Thunalbene, a stilbene derivative from the orchid Thunia alba

Article abstract of DOI:10.1016/S0031-9422(98)00433-6

Thunalbene, a new stilbene derivative, was isolated from the orchid Thunia alba which also afforded six known stilbenoids: batatasin-III, lusianthridin, 3,7-dihydroxy-2,4-dimethoxyphenanthrene, 3,7-dihydroxy-2,4,8- trimethoxyphenanthrene, cirrhopetalanthr

Full text of DOI:10.1016/S0031-9422(98)00433-6

Phytochemistry Vol. 49, No. 8, pp. 2375±2378, 1998
# 1998 Elsevier Science Ltd. All rights reserved
Printed in Great Britain
0031-9422/98/$ - see front matter
PII: S0031-9422(98)00433-6
THUNALBENE, A STILBENE DERIVATIVE FROM THE
ORCHID THUNIA ALBA
P. L. MAJUMDER,* M. ROYCHOWDHURY and S. CHAKRABORTY
Department of Chemistry, University College of Science, 92 Acharya Prafulla Chandra Road,
Calcutta 700009, India
(Received in revised form 20 April 1998)
Key Word IndexÐThunia alba; Orchidaceae; thunalbene; stilbene derivative.
AbstractÐThunalbene, a new stilbene derivative, was isolated from the orchid Thunia alba which also
a€orded six known stilbenoids: batatasin-III, lusianthridin, 3,7-dihydroxy-2,4-dimethoxyphenanthrene, 3,7-
dihydroxy-2,4,8-trimethoxyphenanthrene, cirrhopetalanthrin and ¯avanthrin. The structure of thunalbene,
the ®rst stilbene derivative isolated so far from an Orchidaceae plant, was established as 3,3'-dihydroxy-5-
methoxystilbene from spectral and chemical evidence. # 1998 Elsevier Science Ltd. All rights reserved
INTRODUCTION
RESULTS AND DISCUSSION
Thunalbene (1a), C15H14O3 (M+ at m/z 242),
We reported earlier on the isolation of a fairly large
number of compounds of diverse structural types
from a series of Indian Orchidaceae plants. These
compounds encompass a wide variety of stilbenoids,
i.e. bibenzyls [1], phenanthrenes [2] and their
dimers [3±5], 9,10-dihydrophenanthrenes [6] and
their dimers [7], phenanthropyrans and pyrones [8, 9]
and their 9,10-dihydro derivatives [10], besides a
few other polyphenolics [11, 12], simple aromatic
compounds [13], several triterpenoids [14] and
steroids of biogenetic importance [15]. Our con-
tinued search for new phytochemicals from the
same source has now resulted in the isolation of a
new stilbene derivative, designated thunalbene,
from the orchid Thunia alba which also a€orded six
known stilbenoids, i.e. batatasin-III (3,3'-dihydroxy-
5-methoxybibenzyl) [16, 17], lusianthridin (4,7-di-
hydroxy-2-methoxy-9,10-dihydrophenanthrene) [18],
3,7-dihydroxy-2,4-dimethoxyphenan-threne [19], 3,7-
dihydroxy-2,4,8-trimethoxyphenanthrene [2], cirrho-
petalanthrin (2,2',7,7'-tetrahydroxy-4,4'-dimethoxy-
1,1'-biphenanthryl) [3] and ¯avanthrin (2,2',7,7'-
tetrahydroxy-4,4'-dimethoxy-9,9',10,10'-tetrahydro-
1,1'-biphenanthryl) [7]. While the known com-
pounds were identi®ed by direct comparison with
their respective authentic samples, thunalbene was
shown to have the structure 1a from the following
spectral and chemical evidence.
EtOH
showed the UV absorptions [l
214 and 303 nm
max
(log e 4.30 and 4.72)] expected of a trans-stilbene
derivative. The phenolic nature of the compound
was indicated by its characteristic colour reactions
with FeCl3 (violet) and phosphomolybdic acid re-
agent (intense blue), alkali-induced bathochromic
shifts of its UV maxima and by its IR band at
3360 cm 1. The presence of two phenolic hydroxyl
groups in 1a was con®rmed by the formation of a
diacetyl derivative (1b), C19H18O5 (M+ at m/z 326),
with Ac2O and pyridine. The IR absorption band
1
at 980 cm of 1a indicated the presence of a trans-
double bond in the compound.
The 1H NMR spectrum of 1a showed signals
for an aromatic methoxyl function [d 3.73 (3H,s)],
two phenolic hydroxyl groups [d 5.28 (2H,s);
*Author to whom correspondence should be addressed.
2375

