Chemistry of Glossodoris nudibranchs: specific occurrence of 12-keto scalaranes.

With the aim of establishing whether the oxidation of the tricyclic carbon skeleton of scalaradial (1) is specific to Glossodoris mollusks, the chemical investigation of specimens of Glossodoris pallida from two distinct geographical areas (China and Guam) and of Glossodoris vespa and Glossodoris averni from Eastern Australia was performed. 12-Deacetoxy-12-oxo-scalaradial (4), recently reported from another Glossodoris nudibranch, was the main metabolite of G. pallida from China, G. vespa, and G. averni. A series of scalarane compounds 3 and 5-11, including the unprecedented 12,16-deacetoxy- 12-oxo-scalarafuran 5, was isolated from the mollusks. Interestingly, a population of G. pallida from Guam displayed a different scalarane pattern that was characterized by scalaradial (1), deacetyl scalaradial (2), and deoxoscalarin (3), thus confirming previous reports. The specific occurrence of 12-keto-derivatives in some nudibranchs of the genus Glossodoris is discussed.