Abstract
(+) Usnic acid (2,6-diacetyl-7,9-dihydroxy-8,9b-dimethyl-1,3(2H, 9bH)-dibenzofurandione), an antibiotic, was found in large amount (about 1% based on dry weight) in the Saudi-Arabia lichen Usnea articulata which grows in abundance on the Asir mountains of the South-Western region. This finding was confirmed by various chemical means including mass spectra, NMR, IR, UV spectrometry and polarimetry. Atranorin (3-formyl-2,4-dihydroxy-6-methylbenzoic acid 3-hydroxy-4-(methoxycarbonyl)-2,5-dimethylphenyl ester), of a less known medical importance, was found in a lesser quantity (about 0.1%) in the edible lichen Parmelia tinctorum which is consumed as a food spice in some Arab countries (called in Arabic- Al-Sheba). The presence of atranorin, which is known to be a food deterrent for insects and other herbivores, was also confirmed by the preceeding methods of chemical analysis.