Cis-4-fluoroproline hydrochloride (2) A solution of ester 27 (0.101 g, 0.410 mmol) in 2M aq. HCl (2.0 mL) was refluxed overnight. The solution was cooled, diluted with ethanol, decolourised with activated charcoal, and filtered through Celite ® . The filtrate was concentrated in vacuo to give the title compound as a white solid (59.9 mg, 86%); m.p. 154-156 ºC; 1 H NMR (300 MHz, D 2 O) δ 5.51 (ddd, J = 2.8, 5.6, 51.8 Hz, 1H, FCH), 4.67-4.62 (m, 1H, C(O)CH), 3.90-3.79 (ddd, J = 2.1, 13.9, 19.0

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... 1H, NCHH), 3.67-3.49 (ddd, J = 3.4, 13.9, 37.1 Hz, 1H, NCHH), 2.76-2.55 (m, 2H, CF-CH 2 -CH); 19 F NMR (282 MHz, CDCl 3 ) δ -175.0 (m, 1F); 1 H NMR data in accordance with literature values. [1] Trans-4-fluoroproline hydrochloride (3) A solution of ester 29 (0.198 g, 0.802 mmol) in 2M HCl (3.5 mL) was refluxed overnight, then cooled, diluted with ethanol, decolourised with activated charcoal, and filtered through Celite ® . The filtrate was then concentrated in vacuo to give the title compound as a white solid (0.152 g, >99%); m.p. 104-106 o C; 1 H NMR (300 MHz, MeOD) δ 5.42 (d, J = 51.6 Hz, 1H, FCH), 4.63 (m, 1H, C(O)CH), 3.84-3.49 (m, 2H, NCH 2 ), 2.79-2.60 (m, 2H, CF-CH 2 -CH); 19 F NMR (282 MHz, CDCl 3 ) δ -175.8 (m, 1F); 1 H NMR data in accordance with literature values. [1]