General procedure for the synthesis of 1,4-bis-BOC-2-piperazinecarboxylic acid from 2-piperazinecarboxylic acid and di-tert-butyl dicarbonate: In a 1 L round bottom flask, 15.76 g NaOH (394 mmol) was dissolved in 390 mL of water, followed by the addition of 20 g of 2-piperazinecarboxylic acid (98 mmol) in batches to obtain a colorless solution. A solution of 50 mL of Boc2O (215 mmol) dissolved in 1,4-dioxane was added slowly and dropwise while the temperature was maintained below +5 °C. Upon completion of the reaction, the mixture was cooled to +5 °C and the pH was subsequently adjusted to 3 with 3N HCl. The suspension was extracted with 300 mL of ethyl acetate (AcOEt), the organic phases were combined and dried over sodium sulfate. After vacuum evaporation, 22.7 g of white solid product was obtained. The aqueous phase was acidified with 3N HCl to pH=3 and then extracted with 2×100mL ethyl acetate (AcOEt), and the organic phases were similarly dried with sodium sulfate and then vacuum evaporated to give a second batch of 7.9g of product. The two batches of product were combined to give the final 1,4-bis-BOC-2-piperazinecarboxylic acid (30.6 g, 94% yield). The product was confirmed by 1H-NMR (400 MHz, DMSO-d6): δ 1.33 (18H, m), 2.70-4.50 (8H, m), 12.9 (1H, s); the mass spectrum (ES) showed m/z 330 [M+H]+.
