Synthesis and antioxidant activity of oxazolyl/thiazolylsulfonylmethyl pyrazoles and isoxazoles

Article abstract of DOI:10.1016/j.ejmech.2011.08.010

A new class of oxazolyl/thiazolylsulfonylmethyl pyrazoles (10-13) and isoxazoles (14, 15) were prepared from the synthetically vulnerable intermediate E-styrylsulfonylacetic acid methyl ester (1) and studied their antioxidant activity.

Full text of DOI:10.1016/j.ejmech.2011.08.010

European Journal of Medicinal Chemistry 46 (2011) 5034e5038
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European Journal of Medicinal Chemistry
journal homepage: http://www.elsevier.com/locate/ejmech
Original article
Synthesis and antioxidant activity of oxazolyl/thiazolylsulfonylmethyl pyrazoles
and isoxazoles
*
A. Padmaja , C. Rajasekhar, A. Muralikrishna, V. Padmavathi
Department of Chemistry, Sri Venkateswara University, Tirupati 517 502, Andhra Pradesh, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A new class of oxazolyl/thiazolylsulfonylmethyl pyrazoles (10e13) and isoxazoles (14, 15) were prepared
from the synthetically vulnerable intermediate E-styrylsulfonylacetic acid methyl ester (1) and studied
their antioxidant activity.
Received 24 May 2011
Received in revised form
2 August 2011
Accepted 5 August 2011
Available online 12 August 2011
Ó 2011 Elsevier Masson SAS. All rights reserved.
Keywords:
Oxazoles
Thiazoles
Pyrazoles
Isoxazoles
Antioxidant activity
1. Introduction
oxazole ring is a useful structural motif found in numerous bio-
logically active molecules. Natural product Hennoxazole A, first
The recent literature is enriched with progressive findings about
the synthesis and pharmacological activities of azole based
compounds. The thiazole ring is an interesting building block in
a variety of natural products and bioactive compounds useful as
pharmaceuticals or agrochemical agents [1e5]. Recently, many
natural products containing thiazole moiety were isolated and
most of them exhibit considerable cytotoxicities and antitumor
potentials [6e13]. Tiazofurin, a thiazole-4-carboxamide derivative,
a C-nucleoside having significant activity against both human
lymphoid [14] and lung tumor cell lines [15]. Its biological effects
also include its efficiency in the treatment of acute myeloid
leukemia [16] and chronic myeloid leukemia in blast crisis [17].
Isoxazoles and pyrazoles have attracted much attention since they
play vital role in synthetic organic chemistry over the years and are
important bioactive compounds as anti-cancer [18e20], antiviral
[21], anti-inflammatory [22], antidiabetic [23], antibacterial and
antifungal agents [24e26]. The recent success of pyrazole-based
COX-II inhibitors and their application in medicinal chemistry
have amplified the importance of pyrazoles [27]. Several pharma-
ceutical drugs including celecoxib [27] and rimonabant [28]
possess the pyrazoles as their core molecular entity [29e31]. The
isolated from the marine sponge, displays predominant antiviral
activity against herpes simplex type I [32]. With this background
and to broaden the scope of the ongoing research on developing
new and novel bis heterocycles, we planned to synthesize and to
investigate the antioxidant potentiality of sulfonylmethyl linked
oxazolyl and thiazolyl pyrazoles and isoxazoles.
2. Chemistry
The synthetic intermediate, E-styrylsulfonylacetic acid methyl
ester (1) was prepared as follows. The reaction of styrene with sulfuryl
chloride under nitrogen atmosphere produced E-styrylsulfonyl chlo-
ride. Treatment of latter compound with sodium bicarbonate and
sodium sulfite produced the salt, sodium E-styryl sulfinate. This on
reaction with chloroacetic acid resulted in E-styrylsulfonylacetic acid
which on esterification with methanol in the presence of conc. H2SO4
gave the compound 1 [33]. The two reactive sites, olefin moiety and
ester functionality in 1 were conveniently utilized to construct
different azole rings. The one-pot reaction of 1 with 2-aminoethanol in
the presence of samarium chloride and n-butyllithium resulted in 2-
(2-arylethenesulfonylmethyl)-4,5-dihydrooxazole (2). Similarly the
compound 2-(2-arylehenesulfonylmethyl)-4,5-dihydrothiazole (3)
was prepared by treating 1 with 2-aminoethanethiol (Scheme 1).
