2-(1H-benzotriazol-1-yl)-1,1,3,3-tetramethyluronium tetrafluoroborate (0.33 g) was added to 4-{(S)-1-[(S)-6-(2-hydroxy-2-methyl-propyl)-2-oxo-6-phenyl-[1,3]oxazinan-3-yl]-ethyl}-benzoic acid (0.37 g) and EtNiPr2 (0.41 mL) dissolved in N,N-dimethylformamide (5 mL). After stirring the solution at room temperature for 10 min, hydrazine hydrate (0.23 mL) was added. The solution was stirred at room temperature overnight and then diluted with water. The resulting mixture was extracted with ethyl acetate and the combined extracts were washed with brine and dried (MgSO4). The solvent was removed and the residue was purified by chromatography on silica gel (CH2Cl2/MeOH 98:2→90:10) to afford the title compound as a colorless foam-like solid.
