Combinatorial chemistry: libraries from libraries, the art of the diversity-oriented transformation of resin-bound peptides and chiral polyamides to low molecular weight acyclic and heterocyclic compounds

Parallel Synthesis of Piperazine Tethered Thiazole Compounds with Antiplasmodial Activity

Thiazole and piperazine are two important heterocyclic rings that play a prominent role in nature and have a broad range of applications in agricultural and medicinal chemistry. Herein, we report the parallel synthesis of a library of diverse piperazine-tethered thiazole compounds. The reaction of piperazine with newly generated 4-chloromethyl-2-amino thiazoles led to the desired piperazine thiazole compounds with high purities and good overall yields. Using a variety of commercially available carboxylic acids, the parallel synthesis of a variety of disubstituted 4-(piperazin-1-ylmethyl)thiazol-2-amine derivatives is described. the screening of the compounds led to the identification of antiplasmodial compounds that exhibited interesting antimalarial activity, primarily against the Plasmodium falciparum chloroquine-resistant Dd2 strain. The hit compound 2291-61 demonstrated an antiplasmodial EC50 of 102 nM in the chloroquine-resistant Dd2 strain and a selectivity of over 140.

Polyheterocyclic peptidomimetics: Parallel solid phase synthesis of oligo cyclic guanidines and their inhibition activity against Mycobacterium tuberculosis DNA gyrase

Polyheterocycles are one of the most desired synthetic targets due to their numerous and valuable applications in various fields. We report the design and the parallel synthesis of novel linear oligocyclic guanidine peptidomimetics from predesigned reduced polyamides. A screening of these compounds identified active Mycobacterium tuberculosis DNA gyrase inhibitors which do not inhibit human DNA topoisomerase IIα and topoisomerase I.

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.

1,5-Disubstituted Acylated 2-Amino-4,5-dihydroimidazoles as a New Class of Retinoic Acid Receptor-Related Orphan Receptor (ROR) Inhibitors

A growing body of evidence suggests a pathogenic role for pro-inflammatory T helper 17 cells (Th17) in several autoimmune diseases, including multiple sclerosis, rheumatoid arthritis, inflammatory bowel disease, type I diabetes, and psoriasis-diseases for which no curative treatment is currently available. The nuclear retinoic acid receptor-related orphan receptors alpha and gamma (RORα/γ), in particular the truncated isoform RORγt that is specifically expressed in the thymus, play a critical role in the activation of a pro-inflammatory Th17 response, and RORγ inverse agonists have shown promise as negative regulators of Th17 for the treatment of autoimmune diseases. Our study underscores the screening of a large combinatorial library of 1,5-disubstituted acylated 2-amino-4,5-dihydroimidazoles using a demonstrated synthetic and screening approach and the utility of the positional scanning libraries strategy for the rapid identification of a novel class of ROR inhibitors. We identified compound 1295-273 with the highest activity against RORγ (3.3 µM IC₅₀) in this series, and almost a two-fold selectivity towards this receptor isoform, with 5.3 and 5.8 µM IC₅₀ against RORα and RORβ cells, respectively.

Solid-Phase Synthesis of Pyrrole Derivatives through a Multicomponent Reaction Involving Lys-Containing Peptides

The synthesis of pyrroles has received considerable attention because of their biological and pharmaceutical activities. Herein we describe a solid-phase multicomponent reaction that utilizes Lys as a N donor, β-nitrostyrenes, 1,3-dicarbonyl compounds, and FeCl₃ as an easily accessible catalyst under microwave irradiation to afford the subsequent pyrrole derivatives in high conversions. The strategy combines three of the most powerful tools in modern synthetic chemistry: the solid-phase mode, microwave activation, and a multicomponent reaction. The excellent results in terms of rapidity, versatility, and purity obtained herein support once again that this combined strategy is efficient for gaining chemical diversity.

