Chemoenzymatic enantioselective and stereo-convergent syntheses of lisofylline enantiomers via lipase-catalyzed kinetic resolution and optical inversion approach

Authors:

• Paweł Borowiecki,
• Beata Zdun,
• Maciej Dranka

Abstract

Highly enantioselective enzymatic kinetic resolution (EKR) of racemic lisofylline is presented for the first time. A comprehensive optimization of the key parameters of lipase-catalyzed transesterification of racemic lisofylline revealed that optimal biocatalytic system consisted of immobilized lipase type B from Candida antarctica (Chirazyme L-2, C-3) suspended in a mixture of 3 equiv of vinyl acetate as an acetyl donor and ethyl acetate as a solvent. Under optimal reaction conditions, the 1 g-scale (Chirazyme L-2, C-3)-catalyzed kinetic resolution of racemic lisofylline furnished both the EKR products in a homochiral form (>99 % ee) with the 50 % conv., and the highest possible enantioselectivity. The best results in terms of the reaction yields (47–50 %) and enantiomeric purity of the kinetically-resolved optically active products were achieved when the preparative-scale EKR was carried out for 2 h at 60 °C. In addition, stereoinversion of the less biologically-relevant (S)-lisofylline into its (R)-enantiomer was successfully achieved via acetolysis of the respective optically pure (S)-mesylate by using 2 equiv of ceasium acetate and catalytic amount of 18-Crown-6 in dry toluene, followed by K2CO3-mediated methanolysis of (R)-acetate. The elaborated EKR methodology together with enantioconvergent strategy provided a useful chemoenzymatic protocol for the synthesis of complementary enantiomers of titled API. Moreover, we report on the first single-crystal X-ray diffraction (XRD) analyses performed for the synthesized lisofylline enantiomers. Insight into the source of CAL-B stereoselectivity toward racemic lisofylline was gained by molecular docking experiments. In silico theoretical predictions matched very well with experimental results.