Enynylation of 2-Iodo-4-(phenylchalcogenyl)-1-butenes via Intramolecular Chelation: Approach to the Synthesis of Conjugated Dienynes or Trienynes

2-Iodo-4-(phenylchalcogenyl)-1-butenes 3 and 4, which are derived from methylenecyclopropanes 1, can be enynylated with alkynes catalyzed
by Pd(OAc)2 to give conjugated dienynes 5 and 6 in the absence of any phosphine ligand and copper salt, and trienyne 9a can be obtained
by oxidation of compound 5a. A plausible reaction mechanism has been proposed.