Aldaric acids, sugar oxidation

There is another sugar, (-l-)-gulose, that gives the same aldaric acid on oxidation as does (-l-)-glucose. 

When an oxidizing agent, e.g. nitric acid, is used, a sugar is oxidized at both ends of the chain to the dicarboxylic acid, called aldaric acid. For example, galactose is oxidized to galactaric acid by nitric acid. 

Three kinds of sugar acids can be formally obtained from the corresponding aldoses. They are aldonic acids, produced by oxidation at C-l of the aldose uronic acids, formed by oxidation of the primary alcohol group of the aldose and aldaric acids, formed by oxidation of both the aldehyde and the primary alcohol group. 

Aldonic and aldaric acids are usually obtained by oxidation methods from the corresponding aldoses, whereas a glycoside is the usual starting material for the synthesis of glycuronic acids, important natural sugars that are constituents of many polysaccharides. 

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. 

Oxidation of hydroxy aldehydes may affect only the aldehyde group or both the aldehyde and alcoholic groups. Both types are especially common in sugar chemistry for example, aldoses may be converted into aldonic acids or aldaric acids. 

The oxidation of the primary hydroxyl function at C1 of a sugar alcohol to a carboxyl function leads to the corresponding aldonic acid. Further oxidation of the remaining terminal hydroxyl function of an aldonic acid to a carboxyl function leads to an aldaric acid. Both, aldonic and aldaric acids easily form lactones through intramolecular condensation reactions. As an exam-... 

As depicted in Scheme 21.1 for the oxidation of glucose, three general types of acids can be produced by the oxidation of a sugar aldonic acids in which only the aldehyde of the parent aldose has been oxidized to the acid aldaric acids that are dicarboxylic acids with one carboxy group coming from the oxidation of the aldehyde and the other from the primary alcohol at the other end of the carbon chain or uronic acids derived from the selective oxidation of the primary alcohol without the oxidation of the aldehyde. 

When (-)-arabinose is oxidized with nitric acid, the aldaric acid that is obtained is optically active. This means that the aldaric acid does not have a plane of symmetry. Tlierefore, (-)-arabinose must have the stracture shown on the left because the aldaric acid of the sugar on the right has a plane of symmetry. Thus, (+)-glucose and (-l-)-marmose are represented by sugars 3 and 4. 

Identify A, B, C, and D in the preceding problem if D is oxidized to an optically inactive aldaric acid. A, B, and C are oxidized to optically active aldaric acids, and interchanging the aldehyde and alcohol groups of A leads to a different sugar. 

Aldaric acid A dicarboxylic acid, also called saccharic acid, formed by vigorous oxidation of sugars with... 

Sugar X is known to be a D-aldohexose. On oxidation with HNO3, X gives an optically inactive aldaric acid When X is degraded to an aldopentose, oxidation of the aldopentose gives an optically active aldaric acid Determine the structure of X. 

Carbohydrate diacids (aldaric acids) are derived from oxidation of the terminal carbons of simple aldoses. Several aldaric acids are suggested as being reasonable candidates for commercial development, based on potential applications, and availability and cost of the individual simple sugar precursors. 

We can be sure that the aldotetroses that we obtain from this Kiliani—Fischer synthesis are both D sugars because the starting compound is D-glyceraldehyde and its chirality center is unaffected by the synthesis. On the basis of the Kiliani—Fischer synthesis, we cannot know just which aldotetrose has both —OH groups on the right and which has the top —OH on the left in the Fischer projection. However, if we oxidize both aldotetroses to aldaric acids, one [d-(—)-erythrose] will yield an optically inactive (meso) product while the other [d-(—)-threose] will yield a product that is optically active (see Practice Problem 22.7). 

Oxidation of aldoses yields aldonic acids (onic acids), uronic acids, and glycaric acids (sugar dicarboxylic acids, aldaric acids) (Fig. 36). Aldonic acids easily cyclize to the corresponding y-lactones. Glycaric acids may form dilactones. 

Trioses and tetroses have been separated as their trimethylsilylated oximes by g.l.c. mass-spectral data for these derivatives were also recorded. Mixtures of xylitol, arabinitol, mannitol, glucitol, and maltitol have been separated efficiently by h.p.l.c. of the corresponding 4-nitrobenzoates. Alditols have been determined in the presence of neutral sugars by oxidation with acidified sodium periodate and colorimetric measurement of the formaldehyde released. G.l.c. retention data have been reported for the TMS derivatives of a large number of aldonic acids and their lactones and aldaric acids. ... 

Two general methods for the synthesis of uronic acids have been developed (1) the reduction of the monolactones of aldaric acids, and (2) the oxidation of primary alcoholic groups of sugars or derivatives. The monolactones of dibasic acids can be reduced by sodium amalgam in acid solution... 

FIGURE 22.29 Oxidation of a sugar with nitric acid generates an aldaric acid, which has a carboxylic acid group at each end of the molecule. 

Therefore the only way two different aldoses can be oxidized to give the same aldaric acid is if those two sugars differ by turning the Fischer projection end to end (Fig. 22.50). 

Aldatic acid (Section 22.4) A diacid derived from an aldohexose by oxidation with nitric acid. It has the overall structure HOOC—(CH0H)4—COOH. In an aldaric acid, the old aldehyde and primary alcohol ends of the sugar have become identical. 

Ruff degradation of two d pentoses, A and B, gave two new sugars, C and D. Oxidation of C with HNO3 gave OTeso-2,3-dihydroxybutanedioic (tartaric) acid, that of D resulted in an optically active acid. Oxidation of either A or B with HNO3 furnished an optically active aldaric acid. What are cort5)ounds A, B, C, and D ... 

The structural assignment for the four sugars obtained from o-ribose and D-lyxose, respectively, is again accomplished by oxidation to the corresponding aldaric acids. Both D-aUose and D-galactose give optically inactive oxidation products, in contrast with o-altrose and D-talose, which give optically active dicarboxyUc acids. 

Oxidation of D-glucose should give an optically active aldaric acid, whereas that of D-atlose leads to loss of optical activity. This result is a consequence of turning the two end groups along the sugar chain into the same substituent. 

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