Purinyl N1-directed aromatic C-H oxidation in 6-arylpurines and 6-arylpurine nucleosides

Ruthenium-catalysed direct C-H amidation of 4-aryl-pyrrolo[2,3-d]pyrimidines with acyl/phosphoryl azides

A ruthenium-catalysed arene ortho C-H amidation of 4-aryl-pyrrolo[2,3-d]pyrimidine derivatives with acyl azides or phosphoryl azides as the nitrogen sources toward C-N bond formation was developed. This protocol could offer a novel and direct approach to access a series of amidated and phosphoramidated pyrrolo[2,3-d]pyrimidine derivatives in moderate to good yields, thereby evading the general Curtius rearrangement. The protocol features significant functional group tolerance and a single-step process, with the release of only innocuous molecular nitrogen as the byproduct.

Hydrogen Evolution Enabled Rhodaelectro-Catalyzed [4+2] Annulations of Purines and 7-Deazapurines with Alkynes

A rhodium-catalyzed electrochemical regioselective annulation of 6-phenylpurines and 6-phenyl-7-deazapurines with alkynes has been developed. Electricity is used to recycle the active rhodium-based catalyst, promote the evolution of H2 and help...

Pd-Catalyzed solvent-controlled site-selective arene C-H monoacyloxylation of pyrrolo[2,3-d]pyrimidine derivatives

An efficient and highly regioselective Pd-catalyzed direct arene C(sp²)-H acyloxylation of pyrrolo[2,3-d]pyrimidine derivatives is reported. The key strategy involves the utilization of the unique reactivity of pyrrolo[2,3-d]pyrimidine and the employment of pyrrolo[2,3-d]pyrimidine as the directing group. A variety of monoacyloxylated pyrrolo[2,3-d]pyrimidine derivatives can be achieved by switching the solvents under mild conditions, and they can be further modified and exhibit various biological activities.

When nucleoside chemistry met hypervalent iodine reagents

There has been increasing use of hypervalent iodine reagents in the field of nucleoside chemistry. Applications span: (a) synthesis of nucleoside analogues with sulfur and seleno sugar surrogates, (b) synthesis of unusual carbocyclic and ether ring-containing nucleosides, (c) introduction of sulfur and selenium into pyrimidine bases of nucleosides and analogues, (d) synthesis of isoxazole and isoxazoline ring-containing nucleoside analogues, (e) involvement of purine ring nitrogen atoms for remote C-H bond oxidation, and (f) metal-catalyzed and uncatalyzed synthesis of benzimidazolyl purine nucleoside analogues by intramolecular C-N bond formation. This review offers a perspective on developments involving the use of hypervalent iodine reagents in the field of nucleoside chemistry that have appeared in the literature in the 2003-2017 time frame.

Rhodium(iii)-catalyzed CF3-carbenoid C–H functionalization of 6-arylpurines

An expedient route to access a new family of fluorine-containing purine derivatives via chelation-controlled rhodium(iii)-catalyzed carbenoid C–H functionalization has been developed.

Regioselective C–H and N–H functionalization of purine derivatives and analogues: a synthetic and mechanistic perspective

This perspective provides a systematic and concise overview of the recent development in C–H/N–H bond functionalization in purine derivatives and analogues.

A versatile rhodium(iii) catalyst for direct acyloxylation of aryl and alkenyl C–H bonds with carboxylic acids

Rh(iii)-Catalyzed highly regioselective direct acyloxylation of aryl and alkenyl sp² C–H bonds with various carboxylic acids has been developed.

Pd-catalyzed versus uncatalyzed, PhI(OAc)2-mediated cyclization reactions of N6-([1,1'-biaryl]-2-yl)adenine nucleosides

In this work we have assessed reactions of N⁶-([1,1'-biaryl]-2-yl)adenine nucleosides with Pd(OAc)₂ and PhI(OAc)₂, via a Pd^II/Pd^IV redox cycle. The substrates are readily obtained by Pd/Xantphos-catalyzed reaction of adenine nucleosides with 2-bromo-1,1'-biaryls. In PhMe, the N⁶-biarylyl nucleosides gave C6-carbazolyl nucleoside analogues by C-N bond formation with the exocyclic N⁶ nitrogen atom. In the solvent screening for the Pd-catalyzed reactions, an uncatalyzed process was found to be operational. It was observed that the carbazolyl products could also be obtained in the absence of a metal catalyst by reaction with PhI(OAc)₂ in 1,1,1,3,3,3-hexafluoroisopropanol (HFIP). Thus, under Pd catalysis and in HFIP, reactions proceed to provide carbazolyl nucleoside analogues, with some differences. If reactions of N⁶-biarylyl nucleoside substrates were conducted in MeCN, formation of aryl benzimidazopurinyl nucleoside derivatives was observed in many cases by C-N bond formation with the N1 ring nitrogen atom of the purine (carbazole and benzimidazole isomers are readily separated by chromatography). Whereas Pd^II/Pd^IV redox is responsible for carbazole formation under the metal-catalyzed conditions, in HFIP and MeCN radical cations and/or nitrenium ions can be intermediates. An extensive set of radical inhibition experiments was conducted and the data are presented.

