Aurones and Analogues: Promising Heterocyclic Scaffolds for Development of Antioxidant and Antimicrobial Agents

Article abstract of DOI:10.1134/S1070363219070235

Synthesis of some new multi-functional analogues of 2′-hydroxy chalcone containing isoxazole and pyrazole functions were synthesized, and their pharmacological activity was tested. Chalcone derivatives were synthesized by the reaction of 2′-hydroxy acetophenone with various substituted benzaldehydes in a basic medium. Aurones were isolated upon treatment with mercuric acetate. Synthesized chalcones and aurones were converted into the corresponding isoxazole and pyrazole derivatives upon their reaction with hydroxylamine hydrochloride or hydrazine hydrate, respectively. Structures of the compounds were confirmed by spectroscopic methods. All synthesized compounds were tested for antioxidant and antibacterial potential. Some of those demonstrated excellent antioxidant activity, and one product was identified as a promising antimicrobial candidate.

Full text of DOI:10.1134/S1070363219070235

ISSN 1070-3632, Russian Journal of General Chemistry, 2019, Vol. 89, No. 7, pp. 1519–1527. © Pleiades Publishing, Ltd., 2019.
Aurones and Analogues: Promising Heterocyclic Scaffolds
for Development of Antioxidant and Antimicrobial Agents
M. Irshada*, Q. Alia, F. Iramb, S. A. Ahamadb, M. Saleemc, M. Saadiad,
M. Batoole, A. Kanwala, and S. Tabassuma
a Department of Chemistry, Division of Science and Technology, University of Education Lahore, 54770 Pakistan
*e-mail: misbahchatha@hotmail.com; misbahirshad@ue.edu.pk
b Department of Chemistry, Lahore College for Women University, Lahore, 75500 Pakistan
c Department of Chemistry, University of Education DG Khan Campus, 32200 Pakistan
d Department of Biochemistry, University of Sargodha, Sargodha, 40100 Pakistan
e Department of Chemistry, University of Punjab, Lahore, 54590 Pakistan
Abstract—Synthesis of some new multi-functional analogues of 2'-hydroxy chalcone containing isoxazole and
pyrazole functions were synthesized, and their pharmacological activity was tested. Chalcone derivatives were
synthesized by the reaction of 2'-hydroxy acetophenone with various substituted benzaldehydes in a basic medium.
Aurones were isolated upon treatment with mercuric acetate. Synthesized chalcones and aurones were converted
into the corresponding isoxazole and pyrazole derivatives upon their reaction with hydroxylamine hydrochloride
or hydrazine hydrate, respectively. Structures of the compounds were confirmed by spectroscopic methods. All
synthesized compounds were tested for antioxidant and antibacterial potential. Some of those demonstrated excel-
lent antioxidant activity, and one product was identified as a promising antimicrobial candidate.
Keywords: chalcones, pyrazolic, isoxazolic, aurones, 2'-hydroxyacetophenone, antioxidant, antimicrobial
DOI: 10.1134/S1070363219070235
INTRODUCTION
mentation in plants [4] and help them to resist infections
acting as phytoalexins [5].Among E and Z configurations
of aurones, the latter configuration is stable, accordance
to Austin model 1 computation [6–9] (see the figure).
Chalcones, among other properties, are characterized
by their redox potential, and there is a probability of
electron transfer processes attributed to delocalization
of π-electron system in their molecules [1]. Aurones,
2-benzylidenebenzofuran-3(2H)-ones, are natural hetero-
cyclic compounds [2] originated as secondary metabolites
of flavonoid family [3]. Aurones are responsible for pig-
RESULTS AND DISCUSSION
2'-Hydroxy chalcones 3a–3c were synthesized by
the Claisen-Schmidt condensation of 2'-hydroxy aceto-
(c)
(a)
(b)
OH
O
OH
N
O
O
b
b
3'
1'
1
1'
1
5'
3'
3
3'
3
1''
5
a
a
B
A
B
A
1
O
7'
5'
5
1'
5'
5
3
(d)
(e)
N
(f)
N
N
O
NH
OH
NH
5'
5'
b
1
1
1'
1
3
3
3'
3
'
a
O
A
O
B
7'
7'
5
5'
5
5
General structures of (a) chalcone, (b) aurone, (c, d) their isoxazole and (e, f) pyrazole analogues.
1519

1520
IRSHAD et al.
Scheme 1. Outline of the synthesis of chalcones and aurones analogues.
OH
N
NH
1'
1
3
NH2NH2·H2O (6)
3'
c
R
5'
5
7a-c
OH
O
OH
N
O
1'
1'
1
1
3
3'
3'
NH2OH·HCl (4)
3
b
R
5
5'
5'
5'
5
R
3a3c
5a5c
O
O
3'
OH
O
1
1''
1'
3'
O
1
3
3
5
1'
7'
a
R
5'
9a9c
R
2a2c
1
5
Hg(OAc)2 (8)
d
NH2OH·HCl (4)
NH2NH2·H2O (6)
e
f
N
O
N
NH
5'
1
1
3
3
O
O
'
7
7'
5
R
5
R
10a10c
11a11c
R = OCH3 (2b), NO2 (2c). a: 20 % aq. NaOH, MeOH, room temperature, 24 h; b: ethanol, KOH pellets, reflux, 2 h; c: EtOH,
acetic acid, reflux 4–5 h; d: pyridine, reflux at 110°C, 2 h; e: ethanol, KOH, reflux 24 h; f: acetic acid, reflux 4 h.
