Rates of Diels-Alder cycloadditions of anthracenes with methyl vinyl ketone (MVK) are accelerated significantly by the presence of scandium triflate [Sc(OTf)(3)], Sc(OTf)(3) also promotes photoinduced electron-transfer reactions from various electron donors to MVK significantly. Comparison of the promoting effect of Sc(OTf)(3) in photoinduced electron-transfer reactions of MVK with the catalytic effect of Sc(OTf)(3) in the Diels-Alder reaction of 9,10-dimethylanthracene with MVK has revealed that the MVK-Sc(OTf)(3) complex is a reactive intermediate in both the Diels-Alder and photoinduced electron-transfer reactions. The observed second-order rate constants of the Sc(OTf)(3)-Catalyzed Diels-Alder reactions of anthracenes with MVK are by far larger than those expected from the observed linear Gibbs energy relation for the Diels-Alder reactions of anthracenes with stronger electron acceptors than MVK, which are known to proceed via electron transfer. This indicates that the Sc(OTf)(3)-catalyzed Diels-Alder reactions of anthracenes with MVK does not proceed via an electron-transfer process from anthracences to the MVK-Sc(OTf)(3) complex.