Reduction of hemiacetal ring-opened forms of Asclepiadaceae cardenolide glycosides

Abstract

Reduction of gomphoside with sodium cyanoborohydride occurred with opening of the 2′-hemiacetal to yield the 3-(4,6-dideoxy-β-D-ribo-hexopyranoside) and the 3-(4,6-dideoxy-β-D-arabino- hexopyranoside) of 2α-hydroxyuzarigenin. Sodium borohydride reduction of 3′-didehydrogomphoside and 3′-epi-gomphoside 3′-acetate led instead to 2α-hydroxyuzarigenin 3-(4,6-dideoxy-β-D-lyxo-hexopyranoside). The structures of these stereoisomeric products are shown by ¹H and ¹³C n.m.r. spectra. Borohydride reduction of uscharidin likewise yielded 2α,19-dihydroxyuzarigenin 3-(4,6-dideoxy-β-D-lyxo-hexopyranoside). In contrast, a product with an intact 2′-hemiacetal, viz. 19-hydroxy-3′-epi- gomphoside, was obtained by Reichstein et al. under related conditions.