Monosaccharides hydroxyl groups

Isomerization. Both the carbonyl group and the adjacent hydroxyl group are involved in isomerization of monosaccharides. This reaction can be catalyzed by either a base or an enzyme. By this reaction, an aldose is converted into another aldose and a ketose, and a ketose is converted into two... 

FIGURE 7.21 Amylose and amylopectin are the two forms of starch. Note that the linear linkages are o (1 4), but the branches in amylopectin are o (1 6). Branches in polysaccharides can involve any of the hydroxyl groups on the monosaccharide components. Amylopectin is a highly branched structure, with branches occurring every 12 to 30 residues. 

Monosaccharides behave as simple alcohols in much of their chemistry. For example, carbohydrate -OH groups can be converted into esters and ethers, which are often easier to work with than the free sugars. Because of their many hydroxyl groups, monosaccharides are usually soluble in water but insoluble in organic solvents such as ether. They are aiso difficult to purify and have a tendency to form syrups rather than crystals when water is removed. Ester and ether derivatives, however, are soluble in organic solvents and are easily purified and crystallized. 

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. 

Glycosides are formed by condensation between the hydroxyl group of the anomeric carbon of a monosaccharide, or monosaccharide residue, and a second compound that may—or may not (in the case of an aglycone)—be another monosaccharide. If the second group is a hydroxyl, the O-glycosidic bond is an acetal link because it results from a reaction between a hemiacetal group (formed from an aldehyde and an —OH group) and an-... 

Stevens and coworkers used their c.d. data on the various D-glucans to assign, tentatively, the bands to specific chromophores. They found that derivatives of these polysaccharides that have all of their hydroxyl groups acetylated still exhibit the 177-nm band. They assigned this band (which occurs at somewhat shorter wavelengths for the helical polymers) to the ether of the acetal chromophore. This assignment is essentially consistent with the results obtained by Johnson and coworkers on unsubstituted monosaccharides. 

Monosaccharides can differ in their formulas, their ring sizes, and the spatial orientations of their hydroxyl groups. To analyze the differences between two monosaccharides, begin with structural drawings of the molecules, oriented so the ether linkages are in comparable positions. Then examine the stmctures to locate differences in constituents and bond orientations. 

The activation of an anomeric hydroxyl group from partially protected or unprotected monosaccharides can be achieved via 1,2-cyclic sulfite formation. A subsequent trans-ring opening with azide N3 affords one anomeric derivative exclusively 133... 

Scheme 31. Monosaccharide derivatives obtained by oxidation at secondary hydroxyl groups using PCC in the presence of 3A or 4A MS.

The aldehyde or ketone group of monosaccharides can undergo an intramolecular reaction with one of its own hydroxyl groups to form a cyclic, hemiacetal, or hemiketal structure, respectively (Figure 1.26). In aqueous solutions, this cyclic structure actually predominates. The open-chain aldehyde or ketone form of monosaccharides is in equilibrium with the cyclic form, but the open structure exists less than 0.5 percent of the time in aqueous environments. It is the... 

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