Gukulenins A and B, Cytotoxic Tetraterpenoids from the Marine Sponge Phorbas gukulensis

Gukulenins A (1) and B (2), both having an unprecedented skeleton with a bis-tropolone moiety, were isolated from the Korean marine sponge Phorbas gukulensis. They exhibited significant cytotoxicity against human pharynx, stomach, colon, and renal cancer cell lines in the range 0.05−0.80 μM.