E/Z equilibrium in tertiary amides. Part 2: N-acyl-N′-arylhexahydropyrimidines

The 1H and 13C NMR spectroscopic study of a series of novel N-acyl-N′-arylhexahydropyrimidines 1 is presented. Due to hindered rotation around the (O)C-N bond, tertiary amides 1 exist as a mixture of non-separable E/Z diastereoisomers, which show separate signals in the NMR spectra. For some selected derivatives, differential assignment of the 1H resonances of the E/Z rotamers was made on the basis of the magnitude of ASIS (anisotropic solvent induced shifts) effects and confirmed by NOESY. The corresponding 13C signals were unambiguously attributed by HSQC and HMBC experiments. The detailed conformational study of two representative members was performed employing the ab initio RHF/6-311G++ method. The influence of steric and electronic features of the N-substituents on the relative populations of E/Z rotamers is also analyzed.