The synthesis of 4-isoxazolines via 1,3-dipolar cycloaddition of nitrones generated from allenoates and the subsequent thermal rearrangement to pyrroles is reported. The selection of the reaction conditions allowed the isolation of the initial 1,3-dipolar cycloadducts, the 4-isoxazolines, or the pyrrole derivatives. The isomerization of the parent system (4-isoxazoline) was examined by carrying out quantum chemical calculations and corroborated the favourability of the 4-isoxazoline rearrangement to five-membered heterocycles via 2-acylaziridine intermediates.