To obtain the subject new compound useful as a stable intermediate for chemically synthesizing physiologically active β-lactam compounds.

A compound of formula I [R1, R3 and R4 are each H or a halogen; R2 is H, a halogen or phthalimide; R5 is H, a lower alkyl, an aryl or the like; (n) is 0-2], for example, (3S,4R)-1-[(4S)-2-benzyl-5,5-dimethyl-3-oxo- isothiazolidin-4-yl]-3-bromo-4-chloro-azetidin-2-one. The compound of formula I is obtained by reacting a penicillanic acid sulfoxideamide with a halogenating agent (for example, thionyl chloride) inn an anhydrous aprotic organic solvent (for example, benzene) at -50 to 150°C to open the thiazoline ring of the penicillanic acid sulfoxideamide, subjecting the reaction product to an intramolecular ring-closing reaction to form an isothiazolidinone ring and simultaneously halogenate the azetidine ring and, if necessary, subsequently oxidizing the reaction product.