Anomeric effect rotational orientation

The exo-anomeric effect is illustrated in Fig. 3, which shows three staggered orientations for rotation about the glycosidic bond in both the a and p anomer of methyl D-glycopyranoside. These are referred to as -I-sc),... 

Additional evidence on the selection of conformations by the exo-ano-meric effect is derived from the solid-state structures of saccharides. It was earlier observed that the actual orientation of the anomeric alkoxyl group in pyranosides in the solid state corresponds to the (+sc, +sc) or ap, —sc) conformer, and thus proved that these conformers respectively represent the most stable axial and equatorial forms. As already noted, a particularly clear illustration of the operation of the exo-anomeric effect comes from the nonreducing disaccharide 0 ,a-trehalose, in which the most stable orientation about both exocyclic, C-0 bonds corresponds to the (+sc, +sc) conformer. Analyses of carbohydrate structures - revealed regularities in the distribution of the torsional angle O that are consistent with a restriction of rotation about the exocyclic C-O bond. The torsional angle for equatorial isomers varies from — 50° to — 110 , with a mean value of—79.4°. For the axial isomers, the range is 30-130°, with a mean value of 84.5 ° (see Ref. 29). 

The exo-anomeric effect influences the rotations aroimdthe glycosidic C-1—O bond and is therefore important in determining the relative orientations of saccharide units in carbohydrate chains. The exo-anomeric effect is a balance between electronic and steric effects. The three staggered orientations for rotation about the glycosidic bond are not equivalent the exo-anomeric effect causes preference for the +synclinal orientation of the aglycone group in the a series and -synclinal for the series (see Fig. 18 for the definition of the torsion angle domains). 

---