Radical reactions in aqueous medium using (Me3Si)3SiH

Nitrogen-Centered Radicals Derived from Azidonucleosides

Azido-modified nucleosides have been extensively explored as substrates for click chemistry and the metabolic labeling of DNA and RNA. These compounds are also of interest as precursors for further synthetic elaboration and as therapeutic agents. This review discusses the chemistry of azidonucleosides related to the generation of nitrogen-centered radicals (NCRs) from the azido groups that are selectively inserted into the nucleoside frame along with the subsequent chemistry and biological implications of NCRs. For instance, the critical role of the sulfinylimine radical generated during inhibition of ribonucleotide reductases by 2'-azido-2'-deoxy pyrimidine nucleotides as well as the NCRs generated from azidonucleosides by radiation-produced (prehydrated and aqueous) electrons are discussed. Regio and stereoselectivity of incorporation of an azido group ("radical arm") into the frame of nucleoside and selective generation of NCRs under reductive conditions, which often produce the same radical species that are observed upon ionization events due to radiation and/or other oxidative conditions that are emphasized. NCRs generated from nucleoside-modified precursors other than azidonucleosides are also discussed but only with the direct relation to the same/similar NCRs derived from azidonucleosides.

Radical Reactions in Organic Synthesis: Exploring in-, on-, and with-Water Methods

Radical reactions in water or aqueous media are important for organic synthesis, realizing high-yielding processes under non-toxic and environmentally friendly conditions. This overview includes (i) a general introduction to organic chemistry in water and aqueous media, (ii) synthetic approaches in, on, and with water as well as in heterogeneous phases, (iii) reactions of carbon-centered radicals with water (or deuterium oxide) activated through coordination with various Lewis acids, (iv) photocatalysis in water and aqueous media, and (v) synthetic applications bioinspired by naturally occurring processes. A wide range of chemical processes and synthetic strategies under different experimental conditions have been reviewed that lead to important functional group translocation and transformation reactions, leading to the preparation of complex molecules. These results reveal how water as a solvent/medium/reagent in radical chemistry has matured over the last two decades, with further discoveries anticipated in the near future.

Super silyl-based stable protecting groups for both the C- and N-terminals of peptides: applied as effective hydrophobic tags in liquid-phase peptide synthesis

Tag-assisted liquid-phase peptide synthesis (LPPS) is one of the important processes in peptide synthesis in pharmaceutical discovery. Simple silyl groups have positive effects when incorporated in the tags due to their hydrophobic properties. Super silyl groups contain several simple silyl groups and play an important role in modern aldol reactions. In view of the unique structural architecture and hydrophobic properties of the super silyl groups, herein, two new types of stable super silyl-based groups (tris(trihexylsilyl)silyl group and propargyl super silyl group) were developed as hydrophobic tags to increase the solubility in organic solvents and the reactivity of peptides during LPPS. The tris(trihexylsilyl)silyl group can be installed at the C-terminal of the peptides in ester form and N-terminal in carbamate form for peptide synthesis and it is compatible with hydrogenation conditions (Cbz chemistry) and Fmoc-deprotection conditions (Fmoc chemistry). The propargyl super silyl group is acid-resistant, which is compatible with Boc chemistry. Both tags are complementary to each other. The preparation of these tags requires less steps than previously reported tags. Nelipepimut-S was synthesized successfully with different strategies using these two types of super silyl tags.

Recent advances in electrochemical deoxygenation reactions of organic compounds

As naturally abundant and recyclable industrial feedstock, alcohols and carboxylic acids have drawn tremendous attention in medicinal chemistry and polymer chemistry. The selective C-O cleavage of the hydroxyl group represents an appealing strategy to deliver alkyl and carbonyl moieties into organic molecules. Classical examples of hydroxyl activation include the Appel reaction, Mitsunobu reaction, and Barton-McCombie deoxygenation. However, these early approaches still require large amounts of oxidants or reductants, and suffer from harsh conditions and low atom economy. Electrosynthesis has proven to be an effective and mild way of the modern chemical industry, avoiding the use of chemical oxidants/reductants through the action of an electric current. In this review, we have summarized the recent advances in electrochemical deoxygenation reactions and categorized the deoxygenation methods by different functionalities.

Hydroamination of Unactivated Alkenes with Aliphatic Azides

We report here an efficient and highly diastereoselective intermolecular anti-Markovnikov hydroamination of unactivated alkenes with aliphatic azides in the presence of silane. The system tolerates a wide range of azides and alkenes and operates with alkene as limiting reagent. Mechanistic studies suggest a radical chain pathway that involves aminium radical formation, radical addition to alkenes and HAT from silane to β-aminium alkyl radical. The use of sterically bulky silane is proposed to contribute to the excellent diastereoselectivity for HAT. Computational analysis uncovers the reaction pathway of aliphatic azide activation with silyl radical for aminyl radical formation.

Applications of Halogen-Atom Transfer (XAT) for the Generation of Carbon Radicals in Synthetic Photochemistry and Photocatalysis

The halogen-atom transfer (XAT) is one of the most important and applied processes for the generation of carbon radicals in synthetic chemistry. In this review, we summarize and highlight the most important aspects associated with XAT and the impact it has had on photochemistry and photocatalysis. The organization of the material starts with the analysis of the most important mechanistic aspects and then follows a subdivision based on the nature of the reagents used in the halogen abstraction. This review aims to provide a general overview of the fundamental concepts and main agents involved in XAT processes with the objective of offering a tool to understand and facilitate the development of new synthetic radical strategies.

