Phytochemistry 52 (1999) 1593±1596
Absolute con®gurations of two acyclic triterpenoids from
Ekebergia capensis
Yumi Nishiyama a,*, Masataka Moriyasu a, Momoyo Ichimaru a, Atsushi Kato a, Simon
G. Mathenge b, Joseph N. Nganga c, Francis D. Juma c
a
Department of Natural Medicinal Chemistry, Kobe Pharmaceutical University, 4-19-1, Motoyamakita-machi, Higashinada-ku, Kobe 658-8558,
Japan
b
Department of Botany, University of Nairobi, P.O. Box 30197, Nairobi, Kenya
c
Department of Medicine, University of Nairobi, P.O. Box 30588, Nairobi, Kenya
Received 14 October 1998; received in revised form 1 April 1999
Abstract
The absolute con®gurations of two acyclic triterpenoids 1 and 2, previously isolated from the bark of Ekebergia capensis
(Meliaceae) have been determined by the modi®ed Mosher's method. # 1999 Elsevier Science Ltd. All rights reserved.
Keywords: Ekebergia capensis; Meliaceae; Acyclic triterpenoid; 2,3,22,23-tetrahydroxy-2,6,10,15,19,23-hexamethyl-6,10,14,18-tetracosatetraene;
1,2,3,22,23-pentahydroxy-2,6,10,15,19,23-hexamethyl-6,10,14,18-tetracosatetraene; Modi®ed Mosher's method; Absolute con®guration
1. Introduction
2. Results and discussion
Two new acyclic triterpenoids were previously isolated from the bark of Ekebergia capensis, 2,3,22,23tetrahydroxy-2,6,10,15,19,23-hexamethyl-6,10,14,18-tetracosatetraene (1) and 1,2,3,22,23-pentahydroxy2,6,10,15,19,23-hexamethyl-6,10,14,18-tetracosatetraene
(2)(Nishiyama et al., 1996). The con®gurations of the
double bonds were determined to be all E from the 1H
and 13C NMR data (Nishiyama et al., 1996). However,
the absolute con®guration at the stereogenic centers
has not been established yet. Therefore, the main goal
of this work was to determine the absolute con®gurations of the sterogenic carbinol centers of triterpenoids 1 and 2 using the Mosher ester methodology.
In order to determine the absolute con®gurations of
the sterogenic carbinol centers at C-3 and C-22 in
compound 1, we ®rst tried the Horeau method
(Horeau, 1961, 1962) without success. Therefore, we
applied the modi®ed Mosher ester methodology
(Ohtani, Kusumi, Kashman & Kakisawa, 1991;
Kusumi, Hamada, Ishitsuka, Ohtani & Kakisawa,
1992). The analysis of the DdH (S±R ) data (Fig. 1) of
the (S )- and (R )-MTPA Mosher ester derivatives of
compound 1, showed that the absolute con®gurations
of the chiral centers at C-3 and C-22 were both R.
Compound 2 has three chiral centers at C-2, C-3
and C-22. By application of the modi®ed Mosher's
method, the con®guration of C-22 is determined to be
R. Because C-2 and C-3 are close to each other, the
determinations of these absolute con®gurations is dicult. To solve this problem, we prepared the acetonide
compounds of 2. Namely, on treatment of 2 with 2,2dimethoxypropane, p-toluenesulfonic acid in acetone
(Ngnokam, Massiot, Bliard & Tsamo, 1995), 2 gave
* Corresponding author. Tel.: +81-78-441-7543; fax: +81-78-4417544.
E-mail address: nisiyama@kobepharma-u.ac.jp (Y. Nishiyama).
0031-9422/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 3 1 - 9 4 2 2 ( 9 9 ) 0 0 3 1 7 - 9
1594
Y. Nishiyama et al. / Phytochemistry 52 (1999) 1593±1596
three acetonide compounds, 2a, 2b and 2c. Because 2
contained a vicinal triol moiety (C-1, C-2 and C-3),
three possible acetonides could be formed. The structures of each product were elucidated by MS spectrum,
1
H and 13C NMR and especially 2D NMR spectral
data. The NOESY experimental data on 2a reveal that
the relative con®guration between C-2 and C-3 is
threo, namely the cross peak between H-3 and H2-1 is
observed. In 2c, because a secondary alcohol site (C-3)
remained, the modi®ed Mosher's method was applied.
