Synthesis
The general procedure for the synthesis of N-Boc-4,4-difluoro-L-proline from N-Boc-4,4-difluoro-L-proline methyl ester was as follows:
1. 1.08 g (4.08 mmol) of tert-butyl (S)-2-methyl-4-pyrrolidine-1,4-difluoropyrrolidine-2-carboxylate (purchased from Aldrich, Catalog No. 702463) was dissolved in a mixed solvent of 10 mL of methanol and 10 mL of tetrahydrofuran.
2. 6.12 mL of 2 M aqueous sodium hydroxide was added to the above solution.
3. The reaction mixture was stirred at room temperature for 1 hour.
4. Upon completion of the reaction, the organic solvent was removed under reduced pressure.
5. The remaining aqueous phase was diluted with an appropriate amount of water and then acidified by adding 6.12 mL of 2 M hydrochloric acid.
6. After acidification, the product was extracted with dichloromethane (3 times) and the organic layers were combined.
7. The organic layer was washed with brine, dried with anhydrous magnesium sulfate, filtered and the solvent was removed under reduced pressure.
8. 1.01 g (99% yield) of N-Boc-4,4-difluoro-L-proline was finally obtained as a white solid.LRMS (m/z): 250 (M-1)+.
References
[1] Patent: WO2012/146666, 2012, A1. Location in patent: Page/Page column 138-139
[2] Tetrahedron Letters, 1998, vol. 39, # 10, p. 1169 - 1172
[3] Patent: WO2014/19344, 2014, A1. Location in patent: Paragraph 00451
[4] Patent: WO2014/82380, 2014, A1. Location in patent: Paragraph 00427; 00430
[5] Patent: WO2014/82379, 2014, A1. Location in patent: Page/Page column 158; 159; 161
