Synthesis, stereochemical assignments, and biological activities of homoisoflavonoids

Article abstract of DOI:10.1016/j.bmc.2005.11.031

A series of four naturally occurring homoisoflavonoids and eight analogs have been synthesized starting from an appropriately substituted phenol through chroman-4-one, in four steps. The products were assigned as E-isomers based on NMR spectroscopic data.

Full text of DOI:10.1016/j.bmc.2005.11.031

Bioorganic & Medicinal Chemistry 14 (2006) 2545–2551
Synthesis, stereochemical assignments, and biological activities of
homoisoflavonoids
Vidavalur Siddaiah,a Chunduri Venkata Rao,a Somepalli Venkateswarlu,b
Alluri V. Krishnarajub and Gottumukkala V. Subbarajub,*
aDepartment of Chemistry, Sri Venkatateswara University, Tirupati 517 502, India
bLaila Research Centre, Unit 1, Phase III, Jawahar Autonagar, Vijayawada 520 007, India
Received 14 October 2005; revised 15 November 2005; accepted 16 November 2005
Available online 6 December 2005
Abstract—A series of four naturally occurring homoisoflavonoids and eight analogs have been synthesized starting from an appro-
priately substituted phenol through chroman-4-one, in four steps. The products were assigned as E-isomers based on NMR spec-
troscopic data. The E-isomers were converted into Z-isomers by photoisomerization. The E- and Z-isomers showed distinct
chemical shifts and the differences between (E) and (Z)-homoisoflavonoids in the proton NMR spectra afford a useful method
for ascertaining the stereochemistry. The antioxidant activity of homoisoflavonoids was determined by superoxide (NBT) and
DPPH free radical scavenging methods. The analog 7-hydroxy-3-[(3,4,5-trihydroxyphenyl)methylene]chroman-4-one displayed
excellent activity followed by sappanone A in both the methods and were several times potent than the commercial antioxidants
like BHA, BHT, etc. These compounds were evaluated in vitro for their inhibitory activities against 5-lipoxygenase (5-LOX)
enzyme. The analog 7-hydroxy-3-[(N,N-dimethylaminophenyl)methylene]chroman-4-one was found to possess potent inhibitory
activity and was comparable to that of the standard, nordihydroguiaretic acid. These results suggest that these homoisoflavonoids,
with their potent antioxidant and 5-LOX inhibitory activities, may have useful applications as antioxidants and lead compounds for
asthma and inflammatory diseases.
ꢀ 2005 Elsevier Ltd. All rights reserved.
1. Introduction
sappan L.13 and Caesalpinia japonica Sieb.14 Antioxidant
activity of flavonoids has been well established, but to
the best of our knowledge, these homoisoflavonoids
have not been studied for their antioxidative activity.
In view of the importance of dietary antioxidants in
Homoisoflavonoids (3-benzylidene-4-chromanones) are
related to flavonoids and occur as natural products
and exhibit biological activity.1 These compounds have
been reported to possess antifungal,2 hypocholestero-
lemic,3 antimutagenic,4 and antiviral activities.5 Synthe-
sis of these compounds is based usually on the
condensation of 4-chromanones with aromatic alde-
hydes in the presence of acidic or basic catalyst.6,7 How-
ever, Baylis–Hillman reaction8 was utilized to obtain
homoisoflavonoids, recently. Bonducellin was isolated
from Caesalpinia bonducella9 and Caesalpinia pulcherr-
ima.10 Very recently isobonducellin, the Z-isomer of
bonducellin, and 20-methoxybonducellin (Fig. 1) were
also isolated from C. pulcherrima.11,12 Sappanone A
(Fig. 1) was isolated from the heartwood of Caesalpinia
HO
O
O
OCH
HO
O
O
3
OCH
3
Isobonducellin
Bonducellin
O
OCH
3
HO
OH
OH
HO
O
O
OCH
3
O
Keywords: Homoisoflavonoids; Synthesis; Stereochemical assignments;
Antioxidant activity; 5-LOX.
2'-Methoxybonducellin
Sappanone A
*
Corresponding author. Tel.: +91 866 2541303; fax: +91 866
2546216; e-mail: subbarajugottumukkala@hotmail.com
Figure 1. Chemical structures of natural homoisoflavonoids.
0968-0896/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmc.2005.11.031

2546
V. Siddaiah et al. / Bioorg. Med. Chem. 14 (2006) 2545–2551
chemoprevention of degenerative diseases, such as can-
cer, Alzheimerꢀs, Parkinsonꢀs, and cardiovascular diseas-
es, we have synthesized for the first time bonducellin,
isobonducellin, 20-methoxybonducellin, and sappanone
A along with eight new structural analogs. We have also
screened these compounds for 5-lipoxygenase enzyme
inhibitory activity (5-LOX). 5-Lipoxygenase is the key
enzyme for the biosynthesis of leukotrienes, the impor-
tant mediators for inflammatory, allergic, and obstruc-
tive processes. 5-LOX inhibitors have potential in
treating ashthma and various inflammatory disor-
ders15,16 In this paper, we report the details of synthesis,
stereochemical assignments, and antioxidative and
5-LOX activities of several homoisoflavonoids.
