GaCl3-catalyzed allenyne cycloisomerizations to allenenes.

Cycloisomerizations of allenynes to allenenes have been studied in the presence of catalytic amounts of [Au(PPh3)]SbF6 in dichloromethane or GaCl3 in toluene. Both catalytic systems are quite effective for terminal 1,6-allenynes. However, they showed different reactivities toward allenynes with di-substituents at the allenic terminal carbon. For the GaCl3-catalyzed reactions, allenenes were obtained in reasonable to high yields. However, for a Au(I)-catalyzed reaction, a triene was obtained in a poor yield. Thus, GaCl3 serves as an effective catalyst for the cycloisomerization of allenynes bearing a terminal alkyne to give cyclic allenenes in reasonable to high yields.