Asymmetric synthesis of threonine and partial resolution and retroracemization of .alpha.-amino acids via copper(II) complexes of their Schiff bases with (S)-2-N-(N'-benzylprolyl)aminobenzaldehyde and (S)-2-N-(N'-benzylprolyl)aminoacetophenone. Crystal and molecular structure of a copper(II) complex of glycine Schiff base with (S)-2-N-(N'-benzylprolyl)aminoacetophenone