2,2´-Azobis(2,4-dimethylvaleronitrile) (AMVN) was purchased from FUJIFILM Wako (Osaka, Japan). Ethyl linoleate was purchased from Kanto Chemical (Tokyo, Japan) and utilized after purification using Florisil (FUJIFILM Wako) eluted with hexane. A mixture of ethyl linoleate hydroperoxide isomers was obtained through the air oxidation of ethyl linoleate according to the method described by Terao and Matsushita. 9 N-Benzoylcysteine methyl ester (BCysM) and N,N´-dibenzoylcystine dimethyl ester [(BCysM) 2 ] were synthetically prepared using the previously reported method. The organic solvent extract of rosemary prepared of rosemary leaves was provided by Mitsubishi Chemicals (Yokohama, Japan). Carnosol (CAR) and carnosic acid (CA) as well as the quinone derivative of CAR (CARQ) were prepared according to the reported methods. All solvents and other reagents of extra pure or high-performance liquid chromatography (HPLC) grade were obtained from Nacalai Tesque (Kyoto, Japan). Nuclear magnetic resonance (NMR) spectra were recorded on a JNM-ECZ400S spectrometer (400 MHz; JEOL, Tokyo, Japan), while mass (MS) spectra were recorded on a JMS-T100 spectrometer (JEOL) using direct analysis in real-time (DART) and time-of-flight measurement modes. The molecular formulas of the compounds were obtained from high-resolution mass spectrometry (HR-MS) data using ChemCalc. Analytical HPLC was performed on the reaction products using a PU-2089 quaternary gradient pump (JASCO, Tokyo, Japan) equipped with an MD-2018 photodiode array detector (JASCO). The data were analyzed using ChromNAV software (ver.1.19.02, JASCO). The HPLC system used for lipid peroxide analysis consisted of an LC-10AD pump and an SPD-10Avp UV detector (Shimadzu, Kyoto, Japan). The obtained data were analyzed using ChromNavi Lite (v.2.04.00, JASCO). Preparative HPLC was performed using an LC-6AD pump (Shimadzu) equipped with an SPD-6A UV detector (Shimadzu). (Detailed information for analytical instruments and conditions can be found in section 1 of Supporting Information.) in biological systems, and their effective antioxidant activity has been observed in various assay systems, however, their efficacy depends on the radical species used, the measurement system, and other conditions. In 2014, we discovered that N-acylcysteine esters do not exhibit antioxidant properties against lipid oxidation but cysteine derivatives with free αamino or carboxylate groups strongly inhibit AMVN-induced linoleic acid oxidation, because of the formation of an active thiolate anion by an intramolecular proton shift to α-amino or carboxylate. Comparing bond dissociation enthalpy (BDE) of S-H (ca. 86 kcal/mol) in N-acylcysteine esters to that of hydroperoxides (OO-H, ca. 88 kcal/mol) indicated that the thiol group of BCysM cannot exhibit effective antioxidant activity through lipid peroxyl radical trapping, but rather that degradation began with the withdrawal of a hydrogen atom at α-position with lower BDE (ca. 80 kcal/mol). Thiols are also known to have high nucleophilicity and can be easily conjugate-added to carbonyl compounds.