Synthesis of benzofuran based 1,3,5-substituted pyrazole derivatives: As a new class of potent antioxidants and antimicrobials-A novel accost to amend biocompatibility

Article abstract of DOI:10.1016/j.bmcl.2012.05.061

In search for a new antioxidant and antimicrobial agent with improved potency, we synthesized a series of benzofuran based 1,3,5-substituted pyrazole analogues (5a-l) in five step reaction. Initially, o-alkyl derivative of salicyaldehyde readily furnish corresponding 2-acetyl benzofuran 2 in good yield, on treatment with 1,8-diaza bicyclo[5.4.0]undec-7-ene (DBU) in the presence of molecular sieves. Further, aldol condensation with vanillin, Claisen-Schmidt condensation reaction with hydrazine hydrate followed by coupling of substituted anilines afforded target compounds. The structures of newly synthesized compounds were confirmed by IR, ¹H NMR, ¹³C NMR, mass, elemental analysis and further screened for their antioxidant and antimicrobial activities. Among the tested compounds 5d and 5f exhibited good antioxidant property with 50% inhibitory concentration higher than that of reference while compounds 5h and 5l exhibited good antimicrobial activity at concentration 1.0 and 0.5 mg/mL compared with standard, streptomycin and fluconazole respectively.

Full text of DOI:10.1016/j.bmcl.2012.05.061

Bioorganic & Medicinal Chemistry Letters 22 (2012) 4773–4777
Contents lists available at SciVerse ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis of benzofuran based 1,3,5-substituted pyrazole derivatives:
As a new class of potent antioxidants and antimicrobials-A novel accost
to amend biocompatibility
Javarappa Rangaswamy a, Honnaiah Vijay Kumar b, Salakatte Thammaiah Harini a, Nagaraja Naik a,
⇑
a Department of Studies in Chemistry, University of Mysore, Manasagangotri, Mysore 570006, Karnataka, India
b Department of Organic Chemistry, Indian Institute of Science, Bangalore 560012, Karnataka, India
a r t i c l e i n f o
a b s t r a c t
Article history:
In search for a new antioxidant and antimicrobial agent with improved potency, we synthesized a series
of benzofuran based 1,3,5-substituted pyrazole analogues (5a–l) in five step reaction. Initially, o-alkyl
derivative of salicyaldehyde readily furnish corresponding 2-acetyl benzofuran 2 in good yield, on treat-
ment with 1,8-diaza bicyclo[5.4.0]undec-7-ene (DBU) in the presence of molecular sieves. Further, aldol
condensation with vanillin, Claisen–Schmidt condensation reaction with hydrazine hydrate followed by
coupling of substituted anilines afforded target compounds. The structures of newly synthesized com-
pounds were confirmed by IR, 1H NMR, 13C NMR, mass, elemental analysis and further screened for their
antioxidant and antimicrobial activities. Among the tested compounds 5d and 5f exhibited good antiox-
idant property with 50% inhibitory concentration higher than that of reference while compounds 5h and
5l exhibited good antimicrobial activity at concentration 1.0 and 0.5 mg/mL compared with standard,
streptomycin and fluconazole respectively.
Received 5 February 2012
Revised 23 March 2012
Accepted 15 May 2012
Available online 24 May 2012
Keywords:
Benzofuran based 1,3,5-substituted
pyrazole analogues
Substituted anilines
Antioxidant activity
Antimicrobial activity
Ó 2012 Elsevier Ltd. All rights reserved.
