Subpsoromic acid

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.^[2] The compound has since been reported as a minor metabolite in Herpothallon rubroechinatum^[3] and in Acanthothecis verrucosa.^[4]

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 (δ 2.54 ppm), one methoxy group (δ 3.83 ppm), two meta-coupled aromatic protons, an aldehydic proton (δ 10.56 ppm), and a strongly chelated phenolic hydroxyl (δ 12.36 ppm). 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-dibenzo[b,e][1,4]dioxepin-6-carboxylic acid. The compound's thin-layer chromatography Rf values (0.34–0.38 in standard solvent systems) and HPLC retention index (RI 14; RT ≈ 21.6 min) further permit routine analytical identification.^[2]