[F:1][C:2]1[CH:7]=[C:6]([F:8])[CH:5]=[CH:4][C:3]=1[C@@H:9]1[CH2:13][NH:12][CH2:11][C@H:10]1[C:14]([O:16][CH3:17])=[O:15].C=O.[C:20](O)(=O)C.C(O[BH-](OC(=O)C)OC(=O)C)(=O)C.[Na+]>ClCCl>[F:1][C:2]1[CH:7]=[C:6]([F:8])[CH:5]=[CH:4][C:3]=1[C@@H:9]1[CH2:13][N:12]([CH3:20])[CH2:11][C@H:10]1[C:14]([O:16][CH3:17])=[O:15]To a solution of methyl (3S,4R)-4-(2,4-difluorophenyl)pyrrolidine-3-carboxylate, from preparation 31 (500 mg, 2.07 mmol) and formaldehyde (155 μL, 2.07 mmol) in dichloromethane (20 mL) was added acetic acid (188 μL, 2.07 mmol) followed by sodium triacetoxyborohydride (659 mg, 3.11 mmol) at room temperature. The reaction mixture was stirred for 2 hours, diluted with dichloromethane (10 mL) and partitioned with saturated sodium hydrogen carbonate solution (40 mL). The phases were separated and the organic phase was washed with brine (20 mL), dried over magnesium sulfate, filtered and concentrated in vacuo to give the desired product as a colourless oil, 288 mg (54%).
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