Enantiopure chiral amidic derivatives of sinapic acid were oxidised with hydrogen peroxide using horseradish peroxidase (HRP) as the catalyst to give the aryltetraline dilignol thomasidioic acid. Trans-diastereoselectivity and enantioselectivity in the formation of thomasidioic acid was observed. Computational methods show that the enantioselectivity is controlled by the beta-beta oxidative coupling step, while the diastereoselectivity is controlled by the stability of the reactive conformation of the intermediate quinomethide.