Control of Polymorphs and Crystal Habit in Crystallization of N2-(1(S)-Ethoxycarbonyl-3-Phenylpropyl)-N6-Trifluoroacetyl-L-Lysine

The control of polymorphs and crystal habit in the cooling crystallization from an acetonitrile/water mixed solvent for the purification of N2-(1(S)-ethoxycarbonyl-3-phenylpropyl)-N6-trifluoroacetyl-L-lysine was investigated. The obtained crystals had three polymorphs (I form, II form and III form). The I form is a stable form having the lowest solubility and tends to have a low content of N2-(1(S)-carboxy-3-phenylpropyl)-N6-trifluoroacetyl-L-lysine (carboxyl derivative). So, the selective crystallization of I form was advantageous in terms of the yield and quality. By adjusting the pH of the crystallization system to between 4.5 and 5.5, but not less than 4.5, that goal could be achieved. Besides the polymorphism, two kinds of crystal habit (prismatic form and granular form) were observed in every polymorph. The selective crystallization of granular form was advantageous from the viewpoint of removal of N2-(1(R)-ethoxycarbonyl-3-phenylpropyl)-N6-trifluoroacetyl-L-lysine (diastereomer). It could be achieved by carrying out the crystallization under action of vibration or ultrasonic wave.