Usnic Acid Derivatives with Cytotoxic and Antifungal Activities from the Lichen Usnea longissima

Eight usnic acid derivatives, that is, usenamines A–F (1–6), usone (7), and isousone (8), together with the known (+)-usnic acid (9), were isolated from the lichen Usnea longissima. Their structures were elucidated using 1D and 2D NMR and MS data, and the absolute configurations of compounds 1 and 2 were defined by single-crystal X-ray diffraction analyses. Compounds 1, 2, and 8 showed inhibitory effects on the growth of human hepatoma HepG2 cells with IC50 values of 6.0–53.3 μM compared with methotrexate as the positive control, which had an IC50 value of 15.8 μM. Furthermore, 1 induced apoptosis of HepG2 cells in a dose-dependent manner at concentrations of 0–15.0 μM. The isolated compounds were also evaluated for their antifungal and antibacterial activities, with 7 and 8 exhibiting weak inhibitory effects on fungal Trichophyton rubrum spp. with an MIC value of 41.0 μM.