Fig. 1. DdH values obtained for the MTPA ester of compound 1.
From the results in Fig. 2, absolute con®guration of
C-3 is determined to be R, consequently C-2 is R.
3. Experimental
3.1. General
1
H NMR: 500 MHz;
13
C NMR: 125 MHz, with
Fig. 2. DdH values obtained for the MTPA ester of compound 2c.
Y. Nishiyama et al. / Phytochemistry 52 (1999) 1593±1596
TMS as int. standard; HR-SIMS: 3-nitrobenzyl alcohol matrix+Na2CO3; TLC: silica gel.
3.2. (R)- or (S)-di-MTPA ester of 1
1 (20 mg) was dissolved in dichlromethane (0.5 ml)
and DCC (about 15 mg), (R )- or (S )-MTPA acid
(about 15 mg) and DMAP (about 10 mg) were added.
The mixture was kept at room temperature for 6 h,
then was submitted to pTLC (benzene:diethylether=4:1) and the main ester compound was
obtained (R-di-MTPA ester: 23 mg, S-di-MTPA ester:
18 mg).
3.3. Bis-acetonide of 2
2 (52 mg) was dissolved in acetone (10 ml) and
2,2-dimethoxypropane (1 ml) and p-toluenesulfonic
acid (5 mg) were added. The reaction was completed within 20 min. at room temperature. Hexane
was added and the organic phase was washed with
saturated NaHCO3 and dried over Na2SO4 and
concentrated. The extract was subjected to
pTLC (benzene:diethylether=2:1) and three main
products were obtained (2a: 23.6 mg, 2b: 11.3 mg, 2c:
17.2 mg).
2a: colorless oil. 1H NMR (CDCl3) d 1.06 (3H, s,
CH3-25), 1.10, 1.24 (each 3H, s, CH3-24, 30), 1.33,
1.36, 1.42, 1.46 (each 3H, s, CH3 4), 1.45 01.50 (m),
1.59, 1.60, 1.61, 1.62 (each 3H, bs, CH3-26, 27, 28, 29),
1.63 (2H, m), 2.00 (m), 2.09 (4H, m), 2.20 (2H, m),
3.38 (1H, dd, J = 9.5, 11.5 Hz, HA-1), 3.54 (1H, dd,
J = 3.0, 11.5 Hz, HB-1), 3.66 (1H, dd, J = 3.5, 9.5
Hz, H-22), 4.01 (1H, dd, J = 3.5, 9.5 Hz, H-3), 5.15
(4H, m, H-7, 11, 14, 18). 13C NMR (CDCl3) d 16.02,
16.06 (C-26, 27, 28, 29), 18.86 (C-25), 22.94, 26.08 (C24, 30), 26.69, 26.70, 26.77, 26.87, 28.58, 28.76
(CH3 4), 27.80, 27.97, 28.30, 36.69, 39.71, 39.72,
65.54 (C-1), 77.55 (C-3), 80.10 (C-23), 82.60 (C-2),
82.85 (C-22), 106.43, 107.06, 124.32, 124.34, 124.79 (C7, 11, 14, 18), 134.22, 134.23, 135.04, 135.06 (C-6, 10,
15, 19). SIMS m/z 597[M+Na]+, 559. HR-SIMS
Found: 597.4502 [M+Na]+; C36H62O5Na requires
597.4492.
2b: colorless oil. 1H NMR (CDCl3) d 0.99 (3H, s,
CH3-25), 1.10, 1.24 (each 3H, s, CH3-24, 30), 1.33,
1.41, 1.42, 1.43 (each 3H, s, CH3 4), 1.47 (2H, m),
1.60, 1.61, 1.62 (each bs, CH3-26, 27, 28, 29), 1.61 (m),
2.01 (m), 2.09 (4H, m), 2.20 (2H, m), 3.45, 3.73 (each
1H, d, J = 12.0 Hz, H2-1), 3.56 (1H, dd, J = 4.5, 8.0
Hz, H-3), 3.68 (1H, dd, J = 3.5, 9.5 Hz, H-22), 5.15
(4H, m, H-7, 11, 14, 18). 13C NMR (CDCl3) d 15.85,
16.00, 16.05 (C-26, 27, 28, 29), 18.28, 26.84, 28.56,
29.71 (CH3 4), 18.92 (C-25), 22.91, 26.05 (C-24, 30),
26.03, 26.66, 26.70, 27.76, 28.27, 35.15, 36.67, 39.70,
39.79, 66.89 (C-2), 70.49 (C-1), 75.13 (C-3), 80.07 (C-
1595
23), 82.82 (C-22), 98.91, 106.40, 124.23, 124.28, 124.74,
124.89 (C-7, 11, 14, 18), 134.22, 134.36, 135.04, 135.09
(C-6, 10, 15, 19). SIMS m/z 597[M+Na]+, 559. HRSIMS Found: 597.4481 [M+Na]+; C36H62O5Na
requires 597.4492.
