Synthesis
(1) 1.32 kg of dimethyl malonate and 3.7 kg of 1,1,2-tribromo-2-fluoroethane were added to a 50 L three-necked flask and dissolved in 15 L of anhydrous dimethylformamide. The flask was placed in a 25 °C water bath and 4.4 kg of anhydrous potassium carbonate was added in batches under stirring for 60 hours. After completion of the reaction, 30 L of ice water was added, extracted with ethyl acetate (8 L x 5), the organic phases were combined, washed to neutrality with saturated aqueous sodium chloride, dried over anhydrous sodium sulfate, and the solvent was evaporated. The residue was added to 3 L of ether and stirred in an ice bath for 4 h. It was filtered to yield 1.7 kg of solid A. (2) Solid A was added to a 10 L hydrogenation autoclave, dissolved with 6 L of methanol, and 72 g of palladium-carbon catalyst was added. Hydrogen displaced air 4 times and the reaction was stirred at 5 atm hydrogen pressure for 24 hours. Filter, concentrate the solution, add 10 L of ethyl acetate to the residue, wash with water (3 L x 2), wash with 2 L of saturated brine, dry with anhydrous sodium sulfate, and evaporate the solvent at 78°C to yield 898 g of oily liquid B. (3) Add oily liquid B to a 20 L reactor, add 5 L of dimethylformamide, 184 mL of distilled water, and 299 g of sodium chloride, and reflux the reaction for 35 hrs. The reaction solution was added to 12 L of distilled water, extracted with ethyl acetate (2.5 L x 6), the organic phases were combined, washed with water (2 L x 3), washed with 2 L of saturated brine, dried with anhydrous sodium sulfate, and dried at room temperature to yield 506 g of solid C. (4) Solid C was added to a 10 L reaction flask, dissolved with 4 L of tetrahydrofuran and 2 L of distilled water, and 270 g of lithium hydroxide monohydrate was added in batches with stirring in an ice bath. Keep for 3 hours. 4 mol/L hydrochloric acid was added dropwise to pH 2-3, extracted with ethyl acetate (1 L x 3), the organic phases were combined, washed with 1 L of water, 1 L of saturated brine, dried over anhydrous sodium sulfate, and concentrated. The residue was dissolved in 0.5 L of ethyl acetate, 4.5 L of petroleum ether was added slowly and stirred for 10 h at room temperature, filtered, and the filter cake was washed with petroleum ether and milled to give 380 g of solid D. (5) Solid D was added to a 5 L reaction flask, dissolved in 3 L of ethanol, and 0.5 L of ethanol solution of 425 g of L-leucinamide was added dropwise with stirring. It was heated to 50 °C and stirred for 3 h. It was cooled to room temperature and continued stirring for 3 h. It was filtered and the filter cake was washed with ethanol. The filter cake was dissolved in a mixture of 2.5 L of acetonitrile and 0.5 L of ethanol, heated to 50 °C and stirred for 2 h, cooled to room temperature and stirred for 2 h, filtered, and the filter cake was washed with acetonitrile, and milled to yield 364 g of solid E. (6) The solid E was dissolved in 3 L of distilled water, and 3 mol/L of hydrochloric acid was slowly added dropwise to pH 2-3 under stirring in an ice bath and stirring was continued for 1 hour. The reaction solution was extracted with dichloromethane (1 L x 3), and the organic phase was washed with 0.5 L saturated brine, dried over anhydrous sodium sulfate, and concentrated. The residue was dissolved in a mixture of 0.25 L of ethyl acetate and 2.25 L of petroleum ether, stirred at room temperature for 2 h. It was filtered to give 149 g of solid F. (7) Solid F, 393 mL of diphenylphosphoryl azide, and 315 mL of triethylamine were added to 3 L of the reaction flask, dissolved with 2 L of tert-butanol, and stirred at reflux for 12 hours. The reaction solution was concentrated, extracted by adding 3 L of ethyl acetate, and the organic phase was washed sequentially with 400 mL of saturated ammonium chloride solution, 400 mL of saturated sodium bicarbonate solution, 300 mL of saturated brine, dried over anhydrous sodium sulfate, filtered, and the solvent was evaporated. The residue was dissolved in 800 mL of a mixture of petroleum ether and ethyl acetate (10:1), stirred for 3 h. Filtered to give 167 g of solid G. (8) Dissolved solid G and 470 g of p-toluenesulfonic acid in 3 L of acetonitrile, stirred for 36 h at room temperature. Concentrate, add 600 mL of a mixed solution of ethyl ether and petroleum ether (1:1), stir for 3 h. Filter to obtain 204 g of sitafloxacin ternary ring intermediate (purity 99.2%).
