Enantioselective synthesis of chiral buckybowl and chiral azabuckybowl

Buckybowls are bowl-shaped aromatic hydrocarbons corresponding to a partial fullerene structure or a cap structure of carbon nanotubes. Since the discovery of C₆₀ fullerene in 1985, buckybowls have attracted much attention due to their physical properties, not only as model compounds for fullerenes, but also as unique bowl-shaped aromatic compounds. We have achieved first enantioselective synthesis of chiral buckybowl, trimethylsumanene, and chiral azabuckybowl, triazasumanene. To achieve the synthesis, we have developed Pd-catalyzed syn- selective cyclotrimerization of enantiopure iodonorbornene derivatives, giving enantiopure C₃ symmetric syw-tris(norborneno) benzenes. By using syw-tris(norborneno)benzene derivatives as synthetic intermediates, trimethylsumanene and triazasumanene were synthesized featuring tandem ring-opening/closing metathesis reaction or amide recombination reaction and oxidative aromatization. From the racemization experiments, the bowl inversion/racemization energies of trimethylsumanene and triazasumanene were determined to be 21.6 kcal/mol and 42.2 kcal/mol, respectively.