2,5-diketopiperazines as potent, selective, and orally bioavailable oxytocin antagonists. 2. Synthesis, chirality, and pharmacokinetics

Oxytocin and vasopressin signaling in health and disease

Neurohypophysial peptides are ancient and evolutionarily highly conserved neuropeptides that regulate many crucial physiological functions in vertebrates and invertebrates. The human neurohypophysial oxytocin/vasopressin (OT/VP) signaling system with its four receptors has become an attractive drug target for a variety of diseases, including cancer, pain, cardiovascular indications, and neurological disorders. Despite its promise, drug development faces hurdles, including signaling complexity, selectivity and off-target concerns, translational interspecies differences, and inefficient drug delivery. In this review we dive into the complexity of the OT/VP signaling system in health and disease, provide an overview of relevant pharmacological probes, and discuss the latest trends in therapeutic lead discovery and drug development.

The Discovery of Novel Antimicrobial Agents through the Application of Isocyanide-Based Multicomponent Reactions

Multicomponent reactions (MCR) have been used to synthesize a wide range of analogs from several classes of heterocyclic compounds, with multifaceted medicinal uses. The synthesis of highly functionalized molecules in a single pot is a unique property of MCR, allowing researchers to quickly assemble libraries of compounds of biological interest and uncover novel leads as possible therapeutic agents. Isocyanide-based multicomponent reactions have proven to be extremely effective at swiftly specifying members of compound libraries, particularly in the discovery of drugs. The understanding of structure-activity correlations that drive the development of new goods and technology requires structural variety in these libraries. In today's world, antibiotic resistance is a major ongoing problem that poses risks to public health. The implementation of isocyanide-based multicomponent reactions upholds a significant potential in this regard. By utilizing such reactions, new antimicrobial compounds can be discovered and subsequently used to fight against such concerns. This study discusses the recent developments in antimicrobial medication discovery using isocyanide-based multicomponent reactions (IMCRs). Furthermore, the article emphasizes the potential of IMCRs (Isocyanide-based multicomponent based reactions) in the near future.

Cyclic Dipeptides: The Biological and Structural Landscape with Special Focus on the Anti-Cancer Proline-Based Scaffold

Cyclic dipeptides, also know as diketopiperazines (DKP), the simplest cyclic forms of peptides widespread in nature, are unsurpassed in their structural and bio-functional diversity. DKPs, especially those containing proline, due to their unique features such as, inter alia, extra-rigid conformation, high resistance to enzyme degradation, increased cell permeability, and expandable ability to bind a diverse of targets with better affinity, have emerged in the last years as biologically pre-validated platforms for the drug discovery. Recent advances have revealed their enormous potential in the development of next-generation theranostics, smart delivery systems, and biomaterials. Here, we present an updated review on the biological and structural profile of these appealing biomolecules, with a particular emphasis on those with anticancer properties, since cancers are the main cause of death all over the world. Additionally, we provide a consideration on supramolecular structuring and synthons, based on the proline-based DKP privileged scaffold, for inspiration in the design of compound libraries in search of ideal ligands, innovative self-assembled nanomaterials, and bio-functional architectures.

In silico profiling the interaction mechanism of 2,5-diketopiperazine derivatives as oxytocin antagonists

Oxytocin plays a vital role in the occurrence of preterm birth by interacting with oxytocin receptor (OTR), and thus OTR antagonists provide effective approaches for the treatment of early birth. Presently, for purpose of exploring the structural traits affecting the antagonism potency, the up-to-date largest set of 121 2,5-diketopiperazine derivatives as OTR antagonists was subjected to ligand-based three-dimensional quantitative structure-activity (3D-QSAR) analysis applying comparative molecular field analysis (CoMFA) and comparative molecular similarity indices analysis (CoMSIA) methods. The resultant optimal CoMSIA model displays proper validity and predictability with cross-validated correlation coefficient Q² = 0.614, non-cross-validated correlation coefficient R²_ncv = 0.969 and predicted correlation coefficient R²_pre = 0.912 for the test set of compounds, respectively. In addition, docking study was carried out for further elucidating the binding modes of OTR antagonists. The final docking cavity was located among the TM2-TM7 helices of the target protein and proved to be the same as described by other scholars' researches of other type of OTR antagonists. The major amino acids forming the cavity are Q92, Q96, K116, Q119, V120, M123, G196, I201, Q295, W288, F311 and M315. Representative compound 118 forms two H-bonds with Q119 and two H-bonds with Q295, with also a π - π stacking effect with F311, respectively. Meanwhile, as a supplementary tool to study the antagonists' conformation in the binding cavity, molecular dynamics (MD) simulation was also performed to further elucidate the changes in the ligand-receptor complex. We hope that the obtained models and information may help to provide an insight into the interaction mechanism of OTR antagonists and facilitate the design and optimization of novel antagonists for preventing premature birth.

