Variabilin and related compounds from a sponge of the genus sarcotragus

The bioactivity-directed analysis of the extract from a sponge of the genus Sarcotragus led to the isolation of a series of bioactive sesterterpenes, of which variabilin [1a] was the major component. The sesterterpenes 2a, 3, and 4a, along with the related C 21 furanoterpene [5], were present in lesser amounts. The unequivocal assignment of the stereochemistry of the 20, 21 double bond in variabilin as 20Z was achieved through examination of the 22-O-methyl derivative 1b of variabilin and the isolation of the variabilin isomer 2a with the 20E configuration.