Recent advances in the synthesis of purine derivatives and their precursors

Visible Light-Mediated Late-Stage Thioetherification of Mercaptopurine Derivatives

We disclose herein a novel and general radical approach to alkylthiopurines, encompassing 4 types of thiopurines, as well as their corresponding ribosides. This strategy is achieved through visible light-mediated late-stage functionalization of the sulfur atoms of mercaptopurines. The in situ-generated disulfide was proposed as the pivotal neutral intermediate for this transformation. We present herein a novel photo-mediated homolytic C-S bond formation for the preparation of alkylthiopurines and alkylthiopurine nucleosides. Despite the presence of reactive sites for the Minisci reaction, chemoselective S-alkylation remained the predominant pathway. This method allows for the late-stage introduction of a broad spectrum of alkyl groups onto the sulfur atom of unprotective mercaptopurine derivatives, encompassing 2-, 6-, and 8-mercaptopurine rings. Organoborons serve as efficient and eco-friendly alkylating reagents, providing advantages in terms of readily availability, stability, and reduced toxicity. Further derivatization of the thioetherified nucleosides, together with anti-tumor assays, led to the discovery of potent anti-tumor agents with an IC50 value reaching 6.1 μM (Comp. 31 for Jurkat).

2,4,5-Triaminopyrimidines as blue fluorescent probes for cell viability monitoring: synthesis, photophysical properties, and microscopy applications

Monitoring cell viability is critical in cell biology, pathology, and drug discovery. Most cell viability assays are cell-destructive, time-consuming, expensive, and/or hazardous. Herein, we present a series of newly synthesized 2,4,5-triaminopyrimidine derivatives able to discriminate between live and dead cells. To our knowledge, these compounds are the first fluorescent nucleobase analogues (FNAs) with cell viability monitoring potential. These new fluorescent molecules are synthesized using highly efficient and cost-effective methods and feature unprecedented photophysical properties (longer absorption and emission wavelengths, environment-sensitive emission, and unprecedented brightness within FNAs). Using a live-dead Saccharomyces cerevisiae cell and theoretical assays, the fluorescent 2,4,5-triaminopyrimidine derivatives were found to specifically accumulate inside dead cells by interacting with dsDNA grooves, thus paving the way for the emergence of novel and safe fluorescent cell viability markers emitting in the blue region. As the majority of commercially available viability dyes emit in the green to red region of the visible spectrum, these novel markers might be useful to meet the needs of blue markers for co-staining combinations.

Copper Catalyzed Three-Component Ullmann C-S Coupling in PEG for the Synthesis of 6-Aryl/alkylthio-purines

To tackle the environmental unfriendly issue in existing synthesis strategies for 6-substitued thiopurine derivatives, such as poor step economy, frequent use of malodorous organic sulfur starting materials, toxic organic solvents, and equivalent dosage of base, we have developed a CuI-catalyzed base-free three-component Ullmann C-S coupling synthetic strategy, featured using inorganic salt Na2S as the sulfur source and nontoxic PEG-600 as the solvent. The newly developed strategy is particularly effective for the synthesis of 6-arylthiopurines. The high catalytic efficiency in PEG-600 can be rationalized by the high soluble ability of CuI catalyst, likely due to the presence of multiple oxygen coordination sites in PEG.

Synthesis of 6,8-diaminopurines via acid-induced cascade cyclization of 5-aminoimidazole precursors and preliminary anticancer evaluation

Inspired by the pharmacological interest generated by 6-substituted purine roscovitine for cancer treatment, 5-aminoimidazole-4-carboxamidine precursors containing a cyanamide unit were prepared by condensation of 5-amino-N-cyanoimidazole-4-carbimidoyl cyanides with a wide range of primary amines. When these amidine precursors were combined with acids, a fast cascade cyclization occurred at room temperature, affording new 6,8-diaminopurines with the N-3 and N-6 substituents changed relatively to the original positions they occupied in the amidine and imidazole moieties of precursors. The efficacy and wide scope of this method was well demonstrated by an easy and affordable synthesis of 22 6,8-diaminopurines decorated with a wide diversity of substituents at the N-3 and N-6 positions of the purine ring. Preliminary in silico and in vitro assessments of these 22 compounds were carried out and the results showed that 13 of these tested compounds not only exhibited IC50 values between 1.4 and 7.5 μM against the colorectal cancer cell line HCT116 but also showed better binding energies than known inhibitors in docking studies with different cancer-related target proteins. In addition, good harmonization observed between in silico and in vitro results strengthens and validates this preliminary evaluation, suggesting that these novel entities are good candidates for further studies as new anticancer agents.

