Stereoselective glycosylation based on the conformational restriction of pyranoses

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 alpha/beta-stereo selectivity 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 alpha- and beta-selective glycosylation can be realized by employing conformationally restricted substrates. We showed that the alpha/beta-stereoselectivity was significantly increased by the conformational restriction and was completely inverted by changing the substrate conformation from the C-4(1)-form into the C-1(4)-form in radical and nucleophilic C-glycosylation reactions as well as in O-glycosylation reactions. The conformational restriction of substrates also effectively facilitates the alpha- and beta-selective radical cyclization reaction at the anomeric position. Using the method, C-glucoside trisphosphates designed as Ca2+-mobilizing agents were successfully,synthesized.