Oxidation of Aldonic Acids

Oxidation of Aldonic Acids. Preferential oxidation of the secondary alcoholic group adjacent to the carboxyl group in sugar acids or aldonic acids such as L-gulonic acid (XXV) can be carried out with chromic acid12 or with chlorates in the presence of a vanadium catalyst.13... 

Two useful preparations of lower sugars by degradation of aldonic acids are discussed in the following Chapter. These are the traditional oxidation of salts of aldonic acids with hydrogen peroxide in the presence of ferric acetate (Ruff degradation) and the related oxidation of aldonic acids by hypochlorite, which was developed by Whistler and Schweiger.79... 

Chloric acid, in conjunction with catalysts (particularly vanadium pentaoxide), is used for the oxidation of aldonic acids or lactones to the 2-glyculosonic acids. Thus, D-glucono-1,4-lactone (9) and potassium D-galactonate in methanol, in the presence of phosphoric acid and vanadium pentaoxide, are oxidized by chloric acid to methyl D-arabino-2-hexulosonate (10) and methyl D-/yxo-2-hexulosonate, respectively.38 At moderate temperatures in the absence of a catalyst, aldoses, ketoses, and sucrose are inert to the action of chlorates over a several weeks time period 39 bromates in alkaline solution also exert no oxidative action (Scheme 5). 

A. Abbadi and H. van Bekkum, Highly selective oxidation of aldonic acids to 2-keto-aldonic acids over Pt-Bi and Pt-Pb catalysts, Appl. Catal. A, 124 (1995) 409 117. 

Ruff-Fenton degradation. Shortening of the carbon chain of sugars by the oxidation of aldonic acids (as calcium salts) with hydrogen peroxide and ferric salts. 

One of the most useful methods for shortening the carbon chain of a sugar involves oxidation of a soluble salt of the aldonic acid with hydrogen peroxide in the presence of ferric acetate 184), Prior to the development of this method by Ruff, H. J. H. Fenton had shown that tartaric acid is oxidized by hydrogen peroxide in the presence of ferrous salts, but apparently no oxidative cleavage of carbon-carbon bonds was noted. The ferrous ion-catalyzed oxidation was extended to many carbohydrates by associates of Fenton (Chapter VI) and by other workers 185), The ferric ions used as the catalyst in the Ruff method permit the oxidation of aldonic acids but are inactive with respect to sugars. Ferrous ions are much less selective. 

Chloric acid in conjunction with catalysts, particularly vanadium pent-oxide 204) j has as its principal use the oxidation of aldonic acids or lactones to the 2-keto acids, intermediates in the preparation of ascorbic acid and analogs, as discussed in a preceding section. 

Derivatives of aldoses in which the terminal aldehyde function is oxidized to a car boxylic acid are called aldonic acids Aldonic acids are named by replacing the ose ending of the aldose by omc acid Oxidation of aldoses with bromine is the most com monly used method for the preparation of aldonic acids and involves the furanose or pyranose form of the carbohydrate... 

Although the Tollens reaction is a useful test for reducing sugars, it doesn t give good yields of aldonic acid products because the alkaline conditions cause decomposition of the carbohydrate. For preparative purposes, a buffered solution of aqueous Br2 is a better oxidant. The reaction is specific for aldoses ketoses are not oxidized by aqueous Br2. 

G. Hourdin, The catalysis of the Ruff oxidative degradation of aldonic acids by titanium-containing zeolites, Catal. Lett., 69 (2000) 241-244. 

The aldose (11.1 mmol) was dissolved in water to a final concentration of 0.5 m and subjected to oxidation by addition of glucose oxidase (200 mg, 400 U) and a large excess of catalase (1 mL, 25 kU). The mixture was vigorously stirred under air and the pH was kept constant at 7.5 by means of a pH-stat adding continuously 1 m NaOH. The conversion degree was directly calculated considering the volume of added 1 m NaOH, since 1 mol of NaOH neutralizes 1 mol of aldonic acid formed. 

For a long time, this reaction was conducted exclusively with acety-lated nitriles of aldonic acids, and the products obtained were known in general as "aldose-amides. Fischer2 used this reaction to transform tetra-O-acetyl-L-rhamnononitrile into l,l-bis(acetamido)-l,5-dideoxy-L-arabinitol, whose subsequent hydrolysis and oxidation allowed him to determine the configuration of dextro-tartaric acid (L-threaric acid). 

Aldaric acids may be prepared from aldoses or aldonic acids by oxidation in aqueous solution with oxygen over platinum-charcoal255 or platinum-on-alumina.256 The effect of such promoters as bismuth or gold has also been studied.257 Hydrogen peroxide in the presence of iron salts has been used for the oxidation of uronic acids to aldaric acids.258... 

Bromine (hypobromite) and hypoiodite oxidations are particularly useful for the preparation of aldonic acids from aldoses and of aldaric acids from glycuronic acids. Primary alcohol groups also undergo oxidation by these reagents, although this conversion is of less value glycosides can thus be converted into glycosiduronic acids, and alditols into aldoses and aldonic acids. 

Jeanes and Isbell39 found that, under mild conditions, aldoses are oxidized to aldonic acids, but that nonreducing carbohydrates and ketoses are oxidized only slowly. The rate of oxidation decreases in the order pentoses > hexoses > disaccharides however, in contrast to other oxidants, chlorous acid oxidizes a-hexoses more rapidly than the ft anomers. The yields of aldonic acids are, however, less than those from bromine oxidations.40 The equation for the oxidation in acidic solution was expressed as ... 

The monosaccharides formed during pulping are not completely stable. About 10-20% of them are oxidized to aldonic acids by hydrogen sulfite ions. Simultaneously, hydrogen sulfite is reduced to thiosulfate ... 

Fig. 8-18. Oxidative stabilization of cellulose through formation of aldonic acid end groups. R is cellulose chain. After oxidative formation of a glucosone intermediate this undergoes a benzilic acid rearrangement strongly favoring formation of mannonic acid end groups (2). In addition, cleavage of C-1 to C-2 and C-2 to C-3 bonds gives rise to the formation of arabinonic acid (3) and erythronic acid (4) end groups. (In this simplified scheme only the main products are shown.)...

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