Rotation about aglycon bond

The turning of the key once the complex has formed is a separate issue. In this regard, Lemieux (47) has pointed out that rotation about the glycosidic bond must weaken the exo-anomeric effect and thereby importantly activate the anomeric carbon to nucleophilic attack. Therefore, it seems likely that the role of the key hydroxyl group of the aglycon is to accommodate the rotation prior to the attack by water to form / -D-glucopyranose, which is the first product of the reaction. 

Figure 4. The same as in Figure 3, but for rotation about the aglycon C-0 bond.

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). 

The conformational effect of the aglycon in alkyl glycopyranosides has been examined with the methyl, ethyl, isopropyl, and tert-butyl tri-0-acetyl-/3-D-ribopyranosides, and the corresponding benzoates. In each series, there was a negligible change in conformational population as between the methyl, ethyl, and isopropyl derivatives, but the terf-butyl derivatives contained somewhat less of that con-former [IC(d)] having the aglycon axially attached, presumably because there is loss of some rotational freedom about the C-1-0-1 bond in the tert-butyl derivatives as a result of steric hindrance. 

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