4-Regioisomeric 4-isoxazolines are selectively formed by 1,3-dipolarcycloaddition of N-arylnitrones to substituted alkynes. The reactivity and regioselectivity of the addition process are interpreted by the application of FMO principles. Secondary orbital interactions between reactants affect the transition state for the 4- and 5-regioisomeric 4-isoxazolines, while the steric effect is limited as