Asymmetric synthesis of threonine and retroracemization of amino acids using Ni(II) complexes of Schiff bases with S-2-n-(N′-benzylprolyl)aminobenzaldehyde

Conclusions

• 1. Complexes of Ni(II) with Schiff bases of 2-N-(2′-S-N-benzylpyrrolidin-2-carbonyl)aminobenzaldehyde (S-BPaB) with glycine and valine have been synthesized in which the Ni ion is coordinated with the N atoms of the proline fragment, of the aldimine group, of the ionized amide, and with the carboxyl group.

• 2. The presence has been shown of enantioselective effects in complexes of Schiff bases of S-BPAB with R,S-amino acids with Ni(II) which increase with an increase in volume of the amino acid side chain. The enantiomer excess for the complex with S-Val was 78%.

• 3. The retroracemization of alanine, valine, norvaline, phenylalanine, phenylglycine, and tryptophan was carried out using S-BPAB and Ni(II) with enantiomer yields of 15, 78, 42, 40, 39, and 64% respectively.

• 4. The asymmetric synthesis of S-threonine has been effected by condensing acetaldehyde with (S-BPAB-Gly)·Ni(II) on catalysis with Et₃N with a S-Thr and S-allo-Thr ratio of 1∶2 and enantiomer yield of 86 and 76% respectively. An increase in pH led to a reduction in enantiomer yield and reversal of the sign of the enantioselective effect.

• 5. A method has been developed for enantiomer analysis of tryptophan using high-pressure liquid chromatography as has a GLC method for difficult amino acids using a high-temperature chiral chromatographic phase, viz., N-heptadecanoyl-S-Val tert-butylamide.