A Sigmosceptrella sp. of sponge collected during
trawling operations in the Great Australian Bight, Australia, has yielded a
series of new norterpenes. These include a new bisnorditerpene,
sigmosceptrin-A (5); two new norditerpenes, sigmosceptrin-B (14) and
sigmosceptrin-C (15), isolated as their methyl esters (6) and (7)
respectively; and an ethylated artefact, sigmosceptrin-B ethyl ester (8).
Complete stereostructures were assigned to the sigmosceptrins by spectroscopic
analysis, chemical degradation, derivatization, and by a single-crystal X-ray
structural analysis. A biosynthetic pathway is proposed that requires a common
biosynthetic precursor to both the sigmosceptrins and norterpene cyclic
peroxides.