Synthesis of p-nitrophenyl 2-O-β- and 2-O-α-d-galactopyranosyl-β-d-galactopyranoside

Cleavage of the isopropylidene group of p-nitrophenyl 3-O-benzoyl-4,6-O-isopropylidene-β-d-galactopyranoside afforded p-nitrophenyl 3-O-benzoyl-β-d-galactopyranoside (1). Compound 1 was converted into its 4,6-O-benzylidene derivative (2) by reaction with the benzaldehyde-zinc chloride complex. Compound 2 was also prepared by selective benzoylation of p-nitrophenyl 4,6-O-benzylidene-β-d-galactopyranoside (3), obtained by benzylidenation of p-nitrophenyl β-d-galactopyranoside. The structures of 1, 2, and 3 were established by ¹H- and ¹³C-n.m.r. spectroscopy. Glycosylation of 2 with 2,3,4,6-tetra-O-acetyl-α-d-galactopyranosyl bromide, catalyzed by mercuric cyanide, afforded the protected disaccharide derivatives 4 and 6, which, on deacetalation followed by deacylation, gave the β- and the α-(1→2)-linked disaccharides 8 and 10, respectively. The structures of 4, 6, 8, and 10 were established by n.m.r. spectroscopy. Additionally, the structures of 8 and 10 were confirmed by permethylation, and hydrolysis to 3,4,6-tri-O-methyl-d-galactose. Compounds 8 and 10 were also converted into their fully acetylated derivatives. Compounds 4 and 6 were deacylated, to furnish the corresponding benzylidenated derivatives 5 and 7. The ¹³C-n.m.r. spectra of 5 and 7 are discussed, together with those of the isomeric α- and β-(1→3)-linked disaccharides, and also with that of the β-(1→6)-linked isomer.