Radical "On Water" Addition to the C═N Bond of Hydrazones: A Synthesis of Isoindolinone Derivatives

A radical "on water" addition to the C═N bond of hydrazones has been described. Hydrazone, diphenylsilane, alkyl iodide, and triethylborane afforded the corresponding addition products "on water" in good yields. A significant solvent effect was observed from the water. The developed protocol can be applied to the synthesis of 3-substituted isoindolinone derivatives. Moreover, the process offers environmentally benign tin-free radical reaction conditions.

Reductions of aryl bromides in water at room temperature

Micellar nanoreactors derived from commercially available 'Nok' (SPGS-550-M), in the presence of Fu's catalyst and a mild hydride source (NaBH4), are useful for facile debromination of functionalized aromatic derivatives. This mild and environemntally responsible process is utlized in water at room temperature, and the reaction mixtures can be smoothly recycled.

Synthesis of lemonose derivatives: methyl 4-amino-3-O,4-N-carbonyl-2,4,6-trideoxy-3-C-methyl-α-l-lyxo-pyranoside and its phenyl thioglycoside

Lemonose is a component of the antibiotic lemonomycin and other antibiotics and natural products. Three routes to the synthesis of the title compound, a protected, desmethyl derivative of lemonose, from l-rhamnose or its glycal, were investigated based on electrophilic cyclization, epoxidation-ring opening, and deoxygenation of an intermediate that was used in the synthesis of the amino sugar callipeltose. The deoxygenation route was successful and it provided the title compound, which was then converted to a phenyl thioglycoside.

Synthesis of silafluorenes and silaindenes via silyl radicals from arylhydrosilanes: intramolecular cyclization and intermolecular annulation with alkynes

Effective synthetic methods, involving silyl radical intermediates and furnishing silafluorenes and silaindenes via direct Si–H and C–H cleavage in one step, have been developed.

Investigation of reactions postulated to occur during inhibition of ribonucleotide reductases by 2'-azido-2'-deoxynucleotides

Model 3'-azido-3'-deoxynucleosides with thiol or vicinal dithiol substituents at C2' or C5' were synthesized to study reactions postulated to occur during inhibition of ribonucleotide reductases by 2'-azido-2'-deoxynucleotides. Esterification of 5'-(tert-butyldiphenylsilyl)-3'-azido-3'-deoxyadenosine and 3'-azido-3'-deoxythymidine (AZT) with 2,3-S-isopropylidene-2,3-dimercaptopropanoic acid or N-Boc-S-trityl-L-cysteine and deprotection gave 3'-azido-3'-deoxy-2'-O-(2,3-dimercaptopropanoyl or cysteinyl)adenosine and the 3'-azido-3'-deoxy-5'-O-(2,3-dimercaptopropanoyl or cysteinyl)thymidine analogs. Density functional calculations predicted that intramolecular reactions between generated thiyl radicals and an azido group on such model compounds would be exothermic by 33.6-41.2 kcal/mol and have low energy barriers of 10.4-13.5 kcal/mol. Reduction of the azido group occurred to give 3'-amino-3'-deoxythymidine, which was postulated to occur with thiyl radicals generated by treatment of 3'-azido-3'-deoxy-5'-O-(2,3-dimercaptopropanoyl)thymidine with 2,2'-azobis-(2-methyl-2-propionamidine) dihydrochloride. Gamma radiolysis of N(2)O-saturated aqueous solutions of AZT and cysteine produced 3'-amino-3'-deoxythymidine and thymine most likely by both radical and ionic processes.

Recent applications of the (TMS)3SiH radical-based reagent

This review article focuses on the recent applications of tris(trimethylsilyl)silane as a radical-based reagent in organic chemistry. Numerous examples of the successful use of (TMS)(3)SiH in radical reductions, hydrosilylation and consecutive radical reactions are given. The use of (TMS)(3)SiH allows reactions to be carried out under mild conditions with excellent yields of products and remarkable chemo-, regio-, and stereoselectivity. The strategic role of (TMS)(3)SiH in polymerization is underlined with emphasis on the photo-induced radical polymerization of olefins and photo-promoted cationic polymerization of epoxides.

Application of germyldesulfonylation reactions to the synthesis of germanium-containing nucleoside analogues

Treatment of the protected (E)-5'-deoxy-5'-[(p-toluenesulfonyl)methylene]uridine and adenosine derivatives with tributyl- or triphenylgermane hydride (AIBN/toluene/Delta) effected radical-mediated germyldesulfonylations to give 5'-(tributyl- or triphenylgermyl)methylene-5'-deoxyuridine and adenosine derivatives as single (E)-isomers. Analogous treatment of 2'-deoxy-2'-[(phenylsulfonyl)methylene]uridine with Ph(3)GeH afforded the corresponding vinyl triphenylgermane product. Stereoselective halodegermylation of the (E)-5'-(tributylgermyl)methylene-5'-deoxy nucleosides with N-iodosuccinimide or N-bromosuccinimide provided the Wittig-type (E)-5'-deoxy-5'-(halomethylene) nucleosides quantitatively, while no halodegermylations was observed with the 5'-deoxy-5'-(triphenylgermyl)methylene counterparts. Treatment of the vinyl trialkylgermanes with aqueous trifluoroacetic acid effected protiodegermylation, while vinyl triarylgermanes were stable under the acidic conditions.

Radical-mediated thiodesulfonylation of the vinyl sulfones: Access to (α-fluoro)vinyl sulfides

Radical-mediated thiodesulfonylation of the vinyl and (α-fluoro)vinyl sulfones, derived from aldehydes and ketones, with aryl thiols in organic or aqueous medium provided access to vinyl and (α-fluoro)vinyl sulfides. The vinyl sulfides were formed predominantly with E stereochemistry independent of the stereochemistry of the starting vinyl sulfones.