Oxidation to uronic acids

Unprotected or partially protected glycosides cannot generally be oxidized to uronic acid by the above-described methods. However, for unprotected carbohydrates milder and more selective oxidants have been developed that take advantage of the primary alcohol function being more sterically accessible. An important reagent for this transformation is 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) which is a shelf-stable and commercially available nitrosyl radical... 

The reaction was monitored also by p.c. which showed clearly on increase in the relative proportion of uronic acids related to xylose. After 90 minutes of oxidation, only uronic acids were observed indicating the a-configuration. 

A double mediatory system consisting of modified TEMPO and halide ion or metal ion was also exploited for the oxidation of alcohols [53-55]. A number of carbohydrates have been chemoselectively oxidized at the primary hydroxyl group to uronic acids [56]. 

Partially protected carbohydrates can be selectively oxidized at the primary hydroxy group to uronic acids at the nickel hydroxide electrode. At the same electrode, in polyhydroxy steroids, a preferential oxidation of the sterically better accessible hydroxyl groups is achieved [142]. By way of the mediator, TEMPO, carbohydrates that are only protected at the anomeric hydroxyl group are selectively oxidized at the primary hydroxyl group (Fig. 27) [143-145]. 

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... 

N. J. Davis and S. L. Flitsch, Selective oxidation of monosaccharide derivatives to uronic acids, Tetrahedron Lett., 34 (1993) 1181-1184. 

The oxidation of uronic acids (VIII) leads to the formation of saccharic acids (XIV) mild conditions are necessary to avoid damaging the sensitive uronic acid grouping. In a similar manner, osones (VII) may be converted to 2-keto aldonic acids (XII). 

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. 

The oxidation at Cl can be prevented by protection of the 1-OH group. The glycosidic bond in oligosaccharides offers sufficient protection. Application of weak oxidants offers direct route to uronic acids. 

Oxidation at the terminal secondary alcoholic hydroxyl group (C6-OH) to uronic acid (Fig. 2B). 

Selective Oxidation of Hydroxymethyl Groups. Platinum on charcoal in the presence of air is the conventional method for selectively oxidising hydroxymethyl groups to carboxylic acids, and thus hexoses to uronic acids anomeric protection is required, however, and the mechanism of this heterogeneous reaction is unknown. The reaction fails with polymers, probably for... 

Oxidation (Section 23.19) Bromine oxidizes the aldehyde function of an aldose to a carboxylic acid. The product is an aldonic acid that normally exists as a lactone. The main path to uronic acids, carbohydrates that bear a CO2H group instead of CH2OH, is biosynthetic. Nitric acid oxidizes both the CHO and CH2OH ends of an aldose to CO2H. These compounds are called aldaric acids. 

Nitrogen dioxide oxidizes the primary hydroxyl groups of polysaccharides to uronic acids (161). Side reactions occur to an extent of 10 to 15%, presumably to produce carbonyl groups. These oxidized polysaccharides have some applications, based principally on their conversion to a soluble form by salt formation. 

A review has been published on the oxidation of carbohydrates to carboxylic acids, with particular emphasis on catalytic oxidation of primary alcohol functions to uronic acids by use of molecular... 

Pivaloyl-p-D-glucopyranuronic acid has been prepared in order to identify it as a metabolite of pivaloyloxyalkyl-containing prodrugs, the acid grouping being introduced by catalytic oxidation. i Electrochemical oxidation of some carbohydrate primary alcohols at a nickel hydroxide electrode in alkaline solution has been developed as a route to uronic acids. Examples... 

Uronic acids are monosaccharides in which the terminal primary alcohol group is oxidized to a carboxyhc acid functional group, eg, D-glucuronic acid [6556-12-3] (11). 

---