Synthesis, structural revision, and antioxidant activities of antimutagenic homoisoflavonoids from Hoffmanosseggia intricata

Article abstract of DOI:10.1016/j.bmcl.2006.12.008

Intricatinol and intricatin, the two homoisoflavonoids isolated from Hoffmanosseggia intricata, and two analogs have been synthesized from pyrogallol in three steps. The spectral data of synthetic intricatinol are in good agreement with those of natural metabolite, but the spectral data of intricatin are not corroborative with those of the natural product. The structure of intricatin has been thus revised to 8-methoxybonducellin, a compound isolated from Caesalpinia pulcherrima. The antioxidant activity of all the four homoisoflavonoids was determined by superoxide (NBT) and DPPH free radical scavenging methods. The synthetic analog 7,8-dihydroxy-3-[(3,4-dihydroxyphenyl)methylene]chroman-4-one displayed excellent activity in both methods.

Full text of DOI:10.1016/j.bmcl.2006.12.008

Bioorganic & Medicinal Chemistry Letters 17 (2007) 1288–1290
Synthesis, structural revision, and antioxidant activities
of antimutagenic homoisoflavonoids from
Hoffmanosseggia intricata
Vidavalur Siddaiah,a Muchchintala Maheswara,a Chunduri Venkata Rao,a
Somepalli Venkateswarlub and Gottumukkala V. Subbarajub,*
aDepartment of Chemistry, Sri Venkateswara University, Tirupati 517 502, India
bLaila Research Centre, Unit 1, Phase III, Jawahar Autonagar, Vijayawada 520 007, India
Received 15 August 2006; revised 19 November 2006; accepted 4 December 2006
Available online 15 December 2006
Abstract—Intricatinol and intricatin, the two homoisoflavonoids isolated from Hoffmanosseggia intricata, and two analogs have
been synthesized from pyrogallol in three steps. The spectral data of synthetic intricatinol are in good agreement with those of nat-
ural metabolite, but the spectral data of intricatin are not corroborative with those of the natural product. The structure of intricatin
has been thus revised to 8-methoxybonducellin, a compound isolated from Caesalpinia pulcherrima. The antioxidant activity of all
the four homoisoflavonoids was determined by superoxide (NBT) and DPPH free radical scavenging methods. The synthetic analog
7,8-dihydroxy-3-[(3,4-dihydroxyphenyl)methylene]chroman-4-one displayed excellent activity in both methods.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Homoisoflavonoids (3-benzylidene-4-chromanones) are
related to flavonoids and occur as natural products and
found to exhibit antifungal,1,2 hypocholesterolemic,3
and antiviral activities.4 7,8-Dihydroxy-40-methoxyho-
moisoflavonoid (intricatinol, 1) and 8-hydroxy-40,7-dim-
ethoxyhomoisoflavonoid (intricatin, 2, Fig. 1) were
isolated from Hoffmanosseggia intricata.5 These are the
first examples of antimutagenic homoisoflavonoids
which displayed the inhibition of the mutagenicity of
2AN toward Salmonella typhimurium (T98). Due to
our interest on homoisoflavonoids,6,7 and the impor-
tance of dietary antioxidants in chemoprevention of
degenerative diseases, such as cancer, Alzheimer’s, Par-
kinson’s, and cardiovascular diseases, we have synthe-
sized these natural products along with two new
structural analogs. In this paper, we report the details
of synthesis, structural revision, and antioxidative activ-
ities of these homoisoflavonoids.