The compounds2a and 3a in 1H NMR spectra displayed two triplets at
4.25, 4.31 and 4.42, 3.47 due to C4-H and C5-H, a singlet at 4.12 and 4.02
* Corresponding author. Tel.: þ91 877 2289303; fax: þ91 877 2249532.
E-mail address: adivireddyp@yahoo.co.in (A. Padmaja).
0223-5234/$ e see front matter Ó 2011 Elsevier Masson SAS. All rights reserved.
doi:10.1016/j.ejmech.2011.08.010

A. Padmaja et al. / European Journal of Medicinal Chemistry 46 (2011) 5034e5038
5035
Scheme 1. Synthesis of oxazolyl/thiazolylsulfonylmethyl pyrazoles and isoxazoles.
due to methylene protons. Apart from this, a doublet was observed at
6.57 in 2a and at 6.50 ppm in 3a due to olefin proton HB, while the
other, HA merged with aromatic protons. The 1,3-dipolar cycloaddition
is a facile and an elegant method to exploit activated olefins to develop
pyrazoles and isoxazoles in the presence of appropriate dipolar
reagents. The cycloaddition of ethereal diazomethane to 2 and 3 in the
presence of triethylamine at ꢀ20 to ꢀ15 ꢁC gave 2-(40,50-dihydro-40-
aryl-1’H-pyrazol-30-ylsulfonylmethyl)-4,5-dihydrooxazole (4) and 2-
(40,50-dihydro-40-aryl-1’H-pyrazol-30-ylsulfonylmethyl)-4,5-dihydrot-
hiazole (5) (Scheme 1). The methine and methylene protons of pyr-
azoline ring exhibited an AMX splitting pattern in the 1H NMR spectra
of 4a and 5a. The three double doublets observed at 4.56, 4.24, 3.89 in
4a and at 4.42, 4.18, 3.78 in 5a were assigned to HA, HM and Hx,
respectively. The coupling constant values JAM ¼ 12.1 Hz, JAx ¼ 6.2 Hz,
JMx ¼ 11.0 Hz in 4a and JAM ¼ 11.8 Hz, JAx ¼ 5.9 Hz, JMx ¼ 10.5 Hz in 5a
indicated that HA, HM are cis, HA, Hx are trans and HM, Hx are geminal.
On the other hand, the cycloaddition of nitrile imines generated from
araldehyde phenylhydrazones in the presence of chloramine-T to 2
and 3 resulted in highly substituted pyrazolinyl derivative 2-(40,50-
dihydro-10-phenyl-30,50-diarylpyrazol-40-ylsulfonylmethyl)-4,5-dihy-
drooxazole (6) and 2-(40,50-dihydro-10-phenyl-30,50-diarylpyrazol-40-
ylsulfonylmethyl)-4,5-dihydrothiazole (7) (Scheme 1). The 1H NMR
spectra of 6a and 7a displayed two doublets at 5.21, 5.08 and 5.56,
5.42 ppm due to C4’-H and C5’-H, in addition to the signals due to C4-H,
C5-H and methylene protons. In a much similar way, the cycloaddition
of nitrile oxides generated from araldoximes in the presence of
chloramine-T to compounds 2 and 3 produced 2-(40,50-dihydro-30,50-
diarylisoxazol-40-ylsulfonylmethyl)-4,5-dihydrooxazole (8) and 2-
(40,50-dihydro-30,50-diarylisoxazol-40-ylsulfonylmethyl)-4,5-
doublets at 5.19, 5.02 and 5.69, 5.49 ppm which were assigned to C4’-H
and C5’-H, respectively apart from other signals. The aromatization of
pyrazoline and isoxazoline rings in 4e9 was carried out by treating the
respective compounds with chloranil in xylene. Thus the compounds
