To a solution of tert-butyl 4-oxo-1-phenyl-1,3,8-triazaspiro[4.5]decane-8-carboxylate (0.15 g, 0.45 mmol) and potassium carbonate (0.124 g, 0.9 mmol) in N,N-dimethylformamide (4 mL), was added ethyl-4-bromobutyrate (0.072 mL, 0.5 mmol, d=1.35). After stirring at 60° C. for 60 hours, the reaction mixture was diluted with ethyl acetate (25 mL), washed with dilute citric acid, water and brine. The organic phase was dried over MgSO4, filtered and concentrated in vacuo to obtain tert-butyl 3-(4-ethoxy-4-oxobutyl)-4-oxo-1-phenyl-1,3,8-triazaspiro[4.5]decane-8-carboxylate.
