Cs2CO3 (8.42 g, 25.8 mmol) was added to a solution of 1-t-butyl 2-methyl (2S,4S)-4-{[(4-bromophenyl)sulfonyl]oxy}pyrrolidine-1,2-dicarboxylate (4 g, 8.61 mmol) and the product from Step 1, (3.49 g, 12.92 mmol) in NMP (86 ml) under N2. The mixture was then heated to 60° C. for 6.5 hours, cooled to RT and partitioned between water and EtOAc. The organic layer was washed with water and brine, dried over MgSO4, and the solvent removed in vacuo. The crude product (6.5 g) was purified on silica (gradient elution, 0-100% EtOAc/hexane, then 0-5% MeOH/DCM) to yield the title compound (2.26 g). LRMS ESI+ ((M-Boc)+H)+ 397.3/399.3.
