Zirconocene-catalyzed cationic Diels-Alder reactions

Article abstract of DOI:10.1016/0040-4020(94)01140-U

In situ prepared Cp₂ZrCl⁺ catalyzes the formation of dioxolenium ions from α,β-unsaturated epoxy esters. As a consequence of this activation process, acrylate, methacrylate and crotonate derivatives undergo a rapid and stereoselective cationic [4+2] cycloaddition with a wide range of dienes. Ring-extended carboxylic acid derivatives are formed in 1-7 h at 0-21°C and in 50-90% yield after saponification of the intermediate diol esters. Simple Lewis acid catalysis by Cp₂ZrCl⁺ can be excluded on the basis of the experimental results.