Recent Sino-Italian collaborative studies on marine organisms from the South China Sea

In this communication, selected results obtained in the course of a 10-year Sino-Italian collaborative project are presented. During this collaboration, several studies on marine organisms collected in the South China Sea have been conducted, resulting in the characterization of a large number of novel natural products. The main interest has been focused on sponges, cnidarians, and molluscs. Here, the recent research, including some not yet published studies, on cnidarians and molluscs, in particular, opisthobranchs and pulmonates, will be briefly described. The study of cnidarians and pulmonates led to isolate molecules, in particular, biscembranoids and polypropionates, displaying carbon skeletons already described in previous papers. Our studies resulted in their complete structural characterization including the absolute stereochemistry that was clarified by coupling X-ray diffraction to circular dichroism (CD) spectra analysis. Some unique nitrogenous molecules containing either an isonitrile function or an 1,2,4-oxadiazole moiety, this latter found for the first time in a marine organism, as well as known sesterterpenoids and diterpenoids were isolated from opisthobranchs. Owing to their anatomical location, these metabolites should play a relevant role in the protection of the molluscs. Finally, a series of alkaloids structurally related to known anticancer agents were also characterized.