Heterocyclic ring systems have emerged as powerful scaffolds
for many biological evaluations.1 Heterocyclic compounds provide
scaffolds on which pharmacophores can arrange to yield potent
and selective drugs.2 Benzofuran is an important class of heterocy-
clic compounds that are known to possess important biological
properties. Specifically, several benzofuran ring systems bearing
various substituents at C-2 positions are omnipresent in nature,
the stilbenoids structural compounds, for example an oligostilbene
derivative viniferin are known to possess antimicrobial, antiviral,
antioxidant, antifungal and antitumor activities.3,4 In addition, phe-
nyl (2-phenylbenzofuran-3-yl)methanone derivatives had been
demonstrated to possess neuroprotective and antitumor activities.5
On the other hand, pyrazole scaffold represents a common motif in
many pharmaceutical active and remarkable compounds demon-
strating a wide range of pharmacological activities; the most impor-
tant activities are the anti-inflammatory, antibacterial, antifungal,
hypoglycemic and anti-hyperlipidemic.6,7 Heterocyclic rings and
in particular, the pyrazole ring represent an advantageous choice
for the synthesis of pharmaceutical compounds with different activ-
ities and good safety profiles.8
The literature survey approaching to synthesis of amide linked
pyrazole gathered with benzofuran moiety indicate the lack of ref-
erence available. Promoted with the above mentioned studies and
as a continuation of our research interest in the synthesis and bio-
logical evaluation of novel heterocyclic analogues,9,10 the present
study aimed at gathering the two bioactive entities like benzofuran
with pyrazole amides in one compact structure for the purpose of
increasing the antioxidant and antimicrobial potentials.
In the present work, five step synthetic strategies were em-
ployed for the preparation of benzofuran based 1,3,5-substituted
pyrazole derivatives (5a–l). In the first step, the desired starting
material 2-acetyl benzofuran (2) was obtained by cyclocondensa-
tion reaction. At present, several general methods for the prepara-
tion of 2-acetyl benzofuran are known.11–15 In this communication,
we described a mild variant 1,8-diaza bicyclo[5.4.0]undec-7-ene
(DBU) assisted one pot synthesis of 2-acetyl benzofuran in pres-
ence of molecular sieves Scheme 1. Initially, o-alkylation of salicyl-
aldehyde (1) with chloroacetone in presence of DBU as organic
base furnished o-alkylated salicyaldehyde derivative (a) which
subsequently generates enolate anion undergo intramolecular
cyclocondensation reaction afforded 2-acetyl benzofuran in excel-
lent yield. In the second step, zirconium catalyzed aldol condensa-
tion reaction of appropriately 2-acetyl benzofuran (2) with vanillin
in dichloromethane (DCM) afforded benzofuran chalcone (E)-1-
(benzofuran-2-yl)-3-(4-hydroxy-3-methoxyphenyl)prop-2-en-1-
one (3).16 Generally, chalcones are considered to be useful
⇑
Corresponding author. Tel.: +91 9482959088.
E-mail addresses: rangaswamyteertha@gmail.com (J. Rangaswamy), vijaycftri@
gmail.com (H. Vijay Kumar), st.harini988@gmail.com (S.T. Harini), drnaikchem@
gmail.com (N. Naik).
0960-894X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2012.05.061

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J. Rangaswamy et al. / Bioorg. Med. Chem. Lett. 22 (2012) 4773–4777
O
O
O
DBU, molecular sieves
dry DCM, RT, 6 hr
H
Cl
+
O
(2)
OH
(1)
O
H
O
O
(a)
Scheme 1. Synthesis of 2-acetyl benzofuran (2).
O
H
O
O
ZrCl4, DCM
O
RT, 1 hr
O
OMe
OMe
(3)
(2)
OH
OH
.
reflux
3 hr
NH2NH2 H2O,
EtOH
O
Cl
Cl
Cl
N
NH
O
N
N
O
OMe
OH
TEA, dry THF, RT, 4 hr
O
OMe
OH
(4)
(5)
K2CO3, DCM,
N2 atm, 3-5 hr
R-NH2
R
HN
N
O
N
O
OMe
OH
(5a-l)
Scheme 2. Reaction pathway for the synthesis of benzofuran based 1,3,5-substituted pyrazole derivatives (5a–l).
