Synthesis of the two monomethyl esters of the disaccharide 4-O-α-d-galacturonosyl-d-galacturonic acid and of precursors for the preparation of higher oligomers methyl uronated in definite sequences

Methyl (α-d-galactopyranosyluronic acid)-(1→4)-d-galactopyranuronate and methyl α-d-galactopyranosyluronate-(1→4)-d-galactopyranuronic acid have been synthesized by coupling methyl (benzyl 2,3-di-O-benzyl-β-d-galactopyranosid)uronate (3) or benzyl (benzyl 2,3-di-O-benzyl-β-d-galactopyranosid)uronate (4) with benzyl (phenyl 2,3,4-tri-O-benzyl-1-thio-β-d-galactopyranosid)uronate and methyl (phenyl 2,3,4-tri-O-benzyl-1-thio-β-d-galactopyranosid)uronate, respectively, using N-iodosuccinimide and trifluoromethanesulphonic acid as promoters, followed by removal of the benzyl groups. The 4′-OH unprotected dimers benzyl (methyl 2,3-di-O-benzyl-α-d-galactopyranosyluronate)-(1→4)-(benzyl 2,3-di-O-benzyl-β-d-galactopyranosid)uronate and methyl (benzyl 2,3-di-O-benzyl-α-d-galactopyranosyluronate)-(1→4)-(benzyl 2,3-di-O-benzyl-β-d-galactopyranosid)uronate were prepared from methyl (phenyl 2,3-di-O-benzyl-1-thio-4-O-trimethylsilyl-β-d-galactopyranosid)uronate and benzyl (phenyl 2,3-di-O-benzyl-1-thio-4-O-trimethylsilyl-β-d-galactopyranosid)uronate and acceptors 4 or 3, respectively. These compounds have been designed to serve as precursors for the preparation of higher-membered d-galacturonic acid oligomers methyl esterified in definite positions.