Sulawesins A–C, Furanosesterterpene Tetronic Acids That Inhibit USP7, from a <i>Psammocinia</i> sp. Marine Sponge

Three new furanosesterterpene tetronic acids, sulawesins A–C (<b>1</b>–<b>3</b>), were isolated from a <i>Psammocinia</i> sp. marine sponge, along with the known compounds ircinins-1 (<b>4</b>) and -2 (<b>5</b>). Although ircinins-1 and -2 were previously isolated as (+)- or (−)-enantiomers from marine sponges, we isolated them as enantiomeric mixtures. Sulawesins A and B possess a new carbon skeleton with a 5-(furan-3-yl)-4-hydroxycyclopent-2-enone moiety and were also found to be diastereomeric mixtures of four isomers by an HPLC analysis with a chiral-phase column. Sulawesin C has a dimeric structure of ircinin-1 and is the first dimer in this family. USP7, a deubiquitinating enzyme, is an emergent target of cancer therapy, and the isolated compounds inhibited USP7 with IC<sub>50</sub> values in the range of 2.7–4.6 μM.