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 13C 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.