Preparation of 13C4-6-methyl anthranilic ester via a Diels-Alder-type process. an experimental and theoretical study to characterize an unexpected isotope exchange

Buta-2,3-dienoate reacts with vinyl ketenimine to give the corresponding substituted aniline through a Diels-Alder cycloaddition. Besides the expected Diels-Alder adduct 3a, the aniline 3b was also obtained in a ratio of 91:9. The observed (13)C exchange is explained on the basis of a reversible 2 + 2] cycloaddition competing with the 4 + 2] process. This is supported by B3LYP DFT computations, as a stepwise pathway lies very close in energy to the 4 + 2] concerted one. reaction: see text]