Chiral Phosphathiahelicenes: Improved Synthetic Approach and Uses in Enantioselective Gold(I)-Catalyzed [2+2] Cycloadditions of N-Homoallenyl Tryptamines

A chiral phosphathiahelicene scaffold displaying a CI phosphole and a thiophene unit as the terminal rings of the helical sequence has been synthesized and characterized by spectroscopic methods and X-ray diffraction studies. The phosphine oxides (HelPhos-V oxides) have been obtained following a robust and scalable synthetic approach, based on a nickel-promoted alkyne cyclotrimerization reaction. Then, late-stage functionalization has been carried out via a bromination/palladium coupling reaction sequence. The HelPhos-V gold(I) complexes have been used as catalysts in the unprecedented enantioselective [2 + 2] cyclization of N-homoallenyl tryptamine derivatives, to afford indolenine-fused cyclobutanes in good isolated yields, with enantiomeric excesses up to 93%.