Optical resolution of N-formylphenylalanine succeeds by crystal growth rate differences of diastereomeric salts

Optical resolution of racemic-phenylalanine through its N-formyl derivative with a 1-phenylethylamine resolving agent is an effective procedure. Differential scanning calorimetry, single crystal X-ray diffraction and optical microscopy were used in the investigation of the resolution process. It was found that the thermodynamic properties of the given system would not allow the efficient enantiomer separation. Kinetic effects during the crystal formation have been discovered by the comparison of the crystal morphologies of the two diastereomers. The crystal structure of the less soluble diastereomer (S)-(−)-1-phenylethylammonium (S)-(+)-N-formylphenylalaninate salt has been determined and discussed.