CONFORMATIONAL-ANALYSIS OF 5,6-DIHYDROPYRIMIDINE AND ITS DERIVATIVES

The equilibrium geometry and inversion barriers of 5,6-dihydropyrimidi ne, 6,7-dihydroazolopyrimidines with node nitrogen atoms and their alk yl (Me, Et, Pr-i, Bu(t)) and phenyl derivatives were calculated using a molecular mechanics approach. Annelation with azole cycles and the i ntroduction of substituents have a slight effect on the equilibrium co nformation of the dihydrocycle (distorted sofa). Alkyl substutuents at saturated carbons have an essentially equatorial orientation in 5,6-d ihydropyridimine derivatives and are axial in the annelated analogs. O n the other hand, the equatorial conformers are more stable in phenyl derivatives of dihydroazolopyrimidines. Factors determining the relati ve stability of conformers were analyzed.