Method for preparing ciprofloxacin by piperazine reaction

The invention relates to a method for preparing ciprofloxacin by piperazine reaction. The technical problems to be solved are as follows: (1) reducing amount of a solvent or using no solvent, (2) reducing consumption of the piperazine, and (3) shortening the reaction time, simplifying operation, and enhancing overall yield. The method is characterized by (a) mixing the piperazine as shown in formula (iii) with the cyclopropane carboxylic acid as shown formula (ii) at the mol ratio of 1-10:1, adding metal Lewis acid catalyst with the chemical equivalent of 1%-30% for thorough reaction in an aqueous system at the temperature of 80-150 DEG C to obtain reaction solution, wherein, the metal Lewis acid catalyst is selected from one or more of aluminum (III) Lewis acid and iron (III) Lewis acids; (b) adding inorganic base to the reaction solution at the temperature of 20 DEG C-100 DEG C to regulate pH value of the reaction system to 9-12, and filtering while the system is hot; and (c) regulating the pH value of the filtrate to 7-8 with the inorganic base for recrystallization to obtain the product.