Photochemical transformations of tropolones. Part III. The photo-oxidation of tetra-O-methylpurpurogallin

Abstract

Irradiation of methanolic tetra-O-methylpurpurogallin with a sensitiser under oxygen, using a tungsten light source, gave a single γ-lactone, methyl 3-(5,6,7-trimethoxyphthalidyl)prop-2-enoate. A greatly enhanced rate of reaction was obtained using a new solvent system incorporating carbon disulphide. Likewise tri-O-methylpurpurogallin and 4′,5′-dimethoxy-6,7-benztropolone methyl ether gave the same transformation. Benztropolones have been shown to undergo Diels–Alder reactions. An analogous reaction with singlet oxygen is considerd to be the initial chemical reaction in the photo-oxidation.