Synthesis of Helicene Scaffolds via [2+2+2] Cycloisomerization of Aromatic Triynes

Triynes R-C≡C-CH 2 CH 2 -Ar-C≡C-Ar'-CH 2 CH 2 -C≡C-R were subjected to Co(I)- or Ni(0)-mediated intramolecular [2+2+2] cycloisomerization to provide corresponding tetrahydropentahelicene, tetrahydrohexahelicene, and tetrahydroheptahelicene in good to moderate yields. The reaction tolerates various substituents at pendant acetylene moieties (R = H, CH 3 , trimethylsilyl, not triisopropylsilyl) and fluorine in an aromatic part. By contrast, under Rh(I) or Pd(0) catalysis, triyne with the CH 2 OCH 2 tether lost pendant propargyl moieties and an aromatic spiroketal was formed preferentially.