A REVIEW ON SYNTHESIS AND PHARMACOLOGICAL DIVERSITY OF ISOXAZOLES & PYRAZOLINES

Author : Kuntal Manna *, Udayan Banik, Partha Sakha Ghosh and Manik Das

Page Nos : 37 - 49

Cite Article :

Manna K, Banik U, Ghosh PS, Das M. A Review on Synthesis and Pharmacological Diversity of Isoxazoles & Pyrazolines. NUJPS. 2014;1(1):37–49.

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Introduction
Heterocyclic compounds forms a major
class of compounds in chemistry. These are
cyclic compounds having at-least two
different elements as a part of the ring
system. On the other hand, a ring system
made up of same elements is considered as
homocyclic compounds.
Fused saturated 5-membered rings
naturally occurring in both carboxylic
(Eg. Triquinane) and heterocyclic (Eg.
Pyrrolizidin alkaloids) systems have
interesting pharmacological importance
[1].
Usually 6-membered rings have well
defined and readily determined
conformations like chair and boat, but 5-
membered rings are much flexible and
their conformations are difficult to
ascertain (Figure 1) [2].

Structures of Various 5
Figure 1: Structures of Various 5-
membered Heterocycles
Pyrazolines are well known and important
nitrogen containing 5-membered
heterocyclic compounds. 2-Pyrazolines
show a broad spectrum of pharmacological
activities and are present in a number of
pharmacologically active molecules such
as phenazone, amidopyrine,
methampyrone (analgesic and antipyretic),
azolid, tandearil (anti-inflammatory),
indoxacarb (insecticide) and anturane

(uricosuric) [3]. Numerous pyrazoline
derivatives have been found to possess
considerable biological activities, which
stimulated the research activity in this
field. They have several prominent effects,
such as antimicrobial, antimycobacterial,
antifungal, anti-amoebic,
antiinflammatory, analgesic, antidepressant
and anticancer activities. They also possess
some potent, receptor selective biological
activity like nitric oxide synthase (NOS)
inhibitor and cannabinoid CB1 receptor
antagonist activity [4]. The work provides
an insight view to pyrazolines synthesis
and its biological activities. Isoxazole
(C H NO) is an azole with an oxygen atom 3 3
next to the nitrogen. Isoxazolyl is the
univalent radical derived from isoxazole.
Isoxazole ring is found in some natural
products, such as ibotenic acid. Isoxazoles
also form the basis for a number of drugs,
including the COX-2 inhibitor valdecoxib
(Bextra) (Figure 2). A derivative, furoxan,
is a nitric oxide donor. An isoxazolyl group
is found in many beta-lactamase-resistant
antibiotics such as cloxacillin, dicloxacillin
and flucloxacillin. The synthetic
androgenic steroid danazol also has an
isoxazole ring [5].
Isoxazoles are unsaturated aromatic
heterocyclic compounds containing a ring
with three carbon atoms, one oxygen atom
and one nitrogen atom. The trivial name
for the title five-membered fully
unsaturated heterocycles as “isoxazole”
was originally proposed by Hantszch as it
was the isomer “oxazole” discovered first.
The trivial name follows the Hantszch-
Widman system of nomenclature: the
prefix “iso” represents isomer, “oxa”.

represents the oxygen atom “aza”
represents the nitrogen atom. The suffix
“ole” denotes the ring size as fivemembered;
altogether the derived name is
“isoxazole” [6]. This name has been
accepted in IUPAC and has been used in
Chemical abstracts. In Chemical Abstracts,
the other systematic name 1,2-azole, is also
used. Isoxazole being an azole with an
oxygen atom next to the nitrogen, exhibits
broad spectrum of biological activity and
also forms a part of various biodynamic
agents [7].
Isoxazole derivative having
Figure 2: Isoxazole derivative having
established pharmacological activity

The substituted isoxazoles are also
considered to be important synthons due to
their versatility towards chemical
transformations to useful synthetic
intermediates. Isoxazole derivatives show
hypoglycemic, analgesic,
antiinflammatory, antifungal, anti-bacterial
and HIV-inhibitory activities [8]. Synthesis
of hybrid natural products has gained
momentum in recent years. It is expected
that combining features of more than one
biologically active natural segment in a
single molecule may result in pronounced
pharmacological activity while retaining

