Traceless solid-phase synthesis of 2,6,9-trisubstituted purines from resin bound 6-thiopurines

Sulfonyl Group Dance: A Tool for the Synthesis of 6-Azido-2-sulfonylpurine Derivatives

9-Substituted 2-chloro-6-sulfonylpurines provide 6-azido-2-sulfonylpurine derivatives with 61-83% yields when treated with sodium azide. Under optimized reaction conditions, the title compounds are obtained in a one-pot process, which involves a sequential treatment of 2,6-dichloropurines with a selected sodium sulfinate and sodium azide. Such a sulfonyl group dance (functional group swap) results from a cascade of S_NAr reactions, which are facilitated by azidoazomethine-tetrazole (azide-tetrazole) tautomeric equilibrium. The formation of Meisenheimer-type intermediates as tetrazolopurine tautomers was supported by various spectroscopic methods, including ¹⁵N NMR.

Traceless Solid-Phase Organic Synthesis

Traceless solid-phase synthesis represents an ultimate sophisticated synthetic strategy on insoluble supports. Compounds synthesized on solid supports can be released without a trace of the linker that was used to tether the intermediates during the synthesis. Thus, the target products are composed only of the components (atoms, functional groups) inherent to the target core structure. A wide variety of synthetic strategies have been developed to prepare products in a traceless manner, and this review is dedicated to all aspects of traceless solid-phase organic synthesis. Importantly, the synthesis does not need to be carried out on a linker designed for traceless synthesis; most of the synthetic approaches described herein were developed using standard, commercially available linkers (originally devised for solid-phase peptide synthesis). The type of structure prepared in a traceless fashion is not restricted. The individual synthetic approaches are divided into eight sections, each devoted to a different methodology for traceless synthesis. Each section consists of a brief outline of the synthetic strategy followed by a description of individual reported syntheses.

Solid-Phase Synthetic Strategies for the Preparation of Purine Derivatives

This Review summarizes all of the currently described strategies applicable for the solid-phase synthesis of purine derivatives. The individual approaches are classified according to the immobilization procedure used resulting in a linkage of the final scaffold at various positions.

Synthesis and biological evaluation of guanidino analogues of roscovitine

A series of 2,9-substituted 6-guanidinopurines, structurally related to the cyclin-dependent kinase (CDK) inhibitors olomoucine and roscovitine, has been synthesized and characterized. A new copper-catalyzed method for the synthesis of 2-substituted 6-guanidino-9-isopropylpurines under mild reaction conditions has been developed. All prepared compounds were screened for their CDK1 and CDK2 inhibitory activities, cytotoxicity and antiproliferative effects in the breast cancer-derived cell line MCF7. The most active derivative 16g possessed an identical side chain in the C2 position to roscovitine; this compound displayed approximately five fold higher inhibitory activity towards CDK2/cyclin E and more than ten fold increase in cytotoxicity in MCF7 cells. Interestingly and in contrast to previously described findings, (S)-6-guanidinopurine derivatives were generally more active than their (R)-counterparts. Kinase selectivity profiling of (R)- and (S)-enantiomers 16e and 16g, respectively, revealed that introduction of a guanidino group at the C6 position of the purine moiety decreased selectivity towards protein kinases compared to roscovitine. Nevertheless, increased inhibitory activity and decreased selectivity offer a good starting point for further development of new protein kinase inhibitors.

Roscovitine-derived, dual-specificity inhibitors of cyclin-dependent kinases and casein kinases 1

Cyclin-dependent kinases (CDKs) and casein kinases 1 (CK1) are involved in the two key molecular features of Alzheimer's disease, production of amyloid-beta peptides (extracellular plaques) and hyper-phosphorylation of Tau (intracellular neurofibrillary tangles). A series of 2,6,9-trisubstituted purines, structurally related to the CDK inhibitor roscovitine, have been synthesized. They mainly differ by the substituent on the C-6 position. These compounds were screened for kinase inhibitory activities and antiproliferative effects. Several biaryl derivatives displayed potent inhibition of both CDKs and CK1. In particular, derivative 13a was a potent inhibitor of CDK1/cyclin B (IC 50: 220 nM), CDK5/p25 (IC 50: 80 nM), and CK1 (IC 50: 14 nM). Modeling of these molecules into the ATP-binding pocket of CK1delta provided a rationale for the increased selectivity toward this kinase. 13a was able to prevent the CK1-dependent production of amyloid-beta in a cell model. CDK/CK1 dual-specificity inhibitors may have important applications in Alzheimer's disease and cancers.

Traceless solid-phase synthesis and biological evaluation of purine analogs as inhibitors of multidrug resistance protein 4

The traceless solid-phase syntheses of 6-oxopurines and pyrazolo[3,4-d]pyrimidines are presented. The effects of these compounds on multidrug resistance protein 4 (MRP4/ABCC4) facilitated efflux was examined. Four of the compounds, 7b, 7c, 15a, and 17e, were active in inhibiting MRP4-mediated efflux of the bimane-glutathione conjugate. In addition, all four compounds were also able to reverse MRP4-mediated resistance to the anticancer drug 6-thioguanine. In the presence of 25 microM 15a or 17e, there was complete reversal. The reversal of resistance was achieved without any effects on the uptake and metabolism of 6-thioguanine.

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.

