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Chem. Pharm. Bull. 52(2) 238—243 (2004)
Vol. 52, No. 2
Preparation on Oligostilbenes of Isorhapontigenin by Oxidative Coupling
Reaction
Chun-Suo YAO, Li-Xin ZHOU, and Mao LIN*
Institute of Material Medica, Chinese Academy of Medical Sciences and Peking Union Medical College; Beijing, 100050,
China. Received September 22, 2003; accepted November 18, 2003
Four new compounds 1—4 were obtained from an oxidative coupling reaction of (E)-isorhapontigenin using
FeCl3 as oxidant. Their structures and stereochemistry were determined on the basis of spectroscopic evidence
[UV, IR, MS, 1H-, 13C-NMR, NOE and 2D NMR], and their possible formation mechanisms were also discussed,
respectively.
Key words oxidative coupling reaction; isorhapontigenin; oligostilbene; ferric chloride
An oxidative coupling reaction of (E)-isorhapontigenin (8) NOE experiment (Fig. 2) was carried out. The NOEs be-
using FeCl3 as an oxidant afforded ten oligostilbenes. In pre- tween H-7a and H-2a, H-6a, H-10a, H-14a; H-8a and H-10a,
vious paper, we determined the structures of three major H-14a indicated trans orientation for H-7a and H-8a. There-
products, shegansu B, bisisorhapontigenin A and B.1) Further fore, the stereochemistry of 1 was shown in structure 1.
investigation resulted in the structural identification of four
Compound 2 is a light yellowish amorphous powder. The
minor products: bisisorhapontigenin C (1), bisisorhaponti- HR EI-MS m/z 771.2445 [MϩH]ϩ, in combination with its
genin D (3), triisorhapontigenin A (2) and tetraisorhaponti- 1H- and 13C-NMR spectra revealed the molecular formula of
genin A (4) (Fig. 1). They are all new oligostilbenes of C45H38O12 (771.2442 calcd for C45H38O12), which indicated
isorhapontigenin. The structures of the remaining three com- that 2 could be an isorhapontigenin trimer. Its UV spectrum
pounds have not been identified due to scarcity. This paper was similar to that of 1, suggesting the presence of strong
describes the structure and stereochemistry identification of conjugated system. The IR spectrum of 2 indicated the pres-
the four new minor products on the basis of spectra analysis, ence of hydroxyl, aromatic group and trans olefinic bond.
1
and discussed their possible formation mechanisms.
The H-NMR spectrum of 2 indicated the presence of three
methoxyls, four aliphatic methines due to two dihydrobenzo-
furan moiety and two trans olefinic protons, as well as 16
Results and Discussion
The natural stilbene (E)-isorhapontigenin (8) from Gne- aromatic protons, which were attributed to three sets of ABX
tum montanum was treated with FeCl3 in acetone at room system for ring A1, ring B1 and ring C1, one set of AB2 sys-
temperature for 36 h to afford ten products. The structures of tem for ring A2, and two sets of meta-coupled protons for
four minor products: 1—4 were established as follows:
ring B2 and ring C2. The 13C-NMR spectrum of 2 showed 35
Compound 1 was obtained as light yellowish amorphous signals representing 45 carbons (including 20 quaternary car-
powder. The molecular ion peak at m/z 514 in EI-MS, com- bons, 22 tertiary carbons and 3 methoxyl carbons). The
bined with its elementary analysis, 1H- and 13C-NMR spectra chemical shifts (95—162 ppm) of C-9b, C-9c, C-10b, C-10c,
indicated that the molecular formula of C30H26O8, suggesting C-11b, C-11c in 2 were similar to those of C-9b, C-10b, C-
that 1 was an isorhapontigenin dimer. The UV spectrum of 1 11b in compound 5, indicating that the coupling route of 2
displayed absorption bands at lmax 284, 334 nm, suggesting was similar to that of 5. Therefore, The skeleton of 2 was
the presence of strong conjugated system in the molecule. similar to that of miyabenol C (6),2) a resveratrol trimer. The
The IR spectrum of 1 exhibited the existence of hydroxyl, connectivities for each isorhapontigenin were futher con-
1
aromatic groups and trans olefinic carbons. The H-NMR firmed by HMBC cross-peaks between H-8a and C-9b; H-7a
spectrum of 1 showed the presence of two methoxyls, two and C-11b; H-7b and C-10c, C-11c; H-8b and C-9b, C-10b,
aliphatic protons due to a dihydrobenzofuran moiety, two C-10c, C-11c (Fig. 3).
olefinic protons, and eleven aromatic protons, including two
The stereochemistry of 2 was determined on the basis of
meta-coupled protons for ring B2, two ABX systems for ring NOESY experiment (Fig. 3). The interactions between H-7a
A1 and ring B1, and an AB2 system for ring A2. Its 13C-NMR and H-2a, H-6a, H-10(14)a; H-8a and H-2a, H-6a, H-10(14)a
spectrum exhibited 24 signals representing 30 carbons, in- demonstrated trans orientation of H-7a and H-8a. The NOEs
cluding 13 quaternary carbons, 15 tertiary carbons and two between H-7b and H-2b, H-14b indicated cis orientation of
1
methoxyl carbons. Comparing the H- and 13C-NMR spectra H-7b and ring B2. The cross-peaks between H-8b and H-2b,
of 1 with those of bisisorhapontigenin A (5) showed that the H-6b, H-14b revealed cis orientation of H-8b and ring B1.
chemical shifts of 7a, 8a protons (d ca. 4.5 ppm and ca. These evidences supported a trans orientation of H-7b and
5.5 ppm) in 1H-NMR and 10b, 11b quaternary carbons (d ca. H-8b. Accordingly, the stereochemistry of 2 was clarified as
110 ppm and ca. 162 ppm) in 13C-NMR were similar,1) sug- shown in structure 2 (Fig.1).
gesting that the structure of 1 was similar to that of 5, except
Compound 3 was obtained as light yellowish crystals. The
for the relative positions of ring A1 and A2 which were inter- molecular ion peak at m/z 514 (Mϩ) in EI-MS, combined
changed. Thus, 1 was determined as an isorhapontigenin with the elementary analysis gave the molecular formula
dimer polymerized by head to head (Fig. 1).
of C30H26O8, which suggested that compound 3 was an
In order to clarify the stereochemistry of H-7a and H-8a, isorhapontigenin dimer. The UV spectrum of 3 revealed the
To whom correspondence should be addressed. e-mail: Linmao@imm.ac.cn
© 2004 Pharmaceutical Society of Japan