high diversity and biological relevance [9-
11]. The nitrogen hetero atom is more
pronounced for electron withdrawing
effect, while the oxygen atom is more
pronounced for electron donating effect.
As neutral molecules, isoxazoles undergo
electrophilic substitution rather more
readily at the position-4 than benzene.
Effects of substituents can modify their
behavior.
Substituents at the position-5 apparently
have more activating and deactivating
effect than substituents at the position-3. In
natural product synthesis, isoxazoles are
used as latent synthons, such as masked
new heterocyclic rings, masked fused
rings, masked aromatic rings and masked
aldol and related moieties. The capability
of isoxazole undergoing reaction is
diverse: protonation, quaternization,
complexation, oxidation, reduction,
carbanionic condensations, thermolysis,
photolysis, transformations into other
heterocyclic ring systems and reaction with
electrophiles, nucleophiles and Grignard
reagents [12].
The naturally occurring antibiotic
cycloserine [13]; the monoamine oxidase
inhibitor isocarboxazide; isoxazole
steroids, ibotenic acid; muscimol isolated
from Amanita muscaria [14] and
isoxazoline-5-ones isolated from Legume
seed [15] are potential isoxazole
derivatives.
Methods of synthesis of isoxazoles

Figure 3 showed regioselective cyclocondensation
of acrylophenone dibromide
derivatives with hydroxylamine

hydrochloride giving corresponding
isoxazoles [16].
Synthesis of isoxazoles from 2

Figure 3: Synthesis of isoxazoles from

acrylophenone dibromide

3-Alkyl, 5-aryl isoxazoles can be prepared
from aryl cycolpropanes (Figure 4) with
NaNO in CF COOH [17]. 2 3

Synthesis of isoxazoles from 2
Figure 4: Synthesis of isoxazoles from
aryl cyclopropanes

Solid phase synthesis of isoxazole
derivative from diaryl 1,3-diketones
(Figure 5) can be carried out in presence of
hydroxylamine hydrochloride and silica
gel [18].

Synthesis of isoxazoles from 3
Figure 5: Solid phase synthesis of
isoxazoles from diaryl 1,3-diketones

Reaction of various substituted
acetophenones with diethyl oxalate in the
presence of sodium ethoxide forms
resulting 2,4-diketo esters which on.

treatment with hydroxylamine
hydrochloride furnishes substituted 3-
isoxazole esters (Figure 6) [19].

Synthesis of 3-isoxazoles from
Figure 6: Synthesis of 3-isoxazoles from
acetophenones

Pharmacological aspects of isoxazole
Anti-convulsant activity
The search for antiepileptic compounds
with more selective activity and lower
toxicity continues to be an area of
investigation in medicinal chemistry [20].
Many patients with epilepsy fail to
experience adequate control of their
seizures, despite the optimal use of
available antiepileptic drugs. Other
patients do so only at the expense of
significant toxic side effects. In recent
years it has been established that inhibitors
of GABA transport and in particular as
troglial uptake can act as anticonvulsant
agents and several isoxazole derivative
(compound-A) (Figure 7) [21].
Compound-B is also a synthesized
isoxazole derivative which affects the
sodium channel to show its activity [22].

Isoxazoles having anticonvulsant
Figure 7: Isoxazoles having anticonvulsant
activity

Anti-cancer activity
The effects of curcumin and of its
isoxazole analogue in breast cancer cell
line and in its multidrug-resistant (MDR)
variant were examined. The isoxazole
analogue Compound A has shown more
potent antitumor and molecular activities
both in parental and in MDR tumor cells.
Isoxazole derivatives produces
significantly higher direct inhibition of the
COX-2 catalytic activity than curcumin.
The isoxazole derivatives proved better
because of minimum metal chelation when
compared to curcumin [23]. The
compound B has been found highly
effective against human tumor cell lines
especially on renal cancer, CNS cancer cell
and ovarian cancer cell lines [24]. Recently
NO-NSAID has been established as potent
anti-cancer agents rather than their antiinflammatory
property [25]. Compound C
is a NO donating compound used as anticancer
agent.
Anti-microbial activity
Drazoxolon is a commonly used fungicidal
agent. Acetyl Sulfisoxazole is another
important anti-microbial agent from the
isoxazole family which is widely used in
pediatric suspensions (Figure 8).
Cycloserine is a well-established molecule
widely known for its potency against
Mycobacterium tuberculosis [26-28].
Isoxazoles having
Figure 8: Isoxazoles having
antimicrobial activity

