The Joint Occurrence of Chloroxanthones in Lichens, and a Further Thirteen New Lichen Xanthones

Abstract

The following new xanthones are shown to co-occur in various combinations with other known naturally occurring xanthones: 2-chloronorlichexane (in Lecanora populicola Lecanora sp.), 7-chloronorlichexanthone (in Lecanora populicola, Lecanora sp.), 5-chloro-6-O- methylnorlichex- anthone (in Lecanora contractula), 5-chlorolichexanthone (in Lecanora contractula), 7-chloro-6- 0- methylnorlichexanthone (in Lecanora populicola, L. salina, L. sp.), 2,4-dichloronorlichexanthone (in Lecidella vorax), 2,7-dichloro-3-O-methylnorlichexanthone (in Lecanora behringii, L. salina, L. sp.), 2,7-dichloro-6-O-methylnorlichexanthone (in Lecanora behringii, L. populicola, L. salina, L. sp.), 2,5, 7-trichlorolichexanthone (in Lecanora broccha), 4,5, 7-trichloro-6-O-methylnorlichex- anthone (in Pertusaria pycnothelia var. A), 3-O-methylthiophanic acid (in Lecidella meiococca), and 6-O-methylthiophanic acid (in Micarea isabellina). Griseoxanthone C (3-0-methylnorlichex- anthone) is reported for the first time from a lichen (Lecanora vinetorum). The joint occurrence of these chloroxanthones has been rationalized in biosynthetic terms and represented in a series of pathway diagrams. All were found to constitute coherent arrays of major xanthones and biose- quentially related satellite derivatives. In so doing we have established the methodologyfor predicting expected co-occurrences and completing similar data sets for other xanthone-containing species. The many secondary metabolites found in the lichen-forming fungi continue to play a dominant role in the systematics of these organisms because of the extensive parallels with morphology and their clear ecological significance. Of the varied structural types present, the aromatic polyketides are especial- ly well represented, particularly the depsides, dep- sidones, and dibenzofurans derived biosynthetically from two acetyl-polymalonyl chains (Culberson & Elix 1989; Mosbach 1969). Anthraquinones and xanthones are also common lichen metabolites, but these substances derive from a single polyketide chain and often occur in or are very closely related to products in the nonlichen fungi. Despite their common occurrence in a number of important gen- era, lichen xanthones have not featured prominently in the repertoire of lichen taxonomists for several reasons. Difficulties were experienced in distin- guishing the numerous (and often isomeric) com- pounds involved, a problem compounded by a number of structural misdeterminations in earlier