Hydrochloric Acid → 2-Methyl-L-cysteine--hydrogen chloride (1/1)

First, water (47.6 g) and conc. hydrochloric acid (177.4 g) were added to S-tert-butyl-α-methyl-L-cysteine (25 g, 131 mmol) produced by the method of EXAMPLE 11, and the resultant mixture was refluxed for 41 hours. Furthermore, conc. hydrochloric acid (47.6 g) was added to the mixture, followed by reflux for 3 hours. After the mixture was allowed to cool down to room temperature, isopropyl alcohol (90 mL) was added to the mixture, followed by vacuum concentration. Then, azeotropic dehydration was performed three times using the same amount of isopropyl alcohol. Finally, isopropyl alcohol was added to the residue, and the resultant mixture was concentrated to a volume of about ⅓. Then, the residue was heated to 60° C., and toluene (90 mL) was added to the residue. After the mixture was allowed to cool down to room temperature under stirring, stirring was continued about 1 hour. Then, the precipitated crystals were filtered off, washed with toluene, and dried under reduced pressure to obtain the title compound as a white solid (13.5 g, yield 60.0%).