ChemInform Abstract: An Alternative Reaction Course in O‐Glycosidation with O‐Glycosyl Trichloroacetimidates as Glycosyl Donors and Lewis Acidic Metal Salts as Catalyst: Acid—Base Catalysis with Gold Chloride‐Glycosyl Acceptor Adducts.

Abstract

Gold(III) chloride as catalyst for O-glycosyl trichloroacetimidate activation revealed low affinity to the glycosyl donor but high affinity to the hydroxy group of the acceptor alcohol moiety, thus leading to catalyst–acceptor adduct formation. Charge separation in this adduct, increasing the proton acidity and the oxygen nucleophilicity, permits donor activation and concomitant acceptor transfer in a hydrogen-bond mediated SN2-type transition state. Hence, the sequential binding between acceptor and catalyst and then with the glycosyl donor enables self-organization of an ordered transition-state. This way, with various acceptors, even at temperatures below −60 °C, fast and high yielding glycosidations in high anomeric selectivities were recorded, showing the power of this gold(III) chloride acid–base catalysis. Alternative reaction courses via hydrogen chloride or HAuCl4 activation or intermediate generation of glycosyl chloride as the real donor could be excluded. With partially O-protected acceptors, ...