A range of pyrazolidin-3-ones have been prepared and their activity as catalysts for iminium- ion promoted Diels–Alder reactions evaluated. Systematic variation of the C (5)-and N (2)- substituents indicates that the incorporation of an electron withdrawing substitutent at N (2) and either a Ph or CF3 substitution at C (5) results in optimal catalytic activity. The diastereoisomeric resolution of a model C (5)-Ph substituted pyrazolidinone and its ability ...