Elsevier

Phytochemistry

Volume 51, Issue 8, August 1999, Pages 1069-1075
Phytochemistry

Steroidal saponins from Asparagus africanus

https://doi.org/10.1016/S0031-9422(99)00051-5Get rights and content

Abstract

The structures of two new monodesmosidic spirostanosides and a new bisdesmosidic furostanol glycoside isolated from the roots of Asparagus africanus Lam. (Liliaceae) have been elucidated as (25R)-3β-hydroxy-5β-spirostan-12-one 3-O-{β-d-glucopyranosyl-(1→2)-[α-l-arabinopyranosyl-(1→6)]-β-d-glucopyranoside} (1), (25R)-5β-spirostan-3β-ol 3-O-{β-d-glucopyranosyl-(1→2)-[α-l-arabinopyranosyl-(1→6)]-β-d-glucopyranoside} (2) and 26-O-β-d-glucopyranosyl]-22α-methoxy-(25R)-furostan-3β,26-diol 3-O-{β-d-glucopyranosyl-(1→2)-[β-d-glucopyranoside} (3), respectively, by the combined use of one and two dimensional NMR experiments. The complete 13C and 1H assignments of the peracetyl spirostanosides and the furostanol oligoside were derived. The interconversions between the methoxyl and hydroxyl group at C-22 of the furostanol glycoside was investigated and the genuine furostanol oligoside of A. africanus appears to be the hydroxyl type based on the comparative study of the methanol, pyridine and dioxane extracts.

Introduction

Asparagus africanus (Liliacae) (local names `Seriti' or `K'estancha') is a widely spread thorny shrub about 1 to 3 m long distributed along the great rift valley of Ethiopia. It is used in the indigenous system of medicine for various health ailments Watt, & Breyer-Brandwijk, 1962, Abate, 1989, Abebe, & Ayehu, 1993. Recent phytochemical investigation of the root of this plant has led to the isolation of antiprotozoal compounds muzanzagenin and (+)-nysol (Oketch-Rabah et al., 1997). This paper describes the isolation, structural elucidation and complete 13C and 1H chemical shifts assignments of two new spirostanosides and a new oligofurostanoside from the methanol extract of the roots of this plant.

Section snippets

Results and discussion

Solvent partition and chromatography of the saponin mixture obtained from the methanol extract of the roots of A. africanus produced three major saponins 1, 2 and 3. Compounds 1 and 2 showed spiroacetal absorption in their IR spectra whereas compound 3 showed no spiroacetal absorption but gave an intense red colour with Ehrlich reagent (Stahl, 1965, Kiyosawa et al., 1968; Tschesche, Tjoa, Wulff, & Noronha, 1968). Structural determinations of the three saponins were mainly based on 1D (1H, 13C,

General

Melting points were determined in open capillaries in an electrothermal melting apparatus. FAB-MS was recorded with a ZAB-E (VG analytical (Micromass)); matrix: thioglycerol and `magic bullet'.

Proton correlated NMR spectra were recorded with a VARIAN UNITY INOVA 600 NMR spectrometer at 313 K, 13C spectra with a VARIAN UNITY INOVA 400 NMR spectrometer at 313 K. Concentration 60 mmol/l. Tubes: 5 mm diameter (Kontes). Triple resonance probe head with shielded z-gradient (600 MHz); dual probe with

Acknowledgements

The authors wish to express their gratitude to Dr. M. Schmid (Institute of Pharmaceutical Chemistry, University of Graz) for measurement of capillary electrophoresis and to Dr. F. Bucar (Institute of Pharmacognosy, University of Graz) for measurement of GC–MS spectra of sugars. We thank Professor Dr. A. Hiermann (Institute of Pharmacognosy, University of Graz) for his technical assistance and cooperation. AD gratefully acknowledges ÖAD, Austria, for the scholarship award and EHNRI, Addis Ababa,

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