Signal-enhanced multi-core fiber-based WaveFlex biosensor for ultra-sensitive xanthine detection

In this work, we introduce a novel multimode fiber (MMF) - seven core fiber (SCF) - MMF (MCM) optical fiber biosensor, also known as the WaveFlex biosensor (plasma wave assisted fiber biosensor), based on localized surface plasmon resonance (LSPR) for qualitative detection of xanthine. Xanthine is a purine base widely distributed in human blood and tissues, and commonly used as an indicator for various disease detections. The MCM sensor incorporates a tapered optical fiber structure, fabricated using the combiner manufacturing system (CMS), and is designed with SCF and MMF. By effectively harnessing LSPR, the sensor boosts the attachment points of biomolecules on the probe surface through immobilized tungsten disulfide (WS2)-thin layers, gold nanoparticles (AuNPs), and carbon nitride quantum dots (C3N-QDs). The functionalization of xanthine oxidase (XO) on the sensing probe further enhances the sensor's specificity. The proposed WaveFlex biosensor exhibits a remarkable sensitivity of 3.2 nm/mM and a low detection limit of 96.75 µM within the linear detection range of 100 - 900 µM. Moreover, the sensor probe demonstrates excellent reusability, reproducibility, stability, and selectivity. With its sensitivity, biocompatibility, and immense potential for detecting human serum and fish products, this WaveFlex biosensor presents a promising platform for future applications.

Approaches to the synthesis of heterocyclic C-nucleosides

This review is focused on the synthetic strategies to heterocyclic C-nucleosides and covers the literature from 2011 to 2021. The main attention is paid to the following three approaches: the direct C-C coupling of a carbohydrate moiety with a preformed aglycon unit, the construction of a (pseudo)sugar residue on a pre-formed aglycon, and the construction of an aglycon on a pre-formed (pseudo)sugar. In each Section, the literature data are categorized in terms of the size of aglycon from simple to complex, the advantages and drawbacks of the reviewed approaches are discussed.

Regiospecific way to N9-alkylated thioxanthines

A regiospecific way to N9-alkylated thioxanthines as novel acyclic nucleoside analogues has been developed. This approach is based on a cleavage methodology involving the construction of a target heterocyclic scaffold...

Stereochemical aspects in the synthesis of novel N-(purin-6-yl)dipeptides as potential antimycobacterial agents

The synthesis of purine conjugates with natural amino acids is one of the promising directions in search for novel therapeutic agents, including antimycobacterial agents. The purpose of this study was to synthesize N-(purin-6-yl)dipeptides containing the terminal fragment of (S)-glutamic acid. To obtain the target compounds, two synthetic routes were tested. The first of them is based on coupling of N-(purin-6-yl)-(S)-amino acids to dimethyl (S)-glutamate in the presence of carbodiimide coupling agent followed by the removal of ester groups. However, it turned out that this coupling process was accompanied by racemization of the chiral center of N-(purin-6-yl)-α-amino acids and in all cases led to mixtures of (S,S)- and (R,S)-diastereomers (6:4). Individual (S,S)-diastereomers were obtained using an alternative approach based on the nucleophilic substitution of chlorine in 6-chloropurine or 2-amino-6-chloropurine with corresponding dipeptides as nucleophiles. The enantiomeric purity of the target compounds was confirmed by chiral HPLC. To test the assumption that racemization of the chiral center of N-(purin-6-yl)-α-amino acids occurs with the participation of nitrogen atoms of the imidazole ring via the stage of formation of a chirally labile intermediate, we obtained such structural analogs of N-(purin-6-yl)-(S)-alanine as N-(9-benzylpurin-6-yl)-(S)-alanine and N-(7-deazapurin-6-yl)-(S)-alanine. It was found that coupling of these compounds to dimethyl (S)-glutamate was also accompanied by racemization. This indicates that the imidazole fragment does not play a crucial role in this process. When testing the antimycobacterial activity of some of the obtained compounds, conjugates with moderate activity against the laboratory Mycobacterium tuberculosis H37Rv strain (MIC 3.1–6.25 μg/mL) were identified.

