Practical Synthesis of Natural Amino Acid Derivatives: Hf(OTf)4-Catalyzed Mannich-Type Reaction of Ketene Silyl Acetals or Enol Silyl Ethers with N,O-Acetals as a Glycine Cation Equivalent - Sakai, Norio; Sato, Asuka; Konakahara, Takeo (Synlett, 2009)