Oxidation of Ketoses

Sheppard and Everett reported the production of 5-keto-L-gulonic and 6-keto-L-gluconic acids, respectively, from D-fructose and 

L-sorbose by the action of bromine. The former product represented only one-third of the reducing material. The stability of the keto aldonic acids to bromine had previously been noted by several workers Kiliani prepared a keto-L-rhamnonic acid which did not react within four days with an excess of bromine. Similar results were reported by Everett, Edwards and Sheppard.  

Ruff noticed in the mother liquors of an erythrose preparation obtained by degradation of L-arabonic acid, a small amount of another sugar, apparently a keto tetrose, which was not oxidized by bromine. The ketose gave the same phenylosazone as that obtained from L-ery-throse, but was not isolated otherwise. Neuberg oxidized erythritol to form an optically inactive tetrose solution and claimed that color reactions showed the presence of a keto tetrose. 

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The faster cupric ion oxidation of ketoses compared with aldoses is due to the more strongly reducing nature of the a-hydroxyketone group. Pre-treatment... 

Direct Oxidation of Ketose Sugars. The primary alcoholic group at Cl adjacent to the keto group at C2 of a ketose sugar such as D-fructose (XXI) or l-sorbose, is more sensitive to oxidation than the other primary alcoholic group at C6 or secondary alcoholic groups at C3, C4, and C5. 

Carbon dioxide is not a common oxidation product in periodate work, but it does appear in the oxidation of ketoses,49 a-keto acids,14,39 and a-hydroxy acids,14 39 and it is often a product23 141 of overoxidation. Carbon dioxide analyses have been carried out using the Plantefol apparatus,49 the Warburg apparatus,14 23 and the Van Slyke-Neill mano-metric apparatus,39 and by absorption in standard sodium hydroxide141 followed by back-titration with acid. A most convenient method is the very old, barium hydroxide absorption scheme.16 The carbon dioxide is swept from the reaction mixture into a saturated, filtered barium hydroxide solution by means of a stream of pure nitrogen. The precipitated barium carbonate is filtered, dried, and weighed. This method is essentially a terminal assay. The manometric methods permit kinetic measurements, but involve use of much more complicated apparatus. 

The Mn(II)-catalysed oxidation of glucose by peroxodisulfate ions occurs via a radical-chain mechanism.26 Kinetics of oxidation of thiodiglycollic acid by (trans-cyclohexane-l,2-diaminc-/V, N, N, /V -tetraacetatolmanganateilJI) have been investigated.27 Oxidations of ketoses and aldoses by manganese(IV) in sulfuric acid media have a first-order dependence on sugar and fractional-order dependence on oxidant.28 A mechanism has been proposed for the oxidation of L-malic acid by Mn(III) pyrophosphate in aqueous acid, involving complex formation and radicals.29... 

H. S. Isbell and H. L. Frush, Reactions of carbohydrates with hydroperoxides. Part II. Oxidation of ketoses with the hydroperoxide anion, Carbohydr. Res., 28 (1973) 295-301. 

Kline and Acree studied the hypoiodite oxidation extensively. The alkali and iodine were both added in small portions throughout the reaction. In this manner the concentration of sugar relative to the sodium hypoiodite was kept at a level favorable to the sugar oxidation (reaction 14). At the end of the reaction, the formation of iodate increased rapidly (reaction 15), taking precedence over the oxidation of ketoses and non-reducing sugars. 

The above methods are so designed that the oxidation of ketoses by sodium hypoiodite is restricted to a minimum. Bailey and Hopkins studied the conditions under which ketoses would be oxidized. The reaction rate increased with the temperature, and the extent of the reaction, which was studied over the temperature range of 1 to 35 , went through a minimum at 15°. An excess of alkali in the 17-37 range apparently caused enolization of the D-fructose and increased the extent of the oxidation. When the alkali was added progressively in small amounts, this effect was increased 4-5 times and was independent of the D-fructose concentration. Oxalic acid was isolated and the presence of D-erythronic acid was assumed. 

For selective oxidations in the carbohydrate field, Heyns and coworkers have investigated the use of platinum catalysts in different forms. For reactions in which further oxidation of the desired product is probable (as, for example, in the oxidation of ketose and pentose derivatives ), a milder catalyst of 5 to 10% platinum on activated carbon is recommended. Care must be taken in the preparation of the catalyst otherwise, it is difficult to obtain catalysts of reproducible activity. The platinum is deposited on the carbon by hydrogenation or by reduction with formaldehyde or hydrazine sulfate. The catalyst prepared through formaldehyde... 

Nitric acid oxidation of ketoses results in cleavage of the carbon-carbon links and the formation of lower chain carbon acids. 

Oxidation of ketoses leads to cleavage of the carbon chain, sometimes with the formation of lower aldonic acids, or more generally to extensive degradation. 

Careful control of the conditions is necessary during the oxidation of ketoses with nitric acid. However, whenever it is possible to prepare the... 

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