Total Synthesis of (S)-(+)-Imperanene. Effective Use of Regio- and Enantioselective Intramolecular Carbon−Hydrogen Insertion Reactions Catalyzed by Chiral Dirhodium(II) Carboxamidates

The total synthesis of (S)-(+)-imperanene, a natural product found in Chinese medicine, has been completed in 12 steps from a commercially available cinnamic acid. The key step is highly enantioselective carbon-hydrogen insertion from a diazoacetate using a chiral dirhodium(II) carboxamidate catalyst. An elimination process essential to the construction has been optimized to avoid intramolecular Friedel-Crafts alkylation.