A chemical synthesis of captopril by treatment of -proline with (2S)-3-acetylthio-2-methylpropanoyl chloride under basic conditions (NaOH), followed by aminolysis of the protective acetyl group to unmask the drug's free thiol, is depicted in the figure at right.
Procedure 2 taken out of patent US4105776. See examples 28, 29a and 36.