Nuclear magnetic resonance spectroscopic studies on the tautomerism in tetramic acid analogues and their anilides. Part 4. Interpretation of carbon-13 spectra by molecular orbital calculations

Abstract

The tautomeric structures of 3-acetyltetramic acid (I), 3-acetylterionic acid (III), and their anilides (Ia)–(IIIa) in CDCl₃ have been studied by Fourier-transform ¹³C spectroscopy. On the basis of electronic theory, ¹H-partial and gated decoupling techniques allowed the assignment of all signals in the spectra. Unusual features of the spectra were the large up-field shifts of the olefinic carbon signals, and the appearance of signals for the thiol-lactone carbonyl carbons of (III) and (IIIa) at lower field than those for the amide and lactone carbonyl carbons. These features are explained in terms of the electron densities on the carbons calculated by CNDO/2 or INDO methods.