Cycloaddition reactions of diazoalkanes with olefinic dipolarophiles have been widely studied because the resulting pyrazolines can be converted into a variety of nitrogen-containing molecules.A catalytic synthetic route to highly functionalized chiral 2-pyrazolines was developed by an asymmetric [3+2] cycloaddition reaction of diazoacetates withβ-substituted acyclic enones.In the presence of chiral (S)-oxazaborolidinium ion as a catalyst, the reaction proceeded in high yield with high to excellent enantioselectivity (up to 99％ ee).