Amide Activation in Ground and Excited States

Not all amide bonds are created equally. The purpose of the present paper is the reinterpretation of the amide group by means of two concepts: amidicity and carbonylicity. These concepts are meant to provide a new viewpoint in defining the stability and reactivity of amides. With the help of simple quantum-chemical calculations, practicing chemists can easily predict the outcome of a desired process. The main benefit of the concepts is their simplicity. They provide intuitive, but quasi-thermodynamic data, making them a practical rule of thumb for routine use. In the current paper we demonstrate the performance of our methods to describe the chemical character of an amide bond strength and the way of its activation methods. Examples include transamidation, acyl transfer and amide reductions. Also, the method is highly capable for simple interpretation of mechanisms for biological processes, such as protein splicing and drug mechanisms. Finally, we demonstrate how these methods can provide information about photo-activation of amides, through the examples of two caged neurotransmitter derivatives.

Radiosynthesis and evaluation of 1-substituted 3-(2,3-dihydro-1H-inden-2-yl)-6-(1-ethylpropyl)-(3R,6R)-2,5-piperazinedione derivatives as PET tracers for imaging the central oxytocinergic system

Oxytocin is known to be implicated in a variety of functions, such as learning, stress, anxiety, feeding, and pain perception. Oxytocin is also important for social memory and attachment, human bonding, sexual and maternal behaviour, and aggression. Human disorders characterized by aberrant social interactions, such as autism and schizophrenia, may also involve abnormal oxytocin levels. GSK712043, GSK711320, and GSK664004, three antagonists exhibiting subnanomolar affinity for the human oxytocin receptor (hOTR) and high selectivity over vasopressin receptors were successfully labelled with carbon-11 with suitable yields (0.5-1GBq @EOS), high molar activity (275-700 GBq/μmol), and radiochemical purities. The in vivo regional uptake of these radiotracers was determined in porcine brain. [¹¹ C]GSK711320 baseline scan showed no significant brain uptake, and limited initial uptake was observed following administration of [¹¹ C]GSK712043 or [¹¹ C]GSK664004. The [¹¹ C]GSK712043 and [¹¹ C]GSK664004 kinetics were slow and peaked at around 2%ID/L at 90 minutes post-injection. For both tracers, the distribution of activity was homogeneous throughout the brain. All the tracers showed high uptake in the pituitary gland, especially [¹¹ C]GSK711320; however, its uptake could not be blocked by pretreatment with the known OTR antagonist, L368,899. In vivo evaluation of these candidates demonstrated that they are not suitable as central OTR PET imaging agents.

Recent Advances in the Synthesis of Hydantoins: The State of the Art of a Valuable Scaffold

The review highlights the hydantoin syntheses presented from the point of view of the preparation methods. Novel synthetic routes to various hydantoin structures, the advances brought to the classical methods in the aim of producing more sustainable and environmentally friendly procedures for the preparation of these biomolecules, and a critical comparison of the different synthetic approaches developed in the last twelve years are also described. The review is composed of 95 schemes, 8 figures and 528 references for the last 12 years and includes the description of the hydantoin-based marketed drugs and clinical candidates.

Organocatalytic synthesis of optically active β-branched α-amino esters via asymmetric biomimetic transamination

This paper describes an efficient asymmetric biomimetic transamination of α-keto esters with a quinine-derived chiral base as the catalyst, giving a variety of β-branched α-amino esters in 50-96% yield and 87-95% ee.

