Synthesis and reactivity of nitrogen-containing strained cyclic 1,2,3-trienes

Cyclic 1,2,3-trienes are a class of intermediates that confine a functional group with a preferred linear geometry into a ring. When the ring is sufficiently small, the 1,2,3-triene geometry becomes bent, leading to substantial strain and high reactivity. Whereas advances in cyclic 1,2,3-triene chemistry have focused on carbocyclic derivatives, the corresponding chemistry of their heteroatom-containing counterparts remains unexplored. Here we report the fluoride-mediated generation and trapping of six-membered azacyclic 1,2,3-trienes in a host of reactions, including (4 + 2) and (3 + 2) cycloadditions and σ-bond insertions, allowing for the synthesis of annulated pyridones. Moreover, computational studies provide insight into their structure and reactivity. This study pushes the limits of strained intermediate chemistry, while also laying the foundation for the further strategic use of strained cyclic 1,2,3-trienes as unconventional, but useful synthetic building blocks.