Phytochemistry 55 (2000) 457±459
www.elsevier.com/locate/phytochem
Two iso¯avanones from the stem bark of Erythrina sacleuxii
Abiy Yenesew a, Jacob O. Midiwo a,*, Matthias Heydenreich b,
Dirk Schanzenbach b, Martin G. Peter b
b
a
Department of Chemistry, University of Nairobi, PO Box 30197, Nairobi, Kenya
Institut fuÈr Organische Chemie und Strukturanalytik, UniversitaÈt Potsdam, Am Neuen Palais 10, D-14469 Potsdam, Germany
Received 2 December 1999; received in revised form 24 May 2000
Abstract
From the stem bark of Erythrina sacleuxii two new iso¯avanones, (R)-5,7-dihydroxy-20 ,40 ,50 -trimethoxyiso¯avanone (trivial name,
(R)-2,3-dihydro-7-demethylrobustigenin) and (R)-5-hydroxy-20 ,40 ,50 -trimethoxy-200 ,200 -dimethylpyrano[500 ,600 :6,7]iso¯avanone (trivial
name, (R)-saclenone) were isolated. In addition the known compounds shinpterocarpin, 2,3-dehydrokievitone, abyssinone V,
abyssinone V-40 -methyl ether, erythrinasinate and 40 -O-methylsigmoidin B were isolated. The structures were determined on the
basis of spectroscopic evidence. # 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Erythrina sacleuxii; Leguminosae; Stem bark; Iso¯avanones; (R)-2,3-Dihydro-7-demethylrobustigenin; (R)-Saclenone
1. Introduction
The genus Erythrina is known for its use in traditional
medicinal practice, especially for the treatment of
microbial infections (Mitscher et al., 1987). The presence of ¯avanones, iso¯avones, iso¯avanones and
pterocarpans in this genus have been reported (Dewick,
1994; Barron and Ibrahim, 1996). Some of these ¯avonoids have shown antimicrobial activities against
human pathogens rationalizing the use of Erythrina
species in folk medicine (Mitscher et al., 1987).
In our studies of Erythrina species of Kenya, we have
reported two new ¯avanones from the stem bark of
Erythrina burttii (Yenesew et al., 1998a), and four new
iso¯avones from the stem bark of Erythrina sacleuxii
(Yenesew et al., 1998b). Further investigation of the
stem bark of Erythrina sacleuxii resulted in the isolation
of two new iso¯avanones along with six known compounds. The isolation and characterization of these
compounds is presented here.
2. Results and discussion
In the ®rst compound (1), the presence of an iso¯avanone skeleton was deduced from the UV (lmax 291,
* Corresponding author. Tel.: +254-2-440436; fax: +254-2-446138.
E-mail address: jmidwo@africaonline.co.ke (J.O. Midiwo).
334 nm), 1H ( 4.56, dd, J=11.1 and 12.0 Hz, for H-2ax;
4.41, dd, J=5.6, 11.1 Hz for H-2eq; 4.32, dd, J=5.6,
12.0 Hz for H-3) and 13C ( 71.7 for C-2; 48.3 for C-3
and 198.8 for C-4) NMR spectra. The 1H NMR spectrum further revealed the presence of a chelated
hydroxyl ( 12.35), a free hydroxyl ( 9.68) and three
methoxyl ( 3.84, 3.80 and 3.72) substituents.
In the EIMS the appearance of a fragment ion at m/z
194, resulting from retro±Diels±Alder cleavage of the Cring, is in agreement with the placement of the three
methoxyl groups in B-ring and hence the two hydroxyl
groups should be in A-ring. In the 1H NMR spectrum,
the presence of two meta-coupled protons at 5.95 and
5.97 (J=2.0 Hz) would locate the hydroxyl groups at C5 and C-7, which is expected from biogenetic consideration. Furthermore, in the 1H NMR spectrum, two
singlets at 6.75 and 6.86 are assigned to H-30 and H-60 ,
respectively. The methoxyl groups should then be located at C-20 , C-40 and C-50 of B-ring. In the 13C NMR
spectrum, the chemical shift values (see experimental)
for the carbon atoms of B-ring are in agreement with
such substitution pattern. The identity of this compound
was con®rmed through HMQC and HMBC experiments.
