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. SnCl4 was more successful than TiCl4, with the latter giving the glycosyl chloride by-product in some cases, and both were superior to BF3OEt2. Kinetics study using 1H 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 ß-d-xylopyranoside and ¿-l-arabinopyranoside reactants are conformationally more flexible than ß-l-fucopyranosides.