Convergent synthesis of a blocked trisaccharide fragment related to the exopolysaccharide from Burkholderia cepacia

Abstract

An effective chemical synthesis of the blocked trisaccharide fragment Gal-(1-6)-Man-(1-3)-Glu related to exopolysaccharide of Burkholderia cepacia using a convergent block synthesis approach is described. A selectively protected glucose synthon methyl 2-O-benzoyl-4,6-O-benzylidene-O-α-D- glucopyranoside (1) was condensed with ethyl 2,3,4-tri-O-benzyl-6-O- chloroacetyl-1-thio-α-D-mannopyranoside (8) using N-iodosuccinimide and trimethylsilyl trifluoromethanesulfonate to give the disaccharide methyl 2,3,4-tri-O-benzyl-6-O-chloroacetyl-O-α-D-mannopyranosyl-(1-3) -2-O-benzoyl-4,6-O-benzylidene-O-α-D-glucopyranoside (9). This disaccharide was selectively deprotected to give the acceptor methyl 2,3,4-tri-O-benzyl-O-α-D-mannopyranosyl-(1-3)-2-O-benzoyl-4, 6-O-benzylidene-O-α-D-glucopyranoside (10). Finally compound 10 was condensed with ethyl 2,3,4,6-tetra-O-acetyl-1-thio-β-D-galactopyranoside (11) using N-iodosuccinimide and trimethylsilyl trifluoromethanesulfonate to give the blocked trisaccharide, methyl 2,3,4,6-tetra-O-acetyl-O-β-D- galactopyranosyl-(1-6)-2,3,4-tri-O-benzyl-O-α-D-mannopyranosyl-(1-3) -2-O-benzoyl-4,6-O-benzylidene-O-α-D-glucopyranoside (12).