Photochemical transformations of tropolones. Part IV. The mechanism of the photo-oxidation of tetra-O-methylpurpurogallin

Abstract

Photo-oxidation of tetra-O-methylpurpurogallin in the presence of a sensitizer with a tungsten light source at –77° gave an endo-peroxide. The isolated peroxide was shown to decompose thermally to methyl 3-(5,6,7-trimethoxyphthalidyl)prop-2-enoate (III), the product of the room temperature photo-oxidation. The decomposition was first order in peroxide with a rate constant of 4.7 ± 0.3 sec.^–1. The overall mechanism of this and related reactions is discussed.