Oxidation, monosaccharides uronic acid

Acids related to monosaccharides are also common for the formation of natural polysaccharides. There are three types of acids associated with monosaccharides aldonic acids generated by the oxidation of the aldehyde group of a monosaccharide, uronic acids generated by the replacement of the primary alcohol group of a monosaccharide by a carboxyl group, and saccharic acids generated by simultaneous oxidation and replacement. Aldonic acids have a marked tendency to eliminate water and form lactones. The structural formulas of two common hexuronic acids are... 

Uronic acids are monosaccharides in which the terminal primary alcohol group is oxidized to a carboxyhc acid functional group, eg, D-glucuronic acid [6556-12-3] (11). 

Monosaccharides can be oxidized enzymatically at C-6, yielding uronic acids, such as D-glucuronic and L-iduronic acids (Figure 7.10). L-Iduronic acid is similar to D-glucuronic acid, except for having an opposite configuration at C-5. Oxidation at both C-1 and C-6 produces aldaric acids, such as D-glucaric... 

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. 

There are three possible classes of sugar acids which may be produced by the oxidation of monosaccharides (Figure 9.11). The aldonic acids are produced from aldoses when the aldehyde group at carbon 1 is oxidised to a carboxylic acid. If, however, the aldehyde group remains intact and only a primary alcohol group (usually at carbon 6 in the case of hexoses) is oxidised then a uronic acid is formed. Both aldonic and uronic acids occur in nature as intermediates in... 

N. J. Davis and S. L. Flitsch, Selective oxidation of monosaccharide derivatives to uronic acids, Tetrahedron Lett., 34 (1993) 1181-1184. 

Chemical modifications of the antigens were achieved by oxidation of the monosaccharide residues with periodate and reduction of the carboxyl groups of the uronic acid residues by the carbodiimide and borohydride methods, Fig. (IOC). The periodate oxidation was performed by a procedure described in the literature [37]. The reduction of the uronic acid residues of the antigen was performed by the carbodiimide (CMC) and sodium borohydride method [38], The oxidized and reduced types of antigens no longer reacted with the antibodies, Fig. (IOC). 

In this chapter, methods for oxidation, reduction, and deoxygenation of carbohydrates are presented. In most cases, the reactions have been used on aldoses and their derivatives including glycosides, uronic acids, glycals, and other unsaturated monosaccharides. A number of reactions have also been applied to aldonolactones. The methods include both chemical and enzymatic procedures and some of these can be applied for regioselective transformation of unprotected or partially protected carbohydrates. 

Oxidation of primary alcohols. This selective oxidation system allows the preparation of uronic acids from monosaccharide derivatives. 

Monosaccharide derivatives in which the —CH2OH group at C6 has been specifically oxidized to a carboxyl group are called uronic acids. Their names are based on the monosaccharide from which they are derived. For example, specific oxidation of C6 of... 

Enzyme-catalyzed oxidation of the terminal —CH2OH group of a monosaccharide to a COOH group gives a uronic acid. 

Uronic Acid and Other Monosaccharides. Formation of a uronoside requires the oxidation of the primary hydroxyl group of a hexopyranoside. This type of unit is very important in many natural polysaccharides such as alginates, pectins, or some hemicelluloses. Some important monosaccharides are represented in Figure 11. 

Even though monosaccharides exist largely as cyclic hemiacetals or hemiketals, rapid equilibrium, with trace quantities of carbonyl-containing compounds, means that discussion of the reactions of most sugars must consider both the acyclic and the cyclic structures. Common monosaccharides can be reduced to alkanepolyols (D-manitol, sorbitol) or oxidized to aldaric, aldonic, or uronic acids. 

---