Pentopyranoside and 6-deoxyhexopyranosides, such as those from d-xylose, l-arabinose and l-fucose are components of natural products, oligosaccharides or polysaccharides. Lewis acid promoted anomerisation of some of their alkyl O- and S-glycopyranosides is reported here. SnCl was more successful than TiCl, with the latter giving the glycosyl chloride by-product in some cases, and both were superior to BFOEt. Kinetics study using H NMR spectroscopy showed an order of reactivity: O-xylopyranoside > O-arabinopyranoside > O-fucopyranoside. Benzoylated glycosides were more reactive than acetylated glycosides. The reactivity of S-glycosides was greater than that of O-glycosides for both arabinose and fucose derivatives; the reactivity of O- and S-xylopyranosides was similar. The highest stereoselectivities were observed for fucopyranosides. The β-d-xylopyranoside and α-l-arabinopyranoside reactants are conformationally more flexible than β-l-fucopyranosides.
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