Enantioselective synthesis of (R)-(−)-praziquantel (PZQ)

Praziquantel 8 (2-cyclohexylcarbonyl-1,2,3,6,7,11b-hexahydro-4H-pyrazino[2,1-a]isoquinoline-4-one), a powerful anti-worm drug, has been synthesized in its enantiopure form via asymmetric transfer hydrogenation according to the Noyori protocol. Initially, the reduction of prochiral imine 4 afforded product 5 in 62% ee, but a single crystallization amplified the enantiomeric purity to 98% ee. The final (R)-(−)-praziquantel 8 was prepared in three subsequent steps in 56% chemical yield.

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(1S)-1-Phthalimidomethyl-2-[(2′R)-3′,3′,3′-trifluoro-2′-methoxy-2′-phenylpropanoyl]-1,2,3,4-tetrahydroisoquinolineC28H23N2O4F3[α]D23=+35.0 (c 1.00, CHCl3)Source of chirality: asymmetric transfer hydrogenationAbsolute configuration: (1S,2′R)