Conformational Behavior of Aza-C-Glycosides:  Experimental Demonstration of the Relative Role of the exo-anomeric Effect and 1,3-Type Interactions in Controlling the Conformation of Regular Glycosides

The conformational behavior of different aza-C-glycosides synthesized as glycosidase inhibitors has been studied using a combination of NMR spectroscopy (J and NOE data) and time-averaged restrained molecular dynamics calculations. The obtained results show that the population distribution of conformers around their pseudoglycosidic linkages is mainly controlled by 1,3-syn-diaxial interactions. Electrostatic effects slightly modulate the conformational equilibrium. This result is in contrast with that observed for O-glycosides. For these natural compounds, the conformational behavior around the glycosidic linkage Φ is mainly governed by the exo-anomeric effect. Experimentally based energy values for both the 1,3-syn-diaxial interactions and the stereoelectronic effect have been deduced. Finally, the inhibitory activity of these compounds has been tested again a variety of glycosidase enzymes.