Oxidation of monosaccharides

To determine the structure of carbohydrates the following oxidizing agents are 

Show the cyclic form of fructose when it is dissolved in water. 

This reducing property is used to measure blood sugar in terms of glucose units. If a sample of urea is mixed with Fehling s reagent, a red precipitate is observed. 

First this keto group is converted into an aldehyde group, then it is oxidized to a carboxylic acid. 

All monosaccharides and disaccharides (except saccharose) are reducing sugars. However, polysaccharides are not reducing sugars. 

Although aldoses exist primarily in cyclic hemiacetal forms, these structures are in equilibrium with a small but finite amount of the open-chain aldehyde. These aldehyde groups can be easily oxidized to acids (review Sec. 9.13). The products are called aldonic acids. For example, D-glucose is easily oxidized to D-gluconic acid. 

The most easily oxidized groups in an aldose are the aldehyde at one end and the primary alcohol at the other. Oxidation of CH=0 gives an aldonic acid oxidation of CH2OH gives a uronic acid, and oxidation of both gives an aldaric acid. 

Aldonic acids are named by replacing the -ose ending of the aldose by -onic acid. Similarly, the endings -uronic acid and -aric acid are used in uronic and aldaric acids, respectively. 

The most commonly used method for preparing aldonic acids is by oxidation with bromine in aqueous solution. The species that is oxidized is a furanose or pyranose form of the carbohydrate. 

Aldonic acids spontaneously form five-membered (7) or six-membered (8) lactones. 7-Lactones are normally more stable than 8-lactones. 

What is the structure of the aldonic acid that is produced during the oxidation of L-rhamnose  

Both enzymatic catalysis and chemical catalysis can be applied under proper experimental conditions in order to prepare the desired product, and literature data may assist in making the best choice. According to the pioneering work of Heyns and Paulsen, the chemical oxidation of carbohydrates with finely divided platinum is first directed toward the anomeric carbon. 

When only secondary hydroxyl groups are present in the cyclic system, the axial hydroxyl groups are oxidized in preference to the less reactive equatorial hydroxyl groups [17]. 

Laboratory-scale liquid phase oxidation with heterogeneous catalysis is generally performed under mild conditions by bubbling air or oxygen at 20-50 C and atmospheric pressure in the presence of Pd, Pt, Au, or derived bimetallic catalysts [4]. 

Gold deposited on supports such as C, AI2O3, Ti02, or Ce02 was found to be very active and selective for the oxidation of many monosaccharides and disaccharides [20-22]. 

Unlike from palladium and platinum, gold does not require any promoter. However, with gold catalysts the activity is very sensitive to the size of the metal particles, with activity in the narrow range of 3-7 nm [22]. 

Benedict s reagent is reduced in the process to cuprous oxide. 

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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. 

Kinetics of oxidation of four pentoses by bromamide-T were conducted in alkaline medium at different temperatures and the overall activation parameters have been calculated.52 Aldonic acids were the oxidation products, and a mechanism was suggested in which formation of the enediol anion of the sugar is the rate-limiting step. As aldoses may undergo epimerization in alkaline solutions, the oxidation of monosaccharides with bromamide-T was also performed in hydrochloric acid solution.53 Kinetic parameters revealed a low reactivity of ketoses relative to aldoses, and indicated that the cyclic forms of the latter are involved in the oxidations. 

P. O. I. Virtanen, O. Oikarinen, and H. Nevala, Vanadium(V), Part XXI. A comparison of the rates of oxidation of monosaccharides and selected model compounds with vanadium(V), J. Carbohydr. Chem., 8 (1989) 313-318. 

Oxidation of Monosaccharides Reducing Sugars 1117 23-11 Nonreducing Sugars Formation of Glycosides 1119 23-12 Ether and Ester Formation 1121... 

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... 

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... 

The synthesis and catalytic properties of Pd, Pt, Ru nanoparticles in nanostructures ofpolymers are described. The controlled growth of metal nanoparticles in a polymer matrix was achieved. The catalytic systems have been studied in selective oxidation of monosaccharide. These processes are of particular interest because the products are the intermediates in the synthesis of biology active substances. 

These reactions may be accompanied by unwanted side-reactions, such as oxidation of hypobromite to bromate or its reduction to bromide. The electro-oxidation of 2,3 4,6-di-0-isopropylidene- -L-sorbofuranose (14) is affected by a number of factors, and here the method of mathematical planning of so-called extreme experiments for obtaining the optimal conditions for electrolysis was utilized the conditions are concentration of sodium bromide, 107.7 g per liter concentration of nickel chloride, 0.71 g per liter concentration of 2,3 4,6-di-0-isopropylidene-L-sorbose, 86 g per liter the amount of electric current passed, 1.912 A-hr/g of the diisopropylidene acetal current density, 4.56 A.dm pH of the solution, 9.83 the expected yield, 91.9 0.7%. A more-detailed survey of the mechanism and kinetics of the electrochemical oxidation of monosaccharides and their derivatives, as well as of the effect of experimental conditions on the yields of aldonic acids and of the di-O-isopropylidene-xylo-hexulosonic acid (15) formed, has been given. ... 

Along with other carbohydrates, the oxidation of monosaccharides by supported metal catalysts has been reviewed [16]. Glucose is the most investigated substrate for the synthesis of a variety of products and intermediates (Figure 21.6). 

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