Isocyanide-based multicomponent reactions towards cyclic constrained peptidomimetics

In the recent past, the design and synthesis of peptide mimics (peptidomimetics) has received much attention. This because they have shown in many cases enhanced pharmacological properties over their natural peptide analogues. In particular, the incorporation of cyclic constructs into peptides is of high interest as they reduce the flexibility of the peptide enhancing often affinity for a certain receptor. Moreover, these cyclic mimics force the molecule into a well-defined secondary structure. Constraint structural and conformational features are often found in biological active peptides. For the synthesis of cyclic constrained peptidomimetics usually a sequence of multiple reactions has been applied, which makes it difficult to easily introduce structural diversity necessary for fine tuning the biological activity. A promising approach to tackle this problem is the use of multicomponent reactions (MCRs), because they can introduce both structural diversity and molecular complexity in only one step. Among the MCRs, the isocyanide-based multicomponent reactions (IMCRs) are most relevant for the synthesis of peptidomimetics because they provide peptide-like products. However, these IMCRs usually give linear products and in order to obtain cyclic constrained peptidomimetics, the acyclic products have to be cyclized via additional cyclization strategies. This is possible via incorporation of bifunctional substrates into the initial IMCR. Examples of such bifunctional groups are N-protected amino acids, convertible isocyanides or MCR-components that bear an additional alkene, alkyne or azide moiety and can be cyclized via either a deprotection-cyclization strategy, a ring-closing metathesis, a 1,3-dipolar cycloaddition or even via a sequence of multiple multicomponent reactions. The sequential IMCR-cyclization reactions can afford small cyclic peptide mimics (ranging from four- to seven-membered rings), medium-sized cyclic constructs or peptidic macrocycles (>12 membered rings). This review describes the developments since 2002 of IMCRs-cyclization strategies towards a wide variety of small cyclic mimics, medium sized cyclic constructs and macrocyclic peptidomimetics.

Concise Two-Step Solution Phase Syntheses of four novel Dihydroquinazoline scaffolds

Novel two-step solution phase protocols for the synthesis of dihydroquinazolines and fused dihydroquinazoline-benzodiazepine tetracycles are reported. The methodology employs the Ugi reaction to assemble desired diversity and acid treatment enables ring closing transformations. The protocols are further facilitated by the use of microwave irradiation and n-butyl isocyanide to control the rate of each ring forming transformation.

The design of orally bioavailable 2, 5-diketopiperazine oxytocin antagonists: from concept to clinical candidate for premature labor

A short, efficient and highly stereoselective synthesis has been developed for a series of 6-indanyl-3-alkyl-7-aryl/heterocyclic-(3R, 6R, 7R)-2, 5-diketopiperazine amides that are potent and selective oxytocin (OT) antagonists. Property-based design using an estimate of human oral absorption enabled focus to be directed to those templates with the greatest chance of delivering high bioavailability in humans. This led to the 2', 4'-difluorophenyl dimethylamide 40, a highly potent (pK(i) =9.2) and selective OT antagonist (>1,000-fold selectivity vs. the human vasopressin receptors V1a, V2, and V1b) with good oral bioavailability (>50%) in the rat and dog. Increased solubility and an improved Cyp450 profile was achieved with a range of 2'-substituted 7-(1',3'-oxazol-4'-yl)-(3R,6R,7R)-2,5-diketopiperazine amides and branching at the α-carbon of the 3-butyl group led to a superior rat pharmacokinetic profile that resulted in the discovery of the 2'-methyl-1',3'-oxazol-4'-yl morpholine amide derivative 74 GSK221149A (Retosiban), which had the best oral exposure and bioavailability in the rat. Retosiban has sub-nanomolar affinity (K(i) =0.65 nM) for the oxytocin receptor with >1400-fold selectivity over the closely related vasopressin receptors. It has good solubility, low protein binding and has a good Cyp450 profile with no significant inhibition IC(50) >100 µM. Retosiban is >15-fold more potent at the human oxytocin receptor than atosiban (a marketed i.v, peptide OT antagonist) and it has been shown to be an effective tocolytic by i.v. and by oral administration in rats, and was selected for progression as a potential clinical candidate for preterm labor.

