To a mixture of dimethylformamide (4.2 ml) and tetrahydrofuran (10 ml) was added phosphorus oxychloride (4.2 ml) at 5° C. After stirring at 5° C. for 30 minutes, to the mixture were added successively tetrahydrofuran (60 ml) and N-allyloxycarbonyl-N-methylglycine (8.7 g). The mixture was stirred for 30 minutes at 5° C. (solution A). To a solution of (2S)-1-allyloxycarbonyl-2-(2-methyl-3-oxo-1-butenyl)pyrrolidine (10.0 g) in tetrahydrofuran (160 ml) were added successively triphenylphosphine (5.5 g), 5,5-dimethyl-1,3-cyclohexanedione (dimedone) (5.8 g), acetic acid (4.8 ml) and tetrakis(triphenylphosphine)palladium(0) (2.4 g). After stirring at ambient temperature for 30 minutes, the mixture was taken up into a mixture of ethyl acetate (100 ml) and water (100 ml). After adjusting to around pH 2 with 1N hydrochloric acid, the aqueous layer was separated, washed with ethyl acetate and tetrahydrofuran (200 ml) was added thereto. After adjusting to around pH 8.5 with 20% aqueous sodium hydroxide, to the mixture was added dropwise the solution A. After stirring at 5° C. for 1 hour, the mixture was extracted with ethyl acetate. The organic layer was washed with water and dried over magnesium sulfate. Removal of the solvent gave a residue, which was chromatographed on silica gel (300 ml) eluting with a mixture of n-hexane and ethyl acetate (5.5-3.7 V/V) to give (2S)-1-(N-allyloxycarbonyl-N-methylaminoacetyl)-2-(2-methyl-3-oxo-1-butenyl)pyrrolidine (3.4 g).
