We have investigated the 1,3-dipolar cycloaddition of nitrone 1 to seven, six, and five membered α,β-unsaturated lactones (2, 3, and 4-6) and 2,5-dihydrofuran, 18. In all these reactions, exo adducts are obtained as major or exclusive products. To elucidate the stereochemistry of the obtained heterocyclic compounds, an accurate conformational analysis has been performed. Important differences in their conformational behavior in solution have been found as a function of the lactone ring size and the presence or absence of a carbonyl group. This study points out the existence of an attractive interaction between the nitrogen lone electron pair and a carbonyl group at a fitting distance, which induces the preference for the cis fusion of type E in the furanonic exo adducts (10, 12, 13, 15, and 16). In all the other cases, where this distance is too long (7, 8, 9, 11, 14) or no carbonyl group is present in the molecule (20), the trans invertomers predominate. © 1993.