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)aminobenzophenone (BPBP) and hitherto unknown (S)-2-N-(N'-benzylprolyl)amino-4-methylbenzophenone (4-Me-BPBP), (S)-2-N-(N'-benzylprolyl)amino-5-nitrobenzophenone (5-NO(2)-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- NO(2)-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).