Cyclobutenbriarein A,the first diterpene with a tricyclo[8.4.0.0(3,6)]tetradec-4-ene ring system isolated from the Gorgonian Briareum asbestinum

Cyclobutenbriarein A (2), a novel class of C20-rearranged diterpene briarane possessing an unprecedented tricyclo[8.4.0.0(3,6)]tetradec-4-ene ring system was isolated, along with five new briarane diterpenoids (1 and 3-6), from the organic extracts of the Bahamian gorgonian Briareum asbestinum. The structures of these secondary metabolites were established on the basis of extensive NMR studies and accurate mass measurements (HRFABMS).