New organofluorine building blocks: inhibition of the malarial aspartic proteases plasmepsin II and IV by alicyclic α,α-difluoroketone hydrates

The development of new therapeutic agents against malaria has become urgent during the past few decades, due to an increased prevalence of drug-resistant strains of malaria-causing Plasmodium parasites. Possible targets are the hemoglobin-degrading aspartic proteases, the plasmepsins. While acyclic alpha,alpha-difluoroketone hydrates have been introduced into peptidomimetics to bind to the catalytic Asp dyad of aspartic proteases, alicyclic derivatives were unknown. This paper describes a versatile synthesis of hydrated alicyclic alpha,alpha-difluoro-cyclopentanones and -cyclohexanones, decorated with appropriate substituents to fill the S1/S3 and the "flap-open" pocket at the enzyme active sites. Their biological activity was tested against plasmepsin II and IV, revealing an IC(50) value (concentration of an inhibitor at which 50% maximum initial velocity is observed) of 7 microM for the best ligand. Reference inhibitors with a protonated secondary ammonium centre to address the catalytic dyad showed similar binding affinities. The X-ray crystal structure of a cyclic alpha,alpha-difluoroketone hydrate revealed the ability of these novel building blocks to participate in H-bonding networks. The hydration of difluoroketones was also investigated in solution. An exemplary study showed that the equilibrium constants for the hydration of alpha,alpha-difluorinated cyclohexanones are much higher than those for the corresponding cyclopentanones.