Dehydration aldonic acid

H. Kiliani, as Fischer always emphatically acknowledged, discovered and developed the method of building up the aldose series by the cyanohydrin reaction to give nitriles from the nitrile, the next higher aldonic acid could then be prepared. In 1890, A. Wohl, working in Fischer s Berlin laboratory, elaborated the dehydration of an aldose oxime to the nitrile, from which the next lower aldose could be prepared by loss of hydrocyanic acid. Fischer exploited the possibilities of sugar extension and degradation afforded by the use of these two important methods. 

In vivo, pyruvate lyases perform a catabolic function. The synthetically most interesting types are those involved in the degradation of sialic acids or the structurally related octulosonic acid KDO, which are higher sugars typically found in mammalian or bacterial glycoconjugates [62-64], respectively. Also, hexose or pentose catabolism may proceed via pyruvate cleavage from intermediate 2-keto-3-deoxy derivatives which result from dehydration of the corresponding aldonic acids. Since these aldol additions are freely reversible, the often unfavourable equilibrium constants require that reactions in the direction of synthesis have to be driven by an excess of one of the components, preferably pyruvate for economic reasons, in order to achieve a satisfactory conversion. 

The equilibrium proportions of the constituents vary with temperature, concentration, and solvent, and are characteristic of each individual aldonic acid. Dehydration in vacuo, or by evaporation from suitable solvents, promotes lactone formation. By suitable choice of conditions, many aldonic acids and both types of lactones have been obtained crystalline 1 considerations have been detailed by Isbell and Frush.63... 

Under dehydrating conditions, it is probable that, in addition to lactonization, intermolecular esterification also occurs, yielding esters of aldonic acids and polymers. 

There are three known metabolic pathways to 3-deoxyulosonic acids. In the first, aldonic acids, formed by oxidation of aldopyranoses to the corresponding lactones, are dehydrated to the ulosonic acids (see Fig. 6). Thus, D-arabinose is converted by way of D-arabinonic acid to 3-deoxy-o-ffZj/cero-pentulosonic acid. The latter is then subjected to oxidative cleavage, yielding pyruvic and glycolic acids. L-Arabinose is oxidized and dehydrated to the 3-deoxy-L-g Z2/cero-pentulosonic acid, which is further oxidized to... 

Fig. 6.— Pathway for the Enzymic Formation of 3-Deoxyulosonic Acids by the Dehydration of Aldonic Acids.

This method involves the oxidation of aldoses to aldonic acids by potassium hypoiodite in methanol and the condensation of the isolated potassium salt of the aldonic acid in hot acid with o-phenylenediamine (M3). The specific aldobenzimidazole can be identified by the solubility in water, optical rotation in acid, and melting point. This method is not as simple as chromatography. It requires dehydration of the sample and a relatively high concentration of the sugars to be identified. 

The cyanohydrin synthesis is particularly important in the carbohydrate series Fischer thus converted, e.g., pentoses into hexoses by way of the lactones of the aldonic acids which he later reduced. Admittedly, the nitriles produced as intermediates (best obtained in other cases by dehydration of the corresponding oximes) are generally not isolated. On the other hand, the corresponding amides can often be separated in a pure state, as shown, e.g., by Kiliani166 for heptonamide ... 

A solution supersaturated with respect to both free acid and lactone can often be seeded with the appropriate crystals and the desired product obtained. Normally, the free aldonic acid is obtained by concentration of the aqueous solution at a low temperature in vacuo. The free acid can also be crystallized from a solution of the sodium salt in acetic acid. The lactones are formed by dehydration, often very easily. Water can be removed by distillation with butanol or dioxane or by heating in vojcuo. The lactones are crystallized from an anhydrous solvent in some cases, as with rhamnonic 7-lactone, they are formed very easily and crystallize readily from water. 

Aldonic Acid Preparation and Subsequent Dehydration to Give an Aldonolactone... 

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