A mixture of 8-chloro-1-cyclopropyl-6,7-difluoro-1,4-dihydro-4-oxo-3-quinolinecarboxylic acid (2.2 g), anhydrous acetonitrile (22 ml), trans-3-t-butoxycarbonylamino-4-methylpyrrolidine (reference example 5; 2.4 g) and DBU (1.22 g) was refluxed for 5 hours. The reacting mixture was concentrated under reduced pressure, the resulting residue was dissolved in chloroform and washed with 10% aqueous citric acid solution. The chloroform layer was further washed with saturated aqueous sodium chloride solution, dried over anhydrous sodium sulfate and concentrated under reduced pressure. The resulting residue was dissolved in hot methanol (20 ml) and, after cooling, the resulting precipitate was collected by filtration to give 7-(trans-3-t-butoxycarbonylamino-4-methyl-1-pyrrolidinyl)-8-chloro-1-cyclopropyl-6-fluoro-1,4-dihydro-4-oxo-3-quinolinecarboxylic acid (3.62 g) as yellow needle, mp 210°-213° C. (decompd.).
