Subpsoromic acid


Subpsoromic acid is a depsidone with the molecular formula C17H12O8 that was originally isolated from the lichen species Ocellularia praestans. It belongs to a small subclass of depsidones that combine a β-orcinol A-ring with an orcinol B-ring; few other natural products share that architectural motif.

Natural occurrence and isolation

The compound was discovered during a thin-layer chromatographic survey of O. praestans thalli collected on tree bark in Brazil's coastal Restinga forest. Soxhlet extraction of 0.39 g of dried material with anhydrous diethyl ether, followed by preparative-layer chromatography, yielded 2.4 mg of pure subpsoromic acid—about 0.6 % of the dry thallus mass. The product formed colourless crystals that decomposed above 350 °C, and its homogeneity was confirmed by high-performance liquid chromatography and proton nuclear [magnetic resonance]. The compound has since been reported as a minor metabolite in Herpothallon rubroechinatum and in Acanthothecis verrucosa.

Structure and chemical properties

High-resolution mass spectrometry established the molecular formula and showed that subpsoromic acid contains one fewer methylene unit than the better-known psoromic acid. One- and two-dimensional NMR spectra revealed a single carbon-bound methyl group, one methoxy group, two meta-coupled aromatic protons, an aldehydic proton, and a strongly chelated phenolic hydroxyl. These data, together with diagnostic mass spectral fragments at m/z 179 and 177 arising from the A-ring, support the structure 4-formyl-3-hydroxy-8-methoxy-1-methyl-11-oxo-11H-dibenzodioxepin-6-carboxylic acid. The compound's thin-layer chromatography Rf values and HPLC retention index further permit routine analytical identification.