The hex-5-enose degradation: cleavage of glycosiduronic acid linkages in modified methylated Sterculia gums

Reduction of uronic acid residues in permethylated polysaccharides from Sterculiaurens and S. caudata gums, followed by transformation of the resulting hexose into 6-deoxy-6-iodohexose residues, affords modified polysaccharides that undergo depolymerization on treatment with zinc dust. The main products after reduction with sodium borohydride are hexenitolterminated oligosaccharides. Their structures have been assigned as members of a series of O-[α-L-rhamnopyranosyl]-(1 → 3)-1,2-dideoxyhex-1-enitol derivatives, with and without attendant β-D-galactopyranosyl substituents, on the basis of nuclear magnetic resonance and mass spectral data and compositional analysis of trideuteriomethylated derivatives. The structures of the polysaccharides have been reassessed in the light of these and other experiments, and may now be formulated as containing a backbone of alternating 4-linked α-D-galacturonic acid and 2-linked α-L-rhamnopyranose residues bearing a minimum of three types of side chains attached at specific sites. Characterization of some of the degradation products provides evidence that limited degradation had occurred during methylation of the glycuronans.