Chiral differentiation of some cyclic β‐amino acids by kinetic and fixed ligand methods

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Differentiation of beta-amino acid enantiomers with two chiral centres was investigated by kinetic method with trimeric metal-bound complexes. Four enantiomeric pairs of beta-amino acids were studied: cis-(1R,2S)-, cis-(1S,2R)-, trans-(1R,2R)- and trans-(1S,2S)-2-aminocyclopentanecarboxylic acids (cyclopentane beta-amino acids), and cis-(1R,2S)-, cis-(1S,2R)-, trans-(1R,2R)-, and trans-(1S,2S)-2-aminocyclohexanecarboxylic acids (cyclohexane beta-amino acids). The results showed that the choice of metal ion (Cu(2+), Ni(2+)) and chiral reference compound (alpha- and beta-amino acids) had an effect on the enantioselectivity. Especially, aromaticity of the reference compound was noted to enhance the enantioselectivity. The fixed-ligand kinetic method, a modification of the kinetic method, was then applied to the same beta-amino acids, with dipeptides used as fixed ligands. With this method, dipeptide containing an aromatic side chain enhanced the enantioselectivity.