To a solution of methyl 3-(1-benzyloxycarbonylpiperidin-4-yl)acrylate (5.55 g) and 4-(tert-butoxycarbonyl)-2-piperazinone (3.66 g) in tetrahydrofuran (50 ml) was added potassium tert-butoxide (0.56 g), and the mixture was stirred under nitrogen atmosphere at room temperature for 3 days. The reaction solution was diluted with ethyl acetate (50 ml), washed with 0.5N hydrochloric acid and saturated brine, dried and concentrated, and the residue was purified with silica gel column chromatography (eluent:acetone/hexane=1/2) to give colorless amorphous of methyl 3-(1-benzyloxycarbonylpiperidin-4-yl)-3-[4-(tert-butoxycarbonyl)-2-oxopiperazine-1-yl]propionate (5.51 g).