phenone 1 with substituted benzaldehydes 2a–2c under
basic conditions (Scheme 1). By a multistep synthesis we
approached a series of compounds bearing chalcones as
the central structural block. Isoxazole (5a–5c, 10a–10c)
and pyrazole (7a–7c, 11a–11c) chalcones and aurones
respectively were accumulated under basic conditions.
oxidation of mercuric acetate in acetic acid for minimiz-
ing formation of flavone cyclized product. Thus aurones
(9a–9c) were accumulated as major products (yield
80–84%). Isoxazole derivatives of chalcone 5a–5c and
aurone 10a–10c were obtained upon refluxing of com-
pounds 3a–3c and 9a–9cin ethanol with hydroxylamine
HCl. Pyrazole derivatives 7a–7c and 11a–11c of chalcone
and aurone were obtained upon refluxing of 3a–3c and
9a–9c in acetic acid.
Synthesis of aurones 9a–9c from chalcones 3a–3c
was accompanied by oxidative cyclization [10] under
the action of Hg(II) acetate in pyridine. Here we avoided
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AURONES AND ANALOGUES: PROMISING HETEROCYCLIC SCAFFOLDS
1521
Table 1. Radical scavenging activity (%) of the standard and synthesized compounds
Concentration, mg/mL
Compounds
0.25
0.5
0.75
1.0
1.5
3a
20.56±1.62
47.16±1.37
61.13±0.9
69.24±1.03
73.77±1.58
69.81±1.58
73.1±0.67
72.83±0.67
69.43±2.18
71.24±3.17
80±1.22
3b
14.3±1.58
26.6±1.61
25.09±0.67
25.4±0.45
28.86±0.98
33.58±0.47
42.64±0.67
19.6±1.22
21.23±0.13
20.75±3.15
16.03±0.9
21.69±3.15
26.79±1.22
24.71±1.58
18.16±1.59
16.03±0.9
21.19±0.43
35±1.22
18.3±0.67
31.6±0.47
55.66±1.22
32.56±1.47
35.23±1.04
44.33±0.67
61.88±0.9
19.81±1.58
23.45±0.55
27.16±0.9
27.16±1.03
33.96±1.22
29.81±1.58
41.32±1.61
23.61±0.45
27.16±1.03
27.88±0.79
47±0.67
27.54±1.47
34.9±1.03
59.43±1.03
35.89±0.12
40.89±1.56
56.79±0.67
63.20±1.22
28.67±0.67
27.67±1.02
30.75±0.8
34.52±3.15
42.45±3.15
31.13±1.22
41.88±1.22
28.28±2.19
34.52±3.15
37.72±0.22
66±2.82
33.7±1.22
58.7±1.22
70.56±1.51
56.67±1.17
59.30±2.67
73.58±1.03
80.18±1.58
46.98±1.22
31.65±0.25
67.92±0.6
46.79±1.22
54.52±0.67
33.58±0.67
68.11±1.42
36.13±1.34
46.79±1.22
45.18±2.26
70±1.22
3c
5a
5b
5c
7a
7b
86.41±0.47
71.69±1.7
42.12±2.37
70.94±1.22
71.32±0.67
65.6±1.22
48.68±3.15
79.43±1.8
44.19±1.24
51.32±0.67
56.84±1.92
86±3.15
7c
9a
9b
9c
10a
10b
10c
11a
11b
11c
Gallic acid (standard)
Signals of vinylic protons of the chalcone functional-
ity appeared at 7.64–7.89 for Hα and 7.93–8.46 for Hβ;
the –OH proton was deshielded and it signal appeared
in the downfield region, 12.97 to 13.89 ppm. Isoxazolic
chalcones 5a–5c were identified by the deshielded singlet
at 6.21–6.29 ppm, whereas pyrazolic chalcones 7a–7c
were recorded by two singlets in the regions 12.09–
12.55 ppm (NH) and 4.59–4.75 ppm (CH).
cals was evaluated according to the method of Zhuang
(Table 1). The products demonstrated their scavenging
activity by interaction with the stable free DPPH radical.
This ability of the synthesized compounds was found
to be concentration dependent. In most cases at various
concentrations the compounds 3a, 5a, 7a, and 7b dem-
onstrated higher DPPH radical scavenging activity than
the standard compound gallic acid. Whereas compound
3b was characterized by the lowest activity. Overall, the
compound 7b bearing the methoxy group in position 2 of
B ring was found to be the most active radical scavenger
at all concentrations.
Antioxidant activity. In vitro antioxidant activity of
the synthesized compounds was studied by two different
assays. For this purpose scavenging ability of the pre-
pared compounds was assessed by DPPH (1,1-diphenyl-
2-picryl-hydrazyl) free radical scavenging activity and
reducing power assay FRAP.
Reducing power assay. Ferric reducing power assay
of all the synthesized compounds was calculated by using
the method of Oyaizu (Table 2). Compound 11b contain-
ing isoxazolic ring in aurone skeleton and the methoxy
Interaction with DPPH stable free radical. Ability
of the synthesized compounds to scavenge free radi-
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IRSHAD et al.
Table 2. Ferric reducing power of the synthesized compounds
Table 3. Zones of inhibition of the synthesized compounds
against tested strains
Zones of inhibition, mm
Compounds
Absorbance
(700 nm)
Reducing
power, %
Comp. no.
