Abstract 13C 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 13C 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 13C chemical shifts. Good agreement was found between the predicted molecular mechanics conformations and the 13C NMR spectral data.
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Biekofsky et al. (1991) studied this question.
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