Polysaccharides etherification

Ethers are important derivatives of both monosaccharides and polysaccharides. Etherification is often used in the determination of structures and types of linkages between sugars in oligo- and polysaccharides. Table 2-2 gives examples of the preparation of ethers. Ethers are very stable against both acids and bases. 

Other methylation procedures of value in the pol3rsaccharide field have been advocated. Methylation of a solution of the polysaccharide in tetra-hydrofuran with solid sodium hydroxide and dimethyl sulfate can be used to totally methylate an already partially methylated polysaccharide. Etherification takes place readily the solvent appears to stabilize the alkoxide intermediate in the reaction long enough to facilitate ether formation at the expense of competing side reactions. Another useful methylation procedure involves treatment of the polysaccharide with methyl iodide and thallous hydroxide. 

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. 

The acidic sugars discussed in this Section are glycuronic acids and glycu-losonic acids. Bacterial polysaccharides may also become acidic by substitution of sugar residues, for example by etherification with lactic acid, acetala-tion with pyruvic acid, or phosphorylation, and these possibilities will be discussed in the following Sections. A sugar that does not fall into any of... 

Most of the selective-etherification studies on polysaccharides have been made with cellulose, and nearly all of them have involved quantitative separation of the D-glucose derivatives formed on hydrolysis of the partially substituted celluloses. In view of their stability, ethers of polysaccharides are particularly suited to this approach. 

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 most widely recognized method of selective etherification of sugars and polysaccharides involves the reaction of primary alcohol groups with triphenylmethyl (trityl) chloride. Helferich60 has described the selective character of this reagent, whose rate of reaction with primary hydroxyl groups is many fold that with secondary groups. 

The value of number-average molecular weights of methylated polysaccharides, used in conjunction with methylation end-group assays in the assessment of the degree of branching, has been stressed previously (see p. 435). Etherification is, however, normally carried out under alkaline con-... 

The aim of the methylation is to achieve an etherification of all of the free hydroxyl groups in the polysaccharide. The original procedure, as used by Denham and Woodhouse and by Haworth, in which the polysaccharide in 25-30% aqueous sodium hydroxide is treated with dimethyl is still the standard method. Fresh reagents are usually added to the reaction mixture after the first methylation period, with or without removal of the sodium sulfate formed, and the process is repeated until there is no further increase in methoxyl content. The many slight modifications of this method seem mostly to be matters of habit or convenience. The use of more concentrated sodium hydroxide solutions has been suggested, and occasionally seems to be essential for a satisfactory result. A practical point is that an aqueous solution of 42.2 g. of sodium hydroxide per 100 ml of solution is of exactly the concentration that 2 ml. is equiva-... 

Etherifications (alkylations) are carried out for two reasons structural analysis (O Sect. 8.1) and to change the nature (properties) of the polysaccharide (O Chap. 6.4). The basic reaction used is the classical Williamson ether synthesis, i. e., some of the hydroxyl groups on the polysaccharide are converted into the alkoxy form, then these molecules are reacted with... 

Cellulose, the feedstock for cellulose ethers, is a polysaccharide like xanthane, guar and starch. It is a biopolymer made by photosynthesis and thus a renewable raw material. Cellulose is water insoluble, and only becomes water-soluble after hydrolysis and etherification [104, 105] (Fig. 5). 

Abstract Cellulose is the most important biopolymer in Nature and is used in preparation of new compounds. Molecular structure of cellulose is a repeating unit of p-D-glucopyranose molecules forming a linear chain that can have a crystallographic or an amorphous form. Cellulose is insoluble in water, but can dissolve in ionic liquids. Hemicelluloses are the second most abundant polysaccharides in Nature, in which xylan is one of the major constituents of this polymer. There are several sources of cellulose and hemicelluloses, but the most important source is wood. Typical chemical modifications are esterifications and etherifications of hydroxyl groups. TEMPO-mediated oxidation is a good method to promote oxidation of primary hydroxyl groups to aldehyde and carboxylic acids, selectively. Modified cellulose can be used in the pharmaceutical industry as a metal adsorbent. It is used in the preparation of cellulosic fibers and biocomposites such as nanofibrils and as biofuels. 

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... 

The reaction of monochloroacetic acid with polysaccharides occurs first with the secondary hydroxyl groups, particularly at the beginning of the reaction. However, as in the etherifications already discussed, digestion of 0-(carboxymethyl)cellulose with eellulase reveals that substitution along the chain is non-uniform, accessibility playing an important part. 

During our studies on cellulose chemistry ), we have encountered an unusual pattern of solubilities of various celluloses and related polysaccharides in one of the nonaqueous cellulose solvent systems we investigated, the S02-diethylamine(DEA)-dimethyl-sulfoxide(DMSO) system. In this paper, we propose an interpretation of this pattern in terms of intra- and intermolecular hydrogen bonds in native, mercerized and regenerated celluloses. We also consider parallels with the relative reactivities of hydroxyl groups in glucose residues of cellulose toward etherification, under basic conditions, and the data from solid-state C-NMR reported by Atalla and others ( - ) ... 

A variety of polysaccharide ethers are produced industrially. They include methyl, ethyl, hydroxyethyl, hydroxypropyl, and carboxymethyl ethers. Combinations of these with other subslitulions are often seen. Because of the wide range of properties produced and low cost, etherifications are among the most common industrial modifications. The mechanism for etherification varies, depending on the desired subslitulion. Methylalion can be achieved with a simple Williamson synthesis where the hydroxyl is exposed by the addition of caustic soda [Figure 2] (4). 

A final mechanism worth noting is the reverse Michael reaction, in which an activated vinyl group is added to the polysaccharide through a 0-(2-cyanoethyl) derivative [Figure 4] (5). While other etherification reaction mechanisms have been investigated, they have limited qtplication. 

Guar gum and locust bean gum are galactomannans fiom plant sources. Because of similar chemical compositions, both have similar applications and modifications. Both gums bind well with many polysaccharides which alter both dispersion and viscosity characteristics. Etherification is the most common modification of these gums, and, in general, imparts greater viscosity, solubility, and solution clarity. 

Notes Except in the etherification of aliphatic alcohols in the presence of a Lewis acid, in which case dichloromethane is most frequently used as the solvent (Section 2.9), ethereal and ether—alcoholic solutions are the most commonly used forms of diazoalkanes in alkylation reactions. The substrate may be dissolved in ether (preferred), alcohol, alcohol-water [45] or dimethyl sulfoxide, a solvent which has been successfully applied for acidic polysaccharides [46, 47]. 

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