Facile synthesis of O6-alkyl-, O6-aryl-, and diaminopurine nucleosides from 2'-deoxyguanosine

Bracken Fern Carcinogen, Ptaquiloside, Forms a Guanine O6-Adduct in DNA

Bracken fern (Pteridium sp.) is a viable and vigorous plant with invasive potential, ingestion of which causes chronic illness and cancers in farm animals. Bracken is a suspected human carcinogen, and exposure can result from ingestion of bracken-contaminated water, dairy products, or meat derived from livestock grazing on bracken fern. Bracken is also consumed in the diets of some communities. Ptaquiloside (PTQ), a known bracken carcinogen, is an illudane-type glycoside that forms a highly reactive electrophile, PTQ dienone, known to produce N7-guanine and N3-adenine adducts in DNA. Here, we demonstrate for the first time that PTQ dienone also produces an O6-alkylguanine (O6-PTBguanine) in DNA. Since O6-alkylguanines in DNA can be mutagenic, this work provides a potential mechanistic link between PTQ exposure and carcinogenicity. O6-PTBguanine is poorly repaired by O6-methylguanine-DNA methyltransferase that acts on other O6-alkylguanines, further highlighting the potential risk of exposure to bracken and PTQ.

Base Modifications of Nucleosides via the Use of Peptide-Coupling Agents, and Beyond

Several naturally occurring purine and pyrimidine nucleosides contain an amide linkage as part of the heterocyclic aglycone. Enolization of the amide and conversion to leaving groups at the amide carbon atom permits base modification by addition-elimination types of processes. Although a number of methods have been developed over the years for accomplishing such conversions, the present Personal Account describes efforts from the Lakshman laboratories. Facile activation of the amido groups in nucleobases can be achieved with peptide-coupling agents. Subsequent reaction with nucleophiles then accomplishes the base modifications. In many cases, the activation and displacement steps can be done as two-step, one-pot processes, whereas in other cases, discrete storable activated nucleosides can be isolated for subsequent displacement reactions. Using such an approach a wide range of nucleoside base modifications is readily achievable. In many instances, mechanistic investigations have been conducted so as to understand the activation process.

1,2,3-Triazoles as leaving groups: SNAr reactions of 2,6-bistriazolylpurines with O- and C-nucleophiles

A new approach was designed for the synthesis of C6-substituted 2-triazolylpurine derivatives. A series of substituted products was obtained in SNAr reactions between 2,6-bistriazolylpurine derivatives and O- and C-nucleophiles under mild conditions. The products were isolated in yields up to 87%. The developed C-O and C-C bond forming reactions clearly show the ability of the 1,2,3-triazolyl ring at the C6 position of purine to act as leaving group.

Independent Generation and Reactivity of 2'-Deoxyguanosin-N1-yl Radical

2'-Deoxyguanosin-N1-yl radical (dG(N₁-H)^•) is the thermodynamically favored one-electron oxidation product of 2'-deoxyguanosine (dG), the most readily oxidized native nucleoside. dG(N₁-H)^• is produced by the formal dehydration of a hydroxyl radical adduct of dG as well as by deprotonation of the corresponding radical cation. dG(N₁-H)^• were formed as a result of the indirect and direct effects of ionizing radiation, among other DNA damaging agents. dG(N₁-H)^• was generated photochemically (λ_max = 350 nm) from an N-aryloxy-naphthalimide precursor (3). The quantum yield for photochemical conversion of 3 is ∼0.03 and decreases significantly in the presence O₂, suggesting that bond scission occurs from a triplet excited state. dG is formed quantitatively in the presence of excess β-mercaptoethanol. In the absence of a reducing agent, dG(N₁-H)^• oxidizes 3, decreasing the dG yield to ∼50%. Addition of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodGuo) as a sacrificial reductant results in a quantitative yield of dG and two-electron oxidation products of 8-oxodGuo. N-Aryloxy-naphthalimide 3 is an efficient and high-yielding photochemical precursor of dG(N₁-H)^• that will facilitate mechanistic studies on the reactivity of this important reactive intermediate involved in DNA damage.

