SYNOPSIS
SYNOPSIS
The thesis entitled “DESIGN, SYNTHESIS AND BIOLOGICAL EVALUATION OF SOME NEW
AZOLE HETEROCYCLES”
has been divided into four chapters.
CHAPTER 1: This chapter describes the introduction, natural abundance and biological
significance of azoles i.e. 1,3,4-oxadiazoles, benzoxazolones and cyclic amines i.e.
piperizines.
CHAPTER II: This chapter describes the synthesis of new cyclic amine bearing 1,3,4oxadiazoles (Base free alkylation and an One-pot three-component reaction).
CHAPTER III: This chapter describes the synthesis of new heterocyclic hybrids based
on benzoxazolone and 1,3,4-oxadiazole scaffolds.
(Molecular Hybridization approach: A novel concept in drug design, and base free
alkylation).
CHAPTER IV: This chapter divided into two parts Section-A and Section-B, describes
the Biological evaluation.
Section-A describes the anticancer and antimicrobial activities of cyclic amine bearing
1,3,4-oxadiazoles.
Section-B describes the anticancer and antimicrobial activities of new heterocyclic
hybrids based on benzoxazolone and 1,3,4-oxadiazole scaffolds.
CHAPTER-I
AN OVERVIEW OF AZOLE HETEROCYCLIC COMPOUNDS AND THEIR BIOLOGICAL
SIGNIFICANCE
Heterocyclic chemistry is a very important branch of organic chemistry and
more than half of all known organic compounds are heterocyclic compounds.
Heterocycles are present in a wide variety of drugs, most vitamins, many natural
products, biomolecules, and biologically active compounds, including antitumor,
antibiotic, anti-inflammatory, antidepressant, antimalarial, anti-HIV, antimicrobial,
antibacterial, antifungal, antiviral, antidiabetic, herbicidal, fungicidal, and insecticidal
agents. Today, most of the heterocyclic drugs are not extracted from natural sources
I
SYNOPSIS
but are synthesized from readily available fine chemicals. In this aspect synthesis and
characterization of new molecular entities incorporating heterocyclic structures is of
high importance. As it is difficult to meet the worldwide demand of the requirement of
the natural products due to their low abundance in nature, it is essential produce
synthetic substances in large quantities. This can extend to so-called nature-identical
materials that are natural substances produced synthetically in an identical or with
slight modification of the molecular form, in order to increase the biological activity of
the molecule. Heterocycles are among the most frequently encountered scaffolds in
drugs and pharmaceutically relevant substances. The remarkable ability of heterocyclic
nuclei to serve both as biomimetics and reactive pharmacophores has largely
contributed to their unique value as traditional key elements of numerous drugs. The
development of heterocycles as scaffolds, containing a high degree of diversity has
become a leading focus in modern drug discovery. In the pharmaceutical industry over
75% of the top two hundred branded drugs have heterocyclic fragments in their
structures.
Certain possible modifications on the heterocyclic ring by the addition of diverse
substituents may lead to new products with better biological profiles. As a result of the
biological activity exhibited by the heterocyclic molecules, the development of new
chemical entities (NCEs) is the focus of intense activity in pharmaceutical industry.
Various types of newly synthesized molecules may be very good at blocking the action
or killing the bacteria without harming the human cells so as to prevent or cure the
disease. The nitrogen, oxygen and sulphur heterocycles are an attractive source of
compounds for the identification of new biological probes. The main aim is to design
and synthesize molecules involving the use of structural motif commonly found in
majority of well-established drug molecules. In this research program various diverse
classes of oxadiazole and benzoxazolone derivatives were developed. Firstly, this
research may generate development of new methods for synthesis. Secondly,
Characterization of a set of compounds by spectral means would create benchmarks for
characterization of similar molecules. Finally, biological evaluation of the prepared
compounds may expose lead compounds for further structural fine tuning.
II
SYNOPSIS
1,3,4-OXADIAZOLE DERIVATIVES
1,3,4-oxadiazoles have been known from 1880 and research in this field has
been intensified from 1950 due to large number of uses and applications in most diverse
area. 1,3,4-oxadiazole-2-thiol (Figure 1) is a non-naturally occurring heterocyclic
compound containing an oxygen atom, two carbon atoms and two nitrogen atoms (N=) in a five membered ring and an exocyclic sulphur atom which shows thiol-thione
tautomerism (where R = alkyl or aryl group).
