A mixture of 1-cyclopropyl-6,7-difluoro-1,4-dihydro-8-methoxy-4-oxo-3-quinolinecarboxylic acid (0.40 g), trans-3-t-butoxycarbonylamino-4-methylpyrrolidine (0.41 g), DBU (0.21 g) and anhydrous acetonitrile (5 ml) was refluxed for 2.5 hours and then the reaction mixture was concentrated under reduced pressure. The residue was dissolved in chloroform (40 ml) and washed with 10% aqueous citric acid solution (20 ml) and with saturated saline (20 ml) successively. The organic layer was dried over anhydrous sodium sulfate and then concentrated under reduced pressure. The residue was crystallized from ethanol to give 7-(trans-3-t-butoxycarbonylamino-4-methyl-1-pyrrolidinyl)-1-cyclopropyl-6-fluoro-1,4-dihydro-8-methoxy-4-oxo-3-quinolinecarboxylic acid. To a suspension of these crystals in methanol (5 ml) was added concentrated hydrochloric acid (5 ml) dropwise. After stirring for 1.5 hours at room temperature, the reaction mixture was neutralized with concentrated aqueous ammonia, the resulting crystals were collected by filtration and washed with water sufficiently to give the title compound (0.29 g) as colorless powder, mp 214°-215° C.
