UNEXPECTED PHENOMENA IN GLYCOSYLATIONS OF ACCEPTORS WITH L-IDOSE CONFIGURATION

For the synthesis of Galbeta(1 --> 4)IdoA disaccharides 1, several gly cosylations of L-iduronic acid derivatives 5 and 13 and of L-idopyrano se derivative 21 were performed with differently protected D-galactopy ranosyl donors under various conditions. It was found that these coupl ing reactions mainly led to the formation of the undesired alpha-inter glycosidic linkage, even when the donor contained a participating grou p at C-2. On the other hand, coupling of the corresponding D-glucopyra nosyl donor 19 with 13 led to a much higher beta/alpha ratio (2/1). By changing the conformation of the acceptor into the 1,6-anhydro-L-idop yranose acceptor 23, 90% of the desired beta-coupled disaccharide was obtained in the glycosylation with galactopyranosyl donor 12. Finally it was found that the alpha/beta ratio of the condensation with 25, pr omoted by an insoluble silver salt, was influenced in an unusual way b y the substituents at the galactosyl bromide.