures (MES) and the
subcutaneous pentylenetetrazole (scPTZ) as well as a toxicity screen (rotorod in mice,
positional sense and gait in rats). The MES is a model for generalized tonic-clonic seizures.
The behavioral and electrographic seizures generated in this model are consistent with the
Chapter-I
31
human disorder [189]. This model identifies those compounds which prevent the spread of
seizures. The scPTZ seizure test is a model which primarily identifies compounds that raise
seizure threshold. The behavioral seizure produced is not typical of absence epilepsy but
clonic in nature. Like other rodent models of absence seizures, PTZ-induced seizures are
potentiated by γ-aminobutyric acid (GABA) agonist. With some minor exceptions, the
pharmacological profile of the scPTZ seizure model is consistent with the human condition
[190]. All clinically active anticonvulsants have been found to be protective in at least one
of these two tests.
1.2.5. Neurotoxicity
Neurotoxicity is concerned with the adverse changes in the structure or function of the
nervous system. A neurotoxin is considered to be a substance which elicits a pathological
response primary or specifically on the nervous system. The complexity of the nervous
system results in a broad range of potential targets and adverse squeal, since the activity of
the nervous system balance between all the various organs in the body.
The importance of the potential impact of chemicals on human neurological function
has been recognized by the organization for Economic Co-operation and Development
(OECD). The deleterious effects of chemicals may not become clinically evident for some
time following exposure, aptly called the silent period. Such latent toxicity has been defined
as ‘persistent morphological or biochemical injury’ which means clinically unapparent [191].
Several hypotheses have been proposed to explain the asymptomatic period prior to clinical
expression of neurotoxic injury. Exposure to neurotoxic chemicals may cause cell death of a
subpopulation of neuronal cells, but the total number of cells lost is insufficient to cause
adverse effects because of the reserve capacity regarding this fraction. Alternatively, toxic
exposure may cause subethal injury to neuronal cells in critical areas of the brain such as the
substantia nigra which leads to a progressive loss of function [192]. Initially this can be
compensated for, although overtime loss of function arises due to the lack of plasticity of the
brain [191]. Such hypothesis also exists for developmental neurotoxicity, exposure to
chemicals or infections during the appropriate gestational age can result in fewer
dopaminergic neurons. Although no clinical signs may be evident in early years, the subject
may be predisposed to parkinson traits later on in life. This has lead to particular concern
about exposure to low levels of environmental chemicals and effects on
neurobehavioral/development in children and on the development of neurodegenerative
Chapter-I
32
diseases in the adult. Neurodegenerative diseases are of great concern, bearing in mind the
aging of the population. Alzheimer’s disease and Parkinson’s disease are the most common
neurodegenerative diseases.
1.2.5.1. In vivo models of neurotoxicity
A multidisciplinary approach is required in order to adequately assess potential
neurotoxic effects of compounds due to the diverse function of the nervous system. Many
effects may be measured using neurophysiological (e.g. electroencephalography
measurement of evoked potentials), neuropathological (e.g. microscopy, histochemistry,
immunohistochemistry) or behavioural techniques [193].
The Functional Observational Battery (FOB) is a standardized screening battery for
assessing many aspects of behaviour and neurological functions in rodents and is designed to
detect and quantify major overt behavioural, physiological and neurological signs [194]. The
tests have been validated with many known neurotoxic chemicals and are often used in
conjunction with other measures of toxicity [195]. The FOB comprises a number of tests to
identify specific deficits in motor and sensory functions by measuring neuromuscular,
sensory and autonomic functions.
1.2.5.2. In vitro models of neurotoxicity
To address problems associated with increasing cost and time required for neurotoxicity
testing, the large number of chemicals in commercial use that have not been investigated
and animal welfare issues, considerable effort is being directed at the development of in
vitro alternatives. These may be considered as part of the tiered system with which to
identify potential neurotoxic chemicals, although such approaches have not been validated
as replacements for animal studies. Several in vitro models are commonly used in
neurotoxicity evaluations, including synaptic fractions, primary cultures of rat astrocytes,
rat cerebellar granule neurones, primary motor neurones of dissociated cultures of murine
spinal cord, rat brain region organ cultures and hippocampal slices [196].
