Oxidation of polysaccharides

Oxidation of polysaccharides is a far more attractive route to polycarboxylates, potentially cleaner and less cosdy than esterification. Selectivity at the 2,3-secondary hydroxyls and the 6-primary is possible. Total biodegradation with acceptable property balance has not yet been achieved. For the most part, oxidations have been with hypochlorite—periodate under alkaline conditions. In the 1990s, catalytic oxidation has appeared as a possibiUty, and chemical oxidations have also been developed that are specific for the 6-hydroxyl oxidation. 

The oxidation of polysaccharide on carbon 6 is obviously rather a slow process at low temperatures and starts to be important at temperatures above 100°C. The stoichiometry of the reaction for cellulose is depicted in Scheme 10. 

Oxidation of polysaccharides using 10 mM or greater concentrations of sodium periodate at room temperature results in the cleavage of adjacent hydroxyl-containing carbon-carbon bonds on other sugars besides just sialic acid residues (Lotan et al., 1975). High concentrations... 

Trifluoroacetic acid is volatile, and thus readily removed. This acid was used by Albersheim and coworkers for the hydrolysis of plant cell-walls,39 and has since been employed for cell walls,40-43 plant mucilages,44 blood-group oligosaccharides,45 peptidogalactoman-nans,46 heparin,47 and disaccharides in blood and urine.48,49 It has also been suggested as an alternative to 6 M hydrochloric acid in the determination of amino sugars,50 and for the hydrolysis of polyalcohols produced by periodate oxidation of polysaccharides.503 Lee... 

The linear dextran has also been used in investigating the mechanism of the periodate oxidation of polysaccharides. The results support the proposal that inter-residue, hemiacetal formation is a general occurrence in the later stages of the oxidation.128... 

The periodate oxidation of carbohydrates, an important analytical technique, has been reviewed earlier in this Series.73 The dialdehydes obtained on periodate oxidation of polysaccharides have also been discussed.74 The requisite for the degradations to be discussed here is that part of the sugar residues in a polysaccharide are not oxidized by periodate and can be obtained separated from the oxidized residues as mono-, oligo-, or poly-saccharide derivatives after some chemical treatment. Characterization of these products may then give significant structural information. 

Painter and coworkers demonstrated that inter-residue hemiacetals may be formed during the periodate oxidation of polysaccharides,... 

The polyaldehydes obtained on periodate oxidation of polysaccharides are alkali-labile, as discussed in previous articles in this Series.74,103 The products are, however, generally intractable mixtures, and this mode of degrading polysaccharides (with alkali) has... 

The use of nitrogen dioxide for the selective oxidation of polysaccharides to polyuronic acids was introduced by Kenyon and his coworkers13,63 in 1941. By this means extensive oxidation of the primary alcohol groups in cellulose was obtained, through the mechanism of preferential nitration followed by decomposition of the nitric acid ester with carboxyl forma-tion.68(0< > Apparently some undissociated nitration products also were formed, since infrared absorption studies54 indicated the presence of nitrate radicals in the polyuronic acid. Side reactions produced carboxyl,... 

The effectiveness of various chlorine oxidants, and the influence of the pH, on D-galacturonic acid has been studied (see Ref. 1, p. 1120). The autocatalytic chlorite oxidation of polysaccharides is similar to that described for the bromate oxidation (see Section II.2). Thus, the reactive oxyhalogen species are reduced to halides, as shown in Eq. (26). 

An advantageous variant of the oxidation of polysaccharides by nitrogen oxides in 85% phosphoric acid consists in the use of sodium nitrate, instead of sodium nitrite, as the stoichiometric oxidant and a catalytic amount of sodium nitrite to decrease the induction time.123 The advantage of this variant may be seen by taking into account the stoichiometry of the overall reactions ... 

Polysaccharides.—Very few oxidations of polysaccharides have been carried out with lead tetraacetate, a situation in marked contrast to the voluminous literature on polysaccharide oxidations with periodate.22 The main reason for this discrepancy has undoubtedly been the fact that polysaccharides are insoluble in the solvents generally used for lead tetraacetate oxidation. 

Besemer, A.C., Preparation of Polydicarboxysaccharides by Oxidation of Polysaccharides and Their Use as Detergent Builders, WO Patent 9117189, 1991. 

Bragd, P.L., van Bekkum, H., and Besemer, A.C., TEMPO-mediated oxidation of polysaccharides survey of methods and applications, Tropics Catalysis, 27, 49-66, 2004. 

Floor, M., Glycol-Cleavage Oxidation of Polysaccharides and Model Compounds Calcium Complexation of Dicarboxy Glucans, Ph.D. thesis, Delft University of Technology, Delft, The Netherlands, 1989. 

The oxidation of polysaccharides by hypochlorite proceeds mainly to the carboxylic acid state provided the pH value is above 9. At lower pH values the formation of carbonyl groups dominates, resulting in alkali lability. 

The products from the oxidation of polysaccharides are, generally, easily isolable. As a polysaccharide containing a-glycol groups is composed of a large number of potentially oxidizable units, a variable degree of oxidation (D.O.) from 0-100 % is obtainable. (Where the D.O. is not specified, it is assumed to be 100 %.) The possibilities for hemiacetal and hemialdal formation are much increased, because these links may be intermolecular as well as intramolecular. 

Lead tetraacetate has been used little for oxidation of polysaccharides. Its ineffectiveness for this purpose probably stems mainly from the fact that the organic media commonly employed for the reagent are unable to dissolve polysaccharides. However, the use of dimethyl sulfoxide (containing about 10% of acetic acid), a good solvent for many polysaccharides, has permitted satisfactory... 

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