Copper-Catalyzed Cascade Annulation of o -Hydroxyphenyl Propargylamines with Pyrazolin-5-ones to Access Pyrano[2,3- c ]pyrazoles.

An efficient copper-catalyzed cascade annulation of o -hydroxyphenyl propargylamines and pyrazolin-5-ones is described. This methodology leads to the rapid assembly of a series of valuable pyrano[2,3- c ]pyrazoles with good yields across a wide range of substrates in a simple fashion. This novel reaction involves the formation of alkynyl ortho -quinone methides, a 1,4-conjugate addition, and a subsequent 6-endo cyclization process. The mechanistic elucidation is well supported by control experiment and literature precedents.