Control over [2+2+2] and Carbonylative [4+2] Cycloaddition by CO Pressure in Co-Catalyzed Cycloaddition between Internal Diynes and Cyclopentadiene

The reaction of internal diynes and cyclopentadiene in the presence of 5 mol % of cobalt catalysts proceeded under 5 atm of carbon monoxide pressure to give a high yield of the corresponding [2+2+2] cycloaddition product. By lowering carbon monoxide pressure from 30 atm to 5atm, cyclopentadiene can be used as a dienophile in the cobalt carbonyl-catalyzed [2+2+2] cycloaddition reaction between internal diynes and cyclopentadiene.