Alditols reactions with aldehydes

Several reviews have already been published on the subject, for example, the acetala-tion of alditols [4], of aldoses and aldosides [5,6], and of ketoses [7]. Some aspects of the stereochemistry of cyclic acetals have been discussed in a review dealing with cyclic derivatives of carbohydrates [8], also in a general article [9] and, more recently, in a chapter of a monograph devoted to the stereochemistry and the conformational analysis of sugars [10], Aspects on predicting reactions patterns of alditol-aldehyde reactions are reviewed within a general series of books on carbohydrates [11]. The formation and migration of cyclic acetals of carbohydrates have also been reviewed [12,13],... 

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. 

Other aldehydes (benzaldehyde, formaldehyde) I4S), condense with alditols and sugars to form cyclic acetals 14 )- The analogous reaction of... 

Formaldehyde, as the most reactive aldehyde, undergoes Cannizzaro reaction even with aldehydes that have a hydrogen atom at the carbon. Formaldehyde also reacts to some extent with aldoses that are reduced to sugar alcohols (alditols) and formaldehyde is oxidised to formic acid (Figure 4.69). Reaction of formaldehyde with D-glucose yields D-glucitol, reaction with D-glyceraldehyde... 

Reaction of aldehyde 85 with phosphonate 86 should provide enone 87. After proper functionalization of the Cu-Co bridge followed by removal of the isopropylidene blocks (and subsequent reduction), the C18 linear higher-sugar alditols will be available. We did obtain the desired enone 87, however in a moderate yield. Besides the expected product, we isolated also two other compounds that were identified as alcohols 88 and 89 (Figure 10.28). The removal of the benzyl block from either position C7 or C8 occurred under the basic conditions [62] ... 

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. 

This enzyme [EC 1.1.1.21], also known as aldehyde reductase and polyol dehydrogenase (NADP ), catalyzes the reaction of an alditol with NAD(P) to generate an aldose and NAD(P)H. The enzyme exhibits a broad specificity for the alditol. 

Decarbonylation of aldoses.2 Although this rhodium complex has been known since 1968 to effect decarbonylation of aldehydes, it has been used for decarbonylation of sugars only recently, probably for lack of a compatible solvent. Actually, this reaction when carried out in N-methyl-2-pyrrolidinone (NMP) at 110-130° is extremely useful in the case of simple aldoses, which are converted to the lower alditol with formation of carbonylchlorobis(triphenylphosphine)rhodium(I). The yields are 75-95%. This method of degradation has the further advantage that protecting groups are not necessary. Deoxyaldoses, particularly 2-deoxyaldoses, are decar-bonylated in 75-99% yield. A disadvantage of this reaction is that a full equivalent of the complex is required. 

In the presence of formaldehyde, a Cannizzaro reaction even occurs with aldehydes, which do not have hydrogen atoms on the carbon in position a to the carbonyl group. Formaldehyde is then oxidised to formic acid, and the aldehyde is reduced to the corresponding alcohol (Figure 8.36). Reaction of formaldehyde with aldoses produces small amounts of formic acid and the corresponding alditol. 

Another method for lengthening the carbohydrate chain is the addition of a nitroalkane to the aldehyde group. The nitroalkane is treated with sodium methox-ide to form a carbanion that adds to the aldehyde group to give a 1-nitro-l-deoxyalditol. The alditol can be converted into an aldehyde by treatment with sodium hydroxide, followed by acid this is the Nef reaction [41] (reaction 4.42). 

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