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

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.