The hex-5-enose degradation: cleavage of 6-deoxy-6-iodo-α-D-galactopyranosidic linkages in modified carboxyl-reduced methylated tragacanthic acid

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Abstract
Reduction of galacturonic acid residues in permethylated tragacanthic acid, followed by transformation of the resulting galactose residues to those of 6-deoxy-6-iodogalactose, affords a modified polysaccharide which undergoes depolymerization by the Bernet-Vasella reaction on treatment with Zn dust. The 2 main products of depolymerization were characterized, after reduction and acetylation, as 3,6-di-O-acetyl-1,2-dideoxy-5-O-methyl-4-O-(2,3,4-tri-O-methyl-.beta.-D-xylopyranosyl)-L-lyxo-hex-1-enitol and O-(2,3,4-tri-O-methyl-.alpha.-L-fucopyranosyl)-(1 .fwdarw. 2)-O-(3,4-di-O-methyl-.beta.-D-xylopyranosyl)-(1 .fwdarw. 4)-3,6-di-O-acetyl-1,2-dideoxy-5-O-methyl-L-lyxo-hex-1-enitol. The fragmentation provides the first example of a chemical degradation of an .alpha.-D-galacturonan to furnish oligosaccharides in which side chain units remain attached to modified galacturonic acid residues. Under the conditions used for the introduction of iodo substituents no evidence could be obtained for loss of 3-O-glycosyl substituents through side reactions involving solvolysis of 6-sulfonates.