A preparation of the branched trisaccharide 2-acetamido-2-deoxy-4-O-(.BETA.-D-galactopyranosyl)-3-O-(.BETA.-D-xylopyranosyl)-D-glucopyranose (3-O-.BETA.-D-xylopyranosyl-N-acetyllactosamine).

The title branched trisaccharide (19) was prepared from benzyl 2-acetamido-4, 6-O-benzylidene-2-deoxy-α-D-glucopyranoside (1) by stepwise Koenigs-Knorr condensation followed by removal of the protecting groups. Benzyl 2-acetamido-3-O-(2, 3, 4-tri-O-acetyl-β-D-xylopyranosyl)-2-deoxy-α-D-glucopyranoside (4), prepared by coupling 1 with 2, 3, 4-tri-O-acetyl-α-D-xylopyranosyl bromide followed by debenzylidenation, is a key intermediate in this preparation. Removal of the benzyl and O-acetyl groups of 4 gave a crystalline 2-acetamido-2-deoxy-3-O-(β-D-xylopyranosyl)-α-D-glucopyranose. Preferential etherification and esterification of 4 were investigated. Selective benzylation of 4 afforded the 6-O-benzoate (15). Condensation of 15 with 2, 3, 4, 6-tetra-O-acetyl-α-D-galactopyranosyl bromide provided a crystalline, protected trisaccharide in 41% yield, from which 19 was obtained as a white powder after removal of the protecting groups. Purified β-galactosidase from jack bean did not act on the β-galactosidic linkage in 19.