hydrochloride18 gave 4, in 63% yield, a key intermediate
in the synthesis. Base catalyzed condensation of 4 with
4-methoxybenzaldehyde, 2,4-dimethoxybenzaldehyde,
and 3,4-dihydroxybenzaldehyde afforded bonducellin
(5E),20-methoxybonducellin (6E), and sappanone A
(7E) in 65%, 64%, and 58% yields, respectively (Scheme
1). In order to assign the stereochemistry unambiguous-
ly and to get isobonducellin, 5E was photoisomerized to
5Z using medium pressure mercury lamp.19 The spectral
data (IR, NMR, and MS) of 5E, 5Z, 6E, and 7E were
identical with those of natural products.9,11–13 The other
analogs (8–14) were synthesized using the appropriately
substituted benzaldehydes as shown above (Scheme 1).
In all cases, a single stereo isomer (E) was obtained
and some of them were converted into Z-isomers by
irradiation.
2. Results and discussion
2.1. Synthesis
Due to poor yields in base catalyzed condensation of 4
with 4-chlorobenzaldehyde, a simple method under sol-
vent-free conditions catalyzed by perchloric acid sup-
ported on silica gel was developed (Scheme 2).
The reaction of 3-methoxyphenol (1a) with 3-bromo-
propionic acid using sodium hydride as base furnished
3-phenyloxypropionic acid (2a), which was cyclized
using polyphosphoric acid17 to give 7-methoxychro-
man-4-one (3a). Demethylation of 3a using pyridine
The condensation reaction of 4 with 4-chlorobenzalde-
20
hyde using catalytic HClO4–SiO2 under stirring at
90–100 ꢁC gave homoisoflavonoid (15) in 71% yield.
R1
R2
OH
R1
R2
O
O
O
R1
R2
(i)
(ii)
COOH
2
3
1
1,2,3
(iii)
3a
4 (R1=OH, R2=H)
a
R1=OCH3, R2=H
R1=H, R2=OCH3
b
R4
8
O1 R3
R5
R6
7
O
2'
R1
R1
R2
(v)
(iv)
2
4
4'
3
6
R2
1'
5
9
6'
R3
O
O
Z
E
R4
R6
R5
5
R 1= OH, R5= OCH3, R2=R3=R4=R6=H
R 1= OH, R3=R5=OCH3, R2=R4=R6=H
R 1=R4=R5=OH, R2=R3=R6= H
R 1=R5= OCH3, R2=R3=R4=R6=H
R 1= OH, R4=R5=R6=OCH3, R2=R3=H
10
11
12
13
14
R1= OH, R5= N(CH3)2, R2=R3=R4=R6=
R1=R5=OH, R2=R3=R4=R6=H
R1=R4=R5=R6= OH, R2= R3=H
R1=R5= OH, R4=COOH, R2=R3=R6=H
R2=OCH3, R5=OH, R1=R3=R4=R6=H
6
7
8
9
Scheme 1. Reagents and conditions: (i) Bromopropionic acid, NaH, DMF, 70–80 ꢁC, rt, 16 h, 30%; (ii) PPA, 80 ꢁC, 4 h, 46%; (iii) PyÆHCl, 180–
190 ꢁC, 2 h, 62%; (iv) substituted benzaldehyde, piperidine, 70–80 ꢁC, 2 h, 43–72%; (v) hm, benzene, 4 h, 32–54%.
CHO
HO
O
O
R
HO
O
90-100˚C
30 min
+
R
O
4
15 R = Cl
5
R = OCH3
Scheme 2. Synthesis under solvent-free conditions.

V. Siddaiah et al. / Bioorg. Med. Chem. 14 (2006) 2545–2551
Table 2. Antioxidant activity of homoisoflavonoids
2547
The reaction was completed in 30 min and the product
was isolated by filtration through silica gel. It is an
environment-friendly and cost-effective process. The
generality of the reaction was checked with 4-methoxy-
benzaldehyde and obtained bonducellin (5E) in 68%
yield.
Compound
NBT superoxide
scavenging activity radical
DPPH free
(IC50 in lM)*
scavenging
activity
(IC50 in lM)*
Bonducellin (5E)
Isobonducellin (5Z)
>100
>100
>100
>100
>100
8.3
2.2. Stereochemical assignments
20-Methoxy-bonducellin (6E) >100
Sappanone A (7E)
8
26.3
>100
>100
>100
>100
6.5
The proton NMR data of E and Z homoisoflavonoids
are presented in Table 1. In a,b-unsaturated carbonyl
compounds, the olefinic proton cis to the carbonyl
exhibits a chemical shift lower than that of the corre-
sponding trans proton and this difference could be used
for assignment of configuration.21–23 The present data
also reveal that the olefinic proton is useful to distin-
guish between the two isomers in homoisoflavonoids.
