6-substituted Purine Research Articles

Study of Direct N7 Regioselective tert-Alkylation of 6-Substituted Purines and Their Modification at Position C6 through O, S, N, and C Substituents.

A new N7 direct regioselective method allowing the introduction of tert-alkyl groups into appropriate 6-substituted purine derivatives is developed. This method is based on a reaction of N-trimethylsilylated purines with a tert-alkyl halide using SnCl4 as a catalyst. In this work, we study the structure and optimal reaction conditions leading to the N7 isomer and in some cases also to the N9 isomer. The main goal is devoted to preparing 7-(tert-butyl)-6-chloropurine as a suitable compound for other purine transformations. The stability of the tert-butyl group at the N7 position is tested for classic model reactions, leading to the preparation of new 6,7-disubstituted purine derivatives, which is also interesting from the point of view of possible biological activity.

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.

Abundant extraterrestrial purine nucleobases in the Murchison meteorite: Implications for a unified mechanism for purine synthesis in carbonaceous chondrite parent bodies

Extraterrestrial nucleobases are of key interest owing to their implications for the chemical evolution of primordial molecules in the solar system and their potential contributions to prebiotic chemistry on the early Earth. Recent advances in analytical techniques capable of detecting femto-mole-order nucleobases have allowed us to identify all five exogenous nucleobases—uracil, cytosine, thymine, adenine, and guanine—in carbonaceous chondrites and to quantify uracil obtained from the carbonaceous asteroid (162173) Ryugu. In this study, we optimized the analytical method using hot-water extraction followed by 6 M hydrochloric acid (HCl) extraction from a sample of the CM2 Murchison meteorite to reassess the abundances and distributions of extraterrestrial nucleobases. The target analyses performed using high-performance liquid chromatography paired with electrospray-ionization, high-resolution mass spectrometry revealed that purine nucleobases were substantially more enriched in the subsequent 6 M HCl extract (951 ± 104 parts per billion, ppb) than in the hot-water extract (199 ± 3 ppb). The most abundant nucleobase was guanine (649 ± 103 ppb in total). The 6 M HCl extract not only contained canonical nucleobases but also included rare nucleobase analogs from the purine family, such as 8-substituted purines. Unlike purines, we preferentially detected pyrimidine nucleobases in the hot-water extract and the acid hydrolysate (185 ± 17 ppb and 297 ± 5 ppb, respectively) rather than in the 6 M HCl extract (51 ± 4 ppb). The disparate distributions of purine and pyrimidine bases in the Murchison meteorite suggests that purines are more robustly incorporated than pyrimidines into meteoritic matrices and/or insoluble macromolecular organic matter. We propose a unified formation mechanism for purine nucleobases—which involves the synthesis of 5-aminoimidazole derivatives from hydrogen cyanide and its related molecules—that can account for the molecular distributions of the extraterrestrial purine nucleobases found in the Murchison meteorite.

Hydrogen-Bonding Interactions of 8-Substituted Purine Derivatives.

Hydrogen bonding between nucleobases is a crucial noncovalent interaction for life on Earth. Canonical nucleobases form base pairs according to two main geometries: Watson-Crick pairing, which enables the static functions of nucleic acids, such as the storing of genetic information; and Hoogsteen pairing, which facilitates the dynamic functions of these biomacromolecules. This precisely tuned system can be affected by oxidation or substitution of nucleobases, leading to changes in their hydrogen-bonding patterns. This paper presents an investigation into the intermolecular interactions of various 8-substituted purine derivatives with their hydrogen-bonding partners. The systems were analyzed using nuclear magnetic resonance spectroscopy and density functional theory calculations. Our results demonstrate that the stability of hydrogen-bonded complexes, or base pairs, depends primarily on the number of intermolecular H-bonds and their donor-acceptor alternation. No strong preferences for a particular geometry, either Watson-Crick or Hoogsteen, were found.

