An Oxidant-Free Single-StepSynthesis of Tetra- or Disubstituted Symmetrical Pyridine Derivativesby a Hf(OTf)4-Catalyzed Annulation

The Hf(OTf)4-catalyzed unprecedented annulation of an enamine with triethyl orthoformate leading to the preparation of tetrasubstituted symmetrical pyridine derivatives without an oxidant is described. This method accommodated the single-step synthesis of a disubstituted symmetrical pyridine using an acetophenone derivative, triethyl orthoformate, and ammonium acetate (NH4OAc).