Oxidation of Aldoses to Aldonic Acids

According to the reactivity scale proposed by Heyns et al. [2-4], oxidation at the anomeric position at C] is easier than for primary or secondary alcohol functions. Selectivity higher than 95 % for aldonic acids were indeed observed in the catalytic oxidation of o-glucose, and of reducing disaccharides such as lactose and maltose. Palladium catalysts or palladium-bismuth catalysts are more selective than platinum because they are less active in the oxidation of primary or secondary alcohol functions. 

D-gluconate 2, 2-keto-D-gluconate 3, 5-keto-D-gluconate + o-glucarate 4, D-fructose. Reaction conditions 1.7 mol L, T = 313 K, pH 9 [glucose]/[Pd] = 787 air at atmospheric pressure. 

The selective oxidation of lactose to lactobionate with air on palladium-bismuth catalysts was first reported in patents [47]. Hendriks et al. [16] studied the oxidation of a 0.5 mol solution of lactose as a function of pH, temperature and Pd/Bi ratios of promoted Pd/C catalysts. Sodium lactobionate was obtained with 100% selectivity up to 95% conversion on Pd-Bi/C catalysts (Bi/Pd = 0.5) at 333 K and pH 9. Oxygen mass-transfer limited the maximum initial reaction rate (0.47 mol kg s ). The catalyst was recycled 15 times without any significant loss of activity and selectivity. 

Abbadi et al. [34] studied the oxidation of lactose and sodium lactobionate to 

2-keto-Na lactobionate. The oxidation of lactose on Pt-Bi/C catalysts at pH 7 yielded lactobionate transiently (oxidation of the anomeric center at C,) this 

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The oxidation of reducing disaccharides (V) leads to the formation of aldobionic acids in a manner similar to the oxidation of aldoses to aldonic acids conditions must be such that the glycosidic linkage is not hydrolyzed. 

This method involves the oxidation of aldoses to aldonic acids by potassium hypoiodite in methanol and the condensation of the isolated potassium salt of the aldonic acid in hot acid with o-phenylenediamine (M3). The specific aldobenzimidazole can be identified by the solubility in water, optical rotation in acid, and melting point. This method is not as simple as chromatography. It requires dehydration of the sample and a relatively high concentration of the sugars to be identified. 

There have been reports on the electro-chemical oxidation of glucose to give D-gluconic acid, and its zinc and magnesium salts. The kinetics and mechanism of the Ru04-catalyzed oxidation of aldoses to aldonic acids by alkaline N-bromoacetamide have been smdied, with the conclusion that hypobromite is the reactive oxidizing species. ... 

P. C. C. Smits, B. F. M. Kuster, K. Van der Wiele, and H. S. Van der Baan, The selective oxidation of aldoses and aldonic acids to 2-ketoaldonic acids with lead-modified platinum-on-carbon catalysts, Carbohydr. Res., 153 (1986) 227-235. 

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. 

The indirect electrochemical oxidation of aldoses to the corresponding aldonic acids 273-27 ), which was carried out industrially as early as about 1930, is still used today for production on the tonne scale by Sandoz 27 S) and in India 276). Specific examples are the anodic oxidation of lactose to calcium lactobionate 275,277 278) ... 

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. 

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... 

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... 

Alkaline hypoiodite oxidizes aldoses, under carefully controlled conditions, almost quantitatively to aldonic acids (see preceding chapter). Measurement of the iodine consumed permits quantitation of the amount of aldose originally present [Eq. (19)]. 

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 ... 

In perchloric acid, hexoses and pentoses are oxidized by Ce(IV) via formation of two complex intermediates. The first is partly oxidized following Michaelis-Menten kinetics and partly dissociated to the second, which is oxidized more slowly than the former.180 The first step in the oxidation of aldoses by Tl(III) in the same medium involves the C-l-C-2 cleavage of the aldehydo form of the sugar. Thus, D-glucose gives D-arabinose and formic acid. With an excess of oxidant the final product is carbon dioxide.181 In the presence of a catalytic amount of sulfuric acid in acetic acid, Tl(III) oxidizes maltose and lactose to the corresponding disaccharide aldonic acids. The reaction showed activation enthalpies and enthropies characteristic of second-order reactions.182... 

Many other cations of transition metals have been employed for the oxidation of sugars. For example, the oxidation of aldoses by hexachloroiridate(IV)183,184 and tetrachloroaurate(III)183,185 in hydrochloric acid led to the corresponding aldonic acids or aldonolactones. The observed... 

In 1931, Isbell and Frush reported the electrolytic oxidation of aldoses in the presence of calcium carbonate and a small amount of bromide, which served as a catalyst. The main solutes in the reaction mixture are the aldose and the calcium salt of the aldonic acid. The latter is easily crystallized from solution. The salts of D-gluconic, D-xylonic, lactobionic and maltobionic acids were prepared. A description of the process was published in 1932 and a patent was issued in 1934. German and French patents were issued in 1931 to the Rohm and Haas Co. ... 

Moore and Link developed a method for the oxidation of aldoses with hypoiodite in methanol to produce aldonic acids, which are isolated as the potassium or barium aldonates in 90% yield. The procedure was worked out for the purpose of preparing... 

Oxidation — The oxidation of aldoses (5.24) with bromine or chlorine in alkaline solution (hypobromites and hypochlorites, respectively) leads to aldonic acids that readily self-esterify (lactonize) into S- (5.25) and y- (5.26) lactones residing with free acid (5.27) in equilibrium. y3-Conformers oxidize more readily than a-conformers. 

Iodine is not reactive in an acidic medium. In a basic medium, it is converted into hypoiodite, a powerful oxidant, and quantitatively oxidizes aldoses into aldonic acids according to reaction (6.2), which is of analytical as well as preparative value. 

Oxidation of aldoses by chlorites also results in the formation of the corresponding aldonic acids. The aldopentoses react faster than aldo-hexoses, and monosaccharides are oxidized more rapidly than are the disaccharides. The ketoses remain unaffected unless drastic conditions are employed. The principal reaction has been shown to be as follows. 

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