To a 0° C. solution of triphosgene (959 mg, 3.24 mmol, 0.35 equiv) in THF (45 mL) was slowly added via cannula a solution of tert-butyl 4-(3-amino-2-fluorobenzyl)piperazine-1-carboxylate (2.86 g, 9.25 mmol, 1.0 equiv) and DIPEA (3.2 mL, 18.5 mmol, 2.0 equiv) in THF (25 mL) over approximately 20 minutes. The solution was stirred for an additional 15 min at 0° C., and 4-amino-2-methylpyridine was then added. After stirring at room temperature for 72 h, the reaction was diluted with EtOAc, washed with aq. satd. NaHCO3 and brine, dried over sodium sulfate, filtered and concentrated in vacuo. Purification by reverse phase preparative HPLC provided tert-butyl 4-(2-fluoro-3-(3-(2-methylpyridin-4-yl)ureido)benzyl)piperazine-1-carboxylate (1.77 g, 43%). LCMS m/z (APCI)=444.3 (M+H).
