Synthesis of Phenanthrenes through Copper-Catalyzed Cross-Coupling of <i>N</i>‑Tosylhydrazones with Terminal Alkynes

A novel protocol for the synthesis of phenanthrenes through the copper-catalyzed reaction of aromatic tosylhydrazones with terminal alkynes is explored. The reaction proceeds via the formation of an allene intermediate and subsequent six-π-electron cyclization–isomerization, affording phenanthrene derivatives in good yields. The transformation can be performed in two ways: (1) with N-tosylhydrazones derived from [1,1′-biphenyl]-2-carbaldehydes and terminal alkynes as the starting materials and (2) with N-tosylhydrazones derived from aromatic aldehydes and 2-alkynyl biphenyls as the starting materials. This new phenanthrene synthesis uses readily available starting materials and a cheap copper catalyst and has a wide range of functional group compatibility.