Dye-sensitized photooxidation of vitamin E revisited. New 7-oxaspiro(4,5)dec-1-ene-3,6-dione product by oxygenation and ring contraction of alfa-tocopherol

The dye-sensitized photooxidation of alpha-tocopherol, the major component of the biological antioxidant vitamin E, was reinvestigated under biomimetic conditions. When methylene blue was used as the sensitizer and ethanol-phosphate buffer (pH 6.8; 7:3, v/v) as the solvent, the reaction led to the formation, besides alpha-tocopherol qunione (3) and its epoxide (4), of three hitherto unknown photooxygenation products accounting overall for about 15% of the consumed substrate. These were identified as two diastereoisomers corresponding to the gross structure of 1,2,8-trimethyl-4-methylene-8-phytyl-7-oxaspiro[4.5]dec-1-ene-3,6-dione (8) and one diastereomer of the hydrated derivative 4-hydroxy-1,2,4,8-tetramethyl-8-phytyl-7-oxaspiro[4.5]dec-1-ene-3,6-dione (9). Formation of the 7-oxaspiro[4.5]dec-1-ene-3,6-dione system was envisaged as resulting from a ring-contracting rearrangement of an epoxyquinone hemiketal intermediate (10) or a related species arising from singlet oxygenation of alpha-tocopherol