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 pKa 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.
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Schedule a callMore From: Journal of the Chemical Society, Perkin Transactions 2
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