Independent Photochemical Generation and Reactivity of Nitrogen-Centered Purine Nucleoside Radicals from Hydrazines

Photochemical precursors that produce dA^• and dG(N₂-H)^• are needed to investigate their reactivity. The synthesis of two 1,1-diphenylhydrazines (1, 2) and their use as photochemical sources of dA^• and dG(N₂-H)^• is presented. Trapping studies indicate production of these radicals with good fidelity, and 1 was incorporated into an oligonucleotide via solid-phase synthesis. Cyclic voltammetric studies show that reduction potentials of 1 and 2 are lower than those of widely used "hole sinks", e.g., 8-oxodGuo and 7-deazadGuo, to investigate DNA-hole transfer processes. These molecules could be useful (a) as sources of dA^• and dG(N₂-H)^• at specific sites in oligonucleotides and (b) as "hole sinks" for the study of DNA-hole transfer processes.

Bypass of Mutagenic O(6)-Carboxymethylguanine DNA Adducts by Human Y- and B-Family Polymerases

The generation of chemical alkylating agents from nitrosation of glycine and bile acid conjugates in the gastrointestinal tract is hypothesized to initiate carcinogenesis. O(6)-carboxymethylguanine (O(6)-CMG) is a product of DNA alkylation derived from nitrosated glycine. Although the tendency of the structurally related adduct O(6)-methylguanine to code for the misincoporation of TTP during DNA replication is well-established, the impact of the presence of the O(6)-CMG adduct in a DNA template on the efficiency and fidelity of translesion DNA synthesis (TLS) by human DNA polymerases (Pols) has hitherto not been described. Herein, we characterize the ability of the four human TLS Pols η, ι, κ, and ζ and the replicative Pol δ to bypass O(6)-CMG in a prevalent mutational hot-spot for colon cancer. The results indicate that Pol η replicates past O(6)-CMG, incorporating dCMP or dAMP, whereas Pol κ incorporates dCMP only, and Pol ι incorporates primarily dTMP. Additionally, the subsequent extension step was carried out with high efficiency by TLS Pols η, κ, and ζ, while Pol ι was unable to extend from a terminal mismatch. These results provide a first basis of O(6)-CMG-promoted base misincorporation by Y- and B-family polymerases potentially leading to mutational signatures associated with colon cancer.

C-C cross-coupling reactions of O6-alkyl-2-haloinosine derivatives and a one-pot cross-coupling/O6-deprotection procedure

Reaction conditions for the CC cross-coupling of O(6)-alkyl-2-bromo- and 2-chloroinosine derivatives with aryl-, hetaryl-, and alkylboronic acids were studied. Optimization experiments with silyl-protected 2-bromo-O(6)-methylinosine led to the identification of [PdCl(2)(dcpf)]/K(3)PO(4) in 1,4-dioxane as the best conditions for these reactions (dcpf=1,1'-bis(dicyclohexylphosphino)ferrocene). Attempted O(6)-demethylation, as well as the replacement of the C-6 methoxy group by amines, was unsuccessful, which led to the consideration of Pd-cleavable groups such that C-C cross-coupling and O(6)-deprotection could be accomplished in a single step. Thus, inosine 2-chloro-O(6)-allylinosine was chosen as the substrate and, after re-evaluation of the cross-coupling conditions with 2-chloro-O(6)-methylinosine as a model substrate, one-step C-C cross-coupling/deprotection reactions were performed with the O(6)-allyl analogue. These reactions are the first such examples of a one-pot procedure for the modification and deprotection of purine nucleosides under C-C cross-coupling conditions.

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.

Photolabile N-hydroxypyrid-2(1H)-one derivatives of guanine nucleosides: a new method for independent guanine radical generation

One-electron oxidized guanine is an important reactive intermediate in the formation of oxidatively generated damage in DNA and a variety of methods have been utilized for the abstraction of a single electron from the guanine moiety. In this study, an alternative approach for the site specific, independent generation of the guanine radical, utilizing N-hydroxypyrid-2(1H)-one as a photolabile modifier of guanine, is proposed. Novel photolabile 6-[(1-oxido-2-pyridinyl)oxo]-6-deoxy- and 2',6-dideoxy-guanosine derivatives capable of generating the neutral guanine radical (G(-H)*) upon photolysis were synthesized and characterized. The generation of G(-H)* proceeds through homolysis of the N-O bond and was confirmed through continuous photolysis product analysis and trapping studies, as well as laser flash photolysis experiments.

