The interest in the stereoselective synthesis of pyranose and furanose containing glycans has been growing rapidly over the past decades as they play important roles in biological processes. Oligosaccharides having 1,2-trans pyranosidic and furanosidic linkages are often obtained through neighboring group participation. On the other hand, the ability to generate 1,2-cis pyranosides and furanosides in a stereoselective fashion remains an important synthetic challenge due to the existence of reactions at the SN1-SN2 boundary. While the current approaches have been successful to provide solutions to a number of 1,2-cis pyranoside and furanoside challenges in the oligosaccharide synthesis, achieving desired stereoselectivity and reactivity remains substrate-dependent. Subtle changes to the structure of carbohydrate coupling partners have pronounced effects on the reaction selectivity and reactivity. We present herein a scope and mechanism of stereoselective 1,2-cis pyranoside and furanoside formation catalyzed by phenanthroline. NMR experiments and density-functional theory calculations support an associative mechanism in which rate-determining step occurs from invertive displacement of the faster reacting phenanthrolinium ion intermediate with alcohol nucleophile.

Catalytic stereoselective formation of 1,2-cis pyranosides and furanosides

ACS Spring 2022

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