Two New Diphenylketones and a New Xanthone from Talaromyces islandicus EN-501, an Endophytic Fungus Derived from the Marine Red Alga Laurencia okamurai

Two new diphenylketones (1 and 2), a new xanthone (3), and a known xanthone analogue (4) were isolated and identified from Talaromyces islandicus EN-501, an endophytic fungus obtained from the fresh collected marine red alga Laurencia okamurai. Their structures were elucidated on the basis of NMR spectroscopic and X-ray crystallographic analysis. The joint isolation of benzophenones and xanthones from the same fungal strain supports the biogenesis of xanthones via a benzophenone intermediate. It is worth mentioning that xanthones 3 and 4 have a methyl group at C-6 and C-2, respectively, which is uncommon compared with typical xanthones usually having a methyl group at C-8. Compounds 1-4 exhibited potent antioxidative activities against DPPH (1,1-diphenyl-2-picrylhydrazyl) and ABTS (2,2'-azino-bis(3-ethylbenzothiazoline-6-sulphonate) radicals with IC50 values ranging from 0.58 to 6.92 μg/mL, which are stronger than that of the positive controls BHT (butylated hydroxytoluene) and ascorbic acid. Compounds 1, 3, and 4 also showed inhibitory activities against several pathogenic bacteria.