Heteroelectrocyclic reactions of 3-acyl-4-azido heterocycles: ab initio and density functional calculations on 3-azido-propenal as a model system

 Concerted as well as stepwise reaction pathways for cyclization of 3-azido-propenal to isoxazole have been investigated by density functional (B3LYP) and ab initio methods up to CCSD(T)/cc-pVQZ methods. These calculations clearly establish the pathway with concerted albeit asynchronous nitrogen extrusion and ring closure as the most feasible mechanism. Barriers for cyclization increase in the order Hartree–Fock