Amino acids as selective acylating agents: regioselective N1-acylation of imidazolidin-4-one derivatives of the antimalarial drug primaquine

Primaquine derivatives: Modifications of the terminal amino group

Numerous modifications of the well-known antimalarial drug primaquine, both at the quinoline ring and at the primary amino group, have been reported, mostly to obtain antimalarial agents with improved bioavailability, reduced toxicity and/or prolonged activity. Modifications of the terminal amino group were made with the main idea to prevent the metabolic pathway leading to inactive and toxic carboxyprimaquine (follow-on strategy), but also to get compounds with different activity (repurposing strategy). The modifications undertaken until 2009 were included in a review published in the same year. The present review covers various classes of primaquine N-derivatives with diverse biological profiles, prepared in the last decade by our research group as well as the others. We have summarized the synthetic procedures applied for their preparation and discussed the main biological results. Several hits for the development of novel antiplasmodial, anticancer, antimycobacterial and antibiofilm agents were identified.

Crystal structure, Hirshfeld surface analysis and PIXEL calculations of a 1:1 epimeric mixture of 3-[(4-nitro-benzyl-idene)amino]-2(R,S)-(4-nitro-phenyl)-5(S)-(propan-2-yl)imidazolidin-4-one

A 1:1 epimeric mixture of 2(R,S)-(4-nitro­phen­yl)-3-[(4-nitro­benzyl­idene)amino]-5(S)-(propan-2-yl)imidazolidin-4-one was derived from an initial reaction of 2(S)-amino-3-methyl-1-oxo­butane­hydrazine at its hydrazine moiety to provide a 4-nitro­benzyl­idine derivative, followed by a cyclization reaction with another mol­ecule of 4-nitro­benzaldehyde to form the chiral five-membered imidazolidin-4-one ring.

A 1:1 epimeric mixture of 3-[(4-nitro­benzyl­idene)amino]-2(R,S)-(4-nitro­phen­yl)-5(S)-(propan-2-yl)imidazolidin-4-one, C₁₉H₁₉N₅O₅, was isolated from a reaction mixture of 2(S)-amino-3-methyl-1-oxo­butane­hydrazine and 4-nitro­benz­alde­hyde in ethanol. The product was derived from an initial reaction of 2(S)-amino-3-methyl-1-oxo­butane­hydrazine at its hydrazine group to provide a 4-nitro­benzyl­idene derivative, followed by a cyclization reaction with another mol­ecule of 4-nitro­benzaldehyde to form the chiral five-membered imidazolidin-4-one ring. The formation of the five-membered imidazolidin-4-one ring occurred with retention of the configuration at the 5-position, but with racemization at the 2-position. In the crystal, N—H⋯O(nitro) hydrogen bonds, weak C—H⋯O(carbon­yl) and C—H⋯O(nitro) hydrogen bonds, as well as C—H⋯π, N—H⋯π and π–π inter­actions, are present. These combine to generate a three-dimensional array. Hirshfeld surface analysis and PIXEL calculations are also reported.

Crystal structure of ethyl 2-(4-chlorophenyl)-3-cyclopentyl-4-oxo-1-propyl-imidazolidine-5-carboxylate

The title compound, C₂₀H₂₇ClN₂O₃, was obtained via an original synthesis method. The central heterocyclic ring adopts a shallow envelope conformation, with the N atom bearing the cyclo­pentane ring as the flap [deviation from the other atoms = 0.442 (2) Å]. The cyclo­pentane ring adopts a twisted conformation about one of the C_N—C bonds: the exocyclic C—N bond adopts an equatorial orientation. The dihedral angles between the central ring (all atoms) and the pendant five- and six-membered rings are 10.3 (2) and 87.76 (14)°, respectively. In the crystal, C—H⋯O inter­actions link the mol­ecules into [011] chains. A weak C—H⋯Cl inter­action links the chains into (100) sheets. A mechanism for the cyclization reaction is proposed.

Imidazoquines as antimalarial and antipneumocystis agents

Peptidomimetic imidazolidin-4-one derivatives of primaquine (imidazoquines) recently displayed in vitro activity against blood schizonts of a chloroquine-resistant strain of Plasmodium falciparum. Preliminary studies with a subset of such imidazoquines showed them to both block transmission of P. berghei malaria from mouse to mosquito and be highly stable toward hydrolysis at physiological conditions. This prompted us to have deeper insight into the activity of imidazoquines against both Plasmodia and Pneumocystis carinii, on which primaquine is also active. Full assessment of the in vivo transmission-blocking activity of imidazoquines, in vitro tissue-schizontocidal activity on P. berghei-infected hepatocytes, and in vitro anti-P. carinii activity is now reported. All compounds were active in these biological assays, with generally lower activity than the parent drug. However, imidazoquines' stability against both oxidative deamination and proteolytic degradation suggest that they will probably have higher oral bioavailability and lower hematotoxicity than primaquine, which might translate into higher therapeutic indexes.

Electrospray ionization mass spectrometry as a valuable tool in the characterization of novel primaquine peptidomimetic derivatives

Novel primaquine-derived antimalarials have been extensively characterized by electrospray ionization-ion trap mass spectrometry (ESI-MS). Experiments by in-source collision-induced dissociation (CID) in the nozzle- skimmer region (NSR) or by tandem-MS are shown to be most valuable tools for the physicochemical characterization of these 8-aminoquinolinic drugs that also bear the biologically relevant imidazolidin-4-one scaffold. It was possible to find parallelism between compound stability in the NSR and its reactivity towards hydrolysis at physiological pH and T. Moreover, tandem-MS fragmentation patterns were characteristic for each family, providing a means for structural distinction of isomers and allowing to find interesting correlations between the relative abundance of particular fragments and relevant structure-activity determinants, such as Charton steric parameter, v. In conclusion, this work provides solid grounds to establish ESI-MS as a key tool for the physicochemical characterization of biopharmaceuticals bearing the 8-aminoquinoline and/or the imidazolidin-4-one moieties.