Lactones of sugar acids

Lactones are in principle reducible to hydroxyaldehydes, and when applied to lactones of sugar acids the method works well because cyclic acetal or ketal formation stabilizes the carbonyl function and prevents overreduction [3,96-100]. In particular, d-ribose may be prepared from the cathodic reduction of D-ribono-y-lactone as in Eq. (24) the optimum conditions for this reaction include careful buffering of the electrolyte [97-100]. 

In 1889 Fischer discovered that the lactones of the acids of the sugar group can be reduced by sodium amalgam to yield the corresponding carbonyl compound, an aldose. The addition of this reaction to the cyanohydrin procedure of Kiliani made possible the first synthesis of... 

This conciusion was confirmed " by an investigation of the rate of lactone formation of the phosphoribonic acid. (Levene and Simms had previously shown that if hydroxyls (4) and (5) are both free the formation of both the y- and the 5-lactone can be detected if position (4) is substituted and (5) is free, the 5-lactone is formed at a relatively rapid rate if position (5) is substituted and (4) is free, the y-lactone is slowly formed.) The transformation of the phosphoribonic acid to its lactone in aqueous solution was found to proceed smoothly and very slowly, a property of sugar acids so substituted that they can only give rise to 7-lactones. The formation of 5-lactone was not observed, as was to be expected if position (5) of the sugar is substituted. 

BMB has been used to reduce 2,3,5,6-tetra-O-acyl-D-hexono-y-lactones to the furanoses in high yields. Furanose derivatives are difficult to obtain by other methods, but the new method takes advantage of the preferential conversion of sugar acids into y-lactones. 

Oxidation to Sugar Acids and Lactones. When the aldehyde group of an aldose is oxidized, the resulting compound is an aldonic acid (salt form = aldonate) (11)4. Some aldonic acids are products of carbohydrate metaboHsm. 

Ascorbic acid, which is closely related to the sugars and is no doubt derived from them in the metabolism of the cells of plants, has been shown to be identical with vitamin- C (Szent-Gyorgyi). It has the constitution of a lactone of a-keto-i-gulonic acid (W. N. Haworth, T. Reichstein) ... 

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