Pures-(-)-9-fluoro-6,7-dihydro-8-(4-hydroxypiperidin-1-yl)-5-methyl-1-oxo-1h,-5h-benzo[i,j]quinolizine-2- carboxylic acid l-arginine salt tetrahydrate and a process for its preparation

Pure S-(-)-9-fluoro-6,7-dihydro-8-(4-hydroxypiperidin-1-yl)-5-methyl-1-oxo-1H,-5H-benzo[i,j]quinolizine-2-carboxylic acid L-arginine salt tetrahydrate and a process for its preparation is disclosed. The preparative process comprises (a) dissolving L-arginine in a mixture of acetone and water; (b) adding the compound of formula (I); (c) warming the solution obtained in step (b) to get a clear solution; (d) optionally treating the warm solution obtained in step (c) with activated carbon and filtering the treated solution; (e) diluting the warm solution obtained in step (d) with acetone and refluxing the diluted solution; (f) cooling the solution obtained in step (e) to obtain the solid arginine salt tetrahydrate; and (g) collecting the L-arginine salt tetrahydrate obtained in step (f) by filtration, and optionally washing it with acetone.