The crude 1,1-dimethylethyl (3S)-3-[(3-chloro-4-cyanophenyl)amino]-4-{ethyl[(2-nitrophenyl)sulfonyl]amino}butanoate from the previous step (16.2 mmol), phenyl hydrosulfide (1.0 equiv., 1.78 g, 16.2 mmol), and K2CO3 (325 mesh, 3 equiv., 6.71 g, 48.6 mmol) in DMF (60 mL) were stirred at room temperature for 90 min. The reaction mixture was diluted with H2O (200 mL) and extracted with Et2O (3×). The organic extracts were washed with H2O, and 10% aq. Na2CO3, dried, and concentrated to give the titled product which was contaminated with the 2-nitrophenyl phenyl sulfide by-product. This contaminated product was used directly in the next step.
