NHC-Catalyzed Enantioselective [2+2] and [2+2+2] Cycloadditions of Ketenes with Isothiocyanates

The enantioselective N-heterocyclic carbene-catalyzed formal [2 + 2] and [2 + 2 + 2] cycloaddition of ketenes and isothiocyanates were developed. Reaction with N-aryl isothiocyanates at room temperature favors the [2 + 2] cycloaddition, while reaction with N-benzoyl isothiocyanates at -40 degrees C favors the [2 + 2 + 2] cycloaddition.