1.2.6. Anticancer activity
Cancer is a class of diseases in which a group of cells display uncontrolled growth
(division beyond the normal limits), invasion (intrusion on and destruction of adjacent
tissues) and sometimes metastasis (spread to other locations in the body via lymph or
Chapter-I
33
blood). These three malignant properties of cancer differentiate them from benign tumors
which are self-limited and do not invade or metastasize. Worldwide, about 7.6 million
people die from cancer every year [197]. Research on the anticancer drug discovery has
significant attention over the last few years. The term cancer chemoprevention coined by
Sporn and co-workers [198] in 1976 has been defined as a strategy for reducing cancer
mortality by the prevention, delay or reversal of cancer by pharmaceutical agents capable
of mediating the process of carcinogenesis [199]. The introduction of nitrogen mustard in
1940s can be considered the origin of antineoplastic chemotherapy targeting all tumor cells
[200].
A need for effective anti-cancer therapeutic agents, as well as a well-defined
pharmacokinetic property of the drug was felt. Induction of apoptosis is another indicator
for drug activity in cancer cells [201]. Recently, many chemotherapeutic compounds have
been shown to have antiproliferative effects by inhibiting cells cycle at certain checkpoints.
Furthermore, apoptosis provides a number of clues with respect to effective anticancer
therapy, and many chemotherapeutic agents reportedly exert their antitumor effects by
inducing apoptosis in cancer cells [202]. Considerable attention has been devoted to the
sequence of events with respect to the apoptotic cell death and its role in mediation of the
lethal effects of the diverse antineoplastic agents. These studies have become a focus of
interest in cancer chemotherapy to shed light on the mechanism of action of candidate
drugs.
Leukaemia or leukemia is a cancer of the blood or bone marrow and is characterized
by an abnormal proliferation (production by multiplication) of blood cells, usually white
blood cells (leukocytes). Leukemia is a broad term covering a spectrum of diseases. In
turn, it is part of the even broader group of diseases called hematological neoplasms.
Chronic myelogenous (or myeloid) leukemia (CML) also known as chronic granulocytic
leukemia (CGL) is a cancer of the white blood cells. It is a disease of hematopoietic stem
cells characterized by hyper proliferation of often immature cells of the myeloid,
megakaryocytic and erythroid lineages. The Bcr-abl protein activates multiple signalling
pathways that promote cell proliferation, block apoptosis and decrease cell adhesion,
thereby blocking maturation and causing release of immature cells into the blood. After
several years, CML turns into blast crisis, a rapidly lethal disease resembling acute
leukemia. CML is treated by chemotherapy, interferon therapy and allogeneic stem cell
Chapter-I
34
transplantation. Imatinib mesylate (Gleevec or Glivec), which directly targets a molecular
abnormality in certain types of cancer (CML, gastrointestinal stromal tumors).
New cancer targeted therapies that make use therapeutic antibodies or small
molecules have made treatment more tumors specific and less toxic. Nevertheless, there
remain several challenges to the treatment of cancer, including drug resistance, cancer stem
cells and high tumor interstitial fluid pressure. Cell birth and death rates determine adult
body size and the rate of growth in reaching the appropriate size. In some adult tissues, cell
proliferation occurs continuously as a constant tissue-renewal strategy. Identification of
novel, efficient, selective and less toxic anticancer agents remains an important and
challenging goal in medicinal chemistry. The past two decades have seen a dramatic
change in cancer treatment paradigms. Similarly, cell cycle-mediated apoptosis is also
gaining importance because certain compounds are believed to function via this pathway
[203]. In the recent years, the number of the anticancer drugs such as 5-fluorouracil (5-FU)
[204, 205], doxorubin [206], palcitaxol [207], methotrexate [208], campothecin [209],
cytarabine [210], cis-platinum [211], taxotere [212] and combrestastin [213] have been
reported as clinical anticancer drugs.
