Low-temperature NMR studies with a4-C-methyl-4-O-benzoyl galactopyranosyl donor enable the observation and characterization of ab ridged bicyclic dioxacarbenium ion arising from participation by ad istal ester.V ariable-temperature NMR studies reveal this bridged ion to decompose at temperatures above %À30 8 8C. In the absence of the methyl group,t he formation of ab icyclic ion is not observed. It is concluded that participation by typical secondary distal esters in glycosylation reactions is disfavored in the ground state conformation of the ester from whichi ti ss tereoelectronically impossible.Methylation converts the secondary ester to aconformationally more labile tertiary ester,r emoves this barrier, and renders participation more favorable.N evertheless,t he minor changes in selectivity in model glycosylation reactions on going from the secondary to the tertiary esters at both low and room temperature argue against distal group participation being am ajor stereodirecting factor even for the tertiary system.

Low-temperature NMR studies with a4-C-methyl-4-O-benzoyl galactopyranosyl donor enable the observation and characterization of ab ridged bicyclic dioxacarbenium ion arising from participation by ad istal ester.V ariable-temperature NMR studies reveal this bridged ion to decompose at temperatures above %À30 8 8C. In the absence of the methyl group,t he formation of ab icyclic ion is not observed. It is concluded that participation by typical secondary distal esters in glycosylation reactions is disfavored in the ground state conformation of the ester from whichi ti ss tereoelectronically impossible.Methylation converts the secondary ester to aconformationally more labile tertiary ester,r emoves this barrier, and renders participation more favorable.N evertheless,t he minor changes in selectivity in model glycosylation reactions on going from the secondary to the tertiary esters at both low and room temperature argue against distal group participation being am ajor stereodirecting factor even for the tertiary system.

The use of a phenylthio group (SPh) as a dummy ligand at the 6-position to control the side-chain conformation of a series of hexopyranosyl donors is described. The SPh group limits side-chain conformation in a configuration-specific manner, which parallels that seen in the heptopyranosides, and so influences glycosylation selectivity. With both D-and L-glycero-D-galactoconfigured donors, the equatorial products are highly favored as they are with an L-glycero-D-gluco donor. For the D-glycero-D-gluco donor, on the other hand, modest axial selectivity is observed. Selectivity patterns are discussed in terms of the side-chain conformation of the donors in combination with the electron-withdrawing effect of the thioacetal group. After glycosylation, removal of the thiophenyl moiety and hydrogenolytic deprotection is achieved in a single step with Raney nickel.