Design, Synthesis, and Biological Activity Studies of Istradefylline Derivatives Based on Adenine as A2A Receptor Antagonists

Due to its double bond, istradefylline rapidly isomerizes to Z-istradefylline when exposed to normal daylight in dilute solution. To solve the poor photostability of the istradefylline solution, a series of istradefylline derivatives (in total 17 compounds, including II-1 and II-2 series) were designed and synthesized, and their biological activity in inhibiting cAMP was evaluated. The IC50 values of compounds II-1-3, II-2-1, II-2-2, II-2-3, II-2-4, and II-2-6 were 7.71, 6.52, 6.16, 7.23, 7.96, and 9.68 μg/mL, respectively, which had the same order of activity as that of istradefylline (IC50 value was 1.94 μg/mL). The preliminary structure-activity relationship suggested that the 6-amino in adenine played an important role in binding an A2A receptor. The results of photostability experiments showed that the photostability of the target compounds of II-1 and II-2 series was improved when compared with that of istradefylline.

Coumarin hybrid derivatives as promising leads to treat tuberculosis: Recent developments and critical aspects of structural design to exhibit anti-tubercular activity

Tuberculosis (TB) is a highly contagious airborne disease with nearly 25% of the world's population infected with it. Challenges such as multi drug resistant TB (MDR-TB), extensive drug resistant TB (XDR-TB) and in rare cases totally drug resistant TB (TDR-TB) emphasizes the critical and urgent need in developing novel TB drugs. Moreover, the prolonged and multi drug treatment regime suffers a major drawback due to high toxicity and vulnerability in TB patients. This calls for intensified research efforts in identifying novel molecular scaffolds which can combat these issues with minimal side effects. In this pursuit, researchers have screened many bio-active molecules among which coumarin have been identified as promising candidates for TB drug discovery and development. Coumarins are naturally occurring compounds known for their low toxicity and varied biological activity. The biological spectrum of coumarin has intrigued medicinal researchers to investigate coumarin scaffolds for their relevance as anti-TB drugs. In this review we focus on the recent developments of coumarin and its critical aspects of structural design required to exhibit anti-tubercular (anti-TB) activity. The information provided will help medicinal chemists to design and identify newer molecular analogs for TB treatment and also broadens the scope of exploring future generation potent yet safer coumarin based anti-TB agents.

Biocatalytic routes to anti-viral agents and their synthetic intermediates

An assessment of biocatalytic strategies for the synthesis of anti-viral agents, offering guidelines for the development of sustainable production methods for a future COVID-19 remedy.

Fragment-based approach to novel bioactive purine derivatives

Using purine as a scaffold, the methods for preparation of novel 2-aminopurine and purine derivatives substituted at position C 6 by the fragments of natural amino acids, short peptides, and N-heterocycles, including enantiopure ones, have been proposed. The methods for determination of the enantiomeric purity of the obtained chiral compounds have been developed. Conjugates exhibiting high antimycobacterial or anti-herpesvirus activity against both laboratory and multidrug-resistant strains were revealed among the obtained compounds.

Synthesis of 2-(aminophenyl)adenine derivatives: a simple protocol using the classical iron powder/acetic acid reduction methodology

2-(Nitrophenyl)adenine derivatives were isolated from the reaction of 5-amino-4-amidino-imidazoles with nitrobenzaldehydes. The conversion of the nitro derivatives to 2-(aminophenyl)adenine derivatives was performed using iron/acetic acid as reducing agent, in 70% aqueous ethanol. The products were isolated in good yield and the isolation protocol involves simple filtration and extraction procedures. This methodology is compatible with the presence of functional groups such as amines, ethers, halofluorides, and halochlorides.

Synthesis and fluorescent properties of N(9)-alkylated 2-amino-6-triazolylpurines and 7-deazapurines

The synthesis of novel fluorescent N(9)-alkylated 2-amino-6-triazolylpurine and 7-deazapurine derivatives is described. A new C(2)-regioselectivity in the nucleophilic aromatic substitution reactions of 9-alkylated-2,6-diazidopurines and 7-deazapurines with secondary amines has been disclosed. The obtained intermediates, 9-alkylated-2-amino-6-azido-(7-deaza)purines, were transformed into the title compounds by CuAAC reaction. The designed compounds belong to the push-pull systems and possess promising fluorescence properties with quantum yields in the range from 28% to 60% in acetonitrile solution. Due to electron-withdrawing properties of purine and 7-deazapurine heterocycles, which were additionally extended by triazole moieties, the compounds with electron-donating groups showed intramolecular charge transfer character (ICT/TICT) of the excited states which was proved by solvatochromic dynamics and supported by DFT calculations. In the 7-deazapurine series this led to increased fluorescence quantum yield (74%) in THF solution. The compounds exhibit low cytotoxicity and as such are useful for the cell labelling studies in the future.

