Based on Changing the Configuration of Other Sugars

Methods Based on Changing the Configuration OF Other Sugars 

A number of methods utilized for the synthesis of sugars depend upon changing the configuration of one or more carbon atoms of other sugars. Especially when the configuration of carbon 2 of the aldoses is changed, the process is termed an epimerization. 

The aldoses in such mixtures may be oxidized by bromine, which has no action on the ketoses. The aldonic acids formed in this manner are conveniently separated from the ketoses by ion-exchange resins. 

The direct action of alkali or pyridine on the sugars is of particular value for the preparation of the ketoses, but the action of pyridine on the aldonic acids is solely an epimerization. The action of hot tertiary amines (particularly aqueous pyridine and quinoline), as well as of alkali, on the aldonic acids and their methylated derivatives results in the establishment of an equilibrium between the two epimeric acids 219). 

The sugars are prepared by reduction of the corresponding lactones. From D-arabonic acid, D-ribose 220) is obtained by this procedure, and from D- and L-galactonic acids, d- and L-talose 221) are prepared. 

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