C([BH-](CC)CC)C.[Li+].C1COCC1.[CH3:14][CH2:15][O:16][C:17]([C@@H:19]1[CH2:23][CH2:22][C:21](=O)[N:20]1[C:25]([O:27][C:28]([CH3:31])([CH3:30])[CH3:29])=[O:26])=[O:18].CCN(C(C)C)C(C)C.FC(F)(F)C(OC(=O)C(F)(F)F)=O>C1(C)C=CC=CC=1.CN(C1C=CN=CC=1)C.O>[CH3:14][CH2:15][O:16][C:17]([C@@H:19]1[CH2:23][CH:22]=[CH:21][N:20]1[C:25]([O:27][C:28]([CH3:29])([CH3:31])[CH3:30])=[O:26])=[O:18]At −50° C. a solution of lithium triethylborohydride in THF (1.0 M, 107 mL, 1.07 eq) is added dropwise to a solution of (S)-5-oxo-pyrrolidine-1,2-dicarboxylic acid 1-tert-butyl ester 2-ethyl ester (100 mmol, 1.0 eq) in toluene (170 mL). After 45 min DIPEA (450 mmol, 4.5 eq) and powdered DMAP (1.31 mmol) are added slowly at −50° C. The mixture is treated with trifluoroacetic anhydride (120 mmol, 1.20 eq) while keeping the internal temperature below −45° C., allowed to reach RT within 90 min and stirred for additional 17 h. At 0° C. water (250 mL) is added slowly and the layers are separated. The organic layer is washed with ice-cold water (3×250 mL) and concentrated in vacuo to give the desired product as an orange liquid. LC-MS (acidic): tR=0.93 min; [M+H]+=242.2.
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