X
X
E. coli S. aureus B. subtilis
3a
3b
3c
5a
5b
H
13
11
9
12
9
15
10
10
15
8
2-OCH3
3-NO2
H
3a
H
0.075
0.087
0.093
0.075
0.089
0.064
0.091
0.067
0.096
0.063
0.089
0.088
0.076
0.062
0.093
0.083
0.052
0.058
40
11
12
9
3b
3c
2-OCH3
3-NO2
H
30.4
25.6
32.7
29.4
24.3
27.2
46.4
23.2
31.2
30.4
25.6
28.2
32.4
21.2
33.6
51.4
45.6
13
11
2-OCH3
5a
5c
3-NO2
H
18
13
15
16
8
14
10
16
14
11
9
13
13
15
15
9
5b
5c
2-OCH3
3-NO2
H
7a
7b
2-OCH3
3-NO2
H
7a
7c
7b
7c
2-OCH3
3-NO2
H
9a
9b
2-OCH3
3-NO2
H
9
10
8
9a
9c
10
12
9
9
9b
9c
2-OCH3
3-NO2
H
10a
10b
10c
11a
11b
11c
10
12
11
8
9
2-OCH3
3-NO2
H
8
10a
10b
10c
11a
11b
11c
12
8
9
2-OCH3
3-NO2
H
10
8
2-OCH3
3-NO2
Amoxicillin
DCM
10
11
19
7
9
6
Standard
Control
13
16
2-OCH3
3-NO2
group in position 2 of B ring was found to have the high-
est reduction potential while compound 10c containing
pyrazolic ring in aurone skeleton and the nitro group in
position 3 of B ring was found to have the lowest reduc-
tion potential.
reduction potential in FRAPassay. It is therefore suggest-
ed that the antioxidant activity of the compounds could
be related to their antibacterial activity. Some compounds
with strong antioxidant potential can resist the oxidative
damage induced by the food pathogenic bacteria, though
this is not true for all synthesized compounds.
Antibacterial activity. The compound 5c was deter-
mined to be a highly potent antibacterial agent against
gram-negative bacteria E. coli (Table 3). Compounds
5c, 7b and 7c demonstrated high antibacterial activity
against Staphylococcus aureus. The products 3a, 7a and
10b showed good antibacterial activity but not higher
than amoxicillin. Compounds 3a, 5a, 7a, and 7c exhibited
good activity against Bascillus subtilis and also maximum
EXPERIMENTAL
Reagents, chemicals and solvents were purchased
from local suppliers of Alfa-Aesar, Sigma Aldrich and
Merck. The solvents were of analytical grade. Purity of
synthesized compounds was tested by TLC and visualized
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AURONES AND ANALOGUES: PROMISING HETEROCYCLIC SCAFFOLDS
1523
under UV 254 lamp. Silica gel coated on aluminum plates
was used for TLC. DCM and n-hexane were used as sol-
vents for gradient mobile phase.AGallonkamp MFB-595
apparatus was used to determine melting points, that are
uncorrected. IR spectra (KBr) were recorded on a Jasco-
320-Aspectrophotometer. 1H and 13C NMR spectra were
measured on a Bruker 400 spectrometer using CDCl3 as
a solvent and TMS as an internal standard. EIMS spectra
were measured on a JMS-HX-110 spectrometer.
119.89, 120.64, 126.79, 127.84, 128.17, 130.91, 134.04,
136.14, 137.45, 142.12, 143.01, 145.87, 159.49, 191.76.
EIMS (m/z): 254 [M]+.
(E)-1-(2-Hydroxyphenyl)-3-(3-nitrophenyl)prop-
2-en-1-one (3c). Yellow solid, yield 86%, mp 198°C. IR
spectrum, ν, cm–1: 1438 (NO2), 1521 (C=C aromatic),
1644 (C=C aliphatic), 1685 (C=O, α,β unsaturated,
aliphatic), 2932 (C–H aliphatic), 3010 (C–H, aromatic).
1H NMR spectrum, δ, ppm: 6.92 d (1H, J = 8.6 Hz, H3'),
7.32 m (1H, H5'), 7.51 d.d.d (1H, J = 7.35, 6.99, 0.9
Hz, H4'), 7.82 d (1H, J = 8.24 Hz, H5), 7.89 d (1H, J =
15.89 Hz, Hα), 7.96 d.d (1H, J = 8.16, 2.0 Hz, H6), 8.00
d (1H, J = 7.80 Hz, H4), 8.04 d.d (1H, J = 8.21, 1.54 Hz,
H6'), 8.21 s (1H, H2), 8.46 d (1H, J = 16.5 Hz, Hβ), 13.40
s (1H, OH). 13C NMR spectrum, δ, ppm: 118.12, 118.19,
120.15, 121.23, 121.84, 129.38, 129.17, 131.88, 135.49,
136.12, 137.76, 142.15, 148.49, 162.43, 193.67. EIMS
(m/z): 269 [M]+.
