A mixture of 1 g (3.2 mmoles) of ethyl 1-cyclopropyl-6,7,8-trifluoro-4-oxo-1,4-dihydro-quinoline-3-carboxylate, 0.98 g (4.8 mmoles) of 3-trifluoroacetamidoazetidine hydrochloride and 2 ml of triethylamine is heated in 15 ml of dimethyl sulphoxide for 4 hours at 80°-5° C. It is diluted with water and extracted with chloroform. The organic phase is washed with water and evaporated yielding 0.31 g (22% of ethyl 1-cyclopropyl-6,8-difluoro-1,4-dihydro-7-(3-trifluoroacetamido-1-azetidinyl)-4-oxo-3-quinolinecarboxylate which is then hydrolysed by heating a mixture of 0.1 g (0.22 mmoles) of this ester with 5 ml of 1N sodium hydroxide and 5 ml of ethanol. The mixture is cooled, evaporated and acidified with acetic acid. Filtering and washing with water yields 70 mg (96%) of 1-cyclopropyl-6,8-difluoro-7-(3-amino-1-azetidinyl)-1,4-dihydro-4-oxo-3-quinolinecarboxylic acid, melting at 214°-6° C.
