Marine Natural Products. XX

Two new scalarane-type bishomosesterterpenes, dehydrofoliaspongin (17) and phyllofoliaspongin (18), were isolated together with foliaspongin (4) and scalardysin-B (15) from the Okinawan marine sponge Phyllospongia foliascens. On the basis of chemical and physicochemical evidence including the X-ray crystallographic analysis of a methylated derivative (16) of 15, the structures of 17 and 18 were determined and the previously proposed structure (3) of foliaspongin was revised to 4, having a 4β-ethyl moiety. Phyllofoliaspongin (18) showed cytotoxic, anti-thrombocyte, and vasodilative activities.