Eumitrins C-E Structurally diverse xanthone dimers from the vietnamese lichen Usnea baileyi
Abstract
Three new xanthone dimers, eumitrins C - E (1-3), along with a new depsidone, 3'-O-demethylcryptostictinolide (4) were isolated from the acetone extract of the whole thallus of the lichen Usnea baileyi collected in Vietnam. Their structures were unambiguously established by spectroscopic analyses (HRESIMS, 1D and 2D NMR), as well as comparison to literature data. The absolute configurations of 1-3 were elucidated through electronic circular dichroism (ECD) analyses. The absolute configuration of 2 was validated by comparison between experimental and TDDFT-calculated ECD spectra while that of 3 was based on DFT-NMR calculations and subsequent DP4 probability score. The antiparasitic activities against Plasmodium falciparum as well as the cytotoxic activity against seven cell lines were determined for the new compounds 1-3, and led from null to mild bioactivities.