N NH
N N
R
SH
O
R
thiol
O
S
thione
Figure 1
BIOLOGICAL IMPORTANCE OF 1,3,4-OXADIAZOLE DERIVATIVES
A large number of 1,3,4-oxadiazole containing ring system have been
incorporated into a wide variety of therapeutically interesting drug candidates. Also,
there are known drugs containing the 1,3,4-oxadiazole group (Figure 2). Recently, one
oxadiazole containing compound, an antiretroviral drug for the treatment of HIV
infection, raltegravir (1) has been launched onto the market place and other marketed
antihypertensive agents such as nesapidil (2) and tiodazosin (3) as well as antibiotics
such as furamizole (4).
Figure 2: Marketed oxadiazole drugs
III
SYNOPSIS
A number of compounds are in late stage clinical trials, including zibotentan (5)
as an anticancer agent12 and ataluren (6) for the treatment of cystic fibrosis13 contain
the oxadiazole nucleus (Figure 3).
N
O
O
N
HO
NH
O2S
N N
N
N
O
O
F
N
5
6
Figure 3: Oxadiazoles which are in late stage clinical trials.
BIOLOGICAL IMPORTANCE OF BENZOXAZOLE/BENZOXAZOLONE DERIVATIVES
Benzoxazole derivatives are biologically significant compounds and known to
exhibit various biological activities such as anticancer, antimicrobial, anti HIV and
dopamine D4 agonists. Benzoxazoles are an important class of heterocycles that are
encountered in a number of natural products. Benzoxazole form core of a variety of
cytotoxic natural products. Taniguchi et al. isolated the bis(benzoxazole) natural
product UK-1 from the mycelial cake of an actinomycete strain. UK-1 exhibits a wide
spectrum of potent anticancer activity against leukemia, lymphoma, and certain solid
tumor-derived cell lines.The benzoxazole derivative nataxazole isolated from
Streptomyces sp. exhibited inhibitory action on the growth of tumor cells. The cytotoxic
activity of nataxazole is somewhat better than UK-1 against AGS cells. Caboxamycin ,a
benzoxazole antibiotic, which was isolated from deep-sea sediment collected in the
Canary Basin. A benzoxazole derivative, calcimycin, is a carboxylic polyether antibiotic
from a strain of Streptomyces chartreusis (NRRL 3882).17 Some of the drugs which
contain benzoxazole pharmacophore are benoxaprofen and flunoxaprofen exhibit antiinflammatory activity.
BENZOXAZOLONES IN NATURE
2(3H)-Benzoxazolone (7) was first discovered in nature by Virtanen and Hietala
as an anti-fusarium factor in rye seedlings in 1955. Secondly it was obtained by Dr.
Murty and his coworkers from the leaves of the Indian A. ilicifolius in 1984. This is the
IV
SYNOPSIS
first time that 2(3H)-benzoxazolone (7) was reported from the genus Acanthus. A
dimeric oxazolinone, 5,5'-bis-benzoxazoline-2,2'-dione (8), was reported by D’Souza
and his coworkers from the leaves of the Indian A. ilicifolius. A benzoxazolinone
glucoside (9) has been recently reported from the Chinese A. ilicifolius (Figure 4).
Figure 4
BENZOXAZOLONE DRUGS
2(3H)-Benzoxazolone (7) and its bioisosters are considered ‘privileged
scaffolds’ in the design of pharmacological probes. Various drugs of 2(3H)benzoxazolone, e.g. benzolone (8, myorelaxant), Paraflex or chlorzoxazone (9, sedative
analgesic) and vinizene (10, topical antiseptic) are in market (Figure 8).
Figure 8: Benzoxazolone drugs
PIPERAZINES
The piperazine scaffold has been classified as a privileged structure and is
frequently found in biologically active compounds across a number of different
therapeutic areas. This motif is found in drug candidates displaying anti-allergenic,
antibacterial, anti-anxiety, anti-emetic and antimigraine activity.
CHAPTER-II
SYNTHESIS OF NEW CYCLIC AMINE BEARING 1,3,4-OXADIAZOLE DERIVATIVES
The 1,3,4-oxadiazole ring is associated with many types of biological activities
such as HIV integrase inhibitory, anti-inflammatory, antibacterial and antifungal, and
hypoglycemic activities. Also, there are known drugs such as raltegravir, nesapidil,
tiodazosin, furamizole etc. containing the 1,3,4-oxadiazole group. Several cyclic amine
V
SYNOPSIS
(e.g. substituted piperazine) derivatives were synthesized in the last decade, as useful
chemotherapeutic agents for various diseases, such as Crivixan (Indinavir sulphate) and
delaviridine (Rescriptor), powerful inhibitors for the protease and reverse transcriptase
inhibitor of HIV enzymes, respectively.