Interactions of 2,6-substituted purines with purine nucleoside phosphorylase from Helicobacter pylori in solution and in the crystal, and the effects of these compounds on cell cultures of this bacterium

Helicobacter pylori represents a global health threat with around 50% of the world population infected. Due to the increasing number of antibiotic-resistant strains, new strategies for eradication of H. pylori are needed. In this study, we suggest purine nucleoside phosphorylase (PNP) as a possible new drug target, by characterising its interactions with 2- and/or 6-substituted purines as well as the effect of these compounds on bacterial growth. Inhibition constants are in the micromolar range, the lowest being that of 6-benzylthio-2-chloropurine. This compound also inhibits H. pylori 26695 growth at the lowest concentration. X-ray structures of the complexes of PNP with the investigated compounds allowed the identification of interactions of inhibitors in the enzyme’s base-binding site and the suggestion of structures that could bind to the enzyme more tightly. Our findings prove the potential of PNP inhibitors in the design of drugs against H. pylori.

Diversity-Oriented Synthesis of 2-Substituted Purine Nucleosides from Available Nucleosides via the Late-Stage Nitration/Derivatization

Diversity-Oriented Synthesis of 2-Substituted Purine Nucleosides from Available Nucleosides via the Late-Stage Nitration/Derivatization

Novel Bis- and Mono-Pyrrolo[2,3-d]pyrimidine and Purine Derivatives: Synthesis, Computational Analysis and Antiproliferative Evaluation.

Novel symmetrical bis-pyrrolo[2,3-d]pyrimidines and bis-purines and their monomers were synthesized and evaluated for their antiproliferative activity in human lung adenocarcinoma (A549), cervical carcinoma (HeLa), ductal pancreatic adenocarcinoma (CFPAC-1) and metastatic colorectal adenocarcinoma (SW620) cells. The use of ultrasound irradiation as alternative energy input in Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC) shortened the reaction time, increased the reaction efficiency and led to the formation of exclusively symmetric bis-heterocycles. DFT calculations showed that triazole formation is exceedingly exergonic and confirmed that the presence of Cu(I) ions is required to overcome high kinetic requirements and allow the reaction to proceed. The influence of various linkers and 6-substituted purine and regioisomeric 7-deazapurine on their cytostatic activity was revealed. Among all the evaluated compounds, the 4-chloropyrrolo[2,3-d]pyrimidine monomer 5f with 4,4′-bis(oxymethylene)biphenyl had the most pronounced, although not selective, growth-inhibitory effect on pancreatic adenocarcinoma (CFPAC-1) cells (IC50 = 0.79 µM). Annexin V assay results revealed that its strong growth inhibitory activity against CFPAC-1 cells could be associated with induction of apoptosis and primary necrosis. Further structural optimization of bis-chloropyrrolo[2,3-d]pyrimidine with aromatic linker is required to develop novel efficient and non-toxic agent against pancreatic cancer.

5-Substituted N-(9H-purin-6-yl)-1,2-oxazole-3-carboxamides as xanthine oxidase inhibitors

Synthetic 6-substituted purine derivatives are known to exhibit diverse bioactivity. In this paper, a series of N-(9H-purin-6-yl)-1,2-oxazole-3-carboxamide derivatives were synthesized and evaluated in vitro against xanthine oxidase, an enzyme of purine catabolism. The introduction of aryl substituent at position 5 of the oxazole ring was found to increase the inhibition efficiency. Some of the inhibitors containing 5-substituted isoxazole and purine moieties were characterized by IC50 values in the nanomolar range. According to the kinetic data, the most active N-(9H-purin-6-yl)-5-(5,6,7,8-tetrahydronaphthalen-2-yl)-1,2-oxazole-3-carboxamide demonstrated a competitive type of inhibition with respect to the enzyme-substrate. Molecular docking was carried out to elucidate the mechanism of enzyme-inhibitor complex formation. The data obtained indicate that xanthine oxidase may be one of the possible targets for the bioactive purine carboxamides.

