New cis-configured aziridine-2-carboxylates as aspartic acid protease inhibitors

A series of 52 cis-configured 1-alkyl-3-phenylaziridine-2-carboxylates were synthesized as new pseudo-irreversible inhibitors of Candida albicans secreted aspartic acid protease 1 (SAP1), SAP2, SAP3, and SAP8. Some of the compounds, which were obtained as diastereomers with S,S- and R,R-configured aziridine rings by Cromwell synthesis of racemic (2R,3S+2S,3R)-dibromophenylpropionic acid ester with amines, followed by ester hydrolysis and coupling to hydrophobic amino acid esters, were separated by preparative HPLC. The absolute configuration of the aziridine ring was assigned by a combination of experimental circular dichroism (CD) investigations and quantum chemical CD calculations. In agreement with previous docking studies, the diastereomers all exhibit similar activity. The compounds were found to be more active against the related mammalian enzyme cathepsin D, presumably due to productive interactions of the N-alkyl substituent with the highly lipophilic S2 pocket. The most active inhibitors (5, 9, 10, 21, and 28), characterized by benzyl, cyclohexylmethyl, tert-butyl, or 1,4-dimethylpentyl moieties at the aziridine nitrogen atom, exhibit k(2nd) values between 500 and 900×10³ M⁻¹ min⁻¹ and K(i) values near or below 1 μM for cathepsin D.