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.