General procedure for the synthesis of (S)-2,5-diaminopentanoic acid methyl ester dihydrochloride from methanol and D-ornithine hydrochloride: L-ornithine hydrochloride (180 g, 1.07 mol) was dissolved in methanol (1,800 mL) and stirred for 30 min at 0 °C and under argon protection. Subsequently, thionyl chloride (160 mL, 2.19 mol) was added slowly and dropwise over 40 min at 0 °C. The reaction solution was refluxed for 6 hours after continued stirring at 0°C for 30 minutes. Upon completion of the reaction, the solvent was removed by distillation under reduced pressure to afford L-ornithine methyl ester dihydrochloride (232.1 g). Next, L-ornithine methyl ester dihydrochloride (223 g, 1.02 mol) was dissolved in methanol (2500 mL) and stirred at 0 °C. After addition of sodium methanol (116 g, 2.15 mol), the reaction solution was stirred at room temperature for 2 hours. After removing the solvent by distillation under reduced pressure, the residue was dissolved in ether (600 mL) and stirred at room temperature. After filtration through diatomaceous earth, the solvent was removed by distillation under reduced pressure again. The residue was dissolved in methanol (400 mL) and methanol saturated with HCl (100 mL) was added to give the crude product. Finally, it was recrystallized by a solvent mixture of methanol-isopropanol (1:1) to give (S)-2,5-diaminopentanoic acid methyl ester dihydrochloride (131.2 g, 86% yield).