A simple method for C-6 modification of guanine nucleosides

A facile method for the introduction of various substituents at the C-6 position of guanosine and 2'-deoxyguanosine is reported. In a simple, 1-step transformation, tert-butyldimethylsilyl protected guanosine and 2'-deoxyguanosine were converted to the O(6)-(benzotriazol-1-yl) derivatives via reaction with 1H-benzotriazol-1-yloxy-tris(dimethylamino)phosphonium hexafluorophosphate (BOP) and 1,8-diazabicyclo[5.4.0]undec-7ene (DBU). The easily isolated, stable and storable, O(6)-(benzotriazol-1-yl) guanosine derivatives upon exposure to a range of nucleophiles, under appropriate conditions, led to the C-6 modified 2-amino purine nucleoside analogues in good yields.

O6-(benzotriazol-1-yl)inosine derivatives: easily synthesized, reactive nucleosides

A novel class of O6-(benzotriazol-1-yl)inosine as well as the corresponding 2'-deoxy derivatives can be conveniently prepared by a reaction between sugar-protected or -unprotected inosine or 2'-deoxyinosine nucleosides and 1H-benzotriazol-1-yloxy-tris(dimethylamino)phosphonium hexafluorophosphate (BOP). The reaction appears to proceed via a nucleoside phosphonium salt, and in the absence of any additional nucleophile, the released 1-hydroxybenzotriazole undergoes reaction with the formed phosphonium salt leading to the requisite O6-(benzotriazol-1-yl)inosine or 2'-deoxyinosine derivatives. Isolation and characterization of the phosphonium salt as well as analysis by 31P{1H} NMR appear to be consistent with this reaction pathway. The resulting O6-(benzotriazol-1-yl)inosine derivatives are effective as electrophilic nucleosides, undergoing facile reactions with a variety of nucleophiles such as alcohols, phenols, amines, and a thiol. Unusual and challenging nucleoside derivatives such as an aryl-bridged dimer, a nucleoside-amino acid conjugate, and a nucleoside-nucleoside dimer have also been synthesized from the O6-(benzotriazol-1-yl)-2'-deoxyinosine derivative. Finally, a fully protected DNA building block, the O6-(benzotriazol-1-yl)-2'-deoxyinosine 5'-O-DMT 3'-O-phosphoramidite, has been prepared and a preliminary evaluation of its use for DNA modification has been performed. Results from these studies indicate several important facts: A single, simple methodological approach provides a class of stable, isolable ribo and 2'-deoxyribonucleoside derivatives that possess excellent reactivity for SNAr chemistry with a wide range of nucleophiles. Also, a benzotriazolyl nucleoside phosphoramidite appears to be a suitable reagent for incorporation into DNA for purposes of site-specific DNA modification.

Pd−Xantphos-Catalyzed Direct Arylation of Nucleosides

Direct arylation of the exocyclic amino groups of nucleosides represents a simple approach to N-aryl nucleoside derivatives. To date, one limitation has been that only electron-deficient aryl bromides and triflates possessed adequate reactivity for efficient, direct N-arylation of nucleosides. We demonstrate herein that Pd-Xantphos catalytic systems lead to successful N-arylation of suitably protected 2'-deoxyadenosine and 2'-deoxyguanosine with a wide range of aryl bromides.

Novel synthesis of O6-alkylguanine containing oligodeoxyribonucleotides as substrates for the human DNA repair protein, O6-methylguanine DNA methyltransferase (MGMT)

The human DNA repair protein O6-methylguanine DNA methyltransferase (MGMT) dealkylates mutagenic O6-alkylguanine lesions within DNA in an irreversible reaction which results in inactivation of the protein. MGMT also provides resistance of tumours to alkylating agents used in cancer chemotherapy and its inactivation is therefore of particular clinical importance. We describe a post-DNA synthesis strategy which exploits the novel, modified base 2-amino-6-methylsulfonylpurine and allows access for the first time to a wide variety of oligodeoxyribonucleotides (ODNs) containing O6-alkylguanines. One such ODN containing O6-(4-bromothenyl)guanine is the most potent inactivator described to date with an IC50 of 0.1 nM.

