Thermally Induced [3 + 2] Cycloaddition of Alkynyl-Tethered Diazoamides: Synthetic and Mechanistic Insights

A general and unprecedented thermally induced formal [3 + 2] cycloaddition has been developed that provides a general access to fused lactam derivatives in high to excellent yields with broad substrate scope. In comparison with the reported metal-catalyzed carbene/alkynyl metathesis, this is the only example in this area under catalyst-free conditions with excellent selectivity. Mechanistic studies indicate that the 3H-pyrazole is the key intermediate in this cascade reaction, which is confirmed spectroscopically for the first time.