Nucleic acid related compounds. 116. Nonaqueous diazotization of aminopurine nucleosides. Mechanistic considerations and efficient procedures with tert-butyl nitrite or sodium nitrite

Synthesis, antitumor and antibacterial activities of cordycepin derivatives

Twelve novel cordycepin derivatives were designed and synthesized with modification at positions of 2', 5'-hydroxyl and N⁶ amino groups of cordycepin. The results showed that the inhibitory activities of 3, 4b, 6c and 6d on A549 were comparable to the positive control gefitinib, and the inhibitory activity of 6a on A549 was better than that of gefitinib. Also, the inhibitory activities of twelve cordycepin derivatives against E. coli 1924, S. aureus 4220 and S. mutans 3289 were studied. Among them, 4b showed certain inhibitory on S. mutans 3289, while 6b showed certain inhibition on S. aureus 4220.

The practical acetylation of nucleosides using acetic anhydride/acetic acid as a reusable solvent

The highly practical acetylation of free nucleosides is achieved using acetic anhydride/acetic acid as a reusable solvent and acetylating regent. A series of nucleosides, including ribosyl, deoxyribosyl, arabinosyl, acyclic and pyranosyl, and many clinical drugs were acetylated efficiently, even on large scale (200 g).

Covalent Inhibition of the Histamine H3 Receptor

Covalent binding of G protein-coupled receptors by small molecules is a useful approach for better understanding of the structure and function of these proteins. We designed, synthesized and characterized a series of 6 potential covalent ligands for the histamine H₃ receptor (H₃R). Starting from a 2-amino-pyrimidine scaffold, optimization of anchor moiety and warhead followed by fine-tuning of the required reactivity via scaffold hopping resulted in the isothiocyanate H₃R ligand 44. It shows high reactivity toward glutathione combined with appropriate stability in water and reacts selectively with the cysteine sidechain in a model nonapeptide equipped with nucleophilic residues. The covalent interaction of 44 with H₃R was validated with washout experiments and leads to inverse agonism on H₃R. Irreversible binder 44 (VUF15662) may serve as a useful tool compound to stabilize the inactive H₃R conformation and to study the consequences of prolonged inhibition of the H₃R.

Cladribine Analogues via O⁶-(Benzotriazolyl) Derivatives of Guanine Nucleosides

Cladribine, 2-chloro-2'-deoxyadenosine, is a highly efficacious, clinically used nucleoside for the treatment of hairy cell leukemia. It is also being evaluated against other lymphoid malignancies and has been a molecule of interest for well over half a century. In continuation of our interest in the amide bond-activation in purine nucleosides via the use of (benzotriazol-1yl-oxy)tris(dimethylamino)phosphonium hexafluorophosphate, we have evaluated the use of O⁶-(benzotriazol-1-yl)-2'-deoxyguanosine as a potential precursor to cladribine and its analogues. These compounds, after appropriate deprotection, were assessed for their biological activities, and the data are presented herein. Against hairy cell leukemia (HCL), T-cell lymphoma (TCL) and chronic lymphocytic leukemia (CLL), cladribine was the most active against all. The bromo analogue of cladribine showed comparable activity to the ribose analogue of cladribine against HCL, but was more active against TCL and CLL. The bromo ribose analogue of cladribine showed activity, but was the least active among the C6-NH₂-containing compounds. Substitution with alkyl groups at the exocyclic amino group appears detrimental to activity, and only the C6 piperidinyl cladribine analogue demonstrated any activity. Against adenocarcinoma MDA-MB-231 cells, cladribine and its ribose analogue were most active.

