Efficient Resolution of Racemic N-Benzyl beta3-Amino Acids by Iterative Liquid-Liquid Extraction with a Chiral (Salen)Cobalt(III) Complex as Enantioselective Selector

The efficient (up to 93% ee) resolution of racemic N-benzyl beta3-amino acids has been achieved by an iterative (two cycle) liquid-liquid extraction process using a lipophilic chiral (salen)cobalt(III) complex [CoIII(1)(OAc)]. As a result of the resolution by extraction, one enantiomer of the N-benzyl beta3-amino acid predominated in the aqueous phase, while the other enantiomer was driven into the organic phase by complexation to cobalt. The complexed amino acid was then quantitatively released into an aqueous phase, by a reductive (CoIII → CoII) counter-extraction using L-ascorbic acid. The reductive cleavage allowed for the recovery of the Cobalt(II) selector in up to 90% yield (easily re-oxidable to CoIII with air/AcOH).