Engineered Monoamine Oxidase-Catalyzed Deracemization of Sterically Hindered Acylaminomethyl-tetrahydroisoquinoline: Asymmetric Synthesis of (R)-(−)-Praziquantel and Its Analogues

At present, praziquantel (PZQ) is the only drug utilized in clinics for the control and treatment of all forms of schistosomiasis, which is a neglected tropical disease infecting over 200 million people. As only the R-isomer of PZQ is biologically active and devoid of associated undesired properties, the development of a stereoselective, sustainable, and cost-effective synthesis of (R)-PZQ is of paramount significance. Herein, a known monoamine oxidase variant, MAO-N-D11D, which showed a specific activity of merely 0.003 U/mg toward 1-cyclohexanecarboxylaminomethyl-tetrahydroisoquinoline (THIQ) (1a), a synthetic intermediate to PZQ, was extensively engineered using combinatorial active-site saturation test/iterative saturation mutagenesis and site-saturation mutagenesis, resulting in the identification of an evolved variant, referred to as LJY-FM, exhibiting 134-fold elevated specific activity (0.401 U/mg) and 260-fold enhanced catalytic efficiency (26,989 min–1 M–1) toward 1a relative to MAO-N-D11D, respectively. After optimizing protein expression and reaction conditions, gram-scale deracemization of 50 mM of 1a catalyzed by variant LJY-FM was executed in the presence of ammonia borane, with (R)-1a, a critical intermediate to (R)-PZQ, being furnished with 76% isolated yield and 96% ee. Subsequent transformation of (R)-1a to (R)-PZQ was achieved in 82% yield. Moreover, eight 1-acylaminomethyl-THIQ derivatives, (R)-1b to (R)-1i, were prepared in 44–99% isolated yields with 90 to >99% ee through a LJY-FM-catalyzed deracemization process. To the best of our knowledge, our work represents the first application of monoamine oxidase to the synthesis of (R)-PZQ or its synthetic intermediates. We expect the efficient, stereoselective, and sustainable synthesis of (R)-PZQ established in the current study to be harnessed for the eventual development of a cost-effective, green production route of this antischistosomal agent, thereby leading to the realization of replacing the clinical use of praziquantel by (R)-praziquantel in the future.