A commercially available 0.1 M solution of HClO(4) in dioxane has been shown to catalyze the glycosidation of glycosyl diphenyl phosphates. The per-O-benzyl-protected glucosyl and galactosyl donors and the 3,4,6-tri-O-acetyl-2-azido-2-deoxygalactosyl donor each react with a range of acceptor alcohols in the presence of 0.05-0.2 equiv of HClO(4) in dioxane/Et(2)O (1:1) to afford glycosides in good yields with good to excellent alpha selectivities. The synthetic utility of this glycosidation method was demonstrated by a stereoselective synthesis of the alpha-galactosylceramide KRN7000, an activator of natural killer (NK) T cells through CD1d molecules.

-Acetamido-2-deoxy-d-glycopyranosides occur as common and important structural units in oligosaccharides and glycoconjugates, such as glycolipids, glycoproteins, proteoglycans, and peptidoglycans, and are associated with a wide range of biological processes.[1] Whereas 2-acetamido-2-deoxy-a-dgalactopyranose residues are linked through a glycoside bond to the side-chain hydroxy groups of serine and threonine, the majority of 2-acetamido-2-deoxy-d-glycopyranosides are found as b-linked glycosides.The most general and extensively developed strategy for the synthesis of 1,2-trans-b-glycosides of 2-amino-2-deoxysugars utilizes donors containing a participating group as the amino-protecting functionality, which is replaced by an acetyl group after the glycosidation event.[2, 3] 2-Azido-2-deoxyglycosyl donors have also been used for this purpose. [4,5] Clearly, a direct glycosidation method by using donors with the natural N-acetyl function would constitute an ideal procedure in terms of efficiency and practicality. In practice, however, the reaction of these donors, 1, generally leads to the predominant formation of oxazoline derivatives 3 through neighboring-group participation and the subsequent abstraction of a proton from the NH group (Scheme 1). [6,7] Although oxazolines can react with acceptor alcohols in the presence of Brønsted or Lewis acids to afford 2-acetamido-2-deoxy-bglycosides 4 via oxazolinium ion intermediates 2 or 5 (that is, oxazoline method [8] ), the harsh reaction conditions required for this conversion have precluded its wide application in the synthesis of complex oligosaccharides. As part of a program to extend the recently developed glycosidation method that capitalizes on diethyl phosphite as a leaving group, [9,10] we wish to report the stereoselective synthesis of 2-acetamido-2-deoxy-b-glycosides by a direct glycosidation that does not proceed via an oxazolinium ion intermediate.At the outset of this study, we explored the trimethylsilyl trifluoromethanesulfonate (TMSOTf) promoted glycosidation of 2-acetamido-3,4,6-tri-O-benzyl-2-deoxy-a-d-glucopyr-

The first convergent synthesis of the tetrasaccharide repeating unit of the polymeric O antigen isolated from Acinetobacter baumannii serogroup O18 has been achieved. The ManNAcβ1→4Gal and GalNAcβ1→3Gal units were successfully obtained through β-selective glycosylation with 2-azido-4,6-O-benzylidene-2-deoxymannosyl diphenyl phosphate and Tf2NH-promoted glycosylation with 2-acetamido-2-deoxygalactosyl diethyl phosphite, respectively. The disaccharide units could be coupled with the aid of TMSClO4 as an activator of the diphenyl phosphate leaving group, and global deprotection completed the synthesis of the tetrasaccharide.

Leave this area blank for abstract info. Abstract-A direct and practical method for the construction of β-mannosidic linkages is described. While β-selectivities in the TMSOTfpromoted glycosidation of 2,3,4,6-tetra-O-benzyl-D-mannosyl diethyl phosphite are found to be highly dependent on the reactivity of acceptor alcohols, 2,3-di-O-benzyl-4,6-O-benzylidene-D-mannosyl diethyl phosphite reacts with a wide range of acceptor alcohols in the presence of TMSOTf in CH 2 Cl 2 at -45 °C to give β-mannosides in high yields with good to high β-selectivities.

A direct construction of 1,2-trans-β-linked 2-acetamido-2-deoxyglycosides was investigated. The 3,4,6-tri-O-benzyl- and 3,4,6-tri-O-acetyl-protected glycosyl diethyl phosphites and 4,6-O-benzylidene-protected galactosyl diethyl phosphite each reacted with a variety of acceptor alcohols in the presence of a stoichiometric amount of Tf2NH in CH2Cl2 at -78 °C to afford the corresponding β-glycosides in good to high yields with complete stereoselectivity. Some experiments provided strong evidence that the corresponding oxazolinium ions are not responsible for the reaction. We demonstrated that glycosylations with the corresponding glycosyl imidate and thioglycoside also proceeded at a low temperature, indicating the possibility of these donors being attractive alternatives to the phosphite. A plausible reaction mechanism, which features glycosyl triflimide and contact ion pair as reactive intermediates, is proposed on the basis of the results obtained with 2-acetamido-2-deoxymannosyl donors.