Abstract The hypothesis which explains the diastereoselectivity of the 1, 3-dipolar cycloaddition of the N-glycosylnitrones 1–3 leading to the 5, 5-disubstituted isoxazolidines 4– 6 on the basis of a kinetic anomeric effect predicts that nucleophiles should add to N- glycosylnitrones with a high degree of diastereoselectivity. To test this prediction, the nucleophilic addition of lithium and potassium dialkylphosphites to the crystalline (Z)- ...