Harnessing Free Sulfate Groups in Glycosylation Reactions.

General strategies for synthesizing sulfated oligosaccharides employ the protection of sulfate groups for glycosylation or post-sulfation after the synthesis of oligosaccharides. However, the chemical behavior of free sulfate groups in glycosylation reactions has not been thoroughly studied. We examined several glycosyl donors with free sulfate groups, but neither glycosyl imidates nor thioglycosides achieved products. Conversely, activating 6-sulfated GalNAc donors with either a 2-NTroc group or 2-O-acetyl group using an ortho-hexynylbenzoate group at the anomeric position yielded β-glycosides in good yield. These results indicate that glycosylations with a free sulfate group can be performed and that the neighboring group participation of 2-NHTroc and 2-O-acetyl groups works even with an unprotected sulfate group at the 6-position. Ab initio calculations supported the formation of acyloxonium-cation via 2-O-acyl participation. Additionally, glycosylation reactions under various counter cations of the sulfate group, such as Na, Li, K, Ba, and pyridinium were examined. Results showed that sodium and lithium salt donors yielded the products in good yield. These results were also supported by ab initio calculations. Practical glycosylation reactions between disaccharide donor-acceptor pairs with free sulfate groups successfully yielded the sulfated tetrasaccharides. This study also discusses how a sodium salt acts as a protecting group during glycosylation.