The methylenedioxyphenyl moiety in the structure of sesamin and episesamin was changed into the catechol moieties, (1R,2S,5R,6S)-6-(3,4-dihydroxyphenyl)-2-(3,4-methylenedioxyphenyl)-3,7-dioxabicyclo[3,3,0]octane, (1R,2R,5R,6S)-2-(3,4-dihydroxyphenyl)-6-(3,4-methylenedioxyphenyl)-3,7-dioxabicyclo[3,3,0]octane, (1R,2R,5R,6S)-6-(3,4-dihydroxyphenyl)-2-(3,4-methylenedioxyphenyl)-3,7-dioxabicyclo[3,3,0]octane, (1R,2S,5R,6S)-2,6-bis(3,4-dihydroxyphenyl)-3,7-dioxabicyclo[3,3,0]octane, and (1R,2R,5R,6S)-2,6-bis(3,4-dihydroxyphenyl)-3,7-dioxabicyclo[3,3,0]octane, in supercritical water. These products had same structures as the sesamin metabolites which act as antioxidants in the liver. These features suggested the direct preparation of antioxidants from sesamin by a one-step reaction using supercritical water.