Alkaline Degradation of Monosaccharides

The dissertation26 by de Bruijn, Monosaccharides in alkaline medium isomerization, degradation and oligomerization and other publications by 

The reversible reactions are initiated by an equilibrium between neutral and ionized forms of the monosaccharides (see Fig. 6). The oxyanion at the anomeric carbon weakens the ring C-O bond and allows mutarotation and isomerization via an acyclic enediol intermediate. This reaction is responsible for the sometimes reported occurrence of D-mannose in alkaline mixtures of sucrose and invert sugar, the three reducing sugars are in equilibrium via the enediol intermediate. The mechanism of isomerization, known as the Lobry de Bruyn- 

Alberda van Ekenstein rearrangement, generates the enediol anion intermediate that might undergo nonreversible degradation reactions. 

The first step in the nonreversible degradation reactions is the formation of a reactive a-dicarbonyl species through the p-elimination of a hydroxide ion. The subsequent reaction pathways to all degradation products can be described by just five reaction types, namely, p-elimination, benzilic acid rearrangement, a-dicarbonyl cleavage, aldol condensation, and retro-aldol condensation (see Fig. 7).31 Retro-aldol condensation and a-dicarbonyl cleavage involve C-C bond 

De Bruijn et al.26 30 used chromatographic and spectroscopic techniques to analyze the effect of reaction variables (such as pH and monosaccharide concentration) on the product profile and developed a reaction model (see Fig. 9) that emphasized the role of a-dicarbonyl compounds. Some of the features of the model shown in Fig. 9 are  

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Reagent and carbohydrate concentration affect the nature of the products formed during the alkaline degradation of monosaccharides. 

The mechanism of the alkaline degradation of monosaccharides in aqueous solution has been reviewed in a symposium report. A method for the stepwise degradation of glycosylated aldoses, outlined in Scheme 12, has been developed with a view to its application to the sequencing of branched oligosaccharides. ... 

The consumption of effective alkali in a kraft cook corresponds to about 150 kilogram sodium hydroxide per ton of wood. As a result of the alkaline degradation of polysaccharides, about 1.6 equivalents of acids are formed for every monosaccharide unit peeled from the chain. Of the charged alkali, 60-70% is required for the neutralization of these hydroxy acids, while the rest is consumed to neutralize uronic and acetic acids (about 10% of alkali) and degradation products of lignin (25-30% of alkali). 

The alkaline degradation of reducing monosaccharides involves a series of consecutive reactions and gives many kinds of products [58]. For example, the alkaline degradation of 33 in aqueous calcium hydroxide at 100 °C results in a complex mixture of more than 50 compounds ( Scheme 4). Products obtained by the same degradation of 40 are similar to those from the... 

A new kinetic model for the alkaline isomerization and degradation of monosaccharides has been presented. Computer simulations using the model fit the experimental data and allow determination of all relevant rate constants. The rate limiting ste appears to be enolization for both isomerization and degradation. The mechanism of redox reactions between lead hydroxide and 3-0-methyl-D-glucose has been studied by quantum mechanical calculations. The calculations show that there is a redistribution of electron density in both systems which favours the formation of a carboxyl group in the C(1)-G(3) part of the aldose. 

Apart from sotolon, the other compounds in Fig. 5 can be explained as the products of a Maillard reaction, and their carbon skeletons simply originate from the active Amadori intermediate in other words, they still preserve the straight carbon chain structure of monosaccharides. In spite of being a simple Cg lactone, sotolon has a branched carbon skeleton, which implies another formation process in the Maillard reaction. Sulser e al.(6) reported that ethyl sotolon (ll) was prepared from threonine with sulfuric acid, and that 2-oxobutyric acid, a degradation product of threonine, was a better starting material to obtain II. This final reaction is a Claisen type of condensation, which would proceed more smoothly under alkaline conditions. As we(lO) obtained II from 2-oxobutyric acid (see figure 6) with a high yield in the presence of potassium carbonate in ethanol, a mixed condensation of 2-oxobutyric and 2-oxo-propanoic (pyruvic) acids was attempted under the same conditions, and a mixture of sotolon (22% yield) and II were obtained however, the... 

The reactions of monosaccharides in aqueous alkaline solution is the subject of a review covering initial transformations, alkaline degradation and the Influence of reaction variables on product for-1... 

A C n.m.r. study has been carried out on the alkaline degradation products of monosaccharides th e acidic products with up to six... 

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