Molecular, electronic structure and conformational flexibility of the 1,6-dihydropyrimidine, 4,5-dihydrofuro[2,3-d]pyrimidine and their oxo, imino and methylene derivatives

Abstract

Molecular and electronic structures of 1,6-dihydropyrimidine, 4,5-dihydrofuro[2,3-d]pyrimidine and their oxo, imino and methylene derivatives have been studied using the semiempirical quantum-chemical method AM1. The equilibrium geometry of all molecules is planar. These results were confirmed by X-ray structural investigations of the 1-oxo- and 1-imino derivatives of 2,5-dimethyl-3-(2,2,2-trichloroethyl)-4,5-dihydrofuro[2,3-d]pyrimidine. However, the dihydropyrimidine ring in all compounds possesses a degree of conformational flexibility. The transition from a planar equilibrium conformation to a distorted sofa with the CNCC torsion angle ± 20° causes an increase of energy less than 1.5 kcal mol −1. The factors that determine this property of the dihydroheterocycle (bending strain, non-aromaticity, conjugation, 1,2-allylic strain) have been considered.