Oxidative Transformation to Naphthodithiophene and Thia[7]helicenes by Intramolecular Scholl Reaction of Substituted 1,2-Bis(2-thienyl)benzene Precursors

We present here a strategy to synthesize a variety of substituted naphthodithiophene building blocks through DDQ/acid-mediated oxidative cyclizations. The versatility of the Scholl reaction using the DDQ/acid system was demonstrated by the preparation of a novel substituted tetrathia[7]helicene where three new C-C bonds were formed in a one-pot procedure. The new DDQ/acid method was compared to the known strategies such as FeCl3 oxidation and oxidative photocyclization. By protecting the 1,2-bis(2-thienyl)benzene precursors, it is possible to direct the intermediates to controlled cyclization and effectively suppressing the polymerization. The highly reactive α-position of the terminal thiophenes can allow for further functionalization. The efficient preparation of a variety of naphthodithiophene building blocks, the extension to a nonphotochemical synthesis of [n]helicenes, and the ease of isolation of the products are arguments for the use of DDQ/acid system for this Scholl reaction.