Synthesis and Reaction of a New Chiral Pyridoxamine Analogue; Some Doubt about the Stereochemical Process Tentatively Proposed for a Nonenzymatic Transamination Reaction

A pyridoxamine analogue-like chiral pyridinophane with two sulfonyl groups in the bridging chain, (S)-15-aminomethyl-14-hydroxy-2,8-dithia[9](2,5)pyridinophane S,S,S′,S′-tetraoxide ((S)-7), was prepared by oxidation of the sulfide precursor, (S)-2. The amino group was successfully transferred from (S)-7 to several 2-oxo carboxylic acids in methanol at room temperature in the presence of one-half equimolecular zinc(II) ion, giving (R)-amino acids in excess. The reaction rates of this nonenzymatic transamination using chiral (S)-7 were much smaller than those of the corresponding reaction using chiral (S)-2. The enantiomeric excess of the amino acids obtained through the reactions of (S)-7 was compared with those of (S)-2, showing that (S)-7 was more efficient than (S)-2 for the preparation of (R)-alanine, but was less than that of (S)-2 for the preparations of (R)-valine, (R)-leucine, and (R)-phenylalanine. These results aroused some doubt about the previous explanation of the stereochemical features of such nonenzymatic transamination reactions.