Polysaccharide ester derivatives

Among optically active polymers, polysaccharide derivatives are particularly valuable. Polysaccharides such as cellulose and amylose are the most readily available optically active polymers and have stereoregular sequences. Although the chiral recognition abilities of native polysaccharides are not remarkable, they can be readily converted to the esters and carbamates with high chiral recognition abilities. The chiral recognition mechanism of these derivatives has been clarified to some extent. 

Lithium salts in organic solvents with high solubility parameters have been previously used to dissolve strongly hydrogen-bonded polyamidesfciy and polysaccharides /for viscosity studies and for preparation of films or fibers. We are not aware of any previous attempts to utilize these solvent systems to prepare ester or carbamate polysaccharide derivatives... 

More recent developments in the field of the Pirkle-type CSPs are the mixed r-donor/ r-acceptor phases such as the Whelk-Of and the Whelk-02 phases.The Whelk-Of is useful for the separation of underiva-tized enantiomers from a number of families, including amides, epoxides, esters, ureas, carbamates, ethers, aziridines, phosphonates, aldehydes, ketones, carboxylic acids, alcohols and non-steroidal anti-inflammatory drugs.It has been used for the separation of warfarin, aryl-amides,aryl-epoxides and aryl-sulphoxides. The phase has broader applicability than the original Pirkle phases. The broad versatility observed on this phase compares with the polysaccharide-derived CSPs... 

More complicated compounds related to monosaccharides are also implicated in the formation of certain polysaccharides. Acid esters, acids derived from deoxysugars, aminosugars, N-acetylated aminosugars, sialic acids (nonulosaminic acids which are N-or O- substituted derivatives of neuraminic acid), etc., are not uncommon as... 

One of the first polysaccharide derivatives used for insolubilization was the acid azide (or hydrazide) of 0-(carboxymethyl)cellulose. Treatment of the methyl ester of O-(carboxymethyl) cellulose with hydrazine gives the hydrazide 31 which, with nitrous acid, yields the azide (32). The azido group evidently reacts with the free functional groups of lysyl, tyrosyl, cysteinyl, and seryl residues (see Ref. 436), and evidence for formation of a covalent bond, as in 33, comes from the fact that the... 

With this background, we recently disclosed a method for the isolation of a highly purified, high molecular weight arabinoxylan from com fiber (70). In addition to our interest in the properties of the parent arabinoxylan, we were also very interested in preparing ester and ether derivatives of this polysaccharide and examining their physical properties. In this context, we describe the methods we have developed for esterification of arabinoxylan from com fiber and characterization of these new polysaccharide derivatives. 

Ethers can be formed with the alcohol groups of polysaccharides by reacting an alkaline solution or suspension of the polysaccharide with alkyl halides or epoxides [8]. The ethers produce polysaccharide derivatives that are usually soluble in water. This is in contrast to polysaccharide esters, which are usually insoluble in water and soluble in organic solvents. 

Polysaccharides, the most abundant biopolymers in nature, have also been used in chromatography as CS. Although intrinsically chiral, the materials used for this purpose are polysaccharide derivatives. Among them, esters and carbamates of cellulose and carbamates of amylose, in particular the 3,5-dunethylphenylcarbamates of either of them, are the most popular. When the CSPs are constituted by a silica gel matrix onto which the CS is simply coated, the composition of the mobile phase is limited to those solvents that do not solubilize the polysaccharide derivative. Mixtures of a hydrocarbon, hexane, or heptane, with an alcohol, usually 2-propanol, methanol, or ethanol, are common mobile phases. Although aqueous mobile phases can be used together with acetonitrile or methanol, these conditions tend to provide lower enantioselectivity values. ... 

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. 

Similar instances, in which analytical data were of prime importance and where no products were isolated, are the oxidations of various di-D-fructose dianhydrides,80 inositols,81 the trisaccharide moiety of the alkaloid solamargine,82 2-O-a-L-fucopyranosyl-L-fucitol derived from fucoidin83 (a polysaccharide sulfate ester), trehalose and neotrehalose,84 maltotetraose,85 D-galactopyranosyl-glyceritol,86 and a 3-pentulose.87... 

The aforementioned deuterated derivatives were prepared by way of reduction of a ketone, aldehyde, or ester with sodium borodeu-teride, or by deuteroboration of an alkene. An interesting reaction, perhaps eventually applicable to direct deuteration of polysaccharides, was reported by Koch and Stuart413 and by them and their coworkers,41b who found that treatment of methyl a-D-glucopyranoside with Raney nickel catalyst in deuterium oxide results in exchange of protons attached to C-2, C-3, C-4, and C-6. In other compounds, some protons of CHOH groups are not replaced, but the spectra may nevertheless be interpreted with the aid of a- and /3-deuterium effects. 

In many cases the monosaccharides found in these complex structures are present as one of their chemical derivatives, which may be an oxidation or reduction product, a phosphate or sulphate ester or an amino derivative, etc. However, these modified forms of monosaccharides may themselves have important biochemical roles and are not always found incorporated in polysaccharides. 

The phosphate esters and, to lesser extent, the sulphate esters of monosaccharides are very important naturally occurring derivatives. Metabolism of carbohydrates involves the formation and interconversion of a succession of monosaccharides and their phosphate esters of which glucose-1-phosphate and fruc-tose-6-phosphate are important examples. The sulphate esters of monosaccharides or their derivatives (usually esterified at carbon 6) are found in several polysaccharides, notably chondroitin sulphate, which is a constituent of connective tissues. 

Table I lists physical data for a number of the carbamate and ester derivatives of cellulose, chitin, amylose, amylopectin, and dextran synthesized as described in the Experimental Section. The solubility of the polysaccharide starting materials as well as that of the produced derivatives allows for macromolecular characterization through techniques including UV, NMR, IR, high pressure liquid chromatography, etc.

Derivatives (Table I) formed by the reaction of the chosen polysaccharide with an isocyanate or acid chloride are carbamates and esters respectively. However, chitin or poly[(1+4)(N-acetyl-2-amino-2-deoxy-f3-glucopyranose)] actually used in this experiment contains approximately 16% free amine groups which can form urea and amide derivatives with the above reagents. 

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