Scensidin, a new depsidone from the lichen Buellia canescens(Dicks.) De Not

Abstract

The biogenetically favoured constitution (4) for scensidin isolated from Buellia canescens has been established by total synthesis. Intramolecular oxidative coupling of the tetrahydroxybenzophenone (22) yields the grisadienedione (23) which, by thermal isomerisation (23)→(25) followed by methylation, yields scensidin (4). Alternatively, methylation of the grisadienedione (23) followed by thermal isomerisation also yields scensidin.

The transformation of the grisadienedione (26) into the dibenzofuran (28) occurs both thermally and photochemically.