Under argon protection, 1-tert-butyl 2-methyl-(2S,4R)-4-hydroxypyrrolidine-1,2-dicarboxylate (5.0 g, 20.41 mmol) was dissolved in THF (100 mL), cooled to -16 °C and stirred. LiAlH4 (1 M solution of THF, 21 mL, 21 mmol) was added slowly and dropwise over 5 min and precipitate formation was observed in the reaction. THF (50 mL) was subsequently added to destroy the precipitate. The reaction mixture was gradually warmed to 0 °C with stirring for 3.5 hours. Upon completion of the reaction, the reaction was quenched with EtOAc (5 mL) and stirring was continued for 30 minutes. Then, an excess of Rochelle's salt solution (100 mL) was carefully added, and the mixture was warmed to room temperature and stirred for an additional 1 hour. The mixture was extracted twice with CH2Cl2 (100 mL), the organic phases were combined, dried with MgSO4, filtered and concentrated under reduced pressure to remove the solvent. The product was purified by column chromatography (eluent: 10% MeOH/CH2Cl2) to give Intermediate 19 in quantitative yield.1H NMR (300 MHz, DMSO-d6) δ: 4.82 (br s, 1H), 4.64 (br d, J = 5.5 Hz, 1H), 4.20 (sxt, J = 4.2 Hz, 1H), 3.75 (br s , 1H), 3.16-3.50 (m, 4H), 1.85-2.02 (m, 1H), 1.80 (br s, 1H), 1.39 (s, 9H).