Synthesis of (E)-1-2-(hydroxyphenyl)-3-(substi-
tutedphenyl)prop-2-en-1-one (3a–3c). 2'-Hydroxy
acetophenone (1) and one of substituted benzaldehydes
2a–2c were mixed in 20 mL of methanol. Solution of
NaOH (5 mL, 20%) was added drop wise. Mixture was
stirred upon cooling by ice cold water then for 24 h
at room temperature. Upon completion of the process
(TLC), the reaction mixture was cooled down to 0°C in
an ice water bath, acidified with 10% HCl and extracted
by dichloromethane. Evaporation of the solvent gave a
yellow solid. The following column chromatography
with n-hexane-DCM yielded pure yellow crystals of the
corresponding (E)-1-2-(hydroxyphenyl)-3-(substituted
phenyl)prop-2-en-1-ones (3a–3c).
Synthesis of 2-[3-(substituted phenyl)isoxazol-5-yl]-
phenol (5a–5c) (general procedure). A parent chalcone
3a–3c was dissolved in 20 mL of methanol and heated
upon stirred for 5 min prior to adding hydroxylamine
hydrochloride 4. The mixture was refluxed for 120 min
(TLC). Yellow precipitate formed upon acidifying and
cooling down by water followed by vigorous hand
shaking. The precipitate was filtered off and dried at
room temperature to afford the corresponding isoxazolic
chalcone 5a–5c.
(E)-1-(2-Hydroxyphenyl)-3-phenylprop-2-en-1-
one (3a) (general procedure). Light yellow solid, yield
85%, mp 147°C. IR spectrum, ν, cm–1: 1575 (aromatic),
1637 (C=C, aliphatic), 1685 (C=O, α,β unsaturated, ali-
phatic), 3025 (C–H aromatic). 1H NMR spectrum, δ, ppm:
6.96 d.d.d (1H, J = 7.84, 6.9, 1.2 Hz, H5'), 7.06 t (1H,
J = 6.80 Hz, H4), 7.15 d.d (2H, J = 7.76, 0.8 Hz, H3'),
7.32 d (2H, J = 7.80 Hz, H3, H5), 7.54 d.d.d (1H, J = 7.84,
6.4, 1.5 Hz, H4'), 7.59 d (2H, J = 7.81 Hz, H2, H6), 7.64
d (1H, J = 15.38 Hz, Hα), 7.93 d (1H, J = 16 Hz, Hβ),
7.95 d.d (1H, J = 8.4, 1.8 Hz, H6'), 13.89 s (1H, OH). 13C
NMR spectrum, δ, ppm: 119.04, 120.12, 126.56, 127.96,
127.44, 128.581, 130.76, 135.56, 135.22, 137.77, 140.06,
142.33, 145.62, 159.26, 190.23. EIMS (m/z): 224 [M]+.
2-(3-Phenylisoxazol-5-yl)phenol (5a). Light yellow
solid, yield 79%, mp 198°C. IR spectrum, ν, cm–1: 1218
(C–O aromatic), 1560 (C=C aromatic), 1594 (aromatic),
1
1661 (C=N aromatic), 3010 (C–H aromatic). H NMR
spectrum, δ, ppm: 6.24 s (1H, isoxzolyl), 6.94 d.d.d
(1H, J = 8.16, 7.98, 0.8 Hz, H5'), 7.12 d.d (2H, J = 8.19,
1.6 Hz, H3'), 7.23–7.19 m (1H, H4), 7.38 d (2H, J =
8.12 Hz, H3, H5), 7.51 d.d.d (1H, J = 7.67, 6.8, 1.2 Hz, H4'),
7.82 d (2H, J = 7.84 Hz, H2, H6), 7.90 d.d (1H, J = 8.2,
1.6 Hz, H6'), 12.77 s (1H, OH). 13C NMR spectrum, δ,
ppm: 98.16, 116.13, 117.19, 121.35, 122.34, 126.34,
127.21, 127.71, 128.20, 128.89, 129.36, 130.98, 155.15,
162.49, 169.21. EIMS (m/z): 237 [M]+.
1-(2-Hydroxyphenyl)-3-(2-methoxyphenyl)prop-
2-en-1-one (3b). Yellow solid, yield 87%, mp 237°C. IR
spectrum, ν, cm–1: 1249 (C–O–C),1560 (C=C aromatic),
1637 (C=C aliphatic), 1685 (C=O, α,β unsaturated,
aliphatic), 3036 (C–H aromatic). 1H NMR spectrum, δ,
ppm: 3.98 s (3H, OCH3), 6.89–7.21 m (4H, H3 to H6),
7.41 d.d.d (1H, J = 7.86, 7.12, 1.4 Hz, H5'), 7.53 d.d.d
(1H, J = 8.14, 7.5, 1.2 Hz, H4'), 7.68 d.d (1H, J = 7.84,
1.5 Hz, H3'), 7.76 d (1H, J = 16.48 Hz, Hα), 7.96 d.d (1H,
J = 8.64, 1.3 Hz, H6'), 8.20 d (1H, J = 16 Hz, Hβ), 12.97
s (1H, OH). 13C NMR spectrum, δ, ppm: 55.34, 113.83,
2-[3-(2-Methoxyphenyl)isoxazol-5-yl]phenol (5b).
Yellow solid, yield 75%, 196°C. IR spectrum, ν, cm–1:
1120 (C–O–C), 1530 (C=C aromatic), 1632 (C=N aro-
matic), 3030 (C–H aromatic).1H NMR spectrum, δ, ppm:
3.77 s (3H, OCH3), 6.21 s (1H, isoxzolyl), 7.09 d.d (1H,
J = 8.04, 1.6 Hz, H3'), 7.26–7.34 m (1H, H3, H4), 7.41
d.d.d (1H, J = 7.8, 7.5, 1.6 Hz, H4'), 7.53 d.d.d (1H, J =
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IRSHAD et al.