In this chapter, synthesis of new 2-[3-[4-(substituted)-1-cyclic amine]propyl]thio-5substituted[1,3,4]oxadiazole derivatives is described.
RETRO SYNTHETIC ANALYSIS
We had planned to prepare 2-[3-[4-(substituted)-1-cyclicamine]propyl]thio-5substituted[1,3,4]oxadiazole derivatives as it contains three important part from a
structural point of view as shown in Figure 1.
(i) a pharmacophoric portion constituted by a substituted 1,3,4-oxadiazole,
(ii) a terminal fragment constituted by a substituted cyclic amine like aryl piperazine
which is again a pharmacophore and
(iii) a three carbon linker between these two substructures i.e. pharmacophores.
So idea was that this structural feature could have influence on their biological activity.
pharmacophore
iii
pharmacophore
N N
Ar
N
X
ii
S
O
i
three carbon linker
Figure 1
The
2-[3-[4-(substituted)-1-cyclic
amine]propyl]thio-5-substituted[1,3,4]oxadiazole
derivatives (7-26) could be synthesized
by alkylation of
5-substituted-[1,3,4]
oxadiaazole-2-thiols (4a-h) with 1-(3-chloro propyl)-4-substituted cyclic amines (6a-
e). 5-Substituted-[1,3,4]oxadiazole-2-thiols (4a-h) could be synthesized by well known
method from corresponding aryl carboxylic acids via acid hydrazides (Scheme 1).
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SYNOPSIS
Scheme 1: Retro synthetic analysis
SYNTHESIS OF THE 5-SUBSTITUTED[1,3,4]OXADIAZOLE-2-THIOLS (4a-h)
5-Substituted[1,3,4]oxadiazole-2-thiols (4a-h) were synthesized by following
reported sequence of the reactions from corresponding aryl carboxylic acids (1)
(Scheme 2). The aryl carboxylic acids (1) were converted to corresponding methyl
esters (2) with catalytic amount of sulfuric acid in methanol. This was confirmed by
disappearance of broad carboxylic acid peak in 1H NMR spectra and disappearance of
broad ‘OH’ stretching of carboxylic acid in IR spectra of the product. It was also
supported by appearance of OCH3 peak in 1HNMR spectra of methyl esters (2). The
methyl esters (2) were converted to corresponding acid hydrazides (3) using hydrazine
hydrate in methanol. This was confirmed by disappearance of OCH3 peak and
appearance of NH and NH2 peaks in 1H NMR spectra of the product. It was also
supported by the appearance of NH stretching and C=O stretching of acid hydrazide
in IR spectra of the product.
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SYNOPSIS
O
Ar
O
(i)
Ar
OH
1a-h
O
(ii)
2a-h
(iii)
N NH
(iv)
Ar
O
NH2
3a-h
N N
S
N
H
Ar
OCH3
Ar
O
SH
4a-h
Ar : a = 4-Cl-C6H4, b = 4-CH3-C6H4, c = 2-CH3-C6H4, d = 4-OCH3-C6H4, e = 4-F-C6H4,
f = 2,4-di-Cl-C6H3 , g = 3,4-di-OCH3-C6H3, h = 4-OH-C6H4
Scheme 2. Reagents and condtions: (i) Cat. H2SO4, CH3OH, reflux, 4 h; (ii)
NH2NH2.H2O, CH3OH, reflux, 2-3 h; (iii) CS2/KOH, reflux, over night; (iv) dil.HCl.
The acid hydrazides (3) were reacted with carbon disulfide in the presence of
potassium hydroxide to produce corresponding 5-Substituted[1,3,4]oxadiazole-2-thiols
(4a-h). Some reports states that they mainly exist in thione-thiol tautomeric forms. This
was confirmed by disappearance of C=O peak of hydrazide and appearance of
characteristic C-O-C , C=N stretching in IR spectra which indicates the ring closure of
the 1,3,4-oxadiazole ring and molecular weight, the characteristic peaks in 1H NMR
spectra
of
the
product
had
further
confirmed
the
formation
of
5-
substituted[1,3,4]oxadiazole-2-thiols.
