An efficient synthesis of 2-substituted 6-methylpurine bases and nucleosides by Fe- or Pd-catalyzed cross-coupling reactions of 2,6-dichloropurines

Multicomponent Synthesis of C(8)-Substituted Purine Building Blocks of Peptide Nucleic Acids from Prebiotic Compounds

We have explored the reaction of a three-components mixture of aminomalononitrile, urea and α-amino acid methyl esters for the multicomponent synthesis substituted purines resembling PNA's building blocks. 2,6-diamino-purines, 6-amino-3,9-dihydro-2H-purin-2-one (iso-guanines), and 3,9-dihydro-6H-purin-6-one derivatives, selectively decorated at C(8)-position of the purine ring with different amino acid residues, were obtained from acceptable to good yields. The regio-selectivity of the transformation was controlled by the use of urea in the ternary mixture and by the annulation agent involved in the ring-closure of amino-imidazole carbonitrile intermediates. Solvent free conditions, microwave irradiation and simple one-carbon containing reagents further satisfied the major requirement of atom economy and sustainable chemistry. Due to the prebiotic nature of the three-components mixture and of annulation agents, it also embodies the possibility for the synthesis of novel PNAs bearing purine nucleobases decorated at C(8)-position of the imidazole ring as alternative RNA analogues in molecular evolution.

Metal-free direct C-6-H alkylation of purines and purine nucleosides enabled by oxidative homolysis of 4-alkyl-1,4-dihydropyridines at room temperature

Herein we report the application of 4-alkyl-1,4-dihydropyridines (DHPs), which are easily prepared from inexpensive aldehydes in one step, for the direct site-specific C-H alkylation of purines and purine nucleosides. Despite there being three active C(sp²)-H bonds (C-2-H, C-6-H, and C-8-H) in the structure, the reactions still show high regioselectivity at the purinyl C-6-H position. Importantly, the reactions successfully avoid the use of transition metal catalysts and additional acids. Meanwhile, the protocols are not sensitive to moisture and require only persulfate as an oxidant. Besides, this method displays broad functional group compatibility and is easy to scale up. Notably, pharmaceutical purines, e.g. the natural product 6-hydroxymethyl nebularine isolated from basidiomycetes, can be smoothly prepared using this protocol.

Regiospecificity C(sp2)-C(sp3) Bond Construction between Purines and Alkenes to Synthesize C6-Alkylpurines and Purine Nucleosides Using O2 as the Oxidant

A highly site-selective and Markovnikov-type radical C⁶-H alkylation of purines with alkenes is achieved, allowing fast construction of the C(sp²)-C(sp³) bond at the C-6-position of purines and purine nucleosides using O₂ as a green oxidant and alkenes as cheap alkylation reagents. The route was also a radical route to synthesize C⁶-alkyl-N⁷-substituted purines with potential steric hindrance between C⁶-alkyl groups and N⁷-substituted groups. This reaction is easily scaled up and has excellent functional group compatibility and broad substrate scopes. Moreover, the unstable intermediate was also separated, which was the key evidence for the reaction mechanism.

6-Methyl-7-deazapurine nucleoside analogues as broad-spectrum antikinetoplastid agents

Kinetoplastid parasites are the causative agents of Chagas disease (CD), leishmaniasis and human African trypanosomiasis (HAT). Despite a sustained decrease in the number of HAT cases, more efforts are needed to discover safe and effective therapies against CD and leishmaniasis. Kinetoplastid parasites lack the capability to biosynthesize purines de novo and thus critically depend on uptake and processing of purines from host cells. As such, modified purine nucleoside analogues may act as broad-spectrum antikinetoplastid agents. This study assessed the in vitro activity profile of 7-modified 6-methyl tubercidin derivatives against Trypanosoma cruzi, Leishmania infantum, Trypanosoma brucei brucei and T. b. rhodesiense, and led to the identification of analogues that display activity against all these species, such as 7-ethyl (13) and 7-chloro (7) analogues. These selected analogues also proved sufficiently stable in liver microsomes to warrant in vivo follow-up evaluation.

