General procedure for the synthesis of 3-amino-2-hexanolactam hydrochloride from methyl-2,6-diaminohexane hydrochloride: L-lysine hydrochloride (100.0 g, 547 mmol) was dissolved in methanol (1200 mL) and stirred for 30 min at 0 °C under argon protection. Subsequently, thionyl chloride (80 mL, 1.10 mol) was slowly added dropwise at 0 °C for 20 min. After the dropwise addition, the reaction solution was stirred at room temperature for 1 hour and then refluxed overnight. After completion of the reaction, the solvent was removed under vacuum and the crude product was recrystallized by methanol to afford L-lysine methyl ester dihydrochloride (124.1 g, 97% yield). Next, L-lysine methyl ester dihydrochloride (60.0 g, 257 mmol) was dissolved in methanol (1200 mL) and stirred at room temperature under argon protection. After addition of sodium methanol (48.0 g, 889 mmol), the reaction solution was refluxed for 4 hours. At the end of the reaction, ammonium chloride (20.0 g) was added for hydrolysis, and the reaction solution was filtered and the solvent was removed under vacuum. The residue was dissolved in dimethoxyethane (80 mL), filtered and the solvent was again removed. The residue was dissolved in ethanol (100 mL) and ethanol saturated with hydrogen chloride (20 mL) was added to give the crude product. Finally, the crude product was recrystallized by methanol to give (S)-3-aminoazepin-2-one hydrochloride (27.8 g, 66% yield).
