Cobalt-Mediated Macrocyclizations. Facile Synthesis of 2-Oxo Pyridinophanes via [2 + 2 + 2] Cycloaddition of α,ω-Diynes and Isocyanates

[reaction: see text] Cobalt-mediated [2 + 2 + 2] cycloaddition of alpha,omega-diynes and isocyanates provides a direct approach to macrocyclic 2-oxopyridinophanes. This macrocyclization process, which proceeded most efficiently with aliphatic isocyanates, was conveniently performed at a moderate temperature (85 degrees C) without irradiation or syringe-pump addition.