XY–ZH systems as potential 1,3-dipoles. Part 3. Isoxazolidines from the 1:1 cycloaddition of oximes with dipolarophiles

Abstract

The 2-oximes of 1,2,3-tricarbonyl systems undergo cycloaddition to N-phenylmaleimide to give isoxazolidines. Proton transfer from oxygen to nitrogen, possible via an intramolecular 1,5-H shift, is believed to generate a 1,3-dipole as the reactive intermediate. The products are 1 :1 adducts in contrast to the cycloadducts of simple aldehyde oximes and ketone oximes which incorporate a second molecule of dipolarophile on the isoxazolidine N-atom. Attempts to extend the cycloaddition to several other electronegative olefins were unsuccessful, whilst dihydropyran gave Michael adducts.