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

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.

Synthesis of C-linked α-Gal and α-GalNAc-1'-hydroxyalkanes by way of C2 functionality transfer

Inspired by reports of water sculpted Tn antigen (α-GalNAc-O-Ser/Thr) epitopes and our interest in producing metabolically more stable C-linked analogs of Tn, we explored the utility of C2 functionality on α-Gal-C-alkenes to deliver hydroxyl to the pendant alkenyl chain. Toward this end, a cyclic carbonate approach gave rise to a single C-linked α-Gal-1'(S)-hydroxyethane in 3 steps, and use of a 2-(hydroxyimino)galactoside cyclization transferred an oxygen to a pendant cis-substituted C-linked alkene affording the R-configuration at the newly formed stereocenter (7:1 dr). Reduction and acetylation of the resultant isoxazoline demonstrated this approach as a viable route to C-linked α-GalNAc-1'-hydroxyalkanes.

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.