Gukulenins A and B, Cytotoxic Tetraterpenoids from the Marine Sponge <i>Phorbas gukulensis</i>

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