Directed adenine functionalization for creating complex architectures for material and biological applications

In this feature article, targeted design strategies are outlined for modified adenine nucleobase derivatives in order to construct metal-mediated discrete complexes, ring-expanded purine skeletons, linear and catenated coordination polymers, shape-selective MOFs, and purine-capped nanoparticles, with a wide range of applications from gas and solvent adsorption to bioimaging agents and anticancer metallodrugs. The success of such design strategies could be ascribed to the rich chemistry of purine and pyrimidine derivatives, versatile coordination behavior, ability to bind a host of metal ions, which could be further tuned by the introduction of additional functionalities, and their inherent propensity to hydrogen bond and exhibit π-π interactions. These noncovalent interactions produce stable frameworks and network solids that are useful as advanced materials, and the biocompatibility of these ligand complexes provides an impetus for assessing novel biological applications.

Benzimidazopurine nucleosides from N6-aryl adenosine derivatives by PhI(OAc)2-mediated C-N bond formation, no metal needed

The reaction of a variety of N⁶-aryl 2'-deoxyadenosine and adenosine derivatives with PhI(OAc)₂ in 1,1,1,3,3,3-hexafluoro-2-propanol provides a facile access to benzimidazopurine nucleoside analogues by metal-free C-N bond formation with a purinyl nitrogen atom. These reactions likely proceed via radical-cation/radical processes as indicated by radical inhibition experiments.

Cp*CoIII-catalyzed directed C–H trifluoromethylthiolation of 2-phenylpyridines and 6-arylpurines

Cp*Co^III-catalyzed directed C–H trifluoromethylthiolation using N-trifluoromethylthiodibenzenesulfonimide as an electrophilic SCF₃ source is described.

Nickel-catalyzed C–H activation of purine bases with alkyl halides

C–H alkylations of purine nucleosides were achieved by means of user-friendly nickel catalysis with ample substrate scope and high levels of chemo, site and regio control, which among others enabled the direct fluorescent labeling of purines in terms of late stage diversification.

Purinyl N(3)-Directed Palladium-Catalyzed C-H Alkoxylation of N(9)-Arylpurines: A Late-Stage Strategy to Synthesize N(9)-(ortho-Alkoxyl)arylpurines

A palladium-catalyzed alkoxylation of N(9)-arylpurines with primary or secondary alcohols has been developed successfully, which is a rare C-H activation reaction of polynitrogenated purines and offers a late-stage strategy to synthesize N(9)-(ortho-alkoxyl)arylpurines. Although there are more than four nitrogen atoms present in the purine moiety, the reaction can be effectively conducted by sterically blocking the N(1) site for catalyst coordination and first employing the purinyl N(3) atom as a directing group.

Ruthenium-Catalyzed C-H Bond Activation Approach to Azolyl Aminals and Hemiaminal Ethers, Mechanistic Evaluations, and Isomer Interconversion

