Formation and Hydrolysis of Aldonolactones

The aqueous-bromine oxidation rate was determined for a number of other anomeric pairs of aldoses and it was found that, in general, the jS forms (see page 17) react faster. This phenomenon has also been observed for a-D- and /S-D-galactopyraniironic acid oxidation of these compounds provides a mixture of optically active lactones of galactaric (mucic) acid. This constitutes interesting evidence for the direct oxidation of the cyclic modifications (pyranose and furanose forms), since the oxidation of 

It was reported that, at pH 10.15 to 13.10, the configurationally related sugars D-glucose and D-xylose are oxidized at an approximately equal rate. The similarly related sugars D-galacto.se and L-arabinose showed the same phenomenon, but D-mannose was less reactive the ratio between these rates was 1 1.36 0.24. 

Oxidation of aldoses by chlorites also results in the formation of the corresponding aldonic acids. The aldopentoses react faster than aldo-hexoses, and monosaccharides are oxidized more rapidly than are the disaccharides. The ketoses remain unaffected unless drastic conditions are employed. The principal reaction has been shown to be as follows. 

With chlorous acid as the main oxidizing agent, the reaction is slow in neutral solution and becomes faster under acidic conditions. A solution containing equimolar amounts of sodium chlorite and acetic acid affords a slightly faster rate of oxidation for a-n-glucopyranose than for the 8-d anomer. Thus, the reaction appears to follow a different path from that of the bromine-water and the hypoiodite oxidations, and to be similar to that of cyanohydrin formation.  

From the livers of various animals has been isolated an enzyme named glucose dehydrogenase which, with di- or tri-phosphopyridine nucleotide as coenzyme, reversibly converts D-glucose to D-gluconolactone. It appears to be rather specific for fl-D-glucose, although /3-D-xylose can also be oxidized (at a lower rate). - 

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The rates of hydrolysis of the lactones depend on the parent structure for instance, the 1,4-lactone of D-mannonic acid is more stable than that of D-gluconic acid, and the 1,4-lactones of 2-deoxyaldonic acids are more stable than the corresponding aldonolactones. The final attainment of equilibrium between free aldonic acids and their lactones is reached only after many days at room temperature it is, however, accelerated by the presence of strong acids and by heating. A detailed discussion of the formation and hydrolysis of aldonolactones is available in a review by Shafizadeh,59 and the conformations and stabilities of aldonolactones have been discussed by Lemieux.60 Detailed analyses of D-glucono-1,5-lactone and other lactones have been reported.61 13C NMR spectroscopy proved to be a convenient method for monitoring the equilibria.62... 

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