New A-nor steroids and their antifouling activity from the Chinese marine sponge Acanthella cavernosa.

A chemical examination of the Chinese sponge Acanthella cavernosa resulted in the isolation of three new A-ring contracted steroids, the ethyl esters of 2beta-hydroxy-4,7-diketo-A-norcholest-5-en-2-oic acid (1), 24S-ethyl-2beta-hydroxy-4,7-diketo-A-norcholest-5-en-2-oic acid (2), and 2beta-hydroxy-4,7-diketo-24R-methyl-A-norcholest-5,22(E)-dien-2-oic acid (3), along with four other known steroids (4-7). The structures were determined on the basis of extensive spectroscopic analysis. Compounds 1-3 showed antifouling activity toward the settlement inhibition of Balanus albicostatus with the EC(50) values of 8.2, 23.5, 31.6 microg/mL, respectively.