Methoxy ¹³C NMR Chemical Shift as a Molecular Descriptor in the Structural Analysis of Flavonoids and Other Phenolic Compounds

The interpretation of nuclear magnetic resonance (NMR) chemical shifts plays an increasingly important role in the structure elucidation flavonoids. Despite fact that 3H signal methoxy (OMe) group appears as a singlet 1 H NMR spectrum, its shift is not reliably indicative OMe-substitution aromatic rings, unlike 13 C shift, which can be useful for determination molecular For example, ortho-substitutions at 56 ± ppm flavonoids having one OMe-substituent either ortho or orthó position and 61.5 2 those substituents both positions. Overhauser effect spectroscopy ( δ -OMe→ ortho-aryl-H adjacent substituent) heteronuclear multiple-bond correlation ipso-aryl-C) data have been widely employed analysis to identify site OMe-substitution. Thus, OMe act descriptors. However, we noticed there are several examples accordance with this generalization, reinvestigation such reported seems relevant reconsideration assigned structures, few cited herein.