Oxygenations of Vitamin E (α-Tocopherol) and Its Model Compound. 2,2,5,7,8-Pentamethylchroman-6-ol, in the Presence of Potassium Superoxide Suspended in Tetrahydrofuran and Unusual Acyloin Rearrangements

Article abstract of DOI:10.1021/jo00359a011

In the presence of potassium superoxide (KO2) suspended in tetrahydrofuran under an oxygen atmosphere, α-tocopherol (vitamin E) (1) was converted to 6-hydroxy-2,6,7,8-tetramethyl-2-(4',8',12'-trimethyltridecyl)chroman-5(6H)-one (2) and 5-hydroxy-2,5,7,8-tetramethyl-2-(4',8',12'-trimethyltridecyl)chroman-6(5H)-one (3), and a vitamin E model compound, 2,2,5,7,8-pentamethylchroman-6-ol (4), to 6-hydroxy-2,2,6,7,8-pentamethylchroman-5(6H)-one (5) and 5-hydroxy-2,2,5,7,8-pentamethylchroman-6(5H)-one (6).By the use of oxygen-18, molecular oxygen was found to be incorporated into a ketonic group in 5 and a hydroxy group in 6.Since <6a-CD3>5 was derived from <5a-CD3>4 and since 3 and 6 were converted to 2 and 5, respectively, in the presence of KO2 quantitatively, an acyloin rearrangement is shown to occur in the formation of the end products, 2 and 5.On treatment of potassium hydroxide suspended in tetrahydrofuran, 4 gave 5 though its yield was low.Thus, the intermediates, 3 and 6, are considered to result from the base-catalyzed oxygenations of 1 and 4, respectively.The striking characteristic of the KO2-catalyzed reactions is that the products are obtained in high yield and, in particular, 5 and 6 in quantitative yield.A possible mechanistic scheme for the KO2-catalyzed oxygenation of vitamin E is proposed.Due to the basicity of KO2, 1 gives rise to α-tocopherolate.The reaction of the α-tocopherolate with molecular oxygen leads to its derivative bearing a hydroperoxy group at C5.The O-O bond cleavage of the hydroperoxide, followed by hydrogen abstraction, affords 3, which undergoes acyloin rearrangement to yield 2.