Identification of a small molecule inhibitor of the aminoglycoside 6'-N-acetyltransferase type Ib [AAC(6')-Ib] using mixture-based combinatorial libraries

The aminoglycoside, 6'-N-acetyltransferase type Ib [AAC(6')-Ib] is the most widely distributed enzyme among AAC(6')-I-producing Gram-negative pathogens and confers resistance to clinically relevant aminoglycosides, including amikacin. This enzyme is therefore an ideal target for enzymatic inhibitors that could overcome resistance to aminoglycosides. The search for inhibitors was carried out using mixture-based combinatorial libraries, the scaffold ranking approach, and the positional scanning strategy. A library with high inhibitory activity had pyrrolidine pentamine scaffold and was selected for further analysis. This library contained 738,192 compounds with functionalities derived from 26 different amino acids (R1, R2 and R3) and 42 different carboxylic acids (R4) in four R-group functionalities. The most active compounds all contained S-phenyl (R1 and R3) and S-hydromethyl (R2) functionalities at three locations and differed at the R4 position. The compound containing 3-phenylbutyl at R4 (compound 206) was a robust enzymatic inhibitor in vitro, in combination with amikacin it potentiated the inhibition of growth of three resistant bacteria in culture, and it improved survival when used as treatment of Galleria mellonella infected with aac(6')-Ib-harboring Klebsiella pneumoniae and Acinetobacter baumannii strains.

A C-H Oxidation/Two-Fold Cyclization Approach to Imidazopyridoindole Scaffold under Mild Oxidizing Conditions

An expeditious one-step synthesis of the imidazopyridoindole scaffold was achieved through the C-H oxidation/two-fold cyclization reaction of methyl ketone and tryptamine derivatives. Mild oxidizing conditions were employed to realize the efficient oxidation of C(sp³)-H bonds, while suppressing overoxidation of the intermediate and ensuring the cross-trapping of two in situ generated acylimine intermediates.

Direct Phenotypic Screening in Mice: Identification of Individual, Novel Antinociceptive Compounds from a Library of 734,821 Pyrrolidine Bis-piperazines

The hypothesis in the current study is that the simultaneous direct in vivo testing of thousands to millions of systematically arranged mixture-based libraries will facilitate the identification of enhanced individual compounds. Individual compounds identified from such libraries may have increased specificity and decreased side effects early in the discovery phase. Testing began by screening ten diverse scaffolds as single mixtures (ranging from 17,340 to 4,879,681 compounds) for analgesia directly in the mouse tail withdrawal model. The "all X" mixture representing the library TPI-1954 was found to produce significant antinociception and lacked respiratory depression and hyperlocomotor effects using the Comprehensive Laboratory Animal Monitoring System (CLAMS). The TPI-1954 library is a pyrrolidine bis-piperazine and totals 738,192 compounds. This library has 26 functionalities at the first three positions of diversity made up of 28,392 compounds each (26 × 26 × 42) and 42 functionalities at the fourth made up of 19,915 compounds each (26 × 26 × 26). The 120 resulting mixtures representing each of the variable four positions were screened directly in vivo in the mouse 55 °C warm-water tail-withdrawal assay (ip administration). The 120 samples were then ranked in terms of their antinociceptive activity. The synthesis of 54 individual compounds was then carried out. Nine of the individual compounds produced dose-dependent antinociception equivalent to morphine. In practical terms what this means is that one would not expect multiexponential increases in activity as we move from the all-X mixture, to the positional scanning libraries, to the individual compounds. Actually because of the systematic formatting one would typically anticipate steady increases in activity as the complexity of the mixtures is reduced. This is in fact what we see in the current study. One of the final individual compounds identified, TPI 2213-17, lacked significant respiratory depression, locomotor impairment, or sedation. Our results represent an example of this unique approach for screening large mixture-based libraries directly in vivo to rapidly identify individual compounds.

Carbohydrates in diversity-oriented synthesis: challenges and opportunities

Carbohydrates are attractive building blocks for diversity-oriented synthesis due to their stereochemical diversity and high density of polar functional groups.

Parallel Synthesis of Hexahydrodiimidazodiazepines Heterocyclic Peptidomimetics and Their in Vitro and in Vivo Activities at μ (MOR), δ (DOR), and κ (KOR) Opioid Receptors

In the development of analgesics with mixed-opioid agonist activity, peripherally selective activity is expected to decrease side effects, minimizing respiratory depression and reinforcing properties generating significantly safer analgesic therapeutics. We synthesized diazaheterocyclics from reduced tripeptides. In vitro screening with radioligand competition binding assays demonstrated variable affinity for μ (MOR), δ (DOR), and κ (KOR) opioid receptors across the series, with the diimidazodiazepine 14 (2065-14) displaying good affinity for DOR and KOR. Central (icv), intraperitoneal (ip), or oral (po) administration of 14 produced dose-dependent, opioid-receptor mediated antinociception in the mouse, as determined from a 55 °C warm-water tail-withdrawal assay. Only trace amounts of compound 14 was found in brain up to 90 min later, suggesting poor BBB penetration and possible peripherally restricted activity. Central administration of 14 did not produce locomotor effects, acute antinociceptive tolerance, or conditioned-place preference or aversion. The data suggest these diazaheterocyclic mixed activity opioid receptor agonists may hold potential as new analgesics with fewer liabilities of use.

