The hex-5-enose degradation: zinc dust cleavage of 6-deoxy-6-iodo-α-D-galactopyranosidic linkages in methylated di- and trisaccharides

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Abstract
The Bernet-Vasella reaction of 6-deoxy-6-halohexopyranosides with Zn dust to give hex-5-enoses was explored as the basis for a new procedure for the selective cleavage of glycosidic linkages in complex oligo- and polysaccharides. The overall reaction sequence involves: (a) the generation of primary hydroxyl groups in otherwise fully methylated oligo- or polysaccharides; (b) the formation of 6-deoxy-6-iodohexopyranosides by iodide displacement of reactive sulfonate esters; and (c) treatment with Zn dust followed by the formation of suitable derivatives from the liberated hex-5-enoses. Oligosaccharides carrying .beta.-D-xylopyranosyl substituents at the 2-O-, 3-O-,4-O-, and both 3-O- and 4-O- positions were converted into otherwise fully methylated derivatives, each containing methyl-6-deoxy-6-iodo-.alpha.-D-galactopyranoside residues, to serve as model substrates for different structural environments. In each case the derived 5,6-dideoxy-hex-5-enoses formed on treatment with Zn dust have been characterized by conversion into the corresponding 1,2-dideoxy-hex-1-enitol acetates. During preparations of 6-deoxy-6-iodo-.alpha.-D-galactopyranosides from the corresponding 6-sulfonate esters, solvolytic side reactions were observed which may include the formation of 3,6-anhydro-.alpha.-D-galactopyranosides with intramolecular participation of 3-O-methyl or 3-O-glycosyl substituents and subsequent loss of such substituents.
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