Radical-Scavenging Reactions of Vitamin E and Its Model Compound, 2,2,5,7,8-Pentamethylchroman-6-ol, in a tert-Butylperoxyl Radical-Generating System

Article abstract of DOI:10.1021/ja00200a042

In a tert-butylperoxyl radical-generating system containing di-tert-butyl diperoxyoxalate (DBPO) and tert-butyl hydroperoxide (BOOH), vitamin E (α-tocopherol: 1) is converted into 4a,5-epoxy-4a,5-dihydro-8a-hydroperoxy-2,5,7,8-tetramethyl-2-(4,8,12-trimethyltridecyl)chroman-6(8aH)-one (2) and 8a-(tert-butyldioxy)-4a,5-epoxy-4a,5-dihydro-2,5,7,8-tetramethyl-2-(4,8,12-trimethyltridecyl)chroman-6(8aH)-one (3), and its model compound 2,2,5,7,8-pentamethylchroman-6-ol (4), is converted into 4a,5-epoxy-4a,5-dihydro-8a-hydroperoxy-2,2,5,7,8-pentamethylchroman-6(8aH)-one (5)and 8a-(tert-butyldioxy)-4a,5-epoxy-4a,5-dihydro-2,2,5,7,8-pentamethylchroman-6(8aH)-one (6).The structures of these compounds were deduced on the basis of the spectral data and, in addition, that of 5 was confirmed by X-ray crystallography.The reactions did not proceed under degassed conditions, so the presence of molecular oxygen was suggested to be a requisite for them.In the system without DBPO, 4 gives 5 and 6 in low yields, and without BOOH, 4 gives its dimers (7a and 8a) and trimer (9a).Further, it was observed that, in the presence of BOOH, 5 was derived from a hydroperoxide, 8a-hydroperoxy-2,2,5,7,8-pentamethylchroman-6(8aH)-one (10), and was transformed into 6.Accordingly, 10 and 8a-hydroperoxy-2,5,7,8-tetramethyl-2-(4,8,12-trimethyltridecyl)chroman-6(8aH)-one (11) may be intermediates of 5 and 2, respectively, and 5 and 2 may be intermediates of 6 and 3, respectively.A possible mechanism is shown for the reaction of 1 with the tert-butylperoxyl radical (BOO^.) under the hydroperoxide-rich conditions.The reaction mechanism is discussed with respect to the chain-breaking reaction of 1 against lipid peroxidation.