Usnic acid derivatives from Leprocaulon microscopicum

In addition to known dibenzofuran derivatives such as (-)-usnic acid, (-)-isousnic acid and (-)-placodiolic acid, a Leprocaulon microscopicum acetone extract yielded a new compound, (+/-)-9-O-methylplacodiolic acid in a keto-enol equilibrium focused on the C-ring. Structures were established using mass spectrometry and combination of 1D and 2D NMR spectral data. C-13 assignments of placodiolic acid were revised. Tautomers of the (+/-)-9-O-methylplacodiolic acid were only separated by GC and a thorough fragmentation study confirmed the structural features. To complete the study, evaluation of antiproliferative effects on HT-29 human colorectal cancer cells showed moderate activity for (-)-usnic acid only. (C) 2012 Phytochemical Society of Europe. Published by Elsevier B. V. All rights reserved.