2-(40-aryl-1’H-pyrazol-30-ylsulfonylmethyl)-4,5-dihydrooxazole (10),
2-(40-aryl-1’H-pyrazol-30-ylsulfonylmethyl)-4,5-dihydrothiazole (11),
2-(10-phenyl-30,50-diarylpyrazol-40-ylsulfonylmethyl)-4,5-dihydroox-
azole (12), 2-(10-phenyl-30,50-diarylpyrazol-40-ylsulfonylmethyl)-4,5-
dihydrothiazole (13), 2-(30,50-diarylisoxazol-40-ylsulfonylmethyl)-
4,5-dihydrooxazole (14) and 2-(30,50-diarylisoxazol-40-ylsulfo-
nylmethyl)-4,5-dihydrothiazole (15) were prepared. The absence of
signals corresponding to pyrazoline and isoxazoline ring protons in
10e15 indicated that aromatization took place. The structures of these
compounds were further confirmed by IR and 13C-NMR spectra.
3. Biology
3.1. Antioxidant testing
The compounds 4e15 are tested for antioxidant property by
DPPH [34,35], nitric oxide [36,37] and H2O2 [38] methods.
4. Result and discussion
4.1. Antioxidant testing
The compounds 4e15 were tested for antioxidant property by
1,1-diphenylpicrylhydrazyl (DPPH) (Table 1), nitric oxide (Table 2)
and hydrogen peroxide (Table 3)methods. The compounds 6b, 8b,
10b, 12b, 14b showed good radical scavenging activity in all three
dihydrothiazole (9). The 1H NMR spectra of 8a and 9a showed two

5036
A. Padmaja et al. / European Journal of Medicinal Chemistry 46 (2011) 5034e5038
Table 1
Table 2
The in vitro antioxidant activity of 4e15 in DPPH method.
The in vitro antioxidant activity of 4e15 in nitric oxide method.
Compound
Concentration (
50
m
g/ml)
Compound
Concentration (
50
m
g/ml)
75
100
IC50
75
100
4a
e
e
e
e
4a
e
e
e
4b
4c
38.73
e
43.65
e
49.78
e
100.44
e
4b
4c
45.91
e
49.72
e
51.38
e
5a
e
e
e
e
5a
e
e
e
5b
5c
40.85
e
43.97
e
48.20
e
103.73
e
5b
5c
43.07
e
48.56
e
52.15
e
6a
6b
6c
48.21
64.43
e
53.35
70.24
e
57.97
73.81
e
86.25
67.74
e
6a
6b
6c
46.73
59.07
e
51.44
64.86
e
57.86
70.54
e
7a
e
e
e
e
7a
e
e
e
7b
7c
37.46
e
45.83
e
51.52
e
97.04
e
7b
7c
39.42
e
44.85
e
50.74
e
8a
8b
8c
49.37
58.14
e
55.19
63.26
e
57.93
66.52
e
86.31
75.16
e
8a
8b
8c
53.86
61.93
e
57.23
65.87
e
62.78
71.25
e
9a
e
e
e
e
9a
e
e
e
9b
9c
46.82
e
50.64
e
58.56
e
85.38
e
9b
9c
48.64
e
54.12
e
59.57
e
10a
10b
10c
11a
11b
11c
12a
12b
12c
13a
13b
13c
14a
14b
14c
15a
15b
15c
Ascorbic acid
Blank
50.10
59.83
e
54.32
63.67
e
60.78
68.95
e
82.26
72.51
e
10a
10b
10c
11a
11b
11c
12a
12b
12c
13a
13b
13c
14a
14b
14c
15a
15b
15c
Ascorbic acid
Blank
50.57
59.37
e
54.67
63.51
e
57.45
65.18
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
51.86
65.50
e
56.57
67.63
e
60.62
70.35
e
82.48
71.07
e
54.48
61.59
e
58.95
66.64
e
63.52
68.18
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
61.65
72.94
e
65.76
75.81
e
71.97
79.66
e
69.47
62.76
e
56.72
68.76
e
57.31
73.03
e
62.08
76.18
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
77.15
e
80.98
e
83.82
e
59.65
e
78.23
e
81.46
e
82.79
e
(ꢀ) Showed no scavenging activity.
