Orthoquinones are antioxidant reaction products of catechol-type polyphenols and are targets of the conjugate addition of thiols as carbonyl compounds. As a result, the phenolic hydroxy group, which is essential for antioxidant activity, is restored by the reaction. It is well known that two phenolic hydroxy groups in an ortho relationship exhibit strong antioxidant activity. Several phenolic acids and flavonoids have been reported to exhibit longer antioxidant effect by this mechanism. Figure Furthermore, a small amount of CARQ was produced, while (BCysM) and new compounds 1 and 2 accumulated as the reaction progressed (Figure , panel A). This trend was even more pronounced in the reaction of CAR with two molar equivalents of BCysM, as shown in Figure , panel B. The reaction products in the radical scavenging-antioxidation of catechol-type polyphenols are orthoquinone derivatives. The oxidative reactivity of CARQ was considered to convert BCysM to disulfide (BCysM) . Figure shows the results of HPLC analysis of the reaction products from the mixture of CARQ and equimolar BCysM and from CARQ alone and BCysM alone, under the same analytical conditions. The reaction results showed that CAR and (BCysM) were formed immediately after mixing CARQ and BCysM, as shown in the time course data on the right side of Figure . These results suggest that CARQ, the product of the antioxidant reaction of CAR, is quickly reduced by BCysM to restore CAR and reexhibits antioxidant activity. In conclusion, CAR should be recognized as a catalytic antioxidant since it exhibited excellent antioxidant properties in the presence of thiols in this study. IsoROS, which showed similar antioxidant properties, could be another catalytic antioxidant. Antioxidant catalysts have been examined by several studies. They are mimics of the catalytic sites of redox enzymes or synthetic compounds containing heavy chalcogen atoms with high redox reactivity. This study is the first to report the catalytic antioxidant activity of diterpenoid polyphenols CAR and isoROS, but not of CA, ROS, and epiROS.