New insights into the conformational properties of α-C-glucosides

A series of alkyl α-d-C-glucopyranosides were synthesized and their conformational properties analyzed by CD and NMR spectroscopy. The conformational analysis revealed that the hydroxymethyl group populations (torsion angle ω, O1-C1-C2-O2) and those around the C-glucopyranosidic bond (torsion angle Φ; O2-C6-C7-C8) depend on the structural nature of the C-aglycon. The gt and the exo-syn populations increased as the C-aglycon became more substituted. Linear correlations between these rotational populations and proton chemical shifts versus the Taft's steric parameters revealed the significant role of the C-aglycon in the overall conformation of C-glucosides. The stereoelectronic exo-deoxoanomeric effect, affecting the rotation of the hydroxymethyl group, becomes more important as the steric hindrance of motion increases. A pseudo-anomer rotational comparison study was also performed.