ELECTRONIC-STRUCTURE AND CONFORMATIONAL FLEXIBILITY OF 1,2-DIHYDROPYRIDINE, 1,2-DIHYDROPYRIMIDINE AND 1,6-DIHYDROPYRIMIDINE, AND THEIR YLIDE DERIVATIVES

Molecular and electronic structures of 1,2-dihydropyridine, 1,2- and 1 ,6-dihydropyrimidine, and their ore, imino, and methylene derivatives were studied by the semiempirical quantum-chemical AM1 method. In all compounds, the heterocycle exhibits a high conformational flexibility. The transition from a planar equilibrium conformation to a distorted sofa conformation with the =C-NH-C-C(N)= torsion angle of +/-20 degree s causes an increase in the energy by less than 1.7 kcal mol(-1). All molecules have similar pi-electronic structures, which, apparently, de termines the similarity in their conformational behavior. The bending strain and the nonaromatic character of the cyclic pi-system are the f actors that stabilize the nonplanar conformation of the ring in unsubs tituted dihydroazines and ylide derivatives, respectively.