Methanesulfonic Acid → 2-Methyl-L-proline

Methanesulfonic acid (252 mg, 2.6 mmol) was added to a solution of (S)—N-(tert-butoxycarbonyl)-2-methylproline (500 mg, 2.2 mmol) and 1,2-dichloroethane (2.53 g), and the mixture was stirred at 25° C. for 3 hours. When triethylamine (265 mg, 2.6 mmol) was added thereto, a solid was precipitated. The solid was sufficiently precipitated, and then the solution was stirred at 25° C. for 30 minutes and further stirred at 25° C. for 2 hours. Thereafter, the solid was then separated by filtration under reduced pressure. The solid was washed with 1,2-dichloroethane (0.2 mL), and then was subjected to vacuum drying. Thereby, the titled compound was produced as a white solid (197 mg, yield of 69%).