Additive-Controlled Stereoselective Glycosylations of Oxazolidinone-Protected Glucosamine and Galactosamine Thioglycoside Donors Based on Preactivation Protocol

Based on a pre-activation protocol, the stereoselectivity of oxazolidinone-protected amino sugar thioglycoside donors towards glycosylations can be controlled by additives. Either α- or β-selectivity could be obtained by changing additives. 2,4,6-Tri-tert-butylpyrimidine (TTBP) was the best β-directing additive, while thiophene worked as the best α-directing additive. The bifunctional additives such as tetrabutyl ammonium iodide (TBAI) afforded either α- or β-selectivity depending on the amount added. Poor α-­selectivity of some glycosylations without any additives was greatly improved by adding TBAI or thiophene.