Nucleophilic Addition/Double Cyclization Cascade Processes between Enynyl Fischer Carbene Complexes and Alkynyl Malonates

Two new selective cascade processes for enynyl Fischer carbene complexes 1 are described in their reaction with alkynyl malonates. When carbene complexes 1 react with the sodium enolate of homopropargyl malonates 3 a consecutive Michael-type addition/cyclopentannulation/6-exo cyclization takes place leading, in a regio- and stereoselective way, to n/5/6 angular tricyclic compounds 5. Furthermore, when propargylic malonates are used, a delayed protonation of the reaction mixture allows intermediate 1,4-addition adduct Ia to evolve through a 5-exo cyclization, consisting of an intramolecular nucleophilic attack from the central carbon of the allenylmetallate over the triple C–C bond. Further spontaneous cyclopentannulation of the resulting metallatriene gives rise to bicyclic and linear polycyclic compounds 6 and 7, some of them bearing a polyquinane framework.