Toxic constituents of the asclepiadaceae. Structure elucidation of sarcovimiside A-C, pregnane glycosides of Sarcostemma viminale

The structure elucidation of three related pregnane glycosides, sarcovimiside A, B, and C from Sarcostemma viminale, is based on a detailed study of their highfield 1H and 13C NMR spectra. The results show that all the sugars are (1 → 4)-linked and both (β-D- and α-L-cymarose residues are present in each glycoside chain. The aglycone of sarcovimiside A was identified as 12β-benzoyloxy-3β,8β,14.17-tetrahydroxy-14β,17α-pregn-5-en-20-one whereas in the case of both sarcovimiside B and C, oxidative cleavage of the C(8)–C(14) bond occurred and the aglycone is the 12,20-O,O-dibenzoyl derivative of (20S)-3β,5,12β3,17,20-pentahydroxy-8,14-seco-5β,17α-pregn-6-ene-8,14-dione. Sarcovimiside C is the β-(1→4)-glucopyranosyl derivative of sarcovimiside B.