Cobalt‐Mediated [2+2+2] Cycloaddition versus CH and NH Activation of Pyridones and Pyrazinones with Alkynes: An Experimental Study

The reactivity of a range of pyridone and pyrazinone derivatives towards alkynes in the presence of cyclopentadienylcobaltbis(ethene) has been investigated. Depending on the nature of the substrates, [2+2+2]‐ or [2+2] cycloaddition, CH, or NH activation may occur. In the case of pyridones, the first three predominated with N‐protected derivatives, whereas substrates containing NH bonds followed an NH activation pathway. The [2+2+2] cycloaddition of an N ‐butynylisoquinolone was applied successfully to the total synthesis of anhydrolycorinone. Pyrazinone substrates showed similar patterns of reactivity.