2
OR
1
OCH
O
O
R O
3
7,8-Dihydroxychroman-4-one (8), a key intermediate,
was obtained by the reaction of pyrogallol (6) with acry-
lonitrile in the presence of sodium methoxide followed by
cyclization with aqueous sulfuric acid. Base-catalyzed
condensation of 8 with 4-methoxybenzaldehyde afforded
intricatinol (1) in 60% yield (Scheme 1).8 The spectral
data of synthetic homoisoflavonoid 1 (Table 1) were in
agreement with those of natural homoisoflavonoid 1.5
Selective methylation of homoisoflavonoid 1 with
dimethyl sulfate using sodium bicarbonate as a base
afforded intricatin (2) in 48% yield and dimethoxy deriv-
ative in 15% yield. The other analogs 4 and 5 were
synthesized using appropriately substituted benzaldehy-
des as shown in Scheme 1 and were characterized by their
1. Intricatinol, R1=R2=H
2. Intricatin, R1=CH3, R2=H
3. 8-Methoxybonducellin, R1=H, R2=CH3
Figure 1. Chemical structures of natural homoisoflavonoids.
Keywords: Homoisoflavonoids; Synthesis; Revised structure; Antiox-
idant activity.
*
Corresponding author. Tel.: +91 866 2541303; fax: +91 866
2546216; e-mail: subbarajugottumukkala@hotmail.com
0960-894X/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2006.12.008

V. Siddaiah et al. / Bioorg. Med. Chem. Lett. 17 (2007) 1288–1290
1289
OH
OH
OH
HO
OH
HO
O
N
HO
O
O
(i)
(ii)
(iii)
7
6
8
OH
OH
4'
1
OCH
H CO
3
O
O
3
R
HO
O
4
2
(iv)
1'
6
2
R
9
O
2
1 - 5
1
4
5
R1=OCH3, R2=H
R1=OH, R2=H
R1=R2=OH
Scheme 1. Reagents and conditions: (i) acrylonitrile, NaOMe, 70–80 ꢁC, 7 h, 35%; (ii) aq H2SO4, 95–100 ꢁC, 3 h, 54%; (iii) substituted benzaldehyde,
piperidine, 70–80 ꢁC, 2 h, 53–60%; (iv) DMS, NaHCO3, acetone, rt, 36 h, 48%.
Table 1. 1H NMR data of synthetic and natural homoisoflavonoids
Assignment
Synthetic intricatinol (1)a
Natural intricatinol (1)a
Synthetic intricatin (2)b
Natural intricatin (2)b
8-Methoxy
bonducellin (3)b
H-2
H-5
5.36 (s)
5.39 (d, 2)
7.27 (d, 8.5)
6.57 (d, 8.5)
7.65 (br s)
7.06 (d, 8.5)
7.42 (d, 8.5)
3.83 (s)
5.41 (d, 2)
7.46 (d, 8.8)
6.79 (d, 8.8)
7.71 (s)
5.48 (d, 2)
7.61 (d, 8.8)
6.65 (d, 8.8)
7.72 (br t)
7.06 (d, 8.8)
7.43 (d, 8.8)
3.82 (s)
5.49 (d, 1.8)
7.63 (d, 8.8)
6.66 (d, 8.8)
7.73 (t, 1.8)
7.08 (d, 9)
7.45 (d, 9)
3.84 (s)
7.27 (d, 8.3)
6.57 (d, 8.3)
7.64 (s)
H-6
H-9
H-30,50
H-20,60
Ar-OMe
7.03 (d, 8.3)
7.40 (d, 8.3)
3.80 (s)
7.04 (d, 8.6)
7.40 (d, 8.6)
3.85 (s)
3.91 (s)
3.88 (s)
3.89 (s)
a NMR in DMSO-d6.
b NMR in acetone-d6.
spectral data.9 In all cases, a single geometrical isomer (E)
was obtained. The stereochemistry at the double bond
was confirmed by the characteristic 1H NMR data.7,10
superoxide radical scavenging (NBT method) and
the DPPH free radical scavenging methods, were
chosen to determine the antioxidant potential of
the compounds. Both the methods measure the effi-
cacy of a hydrogen atom transfer from a phenol
to a radical.