Crystal structures of N6-modified-amino acid related nucleobase analogs (II): hybrid adenine-β-alanine and adenine-GABA molecules

H-Bonding networks and anion–π interactions in the crystal structures of N⁶-modified-amino acid adenine analogs are investigated using X-ray crystallography and DFT calculations.

N-Heterocyclic Carbenes Derived from Guanosine: Synthesis and Evidences of Their Antiproliferative Activity

Palladium(II) and platinum(II) complexes bearing N-heterocyclic carbenes derived from guanosine are synthesized via oxidative addition, followed by protonation in the presence of acid. Cytotoxicity of the compounds is evaluated in several cell lines. Compounds 2a, 2b, and 3a are selective for glioblastoma U251 cells and are nontoxic toward healthy human embryonic kidney (HEK293) cells.

Computer-aided molecular design of pyrazolotriazines targeting glycogen synthase kinase 3

Numerous studies have highlighted the implications of the glycogen synthase kinase 3 (GSK-3) in several processes associated with Alzheimer's disease (AD). Therefore, GSK-3 has become a crucial therapeutic target for the treatment of this neurodegenerative disorder. Hereby, we report the design and multistep synthesis of ethyl 4-oxo-pyrazolo[4,3-d][1-3]triazine-7-carboxylates and their biological evaluation as GSK-3 inhibitors. Molecular modelling studies allow us to develop this new scaffold optimising the chemical structure. Potential binding mode determination in the enzyme and the analysis of the key features in the catalytic site are also described. Furthermore, the ability of pyrazolotriazinones to cross the blood-brain barrier (BBB) was evaluated by passive diffusion and those who showed great GSK-3 inhibition and permeation to the central nervous system (CNS) showed neuroprotective properties against tau hyperphosphorylation in a cell-based model. These new brain permeable pyrazolotriazinones may be used for key in vivo studies and may be considered as new leads for further optimisation for the treatment of AD.

Purine derivatives with antituberculosis activity

The review summarizes the data published over the last 10 – 15 years concerning the key groups of purine derivatives with antituberculosis activity. The structures of purines containing heteroatoms (S, O, N), fragments of heterocycles, amino acids and peptides, in the 6-position, as well as of purine nucleosides are presented. The possible targets for the action of such compounds and structure – activity relationship are discussed. Particular attention is paid to the most active compounds, which are of considerable interest as a basis for the development of efficient antituberculosis drugs.

The bibliography includes 99 references.

Marine Alkylpurines: A Promising Group of Bioactive Marine Natural Products

Marine secondary metabolites with a purine motif in their structure are presented in this review. The alkylpurines are grouped according to the size of the alkyl substituents and their location on the purine ring. Aspects related to the marine source, chemical structure and biological properties are considered together with synthetic approaches towards the natural products and bioactive analogues. This review contributes to studies of structure–activity relationships for these metabolites and highlights the potential of the sea as a source of new lead compounds in diverse therapeutic fields.

Synthesis of Lipophilic Guanine N-9 Derivatives: Membrane Anchoring of Nucleobases Tailored to Fatty Acid Vesicles

Covalent or noncovalent surface functionalization of soft-matter structures is an important tool for tailoring their function and stability. Functionalized surfaces and nanoparticles have found numerous applications in drug delivery and diagnostics, and new functionalization chemistry is continuously being developed in the discipline of bottom-up systems chemistry. The association of polar functional molecules, e.g., molecular recognition agents, with soft-matter structures can be achieved by derivatization with alkyl chains, allowing noncovalent anchoring into amphiphilic membranes. We report the synthesis of five new guanine-N9 derivatives bearing alkyl chains with different attachment chemistries, exploiting a synthesis pathway that allows a flexible choice of hydrophobic anchor moiety. In this study, these guanine derivatives were functionalized with C₁₀ chains for insertion into decanoic acid bilayer structures, in which both alkyl chain length and attachment chemistry determined their interaction with the membrane. Incubation of these guanine conjugates, as solids, with a decanoic acid vesicle suspension, showed that ether- and triazole-linked C₁₀ anchors yielded an increased partitioning of the guanine derivative into the membranous phase compared to directly N-9-linked saturated alkyl anchors. Decanoic acid vesicle membranes could be loaded with up to 5.5 mol % guanine derivative, a 6-fold increase over previous limits. Thus, anchor chemistries exhibiting favorable interactions with a bilayer's hydrophilic surface can significantly increase the degree of structure functionalization.