The olefinic proton (@CH) in E-isomers appeared as sin-
glet in the range of d 7.74 and 7.81, while the same pro-
ton in Z-isomers appeared in the range of d 6.78 and
7.07. The protons (CH2) adjacent to double bond are
also useful in distinguishing the isomers. In E-isomers,
a singlet appeared in the range of d 5.20–5.36 and in
Z-isomers these protons were appeared in the range of
d 4.93–4.98. In addition to these, in all Z-isomers, the
protons (H-20 and H-60) of pendant aryl unit appeared
as a doublet at a much lower field than the correspond-
ing protons in E-isomers and these chemical shift differ-
ences are significant and could be used to distinguish the
isomers. Thus, 20,60-protons in Z-isomers appeared as
doublet at about d 7.86, while in E-isomers the same
protons appeared in the range of d 7.02–7.27. These
differences lead to unambiguous assignment of
stereochemistry at the double bond.
>100
>100
>100
>100
3.8
9
10
11
12
13
14
>100
>100
>100
852
>100
>100
>100
25.1
>100
34
15
Vitamin C
Vitamin E acid succinate
BHA
BHT
726
966
381
22.5
BHA, butylated hydroxyanisole; BHT, butylated hydroxytoluene.
The lower the IC50 values, the higher the antioxidant activity.
* This indicates the significant difference at the level of P < 0.05, n = 3.
7-hydroxy-3-[(3,4,5-trihydroxyphenyl)methylene]chroman-
4-one (12E, IC50:6.5 lM), having pyrogallol moiety showed
highest activity followed by sappanone A (7E, IC50:
26.3 lM) having catechol moiety and showed several fold
potentactivity incomparisonwiththoseofthecommercially
available antioxidants like vitamin C (IC50: 852 lM),
vitamin E acid succinate (IC50: 726 lM), BHA (IC50:
966 lM, lit.26 IC50: >1000 lM), and BHT (IC50: 381 lM).
The superior scavenging ability of these compounds (7E
and 12E) lends further support to the fact that the catechol
or pyrogallol system enhances the antioxidant activity.27
2.3. Antioxidant activity
2.3.2. DPPH radical scavenging activity. DPPH (1,1-di-
phenyl-2-picrylhydrazyl) radical scavenging activity of
homoisoflavonoids was determined by the method de-
scribed by Lamaison et al.,28 based on the reduction of
methanolic solution of the colored DPPH radical. From
the IC50 values (Table 2), again 7-hydroxy-3-[(3,4,5-tri-
hydroxyphenyl)methylene]chroman-4-one (12E, IC50:
3.8 lM) and sappanone A (7E, IC50: 8.3 lM) were found
to be powerful scavengers of DPPH free radicals in com-
parison with the commercially available antioxidants
[vitamin C IC50: 25.1 lM (lit.29 IC50: 26.5 lM)].
In the present study, two commonly used antioxidant
evaluation methods, superoxide radical scavenging
(NBT method) and the DPPH free radical scavenging
methods, were chosen to determine the antioxidant po-
tential of the compounds. Both the methods measure
the efficacy of a hydrogen atom transfer from a phenol
to a radical.
2.3.1. Superoxide radical scavenging activity. Superoxide
radicals were generated in vitro by non-enzymatic system
and determined spectrophotometrically (560 nm) by nitro
blue tetrazolium (NBT) photoreduction method of
McCord and Fridovich.24,25 The antioxidant activity of
homoisoflavonoids(5E, 5Z, 6E, and 7E) and their analogs
was expressed as 50% inhibitory concentration (IC50 in
lM) and the values are incorporated in Table 2. From
the superoxide scavenging activity data, the analog,
2.4. 5-Lipoxygenase enzyme inhibitory activity
The homoisoflavonoids (5–15) were screened for their
5-lipoxygenase enzyme inhibitory potential using
colorimetric method,30,31 at different concentrations.
From the inhibitory values (Table 3), the homoisoflavo-
noids, 7-hydroxy-3-[(N,N-dimethylaminophenyl)methy-
lene]chroman-4-one (10E, 39% inhibition at 10 lM),
7-hydroxy-3-[(3,4,5-trihydroxyphenyl)methylene]-
chroman-4-one (12E, 45% inhibition at 50 lM), and
20-methoxybonducellin (6E, 44% inhibition at 50 lM),
displayed good activity. Interestingly, the inhibitory
activity of 10E was comparable with that of NDGA
(37% inhibition at 10 lM).
Table 1. 1H NMR data of E and Z homoisoflavonoids
Compound
E
Z
H-2
H-9
H-20,60
H-2
H-9
H-20,60
5
6
8
5.32
5.20
5.36
7.74
7.78
7.81
7.26
7.02
7.27
4.93
4.98
4.94
6.82
7.07
6.78
7.86
7.86
7.87

2548
V. Siddaiah et al. / Bioorg. Med. Chem. 14 (2006) 2545–2551
Table 3. 5-Lipoxygenase inhibitory activity of homoisoflavonoids
6-Methoxychroman-4-one (3b) was prepared by the lit-
erature procedure.32
Compound
Concentration (lM) % inhibition
Bonducellin (5E)
Isobonducellin (5Z)
50
50
28.8
19.0
44.0
21.3
28.0
19.1
39.2
21.2
45.4
24.9
—
4.2. 3-(3-Methoxyphenyl)-propionic acid (2a)
20-Methoxy-bonducellin (6E) 50
To a mixture of sodium hydride (5 g, 50%, 104 mmol)
in DMF (40 mL) was added 3-methoxyphenol (5 g,
40 mmol) in DMF (10 mL) at 10–15 ꢁC and the mix-
ture was stirred at rt for 0.5 h. Then a solution of
3-bromopropionic acid (7.4 g, 48 mmol) in DMF
(10 mL) was added and the mixture was stirred at rt
for 14 h. The reaction mixture was diluted with meth-
anol (10 mL), acidified with dil HCl, and extracted
with ethyl acetate (2· 100 mL). The combined ethyl
acetate layer was washed with water (50 mL), brine
(30 mL), and dried over sodium sulfate. The residue
obtained after evaporation of the solvent was chro-
matographed over silica gel column using mixtures
of petroleum ether and ethyl acetate (75:25) as eluent
to give the title product (2.4 g, 30%), mp 60–62 ꢁC
(lit.33 mp 80–81 ꢁC).
