The crude (R)-2-((tert-butoxycarbonyl)amino)-3-methoxy-3-methylbutanoic acid (4 g, 16.2 mmol) was converted to a mixed anhydride by adding Et3N (3.38 ml, 24.26 mmol), ethyl chloroformate (1.931 g, 17.79 mmol) in anhydrous THF (32.4 ml, 0.5 M) at −5° C. to 0° C. The mixture was stirred at −5° C. for 30 min. The resulting solid was filtered off and additional anhydrous THF was added to wash the precipitate. The resulting reaction solution was then added to a THF solution of 2,5-difluorobenzimido-hydrazide (2.77 g, 16.18 mmol) at −5° C. Then the reaction was gradually warmed to room temperature and stirred for overnight. Once the reaction was complete, the mixture was partitioned between EtOAc and H2O. The organic layer was separated and washed with H2O, brine, then dried over anhydrous Na2SO4, filtered, and evaporated under reduced pressure to give the crude product, which was purified on silica gel column (50% EtOAc in Hexanes) to afford (R)-tert-butyl (1-(2-((2,5-difluorophenyl)(imino)methyl)hydrazinyl)-3-methoxy-3-methyl-1-oxobutan-2-yl)carbamate (1 g, 15%). LC/MS (uplc): MH+ 401.2, 0.61 min.
