Thermal Hetero-Diels-Alder Reaction of Benzocyclobutenones with Isatins To Form 2-Oxindole Spirolactones

Benzocyclobutenones 1a-1g undergo cycloreversion at 150 degrees C in m-xylene solvent to form transient alpha-oxo-ortho-quinodimethanes or ortho-quinoid ketene methides, which engage in intermolecular [4+2] cycloadditions With isatins 2a-2f to form 2-oxiiidole spirolactones 3a-3I. This process tolerates an array of different functional groups and substitution patterns, and is applicable to unprotected isatins 2b-2f bearing free NH-functionalities. The superior performance of isatins compared to other carbonyl based dienophiles was demonstrated and rationalized with the aid of quantum chemical calculations.