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

摘要

Glycosylation of 4-methoxyphenyl 2,3,6-tri-O-benzoyl-beta-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-beta-D-galactopyranoside (7) as the donor, afforded an alpha- and beta-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-beta-D-galactopyranoside (15) as the donor, glycosylation of 2 gave alpha-linked products only, indicating that 4,6-O-benzylidenation led to alpha-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, glycose 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 alpha-linked products. However, when using 15 as the donor, glycosylation of aglycon alcohol or sugars with 6-OH's yielded normal beta-linked products.