Carbohydrates aldose-ketose isomerization

A study of the formation of oligosaccharides on thermal degradation of lactose has shown that the pyrolysis proceeds via simultaneous aldose-ketose isomerization and condensation of initially-formed monosaccharides without involving any cleavage of the carbohydrate carbon skeleton. 

Two Dutch chemists, Lobry de Bruyn and Alberda van Eckenstein, collaborated in the study of the effects of alkali on carbohydrates. The reaction with alkali produces epimerization of aldoses and ketoses and aldose-ketose isomerization [2]. At pH values of 11-13 and 20°C, alkali catalyzes the transformation of D-glucose into D-fructose and o-mannose. The transformation most probably takes place by the formation of two enediols, although the enolic forms of the sugars have never... 

Isomerization of carbohydrates in alkaline media, considered to embrace both epimerization of aldoses and ketoses and aldose-ketose interconversion ... 

The reactivity of carbohydrates is dominated by the reactivity of the aldehyde group and the hydroxyl on its next-neighbor (/ ) carbon. As illustrated by the middle row of Fig. 2.3, the aldehyde can be isomerized to the corresponding enol or be converted into its hydrate (or hemiketal) form upon reaction with water (or with an hydroxyl-group). These two reactions are responsible for the easy cycliza-tion of sugars in five- and six-membered rings (furanose and pyranose) and their isomerization between various enantiomeric forms and between aldehyde- and ketone-type sugars (aldose and ketose). 

Isomerization reactions are of high importance to the exploitation of carbohydrates. These transformations retain the functionality parameter of the feedstock, while maintaining complete atom-efficiency, since the reaction only involves a reshuffle of atoms within the molecule. Keto-enol tautomerization between aldoses and ketoses as encountered in Fig. 13, as well as the furanose or pyranose ring formation via cychc hemi-acetalization of pentoses and hexoses, are simple examples. The epimerization... 

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