Gold(III) Chloride─tert-Butyl Cyanide Catalyzed Glycosidations with O-Glycosyl Trihaloacetimidates

Activation of O-glycosyl trihaloacetimidates as glycosyl donors with AuCl3 as a catalyst and pivalonitrile (tert-BuCN) as a cocatalyst led to excellent glycosylation results in terms of yield and anomeric selectivity. This way, independent of the glycosyl donor anomeric configuration and without the support of neighboring group or remote group participation, various β-gluco- and β-galactopyranosides were obtained conveniently and efficiently. Also glycoalkaloid solamargine could be readily prepared with this methodology. Studies to the reaction course support adduct formation of the glycosyl donor with the tert-BuCN─AuCl3 complex leading to anomerisation and formation of the thermodynamically most stable anomer prior to the glycosidation event. Then, via hydrogen-bonding of the acceptor to this adduct, intramolecular SN2-type reaction takes place that provides the β-glycoside. This course is closely related to the sequence of events in glycosyltransferase-catalyzed enzymatic glycoside bond formations.