A reappraisal of the Ni-[(Benzylprolyl)amino]benzophenone complex in the synthesis of α,α-disubstituted amino acid derivatives

α,α-Disubstituted alkenyl amino acid derivatives (e.g. Fmoc-S 5-OH) are valuable monomers in the construction of stapled peptide derivatives. Synthetic access to these is possible using the Ni-[(Benzylprolyl)amino]benzophenone (BPB) complex as a chiral auxiliary. We discuss a reappraisal of the use of this, and demonstrate that epimerisation of the proline α-centre occurs during formation of the complex, leading to erosion in the enantiomeric excess of the final product. Modified conditions have been developed, providing the target compounds in high enantiomeric excess