Bisamidate Prodrugs of 2-Substituted 9-[2-(Phosphonomethoxy)ethyl]adenine (PMEA, adefovir) as Selective Inhibitors of Adenylate Cyclase Toxin from Bordetella pertussis

Novel small-molecule agents to treat Bordetella pertussis infections are highly desirable, as pertussis (whooping cough) remains a serious health threat worldwide. In this study, a series of 2-substituted derivatives of 9-[2-(phosphonomethoxy)ethyl]adenine (PMEA, adefovir), in their isopropyl ester bis(L-phenylalanine) prodrug form, were designed and synthesized as potent inhibitors of adenylate cyclase toxin (ACT) isolated from B. pertussis. The series consists of PMEA analogues bearing either a linear or branched aliphatic chain or a heteroatom at the C2 position of the purine moiety. Compounds with a small C2 substituent showed high potency against ACT without cytotoxic effects as well as good selectivity over human adenylate cyclase isoforms AC1, AC2, and AC5. The most potent ACT inhibitor was found to be the bisamidate prodrug of the 2-fluoro PMEA derivative (IC50 =0.145 μM). Although the bisamidate prodrugs reported herein exhibit overall lower activity than the bis(pivaloyloxymethyl) prodrug (adefovir dipivoxil), their toxicity and plasma stability profiles are superior. Furthermore, the bisamidate prodrug was shown to be more stable in plasma than in macrophage homogenate, indicating that the free phosphonate can be effectively distributed to target tissues, such as the lungs. Thus, ACT inhibitors based on acyclic nucleoside phosphonates may represent a new strategy to treat whooping cough.

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.

2,6-Di-chloro-9-(2',3',5'-tri-O-acetyl-β-d-ribo-furanos-yl)-9H-purine

The title synthetic analog of purine nucleosides, C₁₆H₁₆Cl₂N₄O₇, has its acetyl­ated β-furan­ose ring in a 3′β-envelope conformation, with the corresponding C atom deviating by 0.602 (5) Å from the rest of the ring. The planar part of the furan­ose ring forms a dihedral angle of 65.0 (1)° with the mean plane of the purine bicycle. In the crystal, mol­ecules form a three-dimensional network through multiple C—H⋯O and C—H⋯N hydrogen bonds and C—H⋯π interactions.

Pentakis(trifluoromethyl)benzenediazonium Cation: A Useful Building Block for the Syntheis of Trifluoromethyl-Substituted Derivatives

The sterically crowded, highly electron-deficient diazonium salt C₆ (CF₃ )₅ N₂ ⁺ BF₄ ^- has been synthesized and sample dediazotization reactions were carried out. New compounds, C₆ (CF₃ )₅ I, C₆ (CF₃ )₅ NO₂ , and C₆ (CF₃ )₅ NO were isolated and identified. Also, a more convenient route to C₆ (CF₃ )₅ NH₂ that gave good yield was achieved.

Nitric oxide promotes recycling of 8-nitro-cGMP, a cytoprotective mediator, into intact cGMP in cells

8-Nitro-cGMP is an endogenous nucleotide discovered under inflammation conditions as an important mediator of nitric oxide (NO) signaling. Besides cGMP-like behaviour, 8-nitro-cGMP exerts unique cytoprotective effects against oxidative stress. Although the formation of 8-nitro-cGMP from 8-nitro-GTP has previously been proposed, the mechanism by which excess or unused 8-nitro-cGMP is removed from cells remains unknown. In this study, we report a nitric oxide-dependent cellular conversion of 8-nitro-cGMP to intact cGMP in RAW 264.7 macrophage cells. In our experiments, we synthesized isotopically labeled 8-nitro-cGMP as a tool for metabolites analysis and identified 8-amino-cGMP as an initial metabolite of 8-nitro-cGMP using a LC-MS/MS technique. We also proved that endogenous 8-nitro-cGMP can be converted into 8-amino-cGMP by immunocytochemical staining with an antibody that specifically recognizes 8-amino-cGMP. Moreover, we showed that isotopically labeled 8-amino-cGMP is metabolized into cGMP under inflammation conditions. We propose that nitrosylation of 8-amino-cGMP occurs by NO formation under stress conditions and gives putative 8-diazonium-cGMP, which subsequently decomposes into cGMP. To the best of our knowledge, this study is the first to report reductive deamination of aminoguanine nucleotide at the C-8 position. The findings of this study collectively indicate that NO plays a crucial role not only in the production of 8-nitro-cGMP but also in its elimination under oxidative stress or inflammation.