Cytotoxicity assay is widely used in in vitro toxicology studies. The lactate
dehydrogenase (LDH) leakage of cytotoxicity or cell viability following exposure to toxic
substances. MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide) is a
water soluble tetrazolium salt which is converted to an insoluble purple formazan by
cleavage of the tetrazolium ring by succinate dehydrogenase within the mitochondria. The
formazan product is impermeable to the cell membranes assay, protein assay, neutral red
assay and methyl tetrazolium (MTT) assay are the most commonly employed for the
detection and therefore it accumulates in healthy cells. The MTT assay was tested for its
validity in various cell lines [214]. More recent evidence suggests that reduction of MTT
can also be mediated by nicotinamide adenine dinucleotide (NADH) within the cells and
out of mitochondria [215]. Further modification of the initial protocol was proposed [216]
in order to improve the repeatability and the sensitivity of the assay.
1.3. Scope of the present work
Heterocycles containing triazole, oxadiazole and pyrimidine groups were proven to
be biologically very potent and selective. A wide spectrum of pharmacological activities
has been reported for these compounds. In view of the above facts, triazole, oxadiazole and
Chapter-I
35
pyrimidine derivatives were synthesized and characterized by different spectral studies.
These derivatives were screened for their biological activity. It is expected that these would
result in highly potent and selective pharmaceutical agents. Chapter II describes the
synthesis of new fluorinated schiff bases derived from 1,2,4-triazoles and their
antiproliferative activity. New compounds were structurally characterized by 1H NMR, 13C
NMR, LC-MS, FT-IR and elemental analyses. The synthesized compounds showed good
antiproliferative activity. Chapter III describes the synthesis, characterization and in vitro
antimicrobial evaluation of new 5-chloro-8-bromo-3-aryl-1,2,4-triazolo[4,3-c]pyrimidines.
A series of new 5-chloro-8-bromo-3-aryl-1,2,4-triazolo[4,3-c]pyrimidines have been
accomplished in excellent yields by the oxidative cyclization of pyrimidinehydrazines of
various aryl aldehydes with iodobenzene diacetate in methanol. The chemical structures of
the synthesized compounds were confirmed by elemental analysis, LC-MS, FT-IR 1H
NMR and 13C NMR spectral studies. Chapter IV describes the synthesis 1,3,4-oxadiazoles
bearing 5-chloro-2-methoxyphenyl moiety by the reactions of acid hydrazide with different
aromatic carboxylic acid in the presence of phosphorous oxychloride. New compounds
were structurally characterized by different spectral studies and showed good antimicrobial
activity. Chapter V describes the synthesis of some pyrimidine derivatives and their
biological activity. New compounds were structurally characterized by different spectral
studies. The synthesized compounds showed good anticonvulsant and antioxidant activity.
Chapter VI describes the synthesis of N-[{5-aryl-1,3,4-oxadiazole-2-yl}methyl]-4-
methoxyaniline derivatives and their anticonvulsant activity. A series of new of N-[{5-aryl-
1,3,4-oxadiazole-2-yl}methyl]-4-methoxyaniline have been accomplished in excellent
yields by the intramolecular oxidative cyclization of E)-2-(arylbenzylidene)-2-[(4-
methoxyphenyl)amino]acetohydrazides of various aryl aldehydes with iodobenzene
diacetate in methanol. The anticonvulsant activity of the nine newly synthesized 2,5-
disubstituted-1,3,4-oxadiazoles was evaluated by MES induced seizure in rats. Few
compounds were exhibited good anticonvulsant activity. Chapter-VII describes the
synthesis and in vitro antiproliferative activity of 2,5-disubstituted-1,3,4-oxadiazoles
containing trifluoromethyl benzene sulfonamide moiety. The antiproliferative action of the
synthesized compounds was tested against four different cell lines. Chapter VIII describes
the synthesis and in vitro antiproliferative activity of new S-alkylated bis-1,2,4-triazole
derivatives. New compounds were evaluated for their antiproliferative effect using the
MTT assay method against four human cancer cell lines. Hence, there is a need for further
investigations to clarify the features underlying the biological activities of these new
Chapter-I
36
heterocyclic derivatives. Further studies on this line are under progress. A detailed survey of
literature revealed that the compounds synthesized and reported in this thesis have not been
carried out earlier by any other workers.
Chapter-I
37
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