Discovery and optimization of highly ligand-efficient oxytocin receptor antagonists using structure-based drug design

A novel oxytocin antagonist was identified by 'scaffold-hopping' using Cresset FieldScreen molecular field similarity searching. A single cycle of optimization driven by an understanding of the key pharmacophoric elements required for activity led to the discovery of a potent, selective and highly ligand-efficient oxytocin receptor antagonist. Selectivity over vasopressin receptors was rationalized based on differences in the structure of the natural ligands.

Solid-phase and microwave-assisted syntheses of 2,5-diketopiperazines: small molecules with great potential

Diketopiperazines (DKPs) are a well-known class of heterocycles that have recently emerged as a promising biologically active scaffold. Solid-phase organic synthesis has become an important tool in the combinatorial exploration of these privileged structures, expediting the synthesis and, therefore, the discovery of active compounds. To date, certain DKPs have shown potent activities against a range of diseases and biological phenomena, including bacterial infections, various cancers, asthma, infertility, premature labor, and HIV. Recent applications of solid-phase DKP synthesis, with a particular focus on cyclative cleavage and microwave-assisted reactions, are highlighted herein.

Cyclization-activated prodrugs

Many drugs suffer from an extensive first-pass metabolism leading to drug inactivation and/or production of toxic metabolites, which makes them attractive targets for prodrug design. The classical prodrug approach, which involves enzyme-sensitive covalent linkage between the parent drug and a carrier moiety, is a well established strategy to overcome bioavailability/toxicity issues. However, the development of prodrugs that can regenerate the parent drug through non-enzymatic pathways has emerged as an alternative approach in which prodrug activation is not influenced by inter- and intraindividual variability that affects enzymatic activity. Cyclization-activated prodrugs have been capturing the attention of medicinal chemists since the middle-1980s, and reached maturity in prodrug design in the late 1990 s. Many different strategies have been exploited in recent years concerning the development of intramoleculary-activated prodrugs spanning from analgesics to anti-HIV therapeutic agents. Intramolecular pathways have also a key role in two-step prodrug activation, where an initial enzymatic cleavage step is followed by a cyclization-elimination reaction that releases the active drug. This work is a brief overview of research on cyclization-activated prodrugs from the last two decades.

Analysis of antidiuretic effect of arginine-vasotocin and its analogs in primates

Intramuscular injection of synthetic analogs of arginine-vasotocin to monkeys after 3% water loading caused antidiuretic reaction, manifesting by reabsorption of osmotically free water and paralleled by significant changes in excretion of sodium or potassium ions. It is shown that synthetic analogs can be more effective and selective than natural agonists.

Synthesis of Functionalized, Unsymmetrical 1,3,4,6-Tetrasubstituted 2,5-Diketopiperazines

A general and efficient method for the synthesis of unsymmetrical 1,3,4,6-tetrasubstituted 2,5-diketo- piperazines (DKPs) is described. Cyclization of N-amide alkylated dipeptide methyl esters, followed by alkylation, furnished the corresponding tetrasubstituted DKPs in good overall yields. The influence of steric hindrance in the alkylation reactions appeared to be of lesser importance as long as reactive alkylating agents were used. Furthermore, we have demonstrated the use of tetrasubstituted DKPs as a scaffold for further chemical manipulations to produce novel DKPs with desired properties.

1,3,5-Triazepan-2,6-diones as Structurally Diverse and Conformationally Constrained Dipeptide Mimetics: Identification of Malaria Liver Stage Inhibitors from a Small Pilot Library

The development of the 1,3,5-triazepane-2,6-dione system as a novel, conformationally restricted, and readily accessible class of dipeptidomimetics is reported. The synthesis of the densely functionalized 1,3,5-triazepane-2,6-dione skeleton was achieved in only four steps from a variety of simple linear dipeptide precursors. To extend the practical value of 1,3,5-triazepane-2,6-diones, a general polymer-assisted solution-phase synthesis approach amenable to library production in a multiparallel format was developed. The conformational preferences of the 1,3,5-triazepane-2,6-dione skeleton were investigated in detail by NMR spectroscopy and X-ray diffraction. The ring exhibits a characteristic folded conformation which was compared to that of related dipeptide-derived scaffolds including the more planar 2,5-diketopiperazine (DKP). Molecular and structural diversity was increased further through post-cyclization appending operations at urea nitrogens. Preliminary biological screens of a small collection of 1,3,5-triazepane-2,6-diones revealed inhibitors of the underexplored malaria liver stage and suggest strong potential for this dipeptide-derived scaffold to interfere with and to modulate biological pathways.

