Hydride Reduction of aldonolactones to aldoses

Abstract Two new methods for the reduction of aldonolactones to aldoses have been developed for use in small-scale syntheses, such as those of 13 C-enriched sugars. One method involves the action of diborane(6) on a suspension of the lactone in tetrahydrofuran; in the second method, the lactone, as an O -tetrahydropyranyl derivative, is reduced with 1:1 lithium aluminum hydride-aluminum chloride in diethyl ether. On a semi-micro scale, both of these methods gave a higher yield of d -glucose or d -mannose from its respective aldonolactone than established methods; furthermore, they were found to be equally suitable for the preparation of much larger quantities of these sugars. With some aldonolactones, the yield of aldose was relatively low, because the latter was rapidly reduced to the alditol. Reduction mechanisms are discussed, and large differences found in the rates of reaction of various aldoses with diborane(6) are considered in terms of the ease of lactol-ring opening.