Ruthenium-Catalyzed (2 + 2) Intramolecular Cycloaddition of Allenenes.

We report a ruthenium-catalyzed (2 + 2) intramolecular cycloadclition of allenes and alkenes. We have found that the use of the ruthenium complex nH2Cl2(PiPr3)2., which has previously gone unnoticed in catalytic applications, is crucial for the observed reactivity. The reaction proceeds under mild conditions and is fully diastereoselective, providing a practical entry to a variety of bicydo[3.2.O]heptane skeletons featuring cyclobutane rings. [ABSTRACT FROM AUTHOR]