1H, 13C, and 15N nuclear magnetic resonance studies on the tautomerism of the Schiff's bases of 3-acetyl-6-methyl-2H-pyran-2,4(3H)-dione and 3,5-diacetyltetrahydropyran-2,4,6-trione

Abstract

The ¹H and ¹³C n.m.r. spectra of 3-acetyl-6-methyl-2H-pyran-2,4(3H)-dione (I) and 3,5-diacetyltetrahydropyran-2,4,6-trione (III) and their Schiff's bases were studied in CDCl₃ over a range of temperatures for elucidation of possible tautomerism. Whereas compound (I) appears to exist in only one form, at least two forms have been detected for compound (III). The Schiff's bases exist predominantly in keto-amine forms and the rotational barrier due to substantial double bond character of the exocyclic carbon to carbon bonds allows ready detection of rotamers. Structures of the predominant rotamers are deduced from the ¹Hand ¹³C spectra and the assignments of NH proton signals are based on the relative strengths of the intramolecular hydrogen-bonds. The chemical shifts of NH protons correlate linearly with the pK_a of the amine precursors. The ¹⁵N spectra of some ¹⁵N-enriched Schiff's bases were obtained in CDCl₃ and in (CD₃)₂SO and the considerable downfield shifts of the ¹⁵N resonances, compared with those of typical amides, are indicative of extensive electron delocalisation.