6-Nitro-2-benzothiazolyl α-mannoside: A highly efficient mannosyl donor in constructing β-Man(1→4)GlcN linkage and its application to the synthesis of the pentasaccharide core of N-glycans

An efficient and concise synthesis of the β-Man(1→4)GlcN linkage that exists in N-linked glycans has been established. Direct β-mannosylations of the 4-OH group of glucosamine derivatives by using 6-nitro-2-benzothiazolyl 3,6-di-O-allyl-2,4-di-O-benzyl-α-D- mannopyranoside (3α) proceed smoothly in the presence of a catalytic amount of HB(C ₆F ₅) ₄ in CH ₂Cl ₂ at -78°C to afford the desired β-mannosides in high yields. The β-trisaccharide 14β, a key building block for the synthesis of the pentasaccharide core 20, is directly prepared from the mannosyl donor 3α and chitobiose acceptor 10 in high yield. In addition, this mannosylation method has successfully been applied to a wide range of glycosyl acceptors derived from glucosamine in a highly efficient manner. Further, the pentasaccharide core 20 that is commonly present in N-linked glycans can also be synthesized readily according to this direct mannosylation strategy.