Piperidine-4-carboxylic acid (15.00 g, 116 mmol) was added to a solution of sodium hydrogen carbonate (12.7 g, 151 mmol) in water (200 mL) and the mixture was stirred for 10 min. A solution of 1-[[(9H-fluoren-9-ylmethoxy)carbonyl]oxy-2,5-pyrrolidinedione (Fmoc-Osu; 46.9 g, 139 mmol) in tetrahydrofuran (400 mL) was added. The solution was stirred at room temperature for 20 h and then acidified to pH 1 with 3 M HCl (500 mL). The mixture was extracted with ethyl acetate (200 mL then 100 mL) and the combined organic layers were washed with saturated brine (3×100 mL), dried (Na2SO4), filtered and concentrated to approximately 100 mL. Crystallization occurred on concentration. The mixture was allowed to stand for 2 h, then the solid was filtered off, washed with ethyl acetate and dried in a vacuum oven at 60° C. to give N-[(9H-fluoren-9-ylmethoxy)carbonyl]piperidine-4-carboxylic acid (34.51 g, 85%) as a white solid, mp 187-189° C.
