Abstract:
The investigation of chemical constituents of lichen Usnea baileyi (Stirt.) Zahlbr. led to the isolation of ten new bisxanthones (US1−3, 5-11) and a new depsidone (US4). The structures were unambiguously established by the spectroscopic evidence including HRESIMS, 1D and 2D NMR, as well as comparison to literature data. Moreover, the absolute configurations were elucidated through ECD analyses, DFT-NMR calculations and subsequent DP4 probability score. The biological activities of isolated bisxanthones were evaluated for antiparasitic, cytotoxic (US1-3), antibacterial, and enzymatic inhibitory (tyrosinase and α-glucosidase) (US5-11) activities. The results revealed null to mild bioactivities against Plasmodium falciparum (antiparasitic activity) as well as cytotoxic activity against seven cell lines. US5 exhibited good antibacterial activity against Escherichia coli ATCC25922 and Bacillus subtilis ATCC6633 (MIC 62.5 mg/mL for each bacteria). In addition, US6, the same co-structure as US5 revealed good activity against B. subtilis (MIC 62.5 mg/mL). US10 and US11 displayed better activity on α-glucosidase than a positive compound, acarbose with IC50 values 83, 64, and 94 mM, respectively.
Moreover, 11 derivatives of usnic acid derived from Dakin oxidation (UD1-5) and esterification (UE1-6) were prepared, characterized and evaluated for tyrosinase and α-glucosidase inhibitory activities. Interestingly, UD2, UD5, UE5, and UE6 displayed good anti- α-glucosidase activity with IC50 43, 91, 27, and 69 µM, respectively.
