Influence of 3-Thio Substituents on Benzylidene-Directed Mannosylation. Isolation of a Bridged Pyridinium Ion and Effects of 3- O -Picolyl and 3- S -Picolyl Esters.

The influence on glycosyl selectivity of substituting oxygen for sulfur at the 3-position of 4,6- O -benzylidene-protected mannopyranosyl thioglycosides is reported and varies considerably according to the protecting group employed at the 3-position. The substitution of a thioether at the 3-position for the more usual 3- O -benzyl ether results in a significant loss of selectivity. The installation of a 3- S -picolinyl thioether results in a complex reaction mixture, from which a stable seven-membered bridged bicyclic pyridinium ion is isolated, while the corresponding 3- O -picolinyl ether affords a highly α-selective coupling reaction. A 3- O -picolyl ester provides excellent β-selectivity, while the analogous 3- S -picolyl thioester gives a highly α-selective reaction. The best β-selectivity is seen with a 3-deoxy-3-(2-pyridinyldisulfanyl) system. These observations are discussed in terms of the influence of the various substituents on the central glycosyl triflate - ion pair equilibrium.