The general procedure for the synthesis of 1-Boc-4-amino piperidine-4-carboxamide from tert-butyl 4-benzyloxycarbonylamino-4-carboxamide piperidine-1-carboxylate was as follows: firstly, 4-(benzyloxycarbonylamino)-1-(tert-butoxycarbonyl)piperidine-4-carboxylic acid (1A, 2.0 g, 1.0 eq.) was solubilized in DME (0.5 M), NMM (1.0 eq.) and IBCF (1.0 eq.) and reacted at -15 °C for 10 min. Subsequently, ammonia (1.5 eq.) was added and the reaction mixture was stirred at room temperature for 1.5 hours. After the completion of the reaction was confirmed by LCMS analysis, the mixture was partitioned between ethyl acetate and water. The organic phase was washed with brine, dried over Na2SO4, filtered and the solvent was removed under reduced pressure. The residue was purified by crystallization from ether to afford tert-butyl 4-(benzyloxycarbonylamino)-4-carbamoylpiperidine-1-carboxylate (1B, 70% yield). Next, 1B (1.0 equiv.) and a catalytic amount of acetic acid were dissolved in MeOH (0.05 M) and hydrogenated at 50 °C through an H-cube hydrogenator equipped with a Pd/C column. After completion of the reaction, methanol was removed under reduced pressure and the product was washed twice with cold ether to afford tert-butyl 4-amino-4-carbamoyl piperidine-1-carboxylate (1C, 90% yield). Then, 1C (0.3 M) was mixed with 2-chloro-1,1,1-trimethoxyethane (4.0 eq.) and acetic acid (2.0 eq.) and stirred at 118 °C for 12 hours. After removal of the solvent under reduced pressure, the residue was purified by crystallization from cold ether to afford tert-butyl 2-(chloromethyl)-4-oxo-1,3,8-triazaspiro[4.5]dec-1-ene-8-carboxylate (1D, 67% yield). 1D was dissolved in a 2.0 M solution of ammonia in 2-propanol (0.04 M), an excess of ammonia was drummed in and heated at 60 °C for 12 hours. After confirming the completion of the reaction by LCMS analysis, the solvent was removed to afford tert-butyl 2-(aminomethyl)-4-oxo-1,3,8-triazaspiro[4.5]dec-1-ene-8-carboxylate (1E), which could be used without further purification (ESI-MS: m/z 283.1 [M + H]+). Finally, 1E (1.0 eq.) was dissolved in CH2Cl2, and CH2Cl2/TFA solution (2:1, final concentration 0.15 M) was slowly added at 0 °C. The reaction was confirmed to be complete within 30 min at 0 °C by LCMS analysis. Volatiles were removed under reduced pressure to afford the TFA salt of 2-(aminomethyl)-1,3,8-triazaspiro[4.5]dec-1-en-4-one (1G), which was used without further purification (ESI-MS: m/z 183.1 [M + H]+).