The CD curve of this compound showed a positive
Cotton eect at 320 nm which is consistent with R con®guration at C-3 (Yahara et al., 1989; Gale et al.,
1997). The presence of a trans-diaxial relationship
between H-2ax and H-3 (J=12.0 Hz), in the 1H NMR
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458
A. Yenesew et al. / Phytochemistry 55 (2000) 457±459
spectrum, is in agreement with such con®guration. On
these bases this new compound was characterised as 5,7dihydroxy-20 ,40 ,50 -trimethoxyiso¯avanone, and the trivial name (R)-2,3-dihydro-7-demethylrobustigenin is
suggested for this compound by relating it to the corresponding iso¯avone, 7-demethylrobustigenin, earlier
isolated from this plant (Yenesew et al., 1998b).
The second new compound (2) is also an iso¯avanone
derivative, and this was deduced from the UV, 1H and
13
C NMR spectra (see Experimental). The 1H and 13C
NMR spectra further revealed the presence of a chelated hydroxyl (5-OH), three methoxyl and a 2,2-dimethylpyran substituents. The MS (the fragment ion at m/z
194), 1H and 13C NMR spectra (see experimental) of
compound 2 showed that the B-ring substitution pattern
is identical to that of compound 1.
In the A-ring, the chelated hydroxyl group being at C5, the 1H and 13C NMR data would place the 2,2dimethylpyran ring, either between C-6/C-7 (2) or
between C-7/C-8. This was resolved from the HMBC
spectrum (Fig. 1), which showed correlation of H-8 (
5.93) with C-7 ( 161.7) and C-8a ( 162.7), OH-5 ( 12.54)
with C-6 ( 103.1), and also H-400 ( 6.62) with C-6 (
103.1). This allows the placement of the 2,2-dimethylpyran ring between C-6/C-7 (2). A positive Cotton eect
at 334 nm, in the CD curve, and the presence of transdiaxial relationship between H-2ax and H-3 (J=11.9 Hz)
are again consistent with R con®guration at C-3. Thus,
compound 2 is characterized as 5-hydroxy-20 ,40 ,50 -trimethoxy-200 ,200 -dimethylpyrano[500 ,600 :6,7]iso¯avanone for
which the trivial name (R)-saclenone is suggested. It
appears that this compound is derived from compound
1 through prenylation at C-6 and subsequent cyclization
involving the hydroxyl group at C-7 forming the 2,2dimethylpyran ring.
Fig. 1. Signi®cant correlations observed in the HMBC spectrum of
saclenone (2).
The remaining compounds isolated from this plant
were identi®ed as shinpterocarpin (3) (Kitagawa et al.,
1994), 2,3-dehydrokievitone (4) (Hashidoko et al.,
1986), abyssinone V, abyssinone V-40 -methylether, erythrinasinate, and 40 -O-methylsigmoidin B (Yenesew et
al., 1998a). This appears to be the ®rst report on the
occurrence of compounds 3 and 4 in the genus Erythrina, while the other compounds have been reported
from this genus earlier (Yenesew et al., 1998a).
3. Experimental
3.1. General
Analytical TLC: Merck pre-coated silica gel 60 F254
plates. CC on silica gel 60 (70±230 mesh). CD were
recorded on JASCO J-710 Spectropolarimeter. EIMS:
A. Yenesew et al. / Phytochemistry 55 (2000) 457±459
direct inlet, 70 eV on a ssq 710, Fa. Finnigan MAT spectrometer. 1H NMR (300 MHz) and 13C NMR (75 MHz)
on ARX 300 (Bruker) spectrometer using TMS as
internal standard. HMQC and HMBC spectra were
acquired using the standard Bruker software.
3.2. Plant material
The stem bark of Erythrina sacleuxii was collected
from South Coast, Kenya, in February 1996. The plant
was identi®ed at the University Herbarium, Botany
Department, University of Nairobi, where a voucher
specimen is deposited.
3.3. Extraction and isolation
Dried and ground stem bark (1.4 kg) of Erythrina
sacleuxii was extracted with CH2Cl2 by cold percolation. Removal of the solvent aorded a brown gummy
extract (60 g). The extract was subjected to CC on silica
gel (600 g) eluting with hexane containing increasing
percentages of EtOAc. The fraction eluted with 3%
EtOAc in hexane (800 ml) aorded erythrinasinate
(68 mg); the fraction eluted with 5% EtOAc (800 ml)
contains a mixture of two compounds which was separated by PTLC on silica gel plates (solvent, hexane-acetone; 4:1) to give 2 (27 mg) and abyssinone V-40 methylether (143 mg); while elution with 7% EtOAc
(800 ml) gave a mixture of two compounds which were
separated by PTLC (solvent, hexane-acetone; 3:1) to
give 1 (33 mg), and 3 (43 mg); while elution with 9%
EtOAc (800 ml) gave three compounds, which were
separated by PTLC (solvent, hexane-acetone; 3:1) to
give 40 -O-methylsigmoidin B (31 mg), abyssinone V
(26 mg) and 4 (25 mg).
