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.
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