Boronic Acid\nand Phosphoyl Chloride-Mediated Site-Selective\nKetalization of Unprotected Saccharides: In Situ Generation of a Proton\nCatalyst and Multiple Roles of Reagents

In the chemical synthesis or modification of saccharides,\nregioselective\nprotection of the many similar OH groups in saccharides is necessary\nbut remains a major challenge. In particular, the regio- and stereoselective\nconversion of C(1,2)–OH has great synthetic potential in carbohydrate\nsynthesis but has largely remained untapped. Here, an in situ proton-producing\nsystem mediated by boronic acid was found and employed for site-selective\nketalization of various unprotected saccharides. This strategy is\ncharacterized by the controlled production of protons mediated by\nthe dynamic, reversible covalent binding of boronic acid and saccharides.\nThis method provides great convenience for the concise synthesis of\ncomplex saccharides, as illustrated by the streamlined degradation\nand reconstruction of disaccharides.