A concise and practical synthesis of antigenic globotriose, α-d-Gal-(1→4)-β-d-Gal-(1→4)-β-d-Glc

A concise and practical synthesis of the antigenic globotriose, α-d-Gal-(1→4)-β-d-Gal-(1→4)-β-d-Glc (13), was achieved by coupling of a monosaccharide donor, 3-O-allyl-2-O-benzoyl-4,6-O-benzylidene-α-d-galactopyranosyl trichloroacetimidate (4) with a disaccharide acceptor, p-methoxyphenyl 2,3,6-tri-O-benzoyl-β-d-galactopyranosyl-(1→4)-2,3,6-tri-O-benzoyl-β-d-glucopyranoside (8), followed by deprotection. In spite of the existence of a C-2-ester substituent capable of neighboring-group participation in the donor, the coupling gave exclusively the α-linkage in satisfactory yield. The acceptor 8 was readily obtained from selective 3-O-benzoylation of the galactosyl ring of p-methoxyphenyl 2,6-di-O-benzoyl-β-d-galactopyranosyl-(1→4)-2,3,6-tri-O-benzoyl-β-d-glucopyranoside (7), which was prepared from p-methoxyphenyl β-d-lactoside (5) via isopropylidenation, benzoylation, and deisopropylidenation. Donor 4 was obtained from p-methoxylphenyl 3-O-allyl-2,4,6-tri-O-benzoyl-β-d-galactopyranoside (1) via selective 4,6-di-O-debenzoylation, oxidative removal of 1-O-MP, benzylidenation, and trichloroacetimidate formation.