7.86, 7.12, 1.4 Hz, H5'), 7.63 d (1H, J = 7.80 Hz, H5),
7.79 d (J = 7.8 Hz, 1H, H6), 7.81 d.d (1H, J = 8.2, 1.8 Hz,
H6'), 11.23 s (1H, OH).13C NMR spectrum, δ, ppm: 56.56,
98.29, 111.72, 116.15, 117.61, 118.67, 121.82, 121.31,
127.37, 129.76, 130.27, 131.57, 155.18, 157.42, 162.76,
169.46. EIMS (m/z): 265 [M]+.
J = 8.16 Hz, H5'), 7.27–7.34 m (2H, H4, H5), 8.10 d.d
(1H, J = 7.6, 2.0 Hz, H6'), 8.22 d (2H, J = 8.06 Hz, H6),
10.12 s (1H, OH), 12.09 s (1H, NH).13C NMR spectrum,
δ, ppm: 56.65, 99.82, 111.56, 117.95, 118.61, 120.57,
121.24, 121.29, 129.74, 130.39, 131.23, 133.73, 148.81,
149.67, 155.93, 157.52. EIMS (m/z): 266 [M]+.
2-[3-(3-Nitrophenyl)isoxazol-5-yl]phenol (5c).
Greenish yellow solid, yield 70%, mp 192°C. IR spectrum,
ν, cm–1: 1460 (NO2), 1590 (C=C aromatic), 1685 (C=N),
3027 (C–H aromatic). 1H NMR spectrum, δ, ppm: 6.28 s
(1H, isoxzolyl), 7.01 d (1H, J = 7.80 Hz, H3'), 7.39 t (J =
7.8 Hz, 1H, H5'), 7.48 d.d.d (1H, J = 8.15, 7.29, 1.5 Hz,
H4'), 7.88 d (1H, J = 8.00 Hz, H5), 7.94 d.d (1H, J =
8.56, 2.6 Hz, H6), 7.95 d.d (1H, J = 8.26, 2.50 Hz, H6'),
8.25 d (1H, J = 8.15 Hz, H4), 8.34 s (1H, H2), 12.86 s
(1H, OH). 13C NMR spectrum, δ, ppm: 98.21, 116.96,
117.54, 121.72, 123.48, 122.27, 126.36, 130.29, 130.44,
133.81, 134.56, 148.83, 155.35, 167.56, 173.91. EIMS
(m/z): 282 [M]+.
2-[5-(3-Nitrophenyl)-1H-pyrazol-3-yl]phenol (7c).
Brown solid, yield 73%, mp 146°C. IR spectrum, ν, cm–1:
1225 (C–O–C), 1438 (NO2), 1474 (C=C aryl), 1550 (C=C
1
alkene), 1590 (NH), 1664 (C=O), 3207 (C–H aryl). H
NMR spectrum, δ, ppm: 4.75 s (1H, pyrazolyl), 6.99 d
(1H, J = 8.20 Hz, H3'), 7.24 t (J = 7.82 Hz, 1H, H5'), 7.27
d.d.d (1H, J = 7.95, 7.78, 1.7 Hz, H4'), 7.76 d (1H, J =
8.02 Hz, H5), 7.96 d.d (1H, J = 8.23, 2.0 Hz, H6), 8.05 d.d
(1H, J = 8.40, 1.58 Hz, H6'), 8.29 d (1H, J = 8.15 Hz, H4),
8.42 s (1H, H2), 10.12 s (1H, OH), 12.44 s (1H, NH). 13C
NMR spectrum, δ, ppm: 99.63, 117.97, 120.57, 121.48,
122.98, 123.36, 130.28, 130.58, 131.81, 133.84, 147.69,
148.45, 158.67, 160.45. EIMS (m/z): 281 [M]+.
Synthesis of 2-[5-(substituted phenyl)-1H-pyrazol-
3-yl]phenol (7a–7c) (general procedure).An appropriate
compound 3a–3c was mixed with hydrazine hydrate 6
(1 equiv) in 20 mLof ethanol and refluxed for 4–5 h. Upon
completion of the process (TLC), the mixture was cooled
down within 1h. Cold water with crushed ice was added
along with dil HCl, and the obtained solid was filtered off,
washed with ice water, and recrystallized from ethanol to
give the corresponding pyrazolic chalcone 7a–7c.
Synthesis of (Z)-2-(substitutedbenzylidene)benzo-
furan-3(2H)-one (9a–9c) (general procedure).Amixture
of an appropriate compound (3a–3c) was added to the
solution of mercuric acetate 8 in 20 mL of pyridine and
stirred for 2 h at 110°C. When the reaction mixture turned
green it was added to cold water acidified by 10% solution
of HCl. Following extraction by dichloromethanethe and
recrystallization from methanol yielded the corresponding
product 9a–9c.