SYNTHESIS OF 1-(3-CHLORO PROPYL)-4-SUBSTITUTED CYCLIC AMINES (6a-e)
So we have decided first to prepare 1-(3-chloro propyl)-4-substituted cyclic
amines (6a-e) by the reaction of cyclic amines (5a-e) with 1-bromo-3-chloropropane in
presence of activated zinc in THF at ambient temperature (Scheme 3).
Scheme 3: Synthesis of 1-(3-chloropropyl)-4-substituted cyclic amines.
VIII
SYNOPSIS
Method A:
Synthesis of the 2-[3-[4-(substituted)-1-cyclic amine]propyl]thio-5-substituted
[1,3,4]oxadiazole derivatives (7-26)
The synthetic utility of zinc is well established in the literature for various
chemical reactions. Zinc has drawn the attention of many researchers because it is easily
available, inexpensive, and nontoxic. As a part of our continuing efforts to develop
simple, efficient, economical, and environmentally benign synthetic methods using zinc
dust. We have demonstrated a convenient zinc-mediated method for the S-alkylation of
5-aryl-1,3,4-oxadiazole-2-thiol with 1-(3-chloropropyl)-4-substituted piperazines in the
absence of base. The oxadiazole thiol was alkylated with 1-(3-chloropropyl)-4substituted piperazines using K2CO3 and a catalytic quantity of tetra butyl ammonium
iodide (TBAI), which in acetonitrile solvent gave the product in good yield.
Cl
N N
Ar
O
4a-h
SH +
N
N N
Zn, TBAI (cat.)
THF, 1 h, reflux
Ar
O
N
S
X
6a-e
X
7-26
Ar : a = 4-Cl-C6H4, b = 4-CH3-C6H4, c = 2-CH3-C6H4, d = 4-OCH3-C6H4, e = 4-F-C6H4,
f = 2,4-di-Cl-C6H3 , g = 3,4-di-OCH3-C6H3, h = 4-OH-C6H4
X : a = N-4-fluoro phenyl, b = N-phenyl, c = N-benzyl, d = O, e = N-3-chlorophenyl
Method:A
Scheme 4: Synthesis of 2-[3-[4-(substituted)-1-cyclic amine]propyl]thio-5-substituted
[1,3,4]oxadiazoles.
The same reaction in the presence of zinc dust at room temperature did not form
the alkylated product. Equimolar quantities of various 5-substituted-1,3,4-oxadiazole2-thiols were reacted with 1-(3-chloropropyl)-4-substituted piperazines in the presence
of zinc dust and catalytic amount of TBAI in tetrahydrofuran (THF) solvent, which
produced exclusively the S-alkylated product in excellent yield with high purity (Table
1). The reaction was also investigated for the possibility of using zinc dust in catalytical
IX
SYNOPSIS
or less than stoichiometric quantities. High yields of the products with excellent purity
were obtained with 1 equivalent of zinc dust. The products (7-26) were characterized
by their spectral data. The 1H NMR spectrum is the indicative for the determination of
the S-alkylated isomer. The 1H NMR of the compound 10 showed a chemical shift at δ
3.37, which is typical36 for S-CH2 protons.
X-ray crystallography was undertaken on a representative compound (10) to
further confirm the structural isomer. A crystal of compound (10) was grown by slow
evaporation from hexane and the chloroform mixture, and the S-alkylated isomer was
proved by single-crystal x-ray analysis of the compound (10). The reusability of zinc
dust was studied for compounds 8 and 12. The zinc dust was reactivated and reused in
further runs, and no remarkable loss in activity was observed. (Used zinc was treated
with 10% HCl, washed with water and acetone, and dried in an oven for 2 h at 120– 130
o
C.) This makes the process more economical.
Method B:
One-pot
Three-Component
Synthesis
of
2-[3-[4-(substituted)-1-cyclic
amine]propyl]thio-5-substituted[1,3,4]oxadiazole derivatives (7-26)
To our knowledge, no studies are known in the literature exploiting KF/Al2O3
for achieving the conversion of acid hydrazides into 2-[3-[4-(substituted)-1-cyclic
amine]propyl]thio-5-substituted[1,3,4]oxadiazole derivatives. In view of the rapidly
increasing demands for green chemistry, an efficient and convenient method for the
synthesis of 1,3,4-oxadiazole derivatives is highly desirable. We report here a simple
and economical procedure for the synthesis of 2-[3-[4-(substituted)-1-cyclic
amine]propyl]thio-5-substituted[1,3,4]oxadiazole derivatives.
