General procedure for the synthesis of (R)-1-benzyl-N-Boc-proline from di-tert-butyl dicarbonate and (R)-2-benzylpyrrolidine-2-carboxylic acid: a mixture of (R)-2-benzylpyrrolidine-2-carboxylic acid (2.07 g, 10.1 mmol) and tetramethylammonium hydroxide pentahydrate (1.83 g, 10.1 mmol) in acetonitrile (100 mL) was was stirred under nitrogen protection for 90 minutes. Subsequently, di-tert-butyl dicarbonate (Boc2O, 3.31 g, 15.2 mmol) was added. After 48 hours of reaction, a second portion of di-tert-butyl dicarbonate (1.10 g, 5.0 mmol) was added. After continuing the reaction for 24 h, the reaction mixture was concentrated under reduced pressure. The concentrate was partitioned between ether (100 mL) and water (50 mL). The aqueous phase was washed with ether (50 mL) and then acidified with 10% aqueous citric acid (20 mL) to pH 4. The acidified aqueous solution was extracted with ethyl acetate (EtOAc), and the organic phases were combined, washed with brine (30 mL), dried over anhydrous magnesium sulfate (MgSO4), filtered, and concentrated under reduced pressure to afford 1-tert-butyl (R)-2-(phenylmethyl)pyrrolidine-1,2-dicarboxylate ( 1.26 g, 4.13 mmol, 41% yield) as a foamy solid.