Structural revision. The spectral data of synthetic
homoisoflavonoid 2 were not in agreement with those
reported for natural homoisoflavonoid 2.5 In the 1H
NMR data of natural 2 (Table 1), the aromatic proton
adjacent to a methoxyl group was reported to resonate
at d 6.65 (d). However, in the 1H NMR spectrum of syn-
thetic homoisoflavonoid 2, the proton adjacent to a
methoxyl group appeared at d 6.79 (d). Thus, it appears
that the natural intricatin (2) may be an isomeric com-
pound, 8-methoxybonducellin (3). In fact, careful litera-
ture survey revealed that the spectral data of natural
intricatin corroborated with those reported for 8-meth-
oxybonducellin (3), isolated from Caesalpinia pulcherr-
ima11 and Caesalpinia sappan.12 From the foregoing,
structure of intricatin, isolated from Hoffmanosseggia
intricata, has been revised into 8-methoxybonducellin
(3), a known natural homoisoflavonoid.
Superoxide radical scavenging activity. Superoxide radi-
cals were generated in vitro by non-enzymatic system
and determined spectrophotometrically (560 nm) by ni-
tro blue tetrazolium (NBT) photoreduction method of
McCord and Fridovich.13,14 The antioxidant activity
of homoisoflavonoids (1–5) was expressed as 50% inhib-
itory concentration (IC50 in lM) and the values are
incorporated in Table 2. From the superoxide scaveng-
ing activity data, 7,8-dihydroxy-3-[(3,4-dihydroxyphe-
nyl)methylene]-chroman-4-one (5, IC50: 8.5 lM) having
two catechol moieties has shown highest activity and
was several fold more potent activity in comparison with
the commercially available antioxidants like BHA (IC50:
966 lM) and vitamin C (IC50: 852 lM). The superior
scavenging ability of 5 lends further support to the fact
that the catechol system enhances the antioxidant
activity.15
Antioxidant activity. In the present study, two
commonly used antioxidant evaluation methods,

1290
V. Siddaiah et al. / Bioorg. Med. Chem. Lett. 17 (2007) 1288–1290
Table 2. Antioxidant activity of homoisoflavonoids
7. Siddaiah, V.; Venkata Rao, C.; Venkateswarlu, S.; Krish-
naraju, A. V.; Subbaraju, G. V. Bioorg. Med. Chem. 2006,
14, 2545.
a
Compound
(IC50 (lM)
NBT superoxide
scavenging activity
DPPH free radical
scavenging activity
8. General procedure for the preparation of E-homoisofl-
avonoids: A mixture of chroman-4-one 8 (1.22 mmol),
substituted benzaldehyde (1.76 mmol), and piperidine (five
drops) was heated at 70–80 ꢁC for 2 h. The cooled reaction
mixture 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 chromato-
graphed over silica gel column using mixtures of petro-
leum ether and ethyl acetate as eluents to give
homoisoflavonoids.
Intricatinol (1)
2
30.1
>200
26.5
8.5
10.1
>200
8.3
4
5
BHA
Vitamin C
4.5
34
966
852
25.1
BHA, butylated hydroxyanisole.
The lower the IC50 values, the higher the antioxidant activity.
a This indicates the significant difference at the level of P < 0:05, n ¼ 3.
9. Physical and spectral characteristics of compounds, 7,8-
dihydroxy-3-[(4-methoxyphenyl)methylene]chroman-4-
one (1): Light brown powder, mp 192–194 ꢁC (lit.5 mp
196–198 ꢁC). IR (KBr): 3394, 1658, 1602, 1254, 1173,
1150, 1071, 1030, 963 cmꢀ1. For 1H NMR see, Table 1.
13C NMR (DMSO-d6): d 180.2, 160.3, 152.4, 150.4, 135.3,
132.7, 132.2, 129.2, 126.6, 118.4, 115.2, 114.3, 110.5, 67.6,
55.3. MS (ESI, negative ion mode): m/z 297 (M–H)ꢀ.
8-Hydroxy-7-methoxy-3-[(4-methoxyphenyl)methylene]-
chroman-4-one (2). Light yellow powder, mp 96–98 ꢁC
(lit.5 mp 157–159 ꢁC). IR (KBr): 3305, 2932, 1658, 1603,
1288, 1255, 1176, 1102, 1027, 933 cmꢀ1. For 1H NMR see
Table 1. 13C NMR (DMSO-d6): d 181.5, 161.8, 153.9,
150.3, 136.7, 134.4, 133.1, 130.9, 128.0, 119.2, 117.9, 115.2,
106.9, 68.9, 56.7, 55.9. MS (ESI, positive ion mode): m=z
335 (M+Na)+.