The versatile solid-supported reagent, potassium fluoride loaded on alumina,
KF/Al2O3, which was originally introduced by Ando et al. Due to its strongly basic
nature it has been used as a replacement for organic bases in a number of organic
reactions such as Suzuki couplings, Sonogashira couplings, Knoevenagel reactions, and
N-alkylations of amides and epoxidation reactions. Weinstock et al. had argued that
KF/Al2O3 derives its basicity from the formation of KOH in the initial preparation of
the solid supported material by reaction of KF with the alumina support. Ando et al. had
X
SYNOPSIS
concluded that there are three basic species or mechanisms of the appearance of the
basicity of KF/Al2O3: (1) the presence of active fluoride, (2) the presence of [Al–O-] ion
which generates OH- when water is added, and (3) the cooperation of F- and [Al–OH].
It is well-known in the literature that the KF/Al2O3 as a base in water will serve as an
effective reagent in the organic synthesis. Water can be considered the best
environmentally friendly, cleanest and cheapest solvent because its use not only
eliminates the necessity of vigorous drying of solvents and substrates, but also because,
owing to its highly polar character, unique reactivity and selectivity are often observed
in aqueous reactions.
RESULTS AND DISCUSSION
To explore the use of KF/Al2O3 in the synthesis of 1,3,4-oxadiazole , we treated
p-tolylhydrazide (3b), carbon disulfide and 1-(3-chloropropyl)-4-phenylpiperazine (6b)
in the presence of KF/Al2O3 with catalytic amount of TBAI in water (Scheme 5). The
corresponding product (11) was obtained with high yield (92 %). To check the
generality of the reaction, various hydrazides were treated with various 1-(3chloropropyl)-4-substituted piperazines under the same reaction conditions and the
results were summarized in Table 1.
Scheme 5: One-pot three-component synthesis of 2-[3-[4-(substituted)-1-cyclic
amine]propyl]thio-5-substituted[1,3,4]oxadiazoles.
XI
SYNOPSIS
The products were obtained in high yields with excellent purity. For the same reaction,
yields were lower and reaction time was longer when the water was replaced with
acetonitrile. The reaction was unsuccessful when either KF or Al2O3 was used. The S
atom of oxadiazole is a soft base and the N atom is a medium base. 1-(3-Chloropropyl)4-substituted piperazines as soft acid preferred to S rather than N when it reacted with
oxadiazole. The
1
H NMR and
13
C NMR spectra were the indicative for the
determination of the S-alkylated isomer. In the 1H NMR spectra of 11, a triplet for the
methylene group of the propyl chain that connects the piperazine moiety and the
oxadiazole part of the molecule, is observed at a chemical shift of δ 3.37, which is
typical for S–CH2 connectivity. This result was further confirmed by 13C NMR spectra
of 11, where a signal at δ 30.6 was observed which is typical for the S–CH2
connectivity. The plausible mechanism was given in Figure 2.
Figure 2: Plausible mechanism
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SYNOPSIS
CONCLUSION
In conclusion, we have developed a three-component one-pot method for the
synthesis of cyclic amine bearing 1,3,4-oxadiazole derivatives catalyzed by KF/Al2O3 in
aqueous media. The products were obtained in high yields with excellent purity. The
method is economical and environmentally benign. The chemo-selectivity of the
products was confirmed by 1H NMR and 13C NMR spectroscopy.
CHAPTER-III
SYNTHESIS OF NEW HETERO CYCLIC HYBRIDS BASED ON BENZOXAZOLONE AND 1,3,4OXADIAZOLE SCAFFOLDS
2(3H)-Benzoxazolone heterocycles are considered ‘privileged scaffolds’ in the
design of pharmacological probes. These heterocycles have, in fact, high versatility in
chemical modifications, allowing changes to the characteristics of side-chains on a rigid
platform. Usually functionalization of the nitrogen atom (3rd position) is of interest
since the electronic characteristics of this atom can be decisive for the biological
activity. They have received considerable attention from medicinal chemists, due to
their capacity to mimic a phenol or a catechol moiety in a metabolically stable template.
