Benzophenone O-glucoside, a biogenic precursor of 1,3,7-trioxygenated xanthones in Hypericum annulatum.

Two benzophenones, hypericophenonoside (1) and annulatophenone (2) were isolated from the methanol extract of the herb of Hypericum annulatum. The structures of the benzophenones were established as 2'-O-beta-D-glucopyranosyl-2,4,5',6-tetrahydroxy benzophenone (1) and 2,3',5',6-tetrahydroxy-4-methoxybenzophenone (2) based on spectral and chemical evidence. Hypericophenonside is the second benzophenone O-glycoside found in nature. Acid and enzymatic hydrolysis of (1) led directly to the formation of 1,3,7-trihydroxyxanthone (gentisein). This fact confirmed the hypothesis that some xanthones could be formed in plants by dehydration of 2,2'-dihydroxybenzophenones, and the intermediate precursors appear to be benzophenone O-glycosides ortho to the carbonyl function.