To a 100 ml round bottom flask was added L-proline 1a (3.0 g, 26.06 mmol), sodium hydroxide (1.2 g, 28.66 mmol) and tert-butanol/water (20 ml/20 ml), respectively. Di-tert-butyl dicarbonate (6.3 g, 28.66 mmol) was slowly added to the reaction system under stirring at 0C cooling and the temperature was then slowly raised to room temperature. After the end of the reaction was detected by TLC, the tert-butanol was removed by spinning under reduced pressure to obtain the crude product in the aqueous phase, which was washed three times with petroleum ether (3x
30 ml), after which it was adjusted to pH = 6 with 1N hydrochloric acid and extracted with ethyl acetate (3x
50 ml). The combined ethyl acetate extracts were dried over anhydrous sodium sulfate and the solvent was removed by spinning under reduced pressure to give a white solid pyrrolidine-1,2-dicarboxylic acid 1-tert-butyl
ester (5.49 g, 98% yield).
