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.