Gossypetin-3-O-β-D-glucosido-8-O-β-D-glucuronide and 4′-Methylhypolaetin-8-O-β-D-glucuronide, Two Novel Flavonoid Glycosides fromMalva silvestris

Abstract

In Eastern, Southern, and Central Africa, the tuberous roots of Mondia whytei (Hook. F.) Skeels (Asclepiadaceaemilkweed family) are ground to powder and taken orally in porridge, beer, soup, or tea as an aphrodisiac and also to treat anorexia, schistosomiasis, constipation, and gonorrhoea (1). We now report the isolation of a phenolic glycoside 1 from the methanol extract of the tuber using a combination of DCCC, CC, Sephadex LH-20, and MPLC techniques. The structure of 1 was determined by spectroscopy (1H and 13C, DEPT, difference nOe NMR, UV, and IR) and synthesis of the aglycone. H3 CO 0 Powdered tuber was extracted successively at room temperature with CH2CI2, MeOH, and H20. The MeOH extract was separated by DCCC (CHC1,-MeOH-i-PrOH-H20, 5.6: 1 .4, descending mode), followed by Sehadex LH-20 column chromatography (MeOH). Final purification was achieved on RP-8 MPLC (MeOH-H20 step gradient).Acidic hydrolysis of I with 5% H2 SO4 afforded the aglycone, xylose, and glucose. MS data indicated that xylose was the terminal sugar. The interglycosidic linkage was deduced from 13C-NMR data.Synthesis of the aglycone from 2,4-dihydroxybenzoic acid was achieved in three steps. Methylation (to give 2-hydroxy-4-methoxymethyl benzoate), reduction (yielding 2-hydroxy-4-methoxybenzyl alcohol), and partial oxidation of the primary alcohol with pyridinium chlorochromate (2) gave 2hydroxy-4-methoxybenzaldehyde, the NMR data of which were identical to those of the aglycone obtained after hydrolysis of 1. Our investigations about taxonomically significant flavonoid glycosides in Mama silvestris L. leaves that could be used for the chromatographic characterization of the plant material led to the isolation of four 8-hydroxy flavonoid glucuronides. Two of them, gossypetin-3-O-3-D-glucosido-8-O-3-o-glucuromde (1) and 4'-methylhypolaetin-8-O-3-o-glucuronide (2) are novel structures. HOOC HO OH OH Ho$yo1)L)The isolation of the flavonoids from organic plant extracts (80% THF) was done by prepurification over C18-RPmaterial followed by ion-exchange chromatography on an NH2-stationary phase. The final purification step was gel chromatography over Sephadex® LH-20 in the case of 1, the major flavonoid in mallow leaves, while for 2, which occurs only in trace amounts, gel chromatography was followed by preparative RP-HPLC. The structures of both flavonoid glycosides were elucidated by means of FAB-mass, UV/VIS, 1H-NMR, and 13C-NMR spectroscopy.