To obtain the subject new compound of β-lactam analog useful as an intermediate for producing antibiotics, etc., by reacting a penicillanic acid amidosulfoxide with a phosphorus compound and subsequently subjecting the obtained ring-opened compound to a ring-closing reaction in an aprotic organic solvent.

A new 3-imino-2-isopropylidene-7-oxo-4-thia-1- azabicyclo[3.2.0]heptane compound of formula I (R1 is H, a halogen; R2 is H, a halogen, phthalimide; R3 is H, an alkyl, benzyl, a heterocyclic group such as isoxazole group or pyrazole group). The compound of formula I is useful for producing other various β-lactam analogs or as an intermediate for producing β-lactam antibiotics, etc. The compound of formula I is obtained by reacting a penicillanic acid of formula II with a phosphorus compound to open the thiazoline ring, and subsequently subjecting the product to an intramolecular ring-closing reaction in an anhydrous aprotic organic solvent.