2-[5-Phenyl-1H-pyrazol-3-yl]phenol (7a). Light
brown solid, yield 73%, mp 147°C. IR spectrum, ν, cm–1:
1225 (C–O–C), 1474 (C=C aryl), 1550 (C=C alkene),
1590 (NH), 1664 (C=O), 3207 (C–H aryl). 1H NMR spec-
trum, δ, ppm: 4.64 s (1H, pyrazolyl ring), 6.89 t (1H, J =
7.90 Hz, H5'), 7.23 d.d (2H, J = 8.33, 2.1 Hz, H3'), 7.39 t
(1H, J = 8.0 Hz, H4),7.54 d.d.d (1H, J = 8.67, 7.8, 1.4 Hz,
H4'),7.61d(2H,J =8.40Hz,H3,H5),8.11d(2H,J=8.14Hz,
H2, H6), 8.14 d.d (1H, J = 7.8, 1.8 Hz, H6'), 10.14 s (1H,
OH), 12.13 s (1H, NH). 13C NMR spectrum, δ, ppm:
99.53, 117.67, 120.39, 121.71, 127.49, 128.38, 129.31,
130.79, 131.26, 133.96, 147.55, 148.49, 155.71. EIMS
(m/z): 236 [M]+.
(Z)-2-Benzylidenebenzofuran-3(2H)-one (9a).
Light yellow solid, yield 84%, mp 176°C. IR spectrum,
ν, cm–1: 1250 (C–O–C), 1450 (C=C aromatic), 1600
(C=C, C=O), 1745 (C=O, 5-membered ring), 2940 (C–H
aliphatic), 3030 (C–H aromatic). 1H NMR spectrum, δ,
ppm: 6.46 s (1H, H''), 7.24 d (2H, J = 7.81 Hz, H2, H6),
7.35 t (1H, J = 8.40 Hz, H4), 7.39 d (2H, J = 7.86 Hz, H3,
H5), 7.42 d (1H, J = 7.80 Hz, H-7'), 7.48–7.56 m (2H,
H4', H5'), 7.95 d.d.d (1H, J = 8.41, 7.49 1.49 Hz, H6'). 13C
NMR spectrum, δ, ppm: 116.21, 119.63, 123.43, 125.67,
126.21, 127.33, 128.79, 128.41, 129.39, 132.72, 146.36,
169.93, 181.13. EIMS (m/z): 222 [M]+.
(Z)-2-(2-Methoxybenzylidene)benzofuran-3(2H)-
one (9b). Light yellow solid, yield 82%, mp 221°C. IR
spectrum, ν, cm–1: 1250 (C–O–C), 1440 (C=C aromatic),
1640 (C=C conjugated), 1730 (C=O, 5-membered ring),
2-[5-(2-Methoxyphenyl)-1H-pyrazol-3-yl]phenol
(7b). Brown solid, yield 70%, mp 149°C. IR spectrum,
ν, cm–1: 1225 (C–O–C), 1474 (C=C aryl), 1550 (C=C
1
1
alkene), 1590 (NH), 1664 (C=O), 3207 (C–H aryl). H
2940 (C–H aliphatic), 3040 (C–H aromatic). H NMR
NMR spectrum, δ, ppm: 4.59 s (1H, pyrazolyl ring), 6.96
d.d (2H, J = 8.33, 2.1 Hz, H3'), 7.19 d.d (1H, J = 8.24,
2.12 Hz, H4'), 7.21 d (2H, J = 8.60 Hz, H3), 7.23 t (1H,
spectrum, δ, ppm: 3.87 s (3H, OCH3), 6.51 s (1H, H''),
6.89 d.d (1H, J = 8.16, 1.82 Hz, H3), 7.08–7.21 m (2H, H4,
H5), 7.33 d.d (1H, J = 8.20, 1.56 Hz, H6), 7.36 d.d (1H,
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AURONES AND ANALOGUES: PROMISING HETEROCYCLIC SCAFFOLDS
1525
J = 8.25, 1.87 Hz, H6'), 7.66 d.d.d (1H, J = 8.51, 7.42,
1.46 Hz, H5'), 7.84 d.d (1H, J = 7.81, 2.10 Hz, H-7'), 8.28
d.d (1H, J = 7.80, 1.84 Hz, H4'). 13C NMR spectrum, δ,
ppm: 56.09, 106.22, 115.37, 120.58, 120.42, 122.64,
125.78, 124.17, 128.63, 135.88, 147.28, 160.49, 169.72,
181.74. EIMS (m/z): 252 [M]+.
6.84 (br ds, 1H, H3), 6.99–7.16 m (2H, H4, H5), 7.29 d.d
(1H, J = 8.4, 2.4 Hz, H6), 7.36 d.d.d (1H, J = 7.95,7.31,
2.1Hz, H5'), 7.49 t (1H, J = 7.8 Hz, H4'), 7.63 d (1H, J =
7.81 Hz, H3'), 7.77 d.d (1H, J = 8.20, 2.0 Hz, H6'). 13C
NMR spectrum, δ, ppm: 56.23, 107.73, 111.57, 114.92,
116.58, 119.27, 121.39, 123.81, 124.95, 126.76, 129.46,
145.32, 146.26, 155.61, 159.63. EIMS (m/z): 265 [M]+.
(Z)-2-(3-Nitrobenzylidene)benzofuran-3(2H)-one
(9c). Yellow solid, yield 80%, mp 108°C. IR spectrum, ν,
cm–1: 1270 (C–O–C), 1420 (C=C aromatic), 1460 (NO2),
1650 (C=C conjugated), 1715 (C=O, 5-membered ring),
2940 (C–H aliphatic). 1H NMR spectrum, δ, ppm: 6.56 s
(1H, H''), 7.56 d.d (1H, J = 8.46, 2.40 Hz, H5'), 7.65–7.69
m (2H, H5, H6), 7.71 d.d.d (1H, J = 8.31, 7.96, 1.04 Hz,
H6'), 8.09 d.d (1H, J = 7.89, 1.73 Hz, H4'), 8.20 d (1H, J =
8.18 Hz, H4), 8.31 d.d (1H, J = 7.99, 1.75 Hz, H-7'), 8.36
s (1H, H2). 13C NMR spectrum, δ, ppm: 113.09, 120.62,
122.81, 123.23, 124.37, 125.03, 132.62, 133.67, 136.92,
137.84, 146.48, 148.02, 150.93, 169.47, 186.28. EIMS
(m/z): 267 [M]+.
