The conformational analysis of saturated heterocycles. Part XLIX. The conformation of ring NH-groups in piperazines, hexahydropyrimidines, tetrahydro-1,3-oxazines, and tetrahydro-1,3-thiazines
Abstract
Infrared spectral measurements of the νNH first overtone and electric dipole moment measurements indicate that N–H in a piperazine ring prefers the equatorial position by about the same amount as in piperidine. However, where another heteroatom is in the β-position, as in tetrahydro-1,3-oxazines, tetrahydro-1,3-thiazines, and hexahydropyrimidines, there is a strong preference for NH to become axial. The reasons for this behaviour are discussed