Boronic Acid and Phosphoyl Chloride-Mediated Site-Selective Ketalization of Unprotected Saccharides: In Situ Generation of a Proton Catalyst and Multiple Roles of Reagents

In the chemical synthesis or modification of saccharides, regioselective protection of the many similar OH groups in saccharides is necebary but remains a major challenge. In particular, the regio- and stereoselective conversion of C(1,2)-OH has great synthetic potential in carbohydrate synthesis but has largely remained untapped. Here, an in situ proton-producing system mediated by boronic acid was found and employed for site-selective ketalization of various unprotected saccharides. This strategy is characterized by the controlled production of protons mediated by the dynamic, reversible covalent binding of boronic acid and saccharides. This method provides great convenience for the concise synthesis of complex saccharides, as illustrated by the streamlined degradation and reconstruction of disaccharides.