Neighboring Group Participation in Glycosylation Reactions by 2,6-Disubstituted 2-O-Benzoyl groups: A Mechanistic Investigation

Variable yields and glycosylation stereoselectivity were obtained for NIS/TfOH-medi- ated reaction of 4-methoxyphenyl 2,4,6-tetra-O-acetyl-β-D-galactopyranoside and thiogalactosides bearing acetyl, benzoyl, 2,6-dimethoxylbenzoyl, 2,4,6-trimethylbenzoyl, or 2,6-dichlorobenzoyl groups at the 2-positions and acetyl at the remainder. X-ray structures of 4-methylphenyl 2,3,4,6-tetra-O-(2,4,6-trimethylbenzoyl)-1-thio-β-D-galactopyr anoside and 4-methylphenyl 3,4-O-isopropylidene-2,6-di-O-(2,4,6-trimethylbenzoyl)-1-thio-β-D-galactopyranoside revealed slightly distorted 4 C 1 chair conformations. Variable temperature NMR revealed that activation of 4-methylphenyl 2,3,4,6-tetra-O-(2,4,6-trimethylbenzoyl)-1-thio-β-D-galactopyranoside afforded only dioxolenium ion, whereas 4-methylphenyl 3,4,6-tri-O-acetyl-2-O-(2,4,6-trimethylbenzoyl)-1-thio-β-D-galactopyranoside gave a 1:1 mixture of dioxolenium ion and glycosyl triflate. However, the reaction intermediates formed from these deactivated donors do not influence the glycosylation stereoselectivity; instead, it is influenced by steric and electronic interactions at the transition states.