Cycloaddition extrusion reactions in the preparation of pyrroles. A DFT-AM1 theoretical study

The acetylene addition to oxazolium and N-methyloxazolium oxides with subsequent carbon dioxide elimination from the cycloadduct was studied with hybrid Becke3LYP density functional theory (DFT) employing a 6-31G* basis set. An alternative approach using the Becke3LYP/6-31G* energy evaluation on AM1 geometries is offered as a method of choice for the theoretical study. The geometries and energies of the molecules obtained by these two approaches are compared with one another as well as with the experimental results. The applicability of using the Becke3LYP-AM1 approach for studying these reaction types is discussed.