C–H Amination of Purine Derivatives via Radical Oxidative Coupling

Visible-Light-Induced and Iodoform-Promoted Functionalization of Ether with Secondary Sulfonyl Amides

An iodoform-promoted functionalization of ether with secondary sulfonyl amides under visible-light irradiation was developed toward synthesis of hemiaminal skeleton with good to excellent isolated yields. The characterization of the isolated ether and iodoform complex revealed regioselective hydrogen atom transfer to initiate carbon radical formation and enabled the amination reaction with the sulfonamide.

Selectfluor-Induced Oxidative Amination of N-Heteroaromatics with Purine

An oxidative coupling reaction between purines and aromatic N-heterocycles was developed to synthesize a series of N-heteroaryl purine derivatives using Selectfluor as an oxidant at room temperature. This process uses a commercial oxidant, uses no base, metal, or other additives, is simple to carry out, and has a broad range of substrates.

Visible light-induced direct alkylation of the purine C8-H bond with ethers

The one-step visible light-induced direct alkylation of the C₈-H bond for purine derivatives by ethers was developed using Eosin Y as the photocatalyst and t-BuOOH as the oxidant at room temperature. This method describes the coupling of the α-C of the ether to the C₈ of purine. Of particular interest is that substrates include purines with various functional groups and even unprotected 9H-purines. The protocol provides an effective method for the synthesis of 8-alkylpurine derivatives with high atom economy and high regioselectivity.

Potential PDE4B inhibitors as promising candidates against SARS-CoV-2 infection

Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) is an RNA virus belonging to the coronavirus family responsible for coronavirus disease 2019 (COVID-19). It primarily affects the pulmonary system, which is the target of chronic obstructive pulmonary disease (COPD), for which many new compounds have been developed. In this study, phosphodiesterase 4 (PDE4) inhibitors are being investigated. The inhibition of PDE4 enzyme produces anti-inflammatory and bronchodilator effects in the lung by inducing an increase in cAMP concentrations. Piclamilast and rolipram are known selective inhibitors of PDE4, which are unfortunately endowed with common side effects, such as nausea and emesis. The selective inhibition of the phosphodiesterase 4B (PDE4B) subtype may represent an intriguing technique for combating this highly contagious disease with fewer side effects. In this article, molecular docking studies for the selective inhibition of the PDE4B enzyme have been carried out on 21 in-house compounds. The compounds were docked into the pocket of the PDE4B catalytic site, and in most cases, they were almost completely superimposed onto piclamilast. Then, in order to enlarge our study, drug-likeness prediction studies were performed on the compounds under study.

Cross dehydrogenation coupling reaction of purine derivatives with thioethers

A metal-free cross-dehydrogenation coupling method was established to synthesize N⁹ alkylated purine derivatives. Using PhI(OAc)₂ as the oxidant, versatile thioethers were successfully employed as alkylation reagents. Under the optimized conditions, a variety of alkylated purine derivatives and other aromatic N-heterocycles were obtained in moderate to good yields. The regioselectivity of this protocol which involves the reaction of unsymmetrical thioethers with purine derivatives was also studied.

Metal-free site-selective C-H cyanoalkylation of 8-aminoquinoline and aniline-derived amides with azobisisobutyronitrile

Using K₂S₂O₈, an efficient and metal-free site-selective C–H cyanoalkylation of 8-aminoquinoline and aniline-derived amides with AIBN (azobisisobutyronitrile) was developed. Without any catalyst, various substrates and functional groups were compatible to afford corresponding products in moderate to high yields. A mechanism study displayed that a radical–radical coupling process was involved via the N-centered radical generation and delocalization of aryl amides.

An efficient metal-free cyanoalkylation of 8-aminoquinoline and aniline-derived amides was achieved in the presence of K₂S₂O₈. The method showed good substrate tolerance and also suitable for bromination and dimerization reactions.

Decarboxylative N-Alkylation of Azoles through Visible-Light-Mediated Organophotoredox Catalysis

An organophotoredox-catalyzed decarboxylative cross-coupling between azole nucleophiles and aliphatic carboxylic acid-derived redox-active esters is demonstrated. This protocol efficiently installs various tertiary or secondary alkyl fragments onto the nitrogen atom of azole nucleophiles under mild and transition-metal-free conditions. The pyridinium additive successfully inhibits the formation of elimination byproducts from the carbocation intermediate. This reaction is applicable to the synthesis of a protein-degrader-like molecule containing an azole and a thalidomide.

Highly efficient chemical phosphorylation of 6-(4-phenylpiperazine-1-yl)-9-(β-D-ribofuranosyl)-9H-purine

Antimetabolites, which are metabolic antagonists used in the treatment of cancer and viral diseases by replacing metabolites, inhibit the action of metabolic enzymes and disrupt the pathways of synthesis of structural units necessary for the formation of nucleic acids. Purine antagonists, that are subunits of antimetabolites, reduce the production of purine bases, and hence, cause the nucleotide production to stop and bring about the death of cancer cells. Fludarabine (2-fluoro-ara-AMP), which is used in chemotherapy, is an antimetabolite of the purine class containing mono phosphate in its structure. In this study, a protocol was presented to effectively and efficiently synthesis of 6-(4-phenylpiperazine-1-yl)-9-(β-D-ribofuranosyl)-9H-purine-5'- O-phosphate compound in six steps and 25% overall yield starting with commercially available 6-chloropurine.

