(R)-Boc-proline (4.8 g, 22.3 mmol) was suspended in 100 mL of tetrahydrofuran (THF). 1-Ethyl-3-(3-dimethylaminopropyl)carbodiimide (5.13 g, 1.2 eq.), 1-hydroxybenzotriazole (3.62 g, 1.2 eq.) and N-methylmorpholine (3.7 mL, 1.5 eq.) were added sequentially. The reaction mixture was stirred at room temperature for 30 minutes before 35 mL of THF solution of methylamine (2.0 M, 3 eq.) was added. Stirring was continued at room temperature for 18 hours. Upon completion of the reaction, the reaction mixture was concentrated, the residue was dissolved in dichloromethane (CH2Cl2), washed sequentially with dilute sodium bicarbonate (NaHCO3) solution, water, 1N citric acid solution and saturated brine, dried over anhydrous sodium sulfate (Na2SO4), and concentrated to give 4.8 g of crude carboxamide intermediate. The intermediate was dissolved in 100 mL of anhydrous THF and cooled to 0 °C. Borane-THF complex (53 mL, 1.0 M solution, 2.5 eq.) was added slowly and the reaction mixture was gradually warmed to room temperature and stirred for 18 hours. After completion of the reaction, it was cooled to 0 °C and the reaction was quenched by careful addition of 50 mL of methanol. The reaction mixture was concentrated under reduced pressure and the resulting residue was azeotropically dried with 50 mL of methanol and 100 mL of ethyl acetate and concentrated under reduced pressure. The grinding and decompression concentration operations were repeated three times to give almost quantitative amounts of (R)-2-methylaminomethyl-1-BOC-pyrrolidine, which could be used in the next reaction without further purification.
