First Synthesis of Exomethylene Paracyclophanes and Their Structural Properties.

A series of novel exomethylene paracyclophanes was synthesized by the intramolecular benzannulation of conjugated enynes in the presence of tetrakis(triphenylphosphine)palladium(0). Detailed spectral studies revealed that the conformation of the exomethylene paracyclophanes depends on the size of the ring; the alkene moiety does not conjugate with the adjacent phenyl group in the smaller exomethylene paracyclophanes.