ENOLIZATION AND ISOMERIZATION OF MONOSACCHARIDES IN AQUEOUS, ALKALINE-SOLUTION

The initial behaviour of a series of monosaccharides in aqueous, alkaline medium has been studied by using u.v. spectroscopy. An absorption band at 310 nm is observed which may be due to the enediol-anion species as well as to β-elimination intermediates. Comparison of u.v. kinetic data with data from both H/D exchange and degradation reactions shows that the 310-nm band is mainly due to the enediol-anion species. Reaction rate constants have been determined for both the formation and the conversion of the enediol anion by using the overall reaction scheme: sugar + -OH α sugar anion α enediol anion → β-elimination intermediates → products. Calcium (II) and carbonate ions promote the formation of the 310-nm band. Traces of oxygen cause an induction period, due to a rapid oxidation of the enediol anion. The rate of enediol-anion formation is strongly dependent on the configuration of the sugar, whereas the overall rates of conversion of the enediol anion are comparable for the different sugars. For the enolisation and the isomerisation reactions, a molecular picture is presented on the basis of the principle of least motion. It is proposed that the enediol anion is formed through a rate-determining, intramolecular proton-shift. © 1979.