Gold-Catalyzed Cyclization of Yndiamides with Isoxazoles via α-Imino Gold Fischer Carbenes

Gold catalysis is an important method for alkyne functionalization. Here we report the gold-catalyzed formal [3+2] aminative cyclization of yndiamides and isoxazoles in a direct synthesis of polysubstituted diaminopyrroles, which are important motifs in drug discovery. Key to this process is the formation, and subsequent cyclization, of an alpha-imino gold Fischer carbene, which represents a new type of gold carbene intermediate. The reaction proceeds rapidly under mild conditions, with high regioselectivity being achieved by introducing a subtle steric bias between the nitrogen substituents on the yndiamide. DFT calculations revealed that the key to this regioselectivity was the interconversion of isomeric gold keteniminiun ions via a low-barrier pi-complex transition state, which establishes a Curtin-Hammett scenario for isoxazole addition. By using benzisoxazoles as substrates, the reaction outcome could be switched to a formal [5+2] cyclization, leading to 1,4-oxazepines. Gold-catalyzed [3+2] aminative cyclization of yndiamides and isoxazoles affords polysubstituted diaminopyrroles via the intermediacy of an alpha-imino gold Fischer carbene. The reaction proceeds rapidly under mild conditions; high regioselectivity was achieved via a subtle steric bias between the nitrogen substituents, as supported by DFT calculations. The reaction outcome could be switched to 1,4-oxazepines by using benzisoxazoles as substrates.image