1H and 13C NMR study of the conformations of the atropisomers of some 1-(1 1-naphthyl)-2,4-dioxo-(or 2-thio-4-oxo)-hexahydro-pyriminides

Abstract

The 1H and 13C NMR spectra of the atropisomers of 6-(or 5-)-methyl-1-(1 1-naphthyl)-2, 4-dioxo-(or 2-thio-4-oxo)-hexahydro-pyrimidines, 1 to 4, are assigned on the basis of NOE and double resonance experiments. The syn-anti configurations of the isomers of 6-methyl-2,4-dioxo-pyrimidine follow from the close similarity of the NMR parameters to those observed with the isomers of the 6-methyl-2-thio derivative of known configuration. That of the 5-methyl derivatives was assigned from NOE and magnetic anisotropy effects of the naphthalene ring and of the (thio) amide group. Allylic strain between the naphthyl and 5-methyl groups makes the axial confomation prefered int he anti isomers. In the syn isomers, however, the equational and axial conformations are approximately equally populated, apparently due to interference between the remote benzene ring and the axial methyl group. The equatorial conformation is prepared with the 5-methyl derivatives.