High-performance liquid chromatographic enantioseparation of unusual amino acid derivatives with axial chirality on polysaccharide-based chiral stationary phases

• First analytical and semipreparative resolution of axially chiral amino acids by HPLC.
• Use of polysaccharide-based CSPs in the resolution of unusual amino acids.
• The presence of chloroform in the mobile phase increased substantially the loading capacity of the column while providing good selectivity and resolution factors.
• Resolution of about 60–70 mg of axially chiral amino acid per hour.

The successful enantioseparation of axially chiral amino acid derivatives containing a cyclohexylidene moiety on an analytical and semipreparative scale was achieved for the first time by HPLC using polysaccharide-based chiral stationary phases. Racemic methyl N-benzoylamino esters, easily obtained by methanolysis of the corresponding 5(4H)-oxazolones, were subjected to chiral HPLC resolution using chiral stationary phases based on immobilized 3,5-dimethylphenylcarbamate derivatives of amylose (Chiralpak® IA column) or cellulose (Chiralpak® IB column). The behaviour of both selectors under different elution conditions was evaluated and compared. The amylose column showed better performance than the cellulose column for all enantiomers tested. The semipreparative resolution of axially chiral amino acid derivatives with different side chains has been achieved on a 250 mm × 20 mm ID Chiralpak® IA column using the appropriate mixture of n-hexane/chlorofom/ethanol as eluent by successive injections of a solution of the sample in chloroform. Using this protocol up to 120 mg of each enantiomer of the corresponding axially chiral amino acid derivative were obtained from 300 mg of racemate. [(Sa)-2a, 105 mg; (Ra)-2a, 60 mg, [(Sa)-2b, 105 mg; (Ra)-2b, 90 mg, [(Sa)-2c, 120 mg; (Ra)-2c, 100 mg].