Synthesis of the Intermediate of Gemifloxacin by the Chemoselective Hydrogenation of 4-Cyano-3-methoxyimino-1-(N -tert-butoxycarbonyl)pyrrolidine. Part 2. The Palladium Catalysts in Acidic Media

Chemoselective hydrogenation of 4-cyano-3-methoxyimino-1-(N-tert-butoxycarbonyl)pyrrolidine (CMBP) to 4-aminomethyl-3-Z-methoxyiminopyrrolidine methanesulfonate (AMPM), the key intermediate for gemifloxacin, was investigated over Pd catalysts with in situ acid protection. Addition of more than 1.6 equiv of acidic protons for CMBP was found to drastically elevate both the reaction rate and selectivity to 4-aminomethyl-3-Z-methoxyimino-1-(N-tert-butoxycarbonyl)pyrrolidine (Z-AMBP) over Pd catalyst with a complete suppression of the major side reaction to 4-cyano-3-amino-1-(N-tert-butoxycarbonyl)-3,4-pyrroline (CABP). Methanol as the organic solvent was found to increase the hydrogenation rate greatly compared to other solvents with a negligible decrease of selectivity. The leaching of Pd by acid and consequent accumulation of Pd ion in the reaction mixture was negligible in CMBP hydrogenation. The novel process of chemoselective CMBP hydrogenation in acidic media over Pd catalyst was thus much simpler yet more efficient compared to the conventional one. The whole AMPM process time starting from 1-(N-tert-butoxycarbonyl)-4-cyanopyrrolidine-3-one (BCPO) could be reduced by at least approximately 15 h which would result in a great reduction of materials such as catalysts, (t-Boc)2O, and solvent. Additionally, reduction of reaction steps improved the overall yield of AMPM significantly. Employment of methanesulfonic acid as an acidic agent in the hydrogenation step allowed an environmentally benign pathway to AMPM by omission of a neutralization step with an extra reduction in process time and materials consumed.