Antifungal xanthones from Calophyllum brasiliensis heartwood

The heartwood of the tropical tree Calophyllum brasiliensis is known to be highly resistant to fungi and termites. To determine whether resistance to wood-rotting fungi could be caused by bioactive secondary metabolites, a chemical and biological study was carried out. Hexane, acetone, methanol, and water extracts were prepared. The yield of the extracts ranged from 0.04% (hexane) to 4.81% (acetone). Methanol, acetone, and water extracts (5 mg/ml = 0.5%) inhibited the mycelial growth of the brown rot fungus Postia placenta by 83%, 59%, and 21%, respectively. Chromatographic separation of the acetone and methanol extracts afforded five prenylated xanthones: 6-desoxyjacareubin (I), 1,5-dihydroxy-2-(3,3-dimethylallyl)-3-methoxy-xanthone (II), jacareubin (III) and 1,3,5-trihydroxy-2-(3,3-dimethylallyl)-xanthone (IV) and 1,3,5,6-tetrahydroxy-2-(3,3-dimethylallyl)-xanthone (V). Xanthones III, IV, and especially V, were the most abundant constituents of both extracts and inhibited at 0.25 mg/ml the mycelial growth of P. placenta. Inhibitory activity ranged from 55.5% (V) to 68.8% (III and IV mixture). Acetylation of xanthones did not induce a sharp change in the extent of fungistasis compared with parent compounds. The above results suggest that C. brasiliensis xanthones actually play a defensive role against wood decay fungi.