NEW DEPSIDES FROM LICHENS: MICROCHEMICAL METHODOLOGIES APPLIED TO THE STUDY OF NEW NATURAL PRODUCTS DISCOVERED IN HERBARIUM SPECIMENS

Abstract

A high‐performance liquid chromatographic method originally devised to provide a reproducible measure of retention is used to determine chemical structures of new compounds in lichens. The method relates retention to a standard homologous series so that retention indices of members of other homologous series are linear with carbon number. When two or more homologs are known, retention of new members of the same series can be accurately predicted. Combining this method with other microchemical techniques allows the determination of structures of lichen depsides in small samples of crustose species for which the total material is meager and known only from the herbarium. The conditions are sufficiently mild that chemical decompositions of labile depsides are avoided. Eight new orcinol‐type p‐depsides with seven‐ or nine‐carbon oxidized sidechains on one or both rings are reported from an endemic Japanese species, Haematomma pachycarpum. The nine‐carbon sidechain is the longest ever found in lichen depsides. These new compounds are higher homologs of the known lichen products confluentic, 2‘‐O‐methylperlatolic, 4‐O‐methylolivetoric, and 2‘‐C‐methylmicrophyllinic acids. In addition, an isocoumarin with a nine‐carbon sidechain occurs in some specimens. Haematomma pachycarpum has the most complex secondary‐product chemistry thus far reported for any lichen species. The significance of the technique used here lies in providing a way to characterize new compounds in rare or endangered species, thereby completing data sets for phylogenetic reconstructions.