Convergent Synthesis of Complex Diketopiperazines Derived from Pipecolic Acid Scaffolds and Parallel Screening against GPCR Targets

A convergent approach to highly functionalized diketopiperazines (DKPs) using enantioenriched pipecolic acids is described. Scandium triflate-catalyzed [4 + 2] aza-annulation was employed to produce stereochemically well-defined building blocks. A resin "catch and release" strategy was devised to convert annulation products to pipecolic acid monomers. Complex diketopiperazines were efficiently assembled utilizing one-pot cyclodimerization of pipecolic acids. Massively parallel screening of the complex DKPs against a panel of molecular targets identified novel ligands for a number of G-protein-coupled receptors (GPCRs).

2,5-diketopiperazines as potent, selective, and orally bioavailable oxytocin antagonists. 3. Synthesis, pharmacokinetics, and in vivo potency

A short, efficient, and highly stereoselective synthesis of a series of (3R,6R,7R)-2,5-diketopiperazine oxytocin antagonists and their pharmacokinetics in rat and dog is described. Prediction of the estimated human oral absorption (EHOA) using measured lipophilicity (CHI log D) and calculated size (cMR) has allowed us to rank various 2,5-diketopiperazine templates and enabled us to focus effort on those templates with the greatest chance of high bioavailability in humans. This rapidly led to the 2',4'-difluorophenyl-dimethylamide 25 and the benzofuran 4 with high levels of potency (pK(i)) and good bioavailability in the rat and dog. Dimethylamide 25 is more potent (>20-fold) than 4 in vivo and has a high degree of selectivity toward the vasopressin receptors, >10,000 for hV1a/hV1b and approximately 500 for hV2. It has a good Cyp450 profile with no time dependent inhibition and was negative in the genotoxicity screens with a satisfactory oral safety profile in rats.

Synthesis of (Z)-Alkene and (E)-Fluoroalkene-Containing Diketopiperazine Mimetics Utilizing Organocopper-Mediated Reduction−Alkylation and Diastereoselectivity Examination Using DFT Calculations

We have carefully examined the organocopper-mediated reduction-alkylation of gamma-acetoxy or gamma,gamma-difluoro-alpha,beta-unsaturated-delta-lactams for the synthesis of (Z)-alkene- or (E)-fluoroalkene-containing diketopiperazine mimetics. Reduction of acetates 2, 12, 14, and difluorolactam 18 with higher-order cuprate reagents (Me3CuLi2 x LiI x 3 LiBr), followed by trapping the resulting metal dienolate with an electrophile in a one-pot procedure gave alpha-alkylated-beta,gamma-unsaturated-delta-lactams in good yields. Because of side-chain steric repulsion, we found that alkylation using relatively large electrophiles such as BnBr gave mostly 3,6-trans isomers by kinetic trapping of metal enolates. On the other hand, MeI-mediated alkylations predominantly provided the unexpected 3,6-cis isomers despite the presence of a bulky benzyl side chain. Based on density functional theory calculations, we concluded that formation of the 3,6-cis isomers was due to the occurrence of oxa-pi-allyllithium complexes 29 and 31.

Stereoselective Synthesis of 3,6-Disubstituted-3,6-dihydropyridin-2-ones as Potential Diketopiperazine Mimetics Using Organocopper-Mediated anti-SN2‘ Reactions and Their Use in the Preparation of Low-Molecule CXCR4 Antagonists

Organocopper-mediated anti-SN2' reactions of gamma-phosphoryloxy-alpha,beta-unsaturated-delta-lactams were used to prepare highly functionalized diketopiperazine mimetics. The substrate phosphates 24, 32, and 47 were prepared from alpha-amino acid-derived allylic alcohols 10 by a sequence of reactions that included ring-closing metathesis. In the reactions of phosphates with organocopper reagents, the addition of LiCl dramatically improved anti-SN2' selectivity, indicating that an organocopper cluster containing lithium chloride plays an important role in the determination of regioselectivity. This reaction system was applied to the preparation of novel low molecular weight CXCR4-chemokine receptor antagonists.