Asymmetric Synthesis of Diastereo- and Enantiopure Bioxahelicene 2,2-Bipyridines

A versatile asymmetric synthesis of five C-2 symmetric enantio- and diastereopure bioxa[5]- and bioxa[6]helicene 2,2-bipyridines was developed. It relied either on a double intramolecular [2+2+2] cycloisomerization of dicyanotetrayne (forming simultaneously the 2,2-bipyridine unit and biazaoxahelicene backbone) or one-pot/sequential intramolecular [2+2+2] cycloisomerization of triyne accompanied by an intermolecular haloazaoxahelicene reductive homocoupling. We reached an effective central-to-helical-to-axial chirality transfer that was controlled by the 1,3-allylic-type strain and sterically constricted atropoisomerization of the embedded 2,2-bipyridine unit. The chiroptical properties of the bioxahelicene 2,2-bipyridines were studied along with their fluorescence properties.