PURPOSE: To obtain the subject new compound, having excellent pharmacological action such as anti-inflammatory, analgesic, blood platelet agglutination inhibitory actions, etc., and useful as analgesic and antiphlogistic agents, agents for circulatory organs, etc., and useful as an intermediate for antibiotic substances such as cabapenem.

CONSTITUTION: A compound expressed by formula I (R1 and R2 are H, alkyl, etc.; R3 is H, aryl, etc.) or its salt, e.g. 4-difluoromethyl-1-(4-methoxyphenyl)-2- azetidinone. The above-mentioned compound is obtained by reacting a difluoroethaneimine compound expressed by formula II with a compound expressed by formula III (R1' and R2' are H, alkyl, etc., having protected OH; R4 is alkyl) and then eliminating the protecting groups of OH groups from the product. The compound expressed by formula II is preferably reacted with a compound expressed by formula III in a solvent such as THF at -78 to 0°C for 1-10hr while being stirred. Furthermore, the compound expressed by formula II is preferably obtained by reacting a difluoroacetaldehyde alkylhemiacetal with a primary amine expressed by the formula R3-NH2.