Rearranged Diterpenes and Norditerpenes from Three Australian Goniobranchus Mollusks

Three new norditerpenes (1, 6, and 7) and four diterpenes (2–5) with extensively rearranged carbon skeletons have been characterized from Australian nudibranchs. The relative configuration of the cyclopropyl-containing verrielactone (1) from Goniobranchus verrieri was suggested by spectroscopic analysis at 500 MHz informed by a combination of molecular modeling and DFT calculations. The nudibranchs G. splendidus and G. cf. splendidus provided 2–7, for which the structures and stereochemistry were deduced by 2D NMR studies at either 500 or 700 MHz. Each of the seven terpenoids exhibited a carbon skeleton modified from one of the tetrahydroaplysulphurin, spongionellin, or gracilane series of terpenes. A biosynthetic pathway to terpenes 1–7 from spongialactone is proposed.