Combinatorial Libraries As a Tool for the Discovery of Novel, Broad-Spectrum Antibacterial Agents Targeting the ESKAPE Pathogens

Mixture based synthetic combinatorial libraries offer a tremendous enhancement for the rate of drug discovery, allowing the activity of millions of compounds to be assessed through the testing of exponentially fewer samples. In this study, we used a scaffold-ranking library to screen 37 different libraries for antibacterial activity against the ESKAPE pathogens. Each library contained between 10000 and 750000 structural analogues for a total of >6 million compounds. From this, we identified a bis-cyclic guanidine library that displayed strong antibacterial activity. A positional scanning library for these compounds was developed and used to identify the most effective functional groups at each variant position. Individual compounds were synthesized that were broadly active against all ESKAPE organisms at concentrations <2 μM. In addition, these compounds were bactericidal, had antibiofilm effects, showed limited potential for the development of resistance, and displayed almost no toxicity when tested against human lung cells and erythrocytes. Using a murine model of peritonitis, we also demonstrate that these agents are highly efficacious in vivo.

High-Throughput Synthesis of Diverse Compound Collections for Lead Discovery and Optimization

Small-molecule intervention of protein function is one central dogma of drug discovery. The generation of small-molecule libraries fuels the discovery pipeline at many stages and thereby resembles a key aspect of this endeavor. High-throughput synthesis is a major source for compound libraries utilized in academia and industry, seeking new chemical modulators of pharmacological targets. Here, we discuss the crucial factors of library design strategies from the perspective of synthetic chemistry, giving a brief historic background and a summary of current approaches. Simple measures of success of a high-throughput synthesis such as quantity or diversity have long been discarded and replaced by more integrated measures. Case studies are presented and put into context to highlight the cross-connectivity of the various stages of the drug discovery process.

Combinatorial synthesis of oxazol-thiazole bis-heterocyclic compounds

A combinatorial library of novel oxazol-thiazole bis-heterocycles was synthesized in good to excellent overall yields with high purity using a solution and solid-phase parallel synthesis approach. Oxazole amino acids, prepared from serine methyl ester and amino acids via coupling and cyclodehydration, were treated with Fmoc-NCS and α-haloketones for the parallel synthesis of diverse bis-heterocycles. Fmoc-isothiocyanate is used as a traceless reagent for thiazole formation. Oxazole diversity can be achieved by using variety of amino acids, whereas thiazole diversity is produced with various haloketones.

A peptide & peptide nucleic acid synthesis technology for transporter molecules and theranostics--the SPPS

Advances in imaging diagnostics using magnetic resonance tomography (MRT), positron emission tomography (PET) and fluorescence imaging including near infrared (NIR) imaging methods are facilitated by constant improvement of the concepts of peptide synthesis. Feasible patient-specific theranostic platforms in the personalized medicine are particularly dependent on efficient and clinically applicable peptide constructs. The role of peptides in the interrelations between the structure and function of proteins is widely investigated, especially by using computer-assisted methods. Nowadays the solid phase synthesis (SPPS) chemistry emerges as a key technology and is considered as a promising methodology to design peptides for the investigation of molecular pharmacological processes at the transcriptional level. SPPS syntheses could be carried out in core facilities producing peptides for large-scale scientific implementations as presented here.

Scaffold ranking and positional scanning utilized in the discovery of nAChR-selective compounds suitable for optimization studies

Nicotine binds to nicotinic acetylcholine receptors (nAChR), which can exist as many different subtypes. The α4β2 nAChR is the most prevalent subtype in the brain and possesses the most evidence linking it to nicotine seeking behavior. Herein we report the use of mixture based combinatorial libraries for the rapid discovery of a series of α4β2 nAChR selective compounds. Further chemistry optimization provided compound 301, which was characterized as a selective α4β2 nAChR antagonist. This compound displayed no agonist activity but blocked nicotine-induced depolarization of HEK cells with an IC50 of approximately 430 nM. 301 demonstrated nearly 500-fold selectivity for binding and 40-fold functional selectivity for α4β2 over α3β4 nAChR. In total over 5 million compounds were assessed through the use of just 170 samples in order to identify a series of structural analogues suitable for future optimization toward the goal of developing clinically relevant smoking cessation medications.

