Isolation, synthesis, and bioactivity of homoisoflavonoids from Caesalpinia pulcherrima

Article abstract of DOI:10.1248/cpb.57.1139

One new homoisoflavonoid, (3E)-2,3-dihydro-6,7-dimethoxy-3[(3-hydroxy-4- methoxyphenyl)methylene]-4H-1-benzopyran-4-one and four naturally new analogues, (3E)-3-(1,3-benzodioxol-5-ylmethylene)-2,3-dihydro-7-hydroxy-4H-1-benzopyran-4- one, (3E)-3-(1,3-benz

Full text of DOI:10.1248/cpb.57.1139

October 2009
Notes
Chem. Pharm. Bull. 57(10) 1139—1141 (2009)
1139
Isolation, Synthesis, and Bioactivity of Homoisoflavonoids from
Caesalpinia pulcherrima1)
Biswanath DAS, ,a Ponnaboina THIRUPATHI,a Bommena RAVIKANTH,a Rathod ARAVIND KUMAR,a
*
b
Akella Venkata Subramanya SARMA,b and Shaik Jilani BASHA
a Organic Chemistry Division-I, Indian Institute of Chemical Technology; and b NMR Division, Indian Institute of Chemical
Technology; Hyderabad–500 007, India.
Received May 29, 2009; accepted July 29, 2009; published online July 30, 2009
One new homoisoflavonoid, (3E)-2,3-dihydro-6,7-dimethoxy-3[(3-hydroxy-4-methoxyphenyl)methylene]-4H-
1-benzopyran-4-one and four naturally new analogues, (3E)-3-(1,3-benzodioxol-5-ylmethylene)-2,3-dihydro-
7-hydroxy-4H-1-benzopyran-4-one, (3E)-3-(1,3-benzodioxol-5-ylmethylene)-2,3-dihydro-7-methoxy-4H-1-ben-
zopyran-4-one, (3E)-2,3-dihydro-7-hydroxy-3-[(3-hydroxy-4-methoxyphenyl)methylene]-4H-1-benzopyran-4-one
and (3E)-2,3-dihydro-3-[(3,4-dimethoxyphenyl)methylene]-7-methoxy-4H-1-benzopyran-4-one, along with four
known homoisoflavonoids, bonducellin, sappanone A, 2ꢀ-methoxybonducellin and 7-O-methylbonducellin were
isolated from aerial parts of Caesalpinia pulcherrima. The structures of the new compounds were elucidated by
interpretation of their 1D and 2D NMR spectra. Syntheses of the naturally new compounds and the known com-
pounds have also been accomplished. The antibacterial and antifungal activities of the isolated homo-
isoflavonoids were studied.
Key words Caesalpinia pulcherrima; homoisoflavonoid; synthesis; antibacterial activity; antifungal activity
Caesalpinia pulcherrima SWARTZ is a Leguminous, peren- formula was assigned as C19H18O6 from its mass spectrum
nial large shrub or small tree found throughout India. It has ([MꢁH]ꢁ at m/z 343), elemental analysis and 13C-NMR
·
several medicinal properties, used in the treatment of ulcer, spectrum. A comparison of its molecular formula with that
asthma, fever, tumors, and skin diseases.2) Previous studies of the known constituent, 4 (molecular formula: C17H12O5)
on this plant led to the isolation of several diterpenoids,3—6) indicated that 3 contained a methoxy group instead of a hy-
peltogynoids,7) flavonoids,7,8) chalcones,8) and homoiso- droxy group present in 4 along with an additional methoxy
flavonoids.7—13)
group. The appearance of two fragment peaks at m/z 180 and
In continuation of our search for new plant metabo- 162 in the mass spectrum due to retro Diels-Alder cleavage
lites14—17) we carried out the chemical investigation on the suggested the presence of two methoxy group in ring A while
chloroform–methanol (1 : 1) extract of the fresh collection of a hydroxy and a methoxy groups in ring B. The 1H- and 13C-
the aerial parts of C. pulcherrima. The crude extract was sub- NMR spectral data of the compounds also revealed that it
jected to column chromatography over silica gel using
hexane/ethyl acetate mixtures to obtain nine homoiso-
flavonoids 1—9 (Fig. 1). Compounds 1 [(3E)-3-(1,3-benzo-
dioxol-5-ylmethylene)-2,3-dihydro-7-hydroxy-4H-1-ben-
zopyran-4-one], 2 [(3E)-3-(1,3-benzodioxol-5-ylmethylene)-
2,3-dihydro-7-methoxy-4H-1-benzopyran-4-one], 4 [(3E)-
2,3-dihydro-7-hydroxy-3-[(3-hydroxy-4-methoxyphenyl)-
methylene]-4H-1-benzopyran-4-one], and 5 [(3E)-2,3-dihy-
dro-3-[(3,4-dimethoxyphenyl)methylene]-7-methoxy-4H-1-
benzopyran-4-one] are reported here for the first time from a
natural source but they were synthesized before.8,18—24) On
the other hand, compounds 6 [(3E)-2,3-dihydro-7-hydroxy-3-
[(4-methoxyphenyl)methylene]-4H-1-benzopyran-4-one]
(bonducellin),3—6)
7
[(3E)-2,3-dihydro-3[(3,4-dihydroxy-
phenyl)methylene]-7-hydroxy-4H-1-benzopyran-4-one]
(sappanone A),8,9)
[(3E)-2,3-dihydro-7-methoxy-3-[(4-
8
methoxy-phenyl)methylene]-4H-1-benzopyran-4-one] (7-O-
methyl bonducellin)12) and 9 [(3E)-2,3-dihydro-3-[(2,4-
dimethoxyphenyl)methylene]-7-hydroxy-4H-1-benzopyran-
1-one] (2ꢀ-methoxybonducellin)11) were isolated previously
from the title plant. The structures of these naturally new and
known homoisoflavonoids were settled by comparison of
their spectral (IR, 1H- and 13C-NMR and MS) data with those
reported earlier.
Compound 3 is a new constituent. It was isolated as an
yellow amorphous powder, mp 168—170 °C. Its molecular
Fig. 1. Structures of Homoisoflavonoids 1—9
© 2009 Pharmaceutical Society of Japan
∗ To whom correspondence should be addressed. e-mail: biswanathdas@yahoo.com