DPPH radical scavenging activity. DPPH (1,1-diphenyl-
2-picrylhydrazyl) radical scavenging activity of homoi-
soflavonoids was determined by the method described
by Lamaisen et al.16,17 based on the reduction of metha-
nolic solution of the colored DPPH radical. From the
IC50 values (Table 2), again 7,8-dihydroxy-3-[(3,4-
dihydroxyphenyl)methylene]-chroman-4-one (5, IC50:
4.5 lM) was found powerful scavenger of DPPH free
radicals.
In summary, we have accomplished the synthesis of
intricatinol (1) and the proposed structure for intricatin
(2), two plant antimutagenic homoisoflavonoids isolated
from Hoffmanosseggia intricata in three steps, with two
synthetic analogs. The structure of natural intricatin
has been revised into 8-methoxybonducellin, a homoi-
soflavonoid isolated earlier from Caesalpinia pulcherr-
ima and Caesalpinia sappan. These homoisoflavonoids
were evaluated for their antioxidative potential by two
commonly used methods, the superoxide (NBT) and
DPPH free radical scavenging methods. Homoisoflavo-
noid 5 was found to be a potent antioxidant.
7,8-Dihydroxy-3-[(4-hydroxyphenyl)methylene]-chroman-
4-one (4). Light brown powder, mp 246–248 ꢁC. IR (KBr):
1
3371, 1662, 1618, 1244, 1173, 1068, 996 cmꢀ1. H NMR
(DMSO-d6): d 10.08 (2H, s, 2· Ar-OH), 8.76 (1H, s, Ar-
OH), 7.60 (1H, s, H-9), 7.31 (2H, d, J = 8.3 Hz, H-20,60),07.26
(1H, d, J = 8.6 Hz, H-5), 6.87 (2H, d, J = 8.3 Hz, H-3 ,50),
6.56 (1H, d, J = 8.6 Hz, H-6), 5.35 (2H, s, H-2). 13C NMR
(DMSO-d6): d 180.2, 158.9, 152.2, 150.4, 135.6, 132.7, 132.4,
128.3, 125.1, 118.3, 115.7, 115.2, 110.4, 67.7. MS (ESI,
negative ion mode): m/z 283 (M–H)ꢀ. Anal. Calcd for
C16H12O5: C, 67.60; H, 4.26. Found: C, 67.55; H, 4.32.
7,8-Dihydroxy-3-[(3,4-dihydroxyphenyl)methylene]-chro-
man-4-one (5). Light yellow powder, mp 220 ꢁC (dec). IR
(KBr): 3458, 3402, 3200, 1653, 1600, 1272, 1194, 1075,
980 cmꢀ1. 1H NMR (DMSO-d6): d 9.31–9.81 (br m, 4 H, Ar-
OH), 7.52 (1H, s, 0H-9), 7.25 (1H, d, J = 8.8 Hz, H-5), 6.83–
6.85 (3H, m, H-2 ,60), 6.77 (1H, dd, J = 8.3, 1.5 Hz, H-50),
6.55 (1H, d, J = 8.8 Hz, H-6), 5.34 (1H, d, J = 1.0 Hz, H-2).
13C NMR (DMSO-d6): d 180.2, 152.2, 150.3, 147.4,145.4,
135.9, 132.7, 128.1, 125.5, 123.0, 118.3, 117.5, 115.9, 115.2,
110.4, 67.7. MS (ESI, negative ion mode): m=z 299 (M–H)ꢀ.
Anal. Calcd for C16H12O6: C, 64.00; H, 4.03. Found: C,
63.97; H, 4.06.
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.
Srikrishna, Indian Institute of Science, for helpful dis-
cussions and CSIR, New Delhi, for the award of SRF
to one of the authors (V.S.).
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