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
3<i>H</i>-pyrazole is the key intermediate in this cascade
reaction, which is confirmed spectroscopically for the first time
