b) General procedure for the synthesis of 2-(4-(tert-butoxycarbonyl)piperazin-1-yl)acetic acid: to a solution of tert-butyl 4-(2-ethoxy-2-oxoethyl)piperazine-1-carboxylate (1.5 g, 5.51 mmol) in methanol (10 ml) was added an aqueous solution of sodium hydroxide (0.8 g, 22.0 mmol, 4 equiv). The reaction mixture was stirred at room temperature for 2 hours. Subsequently, the mixture was concentrated and extracted according to the method of Intermediate Example 5 (c). The solvent was removed by distillation to give the target product in 76% (1 g) yield. The product was characterized by LC-MS (ESI): calculated mass: 244.29; observed mass: 145.1 [M-Boc + H]+ (retention time: 0.102 min).
