Stereospecific electronegative effects. Part I. The 19F nuclear magnetic resonance spectra of deoxyfluoro-D-glucopyranoses

Abstract

A detailed study has been made of the ¹⁹F n.m.r. spectra of the anomeric pairs of all five deoxyfluoro-D-glucopyranoses and of 2-deoxy-2-fluoro-D-mannopyranose in solution in deuterium oxide. The values of the geminal and vicinal ¹⁹F–¹H spin–spin coupling constants, together with the ¹H n.m.r. parameters and observed equilibrium anomer concentrations, have been used to establish the configurations and conformational properties of these molecules. The small vicinal ¹H–¹⁹F coupling constants found for 2-deoxy-2-fluoro- and 4-deoxy-4-fluoro-D-glucose have been rationalised in terms of the electronegativity of the ring oxygen atom and an empirical expression has been found which enables gauche ¹H–¹⁹F coupling constants to be predicted. The large value of the fluorine coupling to H-5 in 6-deoxy-6-fluoro-D-glucose has been used to show the existence of a favoured rotational isomer in which F-6 is antiparallel to H-5. Vicinal ¹⁹F–¹H coupling is shown to have a stereochemical dependence upon electronegative substituents, and the ¹⁹F chemical shifts and chemical shift differences between paris of anomeric isomers may also be explained in terms of a stereochemical dependence upon electronegative substituents elsewhere in the molecule.