Synthesis of Xanthone-O-glycosides, I Synthesis of 3-0-/3-Glycosides

Xanthone-O-glycosides seem to be characteristic constituents of the Gentianaceae [1]. Various species of the latter have been used in popular medicine both in Europe and the Orient. Thus in the Indian system of medicine, extracts of Canscora decussata and several Swertia species are used in the treatment of certain mental disorders e.g. melancholia. Recently Ghosal et al. [2] performed the pharmacological screening of xanthone-O-glycosides and reported their monoamine oxidase inhibition and CNS depressant action in mice and rats. We therefore undertook a synthesis of various substituted xanthone-O-glycosides in order to achieve more information on structure activity relationships. In this communication we present the first synthesis of some xanthone-3-0-/?-glycosides including that of the naturally occurring gentioside (1 hydroxy 3 0 ß primeverosyl 7 methoxy xan thone) (14). The synthesis of xanthone-O-glycosides has not yet been reported although the synthesis of the corresponding C-glucoside mangiferin is known [3, 4]. The glycosidation of 1,3-dihydroxyxanthone (1) with a-acetobromoglucose was carried out in pyridine solution using Ag2C03 as the acid acceptor. In analogy with known results in flavonoid glycosidations, condensation took place exclusively at OH-3 with 5 8 % yield of 2. Deacetylation with NaOMe in MeOH [6] gave the free xanthone glycoside (3). The negative optical rotation of the condensation product [a]2^ = —49.7° and the free glycoside, [a]2^ = —54.7°, are indicative of a ß-\mkage between aglycone and sugar. The *H NMR spectrum of the glucoside in DMSO-dö showed the