1,2-<i>cis</i>-α-Stereoselective Glycosylation Utilizing a Glycosyl-Acceptor-Derived Borinic Ester and Its Application to the Total Synthesis of Natural Glycosphingolipids

1,2-<i>cis</i>-α-Stereoselective glycosylations were conducted using a 1,2-anhydroglucose donor and mono-ol acceptors in the presence of a glycosyl-acceptor-derived borinic ester. Reactions proceeded smoothly under mild conditions to provide the corresponding α-glycosides with high stereoselectivity in moderate to high yields. In addition, the present method was applied successfully to the direct glycosylation of a protected ceramide acceptor and the total synthesis of natural glycosphingolipids (GSLs)

Glycosyl-Acceptor-Derived Borinic Ester-Promoted Direct and β‑Stereoselective Mannosylation with a 1,2-Anhydromannose Donor

β-Stereoselective mannosylations were conducted using a 1,2-anhydromannose donor and mono-ol acceptors in the presence of a glycosyl-acceptor-derived borinic ester. Reactions proceeded smoothly under mild conditions to provide the corresponding β-mannosides with high stereoselectivity in moderate to high yields. In addition, the present glycosylation method was applied successfully to the total synthesis of acremomannolipin A

Boronic-Acid-Catalyzed Regioselective and 1,2-<i>cis</i>-Stereoselective Glycosylation of Unprotected Sugar Acceptors via S_Ni‑Type Mechanism

Regio- and 1,2-<i>cis</i>-stereoselective chemical glycosylation of unprotected glycosyl acceptors has been in great demand for the efficient synthesis of natural glycosides. However, simultaneously regulating these selectivities has been a longstanding problem in synthetic organic chemistry. In nature, glycosyl transferases catalyze regioselective 1,2-<i>cis</i>-glycosylations via the S_Ni mechanism, yet no useful chemical glycosylations based on this mechanism have been developed. In this paper, we report a highly regio- and 1,2-<i>cis</i>-stereoselective S_Ni-type glycosylation of 1,2-anhydro donors and unprotected sugar acceptors using <i>p</i>-nitrophenylboronic acid (<b>10e</b>) as a catalyst in the presence of water under mild conditions. Highly controlled regio- and 1,2-<i>cis</i>-stereoselectivities were achieved via the combination of boron-mediated carbohydrate recognition and the S_Ni-type mechanism. Mechanistic studies using the KIEs and DFT calculations were consistent with a highly dissociative concerted S_Ni mechanism. This glycosylation method was applied successfully to the direct glycosylation of unprotected natural glycosides and the efficient synthesis of a complex oligosaccharide with minimal protecting groups