Antimalarial and cytotoxic activities of chiral triamines

Chiral triamine antimalarial compounds have been identified following the screening of mixture-based positional scanning libraries made up of 31,320 compounds against P. falciparum. The library, namely N-methyl triamine (TPI 762) was generated following exhaustive reduction of resin-bound acylated dipeptides. Using the PSCL approach, individual compounds were rapidly identified which were only 10 times less active than the standard drugs chloroquine (CQ) and Artemisinin (Artes).

Libraries from Libraries: A Series of Sulfonamide Linked Heterocycles Derived from the Same Scaffold

A libraries from libraries approach is described for the synthesis of five different sulfonamide linked scaffolds. Four of the scaffolds are sulfonamides linked to heterocycles; piperazine, thiourea, cyclic guanidine, and dimethyl cyclic guanidine. The fifth scaffold is a polyamine linked sulfonamide. Three different diversity positions were effectively incorporated into each scaffold providing a number of different compounds with good yields and purity.

Selective agonists and antagonists of formylpeptide receptors: duplex flow cytometry and mixture-based positional scanning libraries

The formylpeptide receptor (FPR1) and formylpeptide-like 1 receptor (FPR2) are G protein-coupled receptors that are linked to acute inflammatory responses, malignant glioma stem cell metastasis, and chronic inflammation. Although several N-formyl peptides are known to bind to these receptors, more selective small-molecule, high-affinity ligands are needed for a better understanding of the physiologic roles played by these receptors. High-throughput assays using mixture-based combinatorial libraries represent a unique, highly efficient approach for rapid data acquisition and ligand identification. We report the superiority of this approach in the context of the simultaneous screening of a diverse set of mixture-based small-molecule libraries. We used a single cross-reactive peptide ligand for a duplex flow cytometric screen of FPR1 and FPR2 in color-coded cell lines. Screening 37 different mixture-based combinatorial libraries totaling more than five million small molecules (contained in 5,261 mixture samples) resulted in seven libraries that significantly inhibited activity at the receptors. Using positional scanning deconvolution, selective high-affinity (low nM K(i)) individual compounds were identified from two separate libraries, namely, pyrrolidine bis-diketopiperazine and polyphenyl urea. The most active individual compounds were characterized for their functional activities as agonists or antagonists with the most potent FPR1 agonist and FPR2 antagonist identified to date with an EC₅₀ of 131 nM (4 nM K(i)) and an IC₅₀ of 81 nM (1 nM K(i)), respectively, in intracellular Ca²⁺ response determinations. Comparative analyses of other previous screening approaches clearly illustrate the efficiency of identifying receptor selective, individual compounds from mixture-based combinatorial libraries.

Parallel Synthesis of Structurally Diverse Aminobenzimidazole Tethered Sultams and Benzothiazepinones

A solid-phase methodology to construct aminobenzimidazole tethered sultams and benzothiazepinones from commercial amino acids, amines, carboxylic acids and sulfonyl chlorides is described. Coupling of Fmoc-Cys(Trt)-OH to resin-bound aminobenzimidazole scaffold provided an essential precursor for the construction of a variety of seven membered benzofused cyclic sulfonamides and thiazepinones via palladium catalyzed Buchwald-Hartwig type intramolecular cyclization.

A novel method for the determination of isokinetic ratios and its application in the synthesis of two new positional scanning libraries

A novel method for the direct evaluation of the equimolarity of the compounds contained in a mixture is presented. We applied the method toward calculating isokinetic ratios for the reaction between the amine termini of a resin bound peptide fragment and a sulfonyl chloride to produce equal molar mixtures of sulfonamides. The results of this study and the application of the method to the synthesis of two new positional scanning synthetic combinatorial libraries (PS-SCL) are discussed.

