The structure of the lichen depsidone hydroxyphysodic acid. A carbon-13 magnetic resonance study

Abstract

Unequivocal evidence for the structure of the lichen depsidone 3-hydroxyphysodic acid has been provided by analysis of long-range carbon-13-hydrogen coupling interactions and carbon-13 chemical shifts. This approach was also used to establish the isocoumarin structure (16) for that of the key decarboxycyclodehydration product obtained by degradation of 3-hydroxyphysodic acid. The breakdown of some carbon-13 additivity effects in the sterically congested ring is discussed. A possible mechanism for the formation of the isocoumarin (16) is outlined.