Two-step, one-pot synthesis of inosine, guanosine, and 2'-deoxyguanosine O6-ethers via intermediate O6-(benzotriazol-1-yl) derivatives

A simple method for the etherification at the O(6)-position of silyl-protected inosine, guanosine, and 2'-deoxyguanosine is described. Typically, a THF solution of the silylated nucleoside is treated with 1H-benzotriazol-1-yloxy-tris(dimethylamino)phosphonium hexafluorophosphate (BOP) and Cs(2)CO(3) under a nitrogen atmosphere. Conversion to the O(6)-(benzotriazol-1-yl) ethers occurs within about 10 min for inosine, and within about 60 min for guanosine and 2'-deoxyguanosine. Then, for reaction with alcohols, the reaction mixture is evaporated and the O(6)-(benzotriazol-1-yl) ether is treated with Cs(2)CO(3) and an appropriate alcohol, at room temperature. On the other hand, for reaction with phenols, Cs(2)CO(3) and the appropriate phenol are added to the reaction mixture without evaporation, and the reaction is carried out at 70°C. Subsequently, workup, isolation, and purification lead to the requisite O(6)-alkyl or O(6)-aryl ethers in good to excellent yields.

Investigations into the origin of the molecular recognition of several adenosine deaminase inhibitors

Inhibitors of adenosine deaminase (ADA, EC 3.5.4.4) are potential therapeutic agents for the treatment of various health disorders. Several highly potent inhibitors were previously identified, yet they exhibit unacceptable toxicities. We performed a SAR study involving a series of C2 or C8 substituted purine-riboside analogues with a view to discover less potent inhibitors with a lesser toxicity. We found that any substitution at C8 position of nebularine resulted in total loss of activity toward calf intestinal ADA. However, several 2-substituted-adenosine, 8-aza-adenosine, and nebularine analogues exhibited inhibitory activity. Specifically, 2-Cl-purine riboside, 8-aza-2-thiohexyl adenosine, 2-thiohexyl adenosine, and 2-MeS-purine riboside were found to be competitive inhibitors of ADA with K(i) values of 25, 22, 6, and 3 μM, respectively. We concluded that electronic parameters are not major recognition determinants of ADA but rather steric parameters. A C2 substituent which fits ADA hydrophobic pocket and improves H-bonding with the enzyme makes a good inhibitor. In addition, a gg rotamer about C4'-C5' bond is apparently an important recognition determinant.

Efficient radiosynthesis of 2-[(18)f]fluoroadenosine: a new route to 2-[(18)f]fluoropurine nucleosides

An efficient method to incorporate the fluorine-18 radionuclide in 2-nitropurine-based nucleosides was developed. The nucleophilic radiofluorination of the labeling precursor with [(18)F]KF under aminopolyether-mediated conditions (Kryptofix 2.2.2/K2CO3) followed by deprotection was straightforward and, after formulation, gave 2-[(18)F]fluoroadenosine, ready for injection with a radiochemical yield of 45 ± 5%, a radiochemical purity of >98%, and a specific radioactivity up to 148 GBq/μmol. A micropositron emission tomography imaging and biodistribution study on rodents was reported.

Synthesis of N6 ,N6-Dialkyl Adenine Nucleosides With In Situ Formed Hexaalkylphosphorus Triamides

Reactions between secondary amines and phosphorus trichloride (PCl₃) leads to the formation of the corresponding tris(dialkylamino)phosphines or hexaalkylphosphorus triamides [HAPT: (R₂N)₃P]. Reaction of silyl-protected 2'-deoxyinosine and acetyl-protected inosine with the in situ formed HAPT and iodine (I₂) leads to the formation of N⁶,N⁶-dialkyl adenosine and 2'-deoxyadenosine. In some cases the stoichiometry of the amine is important as also the use of a tertiary amine base. The effect of amine stoichiometry on the reaction of HAPT with I₂ has been studied by ³¹P{¹H} NMR.