Sappanone A (7E)
8
50
50
50
10
50
50
50
50
50
10
9
10
11
12
13
14
15
NDGA
30.9
36.9
NDGA, nordihydroguiaretic acid.
3. Conclusions
In conclusion, we have accomplished the synthesis of
four natural homoisoflavonoids, bonducellin (5E), iso-
bonducellin (5Z), 20-methoxybonducellin (6E), and sap-
panone A (7E), along with eight analogs (8–15) in
three steps from substituted phenol. Some of the E-iso-
mers were converted into Z-isomers and assigned the
stereochemistry at the double bond based on proton
NMR data. The analog 12E and sappanone A exhibit-
ed excellent antioxidative activity and were several fold
more potent than the commercial antioxidants like
vitamin C, vitamin E acid succinate, BHA, and BHT.
The analogs 10E and 12E exhibited potent 5-LOX
inhibitory activities and were comparable with that of
the standard drug, nordihydroguiaretic acid. Homoi-
soflavonoid 12E with its potent antioxidative and 5-
LOX inhibitory activities may find applications as an
antioxidant and to be useful for asthma and inflamma-
tory diseases.
4.3. 7-Methoxychroman-4-one (3a)
A mixture of 3-(3-methoxyphenyl)-propionic acid (0.5 g)
and polyphosphoric acid (2 g) was stirred at 70–75 ꢁC
for 1.5 h and at rt for 2 h. The mixture was diluted with
ice-cold water and extracted with ether (2· 50 mL). The
combined ethereal solution was washed with aq. sodium
carbonate (10%, 20 mL), water (50 mL), brine (30 mL),
and dried over sodium sulfate. The residue obtained
after evaporation of the solvent was chromatographed
over silica gel column using mixtures of petroleum ether
and ethyl acetate (85:15) as eluent to give 3a (0.21 g,
46%), which was crystallized from chloroform–hexane,
mp 46–48 ꢁC (lit.33 mp 52–54 ꢁC), IR (neat): 2942,
1681, 1610, 1259, 1234, 1159, 1119, 1024, 839 cmꢀ1
;
1H NMR (CDCl3) d 2.76 (2H, t, J = 6.5 Hz, H-3), 3.84
(3H, s, Ar-OCH3), 4.52 (2H, t, J = 6.5 Hz, H-2), 6.41
(1H, d, J = 2.4 Hz, H-8), 6.58 (1H, dd, J = 2.4, 8.8 Hz,
H-6), 7.84 (1H, d, J = 8.8 Hz, H-5); LCMS (positive
scan): m/z 179 (M+H)+.
4. Experimental
4.1. General
4.4. 7-Hydroxychroman-4-one (4)
Melting points were recorded with a Mel-Temp melting
point apparatus, in open capillaries and are uncorrected.
IR spectra were recorded on a Perkin-Elmer BX1 FTIR
Spectrophotometer. 1H NMR (300 MHz), 13C NMR
(75 MHz) spectra were recorded on a Joel JNM k-300
spectrometer using TMS as internal reference, the values
for chemical shifts (d) being given in ppm and coupling
constants (J) in Hertz (Hz). In the 13C NMR spectra, the
nature of the carbons (C, CH, CH2 or CH3) was deter-
mined using DEPT-135 and is given in parentheses.
Mass spectra were recorded on an Agilent 1100 LC/
MSD and elemental analysis on a Vario El Elementar
instrument. Acme silica gel G and silica gel (100–200
mesh) were used for analytical TLC and column chro-
matography, respectively. Nitro blue tetrazolium
(NBT), 1,1-diphenyl-2-picrylhydrazyl (DPPH), and vita-
min E acid succinate were purchased from Sigma–Al-
drich, and other chemicals and solvents were of
commercial grade and used without further purification.
