Antioxidation mechanism of rosmarinic acid, identification of an unstable quinone derivative by the addition of odourless thiol

The antioxidation mechanism of rosmarinic acid was investigated. An AIBN-induced oxidation reaction of linoleate in the presence of rosmarinic acid produced one main product. The DPPH reaction of rosmarinic acid also gave two products, one of which corresponded to the product of the lipid system. Direct isolation of the products failed because of their instability. To the DPPH reaction, the addition of 1-dodecanethiol afforded three isolable adducts. Their structures revealed that the reaction produced quinone derivatives on either the caffeoyl or 2-oxyphenylpropanoyl moieties. However, the lipid oxidation afforded one quinone, which could convert to a thiol adduct. The HPLC of the reaction clarified that the antioxidation product of rosmarinic acid was the unstable quinone on the 2-oxyphenylpropanoyl moiety. This quinone formation was the first stage of the antioxidation of rosmarinic acid in the lipid system. The odourless 1-dodecanethiol is a useful reagent to identify the unstable quinone from polyphenol oxidation.