The 1,3-dipolar cycloadditions of 3,4-dihydro-2H-pyrrole 1-oxide, 1, and 2,3,4,5-  tetrahydropyridine 1-oxide, 2, to five, six, and seven membered α,β-unsaturated lactones, 3-6, have been studied under several reaction conditions. In all these reactions exo adducts are obtained as major or exclusive products. The stereochemistry of the cycloadducts has been established using high field NMR techniques. In the series of adducts derived from nitrone 2 a detailed conformational analysis has been performed. Important differences in their conformational behaviour in solution have been found as a function of the lactone size.