Application of Familiar Reactions to Monosaccharides

Amylopectin. The main chain (biack) is the same as in amyiose. Amyiopectin differs from amylose in having branches (red) iinked to the main chain by a-(l- 6) giycosidic bonds. Except for the giycoside bonds connecting the branches to the main chain, aii other giycoside bonds are a-(1 4). 

In our discussion of carbohydrate sttucture to this point, we have already encountered an important reaction—the formation of glycosides under acid-catalyzed conditions (see Section 23.14). Glycoside formation draws our attention to the fact that an OH group on the anomeric carbcMi of a pyranose or furanose differs in reactivity from the other hydroxyl groups of the carbohydrate. It also demonstrates that what looks like a new reaction is one that we ve encountered before— in this case, acetal formation that occurs in the reactions of alcohols with aldehydes and ketones (see Section 17.8). Many other reactions of carbohydrates are also related to familiar functional-group transformations some of these are recounted in Table 23.2. 

The first two entries in Table 23.2 illustrate reactions that involve nucleophilic addition to the carbonyl group of the open-chain form which, although present in small amounts, is continuously replenished as it reacts. Entry 1 is the sodium borohydride reduction of the carbonyl group of the aldose o-galactose. The reaction is a general one other 

Reduction Carbonyl groups in carbohydrates are reduced by the same methods used for aldehydes and ketones reduction with sodium borohydride or lithium aluminum hydride or by catalytic hydrogenation. 

Reaction of an aldose with HCN gives a mixture of two diastereomeric cyanohydrins. 

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