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S. VENKATESWARLU et al.
Table 1. Antioxidative Activity of Aurones 1a–1f
(3.0 mmol) in acetic anhydride (7.5 ml) was heated at
90 ꢀC for 2 h. The cooled reaction mixture was poured
into ice-cooled water, extracted with diethyl ether, and
the solvent was removed. The residue was dissolved in
methanol (7.5 ml) and HCl (20%, 7.5 ml) and then
refluxed for 1 h. The cooled reaction mixture was diluted
with ice-cooled water and extracted with ethyl acetate.
The residue obtained after evaporating the solvent was
chromatographed in a silica gel column, using chloro-
form-methanol (90:10) as the eluent, to give aurones
1a–1f.
2-[(3,4-Dihydroxyphenyl)methylene]6,7-dihydroxyben-
zo[b]furan-3-one (1a): mp 286–288 ꢀC (lit.11) mp 280–
292 ꢀC); UV ꢁmax (MeOH) nm ("): 205 (63,860), 411
(53,180); IR ꢂmax (KBr) cmꢁ1: 3366, 1604, 1290, 1192,
1125, 1036; NMR ꢃH: 6.63 (1H, s, =CH), 6.74 (1H, d,
J ¼ 8:3 Hz, H-5), 6.86 (1H, d, J ¼ 8:2 Hz, H-50), 7.13
(1H, d, J ¼ 8:3 Hz, H-4), 7.38 (1H, dd, J ¼ 8:4 &
1.8 Hz, H-60), 7.44 (1H, d, J ¼ 1:9 Hz, H-20), 9.1–9.8
(3H, brs, 3 ꢂ Ar–OH), 10.65 (1H, brs, Ar–OH); NMR
ꢃC: 182.0, 155.1, 154.2, 147.9, 145.9, 145.4, 130.1,
124.6, 123.6, 118.4, 116.0, 115.2, 114.6, 112.6, 111.8;
MS (ESI, negative scan): m=z 285 (M ꢁ H)ꢁ.
NBT superoxide DPPH radical
scavenging activity scavenging activity
S. No
Compound
(IC50 ꢀM)
(IC50 ꢀM)
1
2
1a
1b
6.5
9.0
8.3
8.7
3
1c
1d
10.0
12.3
4.3
10.4
10.1
4
5
1e
1f
7.9
11.0
6
20.2
301.8
670.5
530.2
482.5
7
BHT
vitamin C
vitamin E
resveratrol
13.4
23.3
8
9
10
>1000
35.5
mass). To understand the effect of substituents to the
aurone structure on the antioxidative activity, we
synthesized five other analogs, 1b–1f, by the same
method with appropriately substituted benzaldehydes. In
all cases, a single geometric isomer (Z) was obtained.
The stereochemistry at the double bond was confirmed
by diagnostic 13C-NMR data6) (exocyclic olefinic carbon
=CH, resonating at about 111 ppm). It is known that the
reaction proceeds stereoselectively and affords only the
(Z)-aurones. (Z)-Isomer is thermodynamically more
stable than the (E)-isomer.7)
We determined the antioxidative activity of 1a and its
analogs, 1b–1f, by the superoxide free radical scaveng-
ing (NBT) method8) and DPPH method.9) The IC50
values of these compounds are presented in Table 1.
Aurones 1a (IC50: 6.5 ꢀM) and 1e (IC50: 4.3 ꢀM) having
catechol and pyrogallol moieties were the most active
compounds, followed by 1b (IC50: 9 ꢀM), 1c (IC50:
10 ꢀM), 1d (IC50: 12.3 ꢀM) and 1f (IC50: 20.2 ꢀM).
