Configurational assignment in <i>N</i>‐acetylneuraminic acid and analogues <i>via</i> the geminal C,H coupling constants

Abstract

Abstract High field 1 H and 13 C NMR spectra were recorded of N ‐acetylneuraminic acid, its methyl ester and its per‐ O ‐acetylated methyl ester. The sign and the magnitudes of the geminal coupling constants between the H‐3 protons and the C‐2 quaternary anomeric carbon were determined by heteronuclear 2D‐experiments. The C‐2,H‐3ax coupling in the α anomers (−8 Hz) was significantly more negative than that in the β anomers (−3 to −4 Hz). Therefore, the determination of 2 J(C‐2,H‐3ax) offers a unique criterion for anomeric assignment in sialic acid ketosides. The geminal coupling constants are discussed in terms of the number and orientation of the oxygen substituents. Due to their 3‐deoxy function adjacent to the quaternary anomeric centre, sialic acids give novel contributions to the correlation of carbon‐proton coupling constants with molecular geometries in carbohydrates.