Efficient Synthesis of Helicenes

Significance

In this communication, Harrowven and co-workers prepare helicenes in high yields using a stereoselective Wittig reaction followed by a regioselective radical cyclization. The Z-selectivity in the Wittig reaction is attributed to a cooperative effect when ortho substituents are present on both the ylide and aldehyde. Additionally, the ortho-halo or ortho-alkoxy substituents on the aldehyde serve as positional protecting groups to prevent regioisomer formation in the radical cyclization. This approach was used for syntheses of [5]helicenes, [7]helicenes, azahelicenes, and functionalized phenanthrenes in high yields.