The conformational analysis of saturated heterocycles. Part XXIII. Hexahydropyrimidines

Abstract

The preparations of some N-substituted hexahydropyrimidines are described. Their electric dipole moments have been measured in cyclohexane solution and interpreted in terms of their conformations. The conformational free-energy differences between axial and equatorial N-methyl and N-ethyl groups are a little smaller than in the corresponding N-alkylpiperidines and accord with a repulsion between two axial lone electron pairs of about 0·3 kcal./mole. For N-isopropyl groups the conformational free-energy difference is surprisingly high; a possible explanation is suggested.