Under ice-cooling, 1M sodium hydroxide aqueous solution (6.0 ml) and benzyl chloroformate (1.03 g) were sequentially added to a dioxane (50 ml) suspension of tert-butyl 4-cyano-4-({[(4-methylphenyl)sulfonyl]oxy}methyl)piperidine-1-carboxylate (2.15 g), and stirring was performed at room temperature for 16 hours. The reaction liquid was concentrated under reduced pressure, and extracted with EtOAc. The organic layer was concentrated under reduced pressure and the residue was purified by silica gel column chromatography (EtOAc/hexane) to obtain 2-benzyl 7-tert-butyl 2,7-diazaspiro[3.5]nonane-2,7-dicarboxylate (1.68 g).
