Synthesis and NMR spectroscopy of nine stereoisomeric 5,7-diacetamido-3,5,7,9-tetradeoxynon-2-ulosonic acids Original Research Article

Derivatives of 5,7-diamino-3,5,7,9-tetradeoxynon-2-ulosonic acids are essential constituents of some bacterial polysaccharides and glycoproteins. In order to establish reliably the configuration of the natural sugars, nine stereoisomeric 5,7-diacetamido-3,5,7,9-tetradeoxynon-2-ulosonic acids were synthesized, including di-N-acetyl-legionaminic and -pseudaminic acids (the d-glycero-d-galacto and l-glycero-l-manno isomers, respectively) and their isomers at C-4, C-5, C-7, and C-8 having the l-glycero-d-galacto, d-glycero-d-talo, l-glycero-d-talo, d-glycero-l-altro, l-glycero-l-altro, d-glycero-l-manno, and l-glycero-l-gluco configurations. Synthesis was performed by condensation of 2,4-diacetamido-2,4,6-trideoxy-l-gulose, -d-mannose, -d-talose, and -l-allose with oxalacetic acid under basic conditions, the reaction of the last two precursors being accompanied by epimerisation at C-2. The 1H and 13C NMR data of the synthetic compounds are discussed. Acetylated methyl esters of the C-7 and C-8 isomeric nonulosonic acids were prepared and used for analysis of the side-chain conformation by NMR spectroscopy.