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

Isoxazoles as efficient alkyne amination reagents in divergent heterocycle synthesis

During the past decades, the exploration of new alkyne amination reactions has attracted increasing attention due to the high efficiency in heterocycle synthesis. In addition to the well-established alkyne amination reagents (such as nitrogen ylides and azides), isoxazoles and their derivatives have been proven to be efficient amination reagents, especially the N,O-bifunctional reagents of alkynes, in the transition metal-catalyzed transformation of alkynes through metal carbene intermediates. Isoxazole derivatives have been extensively applied to the rapid synthesis of a diverse range of structurally complex N-containing molecules, especially the valuable N-heterocycles in atom-economic manner. In this review, we summarize the latest trends and developments of isoxazole-enabled alkyne amination reactions and their applications in divergent heterocycle synthesis, including amination of ynamides, amination of ynol ethers, amination of thioynol ethers, amination of electron-deficient alkynes, amination of unpolarized alkynes and asymmetric amination of alkynes. Finally, we list the current challenges and opportunities for potential breakthroughs in this field.

Gold-Catalyzed Diyne-Ene Annulation for the Synthesis of Polysubstituted Benzenes through Formal [3+3] Approach with Amide as the Critical Co-Catalyst

The one-step synthesis of tetra-substituted benzenes was accomplished via gold-catalyzed diyne-ene annulation. Distinguished from prior modification methods, this novel strategy undergoes formal [3+3] cyclization, producing polysubstituted benzenes with exceptional efficiency. The critical factor enabling this transformation was the introduction of amides, which were reported for the first time in gold catalysis as covalent nucleophilic co-catalysts. This interesting protocol not only offers a new strategy to achieve functional benzenes with high efficiency, but also enlightens potential new reaction pathways within gold-catalyzed alkyne activation processes.

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 α-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 π-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.