Cytotoxic Diterpenoid Pseudodimers from the Korean Sponge <i>Phorbas gukhulensis</i>

Four new cytotoxic diterpenoid pseudodimers (<b>2</b>–<b>5</b>), along with a previously reported one, gukulenin A (<b>1</b>), were isolated from the marine sponge <i>Phorbas gukhulensis</i> collected off the coast of Gagu-do, Korea. These novel compounds, designated gukulenins C–F (<b>2</b>–<b>5</b>), were determined by extensive spectroscopic analyses to be pseudodimers of the gagunins, like gukulenin A. The termini of the tropolone-containing side chains in gukulenins C–E (<b>2</b>–<b>4</b>) were found to have diverse modifications involving acetamides or taurine, whereas gukulenin F (<b>5</b>) was formed from <b>1</b> by the ring-opening of a cyclic hemiketal. The relative and absolute configurations were assigned by Murata’s and modified Snatzke’s methods using a HETLOC experiment and a CD measurement of a dimolybdenum complex, respectively. All of these compounds exhibited significant cytotoxicity against the K562 and A549 cell lines.