Lewis acid promoted anomerisation of alkyl O- and S-xylo-, arabino- and fucopyranosides

Abstract 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 4 was more successful than TiCl 4 , with the latter giving the glycosyl chloride by-product in some cases, and both were superior to BF 3 OEt 2 . Kinetics study using 1 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.