Redox-Tag-Guided Radical Cation Diels-Alder Reactions: Use of Enol Ethers as Dienophiles

Although radical cation Diels-Alder reactions enable the formation of cyclohexene ring systems between electronically mismatched (both electron-rich) dienes and dienophiles, which is otherwise difficult or impossible to achieve under thermal conditions, the substrate scope has been limited. Herein, we disclose that a radical cation Diels-Alder reaction using an enol ether as an electron-rich (and therefore oxidizable) dienophile is possible through a rationally designed redox tag strategy. Electrochemical and TiO2 photochemical approaches are effective in driving the reaction, where both intermolecular and intramolecular electron transfers are the key.