Pyrazoline
Pyrazole is a π-excessive aromatic
monocyclic heterocycle containing two
nitrogen atoms in a five membered 1,2-
diazole ring. Pyrazoles exhibit aromatic
character with properties resembling those
of both pyrrole and pyridine. 1-pyrazoline,
2-pyrazoline and 3-pyrazoline are the three
partially reduced forms of the pyrazole
structure with different positions of the
double bonds and exists in equilibrium one
with the other [29]. 2-pyrazoline exhibits
the monoimino character and hence more
stable than the rest. Pyrazole is freely basic
and forms salts with inorganic acids. The
imino hydrogen may be replaced by an
acyl group. Pyrazole is very resistant to
oxidation and reduction, but may be
hydrogenated catalytically, first to
pyrazoline and then to pyrazolidine [30].
Phenylbutazone is a pyrazoline derivative
known for its analgesic and antiinflammatory
and anti-pyretic properties
(Figure 9). Muzolimine is a pyrazoline
derivative used as a diuretic which differs
from the structures of other conventional
diuretic compounds.

Pyrazoline derivative having
Figure 9: Pyrazoline derivative having
established pharmacological activity

Forbisen is a byproduct separated during
synthesis of antipyrine. This pyrazoline is
used in bovine analplasomosis.
Oxyphenbutazone is a pyrazoline
derivative similar to phenylbutazone. It is
an active metabolite of phenyl butazone.
The difference with phenyl butazone is the
presence of p-hydroxyphenyl group instead
of phenyl at position 1 of phenylbutazone
[31].
Methods of synthesis of pyrazolines
α, β-unsaturated carboxylic acid esters
reacts with diazomethane to give 2-
pyrazolines (Figure 10). The primary
product of this reaction is a 1-pyrazoline,
formed by 1, 3-dipolar cyclo addition,
which spontaneously isomerizes into its
thermodynamically more stable 2-
pyrazoline isomer by a 1, 3-H shift [32].

Synthesis of pyrazolines from
Figure 10: Synthesis of pyrazolines from
α, β-unsaturated carboxylic acid esters

Figure 11 showed an example of synthesis
of a pyrazoline from the reaction of an α,β-
unsaturated ketone and diazomethane.
Benzylidene-acetone on reaction with
diazomethane by 1,3- dipolar
cycloaddition yield 2-pyrazolines [33-35].
Synthesis of pyrazolines from ketone
Figure 11: Synthesis of pyrazolines from
α, β-unsaturated ketone

Cycloaddition reaction (Figure 12) of
substituted styrenes with p-anisyl
diazomethane at low temperature yield
trans-3, 5-bis-(p-anisyl)-1-pyrazoline [36].

Synthesis of pyrazolines by
Figure 12: Synthesis of pyrazolines by
cycloaddition reaction

Chalcone derivatives on reacting with
hydrazine hydrate and glacial acetic acid,
form corresponding N-phenyl-3,5-diphenyl
pyrazoline & 3,5-diphenyl pyrazoline
derivatives respectively (Figure 13) [37].
Synthesis of pyrazolines from
Figure 13: Synthesis of pyrazolines from
Chalcone derivatives

Pharmacological aspects of pyrazolines:
2-pyrazoline is insoluble in water but
soluble in propylene glycol because of its
lipophilic character [38]. 2-pyrazolines
display a broad spectrum of potential
pharmacological activities and are present
in a number of pharmacologically active
molecules such as phenazone,
amidopyrene, methampyrone (analgesic
and antipyretic), azolid/ tandearil (antiinflammatory),
indoxacarb (insecticidal),
anturane (uricosuric), etc. Considerable
interest has been focused on the pyrazoline
structure. The discovery of this class of
drugs provides an outstanding case history
of modern drug development and also

points out the unpredictability of
pharmacological activity from structural
modification of a prototype drug molecule.
It is having a variety of medicinal
applications. Pyrazoline derivatives were
found to have potential antipyreticanalgesic,
tranquilizing, muscle relaxant,
psycho analeptic, antiepileptic,
antidepressant, anti-inflammatory,
insecticidal, antimicrobial and antihypotensive
activities. Their derivatives
were also found to exhibit cytotoxic
activity, inhibitory activity of platelet
aggregation, herbicidal activity and
cannabinoid CB1-receptor modulators.
Pyrazoline interest is also extended to dyes
and dye couplers [39]. Given below is a
brief account of various modifications
reported on chalcones, which resulted in a
variety of biological and pharmacological
activities.
Anti-cancer agents
Cancer is a leading cause of death
worldwide, accounting for 8.7 million
deaths (around 14% of all deaths) in 2012.
Although many chemotherapeutic agents,
such as cisplatin, 5 fluorouracil and taxol,
have been developed to treat different
kinds of cancer effectively, some side
effects could happen simultaneously.
Therefore, it is important and urgent to
develop novel compounds as anticancer
agents with higher bioactivities and lower
side effects [40, 41]. It is well known that a
number of multi-cyclic compounds
containing heterocycle fragments exhibit a
wide variety of biological activities.
Pyrazoline and pyrazole are important
structural fragments of many bioactive
compounds.

