Alpha and beta L-Fucopyranosyl oxyamines: key intermediates for the preparation of fucose-containing glycoconjugates by oxime ligation.

We report herein the synthesis of new alpha and beta aminooxylated L-fucopyranosyl derivatives for the preparation of glycoclusters through oxime ligation. The glycosylation reaction between activated triacetylated L-fucopyranosyl fluoride and N-hydroxyphthalimide was carried out in the presence of boron trifluoride-diethyl etherate and the stereochemical outcome of glycosylation was compared in dichloromethane, acetonitrile or tetrahydrofuran. Interestingly, an unexpected alpha and beta anomer ratio was obtained in spite of the presence of an acetate participating group at the carbon 2, particularly the 1,2-cis glycosylation was largely favoured in acetonitrile. The resulting alpha and beta N-oxyphthalimido fucopyranosyl derivatives were finally deprotected with methylhydrazine to obtain the corresponding free aminooxylated fucopyranosyls. The structure of single-crystal alpha anomer 12 was analysed by X-ray diffraction.