Synthesis and Biological Activity of Novel 6-Substituted Purine Derivatives

A series of 6-substituted purines were synthesized from commercially available 2-amino-6-chloropurine with appropriate reagents, and nine new compounds 7, 8, and 11-17 have been discovered. The compounds synthesized were identified by elemental analysis, 1H NMR, as well as mass spectral data. All the title compounds were screened for their antifungal activities, under the method of the disc diffusion, using three species of fungi — Bacillus subtillis, Aspergillus niger, and Candida tropicalis— and some of the compounds showed promising activities.

6-Substituted purines as ROCK inhibitors with anti-metastatic activity

Rho-associated serine/threonine kinases (ROCKs) are principal regulators of the actin cytoskeleton that regulate the contractility, shape, motility, and invasion of cells. We explored the relationships between structure and anti-ROCK2 activity in a group of purine derivatives substituted at the C6 atom by piperidin-1-yl or azepan-1-yl groups. Structure-activity relationship (SAR) analyses suggested that anti-ROCK activity is retained, and may be further increased, by substitution of the parent compounds at the C2 atom or by expansion of the C6 side chain. These inhibitors of ROCK can reach effective concentrations within cells, as demonstrated by a decrease in phosphorylation of the ROCK target MLC, and by inhibition of the ROCK-dependent invasion of melanoma cells in the collagen matrix. Our study may be useful for further optimization of C6-substituted purine inhibitors of ROCKs and of other sensitive kinases identified by the screening of a broad panel of protein kinases.

Purines and triazolo[4,3-e]purines containing pyrazole moiety as potential anticancer and antioxidant agents.

Targeting apoptosis regulators such as caspases aiming at inducing apoptosis is an attractive strategy in cancer therapy. 8-substituted purine incorporating pyrazole moiety were designed, synthesized and evaluated for their anticancer and antioxidant activities. Compounds 7a and 8a displayed potent and selective anticancer activity against lung cancer A549 cell line with low cytotoxicity on peripheral blood mononuclear normal cells. Compounds 7a and 8a induced caspase dependent apoptotic death and DNA damage in all cancer cell lines. In addition, compounds 2, 5, 6a, 7a, 8a, 8c, 11a, 11b and 12b showed good antioxidant activity higher than that of the standard ascorbic acid. Compounds 7a and 8a can be considered promising dual anticancer and antioxidant leads inducing caspase-dependent apoptotic death and DNA damage.

PCR Amplification of Base-Modified DNA.

An efficient PCR amplification of various templates (short 57-mer, random 67- and 82-mer, and long DNA) with base-modified nucleoside triphosphates is presented here. Using 5-substituted pyrimidine and 7-substituted-7-deaza- or 8-substituted purine nucleoside triphosphates as substrates for thermostable DNA polymerases [Taq and Vent (exo- )], successful PCR amplification of partially or entirely modified DNA libraries and long DNA constructs (up to 1.5kb) is achieved. Visualization of double-stranded PCR product formation is improved through the use of primers with different fluorescent labels. This allows one to monitor the efficiency of modified substrate incorporation and the enzymatic recognition of the modified template during PCR. The redesigned fully base-modified DNA (denoted 'DZA') can be utilized for the straightforward production of diverse libraries for in vitro selection of aptamer and catalytic nucleic acids as well as for the synthesis of artificial genetic templates, replicons, or complex vectors. © 2018 by John Wiley & Sons, Inc.

A Multitarget Approach toward the Development of 8-Substituted Purines for Photoprotection and Prevention of UV-Related Damage.