A mixture of 3a (0.35 g) and pyridine hydrochloride
(3.5 g) was stirred in N2 atmosphere at 180–190 ꢁC
for 2 h. The cooled mixture was diluted with water
(100 mL), acidified with dil HCl, and extracted with
ethyl acetate (3· 50 mL). The combined ethyl acetate
solution was washed with water (30 mL), brine
(30 mL), and dried over sodium sulfate. The residue
obtained after evaporation of the solvent was chro-
matographed over silica gel column using mixtures
of petroleum ether and ethyl acetate (80:20) as eluent
to give 4 (0.2 g, 62%), mp 146–148 ꢁC (lit.34 mp 145–
146 ꢁC), IR (KBr): 3271, 2924, 1650, 1602, 1244, 1164,
1
1127, 1035 cmꢀ1; H NMR (DMSO-d6) d 2.65 (2H, t,
J = 6.5 Hz, H-3), 4.45 (2H, t, J = 6.5 Hz, H-2), 6.28
(1H, d, J = 2.1 Hz, H-8), 6.44 (1H, dd, J = 2.1,
8.6 Hz, H-6), 7.61 (1H, d, J = 8.6 Hz, H-5), 8.16
(1H, br s, Ar-OH); LCMS (negative scan): m/z 163
(MꢀH)ꢀ.

V. Siddaiah et al. / Bioorg. Med. Chem. 14 (2006) 2545–2551
2549
4.5. General procedure for the preparation of
E-homoisoflavonoids
(lit.35 mp 124–125 ꢁC), IR (neat): 2966, 1660, 1605,
1589, 1259, 1107, 1028, 829 cmꢀ1 1H NMR
;
(CDCl3+DMSO-d6) d 3.84 (3H, s, Ar-OCH3), 3.86
(3H, s, Ar-OCH3), 5.36 (2H, s, H-2), 6.40 (1H, d,
J = 2.1 Hz, H-8), 6.63 (1H, dd, J = 2.1, 8.9 Hz, H-6),
6.96 (2H, d, J = 8.8 Hz, H-30,50), 7.27 (2 H, d,
J = 8.8 Hz, H-20,60), 7.81 (1H, s, H-9), 7.96 (1H, d,
J = 8.9 Hz, H-5); LCMS (positive scan): m/z 297
(M+H)+.
A mixture of chroman-4-one (1.22 mmol), substituted
benzaldehyde (1.76 mmol), and piperidine (5 drops)
was heated at 70–80 ꢁC for 2 h. The cooled reaction mix-
ture was diluted with water (50 mL), acidified with dil
HCl, and extracted with ethyl acetate (3· 50 mL). The
combined EtOAc layer was washed with water
(30 mL), brine (30 mL), and dried over sodium sulfate.
The residue obtained after evaporation of the solvent
was chromatographed over silica gel column using mix-
tures of petroleum ether and ethyl acetate (80:20) as elu-
ent to give homoisoflavonoids (43–72% yield).
4.5.5.
7-Hydroxy-3-[(3,4,5-trimethoxyphenyl)methy-
lene]chroman-4-one (9E). Crystallized from chloro-
form–hexane as a colorless powder (280 mg, 67%), mp
198–200 ꢁC, IR (KBr): 3233, 2925, 1650, 1618, 1583,
1278,
1158,
1126,
1009 cmꢀ1
;
1H
NMR
4.5.1. 7-Hydroxy-3-[(4-methoxyphenyl)methylene]chro-
man-4-one (bonducellin, 5E). Crystallized from chloro-
form–methanol as a pale yellow powder (225 mg,
65%), mp 206–208 ꢁC (lit.10 mp 208 ꢁC), IR (KBr):
(CDCl3+DMSO-d6) d 3.80 (3H, s, Ar-OCH3), 3.86
(6H, s, 2 · Ar-OCH3), 5.37 (2H, s, H-2), 6.32 (1H, d,
J = 2.0 Hz, H-8), 6.53 (1H, dd, J = 2.0, 8.8 Hz, H-6),
6.60 (2H, s, H-20,60), 7.65 (1H, s, H-9), 7.76 (1H, d,
J = 8.8 Hz, H-5), 10.37 (1H, br s, Ar-OH); LCMS (neg-
ative scan): m/z 341 (MꢀH)ꢀ. Elemental analysis,
found: C, 66.62; H, 5.34. Calcd. for C19H18O6: C,
66.66; H, 5.30.
3256, 1656, 1603, 1289, 1260, 1240, 1157, 1028 cmꢀ1
;
1H NMR (CDCl3+DMSO-d6) d 3.85 (3H, s, Ar-
OCH3), 5.32 (2H, d, J = 1.8 Hz, H-2), 6.37 (1H, d,
J = 2.1 Hz, H-8), 6.57 (1H, dd, J = 2.1, 8.6 Hz, H-6),
6.96 (2H, d, J = 8.7 Hz, H-30,50), 7.26 (2H, d,
J = 8.7 Hz, H-20,60), 7.74 (1H, s, H-9), 7.85 (1H, d,
J = 8.6 Hz, H-5); 13C NMR (CDCl3+DMSO-d6) d
180.2 (C-4), 164.4 (C-7), 162.5 (C-8a), 160.0 (C-40),
135.5 (C-9), 131.4 (C-20,60), 129.2 (C-5), 128.7 (C-3),
126.6 (C-10), 114.4 (C-4a), 113.8 (C-30,50), 110.9 (C-6),
102.4 (C-8), 67.3 (C-2), 54.9 (–OCH3); LCMS (negative
scan): m/z 281 (MꢀH)ꢀ.
