Scheme 2. Synthetic routes to a series of dihetero[8]helicenes. (A) Synthesis of common intermediate (rac)-4. (B) Resolution of 4. (C) Conversions of enantiopure 4 to dihetero[8]helicenes. Structures in dashed boxes were obtained by X-ray single crystal diffraction analysis, and all thermal ellipsoids are set at a 50% probability level. Solvent molecules in (rac)-6CX and (rac)-6CF were omitted for clarity. Values of ee in parenthesis are those after recrystallization.