THE EFFECT OF THE AZOLE RING ON THE CONFORMATIONAL MOBILITY OF THE DIHYDROPYRIMIDINE RING IN 4,7-DIHYDROAZOLO[1,5-A]PYRIMIDINES AS JUDGED FROM MNDO CALCULATIONS

The equilibrium geometries of 4,7-dihydropyrazolo[1,5-a] pyrimidine, 4 ,7-dihydrotetrazolo[1,5-a]pyrimidine, 4,7-dihydro-1,2,4-triazolo[1,5-a ]pyrimidine and 1,4-dihydropyrimido[1,2-a]imidazole were calculated by the semiempirical quantum-chemical MNDO method. The dihydropyrimidine ring exhibits high conformational mobility in all of these compounds. The change in the energy occurring in the transition of the molecule to the boat conformation with an angle between the planar fragments of +/-20 degrees does not exceed 1 kcal mol(-1). The mobility of the dih ydro ring increases as interactions between the pi-systems of the azol e ring and the C=C double bond separated by an imino group and a methy lene bridge decrease.