N-Benzyloxycarbonyl-(4R)-hydroxy-L-proline (150 g, 0.565 mol) was used as a raw material, which was dissolved in tetrahydrofuran (1.5 L), and the borane-dimethyl sulfide complex (59.0 ml, 0.622 mol) was added dropwise slowly under stirring at 0 °C. After the dropwise addition, the reaction mixture was heated to reflux and stirred continuously for 3 hours. Subsequently, the reaction mixture was cooled to 0 °C and borane-dimethyl sulfide complex (16.1 ml, 0.170 mol) was added again and stirring was continued for 10 hours. After completion of the reaction, the reaction mixture was cooled and water (500 ml) was added slowly at 0 °C to quench the excess borane-dimethyl sulfide complex. The reaction mixture was extracted with a solvent mixture of ethyl acetate and chloroform-methanol (10:1, v/v), the organic phases were combined, washed with saturated sodium chloride solution and dried over anhydrous sodium sulfate. The solvent was removed by concentration under reduced pressure to afford Cbz-(2S,4R)-4-hydroxyprolinol (122.06 g, 86% yield), the product was a pale yellow oil, which could be used for subsequent reactions without further purification.1H-NMR (CDCl3) δ: 1.52-1.81 (m, 3H), 2.06 (m, 1H), 3.40-3.85 and 4.04- 4.61 (m series, total 6H), 5.15 (s, 2H, ArCH2), 7.20-7.44 (m, 5H, Ar).