Studies designed to determine the mode of the biological action require the synthesis of
many derivatives and, in the last twenty years, this class of compounds has led to the
discovery of a number of derivatives endowed with various biological activities like
anti-inflammatory, analgesic, antitubercular, antimalarial, anticancer, anticonvulsant,
anti-HIV, antinociceptive, antioxidant, 5-HT antagonist, PPARγ agonist, antimicrobial
etc. This background of 2(3H)-benzoxazolone and the medicinal value of 5-chloro2(3H)-benzoxazolone, broad spectrum activity of 5-substituted-1,3,4-oxadiazole-2-thiol
motif is the key for the initiation of the present work.
MOLECULAR HYBRIDIZATION APPROACH: DESIGN OF NEW HYBRID MOLECULES
Combinations of adequate subunits through the molecular hybridization can
create new entities with better biological profiles. In this approach hybrid compounds
were constructed by linking pharmacophore subunits (benzoxazolone and oxadiazole)
directly with spacer agents (Figure 1).
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SYNOPSIS
X
H
N
N N
O
HS
O
Ar
O
1,3,4-oxadiazole-2-thiol
benzoxazolone
Molecular
Hybridization
X
linker
Ar
O
N
O
S
N
N
O
6a-t
Figure 1: Hybrid compounds with pharmacophore moiety of substituted benzoxazolone
and substituted oxadiazole-2-thiol.
SYNTHESIS OF OXADIAZOLYL BENZOXAZOLONE HYBRID MOLECULES
In recent times, metal and metal-salt catalyzed reactions are extensively studied
due to their reusability and easy work-up. Many researchers used zinc because of its
easy availability, inexpensiveness and non-toxic nature. In the present work, zinc dust
was used for alkylation reactions and the details are illustrated as follows.
Various 5-substituted-3-(3-chloropropyl)benzo[d]oxazol-2(3H)-ones (4a-b) and
5-substituted-3-(3-(5-substituted-1,3,4-oxadiazol-2-ylthio)propyl)benzo[d]oxazol2(3H)-ones (6a-t) were synthesized starting from commercially available 2-amino-4chlorophenol (1). 5-substituted-2(3H)-benzoxazolone (2a-b) was synthesized by the
reaction of 2-amino-4-chlorophenol (1) and urea in concentrated HCl at 140-170 °C
(Scheme 1). The product was confirmed by 1H NMR,
13
C NMR, IR and Mass
Spectroscopy. The characteristic ‘N-H’ and ‘C=O” stretching were presence in IR
spectrum of 2a. 5-Chloro-2(3H)-benzoxazolone 2a, K2CO3 and di halide (3) were
reacted in acetonitrile at ambient temperature to obtain the N-alkylated products i.e. 5-
XIV
SYNOPSIS
chloro-3-substituted-1,3-benzoxazol-2(3H)-ones (4a-b) as shown in the Scheme 1. The
products were confirmed by 1H NMR,
13
C NMR, IR and Mass Spectroscopy. The
characteristic aromatic protons of 2a-b and the characteristic protons of 3 were present
in 1H NMR spectra of the products 4a-b. Similarly the characteristic aromatic carbons
of 2a-b and the characteristic carbons of 3 were present in
13
C NMR spectra of the
products 4a-b.
NH2
X
+
OH
O
H2N
(i)
O
NH2
O
Urea
1a-b
H
N
X
2a-b
Cl
2a-b +
Cl
Br
X
(ii)
O
O
4a-b
3
(iii)
N N
4a-b
+
HS
O
5a-j
Ar
N
X
Ar
Method A
O
N
(iv)
O
Method B
O
S
N
N
6a-t
Ar: a = 4-CH3-C6H4, b = 2-CH3-C6H4, c = 4-Cl-C6H4, d = 2,4-di Cl-C6H3, e = 2-OCH3-C6H4,
f = 3-OCH3-C6H4, g = 4-OCH3-C6H4, h = 3,4-di-OCH3-C6H3, i = 4-F-C6H4, j = 3-Br-C6H4;
X = Cl, H
Scheme 1:Reagents and conditions:(i) Conc. HCl, 140-170°C, 6 h; (ii) K2CO3, CH3CN,
r.t, 8 h; (iii)K2CO3, KI, Acetonitrile, reflux, 6 h ; (iv) Zn, cat.TBAI, THF, reflux, 3 h.
