The exo-deoxoanomeric effect in the conformational preferences of C-glycosides

Interglycosidic C5-C6 rotamer distributions of alkyl O-rutinosides

The conformational study of carbohydrates is critical to understand the molecular recognition mechanisms underlying their biological functions. Moreover, the systematic study of their conformational patterns can unlock useful tools to design optimized glycomimetics and drug candidates. Using nuclear magnetic resonance, we studied the interglycosidic rotamer equilibria of ester-protected and deprotected alkyl O-rutinosides (α-L-Rha(1,6)β-D-GlcOR). In the protected series, the equilibrium about the C5-C6 bond distributes among the three possible rotamers gg, gt, and tg, being gt the predominant conformer. In these series, the flexibility about C5-C6 shows a marked dependency on the aglycone's structure, where the increase on the aglycone's volume leads to a progressive increment on the tg contributions at the expense of gt, with gg remaining practically constant along the series. The removal of the protective groups results in rutinosides displaying an equilibrium equally distributed between gg and gt with no tg contributions regardless of the aglycone's structure.

Conformational Properties of Aryl S-Glucosides in Solution

The conformational study of saccharides and glycomimetics in solution is critical for a comprehensive understanding of their interactions with biological receptors and enabling the design of optimized glycomimetics. Here, we report a nuclear magnetic resonance (NMR) study centered on the conformational properties of the hydroxymethyl group and glycosidic bond of four series of aryl S-glucosides. We found that in acetyl-protected and free aryl S-β-glucosides, the rotational equilibrium around the C5-C6 bond (hydroxymethyl group) exhibits a linear dependence on the electronic properties of the aglycone, namely, as the aryl's substituent electron-withdrawing character increases, the dominance of the gg rotamer declines and the gt contribution rises. Likewise, the conformational equilibrium around the glycosidic C1-S bond also depends on the aglycone's electronic properties, where glucosides carrying electron-poor aglycones exhibit stiffer glycosidic bonds in comparison to their electron-rich counterparts. In the case of the α anomers, the aglycone's effect over the glycosidic bond conformation is like that observed on their β isomers; however, we observe no aglycone's influence over the hydroxymethyl group conformation in the α-glucosides.

Determination of the Influence of Side-Chain Conformation on Glycosylation Selectivity using Conformationally Restricted Donors

The synthesis of a series of conformationally locked mannopyranosyl thioglycosides in which the C6-O6 bond adopts either the gauche,gauche, gauche,trans, or trans,gauche conformation is described, and their influence on glycosylation stereoselectivity investigated. Two 4,6-O-benzylidene-protected mannosyl thioglycosides carrying axial or equatorial methyl groups at the 6-position were also synthesized and the selectivity of their glycosylation reactions studied to enable a distinction to be made between steric and stereoelectronic effects. The presence of an axial methoxy group at C6 in the bicyclic donor results in a decreased preference for formation of the β-mannoside, whereas an axial methyl group has little effect on selectivity. The result is rationalized in terms of through-space stabilization of a transient intermediate oxocarbenium ion by the axial methoxy group resulting in a higher degree of SN 1-like character in the glycosylation reaction. Comparisons are made with literature examples and exceptions are discussed in terms of pervading steric effects layered on top of the basic stereoelectronic effect.