Solid Phase Synthesis of Isoxazole and Isoxazoline-carboxamides via [2+3]-Dipolar Cycloaddition Using Resin-bound Alkynes or Alkenes

An efficient approach for the parallel solid phase synthesis of isoxazole and isoxazoline derivatives has been developed. The isoxazoles and isoxazolines were constructed through a 1,3-dipolar cycloaddition reaction of nitrile oxides, with resin-bound alkynes or alkenes. The cycloaddition reaction conditions performed on solid phase supports was optimized, and an array of resin bound carboxylic acid building blocks were utilized for distinct conversions. This methodology presents a new alternative to the diversity oriented synthesis of disubstituted isoxazoles and isoxazolines different from existing routes which are limited in structural diversity.

Increased diversity of libraries from libraries: chemoinformatic analysis of bis-diazacyclic libraries

Combinatorial libraries continue to play a key role in drug discovery. To increase structural diversity, several experimental methods have been developed. However, limited efforts have been performed so far to quantify the diversity of the broadly used diversity-oriented synthetic libraries. Herein, we report a comprehensive characterization of 15 bis-diazacyclic combinatorial libraries obtained through libraries from libraries, which is a diversity-oriented synthetic approach. Using MACCS keys, radial and different pharmacophoric fingerprints as well as six molecular properties, it was demonstrated the increased structural and property diversity of the libraries from libraries over the individual libraries. Comparison of the libraries to existing drugs, NCI diversity, and the Molecular Libraries Small Molecule Repository revealed the structural uniqueness of the combinatorial libraries (mean similarity <0.5 for any fingerprint representation). In particular, bis-cyclic thiourea libraries were the most structurally dissimilar to drugs retaining drug-like character in property space. This study represents the first comprehensive quantification of the diversity of libraries from libraries providing a solid quantitative approach to compare and contrast the diversity of diversity-oriented synthetic libraries with existing drugs or any other compound collection.

Stannylated allyl carbonates as versatile building blocks for the diversity oriented synthesis of allylic amines and amides

Stannylated allylic carbonates are suitable substrates for Pd-catalyzed allylic aminations. In DMF and with [allylPdCl](2) as catalyst the stannylated allyl amines formed can be directly coupled with electrophiles according to the Stille protocol, giving rise to highly functionalized building blocks in excellent yields.

Brominated extracts as source of bioactive compounds

The chemical composition and the biomolecular properties of a crude plant extract were altered through bromination leading to the discovery of an acetylcholinesterase inhibitor.

Inhibitor screening of proprotein convertases using positional scanning libraries

Proprotein convertases represent an important class of biosynthetic enzymes that are increasingly viewed as targets for therapeutic approaches to infection, cancer, and potentially endocrine disorders. The identification of potent inhibitors can be accomplished by screening synthetic combinatorial libraries containing thousands of small molecules to millions of peptides. In this chapter, the screening of positional scanning libraries is described for the identification of PC1/3 and PC2 inhibitors.

Synthesis and application of isocyanates radiolabeled with carbon-11

Carbon-11 labeled isocyanates are efficiently prepared by dehydration of [(11) C]carbamate salts, which in turn are easily formed from cyclotron-produced [(11) C]CO(2) and amines in the presence of a CO(2) fixation agent. The [(11) C]isocyanates are useful radiosynthons for the synthesis of a variety of [carbonyl-(11) C]-labeled asymmetrical ureas and carbamate esters. The method is well suited to incorporate any isotope of carbon, and is especially useful for positron emission tomography (PET) radiotracers for in vivo imaging. This is demonstrated by using the method to make [carbonyl-(11) C]-6-hydroxy-[1,1'-biphenyl]-3-yl cyclohexylcarbamate which is a novel radiotracer for PET imaging of fatty acid amide hydrolase.

Traceless synthesis of diketopiperazine fused tetrahydro-β-carbolines on soluble polymer support

The Pictet-Spengler reaction, using polyethylene glycol immobilized tryptophan ester with a variety of ketones, was achieved by refluxing condition in acidic chloroform. The linear as well as cyclic ketones were employed. All the ketones were reacted within 6-8 h to furnish soluble polymer-supported tetrahydro-β-carboline in good yields. Further expansion at N-terminus of tetrahydro-β-carbolines was achieved through a reaction with chloroacetyl chloride. Finally, the 2,5-diketopiperazine skeleton was constructed over a β-carboline by amination of the resulting N-chloroacetamides and subsequent intramolecular cyclization leading to cleavage of the polymer; constitutes a traceless synthesis of tetracyclic molecular architecture. Significantly, this strategy affords a straightforward and efficient approach for the construction of biological promising molecules with high purity and good yields.

