Opioid activity of 4-imidazolidinone positional analogues of Leu-Enkephalin

Reaction of Picolinamides with Ketones Producing a New Type of Heterocyclic Salts with an Imidazolidin-4-One Ring

Reactions of picolinamides with 1,3-propanesultone in methanol followed by the treatment with ketones led to a series of previously unknown chemical transformations, yielding first pyridinium salts (2a-f), with a protonated endocyclic nitrogen atom, and then heterocyclic salts (3a-j) containing an imidazolidin-4-one ring. The structures of intermediate and final products were determined by IR and 1H, 13C NMR spectroscopy, and X-ray study. The effects of the ketone and alcohol structures on the product yield were studied by quantum-chemical calculations. The stability of salts 3a-j towards hydrolysis and alcoholysis makes them excellent candidates for the search for new types of biologically active compounds.

Diastereo- and Enantioselective Palladium-Catalyzed Cycloadditions of 5-Vinyloxazolidine-2,4-diones with Electrophilic Imines

Despite the importance of the 4-imidazolidinone scaffold in bioactive compounds or organocatalysts, methodologies to access these nitrogenated heterocycles remain scarce. This manuscript describes a novel preparation of 4-imidazolidinones via a diastereo- and enantioselective (3 + 2) cycloaddition between 5-vinyloxazolidine-2,4-diones (VOxD) and electrophilic imines under palladium catalysis. This work supports the synthetic potential of VOxD as a promising equivalent of the C-C(═O)-N synthon.

Peptidomimetics and Their Applications for Opioid Peptide Drug Discovery

Despite various advantages, opioid peptides have been limited in their therapeutic uses due to the main drawbacks in metabolic stability, blood-brain barrier permeability, and bioavailability. Therefore, extensive studies have focused on overcoming the problems and optimizing the therapeutic potential. Currently, numerous peptide-based drugs are being marketed thanks to new synthetic strategies for optimizing metabolism and alternative routes of administration. This tutorial review briefly introduces the history and role of natural opioid peptides and highlights the key findings on their structure-activity relationships for the opioid receptors. It discusses details on opioid peptidomimetics applied to develop therapeutic candidates for the treatment of pain from the pharmacological and structural points of view. The main focus is the current status of various mimetic tools and the successful applications summarized in tables and figures.

Bis-Cyclic Guanidine Heterocyclic Peptidomimetics as Opioid Ligands with Mixed μ-, κ- and δ-Opioid Receptor Interactions: A Potential Approach to Novel Analgesics

The design and development of analgesics with mixed-opioid receptor interactions has been reported to decrease side effects, minimizing respiratory depression and reinforcing properties to generate safer analgesic therapeutics. We synthesized bis-cyclic guanidine heterocyclic peptidomimetics from reduced tripeptides. In vitro screening with radioligand competition binding assays demonstrated variable affinity for the mu-opioid receptor (MOR), delta-opioid receptor (DOR), and kappa-opioid receptor (KOR) across the series, with compound 1968-22 displaying good affinity for all three receptors. Central intracerebroventricular (i.c.v.) administration of 1968-22 produced dose-dependent, opioid receptor-mediated antinociception in the mouse 55 °C warm-water tail-withdrawal assay, and 1968-22 also produced significant antinociception up to 80 min after oral administration (10 mg/kg, p.o.). Compound 1968-22 was detected in the brain 5 min after intravenous administration and was shown to be stable in the blood for at least 30 min. Central administration of 1968-22 did not produce significant respiratory depression, locomotor effects or conditioned place preference or aversion. The data suggest these bis-cyclic guanidine heterocyclic peptidomimetics with multifunctional opioid receptor activity may hold potential as new analgesics with fewer liabilities of use.

One-step synthesis of N,N'-substituted 4-imidazolidinones by an isocyanide-based pseudo-five-multicomponent reaction

A pseudo-five-multicomponent reaction involving an isocyanide, a primary amine, two molecules of formaldehyde and water is reported, which gives N,N'-substituted 4-imidazolidinones when trifluoroethanol is used as the solvent. The reaction proceeds with good yields and with a wide variety of amines and isocyanides, providing an efficient new entry to these heterocycles. A preliminary study of the reaction mechanism suggests that trifluoroethanol, although acting as the solvent, is directly involved as a reagent in the reaction pathway.

Preparation and evaluation at the delta opioid receptor of a series of linear leu-enkephalin analogues obtained by systematic replacement of the amides

Leu-enkephalin analogues, in which the amide bonds were sequentially and systematically replaced either by ester or N-methyl amide bonds, were prepared using classical organic chemistry as well as solid phase peptide synthesis (SPPS). The peptidomimetics were characterized using competition binding, ERK1/2 phosphorylation, receptor internalization, and contractility assays to evaluate their pharmacological profile over the delta opioid receptor (DOPr). The lipophilicity (LogD7.4) and plasma stability of the active analogues were also measured. Our results revealed that the last amide bond can be successfully replaced by either an ester or an N-methyl amide bond without significantly decreasing the biological activity of the corresponding analogues when compared to Leu-enkephalin. The peptidomimetics with an N-methyl amide function between residues Phe and Leu were found to be more lipophilic and more stable than Leu-enkephalin. Findings from the present study further revealed that the hydrogen-bond donor properties of the fourth amide of Leu-enkephalin are not important for its biological activity on DOPr. Our results show that the systematic replacement of amide bonds by isosteric functions represents an efficient way to design and synthesize novel peptide analogues with enhanced stability. Our findings further suggest that such a strategy can also be useful to study the biological roles of amide bonds.

Electrospray ionization-ion trap mass spectrometry study of PQAAPro and PQProAA mimetic derivatives of the antimalarial primaquine

Electrospray ionization-ion trap mass spectrometry (ESI-MS) of imidazolidin-4-one peptidomimetic derivatives of the antimalarial drug primaquine (PQ) is reported. These compounds contain the imidazolidin-4-one moiety either at the N- or the C-terminal of a dipeptide backbone, thus respectively mimicking PQ-Amino Acid-Proline (PQAAPro) and PQProAA derivatives of PQ. Both the peptidomimetics and precursors previously developed by us are promising drug candidates, as they were found to be active against rodent Plasmodium berghei malaria and Pneumocystis carinii pneumonia. Collision-induced dissociation (CID) and tandem-mass spectra (MS) of the title compounds, and fragmentation pathways thereof, led to the following findings: (1) CID patterns present some parallelism with the reactivity towards hydrolysis previously found for the same or related compounds; (2) a positional shift of the imidazolidin-4-one ring is reflected on both degree and pathways of fragmentation, which makes tandem-MS a key tool for differentiation of imidazolidin-4-one isomers; (3) the major MS/MS fragmentation of PQProAA mimetics involves release of a neutral diketopiperazine (DKP), in parallel to the "diketopiperazine pathway" described in tandem-MS studies of oligopeptides; (4) the relative abundance of a major fragment in tandem-MS spectra is inversely correlated with the size of the N-terminal AA in PQProAA mimetics. Overall, this work embodies an original and valuable contribution towards a deeper insight into the molecular properties of novel antimalarials, which can be viewed as representative of both the 8-aminoquinoline and, especially, the imidazolidin-4-one structural classes.