To 1-(methylsulfonyl)piperazine hydrochloride (150 g, 632 mmol) in DCE (3.5 L) was added TEA (192 g, 1.90 mol). The mixture was stirred at rt for 1 h and then acetic acid (94.8 g, 1.58 mol) and 1,1-dimethylethyl 4-oxo-1-piperidinecarboxylate (251 g, 1.26 mol) was added. After stirring another h, the reaction was cooled with an ice water bath and NaBH(OAc)3 (294 g, 1.39 mol) was added in four portions. The mixture was stirred overnight at rt. The reaction mixture was neutralized with saturated Na2CO3 to pH 8-9. The organic phase was washed with brine and H2O, dried (Na2SO4), and rotovapped down to provide the crude Boc-protected amine (A combination of 3 batches, 720 g). This amount was split into 2 batches and used without further purification. To 1,1-dimethylethyl 4-[4-(methylsulfonyl)-1-piperazinyl]-1-piperidinecarboxylate (360 g, 1.04 mol) in MeOH (1 L) was added HCl (6 M in MeOH, 2 L). The mixture was stirred at rt for 30 min. About 1 L of MeOH was rotovapped off. The resultant precipitate was filtered, washed with MeOH, and dried on high vacuum to provide the title compound of Step C (A combination of 2 batches, 600 g, 1.87 mol, 89% over 2 steps). 1H NMR (400 MHz, D2O) δ 1.87-1.91 (m, 2H), 2.33-2.36 (m, 2H), 2.97 (s, 3H), 2.99-3.05 (m, 2H), 3.45-3.59 (m, 11H).
