Synthesis of p-nitrophenyl 3- O-β- d-galactopyranosyl-β- d-galactopyranoside and p-nitrophenyl 3- O-α- d-galactopyranosyl-β- d-galactopyranoside

Abstract

Glycosylation (catalyzed by mercuric cyanide) of p-nitrophenyl 2,4,6-tri- O-acetyl-β- d-galactopyranoside ( 2) with 2,3,4,6-tetra- O-acetyl-α- d-galactopyranosyl bromide in acetonitrile afforded the α-(1→3)- and β-(1→3)-linked disaccharide heptaacetates ( 4 and 6, respectively) in almost equal proportions. Similar glycosylation of p-nitrophenyl 2- O-benzoyl-4,6- O-benzylidene-β- d-galactopyranoside ( 3) gave the β-(1→3)- and the α-(1→3)-linked, fully protected, disaccharide derivatives ( 8 and 10, respectively) in the ratio of 3:1. The structures of 4, 6, 8, and 10 were evidenced by their respective 1H-n.m.r. spectra. O-Deacetylation of 4 and 6 afforded, respectively, p-nitrophenyl 3- O-α- d-galactopyranosyl-β- d-galactopyranoside ( 5) and p-nitrophenyl 3- O-β- d-galactopyranosyl-β- d-galactopyranoside ( 7). O-Deacylation of 8 and 10 furnished the disaccharide derivatives ( 9 and 11). Cleavage of the benzylidene groups of 9 and 11 gave the disaccharides 7 and 5, respectively. The structures of 5, 7, 9, and 11 were established by 13C-n.m.r. spectroscopy. Additionally, the structures of 5 and 7 were confirmed by permethylation, and acid hydrolysis to 2,4,6-tri- O-methyl- d-galactose. The synthesis of triacetate 2, starting from p-nitrophenyl 3,4- O-isopropylidene-β- d-galactopyranoside ( 1), is also described. Compound 1 was obtained as the major product of the isopropylidenation of p-nitrophenyl β- d-galactopyranoside under thermodynamic control, and its structure was likewise established by 13C-n.m.r. spectroscopy, and confirmed by methylation, and acid hydrolysis to 2,6-di- O-methyl- d-galactose. Compound 3 was obtained from p-nitrophenyl 2- O-benzoyl-4,6- O-isopropylidene-β- d-galactopyranoside by cleavage of the isopropylidene group, and acetalation of the resulting triol with benzaldehyde-zinc chloride.