1140
Vol. 57, No. 10
contained three methoxy groups (d 3.95, 3H, s and 3.92, 6H, 1H-NMR spectrum). The 2D-NMR (double quantum filtered
1
s in the H-NMR spectrum and d 56.3, 56.2, and 56.0 in the (DQF)-COSY, NOESY and HMBC) experiments confirmed
13C-NMR spectrum) and a hydroxy group (d 5.72, br s in the the positions of the methoxys at C-6, C-7 and C-4ꢀ while that
of the hydroxy group at C-3ꢀ (Fig. 2). The E-geometry of the
double bond at C-3 and C-9 in 3 was clearly indicated by the
characteristic of the methylene proton (d 5.35, d, Jꢂ1.5 Hz)
at C-2 and the vinylic proton (d 7.75, 1H, br s) at C-9.8) Thus
the structure of 3 was confirmed as (3E)-2,3-dihydro-6,7-
dimethoxy-3[(3-hydroxy-4-methoxyphenyl)methylene]-4H-
1-benzopyran-4-one.
We extended our work to syntheses of isolated homo-
isoflavonoids 1, 2, and 4—9 according to the literatures.25—27)
Thus the piperidine catalyzed condensation of 7-hydroxy-4-
chromanone 10 with 4-methoxy, 3,4-methylenedioxy, 3-hy-
droxy-4-methoxy, 2,4-dimethoxy, and 3,4-dihydroxyben-
zaldyhydes afforded 6, 1, 4, 9 and 7 in 68, 63, 58, 60, and
59% yields, respectively (Chart 1). The condensation of 7-
methoxy-4-chromanone (11) (obtained by methylation of 7-
hydroxy-4-chromanone (10) using iodomethane and K2CO3)
with 3,4-methylenedioxy, 3,4-dimethoxy, and 4-methoxy
benzaldehydes using piperidine gave compounds 2, 5 and 8
in 65, 69 and 61% yields, respectively (Chart 1).
Fig. 2. Significant DQF-COSY ( ) and NOESY ( ) Correlations and
HMBC (ꢀ) for 3
Reagent and conditions (i) CH3I, K2CO3, acetone, 2 h, reflux, (91%); (ii) substituted
benzaldehyde, piperidine, 2 h (58—69%).
Chart 1
Table 1. Antibacterial Activity of the Homoisoflavonoids 1—9a—c)
Gram-positive organisms
Gram-negative organisms
Chromobacterium
Compound
Bacillus subtilis
Bacillus sphaericus
Staphylococcus aureus Pseudomonas aeruginosa
Klebsiella aerogenes
violaceum
A
B
A
B
A
B
A
B
A
B
A
B
1
2
3
4
5
6
7
8
ꢁ
ꢁ
ꢁ
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ꢁ
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ꢁ
ꢁ
9
ꢁ
ꢁ
ꢁ
(Positive controls)
Penicillin G
Streptomycin
ꢁꢁ
ꢁꢁ
ꢁꢁ
ꢁꢁꢁꢁ
ꢁꢁꢁꢁ
ꢁꢁꢁꢁ
a) Results after 24 h. b) Inhibitory zone measured in mm, (ꢃ) no activity; (ꢁ) inhibitory zone 5—10 cm; (ꢁꢁ) inhibitory zone 11—15 cm; (ꢁꢁꢁ) inhibitory zone 16—
20 cm; (ꢁꢁꢁꢁ) inhibitory zone 21—25 cm. c) Compounds were tested with a concentration of 30 mg/ml (A), or 100 mg/ml (B).
Table 2. Antifungal Activity of the Homoisoflavonoids 1—9a—c)
Microorganisms
Compound
Aspergillus niger
Candida albicans
Rhizopus oryzae
B
C
B
C
B
C
1
2
3
4
5
6
7
8
9
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ꢁ
ꢁ
(Positive controls)
Clotrimazole
ꢁꢁꢁꢁ
ꢁꢁꢁꢁ
ꢁꢁꢁꢁ
a) Results after 48 h. b) Inhibitory zone measured in mm, (ꢃ) no activity, (ꢁ) inhibitory zone 5—10 cm; (ꢁꢁ) inhibitory zone 11—15 cm; (ꢁꢁꢁ) inhibitory zone 16—
20 cm; (ꢁꢁꢁꢁ) inhibitory zone 21—25 cm. c) Compounds were tested with a concentration of 100 mg/ml (B), or 150 mg/ml (C).

