Convergent synthesis of steviol glycosides rebaudioside a and M.

Steviol glycosides, a natural health-focused sweetener, is receiving increasing attention. Herein, an efficient and concise approach for the convergent synthesis of Rebaudioside A and M has been devised. This method features a AuCl-tBuCN cooperative catalysis to achieve highly 1, 2-trans stereoselective glycosidation of a C2-branched trisaccharide trichloroacetimidate with tertiary C-13 hydroxy group in steviol aglycone, without relying on neighboring group participation.

Reaction Rate and Stereoselectivity Enhancement in Glycosidations with O-Glycosyl Trihaloacetimidate Donors due to Catalysis by a Lewis Acid-Nitrile Cooperative Effect.

Activation of -glycosyl trihaloacetimidate glycosyl donors with AuCl as a catalyst and pivalonitrile (BuCN) as a ligand led to excellent glycosidation results in terms of yield and anomeric selectivity. In this way, various β-d-gluco- and β-d-galactopyranosides were obtained conveniently and efficiently. Experimental studies and density functional theory (DFT) calculations, in order to elucidate the reaction course, support formation of the BuCN-AuCl-OR(H) AuCl complex as a decisive intermediate in the glycosidation event.