Eumitrins F-H: three new xanthone dimers from the lichen Usnea baileyi and their biological activities

The lichen Usnea baileyi is a fruticose lichen belonging to the Usnea genus. It is well known as a rich source of natural xanthone dimers and possesses various bioactivities. Nevertheless, the chemical investigation on this type of lichen is still rare as most of researches reported its components without structural elucidation. Herein, in the continuous study on this type of lichen, we further isolate xanthone dimers from the dichloromethane extract and explore three new xanthone dimers, eumitrins F - H (1 - 3). Their structures were elucidated unambiguously by spectroscopic analyses, including high resolution electrospray ionisation mass spectrometry (HRESIMS), 1 D and 2 D nuclear magnetic resonance spectroscopy (1 D and 2 D NMR), and DP4 probability. All com- pounds were evaluated for their enzyme inhibition against alpha-glucosidase, tyrosinase, and antibacterial activity. They revealed moderate antimicrobial and weak tyrosinase inhibition. For alpha-glucosidase inhibition, compound 3 displayed the most significant inhibitory against alpha-glucosidase possessing an IC50 value of 64.2 mu M.