As the diverse pharmacological importance
of pyrazoline have already been explained
in the earlier sections, some of the
pyrazoline derivatives showed potent anticancer
activity (Figure 14).
Pyrazolines having Anticancer
Figure 14: Pyrazolines having Anticancer
activity

Compound A showed potential ALK5
inhibitory activity as transforming growth
factor-β type 1 receptor kinase inhibitors
[42]. But this compound showed some
extent of cytotoxicity. This problem is
eradicated when p-methoxy phenyl
pyrazoline fragment is introduced to the
ring-D of steroid in Compound B [43].
Compound C is proven to be active with
selective influence on colon cancer cell
lines [44]. The above mentioned
compounds A and B have been examined
for anti-proliferative activity against
human colon carcinoma and highly
metastatic human breast carcinoma [45].
Anti-microbial agents
The pyrazoline nucleus is a ubiquitous
feature of various compounds possessing
many pharmacological and physiological
activities and therefore they are useful
materials in drug research. It was reported

in the literature that different substituted 2-
pyrazolines possess antimicrobial activities
(Figure 15).

Pyrazolines having Antimicrobial
Figure 15: Pyrazolines having Antimicrobial
activity
Compound A have structural similarities to
siderophores and evaluated as novel
antimicrobials against Mycobacterium
tuberculosis and Yersinia pestis [46].
Compound B and C possess antibacterial
activities against Bacillus cereus,
Staphylococcus aureus, Escherichia coli
and Pseudomonas putida [47].
Hypotensive agent
Clonidine and Clonidine like thiazoloimidazoline
derivatives (Figure 16)
encouraged the development of pyrazoline
derivatives of this class which are proven
to have hypotensive activity. These
derivatives also bear structural and
isosteric relationship to Clonidine [48].

Pyrazoline having hypotensive activity
Figure 16: Pyrazoline having
hypotensive activity

Analgesic agent
Pain is one of the most prevalent
conditions that limits productivity and
diminishes quality of life. Although there
is an arsenal of effective and widely used
analgesics, there is some concern regarding
their safety and side-effects, making their
clinical use problematic. In the recent
years pyrazoline has emerged as Class of
analgesics having very high
pharmacological activity [49]. Dipyrone
(metamizol) is a drug that belongs to the
pyrazole derived family. Reports in the
literature have shown that dipyrone,
although generally considered as belonging
to the nonsteroidal anti-inflammatory drug
family, seems not to present the typical
side effects of this drug class. Antipyrine
(phenazone) is also a widely used
analgesic belonging to pyrazoline family.
Anti-depressant activity
Depression is a serious disorder with
estimates of lifetime prevalence as high as
21% of the general population in some
developed countries [50]. Treatment for
this disease is possible with antidepressant
medications and psychotherapy for some
patients [51].

Pyrazoline having antidepressant
Figure 17: Pyrazoline having antidepressant
activity.

Although antidepressants have been used
in the clinic for several decades, most of
them are inadequate in efficiency and have
many serious adverse side effects.
Pyrazolines were also reported as potential
antidepressant agents (Figure 17) which
act by MAO inhibition. The compound
shown above has been proven as a potent
anti-depressant activity involving
serotonergic system for its activity [52].
Conclusion

Review emphasizes on detailed
information of Isoxazole and Pyrazoline.
Isoxazole, an azole with an oxygen atom
next to the nitrogen, is found in many
natural products and also found in number
of drugs, including COX-2 inhibitor
valdecoxib. Furoxan, is a nitric oxide
donor is containing isoxazolyl group is
found in many beta-lactamase-resistant
antibiotics, such as cloxacillin,
dicloxacillin and flucloxacillin. The
synthetic androgenic steroid danazol also
has an isoxazole ring. Pyrazoline, a fivemembered
heterocycle having two adjacent
nitrogen atoms within the ring, has only
one endocyclic double bond and is basic in
nature. Among its various derivatives, 2-
pyrazolines seem to be the most frequently
studied pyrazoline type compounds. 2-
Pyrazolines can be considered as a cyclic
hydrazine moiety display a broad spectrum
of potential pharmacological activities.
Both ring systems are having many
pharmacological activity including but not
limited to anti-cancer, anticonvulsant,
antimicrobial, etc. These ring systems can
further exploit in future to design novel
pharmacologically active molecules

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