CONFORMATIONAL FLEXIBILITY OF 6-MEMBERED 1,2-DIHYDROCYCLES BASED ON THE RESULTS OF AM1 CALCULATIONS

The conformational flexibility of 2H-pyran, 1,2-dihydropyridine, 2H-th iopyran, their ore, imino, and methylene derivatives, and the 5-oxo, i mino, and methylene derivatives of 1,3-cyclohexadiene was studied by t he semiempirical quantum-chemical AMI method. All compounds have plana r equilibrium conformations and similar electronic structures. The tra nsition to a distorted sofa conformation with the =C-X-C(=Y)-C= torsio n angle (X = CH2, NH, O, or S; Y = H-2, O, NH, or CH2) of +/-20 degree s increases the energy of the molecule by less than 1.5 kcal mol(-1). The reasons for the high conformational flexibility of these dihydrocy cles were analyzed. The nonaromatic character of the cyclic pi-system is the factor that determines the conformational flexibility of the di hydrocycle in ylide derivatives.