Ultraviolet (UV) light is the most abundant and significant modifiable risk factor for skin cancer and many other skin diseases such as early photo-aging. Across the solar radiation spectrum, UV light is the main cause behind skin problems. In the search for novel photoprotective compounds, a new series of 8-substituted purines were synthesized from commercially available 6-hydroxy-4,5-diaminopyrimidine hemisulfate or 4,5-diaminopyrimidine. All title compounds were investigated for their UV filtering, antioxidant, antifungal, and antiproliferative activities. For the photoprotection assays we used a diffuse transmittance technique to determine the sun protection factor (SPF) in vitro, and 2,2-diphenyl-1-picrylhydrazyl (DPPH) and ferric ion reducing antioxidant power (FRAP) tests for evaluating the antioxidant activity of the more potent compounds. Among them, 8-(2,5-dihydroxyphenyl)-7H-purin-6-ol (compound 26) proved to be a good radical scavenger and is also endowed with broad-spectrum UVA filtering capabilities, suitable for further development as a protective molecule.

Substrate Specificity of a Purine Nucleoside Phosphorylase from <i>Aeromonas hydrophila</i> Toward 6-Substituted Purines and its Use as a Biocatalyst in the Synthesis of the Corresponding Ribonucleosides

A purine nucleoside phosphorylase from Aeromonas hydrophila (AhPNP) was found to catalyze the regio- and stereoselective transfer of the ribofuranosyl moiety from 7-methylguanosine iodide (1) to a library of 6- substituted purines, resulting in moderate to high conversions (18-65%) into their ribonucleoside counterparts (26- 34, 36-49). Exploring of substrate recognition by AhPNP with regard to 6-position of purine base provided the necessary information to exploit this biocatalyst for the chemoenzymatic synthesis of nucleoside analogues through a transglycosylation reaction. Keywords: Purine nucleoside phosphorylase, transglycosylation, chemoenzymatic synthesis, 6-substituted purine ribonucleosides, Aeromonas hydrophila.

The synthesis of ψ-(2-aryl-1,3-dioxolan-2-yl) alkyl derivatives of purines and their activity towards HIV reverse transcriptase

Novel non-competitive inhibitors of HIV RT were synthesized by alkylation of 6-substituted purines with different 2-(chloroalkyl)-2-aryl-1,3-dioxolanes and related compounds. The structure-activity relationship within the synthesized compounds was studied.

Immobilized purine nucleoside phosphorylase from Aeromonas hydrophila as an on-line enzyme reactor for biocatalytic applications

We described the development of a biochromatographic system which uses a purine nucleoside phosphorylase from Aeromonas hydrophila (AhPNP) for the evaluation of the substrate specificity on nucleoside libraries. AhPNP has been covalently immobilized on a fused silica Open Tubular Capillary (OTC) via Schiff base chemistry. The resulting bioreactor has been characterized by the determination of kinetic constants (Km and Vmax) for a natural substrate (inosine) and then assayed versus all natural purine (deoxy)ribonucleosides and a small library of 6-substituted purine ribosides. Characterization of the bioreactor has been carried out through a bidimensional chromatographic system with the sample on-line transfer from the bioreactor to the analytical column for the separation and quantification of substrate and product. Comparison with the soluble enzyme showed that the AhPNP-based bioreactor is reliable as the same ranking order, with respect to the standard activity assay, was obtained. The stability of the IMER was also assessed and the system was found to be stable up to 60 reactions.

6-Substituted purines containing thienyl or furyl substituents as artificial nucleobases for metal-mediated base pairing

The artificial nucleobases 6-(2'-thienyl)-purine (6TP) and 6-(2'-furyl)-purine (6FP) have been investigated in terms of their applicability in metal-mediated base pairs. In principle, these nucleobases should be capable of providing an S,N- and O,N-coordination environment, respectively. Two metal complexes have been synthesized and structurally characterized, using the N9-methylated derivatives of 6TP and 6FP as model nucleobases. The silver(I) complex [Ag(9-Me6TP)(CH3CN)(NO3)]n is of polymeric nature. The thienyl substituent is not involved in metal binding. In contrast, the dinuclear copper(II) complex [Cu(9-Me6FP)Cl2(OH2)]2 clearly shows that the furyl substituent can participate in the coordination of the metal ion. Oligonucleotide double helices comprising a 6TP:6TP or a 6FP:6FP mispair are slightly stabilized in the presence of silver(I). However, a distinction between the formation of a metal-mediated base pair and an unspecific binding event cannot be finally made. Based on these studies, it is proposed that the formation of Hoogsteen-type metal-mediated base pairs should be the more promising alternative compared with the originally anticipated Watson-Crick-type metal-mediated base pairs.

