Visible-Light-Promoted [2 + 2 + 2] Cyclization of Alkynes with Nitriles to Pyridines Using Pyrylium Salts as Photoredox Catalysts.

A highly regioselective [2 + 2 + 2] cyclization of aromatic alkynes with nitriles is developed for the preparation of 2,3,6-trisubstituted pyridines under visible-light irradiation using a pyrylium salt as the photoredox catalyst. This cycloaddition is achieved through a photooxidative single-electron-transfer process at room temperature and under metal-free conditions. A variety of aromatic alkynes and nitriles are employed to furnish the annulation products in good yields.