Mechanistic insights into the rearrangement of azetidine N-oxides to isoxazolidines

Various functionalized azetidines were oxidized with mCPBA or hydrogen peroxide, to produce the corresponding N-oxide and study its fate in the [1,2] Meisenheimer rearrangement. This ring expansion leading to isoxazolidines occurs readily, without trapping of the transient N-oxide. Starting with azetidines bearing a nitrile or an ester group at C-2, the rearrangement is regioselective. However, a varying amount of epimerization on the migrating radical is observed, which can also be observed with the related [1,2] Stevens rearrangement.