4.5.6. 7-Hydroxy-3-[(N,N-dimethylaminophenyl)methy-
lene]chroman-4-one (10E). Crystallized from chloro-
form–methanol as a colorless powder (190 mg, 53%),
mp 264–266 ꢁC, IR (KBr): 3435, 1650, 1599, 1298,
1243,
1199,
1169,
1106 cmꢀ1
;
1H
NMR
(CDCl3+DMSO-d6) d 3.05 (6H, s, –N(CH3)2), 5.38
(2H, s, H-2), 6.36 (1H, s, H-8), 6.55 (1H, d,
J = 8.6 Hz, H-6), 6.73 (2H, d, J = 8.4 Hz, H-30,50), 7.23
(2H, d, J = 8.4 Hz, H-20,60), 7.72 (1H, s, H-9), 7.83
(1H, d, J = 8.6 Hz, H-5), 9.95 (1H, br s, Ar-OH); LCMS
(positive scan): m/z 296 (M+H)+. Elemental analysis,
found: C, 73.19; H, 5.86. Calcd. for C18H17NO3: C,
73.20; H, 5.80.
4.5.2. 7-Hydroxy-3-[(2,4-dimethoxyphenyl)methylene]-
chroman-4-one (20-methoxybonducellin, 6E). Crystallized
from chloroform–methanol as a pale yellow amorphous
powder (245 mg, 64%), mp 210–212 ꢁC (lit.11 mp not
given), IR (KBr): 3174, 1628, 1267, 1163, 1033 cmꢀ1
;
1H NMR (CDCl3+DMSO-d6) d 3.85 (3 H, s, Ar-
OCH3), 3.87 (3H, s, Ar-OCH3), 5.20 (2H, s, H-2), 6.30
(1H, d, J = 2.1 Hz, H-8), 6.52 (1 H, dd, J = 2.1,
8.7 Hz, H-6), 6.57 (1H, d, J = 8.2 Hz, H-50), 6.58 (1H,
s, H-30), 7.02 (1H, d, J = 8.2 Hz, H-60), 7.75 (1H, d,
J = 8.7 Hz, H-5), 7.78 (1H, s, H-9), 10.53 (1H, br s,
Ar-OH); 13C NMR (CDCl3+DMSO-d6) d 179.6, 164.1,
162.2, 161.8, 158.9, 131.1, 130.8, 128.8, 128.4, 115.3,
114.1, 110.5, 104.4, 102.1, 97.8, 67.4, 55.1, 54.9; LCMS
(negative scan): m/z 311 (MꢀH)ꢀ.
4.5.7. 7-Hydroxy-3-[(4-hydroxyphenyl)methylene]chro-
man-4-one (11E). Crystallized from chloroform–metha-
nol as a colorless powder (200 mg, 61%), mp 220–
222 ꢁC, IR (KBr): 3279, 1651, 1623, 1604, 1291, 1261,
1
1232, 1161, 1112 cmꢀ1; H NMR (CDCl3+DMSO-d6)
d 5.26 (2H, s, H-2), 6.30 (1H, s, H-8), 6.49 (1H, d,
J = 8.4 Hz, H-6), 6.84 (2H, d, J = 8.2 Hz, H-30,50), 7.10
(2H, d, J = 8.2 Hz, H-20,60), 7.67 (1H, s, H-9), 7.79
(1H, d, J = 8.4 Hz, H-5), 9.32 (1H, s, Ar-OH), 9.81
(1H, s, Ar-OH); LCMS (negative scan): m/z 267
(MꢀH)ꢀ. Elemental analysis, found; C, 71.61; H, 4.55.
Calcd. for C16H12O4: C, 71.64; H, 4.51.
4.5.3. 7-Hydroxy-3-[(3,4-dihydroxyphenyl)methylene]-
chroman-4-one (sappanone A, 7E). Crystallized from
chloroform–methanol as
a
pale yellow powder
(200 mg, 58%), mp 220–222 ꢁC (lit.13 mp 220–221 ꢁC),
IR (KBr): 3401, 1651, 1615, 1585, 1264, 1215, 1162,
4.5.8.
7-Hydroxy-3-[(3,4,5-trihydroxyphenyl)methy-
lene]chroman-4-one (12E). Crystallized from chloro-
form–methanol as a colorless powder (160 mg, 43%),
mp 286–290 ꢁC, IR (KBr): 3436, 1650, 1604, 1285,
1
1109 cmꢀ1; H NMR (CDCl3+DMSO-d6) d 5.34 (2H,
s, H-2), 6.35 (1H, s, H-8), 6.55 (1H, d, J = 8.1 Hz, H-
50), 6.70–6.89 (1H, m, H-6), 6.80–6.89 (2H, m, H-
20,60), 7.66 (1H, s, H-9), 7.82 (1H, d, J = 7.8 Hz, H-5),
8.5–9.0 (2H, br s, 2 · Ar-OH), 9.8–10.2 (1H, br s, Ar-
OH); LCMS (negative scan): m/z 283 (MꢀH)ꢀ.
