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.