Total Synthesis and Structural Revision of Rebaudioside S, a Steviol Glycoside.

The total synthesis of rebaudioside S, a minor steviol glycoside from the leaves of Stevia rebaudiana, was investigated via a modular strategy, culminating not only in the first and highly efficient synthesis of Reb-S and analogues thereof but also in the revision of the originally proposed structure. The modular strategy dictated the application of C2-branched disaccharide Yu donors to forge C-13 steviol glycosidic linkages, posing considerable challenges in stereoselectivity control. Through systematic investigations, the effect of the internal glycosidic linkage configuration on the glycosylation stereoselectivity of 1,2-linked disaccharide donors was disclosed, and the intensified solvent effect by the 4,6-O-benzylidene protecting group was also observed with glucosyl donors. Through the orchestrated application of these favorable effects, the stereoselectivity problems were exquisitely tackled.