Oxidation, monosaccharides aldaric acids

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. 

Monosaccharides can be oxidized at the aldehyde carbon to give carboxylic acids called aldonic acids. Oxidation at both ends of the carbon chain gives aldaric acids. Reduction of the carbonyl group to an alcohol gives polyols called alditols. The -OH groups in sugars, like those in simpler alcohols, can be esterified or etherified. 

The oxidation of monosaccharides, e.g., D-galactose (5.52), with strong oxidants proceeds at both Cl and C6 atoms, leaving dicarboxylic, aldaric acids (5.53). Aldaric... 

Oxidation of secondary hydroxyl groups at C-2 and C-3 leads to acyclic dialde-hydes and dicarboxylic derivatives. Selective oxidation of primary hydroxyl groups is more difficult, but can be achieved by stoichiometric as well as catalytic methods. Nitric acid oxidizes monosaccharides into aldaric acids. Oxidation of cellulose or starch by nitrogen dioxide (N2O4) yields 6-carboxy starch and 6-carboxycellulose respectively. Subsequent hydrolysis under well-controlled conditions (0.5-2 M HCl at 150 °C) of these materials yields o-glucuronic acid (o-GlcAp) [12]. 

If the anomeric centre is not protected, TEMPO-mediated oxidations of monosaccharides with CI2 or Br2 yield aldaric acids, i.e. co-diacids in which the all secondary hydroxyl groups are preserved the use of the 4-acetamido derivative of TEMPO is to be preferred because of its greater stability and lower volatility. This method of making aldaric (saccharic) acids supersedes the use of nitric acid, used in Emil Fischer s classical work to make both ends of... 

Reduction of an aldose forms one alditol reduction of a ketose forms two alditols. Br2 oxidizes aldoses, but not ketoses ToUens reagent oxidizes both. Aldoses are oxidized to aldonic acids or aldaric acids. Aldoses and ketoses react with three equivalents of phenyUiydrazine, forming osazones. C-2 epimers form identical osazones. The Kiliani-Fischer synthesis increases the carbon chain of an aldose by one carbon— it forms C-2 epimers. The Ruff degradation decreases the carbon chain by one carbon. The OH groups of monosaccharides react with acetyl chloride to form esters and with methyl iodide/silver oxide to form ethers. 

A D-aldopentose is oxidized by nitric acid to an optically active aldaric acid. A Ruff degradation of the aldopentose leads to a monosaccharide that is oxidized by nitric acid to an optically inactive aldaric acid. Identify the D-aldopentose. 

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. 

Full oxidation of monosaccharides by nitric acid to aldaric acids has been an established technique for more than a century. Analogously, oxidation of polysaccharides such as cellulose or starch by nitrogen dioxide (N2O4) yields 6-carboxy starch and 6-carboxycellulose respectively [84]. Subsequent hydrolysis at rigorous conditions (0.5-2M HCl at 150°C) of these materials yields D-glucuronic acid. A drawback of the oxidation with nitrogen dioxide is that depolymerisation may be an important side reaction. An improvement of the process with respect to this aspect can be achieved by conducting the reaction when the polysaccharide is dissolved In 85% phosphoric acid and with sodium nitrite as the oxidant better yield versus a more Important depolymerisation [85-86]. 

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