Methyl 3-amino-2,3,6,-trideoxy-L-hexopyranosides in DFT level theory conformational studies

Geometry optimizations at the B3LYP level of density functional theory are reported for the (1)C4 and (4)C1 conformations of four theoretically possible alpha and beta methyl 3-amino-2,3,6-trideoxy-L-hexopyranosides. The Gibbs free energies, relative Gibbs free energies, and geometry parameters are presented for all the optimized structures. Conformational analysis of the pyranose ring is performed for each stereoisomer on the basis of calculated rotamer populations. It is demonstrated that the alpha/beta-L-arabino, alpha/beta-L-lyxo, and alpha-L-ribo stereoisomers adopt the (1)C4 conformation, whereas beta-L-ribo and alpha/beta-L-xylo stereoisomers remain in (1)C4 <==> (4)C1 conformational equilibrium. The preference of the alpha over the beta anomers is due to the endo-anomeric effect. The factors affecting the stability of pyranose ring conformations are discussed, as is the influence of hydrogen bonds on the orientation of the hydroxyl and amino groups. Figures of the most stable conformers are presented.