Synthesis and Antiviral Evaluation of 6-(Alkyl-heteroaryl)furo[2,3-d]pyrimidin-2(3H)-one Nucleosides and Analogues with Ethynyl, Ethenyl, and Ethyl Spacers at C6 of the Furopyrimidine Core

Sonogashira coupling strategies were employed to synthesize new furo[2,3-d]pyrimidin-2(3H)-one (FuPyrm) 2'-deoxynucleoside analogues. Partial or complete reduction of ethyne-linked compounds afforded ethenyl- and ethyl-linked derivatives. Levels of inhibition of varicella-zoster virus (VZV), human cytomegalovirus (HCMV), a broad range of other DNA and RNA viruses, and several cancer cell lines were evaluated in cell cultures. The anti-VZV potency decreased with increasing rigidity of the side chain at C6 of the FuPyrm ring in the order dec-1-yn-1-yl < dec-1-en-1-yl < decan-1-yl. In contrast, compounds with a rigid ethynyl spacer between C6 of the FuPyrm ring and a 4-alkylphenyl moiety were more potent inhibitors of VZV than the corresponding derivatives with an ethyl spacer. Replacement of the phenyl moiety in 6-(4-alkylphenyl) derivatives with a pyridine ring (in either regioisomeric orientation) gave analogues with increased solubility in methanol but reduced anti-VZV potency, and replacement with a pyrimidine ring reduced the anti-VZV activity even further. The pyridine-ring-containing analogues were approximately 20-fold more potent inhibitors of VZV than acyclovir but were approximately 6-fold less potent than BVDU and approximately 60-fold weaker than the most active 6-(4-pentylphenyl)-substituted prototype.

S(N)Ar displacements with 6-(fluoro, chloro, bromo, iodo, and alkylsulfonyl)purine nucleosides: synthesis, kinetics, and mechanism1

SNAr reactions with 6-(fluoro, chloro, bromo, iodo, and alkylsulfonyl)purine nucleosides and nitrogen, oxygen, and sulfur nucleophiles were studied. Pseudo-first-order kinetics were measured with 6-halopurine compounds, and comparative reactivities were determined versus a 6-(alkylsulfonyl)purine nucleoside. The displacement reactivity order was: F > Br > Cl > I (with BuNH2/MeCN), F > Cl approximately Br > I (with MeOH/1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)/MeCN), and F > Br > I > Cl [with K+ -SCOCH3/dimethyl sulfoxide (DMSO)]. The order of reactivity with a weakly basic arylamine (aniline) was: I > Br > Cl > F (with 5 equiv of aniline in MeCN at 70 degrees C). However, those reactions with aniline were autocatalytic and had significant induction periods ( approximately 50 min for the iodo compound and approximately 6 h for the fluoro analogue). Addition of trifluoroacetic acid (TFA) eliminated the induction period, and the order then was F > I > Br > Cl (with 5 equiv of aniline and 2 equiv of TFA in MeCN at 50 degrees C). The 6-(alkylsulfonyl)purine nucleoside analogue was more reactive than the 6-fluoropurine compound with both MeOH/DBU/MeCN and iPentSH/DBU/MeCN and was more reactive than the Cl, Br, and I compounds with BuNH2 and aniline/TFA. Titration of the 6-halopurine nucleosides in CDCl3 with TFA showed progressive downfield 1H NMR chemical shifts for H8 (larger) and H2 (smaller). The major site of protonation as N7 for both the 6-fluoro and 6-bromo analogues was confirmed by large upfield shifts ( approximately 16 ppm) of the 15N NMR signal for N7 upon addition of TFA (1.6 equiv). Mechanistic considerations and resolution of prior conflicting results are presented.

