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

Abstract

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 in 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.