Total synthesis of 3-O-[2-acetamido-6-O-(N-acetyl-alpha-D-neuraminyl-2-deoxy- alpha-D-galactosyl]-L-serine and a stereoisomer.

O-(5-Acetamido-3,5-dideoxy-D-glycero-alpha-D-galacto-2- nonulopyranoxylonic acid)-(2----6)-O-(2-acetamido-2-deoxy-alpha-D-galactopyranosyl)-(1----3) -L-serine, a structural unit occurring in various submaxillary mucins, was synthesized for the first time by using O-[methyl (5-acetamido-4,7,8,9-tetra-O-acetyl-3,5-dideoxy-D-glycero-alpha-D- galacto-2-nonulopyranosyl)onate]-(2----6)-3,4-di-O-acetyl-2- azido-2-deoxy-D- galactopyranosyl trichloroacetimidate (13) and N-(benzyloxycarbonyl)-L-serine benzyl ester as the key intermediates. The trichloroacetimidate 13 was prepared by starting from two monosaccharide synthons, namely, allyl 2-azido-2-deoxy-beta-D-galactopyranoside and methyl (5-acetamido-4,7,8,9-tetra-O-acetyl-3,5-dideoxy-D-glycero-beta-D- galacto-2-nonulopyranosyl chloride)onate, which were coupled in the presence of silver triflate in tetrahydrofuran to give the desired alpha-(2----6)-linked disaccharide in moderate selectivity.