An Oxidant-Free Single-Step Synthesis of Tetra- or Disubstituted Symmetrical Pyridine Derivatives by a Hf(OTf)4-Catalyzed Annulation - Sasada, Toshiaki; Kobayashi, Fuminori; Moriuchi, Masato; Sakai, Norio; Konakahara, Takeo (Synlett, 2011)