The First Approach to the Synthesis of 1-Unsubstituted 2-Arylindolizines by Intramolecular 1,5-Dipolar Cyclization of 2-(2-Arylethenyl)pyridinium Ylides in the Presence of Tetrakis(pyridine)cobalt (II) Dichromate - Zhou, Jian; Hu, Yuefei; Hu, Hongwen (Synthesis, 1999)