Divergent Cyclizative Approaches of In Situ Formed β-Keto-Allenes: Chemo- and Regioselective Synthesis of Bicyclic Furans, and Pyrroles.

A unified strategy for the construction of bicyclic furans and pyrroles is developed from tert-propargyl alcohols and α-acyl cyclic ketones using an alkaline earth catalyst under solvent-free conditions. The reaction proceeds via the formation of a β-keto allene intermediate, which upon treatment with a tert-amine underwent thermodynamic enol formation and a subsequent annulation to form bicyclic furans. Interestingly, the same allene forms bicyclic pyrrole with primary amines. The reaction shows excellent atom economy as water is the only byproduct formed in bicyclic furans. The generality of the reaction is well established. Gram-scale synthesis and synthetic applications are demonstrated.