Synthesis of Main Chain Purine-Based Copolymers and Effects of Monomer Design on Thermal and Optical Properties

The ability to incorporate diverse monomeric building blocks enables the development of advanced polymeric materials possessing a wide range of properties that suits them for myriad applications. Herein, that synthetic toolbox is expanded through the first report of purine-based copolymers in which purines are incorporated directly into the polymer main chain. Stille cross-coupling of dibromopurine monomers with benzodithiophene (BDT) comonomers is used to generate these "poly(purine)s", and variations in the substitution pattern of the purine monomer and BDT side-chains provides insight into the role of monomer design on their resultant thermal and photophysical properties. Specifically, thermal analyses show that poly(purine)s exhibit high thermal stability and high glass transition temperatures depending on the BDT side-chain substituents and substitution pattern of the purine-derived comonomer. Furthermore, optical properties measured via UV-vis and fluorescence spectroscopies show dependence on monomer substitution pattern. These findings demonstrate the viability of synthesizing poly(purine)s via metal-catalyzed cross-coupling reactions and highlight the potential to tailor poly(purine) properties via simple alterations of comonomers.

Discovery of 7-(Prolinol-N-yl)-2-phenylamino-thiazolo[5,4-d]pyrimidines as Novel Non-Nucleoside Partial Agonists for the A2A Adenosine Receptor: Prediction from Molecular Modeling

We describe the identification of 7-(prolinol-N-yl)-2-phenylamino-thiazolo[5,4-d]pyrimidines as a novel chemotype of non-nucleoside partial agonists for the A2A adenosine receptor (A2AAR). Molecular-modeling indicated that the (S)-2-hydroxymethylene-pyrrolidine could mimic the interactions of agonists' ribose, suggesting that this class of compounds could have agonistic properties. This was confirmed by functional assays on the A2AAR, where their efficacy could be associated with the presence of the 2-hydroxymethylene moiety. Additionally, the best compound displays promising affinity, selectivity profile, and physicochemical properties.

Crystal structures of five 6-mercaptopurine derivatives

The crystal structures of five 6-mercaptopurine derivatives, viz. 2-[(9-acetyl-9H-purin-6-yl)sulfan-yl]-1-(3-meth-oxy-phen-yl)ethan-1-one (1), C16H14N4O3S, 2-[(9-acetyl-9H-purin-6-yl)sulfan-yl]-1-(4-meth-oxy-phen-yl)ethan-1-one (2), C16H14N4O3S, 2-[(9-acetyl-9H-purin-6-yl)sulfan-yl]-1-(4-chloro-phen-yl)ethan-1-one (3), C15H11ClN4O2S, 2-[(9-acetyl-9H-purin-6-yl)sulfan-yl]-1-(4-bromo-phen-yl)ethan-1-one (4), C15H11BrN4O2S, and 1-(3-meth-oxy-phen-yl)-2-[(9H-purin-6-yl)sulfan-yl]ethan-1-one (5), C14H12N4O2S. Compounds (2), (3) and (4) are isomorphous and accordingly their mol-ecular and supra-molecular structures are similar. An analysis of the dihedral angles between the purine and exocyclic phenyl rings show that the mol-ecules of (1) and (5) are essentially planar but that in the case of the three isomorphous compounds (2), (3) and (4), these rings are twisted by a dihedral angle of approximately 38°. With the exception of (1) all mol-ecules are linked by weak C-H⋯O hydrogen bonds in their crystals. There is π-π stacking in all compounds. A Cambridge Structural Database search revealed the existence of 11 deposited compounds containing the 1-phenyl-2-sulfanyl-ethanone scaffold; of these, only eight have a cyclic ring as substituent, the majority of these being heterocycles.

Synthesis of novel N-9 substituted 6-(4-(4-propoxyphenyl)piperazin-1-yl)-9H-purine derivatives as inducers of apoptosis in MCF-7 breast cancer cells

A series of N-9 substituted 6-(4-(4-propoxyphenyl)piperazin-1-yl)-9H-purine derivatives (PP05–PP21) were prepared and evaluated for their anticancer activity against a panel of human cancer cell lines.

New developments in direct functionalization of C–H and N–H bonds of purine bases via metal catalyzed cross-coupling reactions

This review provides a concise overview on the cross-coupling reactions in direct functionalization of purine bases in recent years.