intermediate in several cyclisation reactions to produce types of
heterocyclic compounds of diverse biological importance, accord-
ing to the reactants used and the reaction conditions.17 In the next
step, Claisen–Schmidt cyclocondensation of compound (3) with
hydrazine hydrate in absolute ethanol obtained the corresponding
pyrazole derivative 4-(3-(benzofuran-2-yl)-4, 5-dihydro-1H-pyra-
zol-5-yl)-2-methoxyphenol (4).18 Compound (4) further treated
with phosgene in presence of triethylamine (TEA) to furnished
3-(benzofuran-2-yl)-5-(4-hydroxy-3-methoxyphenyl)-4,5-dihy-
dro-1H-pyrazole-1-carbonyl chloride (5). In the final step,
compound (5) was coupled with different substituted anilines in
presence of potassium carbonate accomplished the desired carbox-
amide products (5a–l) Scheme 2. The structures of the compounds
were elucidated by IR, 1H NMR, 13C NMR, mass and elemental anal-
ysis (Table 1).
Evaluation of antioxidant activity for the newly synthesized
analogues was done by using two in vitro assays such as 2,2-diphe-
nyl-1-picryl-hydrazyl (DPPH) radical scavenging activity and inhi-
bition microsomal lipid peroxidation (LPO). The antioxidant
properties were expressed as 50% inhibitory concentration (IC50
)

J. Rangaswamy et al. / Bioorg. Med. Chem. Lett. 22 (2012) 4773–4777
4775
Table 1
Chemical structure and yield of synthesized compounds (5a–l)
Compound No
Entry R
Yield%
Compound No
Entry R
Yield%
64.7
5a
72.0
5g
Br
5b
5c
69.0
81.0
5h
5i
88.5
74.3
OH
Cl
OH
OCH3
5d
83.4
5j
68.0
OCH3
OH
OH
CH3
NO2
F
5e
5f
71.0
77.0
5k
5l
65.4
73.0
OH
OH
Table 2
electron donating methoxy group at 5-substituted phenyl ring and
also presence of the N–H functional group in the scaffold 4.20
Further, coupling of substituted anilines gave the significant
enhancement of activity. Majority of the tested compounds in
these series (5a–l) showed moderate to high activity. Dominant
RSA was observed for compounds 5d and 5f this may be presence
of hydroxy and methoxy group on the 5-substituted phenyl ring of
the pyrazole skeleton as well as in 1-substituted carboxamide phe-
nyl ring. The coupling of substituted anilines (3-methoxy-4-hydro-
xy aniline and 3,4-dihydroxy aniline) to compound 5 have revealed
ten to 13-folds more activity compared to the benzofuran scaffolds
4 and 5. Antioxidant activity of these compounds is related with
their electron- or hydrogen-donating ability to DPPH radical, so
that it become stable diamagnetic molecules. This might be the
reason for the higher antioxidant activity of the compounds
5d and 5f among the synthesize analogues as well as BHA. The
introduction of electron withdrawing (Br, Cl, NO2 and F) substi-
tuted anilines to scaffold 5 was inadequate for the enhanced activ-
ity in compounds 5g, 5h, 5k and 5l. Whereas the compounds
5b, 5c, 5e, 5i and 5j contain electron releasing single hydroxy,
methoxy and methyl groups at positions 3,4,5 exhibited five to six-
folds more RSA activity than that of 4 and 5. Since compound
5a does not have any substituent on the ring it showed least activ-
ity compared to other compounds.
50% Inhibition of DPPH radical and microsomal LPO inhibition by compounds (5a–l).
Each value represents mean SD (n = 3)
a
b
Compound Nos
DPPH activity IC50
M/mL)
LPO inhibition IC50
M/mL)
(l
(l
4
5
100 0.61
135 0.30
121 0.91
17 0.32
19 0.11
10 0.56
21 0.34
9.5 0.41
105 0.52
107 0.27
25 0.79
37 0.60
103 0.14
110 0.45
11 0.10
112 0.88
147 0.97
116 0.13
15 0.21
22 0.46
12 0.45
24 0.12
11 0.64
115 0.53
121 0.71
29 0.96
44 0.45
113 0.23
125 0.31
13.0 0.71
5a
5b
5c
5d
5e
5f
5g
5h
5i
5j
5k
5l
BHA
a
IC50 = the concentration (
IC50 = the concentration (
l
l
M/mL) exhibiting 50% inhibition of DPPH radical.
