Optimisation of the retroracemisation procedure for α-amino acids using (S)-2-[(N-alkylprolyl)amino]benzophenones, recyclable chiral axiliaries

Article abstract of DOI:10.1016/S0957-4166(97)00331-5

The retroracemisation procedure developed by Belokon and coworkers has been re-examined using a variety of new (S)-2-[N-alkylprolyl)amino]benzophenone chiral auxiliaries. It has been found that (S)-2-[(N-benzylprolyl)amino] and (S)-2-[(N-1-(naphthalenyl-1-methyl)prolyl)amino] benzophenones ((S)-BPB and (S)-NPB) when used in conjunction with Ni(NO₃)₂·6H₂O and a racemic α-amino acid preferentially form a single diastereoisomer in the presence of a mild base such as sodium methoxide. Decomposition of this complex under acidic conditions leads to the isolation of the (S)-amino acid in good yield, and in 55 to 99% e.e. The retroracemisation abilities of a polymer supported form of the (S)-BPB ligand have also been investigated and preliminary results for this are presented here.