Gold-Catalyzed One-Pot A3-Coupling/1,5-Hydride Shift/Schmittel-Type Cyclization: From Aldehydes, Amines and Alkynes to the Synthesis of Benzo[b]fluorenes

A step-economic gold-catalyzed one-pot synthesis of benzo[b]fluorenes, from aldehydes, alkynes and amines, proceeding via an A(3)-coupling/1,5-hydride shift/Schmittel-type cyclization is described. The formation of the allene intermediate is not dependent on a previous installation of reaction triggers, and upon cyclization delivered benzo[b]fluorenes. With this method, just by an easy modification of the starting materials in a modular way, nineteen benzo[b]fluorenes, bearing different substituents Ar-1, Ar-2 and Ar-3, were prepared. Overall, three new carbon-carbon bonds are formed in one-pot.