Interestingly 1a and 1e showed several-fold more potent
activity than vitamin C (IC50: 670 ꢀM), vitamin E (IC50:
530 ꢀM), resveratrol (IC50: 482 ꢀM) and BHT (IC50:
301 ꢀM). The same order of activity was followed by
aurones 1a–1f with the DPPH method. Again 1a (IC50:
8.3 ꢀM) and 1e (IC50: 7.9 ꢀM) showed good DPPH free
radical scavenging activity. The superior antioxidative
activity of these compounds lends further support to the
fact that the pyrogallol or catechol system enhances the
antioxidative activity.10)
2-[(4-Hydroxy-3-methoxyphenyl)methylene]6,7-dihy-
droxybenzo[b]furan-3-one (1b): mp 264–266 ꢀC; IR
ꢂmax (KBr) cmꢁ1: 3518, 3361, 1669, 1278, 1198, 1150,
1034; NMR ꢃH: 3.86 (3H, s, –OCH3), 6.74 (1H, s,
=CH), 6.75 (1H, d, J ¼ 8:2 Hz, H-5), 6.92 (1H, d,
J ¼ 8:0 Hz, H-50), 7.13 (1H, d, J ¼ 8:2 Hz, H-4), 7.50–
7.60 (2H, m, H-20, 60), 9.72 (2H, brs, 2 ꢂ Ar–OH), 10.61
(1H, brs, Ar–OH); NMR ꢃC: 182.0, 155.1, 154.4, 148.8,
147.7, 146.1, 130.0, 125.7, 123.6, 116.1, 115.5, 115.4,
114.5, 112.7, 111.6, 55.8; MS (ESI, negative scan): m=z
299 (M ꢁ H)ꢁ. Elemental analysis. Found: C, 63.78; H,
3.98%. Calcd. for C16H12O6: C, 64.00; H, 4.03%.
2-[(4-Hydroxyphenyl)methylene]6,7-dihydroxybenzo-
[b]furan-3-one (1c): mp 296–299 ꢀC; IR ꢂmax (KBr)
cmꢁ1: 3400, 1678, 1631, 1297, 1251, 1162, 1041; NMR
ꢃH: 6.72 (1H, s, =CH), 6.73 (1H, d, J ¼ 8:3 Hz, H-5),
6.90 (2H, d, J ¼ 8:6 Hz, H-30,50), 7.13 (1H, d, J ¼
8:3 Hz, H-4), 7.92 (2H, d, J ¼ 8:6 Hz, H-10,60), 9.80–
10.40 (3H, br s, 3 ꢂ Ar–OH); NMR ꢃC: 182.0, 159.1,
155.0, 154.2, 145.9, 133.4, 130.0, 123.2, 115.9, 115.2,
114.5, 112.7, 111.2; MS (ESI, negative scan): m=z 269
(M ꢁ H)ꢁ. Elemental analysis. Found: C, 66.29; H,
3.67%. Calcd. for C15H10O5: C, 66.67; H, 3.73%.
2-[(2,4-Dihydroxyphenyl)methylene]6,7-dihydroxyben-
zo[b]furan-3-one (1d): mp 226–229 ꢀC; IR ꢂmax (KBr)
cmꢁ1: 3233, 1655, 1615, 1283, 1159, 1095, 1042; NMR
ꢃH: 6.41 (1H, s, =CH), 6.42 (1H, d, J ¼ 8:8 Hz, H-50),
6.73 (1H, d, J ¼ 8:3 Hz, H-5), 7.07 (1H, s, H-30), 7.10
(1H, d, J ¼ 8:3 Hz, H-4), 8.16 (1H, d, J ¼ 8:8 Hz, H-60),
9.49 (1H, brs, Ar–OH), 9.98 (1H, brs, Ar–OH), 10.24
(1H, brs, Ar–OH), 10.50 (1H, brs, Ar–OH); NMR ꢃC:
182.0, 160.8, 159.1, 154.8, 153.9, 145.4, 133.0, 130.1,
115.1, 114.9, 112.6, 110.8, 108.3, 105.9, 102.3; MS
(ESI, negative scan): m=z 285 (M ꢁ H)ꢁ. Elemental
analysis. Found: C, 62.67; H, 3.47%. Calcd. for
In conclusion, we synthesized 1a together with the
five analogs, 1b–1f, and evaluated their antioxidative
potential by the two commonly used methods, the
superoxide and DPPH free radical scavenging methods.
Tetrahydroxyaurone (1a) and pentahydroxyaurone (1e)
were both potent antioxidants.
Experimental
See refs. 8 and 9 for the general experimental and
antioxidative activity determination procedures. 1H-
NMR (400 MHz) and 13C-NMR (100 MHz) data were
recorded in DMSO-d6.
General procedure for the preparation of 1a–1f: A
mixture of 4 (3.0 mmol) and substituted benzaldehyde