2c: colorless oil. 1H NMR (CDCl3) d 1.10, 1.24
(each 3H, s, CH3-24, 30), 1.25 (3H, s, CH3-25), 1.33,
1.407, 1.414, 1.42 (each 3H, s, CH3 4), 1.43 (m), 1.47
(m), 1.599, 1.602, 1.61, 1.62 (each 3H, bs, CH3-26, 27,
28, 29), 1.63 (m), 2.01 (m), 2.08 (m), 2.20 (m), 2.26
(m), 3.48 (1H, td, J = 3.0, 9.5 Hz, H-3), 3.66 (1H, dd,
J = 3.5, 9.5 Hz, H-22), 3.69, 3.87 (each 1H, d, J = 8.5
Hz, H2-1), 5.16 (4H, m, H-7, 11, 14, 18). 13C NMR
(CDCl3) d 16.00, 16.04 (C-26, 27, 28, 29), 19.68 (C-25),
22.91, 26.06 (C-24, 30), 26.67, 26.85, 26.93, 27.28,
28.56 (CH3 4), 27.77, 28.27, 30.46, 36.52, 36.66,
39.70, 71.73 (C-1), 75.52 (C-3), 80.08 (C-23), 82.82 (C22), 83.83 (C-2), 106.40, 109.60, 124.29, 124.76, 124.84
(C-7, 11, 14, 18), 134.21, 134.54, 135.01, 135.03 (C-6,
10, 15, 19). SIMS m/z 597[M+Na]+, 559. HR-SIMS
Found: 597.4501 [M+Na]+; C36H62O5Na requires
597.4492.
3.4. (R)- or (S)-mono-MTPA ester of 2c
2c (15 mg) was dissolved in dichloromethane (2 ml)
and DCC (about 15 mg), (R )- or (S )-MTPA acid
(about 15 mg) and DMAP (about 10 mg) were added.
The mixture was kept for 6 h, then was submitted to
pTLC (benzene:diethylether=8:1) and main ester compound was obtained (R-mono-MTPA ester: 12.6 mg,
S-mono-MTPA ester: 13.7 mg).
3.5. (R)- or (S)-tri-MTPA ester of 2
2 (20 mg) was dissolved in dichloromethane (2 ml)
and DCC (about 20 mg), (R )- or (S )-MTPA acid
(about 20 mg) and DMAP (about 15 mg) were added.
The mixture was kept for 12 h, then was submitted to
pTLC (benzene:diethylether=4:1) and the ester compound was obtained (R-tri-MTPA ester: 15 mg, S-triMTPA ester: 9 mg).
The structures of compounds mentioned above were
determined by 1H and 13C NMR and various two
dimentional 2D NMR data.
Acknowledgements
We thank Dr. Georges Massiot, director of CNRS
for his valuable advice.
References
Horeau, A. (1961). Tetrahedron Lett., 15, 506.
1596
Y. Nishiyama et al. / Phytochemistry 52 (1999) 1593±1596
Horeau, A. (1962). Tetrahedron Lett., 21, 965.
Kusumi, T., Hamada, T., Ishitsuka, M. O., Ohtani, I., & Kakisawa,
H. (1992). J. Org. Chem., 57, 1033.
Ngnokam, D., Massiot, G., Bliard, C., & Tsamo, E. (1995). Nat.
Prod. Lett., 5, 289.
Nishiyama, Y., Moriyasu, M., Ichimaru, M., Tachibana, Y., Kato,
A., Mathenge, S. G., Nganga, J. N., & Juma, F. D. (1996).
Phytochemistry, 42, 803.
Ohtani, I., Kusumi, T., Kashman, Y., & Kakisawa, H. (1991). J.
Am. Chem. Soc., 113, 4092.
Joseph Nganga