New cytotoxic sterol glycosides from the octocoral Carijoa (Telesto) riisei.

Two new steroidal glycosides, 3beta-O-(3'-O-acetyl-beta-D-arabinopyranosyl)-25xi-choles tan e-3beta, 5alpha,6beta,26-tetrol-26-acetate (riisein A, 2) and 3beta-O-(4'-O-acetyl-beta-D-arabinopyranosyl)-25xi-choles tan e-3beta, 5alpha,6beta,26-tetrol-26-acetate (riisein B, 3), were isolated from extracts of the Brazilian telestacean octocoral Carijoa (Telesto) riisei collected near Rio de Janeiro. The new glycosides co-occur with the polyhydroxy sterol, 25xi-cholestane-3beta,5alpha,6beta, 26-tetrol-26-acetate (1), an inseparable diastereomeric mixture previously reported from Telesto riisei collected in Micronesia. The structures of the new glycosides were assigned by spectroscopic methods and by comparison with spectral data for sterol 1. Riiseins A and B showed in vitro cytotoxicity toward HCT-116 human colon adenocarcinoma with IC(50) values of 2.0 microg/mL.