Nonmetathesis Heterocycle Formation by Ruthenium-Catalyzed Intramolecular [2 + 2] Cycloaddition of Allenamide-enes to Azabicyclo[3.1.1]heptanes

We have developed a novel nonmetathesis reaction, namely, ruthenium-catalyzed intramolecular [2 + 2] cycloaddition of allenamide-enes, to give heterocycles (i.e., azabicyclo[3.1.1]heptanes). This is the first example of [2 + 2] cycloaddition using a ruthenium carbene catalyst. The reaction proceeds at room temperature, but not under thermal or radical conditions.