The structure of alectoronic acid and related lichen metabolites

Abstract

Spectroscopic evidence is presented to show that the lichen depsidone alectoronic acid is a tautomeric compound which exists predominantly in a novel, six-membered, pseudo-acid (hydroxy lactone) form. The existence of this pseudo-acid tautomer was confirmed chemically by esterification with acidic methanol, whereupon the corresponding pseudo-ester was produced. P.m.r. spectroscopy indicated that the related depsidone, α-collatolic acid, and the lichen depside, microphyllinic acid, exhibit analogous ring-chain tautomerism.