(+)-Usnic acid is one of the most common and abundant lichen metabolites, and well known its antibiotic and several interesting properties.^1,2) It is reported that the (+)-usnic acid is effective against a large variety Gram-positive bacterial stains including multi-resistant stains of Streptococcus aureus, enterococci and mycobacteria. (+)-Usnic acid also appears to be against Streptococcus mutans.
On the chemical structure, the absolute configuration of (+)-usnic acid at 9b position has determined by X-ray analysis to be R. ^2,3,4) Of plausible tautomers, the low energy 1-oxo, 3-hydroxy confirmation is preferred according to calculations.^1,2,3) The enolic 3-hydroxy group has the strongest acidic character (pKa 4.4) due to an inductive effect of the keto group.²⁾ Recently, (+)-usnic acid derivatives are used as an organic ligand.⁵⁾