A direct, mild and efficient protocol for the preparation of beta-glycosides of N-acetyl glucosamine (GlcNAc) and N-acetyl galactosamine (GalNAc) has been developed using peracetylated beta-GlcNAc and beta-GalNAc as donors. All rare Earth metal triflate promoters screened were found to promote glycosylation with Sc(OTf)(3) being superior in terms of reaction rate. Simple alcohol glycosylation was found to proceed smoothly in refluxing dichloromethane, whereas higher temperatures under microwave conditions were needed to attain acceptable yields with less reactive, carbohydrate based glycosyl acceptors. The protocol developed was applied to provide the first example of direct chemical formation of a disaccharide using both GlcNAc as a glycosyl donor and acceptor. The alpha-acetate donor was found to be significantly less reactive than the corresponding beta-anomer necessitating higher reaction temperatures under which glycoside anomerisation was found to occur. It was established, that the anomerisation only took place in the presence of both Sc(OTf)(3) and acetic acid.

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Carbohydrates U 0500Direct Formation of β-Glycosides of N-Acetyl Glycosamines Mediated by Rare Earth Metal Triflates. -A mild and efficient protocol for the synthesis of β-glycosides of N-acetyl glycosamines is presented, involving the microwave-assisted and Sc (O-Tf)3-catalyzed glycosylation reaction of N-acetylglucos-or galactosamines as donors. Microwave irradiation in a sealed vessel enables the reaction to proceed under higher temperature in a shorter reaction time [cf. (III), (V)]. The first example of direct synthesis of a disaccharide containing two N-acetyl glucosamine units is presented. The reaction proceeds regioselectively at the C-3 position to give the 1->3 linked disaccharide (VII). The α-acetate donor in diastereomixture (VIII) is significantly less reactive and can only be activated by more harsh reaction conditions (microwave irradiation at 110 °C for 5 hours). -(CHRISTENSEN, H.; CHRISTIANSEN, M. S.; PETERSEN, J.; JENSEN*, H. H.; Org. Biomol. Chem. 6 (2008) 18, 3276-3283; Dep. Chem., Univ. Aarhus, DK-8000 Aarhus, Den.; Eng.) -Mischke 04-188