Synthesis and Antiviral Evaluation of N 9-[2-(Phosphonomethoxy)ethyl] (PME) Analogues Derived from 8-Substituted Purines

Synthesis and Antiviral Evaluation of N 9-[2-(Phosphonomethoxy)ethyl] (PME) Analogues Derived from 8-Substituted Purines

Synthesis, anticancer, anti-HIV-1, and antimicrobial activity of some tricyclic triazino and triazolo[4,3-e]purine derivatives

In an effort to etablish new candidates with improved antineoplastic, anti-HIV-1 and antimicrobial activities, the synthesis of some new triazino and triazolo[4,3-e]purine derivatives is described: 6,8-dimethyl-1,4-dihydro-1,2,4-triazino[4,3-e]purine-7,9(6H, 8H)-diones 3–6; 5,7,9-trimethyl-1,2,4-triazolo[4,3-e]purine-6,8(5H, 7H, 9H)-diones 11–13, together with the synthesis of the 8-substituted purine derivative: 8-(3,5-diamino-1H-pyrazol-4-yl)diazenyl-1,3-dimethyl-1H-purine-2,6(3H, 7H)-dione 7. The prepared compounds were tested for their in vitro anticancer, anti-HIV and antimicrobial activities. The results of the in vitro anticancer screening revealed that compound 3 exhibited considerable activity against melanoma MALME-3 M, non-small lung cancer HOP-92 and breast cancer T-47D (GI50 values of 25.2, 31.8, and 32.9 μM, respectively). The anti-HIV-1 results indicated that compounds 7 and 13c displayed moderate activity (maximum % cell protection 30.52 and 35.54 at 2 × 10−4 M, respectively). The in vitro antimicrobial data showed that compound 12 was the most active against P. aeruginosa, it was equipotent to ampicillin (MIC < 100 μg/ml). While compound 11d was the most active against P. vulgaris, it was as active as ampicillin (MIC < 50 μg/ml). In addition, compounds 12 and 13c were the most active against S. aureus (MIC <50 and <25 μg/ml, respectively). On the other hand, the tested compounds devoid of antifungal activity except 6b and 11c which showed weak activity against A. niger.

Interpretation of substituent effects on 13C and 15N NMR chemical shifts in 6-substituted purines

A range of purine derivatives modified at position 6 of the basic purine skeleton exhibit a variety of biological activities. Several derivatives are used or tested nowadays for pharmacological treatments. The present work aims to analyze the effects of substituents on the electron distribution in the purine core as reflected by NMR chemical shifts. We collected a comprehensive set of experimental NMR data for a variety of 6-substituted purines (-NH(2), -NHMe, -NMe(2), -OMe, -Me, -CCH, and -CN) and determined the molecular and crystal structures of three derivatives (-NHMe, -CCH, and -CN) by X-ray diffraction. The density-functional methods calibrated in our recent study (Phys. Chem. Chem. Phys., 2010, 12, 5126) have been employed to enable understanding of the substituent-induced changes in the NMR chemical shifts of the atoms in the purine skeleton. Analyses of the nuclear shielding using localized molecular orbitals (LMOs), specifically the natural LMOs (NLMOs) and Pipek-Mezey LMOs, were used to break down the values of the isotropic (13)C and (15)N NMR chemical shifts and the chemical shift tensors into the contributions of the individual LMOs. The experimental and calculated trends in the chemical shift of the N-3 atom correlate nicely with the Hammett constants (σ(para)) and the calculated natural charges on N-3, whereas the contributions of the LMOs to the N-1 and C-6 chemical shifts are found to be more complex.