(ꢀ) Showed no scavenging activity.
methods due to the presence of electron donating groups such as
eMe and eOMe in benzene ring when compared with the standard
drug ascorbic acid. The compounds 4b, 5b, 6a, 7b, 8a, 9b, 10a, 12a,
13b, 14a, 15b showed moderate antioxidant activity whereas the
other compounds displayed no activity. In general, it was observed
that aromatized compounds 10e15 exhibited greater activity when
compared with the respective dihydro compounds 4e9. The
compounds isoxazole in combination with oxazoline 14 exhibited
greater activity which may be due to the presence of two oxygen
atoms than the compounds having pyrazole and oxazoline 10, 12.
The presence of both electron donating methyl and methoxy
substituents enhances the activity. The IC50 value of the standard
exhibited greater antioxidant activity. The presence of electron
donating substituents on the aromatic ring enhances the activity.
6. Experimental section
6.1. Chemistry
Melting points were determined in open capillaries on a Mel-
Temp apparatus and are uncorrected. The purity of the
compounds was checked by TLC (silica gel H, BDH, ethyl acetate/
hexane, 1:3). The IR spectra were recorded on a Thermo Nicolet IR
200 FT-IR spectrometer as KBr pellets and the wave numbers were
given in cmꢀ1. The 1H NMR spectra were recorded in CDCl3/DMSO-
d6 on a Bruker-400 spectrometer (400 MHz). The 13C NMR spectra
were recorded in CDCl3/DMSO-d6 on a Bruker spectrometer oper-
ascorbic acid in DPPH method was found to be 59.65
100 g/ml whereas the IC50 values of the compounds 6b, 8b,10b,12b
and 14b were found to be 67.74, 75.16, 72.51, 71.07 and 62.76 g/ml,
mg/ml at
m
m
respectively. Further, the analysis of tables 1, 2 and 3 indicates that
radical scavenging activity in DPPH, nitric oxide and hydrogen
peroxide methods increases with increase in concentration.
ating at 100 MHz. All chemical shifts are reported in
d (ppm) using
TMS as an internal standard. The microanalyses were performed on
a PerkineElmer 240C elemental analyzer. The antioxidant property
was carried out by using Shimadzu UV-2450 spectrophotometer.
5. Conclusion
6.1.1. General procedure for the synthesis of 2-(2-arylethenesulfony-
lmethyl)-4,5-dihydrooxazole (2aec)/2-(2-arylethenesulfonylmethyl)-
4,5-dihydrothiazole (3aec)
A new class of bis heterocycles- oxazolyl/thiazolylsulfonylmethyl
pyrazoles and isoxazoles were prepared from the synthetically
vulnerable intermediate E-styrylsulfonylacetic acid methyl ester and
studied their antioxidant activity. It was observed that the
compounds having isoxazole in combination with oxazoline
To
a flask charged with anhydrous samarium chloride
(0.1 mmol), dry toluene and aminoalcohol/aminothiol (2 mmol)
were added followed by n-butyllithium (2.2 mmol) at 0 ꢁC and

A. Padmaja et al. / European Journal of Medicinal Chemistry 46 (2011) 5034e5038
5037
Table 3
refluxed for 20e22 h on a water bath. The precipitated inorganic
salts were filtered off. The filtrate was concentrated and the residue
was extracted with dichloromethane. The organic layer was
washed with water, brine and dried (an. Na2SO4). The solvent was
removed under vacuum. The resultant solid was recrystallized from
ethanol.