The N-alkylation was confirmed by the two observations: (1) The 5chorobenzoxazolone (2a) carbonyl stretching (υ C=O, 1771 cm-1) was retained in the IR
spectrum of 4a (υ C=O, 1774 cm-1). (2) The 13C chemical shift value for the carbonyl
carbon (δ =152.6 ppm) of the 5-choro-2(3H)-benzoxazolone (2a) was retained in the
13
C spectrum of 4a (δ =154.2 ppm). The alkylation of 2a-b with 3 in the presence of
XV
SYNOPSIS
zinc dust yielded the products with less purity in low yield. Hence the reaction was
carried out under conventional method using a base.
Compounds 6a-t were synthesized by means of the following two methods as
shown in the Scheme 1: (i) Compounds 5-substituted-1,3,4-oxadiazole-2-thiols (5a-j )
(synthesis of oxadiazole-2-thiols described briefly in the Chapter II) reacted with
various 5-substituted-3-(3-chloropropyl)benzo[d]oxazol-2(3H)-ones (4a-b)
using
K2CO3 and cat. KI in acetonitrile at refluxed conditions to obtain different 5substituted-3-(3-(5-substituted-1,3,4-oxadiazol-2-ylthio)propyl)benzo[d]oxazol-2(3H)ones (6a-t). (ii) A simple and convenient new synthesis of 6a-t was carried out using
zinc dust under neutral conditions. Compounds substituted-oxadiazole-2-thiol (5a-j)
were reacted with various various 5-substituted-3-(3-chloropropyl)benzo[d]oxazol2(3H)-ones (4a-b) using zinc dust, cat.TBAI in dry THF obtain different 6a-t (Salkylated products). The products were confirmed by 1H NMR, 13C NMR, IR and Mass
Spectroscopy. The 1H NMR spectrum is the indicative for the determination of the Salkylated isomer. The 1H NMR of the compound in 6a showed a chemical shift at δ
3.35, which is typical for S-CH2 protons.
The characteristic aromatic and methylene protons of 4a-b and the characteristic
aromatic protons of 5a-j were present in 1H NMR spectra of the products 6a-t.
Similarly the characteristic aromatic and methylene protons of 4a-b and the
characteristic aromatic protons of 5a-j were present in 13C NMR spectra of the products
6a-t. Thus the synthesis of 6a-t was illustrated by the use of environmentally benign
protocol that involves the utilization of the recyclable metal, zinc dust as the reaction
medium that enables economical, efficient and high yield products.
CONCLUSION
In conclusion, the synthesis of various biologically active new 5-substituted-3(3-(5-substituted-1,3,4-oxadiazol-2-ylthio)propyl)benzo[d]oxazol-2(3H)-ones
(6a-t)
was achieved with good yield. Here we have explored utility of zinc dust as recyclable
catalyst. The method is environmentally friendly and economical.
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SYNOPSIS
CHAPTER-IV
BIOLOGICAL EVALUATION OF NEWLY SYNTHESIZED AZOLE HETEROCYCLES
SECTION-A: BIOLOGICAL EVALUATION OF NEW CYCLIC AMINE BEARING 1,3,4OXADIAZOLE DERIVATIVES
The synthesized cyclic amine bearing 1,3,4-oxadiazole derivatives (7-26) were
evaluated for anticancer, antibacterial and antifungal activities.
ANTICANCER ACTIVITY
The sensitivity of the human leukemic cell lines, HL-60 (myeloid leukemia) and
U937 (leukemic monocyte lymphoma) to the synthetic oxadiazole derivatives was
evaluated. The cytotoxic activities of the derivatives were determined by measuring the
number of live cells after 24 h of treatment (MTT assay). The test compounds, (9) and
(15) were potent antiproliferative agents and inhibited cell growth on HL-60 cells, with
IC50 values of 13.56 and 9.51μg/mL, respectively. However, the other compounds did
not influence any cytotoxic activity on HL-60 cells at concentrations of 100 μg/mL.
In contrary, the test compound, (11) responded exclusively to show cytotoxic activity
on U937 cells with IC50 value of 9.36 μg/mL. Neverthless, others entries exhibited
insignificant cytotoxic activity on U937 cells at all the test concentrations.
