Conformational Domino Effect in Saccharides. 2. Anomeric Configuration Control

Abstract

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.

Conformational Properties of α- or β-(1→6)-Linked Oligosaccharides: Hamiltonian Replica Exchange MD Simulations and NMR Experiments

“…25 However, the overall performance of the parameters for the O5′–C5′–C6′–O6′ torsion is satisfactory in the CHARMM36 force field, as the model captures the trends from NMR spectroscopy and crystallography, i.e., a preferred equilibrium between gt and gg over tg in gluco- or mannopyranosides, whereas the favored equilibrium occurs between the gt and tg rotamers over the gg rotamer in galactopyranosides. 123,125−127 …”

Influence of protecting groups on the anomeric equilibrium; case of the 4,6-O-benzylidene acetal in the mannopyranose series

“…It is known that the population of the different conformers of the C5–C6 bond in glycopyranosides is affected by the aglycone and by the anomeric configuration both in general, 9,48–50 and in the mannopyranosides, 51,52 and that this effect is transmitted through multiple glycosidic linkages in oligosaccharides. 53 Such effects are usually interpreted i) in terms of changes in the hydrogen bonding pattern around the pyranoside ring and ii) as changes in the relative populations of the gauche-gauche ( gg ) and gauche-trans ( gt ) conformers (Fig. 3).…”