Catalytic asymmetric aldol reactions in aqueous media have been developed using Pr (OTf) 3 and chiral bis-pyridino-18-crown-6 1. In the asymmetric aldol reaction using rare earth metal triflates (RE (OTf) 3) and 1, slight changes in the ionic diameters of the metal cations greatly affected the diastereo-and enantioselectivities of the products. The substituents (MeO, Br) at the 4-position of the pyridine rings of the crown ether did not significantly ...