Versixanthones A–F, Cytotoxic Xanthone–Chromanone Dimers from the Marine-Derived Fungus <i>Aspergillus versicolor</i> HDN1009

Six unusual xanthone–chromanone dimers, versixanthones A–F (<b>1</b>–<b>6</b>), featuring different formal linkages of tetrahydroxanthone and 2,2-disubstituted chroman-4-one monomers, were isolated from a culture of the mangrove-derived fungus <i>Aspergillus versicolor</i> HDN1009. The absolute configurations of <b>1</b>–<b>6</b>, representing the central and axial chirality elements or preferred helicities, were established by a combination of X-ray diffraction analysis, chemical conversions, and TDDFT-ECD calculations. The interconversion of different biaryl linkages between <b>1</b> and <b>4</b> and between <b>2</b> and <b>3</b> in DMSO by a retro-oxa-Michael mechanism provided insight into the formation of the xanthone–chromanone dimers and supported the assignments of their absolute configurations. Compounds <b>1</b>–<b>6</b> exhibited cytotoxicities against the seven tested cancer cell lines, with the best IC₅₀ value of 0.7 μM. Compound <b>5</b> showed further inhibitory activity against topoisomerase I