On the chemistry of binding sites VII. Enantioselective binding using chiral boronic acids

For the construction of chiral binding sites in chromatography, various phenylboronic acids have been prepared as model substances carrying, in the ortho (3a‐d) or ortho, ortho position (4a‐d), optically active side‐groups containing basic nitrogen atoms. The compound carrying two prolyl residues shows the highest selectivity in chiral recognition of racemic diols (separation factor α between 1.70‐2.25). Separation occurs via the formation of a cyclic boronic diester of the diol. The thermodynamic properties of this esterification reaction are well suited to chromatography and furthermore the reaction seems to be adequately rapid.