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

► Identifying the first antioxidation product of rosmarinic acid against lipid oxidation was successful. ► The antioxidation product was an unstable quinone derivative on the 2-oxyphenylpropanoyl moiety of rosmarinic acid. ► The structure determination was carried out by addition of an odourless thiol. ► This method is useful to identify the antioxidation reaction position of various multi-catechol polyphenols.