Afroside, a 15β-hydroxycardenolide

Abstract

Afroside (2) has the same carbohydrate as gomphoside (1), namely a 4,6-dideoxyhexosulose doubly linked to the aglycone at the 2α and 3β positions. It differs from gomphoside in having a 15β-hydroxy-group, the location of which is shown by n.m.r. (¹H and ¹³C) and mass spectra, and is established by the formation of a 14,15-cyclic carbonate derivative (2e). The 14β,15β-diol group shows unusual inertness to glycol cleavage and to OO-isopropylidene derivative formation, in contrast with the behaviour of the 2′,3′-diol in the carbohydrate, and the 2α,3β-diol in the genin, afrogenin (4). Degradation of the carbohydrate in afroside gave afrogenin (4), and ‘anhydroafrogenin’(9) which is a 15-ketone with 14α-H. The conformation of the highly crowded ring D is studied using ¹H and ¹³C n.m.r.