C(sp³)-N bond-forming reactions between benzotriazole and 5,6-dimethylbenzotriazole with N-methylpyrrolidinone, tetrahydrofuran, tetrahydropyran, diethyl ether, 1,4-dioxane, and isochroman have been conducted using RuCl₃•3H₂O/t-BuOOH in 1,2-dichloroethane. In all cases, N1 and N2 alkylation products were obtained, and these are readily separated by chromatography. One of these products, 1-(isochroman-1-yl)-5,6-dimethyl-1H-benzotriazole, was examined by X-ray crystallography. It is the first such compound to be analyzed by this method, and notably, the benzotriazolyl moiety is quasi-axially disposed, consistent with the anomeric effect. This has plausible consequences, not observed previously. In contrast to other hemiaminal ether-forming reactions, which proceed via radicals, this Ru-catalyzed process is not suppressed in the presence of a radical inhibitor. Therefore, an oxoruthenium-species-mediated rapid formation of an oxocarbenium intermediate is believed to occur. In the radical-trapping experiment, previously unknown products containing both the benzotriazole and the TEMPO unit have been identified. In these products, it is likely that the benzotriazole is introduced via a Ru-catalyzed C-N bond formation, whereas C-O bond-formation with TEMPO occurs via a radical reaction. We show that reactions of THF with TEMPO are influenced by ambient light. A competitive reaction of THF and THF-d₈ with benzotriazole indicated that C-H bond cleavage occurs ca. 5 times faster than C-D cleavage. This is comparable to other metal-mediated radical reactions of THF, but lower than that observed for a reaction catalyzed by n-Bu₄N⁺I^-. Detailed mechanistic experiments and comparisons are described. The catalytic system was also evaluated for reactions of benzimidazole, imidazole, 1,2,4-triazole, and 1,2,3-triazole with THF, and successful reactions were achieved in each case. In the course of our studies, we discovered an unexpected but significant isomerization of some of the benzotriazolyl hemiaminal ethers. This is plausibly attributable to the pseudoaxial orientation of the heterocycle in the products and the stability of oxocarbenium ions, both of which can contribute to C-N bond cleavage and reformation. Predominantly, the N2-isomers rearrange to the N1-isomers even upon storage at low temperature! This previously unknown phenomenon has also been studied and described.

Cobalt(iii)-catalyzed C–H halogenation of 6-arylpurines: facile entry into arylated, sulfenylated and alkoxylated 6-arylpurines

Cp*Co(iii)-catalyzed C–H halogenation of 6-arylpurines has been reported under mild conditions and this protocol was further applied for the synthesis of arylated, sulfenylated and alkoxylated purine analogues.

Regioselectivity of the C-metalation of 6-furylpurine: importance of directing effects

We report the C-metalation of 6-furylpurine with Pt(2+), Pd(2+), and Hg(2+). The ligand binds the metal ions in a bidentate fashion, involving the N7 purine atom and one of the furyl carbon atoms. The regioselectivity is determined by the metal ion. Pt(2+) and Pd(2+) coordinate the furyl moiety in its β position and Hg(2+) in its electronically preferred α position.

Modification of purine and pyrimidine nucleosides by direct C-H bond activation

Transition metal-catalyzed modifications of the activated heterocyclic bases of nucleosides as well as DNA or RNA fragments employing traditional cross-coupling methods have been well-established in nucleic acid chemistry. This review covers advances in the area of cross-coupling reactions in which nucleosides are functionalized via direct activation of the C8-H bond in purine and the C5-H or C6-H bond in uracil bases. The review focuses on Pd/Cu-catalyzed couplings between unactivated nucleoside bases with aryl halides. It also discusses cross-dehydrogenative arylations and alkenylations as well as other reactions used for modification of nucleoside bases that avoid the use of organometallic precursors and involve direct C-H bond activation in at least one substrate. The scope and efficiency of these coupling reactions along with some mechanistic considerations are discussed.

Late-stage diversification of biologically active pyridazinones via a direct C-H functionalization strategy

Divergent C-H functionalization reactions (arylation, carboxylation, olefination, thiolation, acetoxylation, halogenation, naphthylation) using a pyridazinone moiety as an internal directing group were successfully established. This approach offers a late-stage, ortho-selective diversification of a biologically active pyridazinone scaffold. Seven series of novel pyridazinone analogues were synthesized conveniently as the synthetic precursors of potential sortase A (SrtA) inhibitors.

Catalytic C–H bond functionalisation of purine and pyrimidine nucleosides: a synthetic and mechanistic perspective

C–H bond functionalisation of heteroarenes, especially nucleosides, has received a lot of attention in the past few years. This review describes the state-of the art in this area with a global aspiration for possibly functionalising purine and pyrimidine moieties in complex biomolecular systems.

Palladium-catalysed ortho-acylation of 6-anilinopurines/purine nucleosides via C–H activation

Purinyl N1-directed ortho-acylation of 6-anilinopurines was achieved in the presence of [Pd]-catalyst using aldehydes/α-oxocarboxylic acids as the acylating sources.

A versatile methodology for the regioselective C⁸-metalation of purine bases

Purine nucleobases are excellent ligands for metal ions, forming normally coordinative Werner-type bonds by utilizing the N donor atoms of the nucleobase skeleton. Here we show that purines such as 8-chlorocaffeine and 8-bromo-9-methyladenine react with [Pt(PPh3)4] under oxidative addition of the C(8)-halogen bond to the metal center. The resulting Pt(II) complexes feature a C(8)-bound ylidene ligand. Protonation of the ylidene at the N(7/9)-atom yields complexes bearing a protic N-heterocyclic carbene ligand derived from the purine base with an NMe,NH-substitution pattern.