Antarctic Marine Metabolites: New Polyhydroxylated Steroidal Glycosides from the Starfish Odontaster validus

Two new asterosaponins, validosides A and B, were isolated from the butanolic extract of the starfish Odontaster validus and their structures established as (24S)-24-O-[2,4-di-O-methyl-B-xylopyranosyl-(1-2)-a-arabinofuranosyl]-5a-chol-e stane-3B,4B,6a,8,15B,24-hexaol 6-O-sodium sulfate (1) and (24S)-24-O-(3-O-methyl-B-xylopyranosyl)-5a-cholestane-3B, 6a, 8,15B, 24-pentaol 2'-O-sodium sulfate (2) on the basis of extensive spectroscopic investigations and chemical correlations. Validoside A and B are extremely rarely occurring saponins due to the presence of a sulfated group at C-6 of the aglycone in 1 and at C-2' of a pyranose in 2. The absolute configuration at C-24 was found to be S by the application of the Trost-Mosher methodology.