Diterpenes from marine opisthobranch molluscs

Marine opisthobranchs represent a very interesting source of bioactive natural products, which exhibit an extraordinary variety of chemical structures. Opisthobranchs are naked molluscs, apparently unprotected by the physical constrain of a shell. In spite of this, they appear to be free of predation. Their survival is assured by a series of alternative defensive strategies which include the use of chemicals. These bioactive compounds, which are often localized in selected parts of body and in the defensive mucous secretion, can be derived from the diet or de novo biosynthesized. Terpenes represent the major group of secondary metabolites isolated from opisthobranchs. Most of these compounds show both interesting biological properties and unique chemical structures. In particular, diterpenes include some of the most interesting examples of bioactive molecules. This review covers the chemistry of diterpenes from marine opisthobranchs, until approximately 1999. Isolation, synthesis and biosynthesis are separately discussed throughout the paper. Relevant data regarding biological activities are also reported.