Synthesis of compound 19: 1,4-Butanediamine (1 g, 11.36 mmol, 5 eq.) was dissolved in dichloromethane (15 ml). To this solution was slowly added di-tert-butyl dicarbonate (Boc2O, 0.619 g, 2.83 mmol, 1 eq.) dissolved in dichloromethane (15 ml). The reaction mixture was stirred at room temperature for 16 hours. The reaction progression was monitored by thin layer chromatography (TLC, unfolding agent ratio CH2Cl2:MeOH:33% NH4OH aqueous solution = 8:2:0.2). Upon completion of the reaction, the reaction mixture was filtered to remove insoluble impurities and the filtrate was concentrated by rotary evaporator. The resulting oily residue was dissolved with ethyl acetate (15 ml) and washed with saturated sodium chloride solution (3 x 10 ml) to remove unreacted 1,4-butanediamine. The organic phase was dried over anhydrous sodium sulfate and concentrated under reduced pressure to give 0.520 g of yellow oily product in 97.65% yield. The structure of the product was confirmed by 1H NMR (400 MHz, CDCl3): δ 1.38 (s, 9H), 1.52-1.59 (m, 4H), 2.34 (brs, 2H, D2O exchange), 2.72-2.79 (m, 2H), 3.19-3.24 (m, 2H), 5.95 (brs, 1H, D2O exchange).
