Stereospecific Synthesis of Enantiopure [6]Helicene Containing a Seven‐Membered Ring and [7]Helicene by Acid‐Promoted Stepwise Alkyne Annulations of Doubly Axial‐Chiral Precursors**
Tomoyuki Ikai, Kosuke Oki, Shoya Yamakawa, Eiji Yashima
Abstract
Enantiopure [6]helicene containing an embedded seven-membered ring and carbo[7]helicene (>99 % ee) with 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 stereocontrolled by the doubly axial chirality of the precursors as a result of complete axial-to-helical chirality 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 opposite helicity.