Combined Organo- and Transition-Metal Catalysis Cascade to Cyclopentenes

Significance

The authors report an efficient synthesis of various cyclopentenones from α,β-unsaturated ketones 1 and alkyne-tethered malonate pro-nucleophiles 2. The reaction presumably occurs via a complex mechanism, combining pyrrolidine and a copper(II) species as catalysts. Iminium-ion formation from α,β-unsaturated ketones 1 and pyrrolidine is followed by Michael addition of malonate nucleophiles 2. Subsequently the resulting enamine attacks the adjacent alkyne functionality, which is activated by the copper(II) catalyst. Carbon-carbon bond formation, followed by hydrolysis and protonolysis provides products 3 and recycles the catalysts.