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To a solution of 24.8 g (0.15 mol) of L-Proline methyl ester hydrochloride in 500 mL of CH2Cl2 was added 70 mL (0.5 mol) of triethylamine with stirring to give copious white precipitate. The mixture was filtered, and the filtrate cooled to 0° C. (ice bath) with stirring. To the cooled solution was added a solution of 36.8 g (0.15 mol) of 3,5-dichlorobenzenesulfonyl chloride in 70 mL of CH2Cl2 dropwise quickly over five minutes. The addition funnel was rinsed with an additional 30 mL of CH2Cl2, and the cloudy yellow mixture was allowed to warm to room temperature with stirring overnight. The mixture was washed 2× with 400 mL of 1N HCl, 2× with 400 mL of 1N NaOH, then brine, then dried (MgSO4), filtered, and concentrated to a yellow oil which crystallized on standing. The material was recrystallized three times from ethyl acetate/hexanes to give 39.3 g (0.116 mol, 77%) of N-(3,5-dichlorobenzenesulfonyl)-Proline methyl ester (MW=338) as white needles (TLC on silica vs. 2:1 hexanes/ethyl acetate, Rf=0.51). M/z=339.3 (M+H+).

1. customto give copious white precipitate
2. filtrationThe mixture was filtered
3. stirringwith stirring
4. washThe addition funnel was rinsed with an additional 30 mL of CH2Cl2
5. temperatureto warm to room temperature
6. stirringwith stirring overnight
7. washThe mixture was washed 2× with 400 mL of 1N HCl, 2× with 400 mL of 1N NaOH
8. dry with materialbrine, then dried (MgSO4)
9. filtrationfiltered
10. concentrationconcentrated to a yellow oil which
11. customcrystallized
12. customThe material was recrystallized three times from ethyl acetate/hexanes