The cycloadditions of nitrone 2 to α,β-unsaturated lactones 3-6 have been studied under several reaction conditions. The adducts derived from pentenolide 5 and hexenolide 6 are the first examples of the pyrano[3,4-d]pyrrolo[1,2-b]isoxazole and oxepino[3,4-d]pyrrolo[1,2-b]isoxazole systems respectively. The stereochemistry of the products has been established using high field nmr techniques. In all cases the major cycloadduct derives from an exo orientation of the transition state. © 1993.