Regioselective Glycosylation of Mannoside and Galactoside Acceptors Containing 2,4-OH Achieved by Altering Protecting Groups at the 1,3,6-Positions.

In this study, we systematically investigated the regioselective glycosylation of 2,4-OH mannoside and galactoside acceptors since regioselective protection of their 3- and 6-OHs is readily achieved. By altering the protecting groups at 1-, 3-, and 6-positions of such acceptors, we finally screened p -methoxyphenyl 3-OBn, 6-OTBDPS, α-mannoside, and β-galactoside acceptors whose 2-OHs exhibited excellent selectivity for glycosylation with various glycosyl donors, leading to 1,2-linked products in 70-82% yields. By utilizing such acceptors, a series of 2,4-linked trisaccharide products (53-65% yields over two steps) have been highly efficiently synthesized without the need for complex protection/deprotection operations at the 2- and 4-positions of these acceptors.