Alditols with ketones

Treatment of an aldose or ketose with NaBH4 reduces it to a polyalcohol called an alditol. The reduction occurs by reaction of the open-chain form present in the aldehyde/ketone hemiacetal equilibrium. Although only a small amount of the open-chain form is present at any given time, that small amount is reduced, more is produced by opening of the pyranose form, that additional amount is reduced, and so on, until the entire sample has undergone reaction. 

Much of the chemistry of monosaccharides is the familiar chemistry of alcohols and aldehydes/ketones. Thus, the hydroxyl groups of carbohydrates form esters and ethers. The carbonyl group of a monosaccharide can be reduced with NaBH4 to form an alditol, oxidized with aqueous Br2 to form an aldonic acid, oxidized with HNO3 to form an aldaric acid, oxidized enzymatically to form a uronic acid, or treated with an alcohol in the presence of acid to form a glycoside. Monosaccharides can also be chain-lengthened by the multistep Kiliani-Fischer synthesis and can be chain-shortened by the Wohl degradation. 

All monosaccharides and their derivatives that possess aldehyde or ketone groups (that is, excepting derivatives such as alditols and aldonic acids) will have reducing properties. Moreover, those with the appropriate number of carbon atoms can form rings occurring in two forms (anomers) and in which the potential reducing carbon is called the anomeric carbon. 

This reagent, stable in water in slightly alkaline conditions, is ideal for reducing sugars. The aldehyde function is reduced to a primary alcohol and the ketone function to a mixture of secondary alcohol epimers. The simplest treatment after reduction consists in eliminating the sodium on a cation exchanger column, then reaction with boric acid in the form of volatile methyl borate by co-evaporation with methanol. The alditol is recovered by evaporation of the solvent to dryness or lyophilization. 

Aldonic acid thioamide acetates (619) condensed with various a-halo-ketones to afford the 2-(alditol-l-yl)lhiazoles 620 [54AQ(B)609, 54CB78 69ACH(62)179, 69T3413 87H947 95TL3781] (Scheme 163). 

The alditols form acetals readily with aldehydes and ketones. These are discussed in Chapter IV. 

Aldoses and ketoses are reduced by the same types of reducing agents that convert aldehydes and ketones into alcohols. The resulting polyhydroxy compounds are called alditols. For example, D-glucose gives D-glucitol (older name, D-sorbitol) when treated with sodium borohydiide. 

Amino-Alditols.- Anti-selective reduction of acyclic or-alkoxy and of, -dialkoxy ketone oximes occurs with aluminium hydride reagents, L-threo-ketotetrose derivatives (24) giving L-xylitol... 

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