Improved Synthesis of Proline-Derived Ni(II) Complexes of Glycine:  Versatile Chiral Equivalents of Nucleophilic Glycine for General Asymmetric Synthesis of α-Amino Acids

A synthetically practical and operationally convenient method for preparing (S)-2-[N-(N ‘-benzylprolyl)amino]benzophenone (BPBP) and hitherto unknown (S)-2-[N-(N ‘-benzylprolyl)amino]-4-methylbenzophenone (4-Me-BPBP), (S)-2-[N-(N ‘-benzylprolyl)amino]-5-nitrobenzophenone (5-NO2−BPBP), and their corresponding Ni(II) complexes with glycine [GlyNi(II)BPBP], a widely used chiral equivalent of nucleophilic glycine, and new analogues [GlyNi(II)-4-Me-BPBP] and [GlyNi(II)-5- NO2-BPBP] is described. The key step of the method is the synthetically efficient amid bond formation between the corresponding o-aminobenzophenones, featuring significant steric shielding and low nucleophilicity of the amino functionality as well as sterically constrained (S)-N-benzylproline (BP).