Stereoselective Glycosylation Based on the Conformational Restriction of Pyranoses

Abstract

Despite considerable progress and extensive effort, a general method for highly stereoselective glycosylation particularly for the 1, 2-cis-glycosylation has not yet been developed and therefore is required. The α/β-stereoselectivity in glycosylation can be affected by the steric and stereoelectronic (anomeric) effects around the anomeric center, which depend on the conformation of the glycosyl donor substrates. Therefore, we hypothesized that highly α- and β-selective glycosylation can be realized by employing conformationally restricted substrates. We showed that the α/β-stereoselectivity was significantly increased by the conformational restriction and was completely inverted by changing the substrate conformation from the 4C1-form into the 1C4-form in radical and nucleophilic C-glycosylation reactions as well as in O-glycosylation reactions. The conformational restriction of substrates also effectively facilitates the α- and β-selective radical cyclization reaction at the anomeric position. Using the method, C-glucoside trisphosphates designed as Ca2+-mobilizing agents were successfully synthesized.