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Powdery potassium hydroxide (36 mg, 0.64 mmole) and tetrabutylammonium bromide (21 mg) were suspended in tetrahydrofuran (3 ml). To the suspension was slowly added dropwise a solution of 4-methylthio-2-azetidinone (72.5 mg, 0.62 mmole) and p-nitrobenzyl iodoacetate (493 mg, 1.25 mmoles) in tetrahydrofuran (1 ml) at -20° C., with stirring. The temperature of the mixture was elevated to room temperature step by step and the mixture was stirred at room temperature for 1.5 hours. After completion of the reaction, the mixture was diluted with ethyl acetate and subjected to a natural filtration. The filtrate was concentrated under reduced pressure and the resulting residue was purified by preparative thin layer chromatography (eluent: a 1:1 by volume mixture of ethyl acetate and benzene), to give 49 mg of the desired compound as crystals.

1. customwas elevated to room temperature
2. stirringthe mixture was stirred at room temperature for 1.5 hours
3. customAfter completion of the reaction
4. filtrationsubjected to a natural filtration
5. concentrationThe filtrate was concentrated under reduced pressure
6. customthe resulting residue was purified by preparative thin layer chromatography (eluent
7. additiona 1:1 by volume mixture of ethyl acetate and benzene),