Conformations, pyranose rings glycosides

FIGURE 7-19 Conformation at the glycosidic bonds of cellulose, amylose, and dextran. The polymers are depicted as rigid pyranose rings joined by glycosidic bonds, with free rotation about these bonds. Note that in dextran there is also free rotation about the bond between C-5 and C-6 (torsion angle [Pg.251]

Glycosides - formed between a sugar molecule and an alcohol - are in some sense similar to ethers. Glycosides are formed between the sugar molecule in a ring conformation (pyranose or furanose form) and an alcohol. The example below shows D-glucose (2.31), in equilibrium with p-D-glucopyranose (2.32). 

In the case where the electronegative substituent at Cl is not torsionally symmetrical - as with pyranoses and pyranosides - an exactly similar effect is observed with respect to rotation about the 01—Cl bond. This is termed the exo-anomeric effect [15], and ensures that the preferred conformation of an a-glycoside is as shown the other rotamer not disfavoured by the exo-anomeric effect has R exactly under the pyranose ring, where there are severe non-bonded interactions (Fig. 4). 

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