6-nitro-2-benzothiazolyl alpha-glucoside and alpha-mannoside in beta-selective glycosylations

Highly beta-selective glucosylations of glycosyl acceptors having a primary hydroxy group with a 6-nitro-2-benzothiazolyl a-glucoside donor 3alpha proceeded smoothly in the presence of a catalytic amount of trifluoromethanesulfonic acid (TfOH) in CH2Cl2 at -78 degreesC to afford the corresponding glycosides in high yields. With the use of 3alpha, beta-saccharides could be obtained more dominantly than other alpha-glucosyl donors such as thioform- and trichloroacet-imidates or fluoride in the glucosylation under the same conditions. Similarly, highly beta-selective manno sylations of glycosyl acceptors with a 6-nitro-2-benzothiazolyl alpha-mannoside donor 18alpha 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; 18alpha apparently behaved as a potent donor here for the construction of beta-mannoside linkage. Interestingly, in situ anomerization from 18beta to 18alpha was observed when beta-mannosyl donor 18beta was treated with a catalytic amount of H[B(C6F5)(4)] in CH2Cl2.