October 2009
1141
which was subjected to column chromatography to yield the pure 7-
methoxy-4-chromanone (11, 91%) as a colorless amorphous powder, mp
51—53 °C.
Syntheses of Homoisoflavonoids To a mixture of substituted 4-chro-
manone (10 or 11) (2 mmol) and appropriate substituted benzaldehyde
In all the cases single geometrical isomer (E) was ob-
tained. The stereochemistry at the double bond was con-
1
firmed by the characteristic H-NMR spectral value of H-9
which appeared around d 7.5.18) The spectral (NMR and MS)
data of 1, 2, and 4—9 were identical with those of the natural (2.2 mmol) piperidine (6 drops) was added and the total mass was heated at
70—80 °C for 2 h. The mixture was cooled and diluted with water (100 ml),
products.
acidified with dil. HCl and extracted with EtOAc (3ꢄ50 ml). The combined
EtOAc layer was washed with water (50 ml) and dried over Na2SO4. The
residue obtained after evaporation of the solvent was chromatographed over
Activity of Homoisoflavonoids The homoisoflavonoids
possess antibacterial and antifungal activities. The antibacter-
ial activity of the homoisoflavonoids, 1—9 was evaluated
(Table 1) following the reported.4) Agar cup bioassy method.
All the compounds showed moderate activity against the
Gram-positive organisms, Bacillus subtilis, Bacillus sphaeri-
cus and Staphylococcus aureus. However, they were inactive
against the Gram-negative organism, Pseudomonas aerugi-
nosa, and weakly active against Klebsiella aerogenes and
Chromobacterium violaceum.
silica gel using mixtures of n-hexane and EtOAc as eluent to afford pure ho-
moisoflavonoid (1, 2, 4—9 in 58—69%). The physical (Rf and mp) and
1
spectral (IR, H- and 13C-NMR and MS) properties of the compounds were
identical to those of the natural products.
Studies on Antibacterial and Antifungal Activities The methods have
exactly been followed from our earlier work.8)
Acknowledgements The authors thank CSIR and UGC, New Delhi, for
financial assistance.
The antifungal activity of all the compounds 1—9 (Table
2) was also moderate against the organisms, Apergillus niger
and Candida albicans but they were inactive against Rhyzo-
pus oryzae.
References and Notes
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Experimental
General Experimental Procedures Melting points were measured in a
Buchi-510 instrument and are uncorrected. Spectra were recorded with the
following instruments: IR: Perkin-Elmer spectrophotometer and 1H- and
13C-NMR; Gemini 200 spectrometer using CDCl3 and DMSO-d6 with TMS
as an internal standards. HSQC, DQF-COSY, HMBC, and phase sensitive
NOESY (with 150 ms mixing time) experiments were carried out using the
standard pulse sequences. EI-MS were recorded on VG-micromass 7070H
(70 eV) and ESI-MS on Thermo Finnigan LCQ ion trap mass spectrometer.
Column chromatography was performed on silica gel (BDH 100—200
mesh) and TLC with silica gel GF 254.
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Plant Material The aerial parts of C. pulcherrima were collected from
Osmania University campus in May, 2007 and identified botanically. A
voucher specimen (No CP-AP-1) was preserved in our laboratory and an-
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EtOAc in hexane afforded compound 5 (13 mg). A mixture of three com-
1193—1195 (2006).
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hexane. These were separated by preparative TLC using 10% EtOAc in
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(10 mg). The column was next eluted with 35% EtOAc in hexane to produce
the compound 9 (7 mg). Further elution of the column with 40% EtOAc in
hexane afforded 4 (12 mg) and 7 (5 mg).
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methoxyphenyl)methylene]-4H-1-benzopyran-4-one Yellow amorphous
powder, mp 168—170 °C; IR (KBr) nmax: 3382, 2928, 1610, 1507, 1265,
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1129, 756 cmꢃ1; H-NMR (CDCl3) d: 7.75 (1H, br s, H-9), 7.42 (1H, s, H-
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6ꢀ), 115.8 (C-2ꢀ), 114.2 (C-4a), 110.5 (C-5ꢀ), 107.4 (C-5), 100.0 (C-8), 68.1
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