Isomalabaricane-type compounds from the marine sponge Rhabdastrella aff. distincta.

Fractionation of the marine sponge Rhabdastrella aff. distincta led to the isolation and characterization of four new isomalabaricane analogues, isogeoditin A (1), 13-(E)-isogeoditin A (2), isogeoditin B (3), and 22,23-dihydrostellettin B (4), along with seven known isomalabaricane derivatives (5-11). Methylation of 10 and 11 afforded methyl esters 12 and 13. The structures of compounds 1-4 were determined on the basis of spectroscopic data analysis. All compounds were tested against a small panel of human tumor cell lines.