Chromene Dimers from Cultures of Basidiomycete <i>Panus similis</i>

Four chromene dimers, panusimilins A-D (<b>1</b>-<b>4</b>) (racemic mixtures), and two previously undescribed chromanes (<b>5a</b>/<b>5b</b> and <b>6a</b>/<b>6b</b>) were isolated from cultures of basidiomycete <i>Panus similis</i> TBRC-BCC 52578. Interestingly, synthetic compounds closely related to panusimilins A-C (<b>1</b>-<b>3</b>) were previously reported, which were produced by Lewis acid promoted dimerization of a plant-derived chromene, precocene II. In the present study, panusimilins were isolated from the culture broth extract of the fungus <i>P. similis</i>. The chromane monomers were shown to be a non-racemic mixture of enantiomers, and their absolute configurations were elucidated by conversion to Mosher ester derivatives. The isolated compounds were evaluated for their cytotoxic activities. Among them, 2,2-dimethyl-3,4,6-trihydroxychromane (<b>6a</b>/<b>6b</b>) showed selective activity to NCI-H187 cells (IC₅₀ 9.1 μM). On the other hand, panusimilin B (<b>2</b>) exhibited antioxidant activity in the DPPH radical scavenging assay (IC₅₀ 66 μM).