Dimeric tetrahydroxanthones from the lichen Usnea aciculifera

Abstract

Four unusual heterodimeric tetrahydroxanthones, usneaxanthones A-D (1-4) were isolated from lichen Usnea aciculifera Vain (Parmeliaceae). Their structures and absolute configurations, particularly the central and axial chiralities, were unambiguously demonstrated by a combination of spectroscopic data (1D, 2D NMR, HRESIMS), electronic circular dichroism (ECD) experiments, and single-crystal X-ray crystallographic analyses. Cytotoxic effects of isolated compounds (1, 2 and 4) were evaluated on HT-29 human colorectal cancer cells. Compound 4 showed potent cytotoxicity against HT-29 with IC₅₀ values of 2.41 μM.

Keywords: Cytotoxicity; Dimeric xanthones; Tetrahydroxanthone; Usnea aciculifera; Usneaxanthones A–D.