M/mL) exhibiting 50% inhibition of LPO oxidation.
b
values (Table 2). The DPPH radical scavenging evaluation is a
standard assay in antioxidant activity studies and offers a rapid
technique for screening the radical scavenging activity (RSA) of
specific compounds.19 The reaction of synthesized compounds
with stable DPPH free radical indicates their free radical scaveng-
ing ability. Initially, benzofuran scaffolds 4 and 5 exhibited consid-
erable activities, this could be due to the presence of hydroxy and
Inhibition of lipid peroxidation property of newly synthesized
compounds was performed by the formation of thiobarbutaric acid
reactive species (TBARS) using liver excised from adult male Wister
rats. Lipid free radicals produced by ferric chloride and vitamin C
rapidly react with oxygen molecule to give peroxy radicals with

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J. Rangaswamy et al. / Bioorg. Med. Chem. Lett. 22 (2012) 4773–4777
Table 3
Antibacterial activity of the compounds (5a–l). Inhibitory zone (diameter) mm of the synthesized compounds against tested bacterial strains by well plate method
Compound Nos
Escherichia coli
Staphylococcus aureus
Pseudomonas aeruginosa
Concentration in mg/mL
1
0.5
1
0.5
1
0.5
Control
00
00
00
00
00
00
Streptomycin
4
16 0.24
00
10 0.13
00
15 0.24
00
10 0.26
00
16 0.20
00
13 0.23
00
5
03 0.12
02 0.33
03 0.24
03 0.30
02 0.32
03 0.35
02 0.21
16 0.17
17 0.23
13 0.34
05 0.10
15 0.32
18 0.21
02 0.21
01 0.17
02 0.12
01 0.11
01 0.18
01 0.13
01 0.16
10 0.14
11 0.20
01 0.15
04 0.21
09 0.24
12 0.13
02 0.10
03 0.12
02 0.13
02 0.19
03 0.14
02 0.18
02 0.16
08 0.17
06 0.13
02 0.15
04 0.20
08 0.14
16 0.10
01 0.13
02 0.17
01 0.14
01 0.19
02 0.16
01 0.13
01 0.11
06 0.35
04 0.27
01 0.11
03 0.23
06 0.24
14 0.10
02 0.23
02 0.21
03 0.24
04 0.25
02 0.26
03 0.27
05 0.18
10 0.24
18 0.19
07 0.26
04 0.34
09 0.22
19 0.13
02 0.21
01 0.34
02 0.25
02 0.26
01 0.32
02 0.21
03 0.10
07 0.24
15 0.37
05 0.36
03 0.14
07 0.10
16 0.12
5a
5b
5c
5d
5e
5f
5g
5h
5i
5j
5k
5l
Table 4
The antifungal activity of the compounds (5a–l). Inhibitory zone (diameter) mm of the synthesized compounds against tested fungal strains by well plate method
Compound Nos
Aspergillus flavus
Chrysosporium keratinophilum
Candida albicans
Concentration in mg/mL
1
0.5
1
0.5
1
0.5
Control
00
00
00
00
00
00
Fluconazole
4
5
13 0.20
00
10 0.12
00
17 0.20
01 0.11
03 0.10
03 0.10
02 0.22
04 0.11
03 0.15
02 0.24
02 0.28
10 0.17
09 0.16
04 0.29
03 0.13
04 0.14
15 0.10
15 0.25
00
22 0.22
01 0.32
04 0.11
02 0.15
04 0.14
04 0.16
02 0.18
03 0.11
02 0.13
19 0.20
20 0.10
05 0.11
03 0.14
05 0.10
24 0.24
20 0.20
01 0.11
03 0.23
01 0.13
02 0.14
03 0.10
01 0.10
01 0.19
01 0.27
15 0.26
17 0.35
03 0.12
01 0.14
04 0.10
21 0.21
02 0.12
02 0.31
03 0.15
02 0.34
02 0.18
02 0.26
03 0.24
06 0.14
10 0.11
04 0.21
03 0.10
05 0.15
11 0.10
01 0.22
01 0.10
02 0.11
01 0.11
01 0.10
01 0.37
02 0.29
04 0.27
07 0.18
03 0.16
02 0.15
03 0.25
09 0.12
02 0.31
02 0.11
01 0.15
02 0.13
02 0.11
01 0.12
01 0.28
09 0.27
07 0.14
02 0.16
01 0.13
03 0.15
11 0.15
5a
5b
5c
5d
5e
5f
5g
5h
5i
5j
5k
5l
the subsequent formation of the final product, malondialdehyde.21
The results showed that all newly synthesized compounds (5a–l)
inhibited the ferric chloride induced lipid peroxidation and varying
