Ab initio study of heterodienophile addition to oxazole

Abstract

The Diels–Alder additions of formaldehyde, formaldehyde imine, (Z)-diazene, (E)-diazene and nitrosyl hydride to oxazole have been investigated by frontier molecular orbital theory and by ab initio methods. The former cannot account for steric and other electronic interactions in the transition states and is of limited value in predicting the reactivity of these dienophiles. The transition state structures of all possible isomers for the concerted cycloadditions were located at the restricted Hartree Fock 3-21G level and the energies evaluated by means of the second-order Møller–Plesset perturbation theory with the 6-31 G* basis set. The heteroatom lone pairs play a decisive role in directing the regioselectivity of these additions.