Enzymatic Resolution And Its Application In The Preparation Of The Key Chiral Intermediate Of Moxifloxacin

As a good antibacterial agent of fourth generation quinolones, moxifloxacin is used widely in clinical treatment with marked effectiveness, which results in great and enormous market potential. Currently, chemical method is mainly used for the resolution and synthesis of the chiral intermediate of moxifloxcain. However, there are low optical purity of the product, the harsh reaction conditions, resolving agent of difficult recovery and other issues in the conventional chemical resolution. More and more attention has been paid to enzymatic biocatalysis for its good substrate specificity in resolution reaction. Especially, immobilized enzyme resolution techniques possesses many advantages, such as mild reaction conditions, high optical purity of product, easy product separation and enzyme of easy recycling, which are widely used in the resolution of chiral compounds. In aaditon, microwave technology used in enzymatic biocatalysis to accelerate reaction would promote the enzyamtic resolution. In this paper, in order to overcome the problem in conventional chemical resolution, the chiral intermediate of moxifloxacin was preparaed and synthesized using the immobilized enzymes under microwave irradiation.In our work, lipase was used as biocatalyst to catalyze the enzymatic kinetic resolution of sterically hindered racemic cyclic secondary amine, cis-6-benzyltetrahydro-1H-pyrrolo[3,4-b]pyridine-5,7(6H,7aH)-dione, in organic solvents. First, allyl phenyl carbonate was used as donors and toluene as solvent for the selection of enzyme source. We found lipase from Candida antarctica B can accept larger hindered cyclic secondary amine substrates and has good selectivity(E> 200), but the catalytic activity is low. Moreover, solvent, reaction temperature, the donor concentration, additive concentration and other reaction conditions were optimized. The target product was obtained in 50% conversion and >99.99% enantiomeric excess when Candida antarctica lipase B and phenyl allyl carbonate were combined in the tert-butyl methyl ether(TBME) medium at 45 ℃ for 15 h.To further improve the rate of enzymatic resolution reaction, we herein introduced the microwave technique in enzymatic biocatalysis. The effects of conventional heating and microwave radiation on the enzymatic effect has beend examined. We found the solvents has great impact on the enzymatic reaction and Candida antarctica lipase B has higher activity under microwave radiation, The enzymatic reaction rate under microwave radiation is about 1.5 times of that under the conventional heating. In the end, the reaction conditions were also optimized, including the stirring speed, the impact of microwave power and temperature. We found the reaction carried out in the optimal conditions for 4 hours; the product conversion rate reached 46% and enantiomeric excess > 99.99%.Using enzymatic biocatalysis under microwave irradiation, the sterically hindered racemic cyclic secondary amine cis-6-benzyl-tetrahydro-1H-pyrrolo[3,4-b] pyridine-5,7(6H,7aH)-dione was resolved by CAL-B and the(4aR,7aS)-6-benzyltetrahydro-1H-pyrrolo[3,4-b]pyridine-5,7(6H,7aH)-dione was successfully prepared. This research provide a new way for preparing the chiral intermediate of moxifloxacin to avoid the problems in chemical resolution process, including difficultly recovered resolving agent, low optical purity for product, repeated recrystallization, harsh reaction conditions, not suitable for large-scale production. Thus, this method used in our work enrichs the research in the resolution of the sterically hindered cyclic secondary amines.