Preparation of C₃-symmetric homochiral syn-trisnorbornabenzenes through regioselective cyclotrimerization of enantiopure iodonorbornenes

C₃-symmetric homochiral (-)-syn-trisoxonorbornabenzene 1 possessing a rigid cup-shaped structure was synthesized through a novel regioselective cyclotrimerization of enantiopure iodonorbornenes catalyzed by palladium nanoclusters. The yield of the cyclotrimerization was dependent on the stability of the palladium clusters, which was ascertained from the appear- ance and TEM images of the reaction mixtures. The efficient preparation of (-)-syn-1 was established in short steps, including the newly developed cyclotrimerization reaction. The thus- prepared homochiral (-)-syn-1 can serve as a key intermediate for the synthesis of C₃-symmetric homochiral cup-shaped molecules with a helical arrangement of substituents. Introduction of several types of substituents was well demonstrated through palladium-catalyzed coupling reactions with the corresponding phosphate and triflate of (-)-syn-1.