ANTIBACTERIAL ACTIVITY
In the search of new antibacterial agents, all the twenty compounds (7-26) were
evaluated for antibacterial activity. The minimum inhibitory concentrations (MIC) of
various synthetic compounds were tested against three representative Gram-positive
organisms viz. Bacillus subtilis (MTCC 441), Staphylococcus aureus (MTCC 96),
Staphylococcus epidermidis and Gram-negative organisms viz Escherichia coli (MTCC
443), Pseudomonas aeruginosa (MTCC 741), and klebsiella pneumoniae (MTCC 618)
by broth dilution method recommended by National Committee for Clinical Laboratory
(NCCL) standards. The compounds (22), (23), (25) and (26) had shown comparatively
more antibacterial activity against Staphylococcus aureus (MTCC 96) than other
compounds may be due to the halogen substituents on aryl ring (except 26). The other
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compounds had not shown any remarkable antibacterial activity than Streptomycin,
Penicillin.
SECTION-B: BIOLOGICAL EVALUATION OF HETERO CYCLIC HYBRIDS BASED ON
BENZOXAZOLONE AND 1,3,4-OXADIAZOLE SCAFFOLDS
ANTICANCER ACTIVITY
The cytotoxicity studies of the compounds 6a-t were determined against five
human cancer cell lines, namely cervical (HeLa), liver (Hep3B), colon (HCT116), skin
(A375), lung (A549) cancer cell lines using MTT assay. The IC50 values of the test
compounds 6a-t and the percentage cell viability induced by the different
concentrations of the drugs in the cervical cancer cell line, He La , the liver cancer cell
line, Hep3B, the colon cancer cell line, HCT116 , skin cancer cell line, A375 and lung
cancer cell line, A549 are presented. The percentage cell viability data indicates the
percentage cytotoxic activity (dose dependent) of the synthesized compounds at
concentrations ranging from 25 to 250 µM.
The compounds 6c and 6e shown very good activity towards cervical, liver,
colon, lung cancer cells (IC50 : 46- 99 µM) and 6j, 6k, 6n, 6o shown very good activity
towards liver, colon, lung cancer cells (IC50 : 44- 101 µM). Compound 6l induced
cytotoxicity in cervical, colon, lung cancer cells (IC50 : 24- 76 µM), and compound 6s
shown very good activity towards liver, lung cancer cell lines (IC50 : 75- 76 µM).
Compound 6q shown very good activity towards liver, colon cancer cell lines (IC50: 2563 µM) and 6t shown very good activity towards liver, colon, skin and lung cancer cell
lines (IC50 : 19- 78 µM).
From the results it is evident that these compounds are comparatively more
effective on human lung cancer cells (A549) and liver cancer (Hep3B) than colon
cancer (HCT-116) cells and cervical cancer (HeLa). Among twenty compounds, ten
compounds 6c, 6e, 6j, 6k, 6n, 6o, 6l, 6s, 6q and 6t are the potent molecules
possessing anti-proliferative activity. These compounds also may be considered for
further studies in more cell lines. Thus, the activity profile of these hybrid compounds
can be used as new lead molecules in the development of effective anticancer agents
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especially liver cancer cell lines (Hep3B), colon cancer cell lines (HCT116) and human
lung cancer cell lines (A549). This study provides a good starting point to develop
unsymmetrical benzoxazoles as novel anticancer agents. In short, our findings might
be beneficial as leads for designing new compounds with potential antitumor activity.
ANTIBACTERIAL ACTIVITY
All the twenty compounds 6a-t was evaluated for antibacterial activity. The
minimum inhibitory concentrations (MIC) of synthesized compounds were tested
against three representative Gram-positive organisms viz. Bacillus subtilis (MTCC
441), Staphylococcus aureus (MTCC 96), Staphylococcus epidermidis and Gramnegative organisms viz Escherichia coli (MTCC 443), Pseudomonas aeruginosa
(MTCC 741) and klebsiella pneumoniae (MTCC 618). The results indicate that no
compound had shown significant antibacterial activity.
ANTIFUNGAL ACTIVITY
In the search of new antifungal agents, all the twenty synthesized compounds
6a-t were evaluated for antifungal activity. Most of the compounds had shown
moderate activity especially against C.rugosa. The results indicate that the hetero cyclic
hybrids 6a-t have shown only moderate antifungal activity and the substitutions at C-2,
C-3, C-4 positions of phenyl ring of 1,3,4-oxadiazole nucleus and C-5 positions of the
benzoxazole moiety have no significant contribution to the antifungal activity of these
compounds. None of the compounds have shown higher antifungal activity than the
drugs used as a standard, Amphotericin-B.
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