Conformational studies on pyranoid sugar derivatives by NMR spectroscopy. Correlations of observed proton—proton coupling constants with the generalized Karplus equation

Proton spin‐spin coupling values measured for a wide range of tetrasubstituted tetrahydropyran ring‐systems obtained from aldopentopyranose sugars have been correlated with values calculated from a generalized version of the Karplus equation which takes into account the electronegativities of the various atoms in the molecule and the configurations of these substituents. Minor discrepancies between observed and calculated values can be attributed to slight flattening of the tetrahydropyran ring as a result of steric interactions of the substituents. The magnitude of the geminal coupling J 5e,5a of these aldopentopyranose derivatives provides a rough measure of the conformational population; when H‐4 is axial [ Cl (D) or 1C ( L ) conformation] J 5e,5a is ∼10.9 Hz, and is ∼13.4 Hz when H‐4 is equatorial [ 1C ( D ) or Cl ( L ) conformation].