1,2-Enediol, sugar transformations

The complex reactions of alkalies with reducing sugars have been described extensively. The origin of the initial products that are obtained is usually explained by the classical Lobry de Bruyn and Alberda van Ekenstein transformation,80 in which an enediol (XLVI) is proposed as the key intermediate. In recent studies Sowden and Schaffer61 used D-glucose-l-C14, D-fructose-l-C14, and D-glucose in D20 to... 

Fig. 2-32. Lobry de Bruyn-Alberda van Ekenstein transformation of sugars. 1, D-Glucose 2, 1,2-enediol 3, D-mannose 4, D-fructose 5, 2,3-enediol 6, D-allulose.

RedwUmes appear to be formed in only small concentrations during Lobry de Bruyn-Alberda van Ekenstein transformations, and their occurrence is restricted to alkaline reaction mixtures. Nevertheless, their formation is of some significance, since, in the past, they may have been confused with the hypothetical enediols of the sugars (for further discussion of this point, see Section VII of this Chapter). 

In the Lobry de Bruyn-Alberda van Ekenstein transformation (reviewed by Speck ) of a ketose to the epimeric aldoses, formation of the 3-deoxy-uloses is normally considered to be a side reaction, and both reactions are considered to proceed through a common intermediate, the 1,2-enediol of the sugar. Under the conditions used for preparation of the 3-deoxy-hexos-uloses from the diketose-(amino acids), the former were, however, the main products and the epimeric aldoses only minor products. Furthermore, under these conditions, both reactions were irreversible and the products so stable that the amounts of the two t3rpes of compound were a measure of their rates of formation. The rapid rate of decomposition of the diketose-(amino acids) was probably due to their ready enolization, even in the absence of strong alkali or acid, to give the 1,2-enolammonium compound... 

The calcium-catalyzed epimerization is clearly different from the LdB-AvE transformation. It does not proceed through the enediol when it is conducted in deuterium oxide, there is no incorporation of deuterium and, if C-1 is substituted by the substituent shifts to become C-2. The mechanism is therefore the same as in the Bilik and in the nickel-amine catalyzed epimerization, a carbon-carbon migration. This mechanism is explained [52] by the formation of a complex between calcium cations and the anionic form of the sugar this holds the sugar in a conformation suitable for the migration of the bond from C-2 to C-3, just as molybdic acid and nickel-amine do. To convert most of the sugar to this complex, an amount of calcium hydroxide equivalent to the sugar is required, or even more this is where the reaction conditions differ from those of the LdB-AvE reaction. 

Enediol forms of a-hydroxycarbonyl compounds react with a-dicarbonyl compounds with interconversion, in which a-hydroxycarbonyl compounds are transformed into a-dicarbonyl compounds and vice versa (Figure 4.66). The reaction explains a number of oxidation-reduction reactions that occur in degradation products of sugars and in the Maillard reaction. The transfer of two protons takes place within the complex of the a-dicarbonyl compound, with the endiol in either the basic energy singlet state or in the excited triplet state, through the more advantageous biradical mechanism. 

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... 

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