Enantioselective Synthesis of Dithia[5]helicenes and their Postsynthetic Functionalization to Access Dithia[9]helicenes

A highly enantioselective synthesis of 5,13‐disubstituted dibenzo[d,d′]benzo[1,2‐b:4,3‐b′]dithiophenes is reported. Key for the successful assembly of these helical architectures is the last two successive Au‐catalyzed intramolecular alkyne hydroarylation events. Specifically, the second cyclization is the enantiodetermining step of the whole process and provides the desired helicenes with excellent ee values when a TADDOL‐derived 1,2,3‐(triazolium)phosphonite moiety (TADDOL: α,α,α′,α′‐tetraaryl‐1,3‐dioxolane‐4,5‐dimethanol) is employed as an ancillary ligand. The absolute stereochemistry of the newly prepared structures has been determined by X‐ray crystallography to be P; the optical properties of these heterohelicenes are also reported. A three‐step procedure was subsequently developed that allows the transformation of the initially obtained dithia[5]helicenes into dithia[9]helicenes without erosion of the enantiopurity.