1
1235, 1200, 1102, 1041 cmꢀ1; H NMR (DMSO-d6) d
5.32 (2H, s, H-2), 6.30 (1H, d, J = 1.9 Hz, H-8), 6.36
(2H, s, H-20,60), 6.51 (1H, dd, J = 1.9, 8.1 Hz, H-6),
7.42 (1H, s, H-9), 7.70 (1H, d, J = 8.1 Hz, H-5), 8.78
(1H, br s, Ar-OH), 9.17 (2H, br s, 2 · Ar-OH), 10.59
(1H, br s, Ar-OH); LCMS (negative scan): m/z 299
(MꢀH)ꢀ. Elemental analysis, found; C, 63.97; H, 4.09.
Calcd. for C16H12O6: C, 64.00; H, 4.03.
4.5.4. 7-Methoxy-3-[(4-methoxyphenyl)methylene]chro-
man-4-one (8E). Crystallized from chloroform–hexane
as a colorless powder (245 mg, 68%), mp 130–132 ꢁC

2550
V. Siddaiah et al. / Bioorg. Med. Chem. 14 (2006) 2545–2551
4.5.9. 7-Hydroxy-3-[(3-carboxy-4-hydroxyphenyl)methy-
lene]chroman-4-one (13E). Crystallized from chloro-
form–methanol as a colorless powder (200 mg, 52%),
mp 236–238 ꢁC, IR (KBr): 3433, 3062, 1657, 1602,
The residue was chromatographed over silica gel column
to give the Z-isomers (32–54% yield).
4.8.1. 7-Hydroxy-3-[(4-methoxyphenyl)methylene]chro-
man-4-one (isobonducellin, 5Z). Crystallized from chlo-
roform–hexane as a pale yellow powder (120 mg,
42%), mp 152–154 ꢁC (lit.11 mp 156–158 ꢁC), IR
(KBr): 3130, 1639, 1602, 1572, 1248, 1181, 1159, 1124,
1292, 1253, 1200, 1169, 1106, 1038 cmꢀ1 1H NMR
;
(DMSO-d6) d 5.36 (2H, s, H-2), 6.31 (1H, s, H-8), 6.52
(1H, d, J = 8.3 Hz, H-6), 6.74 (1H, d, J = 7.8 Hz, H-
50), 7.30 (1H, s, H-60), 7.56 (1H, s, H-20), 7.71 (1H, d,
J = 8.3 Hz, H-5), 7.72 (1H, s, H-9), 8.46 (1H, br s, Ar-
OH), 10.30 (1H, br s, Ar-OH); LCMS (negative scan):
m/z 311 (MꢀH)ꢀ. Elemental analysis, found; C, 65.37;
H, 3.89. Calcd. for C17H12O6: C, 65.39; H, 3.87.
1
1028 cmꢀ1; H NMR (CDCl3+DMSO-d6) d 3.81 (3 H,
s, Ar-OCH3), 4.93 (2 H, s, H-2), 6.35 (1H, d,
J = 2.0 Hz, H-8), 6.52 (1H, dd, J = 2.0, 8.8 Hz, H-6),
6.82 (1H, s, H-9), 6.85 (2H, d, J = 8.6 Hz, H-30,50),
7.78 (1H, d, J = 8.8 Hz, H-5), 7.86 (2H, d, J = 8.6 Hz,
H-20,60); 13C NMR (CDCl3+DMSO-d6) d 181.6 (C-4),
164.4 (C-7), 162.6 (C-8a), 160.2 (C-40), 138.6 (C-9),
132.5 (C-20,60), 129.2 (C-5), 127.3 (C-3), 126.7 (C-10),
115.6 (C-4a), 112.9 (C-30,50), 110.7 (C-6), 102.2 (C-8),
75.1 (C-2), 54.9 (–OCH3); LCMS (negative scan): m/z
281 (MꢀH)ꢀ.
4.5.10. 3-[(4-Hydroxyphenyl)methylene]-6-methoxy-chro-
man-4-one (14E). Crystallized from chloroform–hexane
as a colorless powder (250 mg, 72%), mp 184–185 ꢁC;
1
IR (neat): 3410, 1648, 1610, 1280, 1161, 1022 cmꢀ1; H
NMR (CDCl3): d 3.84 (3H, s, Ar-OCH3), 5.33 (2H, d,
J = 1.5 Hz, H-2), 5.36 (1H, s, Ar-OH), 6.91 (2H, d,
J = 8.8 Hz, H-30,50), 6.92 (1H, d, J = 8.7 Hz, H-8), 7.10
(1H, dd, J = 2.3, 8.7 Hz, H-7), 7.24 (2H, d, J = 8.8 Hz,
H-20,60), 7.44 (1H, d, J = 2.3 Hz, H-5), 7.83 (1H, s, H-
9); LCMS (positive scan): m/z 283 (M+H)+. Elemental
analysis, found; C, 72.29; H, 5.04. Calcd. for
C17H14O4: C, 72.33; H, 5.00.