The purines: potent and versatile small molecule inhibitors and modulators of key biological targets

The goal of this review is to highlight the wide range of biological activities displayed by purines, with particular emphasis on new purine-based agents which find potential application as chemical-biology tools and/or therapeutic agents. The expanding interest in the biological properties of polyfunctionalized purine derivatives issues, in large part, from the development of rapid high-throughput screening essays for new protein targets, and the corresponding development of efficient synthetic methodology adapted to the construction of highly diverse purine libraries. Purine-based compounds have found new applications as inducers of interferon and lineage-committed cell dedifferentiation, agonists and antagonists of adenosine receptors, ligands of corticotropin-releasing hormone receptors, and as inhibitors of HSP90, Src kinase, p38alpha MAP kinase, sulfotransferases, phosphodiesterases, and Cdks. The scope of application of purines in biology is most certainly far from being exhausted. Testing purine derivatives against the multitude of biological targets for which small molecule probes have not yet been found should thus be a natural reflex.

Solid-phase synthesis of 7-substituted 3H-imidazo[2,1-i]purines

A method for solid-supported synthesis of N,N-disubstituted (3H-imidazo[2,1-i]purin-7-yl)methyl amines has been developed. The key features of this library synthesis are: (i) immobilization of commercially available N6-benzoyl-5'-O-(4,4'-dimethoxytrityl)-2'-deoxyadenosine 3'-(2-cyanoethyl N,N-diisopropylphosphoramidite) by phosphitylation to a hydroxyl-functionalized support, (ii) quantitative conversion of the deprotected adenine base to 3H-imidazo[2,1-i]purine-7-carbaldehyde with bromomalonaldehyde in DMF in the presence of formic acid and 2,6-lutidine, (iii) reductive amination of the formyl group followed by N-alkylation or N-acylation, and (iv) release from the support by acidolytic cleavage of the N-glycosidic bond. Steps (ii) and (iii) have been optimized in some detail by using (adenin-9-yl)acetic acid anchored to a Phe-Wang resin as a model compound.

Parallel Solution-Phase Synthesis of a 2,6,8,9-Tetrasubstituted Purine Library via a Sulfur Intermediate

Purine analogues exhibiting a wide range of pharmacological activities have been considered a privileged structure in medicinal chemistry. In addition, the purine core consisting of four points of structural diversity is a well-sought scaffold in combinatorial chemistry. Although most of the efforts have been focused on 2,6,9-, 6,8,9-, or 2,8,9-trisubstituted purines, syntheses of 2,6,8,9-tetrasubstituted purines are rare. This paper presents a parallel solution phase approach for the synthesis of fully substituted purines via a 6-sulfur-substituted pyrimidine as the key intermediate. This strategy combining construction and modification of the purine ring thus increases the structural diversity of the final products. Sequential substitution of chlorines in 4,6-dichloro-2-methyl-5-nitropyrimidine with primary amine and benzylmercaptan afforded the 4-(substituted)amino-6-benzylthio-5-nitropyrimidine, which was readily converted to its diaminopyrimidine analogue by reduction of the nitro group. The diaminopyrimidine intermediate was cyclized to construct the purine ring with a C-8 substituent. Eventual oxidation of sulfur to sulfone and subsequent displacement by a primary or secondary amine provided the desired 2,6,8,9-tetrasubstituted purine analogues. This synthetic methodology was validated with the synthesis of a 216-member purine library.

Synthesis and in vitro antimicrobial activities of 2-hydroxy-6-methyl-7-(arylamino)-1,7-dihydropurin-8-ones

A number of 2-hydroxy-6-methyl-7-(arylamino)-1,7-dihydropurin-8-ones have been synthesized. 3-Oxo-2-(arylhydrazono)butyric acid ethyl ester were acetylated and treated with triethyl amine and formamide in presence of 1,4-dioxane to yield N-(5-acetyl-4-ethoxy-2-oxo-2,5-dihydro-imidazol-1-yl)-N-arylacetamide, which on refluxation with urea and freshly prepared sodium ethoxide yielded the title compound. All the newly synthesized compounds have been characterized by spectroscopic and elemental analysis data. The synthesized compounds were screened against a representative panel of susceptible and resistant Gram-positive and Gram-negative bacteria using a standard antibiotic drug purinthol as control. Quantitative structure-activity relationship has also been interpreted in terms of correlation of biological activity with molecular refractive index parameters (M(R)) and Hammett substituent constant (sigma).

A “Catch and Release” Strategy for the Parallel Synthesis of 2,4,5-Trisubstituted Pyrimidines

A resin capture and release strategy for making a combinatorial array of 2,4,5-trisubstituted pyrimidines is demonstrated by capturing beta-ketoesters and beta-ketoamides on a solid-supported piperazine. Through a cyclocondensation reaction, the solid-supported enaminone is reacted with several guanidines under heating or microwave irradiation affording the corresponding pyrimidines in good yield and chemical purity directly on solution. After this final step, the support can be effectively recycled.

Fluorous synthesis of disubstituted pyrimidines

[reaction: see text] The fluorous synthesis of disubstituted pyrimidines is carried out by attaching 2,4-dichloro-6-methylpyrimidine with 1H,1H,2H,2H-perfluorodecanethiol. The tagged substrate is substituted with 3-(trifluoromethyl)pyrazole followed by thioether oxidation and tag displacement with amines or thiols. The fluorous chain serves as a phase tag for intermediate and product purification over FluoroFlash SPE cartridges.