Iodine-catalyzed oxidative functionalization of purines with (thio)ethers or methylarenes for the synthesis of purin-8-one analogues

An efficient oxidative functionalization of purine-like substrates with (thio)ethers or methylarenes under mild conditions is described. Using I2 as the catalyst, and TBHP as the oxidant, this protocol provides a valuable synthetic tool for the assembly of a wide range of 9-alkyl(benzyl)purin-8-one derivatives with high atom- and step-economy and exceptional functional group tolerance.

Efficient Synthesis of α-Branched Purine-Based Acyclic Nucleosides: Scopes and Limitations of the Method

An efficient route to acylated acyclic nucleosides containing a branched hemiaminal ether moiety is reported via three-component alkylation of N-heterocycle (purine nucleobase) with acetal (cyclic or acyclic, variously branched) and anhydride (preferentially acetic anhydride). The procedure employs cheap and easily available acetals, acetic anhydride, and trimethylsilyl trifluoromethanesulfonate (TMSOTf). The multi-component reaction is carried out in acetonitrile at room temperature for 15 min and provides moderate to high yields (up to 88%) of diverse acyclonucleosides branched at the aliphatic side chain. The procedure exhibits a broad substrate scope of N-heterocycles and acetals, and, in the case of purine derivatives, also excellent regioselectivity, giving almost exclusively N-9 isomers.

Direct Sulfenylation of the Purine C8-H Bond with Thiophenols

The one-step copper-mediated regioselective formation of the C₈-S bond for purine derivatives with arylthiols was achieved using air as the green oxidant in the presence of 1.0 equiv of Na₂CO₃ and stoichiometric CuCl and 1,10-phenanthroline monohydrate. This method provides an economical, easy-to-handle, and effective method for the synthesis of 8-sulfenylpurine derivatives in moderate to excellent yields. The reaction is selective for C8 over C2 and C6. It also tolerates a free amine on the purine, and it has a wide substrate scope.

Direct functionalization of alkyl ethers to construct hemiaminal ether skeletons (HESs)

Hemiaminal ether skeletons (HESs) are important backbones generally found in organic compounds. Compared to traditional functional group transformations, strategies for the C-H functionalization of alkyl ethers to construct HESs have drawn much attention in the past decade. In this review, we focus on the recent progress in developing various pathways to construct HESs via the direct functionalization of alkyl ethers, including: (a) the transition-metal-catalyzed nitrene insertion pathway, (b) the transition-metal-free organonitrenoid insertion pathway,

Construction of N-S and C-N Bonds from Reactions of Benzofuroxans with DMSO or THF

Novel ring-opening reactions are achieved employing benzofuroxan as a new type of iminating or aminating reagent. These diverse transformations give access to three types of molecular scaffolds, N-aryl dimethylsulfoximines, methanesulfonamides, and hemiaminal ethers, which are important structural motifs in organic and medicinal chemistry. The procedures feature solvent-involved reactions, easily available starting materials, operational simplicity, high atom economy, and the potential further transformation of nitro group.

Metal-free oxidative cross-dehydrogenative coupling of quinones with benzylic C(sp3)–H bonds

A metal-free cross-dehydrogenative coupling of quinones with toluene derivatives has been established. A series of quinones were subjected to reaction with toluene derivatives in the presence of di-tertbutyl peroxide (DTBP) for direct synthesis of benzylquinones. The method exhibits good functional group tolerance, and desired products were obtained in moderate to good yields. Meanwhile, a radical pathway was proposed to describe the cross-dehydrogenative coupling of quinones with toluene derivatives.

A metal-free cross-dehydrogenative coupling of quinones with toluene derivatives has been established. A series of quinones were subjected to reaction with toluene derivatives in the presence of DTBP for direct synthesis of benzylquinones.

Silver(i)-mediated oxidative C(sp3)–H amination of ethers with azole derivatives under mild conditions

A silver(i)-mediated oxidative N–H/C(sp³)–H coupling of NH-azoles with ethers has been developed.

Efforts in redesigning the antileukemic drug 6-thiopurine: decreasing toxic side effects while maintaining efficacy

6-Thiopurine (6TP) is a currently prescribed drug in the treatment of diseases ranging from Crohn's disease to acute lymphocytic leukemia. While its potent mode of action is through incorporation into DNA as a thiol mimic of deoxyguanosine, severe toxicities are associated with its administration which hinder the potential therapeutic application. We have previously reported in vitro that the oxidative metabolites of 6TP, specifically 6-thiouric acid (6TU, K _i 7 μM), are potent inhibitors of UDP-glucose dehydrogenase (UDPGDH), an enzyme that is responsible for the formation of UDP-glucuronic acid (UDPGA), an essential substrate that is used in detoxification processes in the liver. An in vivo investigation was undertaken to probe if 6TU inhibits UDPGDH in rat hepatocytes, and it was observed that 6TU does greatly suppress the conjugation of bilirubin with UDPGA. The failed excretion of bilirubin is linked to a majority of the reported toxicities associated with 6TP administration. Efforts were undertaken for the construction of 6TP analogs, substituted at the C8 position, to reduce inhibition of UDPGDH while retaining therapeutic efficacy. Three new 6TP analogs bearing a halogen (Br, Cl, and F) at the C8 position have been achieved over five-synthetic steps in overall yields of 16 to 32%. Each of these analogs were shown to have reduced inhibition towards UDPGDH, with K _i values of 192, 163, 215 μM, respectively. In addition, the bromine, chlorine, and fluorine analogs were shown to possess cytotoxicity towards the REH cell line (acute lymphocytic leukemia) having IC₅₀ values of 9.54 μM (±0.97), 3.95 μM (±1.94), and 4.71 μM (±1.40), respectively. These three new 6TP analogs represent the first steps in the redesign of this potent anticancer agent into a better drug that possesses reduced toxic side effects while retaining therapeutic potency.

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.