Aldobiouronic acids from acidic xylans

Analyses of the hemicelluloses of woods indicate the presence of a high proportion of D-xylose residues in association with those of a methyl ether of a hexuronic acid.64 The first evidence for the chemical structure of such wood xylans came from the isolation of xylobiose, xylotriose,65 and the aldobiouronic acid 0-(4-0-methyl-a-D-glucosyluronic)-(l —> 2)-D-xylose acid (IX)22 from partial hydrolyzates of aspen wood. The role of such oligosaccharide fragments in wood xylans became more apparent as the result of a study of a xylan from European beechwood,26 in which it was shown... 

There is some evidence that a small, perhaps variable, fraction of the acid side-chains in softwood xylans lacks the 4-0-methyl group. Roudier, in his extensive investigations on the uronic acids present in maritime pine, " isolated and identified 2-0(-a-D-glucopyranosyluronic acid)-D-xylose after partial hydrolysis of the wood. Hydrolysis of the xylan gave a small proportion of the same aldobiouronic acid. As previously mentioned (see Part I, p. 273), D-glucurono-6,3-lactone has been isolated from the wood of black spruce. " It has become evident that, despite the mild conditions often used, demethylation of the 4-0-methyl-D-glu-curonic acid residues may occur during hydrolysis of the above xylans. The presence or absence of D-glucuronic acid side-chains in hardwood or softwood xylans has, therefore, yet to be ascertained. 

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