β-Selective Glycosylation Using Axial-Rich and 2-<i>O</i>-Rhamnosylated Glucosyl Donors Controlled by the Protecting Pattern of the Second Sugar

Herein, we describe two counterexamples of the previously reported β/α-selectivity 96/4 for glycosylation using ethyl 2-O-[2,3,4-tris-O-tert-butyldimethylsilyl (TBS)-α-L-rhamnopyranosyl]-3,4,6-tris-O-TBS-thio-β-D-glucopyranoside as glycosyl donor. Furthermore, investigated effects protecting group on rhamnose moieties in with cholestanol and revealed that β-selectivity originated from TBS groups at 3-O 4-O positions rhamnose. In contrast, 2-O position hampered β-selectivity. Finally, during was enhanced to ≥99/1. The results obtained herein suggest sugar connected a donor axial-rich conformation can control stereoselectivity glycosylation.