Antifouling 26,27-Cyclosterols from the Vietnamese Marine Sponge <i>Xestospongia testudinaria</i>

Three new C₂₉ sterols with a cyclopropane ring cyclized between C-26 and C-27 of the side chain, aragusterol I (<b>1</b>), 21-<i>O</i>-octadecanoyl-xestokerol A (<b>4</b>), and 7β-hydroxypetrosterol (<b>5b</b>), were isolated from the Vietnamese marine sponge <i>Xestospongia testudinaria</i>, along with the known compounds, aragusterol B (<b>2</b>), xestokerol A (<b>3</b>), 7α-hydroxypetrosterol (<b>5a</b>), 7-oxopetrosterol (<b>6</b>), and petrosterol (<b>7</b>). The structures of the new compounds were established by analysis of spectroscopic data including 1D and 2D NMR, and high-resolution electrospray ionization mass spectrometry (HRESIMS). Their capacity to inhibit the adhesion of isolated bacteria from marine biofilms was evaluated against the bacterial strains <i>Pseudoalteromonas</i> sp. D41, <i>Pseudoalteromonas</i> sp. TC8, and <i>Polaribacter</i> sp. TC5. Aragusterol B (<b>2</b>) and 21-<i>O</i>-octadecanoyl-xestokerol A (<b>4</b>) exhibited the most potent antifouling activity with EC₅₀ values close to these reported in the literature for tributyltin oxide, a marine anti-biofouling agent now considered to be a severe marine pollutant. Due to its comparable activity to tributyltin oxide and its absence of toxicity, the new 26,27-cyclosterol, 21-<i>O</i>-octadecanoyl-xestokerol A (<b>4</b>) constitutes a promising scaffold for further investigations