Process for the preparation of l-proline derivatives

A process for the preparation of an L-proline derivative represented by the following formula (III):

• ##STR8## wherein R.sub.2 represents a lower alkyl group by reacting a D-.alpha.-alkyl-.beta.-acylthiopropionic acid halide, which is represented by the following formula (I):
• ##STR9## wherein R.sub.1 represents an acyl group, X represents a halogen atom and R.sub.2 has the same meaning as defined above, with L-proline, which comprises:
• adding D-.alpha.-alkyl-.beta.-acylthiopropionic acid halide (I) to an aqueous medium containing L-proline and an alkali metal hydroxide for deacidification while maintaining the reaction temperature at 12.degree. C. or lower and pH within a range of 10.5 to 11.5, to obtain a compound represented by the following formula (II):
• ##STR10## wherein R.sub.1 and R.sub.2 have the same meanings as defined above;
• and subjecting the compound (II) to deacylation without isolating the compound (II) from the reaction mixture by raising the temperature to at least 40.degree. C. and maintaining the pH between 10.5 and 11.5 by adding an alkali metal hydroxide.