The structures of acylpyran-diones, -triones and acylpyridinediones have been studied primarily by deuterium isotope effects on 13C chemical shifts. The 3,5-diacetyltetrahydropyran-2,4,6-trione forms a double tautomeric system involving one of the carbonyl carbons of the anhydride moiety. This compound also exists as a minor symmetrical isomer with two intramolecular hydrogen bonds to the same acceptor. This isomer shows isotopic perturbation of the OH proton resonance upon deuteriation. A similar situation is found for 1,5-diphenylpentane-1,3,5-trione.The 3-acetyl-6-methyl-2H-pyran-2,4(3H)-dione is found to be tautomeric and mainly in the 4-hydroxy form. The corresponding 5-acetyl derivative forms a very weak hydrogen bond as is also found in the 5-ethoxycarbonyl-6-methylpyridine-2,4(3H)-dione. The same pattern is found for 3- and 5-acetyl-6-methylpyridine-2,4(3H)-dione. This difference in the two-bond deuterium isotope effect is related to the bond orders of the bonds linking the hydrogen bond donors and acceptors and reflects the strength of the intramolecular hydrogen bonds. The 3-acetyl-4-hydroxy-2(1H)-quinolones are tautomeric in a similar fashion.The formal hydroxypyridines are shown by isotope effects to be of the 2-pyridone form.The formal imines of most of the above compounds have also been studied and are shown to exist in their keto-enamine forms. In the case of 3-(1-amino)ethylidenequinoline-2,4(1 H,3H)-diones and 2-(1amino)ethylidene-6,7-dihydro-5H-benzo[ij]quinolizine-1,3(2H)-diones two different forms with hydrogen bonds to either the carbonyl at C-4 or the amide carbonyl group at C-2 are observed. Deuterium isotope effects on chemical shifts again turned out to be crucial in the structure elucidation.
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Hansen et al. (1995) studied this question.