as precursors for the preparation of N-methyltetramic acids is described. The presence of the bulky urethane protecting group on the nitrogen atom gives rise to rotational isomers detectable in the NMR spectra of the compounds, along with the keto/enol tautomerism. The mechanism of the cyclisation reaction of -amino--oxo esters to tetramic acids was studied theoretically by the B3LYP hybrid density