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 2β-hydroxy-4,7-diketo-A-norcholest-5-en-2-oic acid (1), 24S-ethyl-2β-hydroxy-4,7-diketo-A-norcholest-5-en-2-oic acid (2), and 2β-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 EC50 values of 8.2, 23.5, 31.6 μg/mL, respectively.