Δ-Keto-acid/hydroxy-lactone isomerization in some lichen depsides, depsidones and diphenyl ethers

In some compounds in lichens, the carboxylic acid is ortho-substituted by an 2-oxoalkyl chain. This particular structure induces the existence of δ-keto-acid or hydroxy-lactone isomers, clearly identified by their NMR data and chemical properties, such as dehydration, methylation and behaviour in thermal conditions. Internal hydrogen bonding between the carboxylic acid and substituent in the ortho′ position is proposed as an isomerization modulator: an H-bond acceptor (OCH3) leads to isomers, whereas isomers are obtained with an H-bond donor (OH).