One-Pot Synthesis of Fused Tricyclic Heterocycles with Quaternary Carbon Stereocenter by Sequential Pauson-Khand Reaction and Formal [3+3] Cycloaddition

A novel RhI-catalyzed formal cycloaddition reaction of 1-ene-6,8-diynes leading to fused bicyclic 2-(alkyn-1-yl) enones in moderate to good yields was investigated. A solution of enediyne was refluxed under a positive pressure of CO. The reaction was complete in 24 hours, as determined by thin-layer chromatography. The residue was purified by column chromatography on silica gel. The solvent was removed in vacuo and the residue was dissolved in DMF. A solution of acetylacetone in DMF and K2CO3 were added to the reaction mixture, which was then stirred at room temperature. It was observed that all the three starting materials are readily available and the cycloadducts are readily converted into more complex ring systems with many convertible functional groups.