Polar, functionalized guanine-O6 derivatives resistant to repair by O6-alkylguanine-DNA alkyltransferase: implications for the design of DNA-modifying drugs

The protein O6-alkylguanine-DNA alkyltransferase (Atase) is responsible for the repair of DNA lesions generated by several clinically important anti-cancer drugs; this is manifest as active resistance in those cancer cell lines proficient in Atase expression. Novel O6-substituted guanine analogues have been synthesized, bearing acidic, basic and hydrogen bonding functional groups. In contrast to existing O6-modified purine analogues, such as methyl or benzyl, the new compounds were found to resist repair by Atase even when tested at concentrations much higher than O6-benzylguanine, a well-established Atase substrate active both in vitro and in vivo. The inactivity of the new purines as covalent substrates for Atase indicates that agents to deliver these groups to DNA would represent a new class of DNA-modifying drug that circumvents Atase-mediated resistance.

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.

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.

Facile Pd-catalyzed cross-coupling of 2'-deoxyguanosine O6-arylsulfonates with arylboronic acids

[reaction: see text]. The O6-(2-mesitylenesulfonyl) derivative of 2'-deoxyguanosine undergoes a facile palladium-mediated C-C cross-coupling with arylboronic acids. Demonstrating the general applicability of this method, the synthesis of a previously undescribed class of 2-amino-6-arylpurine 2'-deoxynucleosides has been accomplished. The study also describes an evaluation of the O6-(2,4,6-triisopropylphenylsulfonyl) and the O6-(4-toluenesulfonyl) derivatives for the cross-coupling.

6-bromopurine nucleosides as reagents for nucleoside analogue synthesis

Surprisingly facile direct substitution reactions with acetyl-protected 6-bromopurine nucleosides are described. Included in the series of bromonucleosides studied is the guanosine derivative N(2)-2',3',5'-tetraacetyl-6-bromopurine ribonucleoside, the synthesis of which is reported here for the first time. Brominated nucleosides had not previously been considered optimal substrates for S(N)Ar reactions given the general reactivity trend for halogenated aromatic systems (i.e. F > Cl > Br > I). However, even weakly nucleophilic aromatic amines give high yields of the substitution products in polar solvents with these 6-bromopurine nucleosides. For primary aromatic amines, secondary aliphatic amines, and imidazole, reaction takes place only at C6, with no effect on the acetyl-protected ribose. In addition, we report the first synthesis of 3',5'-di-O-acetyl-6-bromopurine-2'-deoxyribonucleoside and its reaction with an arylamine in MeOH in the absence of added metal catalyst. Thus, C6-arylamine derivatives of both adenosine and 2'-deoxyadenosine can be prepared via simple S(N)Ar reactions with the corresponding 6-bromo precursor. We also describe high yielding and C6-selective substitution reactions with 6-bromonucleosides using alcohol and thiol nucleophiles in the presence of added base (DBU). Finally, C6-bromonucleosides are shown to be readily hydrogenated to give purine or 2-aminopurine products in good yield. This work increases the arsenal of reactions and strategies available for the synthesis of nucleoside analogues as potential biochemical tools or new therapeutics.

Mild and efficient functionalization at C6 of purine 2'-deoxynucleosides and ribonucleosides

[reaction: see text] Treatment of sugar-protected inosine and 2'-deoxyinosine derivatives with a cyclic secondary amine or imidazole and I(2)/Ph(3)P/EtN(i-Pr)(2)/(CH(2)Cl(2) or toluene) gave quantitative conversions into 6-N-(substituted)purine nucleosides. S(N)Ar reactions with 6-(imidazol-1-yl) derivatives gave 6-(N, O, or S)-substituted products. The 6-(benzylsulfonyl) group underwent S(N)Ar displacement with an arylamine at ambient temperature.