Trapped Cycloadducts of 1-Azabutadienes via Microwave-Assisted Ring Opening of N -Acyl-2-azetines

-acyl-2-azetines can occur under microwave irradiation or conventional heating to give the corresponding 1-azabutadiene systems, under more mild and accessible conditions than the previously reported flash vacuum pyrolysis conditions. The intermediate 1-azabutadienes, successfully detected by NMR analysis in the case of the 2-naphthoyl system, subsequently undergo intermolecular Diels-Alder reactions with the parent azetine systems to form heterobicyclic dimers, as confirmed by X-ray crystallography. Alternatively, if the 1-azabutadiene is generated in the presence of an electron-rich dienophile (e.g., enol ether or enamine), dimer formation is partially or fully suppressed, giving the corresponding cycloadducts instead. These observations are in agreement with the included theoretical calculations.