Polysaccharides, esterification

Hydroxyl-group reactions, such as etherification, acetalatlon (41), esterification (42-51). and oxidation, can be done in the same manner as they are on other polysaccharides. Esterifications... 

Carboxylic acid esters of xylan are prepared under typical conditions used for polysaccharide esterification, i.e., activated carboxylic acid derivatives are allowed to react >vith the polymer both heterogeneously and homogeneously (Figure 1). The heterogeneous esterification of oakwood sawdust and wheat bran... 

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. 

Every polysaccharide contains glycosyl units with unsubstituted hydroxyl groups available for esterification or etherification. Polysaccharide derivatives are described by their degree of substitution (DS), which is the average number of substituent groups per glycosyl unit. Because each monomeric unit of cellulose molecules has free hydroxyl groups at C-2, C-3, and C-6, the maximum DS for cellulose, and all polysaccharides composed exclusively of neutral hexosyl units, the majority of polysaccharides, is 3.0. 

Kim, J-B., Caipita, N.C. (1992), Changes in esterification of the uronic acid groups of cell wall polysaccharides during elongation of maize coleoptiles. Plant Physiol. 98, 646-653. 

Heinze, T., Liebert, T., Koschella, A. Esterification of Polysaccharides ISBN 3-540-32103-9... 

Another potential site of reactivity for anhydrides in protein molecules is modification of any attached carbohydrate chains. In addition to amino group modification in the polypeptide chain, glycoproteins may be modified at their polysaccharide hydroxyl groups to form ester derivatives. Esterification of carbohydrates by acetic anhydride, especially cellulose, is a major industrial application for this compound. In aqueous solutions, however, esterification may be a minor product, since the oxygen of water is about as strong a nucleophile as the hydroxyls of sugar residues. 

Esterification is not difficult if xylan is first carefully precipitated from solution and dried to a fluffy, non-horny powder, or if the xylan can be highly swollen so as to make the polysaccharide molecules easily accessible to the esterifying reagents. Solechnik101 finds that swollen undried xylan, when treated with acetic anhydride containing sulfuric... 

Here we report an overview of the different heterogeneously-catalyzed pathways designed for the selective conversion of carbohydrates. On the basis of these results, we shall try to determine the key parameters allowing a better control of the reaction selectivity. Water being commonly used as solvent in carbohydrate chemistry, we will also discuss the stability of solid catalysts in the aqueous phase. In this review, heterogeneously-catalyzed hydrolysis, dehydration, oxidation, esterification, and etherification of monosaccharides and polysaccharides are reported. 

The enzymatic esterification of polysaccharides, e.g., glucomannan, a copolymer of glucose and mannose, has also been reported [118]. 

It was established by Braudo and coworkers (1998) that an increase in the degree of pectin esterification leads to a greater extent of pectin association in aqueous solution (an increase in Mw). This is illustrated in Figure 6.7. The strength of the attractive interactions between the polysaccharide associates increases with the degree of pectin esterification, as reflected in the lowering of the value of the second virial coefficient. 

In die -elimination reactions discussed next, the electron-withdrawing groups are first introduced chemically into the polysaccharide this may be done by esterification of a carboxylic acid group, by oxidation of an alcohol group to a carbonyl group, or by nucleophilic-displacement reactions. 

On esterification, the carboxyl group in uronic acid residues becomes a carboxylate that is electron-withdrawing, and, on subsequent treatment with base, / -elimination will occur. The p-elimination reaction for uronic acids and their derivatives has been reviewed in this Series, 6 and only its applications to structural polysaccharide chemistry will be discussed here. 

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