Identification of two novel, potent, low-liability antinociceptive compounds from the direct in vivo screening of a large mixture-based combinatorial library

Synthetic combinatorial methods now make it practical to readily produce hundreds of thousands of individual compounds, but it is clearly impractical to screen each separately in vivo. We theorized that the direct in vivo testing of mixture-based combinatorial libraries during the discovery phase would enable the identification of novel individual compounds with desirable antinociceptive profiles while simultaneously eliminating many compounds with poor absorption, distribution, metabolism, or pharmacokinetic properties. The TPI 1346 small-molecule combinatorial library is grouped in 120 mixtures derived from 26 functionalities at the first three positions and 42 functionalities at the fourth position of a pyrrolidine bis-cyclic guanidine core scaffold, totaling 738,192 compounds. These 120 mixtures were screened in vivo using the mouse 55 degrees C warm water tail-withdrawal assay to identify mixtures producing antinociception. From these data, two fully defined individual compounds (TPI 1818-101 and TPI 1818-109) were synthesized. These were examined for antinociceptive, respiratory, locomotor, and conditioned place preference effects. The tail-withdrawal assay consistently demonstrated distinctly active mixtures with analgesic activity that was blocked by pretreatment with the non-selective opioid antagonist, naloxone. Based on these results, synthesis and testing of TPI 1818-101 and 1818-109 demonstrated a dose-dependent antinociceptive effect three to five times greater than morphine that was antagonized by mu- or mu- and kappa-opioid receptor selective antagonists, respectively. Neither 1818-101 nor 1818-109 produced significant respiratory depression, hyperlocomotion, or conditioned place preference. Large, highly diverse mixture-based libraries can be screened directly in vivo to identify individual compounds, potentially accelerating the development of promising therapeutics.

Identification, structure-activity relationships and molecular modeling of potent triamine and piperazine opioid ligands

Opioid receptors are important targets for pain management. Here, we report the synthesis and biological evaluation of three positional scanning combinatorial libraries, consisting of linear triamines and piperazines. A highly potent (14 nM) and selective (IC(50(mu))/IC(50(kappa))=71; IC(50(delta))/IC(50(kappa))=714) triamine for the kappa-opioid receptor was found. In addition, non-selective mu-kappa binders were obtained, with binding affinities of 54 nM and 22 nM for mu- and kappa-opioid receptors, respectively. Structure-activity relationships of each subset are described. 3D molecular alignments based on shape similarity to internal and external query molecules were carried out. For the combinatorial chemistry dataset studied here a 1.3 similarity cut-off value was observed to be efficient in the rocs-based alignment method. Interactions from the overlays analyzed in the binding sites of homology models of the receptors revealed specific substitution patterns for enhancing binding affinity in the piperazine series. Pharmacophore modeling of the compounds found from the three combinatorial libraries was also performed. The pharmacophore model indicated that the important feature for receptor binding activity with the mu-receptor was the presence of at least one hydrogen bond acceptor and one aromatic hydrophobic group. Whereas for the kappa-receptor two binding modes emerged with one set of compounds employing the hydrogen bond acceptor and aromatic hydrophobic group, and a second set possibly via interactions with the receptor by hydrophobic and ionic salt-bridges.

Microwave-Assisted Synthesis of Tetracyclic 2,5-Diketopiperazines on a Soluble Polymer Support: A Structural Analogue of Tadalafil

Structural analogues of tadalafil that contain two diversity points have been synthesized from a soluble polymer support employing a Pictet–Spengler reaction using focussed microwave irradiation. Polymer-bound deprotected tryptophan reacts with various aldehydes to generate the tetrahydro-β-carbolines on the support. Subsequently immobilized tetrahydro-β-carboline underwent a highly efficient intramolecular N-heterocyclization in a traceless fashion from various in-situ generated α-alkyl and heteroalkyl amides in two steps to generate tetracyclic 2,5-diketopiperazines in high purity. All the compounds were isolated as cis and trans isomers with good yields.

Application of natural product-inspired diversity-oriented synthesis to drug discovery

Natural products have played a critical role in the identification of numerous medicines. Synthetic organic chemistry and combinatorial chemistry strategies such as diversity-oriented synthesis (DOS) have enabled the synthesis of natural product-like compounds. The combination of these approaches has both improved the desired biological properties of natural products as well as the identification of novel compounds. Diversity concepts and strategies to access novel compounds inspired by natural products will be reviewed.