[C:1]([O:5][C:6]([N:8]1[CH2:12][CH:11]=[CH:10][C@H:9]1[C:13]([OH:15])=[O:14])=[O:7])([CH3:4])([CH3:3])[CH3:2].C(=O)([O-])[O-].[Cs+].[Cs+].[CH2:22](Br)[C:23]1[CH:28]=[CH:27][CH:26]=[CH:25][CH:24]=1>CN(C=O)C>[C:1]([O:5][C:6]([N:8]1[CH2:12][CH:11]=[CH:10][C@H:9]1[C:13]([O:15][CH2:22][C:23]1[CH:28]=[CH:27][CH:26]=[CH:25][CH:24]=1)=[O:14])=[O:7])([CH3:4])([CH3:2])[CH3:3]To a solution of (S)-1-(tert-butoxycarbonyl)-2,5-dihydro-1H-pyrrole-2-carboxylic acid (10 g, 46.9 mmol) in DMF (249 mL) cooled at 0° C. were added cesium carbonate (16.8 g, 51.6 mmol) then benzyl bromide (6.69 mL, 56.3 mmol) and the mixture was allowed to warm up to RT overnight. The reaction mixture was quenched with water (500 mL) and extracted with ethyl acetate (3×200 mL). The organic layers were combined and washed with brine, dried (Na2SO4), filtered and concentrated. The crude residue was purified by flash column chromatography on silica gel (c-hexane to c-hexane/EtOAc 4:1) to give the desired material. TLC, Rf (c-hexane/EtOAc 4:1)=0.25; MS (UPLC): 326 [M+Na]+, 204 [MH−Boc]+; tR (HPLC conditions f): 2.35 min.
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