Direct access to benzo[b]fluorenes via a gold-catalysed A3-coupling strategy

A one-pot atom-and step-economical method for the synthesis of benzo[ b] fluorenes, from aldehydes, alkynes and amines, proceeding via an A3-coupling/formal [ 4 + 2] thermal cyclization is described. The transformation generates water as the only by-product and three new carbon-carbon bonds are simultaneously formed. The scope was demonstrated by the synthesis of 32 benzo[b] fluorenes and, by specific designing of starting materials, regiocontrol could be accessed for some substrates.