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.
Ring closure reactions O 0130Palladium(0)-Catalyzed Cyclization of Electron-Deficient Enynes and Enediynes.-The outcome of the cyclization concerning 1,6-enyne esters is found to depend on the nature of the educt as well as on the conditions used. For example, in the presence of Pd2(dba)3 or PBN, the enyne ester (I) undergoes regioselective cyclodimerization. Addition of phosphine ligands results in unusual formation of a 6-membered ring. Under these conditions, the analogues (Ve) and (Vf) undergo normal cycloisomerization yielding the 5-membered ring systems (X) and (XI), respectively. A terminal ester group as well as the presence of heteroatom in 1,6-enyne educts are found to be crucial for successful cyclodimerization. Enediyne esters undergo [2 + 2 + 2]cycloisomerization to afford products of type (XV), (XVI), and (XVIII). -(YAMAMOTO*, Y.; KUWABARA, S.; ANDO, Y.; NAGATA, H.; NISHIYAMA, H.; ITOH, K.; J. Org.
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