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New safe and effective therapies are needed for Chagas disease and leishmaniasis.

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The causative agents rely on the acquisition of purine nucleobases and nucleosides from host cells to grow and multiply.

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New 7-substituted 6-methyl-7-deazapurine ribonucleosides were synthesized.

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A 7-ethyl and 7-chloro analogue display low to submicromolar activity against T. brucei, T. cruzi and L. infantum.

Tuning Cross-Coupling Approaches to C3 Modification of 3-Deazapurines

A general approach to C3 modification of purine scaffold through various types of cross-coupling reactions has been established. Tuning substrate electronics and reaction conditions resulted in the development of highly efficient sp²-sp, sp²-sp², and sp²-sp³ cross-coupling conditions for modification of 3-deazaadenine to access C3-modified adenine and hypoxanthine scaffolds. The optimized methodologies to access the corresponding 3-deazaadenosine phosphoramidites for solid-phase DNA synthesis have been demonstrated.

Iron-Catalyzed C-C Cross-Couplings Using Organometallics

Over the last decades, iron-catalyzed cross-couplings have emerged as an important tool for the formation of C-C bonds. A wide variety of alkenyl, aryl, and alkyl (pseudo)halides have been coupled to organometallic reagents, the most currently used being Grignard reagents. Particular attention has been devoted to the development of iron catalysts for the functionalization of alkyl halides that are generally challenging substrates in classical cross-couplings. The high functional group tolerance of iron-catalyzed cross-couplings has encouraged organic chemists to use them in the synthesis of bioactive compounds. Even if some points remain obscure, numerous studies have been carried out to investigate the mechanism of iron-catalyzed cross-coupling and several hypotheses have been proposed.

2-Substituted 6-(Het)aryl-7-deazapurine Ribonucleosides: Synthesis, Inhibition of Adenosine Kinases, and Antimycobacterial Activity

A series of 6-(hetero)aryl- or 6-methyl-7-deazapurine ribonucleosides bearing a substituent at position 2 (Cl, F, NH2, or CH3) were prepared by cross-coupling reactions at position 6 and functional group transformations at position 2. Cytostatic, antiviral, and antimicrobial activity assays were performed. The title compounds were observed to be potent and selective inhibitors of Mycobacterium tuberculosis adenosine kinase (ADK), but not human ADK; moreover, they were found to be non-cytotoxic. The antimycobacterial activities against M. tuberculosis, however, were only moderate. The reason for this could be due to either poor uptake through the cell wall or to parallel biosynthesis of adenosine monophosphate by the salvage pathway.

Synthesis, cytostatic, antimicrobial, and anti-HCV activity of 6-substituted 7-(het)aryl-7-deazapurine ribonucleosides

A series of 80 7-(het)aryl- and 7-ethynyl-7-deazapurine ribonucleosides bearing a methoxy, methylsulfanyl, methylamino, dimethylamino, methyl, or oxo group at position 6, or 2,6-disubstituted derivatives bearing a methyl or amino group at position 2, were prepared, and the biological activity of the compounds was studied and compared with that of the parent 7-(het)aryl-7-deazaadenosine series. Several of the compounds, in particular 6-substituted 7-deazapurine derivatives bearing a furyl or ethynyl group at position 7, were significantly cytotoxic at low nanomolar concentrations whereas most were much less potent or inactive. Promising activity was observed with some compounds against Mycobacterium bovis and also against hepatitis C virus in a replicon assay.

Chemoselective sp2-sp3 cross-couplings: iron-catalyzed alkyl transfer to dihaloaromatics

The chemoselective functionalization of a range of dihaloaromatics with methyl, cyclopropyl, and higher alkyl Grignard reagents via iron-catalyzed cross-coupling is described. The site selectivity of C-X (X = halogen) activation is determined by factors such as the position of the halogen on the ring, the solvent, and the nucleophile. A one-pot protocol for the chemoselective synthesis of mixed dialkyl heterocycles is achieved solely employing iron catalysis.

