A new xanthone from the lichen Parmotrema praesorediosum (Parmeliaceae)

Two xanthone compounds, praesorexanthone A (1) and praesorexanthone B (2), were isolated from the lichen Parmotrema praesorediosum (Nyl.) Hale (Parmeliaceae). Among them, praesorexanthone B is a new natural compound. Their chemical structures were unambiguously determined by the analysis of 1D and 2D NMR and high resolution ESI mass spectroscopic data, as well as by comparison of their NMR data with the ones in the literature.