Application of the Intramolecular Didehydro-Diels–Alder (IMDDA) Reaction for the Synthesis of Thieno[2,3-f]isoindole-4(8)-carboxylic Acids

The reaction of 3-(thienyl)propargylamines with maleic anhydride, followed by a domino sequence involving successive acylation/[4+2] cycloaddition steps, leads to the formation of the thieno[2,3-f]isoindole core. The key step, the intramolecular didehydro-Diels–Alder (IMDDA) reaction, proceeds with a high level of selectivity, yielding the target products – 7-oxo-5,6,7,8-tetrahydro-4H-thieno[2,3-f]isoindole-4(8)-carboxylic acids – in good yields. X-ray analysis and DFT calculations have revealed that the key step, the IMDDA reaction, proceeds via an exo transition allene state, resulting in the exclusive formation of a single isomer of the target heterocycle.