Proton‐coupled 13C NMR and {14N}—1H‐INDOR of highly‐substituted 6‐membered heterocycles. I—trisubstituted pyridones and isomeric aminopyrones; tetrasubstituted pyridines

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Abstract
The structural elucidation by NMR spectroscopy of trisubstituted α‐pyridones and the isomeric 2‐amino‐γ‐pyrones as well as their internal and external pyrylium salts is described. The most useful parameter for the differentiation between α‐pyridones and isomeric γ‐pyrones is the geminal coupling constant 2J(C‐6, H‐5) which changes from ∼2.5 Hz to ∼7 Hz whenever the cyclic amide group is replaced by an oxa‐function; this applies to both the γ‐pyrones and their pyrylium salts. The value of J(C‐6, H‐5) in the pyridones resembles that of the analogous coupling in N‐vinylacetamide, whose sign determination by the selective population inversion (SPI) technique is reported. The 13C chemical shifts of seven pyridones, pyrones and pyrylium salts are reported and their structural correlations are discussed. Quick structural assignments in these classes of compounds may also be performed by evaluating the 14N chemical shifts, which often are accessible by the {14N}—1H‐INDOR technique. The proton coupled 13C NMR spectra of two tetrasubstituted pyridines are also reported, and empirical correlations between long range CH coupling constants and substituent electronegativity are discussed.