New terpenoids from the brown alga Stypopodium zonale.

Three new terpenoids, 1a-3a, and the known metabolites atomaric acid, stypoquinonic acid, and 5'a-desmethyl-5'-acetylatomaric acid have been isolated from the tropical brown alga Stypopodium zonale. The structures and relative stereochemistry of the methyl ester of 1a-3a were determined by spectral methods. The methyl ester of 3a exhibits in vitro cytotoxic activity against human lung and colon carcinoma.