degree compared with standard antioxidant BHA Table 2. Com-
pound 5d which contains p-OH, m-OCH3 and compound 5f having
3,4-dihydroxy group at 1-substituted carboxamide phenyl ring
showed maximum inhibition and even higher than that of the ref-
erence compound BHA. In general, the presence of electron-donat-
ing groups on the phenyl ring favors the activity. This might be the
reason for the four-seven times enhancement of activity of the
other compounds, which showed moderate to good activities in
the order 5b > 5c > 5e > 5i > 5j which has free hydroxy, methoxy
and methyl as the electron donating substituent’s. The presence
of electron withdrawing groups on the phenyl ring was not favor
for the activity, thus might be the reason for the decreased activity
for the compounds 5g, 5h, 5k and 5l.
The antibacterial activities of newly synthesized compounds
(5a–l) were determined by well plate method.22 In this work, Esch-
erichia coli ATCC 25922, Staphylococcus aureus ATCC 25923, and
Pseudomonas aeruginosa ATCC 27853 were used to investigate the
activity. The test compounds were dissolved in dimethyl sulfoxide
(DMSO) at concentrations of 1 and 0.5 mg/mL. The antibacterial
screening revealed that some of the tested compounds showed
good inhibition against various tested microbial strains Table 3.
Benzofuran scaffold, compound 4 possessed least antibacterial
activity. Whereas, compound 5 exhibited considerable activity.
Further, introducing of substituted anilines into pyrazole ring ac-
counted for the enhanced activity. The results indicated that
among the tested compounds 5h and 5l possessing p-chloro and
p-fluro substituent on the 1-substituted phenyl ring showed excel-
lent activity against P. aeruginosa and E. coli at concentrations of 1
and 0.5 mg/mL compared to standard drug streptomycin. The
activity is considerably affected by halogens substituents present
at the para position of phenyl ring. Compounds 5g (p-bromo) and
5k (p-nitro) showed similar activity as that of standard, against
E. coli at 1 and 0.5 mg/mL concentrations. The remaining com-
pounds 5a–f and 5i–j showed very least activity against all of the
three tested bacterial strains.
Newly synthesized compounds (5a–l) are also screened for their
antifungal activity against Aspergillus flavus MTCC 3306, Chrysospo-
rium keratinophilum MTCC 3017 and Candida albicans MTCC 2827.
The compounds were dissolved in dimethyl sulphoxide (DMSO)
and antifungal activity was determined by well plate method23
at concentration of 1 and 0.5 mg/mL. Among the tested com-
pounds, compound 5l possessing fluro moiety on phenyl ring at
para- position, accounted for the enhanced antifungal activity
against C. albicans compared with standard, fluconazole and as well
as other compounds Table 4. Whereas compounds 5g and 5h
showed moderate to good activity against C. albicans but slightly
less than that of standard. The remaining compounds 5a–f and
5i–k showed less activity against all the tested microorganisms
compared to standard.

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furnished benzofuran based 1,3,5-substituted pyrazole derivatives
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Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.bmcl.2012.05.
061.
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