CONFORMATIONAL-ANALYSIS OF 5,6,7-TRISUBSTITUTED FLAVONES - C-13 NMR AND MOLECULAR MECHANICS STUDY

C-13 NMR spectra of six flavones from Gomphrena martiana were analysed and compared with those of two related flavones isolated from Popowia cauliflora. This series of compounds has a similar substitution pattern on ring A, all compounds having oxygenated substituents on C-5, C-6 and C-7. Consistent trends in relating conformation to substituent chemical shift effects (SCS) were found by analysing the C-13 NMR data for these compounds. Chemical shifts for ring A carbons were calculated using the principle of additivity of substituents and by taking into account the conformation of the substituents. A reasonable agreement between the calculated shifts and the experimental values was found. A molecular mechanics study of these compounds was undertaken in order to obtain a set of consistent molecular structures to search for correlations between molecular structure and measured C-13 chemical shifts. Good agreement was found between the predicted molecular mechanics conformations and the C-13 NMR spectral data.