Relative stability of 5-methyl- and 3,5-dimethyltetrahydro-1,3-oxazine conformers

Abstract

According to HF and DFT quantum chemical calculations and ¹H NMR data, the chair–chair conformational equilibrium of 5-methyltetrahydro-1,3-oxazine shifts toward the conformer with equatorial orientation of the methyl group on C⁵ and axial orientation of the NH proton. 3,5-Dimethyltetrahydro-1,3-oxazine exists preferentially as chair conformer with equatorial orientation of the 5-methyl group; orientation of the N-methyl group almost does not affect the conformational equilibrium.