Conformational domino effect in saccharides. 2. Anomeric configuration control.

A series of alkyl beta-D-glucopyranosyl-(1-->6)-alpha-D-glucopyranosides were synthesized and analyzed by NMR and CD techniques. As in their beta-anomer series, the rotational populations of the hydroxymethyl group involved in the interglycosidic linkage (torsion angle omega) are shown to depend on the aglycon and the solvent. However, for this alpha-anomer series the rotational dependence arises directly from steric effects. Correlations between rotational populations and molar refractivity (MR) steric parameters, but not Taft's steric parameters (beta-anomers), of the alkyl substituents were observed. The conformational domino effect previously predicted from alkyl beta-(1-->6)-diglucopyranosides is now supported by the conformational properties of their alpha-anomers, the anomeric configuration controlling the domino effect. In addition, the rotational populations around the C5'-C6' bond (torsion angle omega') depend weakly on the structure of the aglycon and the anomeric configuration.