Acetonitrile → Gatifloxacin

An acetonitrile solution (30 mL) containing 1-cyclopropyl-6,7-difluoro-1,4-dihydro-8-methoxy-4-oxo-3-quinolinecarboxylic acid (3 g), 2-methylpiperazine dihydrochloride (1.93 g), triethylamine (1.43 mL), and boron trifluoride-tetrahydrofuran complex (2.84 g) was stirred at room temperature for one hour. Triethylamine (5.71 mL) was further added to the reaction mixture, followed by stirring at room temperature for 24 hours. The solvent was evaporated under reduced pressure, and methanol (30 mL) was added to the residue. The mixture was refluxed for 5 hours, and the solvent was evaporated under reduced pressure. 80% Hydrated methanol (30 mL) was added to the residue, and the pH of the product was adjusted to 7 by use of 5N aqueous sodium hydroxide solution, followed by stirring at room temperature for 16 hours. The thus-produced crystals were recovered through filtration and dried, to thereby yield 3.55 g of the title compound (yield: 91%).