3-(3-Nitrophenyl)benzofuro[3,2-c]isoxazole (10c).
Dark brown solid, yield 65%, mp 193°C. IR spectrum,
ν, cm–1: 1270 (C–O–C), 1460 (C=C aromatic), 1540
(NO), 1600 (C=C conjugated), 1610 (C=N), 1715 (C=O,
5-membered ring), 2940 (C–H aliphatic), 3030 (C–H
aromatic). 1H NMR spectrum, δ, ppm: 7.47 d.d (1H, J =
7.60 Hz, H4'), 7.52 d (1H, J = 8.46,7.93, 0.9 Hz, H5'),7.68
t (1H, J = 7.64 Hz, H5), 7.71 d (1H, J = 8.23 Hz, H6), 7.78
d (1H, J = 8.24 Hz, H3'), 7.84 d.d (1H, J = 8.21, 1.8 Hz,
H6'), 8.12 d (1H, J = 8.40 Hz, H4), 8.29 s (1H, H2). 13
C
NMR spectrum, δ, ppm: 106.22, 111.47, 119.46, 122.51,
122.68, 123.73, 124.97, 130.48, 131.21, 145.57, 148.29,
156.13, 158.19. EIMS (m/z): 280 [M]+.
Synthesis of 3-(substituted phenyl)benzofuro[3,2-
c]isoxazole (10a–10c) (general procedure). Mixture of
a (Z)-2-(substitutedbenzylidene)benzofuran-3(2H)-one
(9a–9c) with hydroxylamine hydrochloride 4 was re-
fluxed in 30 mL of ethanol for 24 h. A solution of KOH
(2 g) in H2O (5 mL) was added to the reaction mixture
while stirring. The reaction mixture was treated with cold
water then extracted with dichloromethane. Recrystalliza-
tion from ethanol yielded the corresponding compound
10a–10c.
Synthesis of pyrazolic aurones (11a–11c) (general
procedure). Stirring of a mixture of an appropriate com-
pound (9a–9c), hydrazine hydrate 6 and 20 mL of glacial
acetic acid was carried out for 4 h. Ice cold water was
poured into the mixture (TLC) and it was extracted by
dichloromethane. Following recrystallization from etha-
nol yielded the corresponding 3-(substituted phenyl)-2H-
benzofuro[3,2-c]pyrazole (11a–11c).
3-Phenyl-2H-benzofuro[3,2-c]pyrazole (11a).
Brownish yellow solid, yield 63%, mp 333°C. IR spec-
trum, ν, cm–1: 1245 (C–O–C), 1350 (C–N), 1440 (C=C
aromatic), 1640 (C=N), 3050 (C–H aromatic), 3320
(N–H). 1H NMR spectrum, δ, ppm: 7.07–7.25 m (3H, H3
toH5), 7.31 d.d (1H, J = 8.46, 2.4 Hz, H5'), 7.39 d (2H,
J = 7.8 Hz, H2, H6), 7.52 d.d (1H, J = 8.01, 2.20 Hz, H4'),
7.62 d (1H, J = 8.24 Hz, H3'), 7.76 d.d (1H, J = 7.65, 2.0 Hz,
H6'), 12.66 s (1H, NH). 13C NMR spectrum, δ, ppm:
106.58, 115.73, 119.37, 123.28, 124.62, 126.92, 127.39,
128.83, 129.38, 145.03, 150.74, 156.67. EIMS (m/z):
234 [M]+.
3-Phenylbenzofuro[3,2-c]isoxazole (10a). Dark
brown solid, yield 64%, mp 173°C. IR spectrum, ν, cm–1:
1270 (C–O–C), 1440 (C=C aromatic), 1530 (NO), 1610
(C=N), 1600 (C=C conjugated), 1715 (C=O, 5-membered
1
ring), 2940 (C–H aliphatic), 3030 (C–H aromatic). H
NMR spectrum, δ, ppm: 7.18 d (2H, J = 8.20 Hz, H3,
H5), 7.33 t (1H, J = 7.46 Hz, H4), 7.37 d.d.d (1H, J =
8.46, 7.93, 0.9 Hz, H5'), 7.45 d (2H, J = 8.00 Hz, H2, H6).
7.52 t (1H, J = 7.6 Hz, H4'), 7.69 d (1H, J = 7.89 Hz, H3'),
7.82 d.d (1H, J = 8.35, 2.4 Hz, H6'). 13C NMR spectrum,
δ, ppm: 106.39, 119.46, 111.72, 123.59, 124.61, 125.24,
128.93, 129.69, 131.49, 145.41, 156.83, 159.73. EIMS
(m/z): 235 [M]+.
