A Facile Synthesis of 2 H ‐Chromenes and 9‐Functionalized Phenanthrenes through Reactions between α,β‐Unsaturated Compounds and Arynes

Facile syntheses of 2 H ‐chromenes or 9‐functionalized phenanthrenes under mild conditions in moderate to good yields have been developed. They each involve annulations of arynes with α,β‐unsaturated compounds bearing different electron‐withdrawing groups (EWGs). Depending on the natures of the different EWGs, the reactions proceed by different pathways: enals react with arynes through a tandem [2+2] cycloaddition/thermal electrocyclic ring‐opening/6e‐electrocyclization sequence to afford 2 H ‐chromenes, whereas acyl‐/ethoxycarbonyl‐/cyano‐substituted styrenes undergo Diels–Alder reactions with arynes followed by aromatization to afford 9‐functionalized phenanthrenes. The scope, limitations, regioselectivities and mechanisms have been studied and are discussed in detail.