Palladium(0)-Catalyzed Cyclization of Electron-Deficient Enynes and Enediynes

In the presence of a Pd(0) precatalyst, Pd2(bq)2(nbe)2 or Pd2(dba)3, 1,6-enyne esters were heated in
refluxing benzene to give cyclodimers as single regioisomers. On the other hand, the combination
of the Pd(0) precatalyst and triphenyl phosphite gave rise to various cycloisomerization products
depending on the substitution pattern of the enyne esters. Six-membered ring cycloisomerization
products were predominantly obtained from enyne esters bearing methallyl or 2-phenyl-2-propenyl
moieties, while other enyne esters afforded normal five-membered ring cycloisomerization products.
Intramolecular [2 + 2 + 2] cyclocotrimerization of enediyne esters also proceeded in the presence
of the Pd(0) precatalyst and triphenylphosphine to give fused cyclohexadienes.