Uronic acids reductic acid from

In most cases, uronic acids are liberated from acidic polysaccharides by hydrolysis leading to irreproducible concomitant formation of lactones. Several methods to circumvent this problem have been published describing conversion of the uronic acid into methyl esters followed by reduction with borohydride or borodeuteride reagents and subsequent hydrolysis and GC-MS detection [129]. Other techniques are based on the liberation and quantification of carbon dioxide. Direct determination of uronic acid residues in hydrolyzates has frequently been performed according to colorimetric assays, which are rather insensitive and have thus mostly been replaced by high-performance anion exchange chromatography (HP-AEC) methods [130-132]. 

Dermatan sulfate, also termed chondroitin sulfate B, a related glycosaminoglycan constituent of connective tissue, was known to be composed of galactosamine and a uronic acid, originally believed to be glucuronic acid but then claimed to be iduronic acid based largely on color reactions and paper chromatography. However, the d or L-enantiomer status of the latter monosaccharide was not clear. Jeanloz and Stoffyn unequivocally characterized the monosaccharide as L-iduronic acid by consecutive desulfation, reduction, and hydrolysis of the polysaccharide, followed by isolation of the crystalline 2,3,4-tri-0-acetyl-l,6-anhydro-/ -L-idopyranose, which was shown to be identical to an authentic specimen synthesized from 1,2-0-isopropylidene-/ -L-idofuranose.34... 

The synthesis of L-ribofuranose derivatives from 16a has been carried out by Walker and Hogenkamp (35). The procedure involves oxidation of the hydroxymethyl group of 16a with dimethyl sulfoxide N,N dicyclohexylcarbodiimide and acid hydrolysis of the protecting group to give L-riburonic acid, which was converted into methyl (methyl a,/ -ribofurano-sid)uronates (26). Reduction of 26 with sodium bis(2-methoxyethoxy)alu-minum hydride gave the chromatographically separable anomers of methyl L-ribofuranoside (27). 

Aso98 first proposed 5-hydroxy-2-oxo-3-pentenal (94) as an intermediate in the conversion of uronic acids to reductic acid,190 191 but this proposal does not appear to have been experimentally tested, although the intermediate was prepared.190 Isbell121 suggested a mechanism in which the formation of reductic acid and 2-furaldehyde from pentoses and uronic acids results from the reaction of different tautomers of 94. Although other mechanisms have been suggested,100 102 115 Isbell s original scheme seems adequate to explain the experimental facts. 

The formation of reductic acid and 2-furaldehyde from uronic acids is believed to occur through the same intermediate (72a) that is generated on decarboxylation of 3,4-dideoxy-D-g/t/cero-hex-3-enos-uluronic acid (71) (see Section III,3 p. 191). However, little is... 

Deamination studies have aided attempts to locate the positions of the O-sulfate groups in heparin. From the results of deamination and periodate-oxidation studies, it was concluded239 that half of the uronic acid residues are sulfated at C-2, and sequential periodate oxidation, reduction with sodium borohydride, acid treatment, and deamination gave 2,5-anhydro-D-mannose 6-sulfate.240,241 Sulfated O-(hexosyluronic acid)-2,5-anhydro-D-mannose and sulfated uronic acids were subsequently isolated, and the isolation (in 65% yield) of the disulfated 0-(idosyluronic acid)-2,5-anhydro-D-mannose (137) from heparin, together with the 13C n.m.r. spectra of 137 and heparin, suggested243 that at least two thirds of the heparin structure consists of the repeating unit 138. 

The methylated D-galactose derivatives 66 and 67 were thereafter released by mild hydrolysis with acid. The product was reduced with sodium borodeuteride, the reduction product methylated with trideuteriomethyl iodide, and the ether analyzed. From the disappearance of the ethers derived from the uronic acid and the 2-substi-tuted L-rhamnose, and the appearance of the ethers 68 and 69, the... 

Like the parent compounds, the methyl ethers of aldobiouronic acids are resistant to acid hydrolysis, and it is difficult to carry out hydrolysis without some decomposition of the product. This difficulty has recently been overcome by reduction of the uronic acid residue with lithium aluminum hydride66-67 the resulting disaccharide then undergoes hydrolysis without difficulty. The first reduction of the uronic acid residue of a methylated aldobiouronic acid methyl ester was accomplished by Levene, Meyer and Kuna,69 who reduced the methylated aldobiouronic acid from gum arabic with hydrogen in the presence of copper chromite catalyst under the conditions previously used701 for reducing the acety-... 

Uronic acids are important components in many naturally occurring polysaccharides. By chemical or enz)matic degradation of these polysaccharides, smaller uronic acid units ranging from monosaccharides to smaller oligosaccharides can be prepared. In this connection, reduction of the carboxyl group can serve both synthetic and analytical purposes. 

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