Thermal reactions of hitherto α-(3-pyridyl)-N-phenylnitrone (1) with mono-substituted electron-rich and electron-neutral dipolarophiles are regio-, and stereo-selective (exo-selective), controlled by LUMO -dipole -HOMO-dipolarophile interaction, and furnish syn-5-substituted-3-(3-pyridyl)-isoxazolidines (5) in high yields. With electron deficient dipolarophiles such as acrylonitrile there is observed a loss of regioselectivity as well as stereoselectivity and the regioselectivity is reversed in reactions with methyl vinyl ketone and methyl acrylate, due to intervention of HOMO-dipole -LUMO-dipolarophile interaction, affording 4-substituted-3-(3-pyridyl)-isoxazolidines (7) as major products. Reactions of nitrone ( 1) with disubstituted dipolarophiles such as methyl methacrylate and ethyl coronate furnish methyl syn-5-methy-3-pyridyl-1-phenylisoxazolidine-5-carboxylate ( 8) and ethyl anti-5-methy-3-pyridyl-1-phenyl-isoxazolidine-4-carboxylate (10), respectively, in high yields. Reaction with N-Phenylmaleimide affords novel isoxazolidino-pyrrolidinediones bearing a 3-pyridyl moiety (11,12). A mechanistic rationalization of the obtained results in terms of electronic, steric and secondary interactions is proffered.