2376
P. L. MAJUMDER et al.
disappeared on deuterium exchange], seven aro- in 1a and 1b. Thus, the dC values of C-3, C-3', C-5
matic protons at d 6.27±7.14, and a two-proton and C-5' of 1a and 1c were virtually identical, while
singlet at d 6.96 which is typical of the vinylic those of C-1 and C-1' of 1a showed up®eld shifts of
protons of a trans-stilbene derivative. The relative ca. 4±5 ppm compared to the corresponding carbon
positions of the methoxyl and hydroxyl groups in atoms of 1c due to the change in the state of hy-
the two phenyl rings of 1a were ascertained from bridizations of C-a and C-a' from sp3 in 1c to sp2
the chemical shifts and the splitting patterns of the in 1a. The observed up®eld shifts of C-2, C-2', C-6
aromatic protons of the compound and its diacetyl and C-6' by ca. 2±2.5 ppm and the down®eld shifts
derivative 1b. Thus, the chemical shifts and the of C-4 and C-4' by ca. 1.5±1.8 ppm of 1a compared
splitting patterns of the aromatic protons of 1a to the corresponding carbon atoms of 1c may also
resonating at d 7.14 (1H, appt t; J1=8.1 Hz and be attributed to the di€erent states of hybridizations
J2=7.8 Hz), 6.97 (1H, br d; J = 7.5 Hz), 6.96 (1H, of C-a and C-a' of the two compounds, which, as
br signal; obscured in the signal of the ole®nic pro- expected appeared at ca. dC 129 in 1a as against ca.
tons), 6.67 (1H, dd; J1=7.8 Hz and J2=1.8 Hz) and dC 38 in 1c. The dC values of 1b are also compatible
6.55, 6.51 and 6.27 (each 1H, br signal) are strik- with the placement of the two hydroxyl groups at
ingly similar to those of H-5', H-6', H-2', H-4', H-6, C-3 and C-3' and the methoxyl group at C-5 in 1a
H-2 and H-4, respectively, of batatasin-III (1c) [16], and exhibited expected down®eld shifts of C-2, C-4,
except that the signals corresponding to H-2, H-2', C-6, C-2', C-4' and C-6'. The same trend in the
H-6 and H-6' of 1a showed characteristic down®eld changes of C-1, C-1', C-2, C-2', C-4, C-4', C-6 and
shifts compared to the corresponding protons of 1c C-6' resonances of 1a compared to the correspond-
due to the diamagnetic anisotropic e€ect of the ole- ing carbon atoms of 1c are also discernible in the
®nic double bond between Ca±Ca' in 1a. This was dC values of the above carbon atoms of 1b, when
also corroborated by the similarities of the chemical compared with the corresponding carbon reson-
shifts and the splitting patterns of the aromatic pro- ances of 1d.
ton signals of thunalbene diacetate (1b) and batata-
The structure of 1a was ®nally con®rmed by the
sin-III diacetate (1d) exhibiting the same di€erences conversion of its diacetyl derivative 1b to batatasin-
in regard to their H-2, H-2', H-6 and H-6' reson- III diacetate (1d) by hydrogenation of 1b over
ances. The above 1H NMR spectral data of 1a and PtO2.
1b, thus, not only indicated identical substitution
It is interesting to note that although several stil-
patterns of the hydroxyl and methoxyl functions in bene derivatives were reported from a number of
both 1a and 1c, but also implied that the former plant species [21], i.e. Gnetum ula [22], Alnus
was the corresponding stilbene derivative of the virides [23], Virola elongata [24], Cassia
latter.
roxburghii [25], Diphysia robinioides [26], Phoenix
The structure of thunalbene (1a) was further sup- dactylifera [27] and Combretum ca€rum [28], all
ported by the 13C NMR spectral data of the com- belonging to botanical families other than
pound and its diacetyl derivative 1b (Table 1). The Orchidaceae, the isolation of thunalbene (1a) from
degree of protonation of the carbon atoms of each the orchid Thunia alba constitutes the ®rst report of
compound was con®rmed by APT experiments and the occurrence of a stilbene derivative in an orchid.
the assignments of the carbon chemical shifts of 1a This is despite the fact that the large number of
and 1b were made by comparison with the dC orchids so far chemically investigated were shown
values of structurally similar compounds like bata- to elaborate preponderantly a wide range of stilbe-
tasin-III (1c) [20] and its diacetate 1d [17] taking noids including a fairly large number of bibenzyl
into consideration the alteration in additive par- derivatives. In the light of the above observations,
ameters caused by the change of the state of hybrid- the isolation of thunalbene is of considerable bio-
ization of C-a and C-a' from sp3 in 1c and 1d to sp2 genetic and chemotaxonomical importance.
Table 1. 13C NMR spectral data of compounds 1a, 1b, 1c and 1d
C
1a
1b
1c
1d
C
1a
1b
1c
1d
1
2
3
4
5
6
1'
2'
3'
139.5a
106.8
159.4b
101.6
161.7
104.1
140.1a
113.7c
158.4b
138.5e
111.9
151.8f
107.0
160.5
109.9
139.1e
119.3g
151.0f
144.3i
108.8
159.2j
99.9
161.9
106.3
145.0i
116.2
158.2j
143.7l
113.7
4'
5'
6'
115.4c
130.2
118.1
129.3d
129.4d
55.3
121.0g
128.9h
124.2
129.5h
128.8h
55.4
169.3
169.2
21.0
113.6
130.0
120.4
38.4k
38.1k
55.3
119.1n
129.1
125.8
37.3o
37.0o
55.2
151.5m
105.2
160.2
a
a'
OMe
OAc
111.8
143.0l
121.4n
150.7m
169.3
21.0
Spectra were run in d6-acetone and chemical shifts were measured with d (TMS) = d (d6-acetone) + 29.6 ppm.
Spectra were run in CDCl3 and chemical shifts were measured with d (TMS) = d (CDCl3) + 76.9 ppm.
a±oValues are interchangeable within each column.

