PURPOSE: A process for preparing erdosteine is provided, thereby cheaply preparing high purity of erdosteine in higher yield without side reactions under mild conditions by regulating the amount of organic bases. CONSTITUTION: The process for preparing erdosteine comprises the steps of: reacting D,L-homocysteine thiolactone chloride with bromoacetyl bromide in the presence of organic base such as triethylamine in organic solvent such as dichloromethane to prepare 3-bromoacetylamido-2-oxo-tetrahydrothiophen; and reacting 3-bromoacetylamido-2-oxo-tetrahydrothiophen with mercaptoacetic acid in the presence of organic base such as triethylamine in organic solvent such as dichloromethane, wherein a step of adding inorganic base into the reaction product of 3-bromoacetylamido-2-oxo-tetrahydrothiophen with mercaptoacetic acid and extracting erdosteine from a water-soluble layer may be further added.
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