Oxidative chemistry of the natural antioxidant hydroxytyrosol: hydrogen peroxide-dependent hydroxylation and hydroxyquinone/o-quinone coupling pathways

Oxidation of the natural antioxidant hydroxytyrosol () with peroxidase/H2O2 in phosphate buffer at pH 7.4 led to the formation of two main ethyl acetate-extractable products. These could be isolated by preparative TLC after reduction and acetylation, and were identified as the tetraacetyl derivative of 2-(2,4,5-trihydroxyphenyl)ethanol () and the heptaacetyl derivative of the pentahydroxybiphenyl by 2D NMR and MS analysis. Similar oxidation of 4-methylcatechol gave, after the same work-up, the acetylated derivatives of 1,2,4-trihydroxy-5-methylbenzene () and the pentahydroxybiphenyl . Mechanistic experiments suggested that hydrogen peroxide affects the course of the oxidation of by adding to the first formed o-quinone to give a hydroxyquinone intermediate. This could bring nucleophilic attack to the o-quinone of to give the dimer . These results disclose novel oxidative pathways of 4-alkylcatechols and provide an improved chemical basis to enquire into the mechanism of the antioxidant action of .