Bioactive metabolites from the South African marine mollusk Trimusculus costatus

A reinvestigation of extracts of the endemic South African intertidal limpet Trimusculus costatus yielded the known labdane diterpenes 6 beta,7 alpha-diacetoxylabda-8,13E-dien-15-ol (1) and 2 alpha,6 beta,7 alpha-triacetoxylabda-8,13E-dien-15-ol (2) and three new metabolites, 6 beta,7 alpha,15-triacetoxylabda-8,13E-diene (3), 3 alpha,11-dihydroxy-9,11-seco-cholest-4,7-dien-6,9-dione (4), and cholest-7-en-3,5,7-triol (5). Chiral derivatization and X-ray analysis were used to confirm the labdane absolute configuration of 2. Compounds 1, 2 and 4 exhibited moderate activity (3-25 mu M) against the WHCO1 human esophageal cancer cell line.