N,O-heterocyclics13;-14conversion of isoxazolidines into α,β enones

Substituted isoxazolidines formed by 1,3-dipolar cycloaddition of nitrones to alkenes undergo ring-opening elimination to α,β-enones when treated with trimethyl phosphate. The reaction involves initial alkylation giving the isoxazolidinium intermediate which collapses to the α,β-enone by a Hofmann-like elimination having an orientation controlled by electronic factors, the first step being rate-determining.