Probing the Influence of Silyl-Protecting Group on the Stereoselectivity of Glucosyl Donors towards A-1,2-Cis Glucosylation

In the context of oligosaccharide synthesis, 1,2-cis-α-glycoside linkages represent a major subject of interest. We observed that a strongly electron-donating tert-butyldimethylsilyl (TBS) protecting group at the C-2 hydroxy group of a glycosyl donor induces 1,2-cis-α-glucosylation reactions, mediated by TBS-assisted stabilization of the cationic intermediate, which favours α-attacks of the glycosyl acceptor (Scheme 1A).[1] To demonstrate the efficiency and utility of the silyl-assisted α-glucosylation, we conducted reaction of the same donor with several acceptors having different OH-position, resulted in good to excellent α-selectivities for all the examples. 

To elucidate the stereo-direction factor of the silyl-assisted α-glucosylation, we then compared stereoselectivites of glycosylations using a series of glucosyl donors having a TBS-group introduced at different OH-positions (Scheme 1B). The resulting stereoselectivities clearly showed that the α-selectivity of the glucosylation decreases as the introduction position of the TBS group moves away from the anomeric position of glycosyl donors. According to these systematic experimental results, contribution of the silyl-group toward α-selectivity appears to be dependent on electronic factors.