6-Nitro-2-benzothiazolyl α-Glucoside and α-Mannoside in β-Selective Glycosylations

Highly β-selective glucosylations of glycosyl acceptors having α primary hydroxy group with a 6-nitro-2-benzothia-zolyl α-glucoside donor 3α proceeded smoothly in the presence of a catalytic amount of trifluoromethanesulfonic acid (TfOH) in CH ₂Cl₂ at -78 °C to afford the corresponding glycosides in high yields. With the use of 3α, β-saccharides could be obtained more dominantly than other α-glucosyl donors such as thioform- and trichloroacet-imidates or fluoride in the glucosylation under the same conditions. Similarly, highly β-selective mannosylations of glycosyl acceptors with a 6-nitro-2-benzothiazolyl α-mannoside donor 18α were carried out smoothly in the presence of a catalytic amount of tetrakis(pentafluorophenyl)boric acid H[B(C₆F₅) ₄] to afford the corresponding disaccharides in good to high yields; 18α apparently behaved as a potent donor here for the construction of β-mannoside linkage. Interestingly, in situ anomerization from 18β to 18α was observed when β-mannosyl donor 18β was treated with a catalytic amount of H[B(C₆F₅)₄] in CH ₂Cl₂.