Reaction of Aromatic N-Oxides with Dipolarophiles. XIII

In connection with the stereoselective exo cycloaddition in the 1, 3-dipolar reaction of 3, 5-lutidine N-oxide and N-arylmaleimides, the cycloaddition behavior of some nitrones toward N-phenylmaleimide was investigated. In the reaction of cyclic nitrones with N-phenylmaleimide, pairs of isomers were isolated in a crystalline state. The low-temperature nuclear magnetic resonance measurements of the adducts suggested that the compounds are conformational isomers with regard to the ring inversion brought about by the mobility of the lone pair on nitrogen. The observed reactivity and regiochemistry are discussed in terms of frontier molecular orbital considerations.