The in vitro antioxidant activity of 4e15 in hydrogen peroxide method.
Compound
Concentration (
m
g/ml)
75
50
100
4a
4b
4c
e
e
e
41.26
e
42.12
e
46.76
e
5a
5b
5c
6a
6b
6c
7a
7b
7c
8a
8b
8c
9a
9b
9c
e
e
e
6.1.4. General procedure for the synthesis of 2-(4’,5’-dihydro-3’,5’-
diarylisoxazol-4’-ylsulfonylmethyl)-4,5-dihydrooxazole (8aec)/2-
(4’,5’-dihydro-3’,5’-diarylisoxazol-4’-ylsulfonylmethyl)-4,5-
dihydrothiazole (9aec)
The compound 2/3 (1 mmol), araldoxime (1.2 mmol) and
chloramine-T (1.2 mmol) in methanol (20 ml) were refluxed for
16e18 h on a water bath. The precipitated inorganic salts were
filtered off. The filtrate was concentrated and the residue was
extracted with dichloromethane. The organic layer was washed
with water, brine and dried (an. Na2SO4). The solvent was removed
under reduced pressure. The solid obtained was purified by
recrystallization from ethanol.
39.84
e
42.17
e
45.05
e
44.37
62.47
e
45.54
64.26
e
48.07
67.74
e
e
e
e
42.61
e
43.88
e
47.89
e
51.72
65.93
e
53.31
66.57
e
56.57
70.94
e
e
e
e
47.51
e
49.67
e
52.63
e
10a
10b
10c
11a
11b
11c
12a
12b
12c
13a
13b
13c
14a
14b
14c
15a
15b
15c
Ascorbic acid
Blank
53.34
60.71
e
55.64
62.03
e
58.48
65.48
e
6.1.5. General procedure for the synthesis of 2-(4’-aryl-1’H-pyrazol-3’-
ylsulfonylmethyl)-4,5-dihydrooxazole (10aec)/2-(4’-aryl-1’H-pyrazol-
3’-ylsulfonylmethyl)-4,5-dihydrothiazole (11aec)/2-(1’-phenyl-3’,5’-
diarylpyrazol-4’-ylsulfonylmethyl)-4,5-dihydrooxazole (12aec)/2-(1’-
phenyl-3’,5’-diarylpyrazol-4’-ylsulfonylmethyl)-4,5-dihydrothiazole
(13aec)/2-(3’,5’-diarylisoxazol-4’-ylsulfonylmethyl)-4,5-dihy-
drooxazole (14aec)/2-(3’,5’-diarylisoxazol-4’-ylsulfonylmethyl)-4,5-
dihydrothiazole (15aec)
A solution of 4/5/6/7/8/9 (1 mmol) in xylene (10 ml) and chlor-
anil (1.2 mmol) was refluxed for 25e30 h. Then, it was treated with
a 5% sodium hydroxide solution. The organic extract was separated,
repeatedly washed with water and dried (an. Na2SO4). The solvent
was removed in vacuo. The resultant solid was recrystallized from
2-propanol.
e
e
e
e
e
e
e
e
e
55.57
67.58
e
57.25
68.71
e
59.14
72.53
e
e
e
e
e
e
e
e
e
e
57.23
70.02
e
58.08
71.95
e
60.11
77.08
e
e
e
e
e
e
e
e
e
e
77.68
e
79.27
e
83.16
e
6.2. Biological assays
(ꢀ) Showed no scavenging activity.
6.2.1. Antioxidant testing
The compounds 4e15 are tested for antioxidant property by
DPPH, nitric oxide and H2O2 methods.
stirred for 20 min at the same temperature. Then, the reaction
mixture was warmed to reflux at 100 ꢁC and E-styrylsulfonylacetic
acid methyl ester (1) (1 mmol) was added to the contents and
refluxion was continued for an additional 10e12 h. The suspension
was cooled to room temperature and filtered. The filtrate was
extracted with chloroform and washed with water. Evaporation of
the solvent in vacuo resulted in crude product which was purified
by passing through column of silica gel (60e120 mesh) using ethyl
acetate: hexane (1:3) as eluent.
