Synthesis of uronic acid-containing xylans found in wood and pulp

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Abstract
Two uronic acid-containing trisaccharides, (4-deoxy-β-L-threo-hex-4-enopyranosyluronic acid)- and (4-O-methyl-α-D-gluropyranosyluronic acid)-(1→2)-β-D-xylopyranosyl-(1→4)-D-xylopyranose, found in enzyme hydrolysates from pulp are synthesised. A common dixyloside 2′-OH acceptor, p-methoxyphenyl [3,4-O-(2′,3′-dimethoxybutane-2′,3′-diyl)-β-D-xylopyranosyl]-(1→4)-2,3-di-O-benzoyl-β-D-xylopyranoside, is constructed and coupled with two glucuronate thioglycoside donors differently substituted in the 4-position, O-methyl and O-mesyl, respectively, to give trisaccharides. DMTST as promoter in diethyl ether gives exclusively the α-linked products in high yield. Treatment of the 4″-O-mesyl trisaccharide with DBU then gives the α,β-unsaturated uronic acid derivative. The protection pattern introduced in the acceptor allows continued synthesis of larger oligosaccharides. Removal of the butanedione acetal produces 3′,4′-acceptors, and the p-methoxyphenyl glycoside can be transformed into various glycosyl donors, e.g. thioglycosides and sugar halides. Complete deprotection gives the two target reducing trisaccharides.

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