Phenanthroline Catalysis in Stereoselective 1,2-cis Glycosylations.

Abstract

The National Research Council's report in 2012 recognized glycosidic bond forming (glycosylation) reactions as critical due to the central importance of carbohydrates to the glycosciences. This report emphasized the need for the development of reproducible and broadly applicable glycosylation technologies to facilitate the stereoselective synthesis of biomedically relevant glycan libraries for tool development and for research applications by nonspecialists. In response to this report with NIH Common Fund support, the publications of new catalytic diastereoselective glycosylation protocols, some with broad generality under mild conditions, have been recently reported by our group and others. These recent discoveries have also advanced the understanding of the glycosylation reaction mechanism involving the coupling of a sugar electrophile bearing a leaving group at its C1-anomeric center with an alcohol nucleophile. This glycosidic bond forming reaction can lead to a mixture of two stereoisomers that differ in the configuration of the anomeric center.In our group, we discovered that readily available phenanthroline, a rigid and planar organic compound with two fused pyridine rings, could be utilized as a nucleophilic catalyst to promote highly diastereoselective glycosylation of an alcohol nucleophile with a sugar bromide electrophile. The phenanthroline catalysis process allows access to a myriad of high yielding and diastereoselective 1,2-cis pyranosides and furanosides. This catalyst-controlled approach has been applied to the synthesis of a potential vaccine adjuvant α-glucan octasaccharide. For pyranosyl bromide electrophiles, an extensive mechanistic investigation illustrated that two phenanthrolinium ion intermediates, a 4C1 chair-liked equatorial-conformer and a B2,5 boat-like axial-conformer, are formed in a ratio of 2:1 (equatorial/axial). To obtain high levels of axial-1,2-cis selectivity, a Curtin-Hammett scenario was proposed wherein interconversion of the 4C1 equatorial-conformer and B2,5 axial-conformer is more rapid than nucleophilic addition. Hydroxyl attack takes place from the axial-face of the more reactive 4C1 chairlike equatorial intermediate to afford an axial-1,2-cis glycoside product. The phenanthroline catalysis system is applicable to a number of furanosyl bromide electrophiles to provide the challenging 1,2-cis substitution products in good yield and diastereoselectivity. NMR experiments and density-functional theory (DFT) calculations support an associative mechanism in which the rate-determining step takes place from an invertive displacement of the faster reacting furanosyl phenanthrolinium ion intermediate with an alcohol nucleophile. Overall, this work stands at the underdeveloped intersection of operationally simple conditions, catalysis, and stereocontrolled glycosidic bond formation, each of which represents an important theme in the preparation of biologically important oligosaccharides and glycopeptides for applications to human health and medicine.