A FURANOLABDANE DITERPENE ALCOHOL FROM ... - DSpace

Journal of Chemistry, Vol. 43 (3), P. 375 - 378, 2005
A Furanolabdane Diterpene Alcohol from
Alpinia tonkinensis Gagnep.
Received 24 th -June-2004
Phan Minh Giang 1 , Hideaki Otsuka 2 , Phan Tong Son 1
1 Faculty of Chemistry, College of Natural Science, Vietnam National University
2 Graduate School of Biomedical Sciences, Hiroshima University, Japan
summary
A furanolabdane diterpene alcohol was isolated from the rhizomes of the endemic
Vietnamese medicinal plant Alpinia tonkinensis Gagnep. (Zingiberaceae). Its absolute
stereostructure was established to be (12S)-15,16-epoxy-8(17),13(16),14-labdatrien-12-ol from
spectroscopic data and by applying the modified Mosher’s method. It is first report on the natural
occurrence and absolute stereochemistry of the diterpene which was previously known as a
synthetic intermediate.
Keywords: Alpinia tonkinensis; Zingiberaceae; furanolabdane diterpenoid; absolute
stereostructure; modified Mosher’s method.
Alpinia tonkinensis Gagnep. (Zingiberaceae)
is an endemic medicinal plant to
Vietnam and has been only found in some
provinces (Nam Ha, Ninh Binh, Vinh Phuc) in
North Vietnam [1, 2] The plant was also
introduced to Guangxi and Hainan of China [3].
The rhizomes of the plant were known to
contain a large amount of essential oil [4].
Therefore, the plant possesses antifungal and
antibacterial properties and is used as a natural
antibiotic and a condiment in Vietnam.
Previously, we investigated the chemical
constituents of the hydrodistilled essential oil
from the rhizomes of A. tonkinensis; myrcene,
1,8-cineol and camphor were found as the major
components [4]. This paper describes the
isolation and structural elucidation of the
absolute configuration of a furanolabdane
diterpene alcohol 1 (Fig. 1) which has not
previously reported as natural products.
Compound 1, obtained as a white
amorphous powder with a positive optical
2
3
19
1
4
18
20
10
5
H
11
9
6
16
12
8
7
O
13
15
14
OH
Fig. 1: Chemical Structure of Compound 1
rotation [[] 23 D +15.5 o (c 1.3, in CHCl 3 )], gave
the molecular formula C 20 H 30 O 2 (negative-ion
high-resolution (HR)FAB-MS): see
Experimental). The 13 C-NMR of 1 (table 1)
displayed six sp 2 carbons which were attributed
to a monosubstituted furane ring and an
exocyclic double bond. The assignments were
confirmed by the proton signals of the 1 H-NMR
spectrum of 1 (table 1) [ 4.72 (brs), 4.88 (d,
17
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1.4) of the exocyclic double bond, 6.42 (brd,
1.2), 7.41 (t, 1.2) and7.34 (brs) of the furane
ring]. A secondary alcohol group was found to
be in the structure of 1 on the basis of the signal
at H 4.71, C 66.3 (based on the correlation in
the HMQC spectrum of 1). Furthermore, three
singlet signals of tertiary methyl groups ( 0.70,
0.78, and 0.83) were seen in the 1 H-NMR
spectrum of 1. Taken all together, 1 was
suggested as a labdane-type diterpenoid with an
exocyclic double bond at the 8-position.
Comparison of the 13 C-NMR spectroscopic data
of 1 with the known labd-8(17)-enes [5, 6]
disclosed the location of the hydroxyl group in
the alicyclic side chain (C-11C-16). The
coupling constant of the secondary carbinol
proton (dd, J = 9.6, 4.8) clearly indicated the
location of the group at C-12 since the furane
ring was attached to C-13. The HMBC spectrum
of 1, as shown in Fig. 2, was in full agreement
with the suggested structure. Generally, the
labdane-type diterpenes are characteristic of the
genus Alpinia [7]. The diterpene 1, therefore,
was also assigned to the normal labdane-type
diterpenoids and determined to be 15,16-epoxy-
8(17),13(16),14-labdatrien-12-ol, but the
absolute stereochemistry at the 12-position
remained to be elucidated.
376
O
OH
Fig. 2: HMBC Correlations (HC) of
Compound 1
Literature search showed the presence of
two diastereoisomeric hydroxides, prepared
semisynthetically [8, 9], owing to the
asymmetric center at the 12-position, but their
absolute stereochemistry remained in doubt.
Compound 1 showed the NMR data (table 1)
superimposable with those of one of the
diastereomers, and thus is the first reported
natural product possessing 15,16-epoxy-
8(17),13(16),14-labdatrien-12-ol structure.
Table 1: 1 H-NMR and 13 C-NMR Spectroscopic
data of compounds 1
(CDCl 3 , in ppm, J in parentheses in Hz) a)
C/H
13 C
1
1 H
1
0.80 m
38.9
1.70 brd (13.1)
2
1.44 m
19.4
1.55 qt (13.8, 3.2)
3
1.10 ddd (13.8, 13.3, 4.2)
42.1
1.35 brd (13.3)
4 33.6
5 55.4 0.98 dd (12.6, 2.8)
6
1.69 m
24.4
1.31 qd (12.6, 4.2)
7
1.90 m
38.3
2.37 ddd (12.6, 4.2, 2.3)
8 149.1
9 53.1 1.42 brd (11.2)
10 39.6
11
1.93 m
32.0
1.90 m
12 66.3 4.71 dd (9.6, 4.8)
13 128.9
14 108.3 6.42 brd (1.2)
15 143.5 7.41 t (1.2)
16 139.7 7.34 brs
17A
4.72 brs
106.7
B
4.88 d (1.4)
18 33.5 0.83 s
19 21.7 0.78 s
20 14.6 0.70 s
a) Assignments were made on the basis of the HMQC
and HMBC experiments