Synthesis of purine modified 2'-C-methyl nucleosides as potential anti-HCV agents

Based on the anti-hepatitis C activity of 2'-C-methyl-adenosine and 2'-C-methyl-guanosine, a series of new modified purine 2'-C-methyl nucleosides was prepared as potential anti-hepatitis C virus agents. Herein, we report the synthesis of both 6-modified and 2-modified purine 2'-C-methyl-nucleosides along with their anti-HCV replication activity and cytotoxicity in different cells.

The synthesis of the 8-C-substituted 2,6-diamino-9-[2-(phosphonomethoxy)ethyl]purine (PMEDAP) derivatives by diverse cross-coupling reactions

Diisopropyl 8-bromo-2,6-diamino-9-[2-(phosphonomethoxy)ethyl]purine was used as a starting material for the synthesis of the 8-C-substituted 2,6-diamino-9-[2-(phosphonomethoxy)ethyl]purine (PMEDAP) analogues. A systematic screening of diverse cross-coupling reactions was carried out. Stille, Suzuki–Miyaura, Negishi, and Sonogashira cross-couplings, as well as Pd-catalysed reactions with trialkylaluminiums, were employed for the introduction of various alkyl, alkenyl, alkynyl, aryl, and hetaryl substituents to the C-8 position of the 2,6-diaminopurine moiety. In contrast to the potent parent compound PMEDAP, which exhibits potent antiretroviral and antitumor activity, none of the sixteen newly synthesized 8-C-substituted analogues of PMEDAP showed any specific antiviral activity.

Microwave promoted C6-alkylation of purines through S(N)Ar-based reaction of 6-chloropurines with 3-alkyl-acetylacetone

C6-Alkylated purine analogues were obtained in good to excellent isolated yields by S(N)Ar reaction of 6-chloropurine derivatives with 3-alkyl-acetylacetone. 3-Alkyl-acetylacetones were employed as alkylating agents and C6-alkylated purines were obtained highly selectively within short reaction time under microwave irradiation conditions. This work is complementary to the classical coupling reactions for the synthesis of C6-alkylated purine analogues.

Palladium-catalyzed C–H bond functionalization of C6-arylpurines

A highly regioselective Pd-catalyzed C_Ar–H bond activation method was developed for the modification of purines (nucleosides) with different functional groups by using purine as a directing group. This approach provides a new access to a variety of functionalized purines (nucleosides).

Coming of age: sustainable iron-catalyzed cross-coupling reactions

Iron-catalyzed cross-coupling reactions have, over the past years, developed to maturity and today are an integral part of the organic chemist's toolkit. They benefit from low costs, operational simplicity, and high reactivity and thus constitute the "green" sister of the palladium and nickel establishment. This timely Review traces back major achievements, discusses their mechanistic background, and highlights numerous applications to molecular synthesis.Iron-catalyzed carbon-carbon bond-forming reactions have matured to an indispensable class of reactions in organic synthesis. The advent of economically and ecologically attractive iron catalysts in the past years has stepped up the competition with the established palladium and nickel catalyst systems that have dominated the field for more than 30 years, but suffer from high costs, toxicity, and sometimes low reactivity. Iron-catalyzed protocols do not merely benefit from economic advantages but entertain a rich manifold of reactivity patterns and tolerate various functional groups. The past years have witnessed a rapid development with ever-more-efficient protocols for the cross-coupling between alkyl, alkenyl, alkynyl, aryl, and acyl moieties becoming available to organic chemists. This Review intends to shed light onto the versatility that iron-catalyzed cross-coupling reactions offer, summarize major achievements, and clear the way for further use of such superior methodologies in the synthesis of fine chemicals, bioactive molecules, and materials.