6.2.1.1. DPPH radical scavenging activity. The hydrogen atom or
electron donation ability of the compounds was measured from the
bleachingof the purple colored methanol solution of2,2,-diphenyl-1-
picrylhydrazyl (DPPH). The spectrophotometric assay uses the stable
radical DPPH as a reagent. 1 ml of various concentrations of the test
compounds (25, 50, 75, 100 and 250 mg/ml) in methanol were added
to 4 ml of 0.004% (w/v) methanol solution of DPPH. After a 30 min
incubation period at room temperature, the absorbance was read
against blank at 517 nm. The percent of inhibition (I %) of free radical
production from DPPH was calculated by the following equation
6.1.2. General procedure for the synthesis of 2-(4’,5’-dihydro-4’-
aryl-1’H-pyrazol-3’-ylsulfonylmethyl)-4,5-dihydrooxazole (4aec)/
2-(4’,5’-dihydro-4’-aryl-1’H-pyrazol-3’-ylsulfonylmethyl)-4,5-
dihydrothiazole (5aec)
To a well cooled solution of 2/3 (2.5 mmol) in dichloromethane
(20 ml), an ethereal solution of diazomethane (40 ml, 0.4 M) and
triethylamine (0.12 g) were added. The reaction mixture was kept
at ꢀ20 to ꢀ15 ꢁC for 40e48 h. The solvent was removed under reduced
pressure. The solid obtained was purified by column chromatography
(silica gel, 60e120 mesh) using ethyl acetate: hexane (1:2.5) as eluent.
I% ¼ ½ðA control ꢀ A sampleÞ=A blankꢂ ꢃ 100
where A control is the absorbance of the control reaction (con-
taining all reagents except the test compound) and A sample is the
absorbance of the test compound. Tests were carried at in triplicate.
6.2.1.2. Nitric oxide (NO) scavenging activity. Nitricoxidescavenging
activity was measured by slightly modified methods of Green et al.
and Marcocci et al. Nitric oxide radicals (NO) were generated from
sodium nitroprusside. 1 ml of sodium nitroprusside (10 mM) and
1.5 ml of phosphate buffer saline (0.2 M, pH 7.4) were added to
6.1.3. General procedure for the synthesis of 2-(4’,5’-dihydro-1’-
phenyl-3’,5’-diarylpyrazol-4’-ylsulfonylmethyl)-4,5-dihydrooxazole
(6aec)/2-(4’,5’-dihydro-1’-phenyl-3’,5’-diarylpyrazol-4’-
ylsulfonylmethyl)-4,5-dihydrothiazole (7aec)
A
mixture of 2/3 (1 mmol), araldehyde phenylhydrazone
different concentrations (25, 50, 75 and 100
mg/ml) of the test
(1.2 mmol) and chloramine-T (1.2 mmol) in methanol (20 ml) was
compounds and incubated for 150 min at 25 ꢁC. After incubation 1 ml

5038
A. Padmaja et al. / European Journal of Medicinal Chemistry 46 (2011) 5034e5038
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sulfanilamide, 2% H3PO4 and 0.1% naphthylethylenediamine dihy-
drochloride). The absorbance of the chromatophore was measured at
546 nm. Butylated hydroxyl toluene was used as standard. Nitric
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% of scavenging ¼ ½ðA control ꢀ A sampleÞ=A blankꢂ ꢃ 100
where A control is the absorbance of the control reaction (con-
taining all reagents except the test compound) and A sample is the
absorbance of the test compound. Tests were carried at in triplicate.
6.2.1.3. Hydrogen peroxide (H2O2) scavenging activity. The H2O2
scavenging ability of the test compound was determined according
to the method of Ruch et al. A solution of H2O2 (40 mM) was
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prepared in phosphate buffer (pH 7.4). 10, 25, 50, 75 &100
m
g/ml
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