Synthesis of rhamnosylated diosgenyl glucosides as mimetics of cytostatic steroidal saponins from Ornithogalum saundersiae and Galtonia candicans

Abstract

The synthesis of mimetic 3 of the steroid saponins 1 and 2 was investigated. As a substitute for the complex 22-homo-23-nor-steroid moieties A and B in 1 and 2 diosgenin C was introduced. The silyl protected thioorthoester 20 was successfully employed for glucosylation. After selective 2-O-deacetylation, the glucosylated diosgenyl acceptor 23 was rhamnosylated. The 4-O-p-methoxybenzoylated donor 12 gave only minor yields. By using the tri-O-benzoyl protected donor 15 the α-L-rhamnopyranosyl-(1→2)-β-D-glucopyranosyl-(1→3β)-diosgenin derivative 25 was obtained.