Human DNA polymerase alpha uses a combination of positive and negative selectivity to polymerize purine dNTPs with high fidelity

DNA polymerases accurately replicate DNA by incorporating mostly correct dNTPs opposite any given template base. We have identified the chemical features of purine dNTPs that human pol alpha uses to discriminate between right and wrong dNTPs. Removing N-3 from guanine and adenine, two high-fidelity bases, significantly lowers fidelity. Analogously, adding the equivalent of N-3 to low-fidelity benzimidazole-derived bases (i.e., bases that pol alpha rapidly incorporates opposite all four natural bases) and to generate 1-deazapurines significantly strengthens the ability of pol alpha to identify the resulting 1-deazapurines as wrong. Adding the equivalent of the purine N-1 to benzimidazole or to 1-deazapurines significantly decreases the rate at which pol alpha polymerizes the resulting bases opposite A, C, and G while simultaneously enhancing polymerization opposite T. Conversely, adding the equivalent of adenine's C-6 exocyclic amine (N-6) to 1- and 3-deazapurines also enhances polymerization opposite T but does not significantly decrease polymerization opposite A, C, and G. Importantly, if the newly inserted bases lack N-1 and N-6, pol alpha does not efficiently polymerize the next correct dNTP, whereas if it lacks N-3, one additional nucleotide is added and then chain termination ensues. These data indicate that pol alpha uses two orthogonal screens to maximize its fidelity. During dNTP polymerization, it uses a combination of negative (N-1 and N-3) and positive (N-1 and N-6) selectivity to differentiate between right and wrong dNTPs, while the shape of the base pair is essentially irrelevant. Then, to determine whether to add further dNTPs onto the just added nucleotide, pol alpha appears to monitor the shape of the base pair at the primer 3'-terminus. The biological implications of these results are discussed.

Iron-catalyzed aryl–aryl cross-coupling reaction tolerating amides and unprotected quinolinones

The iron(III)-catalyzed cross-coupling reaction between functionalized arylcopper reagents and aromatic iodides bearing an amide function or an unprotected quinolinone leads smoothly to polyfunctionalized biphenyls in excellent yields due to an intramolecular chelating effect of the amide group.

2,6-disubstituted and 2,6,8-trisubstituted purines as adenosine receptor antagonists

Purines have long been exploited as adenosine receptor antagonists. The substitution pattern about the purine ring has been well investigated, and certain criteria have become almost a prerequisite for good affinity at the adenosine A(1) receptor. The adaptation of the pharmacophore and the initial series of pyrimidines developed in an earlier publication resulted in a series of purines with an entirely new substitution pattern. One compound in particular, 8-cyclopentyl-2,6-diphenylpurine (31, LUF 5962) has been shown to be very promising with an affinity of 0.29 nM at the human adenosine A(1) receptor.

Suzuki-type Pd(0) coupling reactions in the synthesis of 2-arylpurines as Cdk inhibitors

A new series of 2-aryl-substituted purine derivatives has been synthesized by Suzuki Pd(0) coupling reactions. Moderate in vitro inhibitory activity against Cdk1 and Cdk5 was observed. These compounds are inactive against GSK3.

Selective Magnesiation of Chloro-iodopurines: An Efficient Approach to New Purine Derivatives

[reaction: see text] Both 6-chloro-2-iodo-9-isopropylpurine (1) and 2-chloro-6-iodo-9-benzylpurine (4) undergo a selective I/Mg exchange reaction with iPrMgCl at -80 degrees C. The reaction course at 0 degrees C is different. Magnesiation of 1 proceeds with the migration of magnesium to the 8 position of the purine nuclei. In the case of 4, substitution of iodine with an alkyl group from the Grignard reagent accompanied with a Cl/Mg exchange reaction takes place, and 6-alkyl-2-magnesiated purines (9) are formed. Thus prepared Grignard reagents afford the corresponding alcohols by the reaction with aldehydes.