On the dimers of β-tocopherol

Upon oxidation in aprotic media, β-tocopherol (2) forms a spiro-dimer (10) as the main product. The reaction mechanism is a hetero-Diels-Alder process with inverse electron demand of two intermediate ortho-quinone methide molecules. The spiro-dimer can be reduced to the corresponding symmetric ethano-dimer (11). In contrast to the well-studied α-tocopherol case, spiro-dimer and ethano-dimer do not form a reversible redox pair, their interconversion is accompanied by coupling reactions at C-7 with 7a-(β-tocopher-5a-yl)-β-tocopherol (13) as the main byproduct besides some oligomeric material. The full NMR assignments (1H, 13C) of the β-tocopherol oxidation products are given.

Graphical abstractUpon oxidation, β-tocopherol forms a spiro-dimer as the main product, which can be reduced to the corresponding ethano-dimer. In contrast to the α-tocopherol case, the interconversion between the two dimers is not quantitative, but accompanied by coupling reactions at C-7. The bond fluctuation of the β-tocopherol spiro-dimer is slightly slower than that of the α-counterpart.Download full-size image