1,2,4-Oxadiazole-4-oxides as nitrones in 1,3-dipolar cycloaddition reactions to vinyl ethers

1,2,4-Oxadiazole 4-oxides display nitronic reactivity and selectivities identical to those of N-methyl-C-phenyl nitrone, which is a typical acyclic nitrone, affording comparable amounts of endo- and exo-5-alkoxyisoxazolidines. The exo stereoisomers undergo an easy rearrangement under the reaction conditions to yield oxadiazolinic esters. The structures of the adducts have been confirmed by X-ray structures and spectroscopic data. A donor p-methoxyphenyl at the nitronic carbon slows down the cycloaddition rate, while an acceptor p-nitrophenyl retards the rearrangement of the exo adduct.