Synthesis and Reactions of 2-Chloro- and 2-Tosyloxy-2‘-deoxyinosine Derivatives

Convenient syntheses of 2-chloro- and 2-tosyloxy-2'-deoxyinosine as their tert-butyldimethylsilyl ethers are described. Both compounds can be synthesized via a common route and rely on commercially available 2'-deoxyguanosine. The present method leading to the chloro nucleoside is operationally simpler compared to previously reported glycosylation techniques where isomeric products were obtained. Both electrophilic nucleosides can be used for the preparation of N-substituted 2'-deoxyguanosine analogues via displacement of the leaving groups, and a comparison of their reactivities shows the chloro analogue to be superior. Interestingly, a Pd catalyst-mediated, two-step, one-pot conversion of an allyl-protected chloro nucleoside intermediate to the final modified 2'-deoxyguanosine derivatives is also feasible. On the basis of these observations, initial assessments of Pd-catalyzed aryl amination as well as a C-C cross-coupling have also been performed with the chloro and tosyloxy nucleoside substrates. Results indicate a potentially high synthetic utility of 2-chloro-2'-deoxyinosine and in many instances this derivative can supplant the bromo and fluoro analogues that are more cumbersome to prepare or are not readily available.

Synthesis and biological evaluation of 2',3'-didehydro-2',3'-dideoxy-9-deazaguanosine, a monophosphate prodrug and two analogues, 2',3'-dideoxy-9-deazaguanosine and 2',3'-didehydro-2',3'-dideoxy-9-deazainosine

2',3'-Didehydro-2',3'-dideoxy-9-deazaguanosine (1), its monophosphate prodrug (2), and two analogues, 2',3'-dideoxy-9-deazaguanosine (3) and 2',3'-didehydro-2',3'-dideoxy-9-deazainosine (4), have been synthesized from benzoylated 9-deazaguanosine (5). Basic hydrolysis of 5, selective protection of the 2-amino and 5'-hydroxy functions with isobutyryl and silyl groups, respectively, followed by reaction with thiocarbonyldiimidazole gave the cyclic thiocarbonate, which, upon reaction with triethyl phosphite, followed by deprotection, afforded 1. Treatment of 1 with phenyl methoxyalaninylphosphochloridate and N-methylimidazole gave 2. Catalytic hydrogenation of 1 gave 3. Hydrodediazoniation of 1 with tert-butyl nitrite and tris(trimethylsilyl)silane gave 4. Compounds 1-4 were found to be inactive against the human immunodeficiency virus and exhibited minimal to no cytotoxic activity against the L1210 leukemia, CCRF-CEM lymphoblastic leukemia, and B16F10 melanoma in vitro.

Synthesis and biological evaluation of 2- and 7-substituted 9-deazaadenosine analogues

A series of 2-halogen and 7-alkyl substituted analogues of 9-deazaadenosine and 2'-deoxy-9-deazaadenosine was synthesized by new efficient methodology involving transformation of corresponding 9-deazaguanosine and 2'-deoxyguanosine, which in turn were synthesized by direct C-glycosylation of 1-benzyl-9-deazaguanine with 1-O-acetyl-2,3,5-tri-O-benzoyl-D-ribofuranose and methyl 2-deoxy-3,5-di-O-(p-toluoyl)-D-ribofuranoside, respectively. Deoxychlorination of C6 and diazotization/chloroor fluoro-dediazoniation of the sugar-protected 9-deazaguanosine, followed by selective ammonolysis at C6 and deprotection of the sugar moiety, gave 2-chloro- and 2-fluoro-9-deazaadenosine (6 and 9). Substitution of the 7-position of the dihalogen-intermediate with alkyl groups, followed by ammonolysis and deprotection, provided 2-chloro-7-alkyl-9-deazaadenosines (13a-e) and 2-fluoro-7-benzyl-9-deazaadenosine (13f). Catalytic hydrogenation of 13a-e gave 7-alkyl-9-deazaadenosines 14a-e. Similarly, 2-chloro-2'-deoxy-9-deazaadenosine (21), 2-chloro-2'-deoxy-7-methyl-9-deazaadenosine (25), 2'-deoxy-9-deazaadenosine (22), and 2'-deoxy-7-methyl-9-deazaadenosine (26) were prepared from sugar-protected 2'-deoxy-9-deazaguanosine. Among these compounds, 7-benzyl-9-deazaadenosine (14b) showed the most potent cytotoxic activity, with IC50 values of 0.07, 0.1, 0.2 and 1.5 microM, while both 7-methyl-9-deazaadenosine (14a) and 2-fluoro-9-deazaadenosine (9) also demonstrated significant cytotoxic activity with IC50 values of 0.4, 0.7, 0.3, and 1.5 microM, and 1.5, 0.9, 0.3, and 5 microM against L 1210 leukemia, P388 leukemia, CCRF-CEM lymphoblastic leukemia, and B16F10 melanoma cells, respectively.

