Alkyl Galactopyranosides: Rotational Population Dependence of the Hydroxymethyl Group on the Aglycon and Its Absolute Configuration and on the Anomeric Configuration

Abstract

The rotational populations of the hydroxymethyl group in chiral and nonchiral alkyl α- and β-galactopyranosides proved, on the basis of circular dichroism (CD) and 1H NMR data, to be dependent on the structure of the aglycon: the population of the gt and tg rotamers increased and decreased, respectively, as the pKa of the bonded alcohol (aglycon) increased, while the population of the gg rotamer remained practically constant. Furthermore, low-temperature CD measurements proved that the most stable rotamer is the gt, and not the tg. In addition, a clear correlation between the rotational populations and the absolute configuration of the chiral aglycon was observed; namely, higher and smaller gt and tg populations were observed for the (S)-alkyl β-d-galactopyranosides than for their (R)-alkyl β-d-galactopyranoside counterparts, the opposite behavior being observed for the α-anomers. The results point to the exo anomeric effect as being responsible for these rotational dependencies, as well as to nonbonding interactions between the aglycon and the chromophore at C6 for those stereoisomers having in their aglycon a bulky substituent syn to the endocyclic oxygen (O5). In addition, the chemical shift differences (Δδ) of the aglyconic protons of galactosylated chiral alcohols proved to be characteristic of the absolute configuration of the bonded chiral alcohol.