Stereospecific Synthesis of Enantiopure [6]Helicene Containing a Seven-Membered Ring and [7]Helicene via Acid-Promoted Stepwise Alkyne Annulations of Doubly Axial-Chiral Precursors

The enantiopure seven-membered ring embedded [6]helicene and carbo[7]helicene (>99% ee) with an opposite helicity were simultaneously and quantitatively (>99%) synthesized with a perfect stereospecificity through stepwise acid-promoted intramolecular alkyne annulations of doubly axial-chiral cyclization precursors. The helical handedness of the [6]- and [7]helicenes was fully stereo-controlled by the doubly axial chirality of the precursors as a result of complete axial-to-helical asymmetry transfer. The cyclizations proceeded in a stepwise manner; the first six-membered ring formation was followed by the kinetically-controlled seven- or six-membered ring formation with or without helix-inversion of a [4]helicene intermediate generated during the first cyclization step, thus quantitatively producing enantiopure circularly polarized luminescent [6]- and [7]helicenes with an opposite helicity, respectively.