Studies on the Selectivity Between Nickel-Catalyzed 1,2-<i>cis</i>-2-Amino Glycosylation of Hydroxyl Groups of Thioglycoside Acceptors with C(2)-Substituted Benzylidene <i>N</i>‑Phenyl Trifluoroacetimidates and Intermolecular Aglycon Transfer of the Sulfide Group

Abstract

The stereoselective synthesis of saccharide thioglycosides containing 1,2-<i>cis</i>-2-amino glycosidic linkages is challenging. In addition to the difficulties associated with achieving high α-selectivity in the formation of 1,2-<i>cis</i>-2-amino glycosidic bonds, the glycosylation reaction is hampered by undesired transfer of the anomeric sulfide group from the glycosyl acceptor to the glycosyl donor. Overcoming these obstacles will pave the way for the preparation of oligosaccharides and glycoconjugates bearing the 1,2-<i>cis</i>-2-amino glycosidic linkages because the saccharide thioglycosides obtained can serve as donors for another coupling iteration. This approach streamlines selective deprotection and anomeric derivatization steps prior to the subsequent coupling event. We have developed an efficient approach for the synthesis of highly yielding and α-selective saccharide thioglycosides containing 1,2-<i>cis</i>-2-amino glycosidic bonds, via cationic nickel-catalyzed glycosylation of thioglycoside acceptors bearing the 2-trifluoromethylphenyl aglycon with <i>N</i>-phenyl trifluoroacetimidate donors. The 2-trifluoromethylphenyl group effectively blocks transfer of the anomeric sulfide group from the glycosyl acceptor to the C(2)-benzylidene donor and can be easily installed and activated. The current method also highlights the efficacy of the nickel catalyst selectively activating the C(2)-benzylidene imidate group in the presence of the anomeric sulfide group on the glycosyl acceptors