Functionalization of guanosine and 2'-deoxyguanosine at C6: a modified Appel process and S(N)Ar displacement of imidazole

Treatment of sugar-protected 2-N-trityl derivatives of guanosine and 2'-deoxyguanosine with imidazole/triphenylphosphine/iodine/ethyldiisopropylamine gives the corresponding 6-(imidazol-1-yl)-2-(tritylamino)purine nucleosides. S(N)Ar displacement of the imidazole moiety with nucleophiles provides 2-amino-6-substituted-purine nucleosides and 2'-deoxynucleosides.

Azoles as Suzuki Cross-Coupling Leaving Groups: Syntheses of 6-Arylpurine 2‘-Deoxynucleosides and Nucleosides from 6-(Imidazol-1-yl)- and 6-(1,2,4-Triazol-4-yl)purine Derivatives1

[reaction: see text] 6-(Imidazol-1-yl)-, 6-(benzimidazol-1-yl)-, and 6-(1,2,4-triazol-4-yl)purine nucleosides undergo a nickel-mediated C-C cross-coupling of azole-substituted purine derivatives with arylboronic acids to give good yields of 6-arylpurine nucleosides.

SNAr Iodination of 6-Chloropurine Nucleosides: Aromatic Finkelstein Reactions at Temperatures Below −40 °C1

Mesitoyl or toluoyl esters of inosine and 2'-deoxyinosine were deoxychlorinated at C6 to give the crystalline 6-chloropurine nucleoside derivatives, which underwent quantitative conversion to the 6-iodo analogues with NaI/TFA/butanone at -50 to -40 degrees C. The 6-iodo compounds were efficient substrates for SNAr, Sonogashira, and Suzuki-Miyaura reactions, in contrast with the 6-chloro analogues, and gave good to high yields of C-N and C-C coupled products.

Efficient syntheses of 2-chloro-2'-deoxyadenosine (cladribine) from 2'-deoxyguanosine(1)

We report efficient syntheses of the clinical agent cladribine (2-chloro-2'-deoxyadenosine, CldAdo), which is the drug of choice against hairy-cell leukemia and other neoplasms, from 2'-deoxyguanosine. Treatment of 3',5'-di-O-acetyl- or benzoyl-2'-deoxyguanosine (1) with 2,4,6-triisopropyl- or 4-methylbenzenesulfonyl chloride gave high yields of the 6-O-arylsulfonyl derivatives 2 or 2'b. Deoxychlorination at C6 of 1 also proceeded to give the 2-amino-6-chloropurine derivative 5 in excellent yields. The nonaqueous diazotization/chloro dediazoniation (acetyl chloride/benzyltriethylammonium nitrite) of 2, 2'b, and 5 gave the 2-chloropurine derivatives 3, 3'b, and 6, respectively. The selective ammonolysis at C6 (arylsulfonate with 3 or chloride with 6) and accompanying deprotection of the sugar moiety gave CldAdo (64-75% overall yield from 1).