On the roles of protic solvents in imidazolidinone-catalyzed transformations

Step by step: The effect of protic solvents on the rate and stereochemical outcome of the imidazolidinone-catalyzed Diels–Alder cycloaddition was rationalized. The following picture emerges: the solvent accelerates iminium ion formation (step 1), the Diels–Alder cycloaddition is reversible (step 2), and the solvent intercepts the iminium ion adduct (step