摘要:
In a project targeting the synthesis of large oligosaccharide structures corresponding to the Cryptococcus neoformans GXM capsular polysaccharide, an easy access to thiodisaccharide building blocks comprising a beta-linked glucuronic acid moiety and a 6-O-acetyl group was required. Several pathways to such building blocks have been investigated, addressing the problem of constructing a beta-linked glucuronic acid residue protected with groups that are orthogonal to a primary acetyl group. Two efficient routes have been developed, one using benzoylated glucosyl donors to form the beta-linkage followed by a change of protecting groups to benzyls and subsequent introduction of the carboxyl function and the acetyl group. The second route explored the possibility to achieve beta-selectivity using glucuronyl donors without acyl protecting groups. BF3- etherate promoted glycosylations with benzyl (2,3,4-tri-O-benzyl-alpha-D-glucupyranosyl)uronate trichloroacetimidate in the presence of nitrile solvents and at low temperatures reproducibly gave good yields of disaccharides with high beta-selectivity. Furthermore, the use of recently reported glucuronyl thioglycoside donors protected with a cyclic 2,4-silylene acetal was found to represent another efficient and completely beta-selective way to desired disaccharide building blocks. (C) 2014 Elsevier Ltd. All rights reserved.