Tandem catalysis strategy for direct glycosylation of 1-hydroxy sugars. Methoxyacetic acid as an effective catalytic mediator

1-Hydroxy sugars were dehydratively coupled with a variety of alcohols including thiophenol in the presence of a catalytic amount of ytterbium(III) triflate [Yb(OTf)₃] and methoxyacetic acid to give the corresponding glycosides in good to excellent yields, and in some cases, with high stereoselectivities. The reaction proceeds through selective esterification of 1-hydroxy sugars with methoxyacetic acid in the presence of free alcohols as glycosyl acceptors. The mechanism of methoxyacetic acid-mediated tandem catalysis with Yb(OTf)3 is discussed.