Ruthenium Catalysis Unlocks Terminal-Selective Allene–Ene Cycloisomerization

The cycloisomerization of allenes offers efficient access to structurally complex carbocycles and heterocycles, yet ruthenium-catalyzed variants remain scarce beyond alkyne-based systems. We report a cationic ruthenium-catalyzed allene–ene cycloisomerization that displays an unusual site-selectivity profile. In contrast to previous ruthenium-catalyzed unactivated allene–ene cycloisomerizations, the reaction selectively engages the terminal rather than the internal allene double bond, providing cyclic electron-rich dienamides with complementary molecular topology. The transformation proceeds under mild conditions, tolerates a broad range of substrates, and can be integrated with catalyst-free Diels–Alder reactions for rapid access to densely functionalized polycyclic nitrogen heterocycles. Mechanistic studies suggest a ruthenacycle-mediated process in which β-hydride elimination governs product formation. This process expands the synthetic potential of allenes in transition-metal catalysis.