NMR and computational study on the anomeric effect in cis/trans-3,4-dihydro-2-alkoxy-4-substituted-2H,5H-pyrano[3,2-c][1]benzo pyran-5-one derivatives

The configuration and conformation of cis/trans-3,4-dihydro-2-alkoxy-4-(alkyl-or aryl-substituted)-2H,5H-pyrano[3,2-c][1]benzopyran-5-one derivatives were studied by combined NMR and computational analyses. The cis/trans configurational assignments of all diastereoisomers were achieved via 2D NOESY experiments, The conformational analysis of eis compounds, performed via NMR and computational studies, allowed the establishment of a preference for the conformer with both substituents in the pseudo-equatorial orientations. For trans derivatives only the molecular mechanics analysis produced meaningful results, suggesting a general prevalence for the conformer bearing the 2-alkoxy group in a pseudo-axial and the 4-substituent in a pseudo-equatorial orientation. (C) 1997 John Wiley & Sons, Ltd.