Thunalbene, a stilbene derivative from the orchid Thunia alba
2377
EXPERIMENTAL
nmax cm 1: 1230 and 1780 (OAc), 1635, 1390, 930,
915, 890 and 810 (phenyl nucleus) and 990 (trans-
double bond); 1H NMR: d 7.24±7.30 (2H, m; H-5'
and H-6'), 7.14 (1H, br signal; H-2'), 6.90±6.99 (3H,
m; H-4', H-a and H-a'), 6.80, 6.77 and 6.48 (each
1H, br signal; H-2, H-6 and H-4), 3.78 (3H, s;
ArOMe), 2.22 and 2.23 (each 3H, s; 2 ÂOAc); MS
m/z (rel. int.): 326 [M+], (23), 284 (24), 242 (82),
226 (2), 225 (2), 198 (2), 181 (9), 152 (6) and 115
(6).
M.p.'s: Uncorr.; CC: silica gel (100±200 mesh);
MPLC: silica gel (230±400 mesh); TLC: silica gel
G; UV: 95% aldehyde-free EtOH; IR: KBr discs;
1H and 13C NMR: 300 and 75 MHz, respectively,
in CDCl3 and d6-acetone using TMS as an int. stan-
dard. Chemical shifts are expressed in d (ppm). MS:
direct inlet system, 70 eV. All analyt. samples were
routinely dried over P2O5 for 24 h in vacuo and
were tested for purity by TLC and MS. Na2SO4
was used for drying organic solvents and the petrol
used had b.p. 60±808. Plant materials were collected
from Darjeeling, India in September, 1993. A vou-
cher specimen is deposited in the Herbarium of the
Department of Botany, University of Calcutta
(CUH).
The later frs in the above MPLC a€orded a mix-
ture of 3,7-dihydroxy-2,4-dimethoxyphenanthrene
and
3,7-dihydroxy-2,4,8-trimethoxyphenanthrene.
Further MPLC of this mixture using petrol±EtOAc
(1:1) as the solvent ®nally gave pure 3,7-dihydroxy-
2,4-dimethoxyphenanthrene (0.025 g) as a semisolid
mass in the early frs and pure 3,7-dihydroxy-2,4,8-
trimethoxyphenanthrene (0.015 g) in the later frs.
Further elution of the main column with petrol±
EtOAc (1:1) eluate gave pure cirrhopetalanthrin
(0.015 g), crystallized from petrol±EtOAc mixture,
m.p. 2968, in the early frs and pure ¯avanthrin
(0.03 g), crystallized from petrol±EtOAc mixture,
m.p. 2858, in the later frs.
Isolation of thunalbene (1a), batatasin-III (1c),
lusianthridin, cirrhopetalanthrin, ¯avanthrin, 3,7-dihy-
droxy-2,4-dimethoxyphenanthrene and 3,7-dihyhy-
droxy-2,4,8-trimethoxyphenanthrene from Thunia
alba
Air-dried powdered whole plants (2 kg) of T. alba
were soaked in MeOH (7 l) for 3 weeks. The MeOH
extract was then drained o€, concd under red. pres.
to ca. 100 ml, diluted with H2O (500 ml) and the
liberated solids exhaustively extracted with Et2O.
The Et2O extract was fractionated into acidic and
non-acidic frs with 2 M NaOH. The aq. alkaline
soln was acidi®ed in the cold with conc. HCl and
the liberated solids extracted with Et2O, washed
with H2O, dried and the solvent removed. The resi-
due was chromatographed. The early frs of the pet-
rol±EtOAc (10:1) eluate a€orded a mixture of
lusianthridin and 1c which on rechromatography
using petrol±EtOAc (20:1) as the eluent gave in the
early frs pure lusianthridin (0.05 g), crystallized
from petrol±EtOAc, m.p. 1628, and 1c (0.03 g) as a
semisolid mass in the later frs. Elution of the main
column with petrol±EtOAc (5:1) a€orded a mixture
of 3,7-dihydroxy-2,4-dimethoxyphenanthrene, 3,7-
dihydroxy-2,4,8-trimethoxyphenanthrene and 1a,
which was subjected to MPLC using petrol±
EtOAc (1:1) as the solvent. The early frs a€orded
pure 1a (0.04 g) as a semisolid mass (found: C,
Catalytic hydrogenation of 1b
A soln of 1b (0.02 g) in EtOH (20 ml) containing
PtO2 (0.005 g) was stirred under H2 atmosphere for
4 h. The catalyst was ®ltered o€ and the ®ltrate on
evaporation gave a semisolid residue (0.018 g)
which was identical in all respects to batatasin-III
diacetate (1d).
AcknowledgementsÐThe work was supported by
the CSIR and UGC, New Delhi.
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