Potassium tert-butoxide-catalysed oxygenations of vitamin E and its model compound 2,2,5,7,8-pentamethylchroman-6-ol

Abstract

Oxygenation of vitamin E [1a, (RRR)-α-tocopherol] in tetrahydrofuran in the presence of potassium tert-butoxide under oxygen gave products 2a, 3a, 4a, 5a and 6a arising from oxidation of the aromatic moiety. Under similar conditions, a vitamin E model compound, 2,2,5,7,8-pentamethylchroman-6-ol 1b, gave the analogous products 2b, 3b, 4b, 5b and 6b. Initial attack at the 5 position leads to the acyloin 6b, which is converted into the isomer 5b. The hydroperoxide 7b is derived from the acyloin 5b and transformed into the 7-methylene compound 3b. The 8-methylene compound 2b is converted into the carbolactone 4b. The molecular structures of compounds 2b, 3b, 4b and 7b were confirmed by X-ray crystallographic analysis. Possible reaction pathways for the product formation and relationships between the product distribution and the basicity of reaction media are discussed.