Dendrillic Acids A and B: Nitrogenous, Rearranged Spongian Nor-Diterpenes from a Dendrilla sp. Marine Sponge

Two nitrogenous rearranged spongian nor-diterpenoids, dendrillic acids A and B, were isolated from a marine sponge Dendrilla sp. (order: Dendroceratida; family: Darwinellidae). The structures of the metabolites were elucidated on the basis of spectroscopic analysis as well as density functional theory prediction of NMR chemical shifts and application of the DP4+ algorithm. The absolute configuration of the metabolites was established via comparison of experimental and time-dependent density functional theory predicted electronic circular dichroism data. An unusual epimerization reaction was observed leading to the interconversion of the metabolites upon storage in dimethyl sulfoxide solution, which is proposed to proceed via an anionic pathway as probed via isotopic incorporation experiments. Evaluation against a panel of micro-organisms and cell lines revealed that the compounds were devoid of any significant biological activity against all organisms tested, with the exception of mild antiprotozoal activity displayed by dendrillic acid B (2) against Giardia duodenalis.