Chromene Dimers from Cultures of Basidiomycete Panus similis

Four chromene dimers, panusimilins A-D (1-4) (racemic mixtures), and two previously undescribed chromanes (5a/5b and 6a/6b) were isolated from cultures of basidiomycete Panus similis TBRC-BCC 52578. Interestingly, synthetic compounds closely related to panusimilins A-C (1-3) 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 P. similis. 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 (6a/6b) showed selective activity to NCI-H187 cells (IC₅₀ 9.1 μM). On the other hand, panusimilin B (2) exhibited antioxidant activity in the DPPH radical scavenging assay (IC₅₀ 66 μM).