Usimines A-C, bioactive usnic acid derivatives from the Antarctic lichen Stereocaulon alpinum.

Usimines A-C ( 1- 3), three new usnic acid derivatives, have been isolated from a MeOH extract of the Antarctic lichen Stereocaulon alpinum by various chromatographic methods. The structures of 1- 3 were determined by analysis of their spectroscopic data (NMR, UV, MS) and by chemical methods. The known compound usnic acid ( 4) was also obtained. Compounds 1- 4 showed moderate inhibitory activity against therapeutically targeted protein tyrosine phosphatase 1B (PTP1B).