4.8.2. 7-Hydroxy-3-[(2,4-dimethoxyphenyl)methylene]-
chroman-4-one (6Z). Crystallized from chloroform as a
colorless powder (100 mg, 32%), mp 172–174 ꢁC, IR
(KBr): 3156, 1653, 1613, 1268, 1160, 1112, 1037 cmꢀ1
;
1H NMR (CDCl3+DMSO-d6) d 3.78 (6H, s, 2 · Ar-
OCH3), 4.98 (2H, s, H-2), 6.31 (1H, d, J = 2.0 Hz, H-
8), 6.46–6.54 (3H, m, H-30,50,6), 7.07 (1H, s, H-9), 7.67
(1H, d, J = 8.8 Hz, H-5), 7.86 (2H, d, J = 8.3 Hz, H-
20), 10.61 (1H, br s, Ar-OH); LCMS (negative scan):
m/z 311 (MꢀH)ꢀ.
4.6. Preparation of HClO4–SiO2
Perchloric acid (1.25 g, 12.5 mmol, as a 70% aqueous
solution) was added to the suspension of silica gel
(23.75 g, 200–400 mesh) in diethyl ether. The mixture
was concentrated and the residue was heated at 100 ꢁC
for 72 h under vacuum to afford HClO4–SiO2 reagent,
as a free-flowing powder.
4.8.3. 7-Methoxy-3-[(4-methoxyphenyl)methylene]chro-
man-4-one (8Z). Colorless oil (160 mg, 54%), IR (neat):
2927, 1663, 1606, 1246, 1179, 1158, 1104, 1029,
1
968 cmꢀ1; H NMR (CDCl3+DMSO-d6) d 3.80 (6H, s,
4.7. 7-Hydroxy-3-[(4-chlorophenyl)methylene]chroman-4-
one (15E)
2 · Ar-OCH3), 4.94 (2H, s, H-2), 6.39 (1H, d,
J = 2.1 Hz, H-8), 6.59 (1H, dd, J = 2.1, 8.8 Hz, H-6),
6.78 (1H, s, H-9), 6.87 (2H, d, J = 8.8 Hz, H-30,50),
7.87 (2H, d, J = 8.8 Hz, H-20,60), 7.93 (1H, d,
J = 8.8 Hz, H-5); LCMS (positive scan): m/z 297
(M+H)+.
A
mixture of 7-hydroxychroman-4-one (328 mg,
2 mmol), 4-chlorobenzaldehyde (393 mg, 2.8 mmol),
and a catalytic amount of HClO4–SiO2 ( ꢁ 50 mg)
was heated under stirring at 90–100 ꢁC for 30 min.
After completion of the reaction, the residue was
chromatographed over silica gel column using mix-
tures of hexane and ethyl acetate as eluents to give
15 (407 mg, 71%), which was crystallized from chloro-
form–methanol as a colorless powder, mp 196–198 ꢁC,
IR (KBr): 3224, 1653, 1619, 1591, 1284, 1221, 1157,
4.9. Superoxide free radical scavenging activity
The superoxide free radical scavenging activity of
homoisolfavonoids (5–15) was determined by the
NBT method.24,25 The assay mixture contained EDTA
(6.0 lM), NaCN (3 lg), riboflavin (2 lM), NBT
(50 lM), various concentrations of the test substances
in ethanol, and a phosphate buffer (58 mM, pH 7.8) in
a final volume of 3 mL. The tubes were shaken well,
and the absorbance was measured at 560 nm and
mean control OD is 0.8247. The test tubes were uni-
formly illuminated with an incandescent lamp for
15 min, after which the absorbance was measured
again at 560 nm. The percent inhibition of superoxide
radical generation was measured by comparing the
mean absorbance values of control tubes and those
of the test substances. IC50 values were obtained from
the plot drawn of concentration in lg versus percent
inhibition and were converted into lM. All the tests
were run in triplicate and data were analyzed using
one-way ANOVA.
1115, 1017 cmꢀ1
;
1H NMR (CDCl3+DMSO-d6) d
5.28 (2H, s, H-2), 6.32 (1H, d, J = 1.7 Hz, H-8), 6.54
(1H, dd, J = 1.7, 8.8 Hz, H-6), 7.33 (2H, d,
J = 8.3 Hz, H-30,50), 7.45 (2H, d, J = 8.3 Hz, H-20,60),
7.67 (1H, s, H-9), 7.77 (1H, d, J = 8.8 Hz, H-5);
LCMS (negative scan): m/z 285, 287 (MꢀH)ꢀ. Ele-
mental analysis, found: C, 67.01; H, 3.89. Calcd. for
C16H11ClO3: C, 67.03; H, 3.87.
4.8. General procedure for the preparation of
Z-homoisoflavonoids
A solution of E-homoisoflavonoids (1 mmol) in benzene
(100 mL) was irradiated using a medium pressure
mercury lamp for 4 h and the solvent was evaporated.

V. Siddaiah et al. / Bioorg. Med. Chem. 14 (2006) 2545–2551
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Acknowledgments
We sincerely thank Sri G. Ganga Raju, Chairman, and
Mr. G. Rama Raju, Director of Laila Impex, for
encouragement. We are also thankful to Prof. A. Srik-
rishna, Indian Institute of Science, for helpful discus-
sions and CSIR, New Delhi, for the award of SRF to
one of the authors (V.S.).
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