3-(2-Methoxyphenyl)-2H-benzofuro[3,2-c]pyrazole
(11b). Yellow solid, yield 64%, mp 381°C. IR spectrum,
ν, cm–1: 1255 (C–O–C), 1355 (C–N), 1645 (C=N), 3330
(N–C). 1H NMR spectrum, δ, ppm: 3.82 s (3H, OCH3),
6.76 (brds, 1H, H3), 6.82–7.02 m (2H, H4, H5), 7.26 br.d
(1H, J = 8.9 Hz, H6), 7.36 t (1H, J = 7.68 Hz, H5'), 7.49
d.d (1H, J = 7.69, 2.2 Hz, H4'), 7.63 br.d (1H, J = 8.10 Hz,
H3'), 7.77 d (1H, J = 8.42 Hz, H6'), 12.72 s (1H, NH). 13C
3-(2-Methoxyphenyl)benzofuro[3,2-c]isoxazole
(10b). Brown solid, yield 67%, mp 112°C. IR spectrum,
ν, cm–1: 1270 (C–O–C), 1450 (C=C aromatic), 1520
(NO), 1600 (C=C conjugated), 1610 (C=N), 1715 (C=O,
5-membered ring), 2940 (C–H aliphatic), 3030 (C–H aro-
matic). 1H NMR spectrum, δ, ppm: 3.85 s (3H, OCH3),
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1526
IRSHAD et al.
NMR spectrum, δ, ppm: 56.72, 106.93, 111.47, 112.39,
118.44, 119.28, 121.94, 123.27, 124.57, 129.36, 131.82,
145.63, 150.86, 156.21, 157.36. EIMS (m/z): 264 [M]+.
synthetic flavonoid was mixed in 1 mL of ethanol. Anti-
microbial activity of compounds was evaluated against
three food pathogenic strains: Staphylococcus aureus
(Gram-positive), Bascillus subtilis (Gram-positive) and
Escherichia coli (Gram-negative) by the disc diffusion
method. In glass bottle, nutrient agar medium was pre-
pared by mixing 2 g of agar in 100 mL of distilled water
and sterilized for 30 min in an autoclave at 121°C [13].
3-(3-Nitrophenyl)-2H-benzofuro[3,2-c]pyrazole
(11c). Yellow solid, yield 67%, mp 155°C. IR spectrum,
ν , cm–1: 1250 (C–O–C), 1360 (C–N), 1370 (NO2), 1650
(C=N), 2930 (C–H aliphatic), 3030 (C–H aromatic), 3300
(N–H). 1H NMR spectrum, δ, ppm: 7.52 d.d.d (1H, J =
7.88, 7.76, 1.40 Hz, H4'), 7.61(t, 1H, J = 7.63 Hz, H5'),
7.63 d.d (1H, J = 8.02, 1.78 Hz, H5), 7.74 d (1H, J =
8.0 Hz, H6), 7.78 d.d (1H, J = 8.0, 2.1 Hz, H3'), 7.79 d
(1H, J = 7.97 Hz, H6'), 8.08 br.d (1H, J = 8.80 Hz, H4),
8.23 s (1H, H2), 12.78 s (1H, NH). 13C NMR spectrum,
δ, ppm: 106.25, 111.71, 119.83, 122.11, 122.49, 123.53,
123.81, 124.73, 130.61, 133.53, 145.73, 148.64, 152.36,
159.63. EIMS (m/z): 279 [M]+.
The pure isolated colonies (10 μL) of freshly grown
bacteria were transferred to Petri dishes containing
growth media. Whatman No.1 filter paper discs (6 mm)
were impregnated with the samples and the standard drug
amoxicillin (10 mg/mL) and placed over the nutrient agar
plates. The plates were then stored for incubation for 24
to 48 h at 37°C.
CONCLUSIONS
Antioxidant potential .Antioxidant potential of the
synthesized compounds was calculated by using reduc-
ing power (FRAP) assays and free radical scavenging
activity (DPPH assay).
Several new multi-functional analogues of 2'-hydroxy
chalcone containing isoxazolic and pyrazolic function-
alities are synthesized. Among the synthesized products
that demonstrate the pronounced biological activity, the
compound bearing the methoxy group in ring B and pyr-
azolic ring exhibits excellent antioxidant and antibacterial
activities. The present study indicates that further tests of
the synthesized compounds are needed for determining
their biological potential.
To evaluate bioactivity of synthesized compounds,
DPPH test was carried out [11]. Dissolutions were made
for all compounds and gallic acid in dichlorometane (0.25,
0.5, 0.75, 1.0, 1.5 mg/mL). In each dissolution, 2 mL of
0.01 mM DPPH was incubated for 20 min in darkness.
Absorbance of each sample and standard was measured at
700 nm. Percent radical scavenging ability of all sample
compounds was calculated:
CONFLICT OF INTEREST
No conflict of interest was declared by the authors.
REFERENCES
Radical scavenging = (Csolvent – Csample)/Csolvent ×100%.
Ferric reducing power assay (FRAP). The Oyaizu
procedure [12] was used to determine ferric reducing
power of the synthesized compounds. Each compound
were dissolved in ethanol (0.1 mg/mL), and 2.5 mL of
1 % potassium ferricyanide and 2.5 mL of 0.2 M sodium
phosphate buffer (pH 6.5) were mixed with the solution.
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ferric chloride (0.1%) and distilled water (2.5 mL) were
added. Absorbance was measured at 700 nm and com-
pared with that of the control. Rise in absorbance of the
prepared mixtures of synthesized compounds indicated
increasing reduction power of the compounds.
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