Unusual α-glycosylation with galactosyl donors with a C2 ester capable of neighboring group participation

Abstract

Glycosylation of 4-methoxyphenyl 2,3,6-tri- O-benzoyl-β- d-glucopyranoside ( 2 ) with isopropyl 3- O-allyl-2,4,6-tri- O-benzoyl- ( 9 ) or 6- O-allyl-2,3,4-tri- O-benzoyl-1-thio-β- d-galactopyranoside ( 7 ) as the donor, afforded an α- and β-linked mixture, whereas with isopropyl 3- O-chloroacetyl-2- O-benzoyl-4,6- O-benzylidene- ( 13 ) and isopropyl 3- O-allyl-2- O-benzoyl-4,6- O-benzylidene-1-thio-β- d-galactopyranoside ( 15 ) as the donor, glycosylation of 2 gave α-linked products only, indicating that 4,6- O-benzylidenation led to α-stereoselectivity in spite of the C2 ester capable of neighboring group participation. Using 15 as the donor, glycosylation of mannose derivatives with 2- or 3-OH's, glucose with 2- or 3-OH's, galactose with 2-, or 3-, or 4-OH's, glucosamine and glucuronic acid with a 4-OH, and a lactose derivative with a 4-OH, also furnished α-linked products. However, when using 15 as the donor, glycosylation of aglycon alcohol or sugars with 6-OH's yielded normal β-linked products.