Alditol

The carbonyl group of carbohydrates can be reduced to an alcohol function Typi cal procedures include catalytic hydrogenation and sodium borohydnde reduction Lithium aluminum hydride is not suitable because it is not compatible with the solvents (water alcohols) that are required to dissolve carbohydrates The products of carbohydrate reduc tion are called alditols Because these alditols lack a carbonyl group they are of course incapable of forming cyclic hemiacetals and exist exclusively m noncyclic forms... 

The reaction is used for the chain extension of aldoses in the synthesis of new or unusual sugars In this case the starting material l arabinose is an abundant natural product and possesses the correct configurations at its three chirality centers for elaboration to the relatively rare l enantiomers of glucose and mannose After cyanohydrin formation the cyano groups are converted to aldehyde functions by hydrogenation m aqueous solution Under these conditions —C=N is reduced to —CH=NH and hydrolyzes rapidly to —CH=0 Use of a poisoned palladium on barium sulfate catalyst prevents further reduction to the alditols... 

Alditol (Section 25 18) The polyol obtained on reduction of the carbonyl group of a carbohydrate Aldol addition (Section 18 9) Nucleophilic addition of an aldehyde or ketone enolate to the carbonyl group of an aide hyde or a ketone The most typical case involves two mole cules of an aldehyde and is usually catalyzed by bases... 

Table 2. Price of Sucrose, Alditols, and Synthetic Sweeteners...

Sucrose occupies a unique position in the sweetener market (Table 3). The total market share of sucrose as a sweetener is 85%, compared to other sweeteners such as high fmctose com symp (HFCS) at 7%, alditols at 4%, and synthetic sweeteners (aspartame, acesulfame-K, saccharin, and cyclamate) at 4%. The world consumption of sugar has kept pace with the production. The rapid rise in the synthetic sweetener market during 1975—1995 appears to have reached a maximum. 

The small optical rotations of the alditols arise from the low energy barrier for rotation about C—C bonds, permitting easy iaterconversion and the existence of mixtures of rotational isomers (rotamers) ia solution (12). 

Teichoic acids (16) are bacterial polymers in which alditols, glycerol, or ribitol are joined through the primary hydroxyl groups via phosphate diester linkages. 

Reduction. Mono- and oligosaccharides can be reduced to polyols (polyhydroxy alcohols) termed alditols (glycitols) (1) (see Sugar alcohols). Common examples of compounds in this class ate D-glucitol (sorbitol) [50-70-4] made by reduction of D-glucose and xyhtol [87-99-0] made from D-xylose. Glycerol [56-87-5] is also an alditol. Reduction of D-fmctose produces a mixture of D-glucitol and D-mannitol [69-65-8],... 

Alditols are sweet. Xyhtol has essentially the same sweetness as sucrose sorbitol is about half as sweet as sucrose. In chewing gum, polyols provide texture, sweetness, and mouthfeel and reduce the iacidence of dental caries. 

Reduction (Section 25.18) The carbonyl group of aldoses and ketoses is reduced by sodium borohydride or by catalytic hydrogenation. The products are called alditols. 

Alditol (Section 25.18) The polyol obtained on reduction of the carbonyl group of a carbohydrate. 

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. 

D-Glucitol, the alditol produced by reduction of D-glucose, is itself a naturally occurring substance present in many fruits and berries. It is used under its alternative name, D-sorbitol, as a sweetener and sugar substitute in foods. 

Problem 25.17 I Reduction of D-glucose leads to an optically active alditol (D-glucitol), whereas reduc-I tion of D-galactose leads to an optically inactive alditol. Explain. 

Problem 25.18 I Reduction of L-gulose with NaBH4 leads to the same alditol (D-glucitol) as reduction of D-glucose. Explain. 

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. 

What other d aldohexose gives the same alditol as D-talose ... 

The mass spectrum of alditol acetates are easy to interpret as they fragment at each C—C bond as shown. 

The polyhydric alcohols arising formally from the replacement of a carbonyl group in a monosaccharide with a CHOH group are termed alditols (see 2-Carb-19). 

The term polysaccharide has also been widely used for macromolecules containing glycose or alditol residues in which both glycosidic and phosphate diester linkages are present. 

If the anomeric hydroxy group is replaced by a hydrogen atom, the compound is named as an anhydro alditol (2-Carb-26). 

The name of an aldose derivative in which the aldehyde group has been replaced by a terminal CH3 group is derived from that of the appropriate alditol (see 2-Carb-19) by use of the prefix deoxy- . 

The alditols from fucose and rhamnose are frequently termed fucitol and rhamnitol (see 2-Carb-19.1). 

The imino analogue of a monosaccharide may be named as an imino-substituted deoxy alditol. 

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