The secondary function of the alcohol group
at C-12 allowed us to determine the absolute
stereochemistry at C-12 applying the modified
Mosher’s method [10]. Treatment of 1 with (R)-
and (S)--methoxy--trifluoromethylphenylacetic
acid (MTPA) in the presence of 1-ethyl-
3-(3-dimethylaminopropyl)carbodiimide hydrochloride
(EDC) and 4-dimethylaminopyridine
(DMAP) afforded the 12-(R)- and 12-(S)-MTPA
esters (1a and 1b, respectively) [11] The
chemical shifts for the proton signals of 1a and
1b were assigned by comparison with the
spectroscopic data of 1 and
1 H- 1 H COSY
experiments. The differences in the 1 H chemical
shifts between the diastereomers 1b and 1a (
= S R ) indicated an S-configuration for C-12
and therefore the absolute stereostructure of 1
was determined to be (12S)-15,16-epoxy-
8(17),13(16),14-labdatrien-12-ol. In the
NOESY spectrum of 1 (Fig. 3), no NOEs were
observed between H-12 and H-9, and between
H-12 and H 3 -20. The NOE interaction between
H-12 and H-17A ( 4.72; H-17B appeared at
4.88 showing a NOE interaction with H-7 at
2.37) (dotted line) was possible but uncertain to
be confirmed in CDCl 3 since their overlapping
signals ( 4.71 and 4.72, respectively).
H
H
H
H 3 C
H CH 3
H
H
H
CH 3
H
Fig. 3: NOESY Correlations of Compound 1
General Procedure
Experimental
H
H
H
HO
H
H
H
H
H A
H B
Optical rotation was measured on a Union
Giken PM-101 digital polarimeter. 1 H-NMR
(500 MHz) and 13 C-NMR (100 MHz) spectra
were obtained on JEOL JNM-ECP 500 and
JEOL JNM -400 NMR spectrometers,
respectively, with tetramethylsilane as an
internal standard. Negative-ion mode spectrum
of 1 was recorded on a JEOL JMS SX-102.
Preparative HPLC was carried out with JASCO
PU-1580 pump and UV-2075 Plus detector (set
at 210 nm) on an YMC ODS column (150×20
mm i.d.) at the flow rate of 5 ml/min. Silica gel
60 (0.063 - 0.200 mm, Merck) was used for
open column chromatography. TLC was carried
out on pre-coated TLC plates (Kieselgel 60 F 254 ,
MERCK Art. 5747), and detected by spraying
with 10% H 2 SO 4 in 50% EtOH followed by
heating on a hot plate at 200 o C. (R)- and (S)-
MTPAs were purchased from Nacalai Tesque
Co., Ltd.
Plant material
The fresh rhizomes of A. tonkinensis
Gagnep. (4 kg) were collected in Vinh Phuc
Province, Vietnam, in August 2003. The plant
was identified by Mr Nguyen Quoc Binh, a
botanist of the Institute of Ecology and
Biological Resources, Vietnam National Center
for Natural Science and Technology, Hanoi,
Vietnam. A voucher specimen (No. VN1295)
was deposited in the Herbarium of the same
Institute.
Extraction and Isolation
The powdered dried rhizomes of A.
tonkinensis were extracted three times with
MeOH by percolation at room temperature.
Removal of the solvent from the combined
MeOH extracts gave a black sirup (22.3 g). This
sirup was suspended in water and partitioned
with solvents of increasing polarity (n-hexane,
EtOAc, and 1-BuOH). The n-hexane-soluble
fraction (5.5 g) was subjected to a silica gel
column chromatography eluted with n-hexane-
EtOAc solvent systems (9 : 1, 8 : 1, 4 : 1 and
EtOAc), six fractions were collected on the
basis of TLC spots. Fraction 4 (160 mg) was
chomatographed on silica gel using n-hexane-
EtOAc (9 : 1) as solvent system, the main
377

subfraction was purified by HPLC on an ODS
column using MeOHH 2 O (4 : 1) as solvent
system to afford compound 1 (12 mg).
(12S)-15,16-epoxy-8(17),13(16),14-
labdatrien-12-ol (1): Amorphous powder. [] 23 D
+15.5 o (c 1.3, CHCl 3 ). 1 H-NMR (CDCl 3 ) and
13 C-NMR (CDCl 3 ): Table 1. HRFAB-MS
(negative-ion mode) m/z : 301.2189 (Calcd for
C 20 H 29 O 2 : 301.2168).
Preperation of 12-(R)- and 12-(S)-MTPA
esters of 15,16-epoxy-8(17),13(16),14-
labdatrien-12-ol (1a and 1b, respectively): see
reference 11.
Acknowledgements: This research was
supported by the International Foundation for
Science, Stockholm, Sweden, through a Grant to
Dr. Phan Minh Giang, and the Basic Research
Program in Natural Science of Vietnam.
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