Highly enantioselective (96% ee) and endo-selective (>99:1) cycloaddition reactions were observed between carbonyl ylides, generated from o-( p-bromobenzyloxy)carbonyl-α-diazoacetophenone, and 3-crotonoyl-2-oxazolidinone using (4 S,5 S)-Pybox-4,5-Ph 2-Yb(OTf) 3 (20 mol %) as the chiral Lewis acid catalyst. In contrast, high exo-selectivity ( exo/ endo=82:18; 96% ee, exo) was observed for the reaction of o-methoxycarbonyl-α-diazoacetophenone with 3-acryloyl-2-oxazolidinone under similar conditions as reported previously. In the case of cycloaddition reactions between 2-benzopyrylium-4-olate, generated from o-methoxycarbonyl-α-diazoacetophenone, and 3-cinnamoyl- or 3-[( E)-3-(ethoxycarbonyl)propenoyl]-2-oxazolidinones, using the same chiral Lewis acid, the reaction favored the endo-adduct with relatively good enantioselectivity (72 and 78% ee, respectively).