Described herein are improved methods of making Compound 1 (4S,7S,10aS)-4-((S)-2- mercapto-3-phenylpropanamido)-5-oxooctahydro-7H-pyrido[2,1-b][1,3]thiazepine-7-carboxylic acid, or Omapatrilat, and purified Omapatrilat obtained from the improved methods.