Abstract: The 4+2 and 2+2 cycloaddition reactions of formylthioketen e and methylenimine have been studied by using ab initio method at HF/6?31G  l evel.All the geometries of the stationary points on the reaction path have been o ptimized with the energy gradient technique.The single point energy calculations of the stationary points have been performed by MP2/6?31G  method.All the t r ansition states were characterized by vibration frequency analysis.The 4+2 react ion (1) formatting six?membered heterocyclic...