Transformations of sugars in alkaline solutions

Abstract

A comparative study has been made of the rates of enolization of sugars in alkaline, tritiated water by measurement of their rates of tritium uptake; the rates for two inososes were found to be more than 1000 times the rate for D-glucose. The low rates of reaction of the sugars in comparison with those for the inososes suggest that one of the factors controlling the rate of enolization is the concentration of an intermediate carbonyl form of the sugar. To account for differences in the behavior of epimeric sugars in alkaline solution and the well-known differences between the rates of mutarotation and of exchange of 18O of an aldose- 1- 18O with 16O of water, it is suggested that the mutarotation reaction proceeds through pseudo-acyclic intermediates that possess some characteristics of the respective ring-forms. This new concept, applied to the interpretation of enolization reactions, leads to the possibility of formation of cis- and trans-enediols, the proportions of which may differ from sugar to sugar. Enolization experiments were conducted for a variety of sugars in a 0.5 M solution of sodium carbonate in water- t at 25°. The rates of tritium uptake, based on the rate for D-glucose as unity, range from 0.5 to 4.8 for aldohexoses, 0.5 to 10.7 for aldoheptoses, 4.1 to 28 for aldopentoses, 2.6 to 25 for various ketoses, 0.3 to 0.8 for some 6-deoxy sugars, and 90 to 155 for some 2-deoxy sugars. The comparative rates found for the disaccharides studied range from 2.0 to 240; of these, turanose was the most reactive sugar.