3.4. (R)-2,3-Dihydro-7-demethylrobustigenin (1)
Amorphous powder. UV lmax (MeOH) nm: 291, 334.
[a]D ÿ28 (MeOH, c 0.1). CD (MeOH, c 0.001): []320
+2366, []296 ÿ263, []265+3852, []240 ÿ2666. 1H
NMR (acetone-d6, 300 MHz): 4.56 (1H, dd, J=11.1,
12.0 Hz, H-2ax), 4.41 (1H, dd, J=5.6, 11.1 Hz, H-2eq),
4.32 (1H, dd, J=5.6, 12.0 Hz, H-3), 5.97 (1H, d,
J=2.0 Hz, H-6), 5.95 (1H, d, J=2.0 Hz, H-8), 6.75 (1H,
s, H-30 ), 6.86 (1H, s, H-60 ), 12.35 (1H, s, 5-OH), 9.68
(1H, s, 7-OH), 3.84 (3H, s, OMe), 3.80 (3H, s, OMe),
3.72 (3H, s, OMe). 13C NMR (acetone-d6, 75 MHz):
71.7 (C-2), 48.3 (C-3), 198.8 (C-4), 104.2 (C-4a), 166.3
(C-5), 97.6 (C-6), 167.7 (C-7), 96.3 (C-8), 165.2 (C-8a),
116.0 (C-10 ), 153.8 (C-20 ), 100.4 (C-30 ), 151.6 (C-40 ),
145.0 (C-50 ), 117.3 (C-60 ), 56.8 (OMe), 57.0 (OMe), 57.5
(OMe). EIMS m/z (rel. int.): 346 (15, [M]+), 194 (100)
179 (34), 151 (20).
459
3.5. (R)-Saclenone (2)
Amorphous powder. UV lmax (MeOH) nm: 285, 334.
[a]D ÿ22 (MeOH, c 0.1). CD (MeOH, c 0.06): []334
+5127, []307 ÿ8571, []284 ÿ14370, []243 ÿ13084. 1H
NMR (CDCl3, 300 MHz): 4.41 (1H, dd, J=5.6,
10.9 Hz, H-2eq), 4.54 (1H, dd, J=11.9, 10.9 Hz, H-2ax),
4.27 (1H, dd, J=5.6, 11.9 Hz, H-3), 5.93 (1H, s, H-8),
6.57 (1H, s, H-30 ), 6.67 (1H, s, H-60 ), 5.50 (1H, d,
J=10.1 Hz, H-300 ), 6.62 (1H, d, J=10.1 Hz, H-400 ), 12.54
(1H, s, 5-OH), 1.45 (6H, s, Me2-200 ), 3.89 (3H, s, OMe),
3.81 (3H, s, OMe), 3.78 (3H, s, OMe). 13C NMR
(CDCl3, 75 MHz): 70.3 (C-2), 47.2 (C-3), 197.2 (C-4),
103.0 (C-4a), 158.8 (C-5), 103.1 (C-6), 161.7 (C-7), 95.9
(C-8), 162.7 (C-8a), 113.9 (C-10 ), 151.9 (C-20 ), 98.1 (C30 ), 149.6 (C-40 ), 143.3 (C-50 ), 114.3 (C-60 ), 78.2 (C-200 ),
126.1 (C-300 ), 115.4 (C-400 ), 28.4 (Me2-200 ), 56.7 (OMe),
56.4 (OMe), 56.1 (OMe). EIMS m/z (rel. int.): 412 [M]+
(9), 397 [MÿMe]+ (16), 203 (16), 194 (68), 181 (100),
151 (35).
Acknowledgements
A.Y. is grateful to the German Academic Exchange
Service (DAAD) for a research visit to the University of
Potsdam. S.G. Mathenge is thanked for identi®cation of
the plant material.
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Abiy Yenesew
Jacob Midiwo