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.