A practical synthesis of 2-acetamido-2-deoxy-3,4-di-O-β-d-galactopyranosyl-d-galactopyranose

Two different sugar derivatives having free hydroxyl groups have been employed for synthesis of the title trisaccharide. In one attempt, benzyl 2-acetamido-6-O-benzyl-2-deoxy-α-d-galactopyranoside (8) was treated with an excess of 2,3,4,6-tetra-O-acetyl-α-d-galactopyranosyl bromide (11), to give a mixture of products which, on fractionation, afforded benzyl 2-acetamido-6-O-benzyl-3,4-di-O-(2,3,4,6-tetra-O-acetyl-β-d-galactopyranosyl)- α-d-galactopyranoside (15) in 21% yield. However, in another, preferable approach, benzyl 2-acetamido-6-O-benzyl-2-deoxy-3-O-(2,3,4,6-tetra-O-acetyl-β-d- galactopyranosyl)-α-d-galactopyranoside (13) was treated with 11, to produce 15 in 69% yield. Both 8 and 13 were conveniently prepared via reductive ring-opening of the respective 4,6-benzylidene acetals. O-Deacetylation of 15, followed by hydrogenolysis, provided the title trisaccharide. The structure of the final product, and of various other intermediates, was established by ¹H- and ¹³C-n.m.r. spectroscopy.