Synthesis of a pentasaccharide alpha-D-Man-(1-->3)-[alpha-D-Man-(1-->6)] -beta-D-Man-(1-->4)-beta-D-GlcNac(1-->6)]beta-GlcNAc-->Omp (2) is described. A comparison between the 1H NMR data of 2 and those of a novel Asn-linked core structure 1 containing a new GlcNAc residue suggests an alpha-D-configuration for the new linkage.