Cyclization of nitrone 5a occum at 25" C to give isoxazolidine 6a; 6a is unstable at higher reaction temperatures and rearranges to isoxazolidine 7a presumably via a 1, 3-dipolar cycloreversion to regenerate 5a. Structural assignments for 6a and 7a were made by X-ray diffraction analyses. Only the linearly fused isoxazolidine 7b was obtained by cyclization of nitrone 5b. Isoxazolidine 7b is stable in refluxing benzene but in refluxing methanol ...
