Single Electron Transfer‐Induced Selective α‐Oxygenation of Glycine Derivatives

Venugopal, Navyasree; Moser, Johannes; Vojtíčková, Margaréta; Cı́sařová, Ivana; König, Burkhard; Jahn, Ullrich

doi:10.1002/adsc.202100964

Cited by 3 publications

(4 citation statements)

References 49 publications

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“…As already mentioned above, highly electron-rich double bonds are directly oxidized by TEMPO. Along these lines, deprotonated α-amino esters 96 could be oxidized with TEMPO to the α-oxygenated esters 98 within 1 h at 0 °C (Scheme b) . Longer reaction times and higher temperatures were necessary for oxidizing α-chalcogenyl carbonyl compounds 96 without any prior deprotonation .…”

Section: Oxidation Reactionsmentioning

confidence: 99%

“…Along these lines, deprotonated α-amino esters 96 could be oxidized with TEMPO to the α-oxygenated esters 98 within 1 h at 0 °C (Scheme 34b). 787 Longer reaction times and higher temperatures were necessary for oxidizing α-chalcogenyl carbonyl compounds 96 without any prior deprotonation. 788 A more general approach was presented by the group of Renaud in collaboration with our group (Scheme 34c).…”

Section: Oxidations Of Enol(ate)s Enolethers Enamines Ynamides and Ke...mentioning

confidence: 99%

“…This radical either can be directly trapped by TEMPO, forming the corresponding αaminoxylated TEMPO ester, 806 or can engage in typical radical reactions before being trapped by a second equivalent of Scheme 34. (a) Dehydrogenation of Alkyl Perfluoroalkyl Ketones; 784 Oxidation of (b) Electron-Rich Enols or Enolates 787,788 and (c) Catecholboron Enolates; 789 TEMPO. 807−819 A similar reactivity was observed in the oxidation of ynamides.…”

Section: Oxidations Of Enol(ate)s Enolethers Enamines Ynamides and Ke...mentioning

confidence: 99%

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Organic Synthesis Using Nitroxides

Leifert

Studer

2023

Chem. Rev.

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Nitroxides, also known as nitroxyl radicals, are long-lived or stable radicals with the general structure R 1 R 2 N−O • . The spin distribution over the nitroxide N and O atoms contributes to the thermodynamic stability of these radicals. The presence of bulky N-substituents R 1 and R 2 prevents nitroxide radical dimerization, ensuring their kinetic stability. Despite their reactivity toward various transient C radicals, some nitroxides can be easily stored under air at room temperature. Furthermore, nitroxides can be oxidized to oxoammonium salts (R 1 R 2 N�O + ) or reduced to anions (R 1 R 2 N−O − ), enabling them to act as valuable oxidants or reductants depending on their oxidation state. Therefore, they exhibit interesting reactivity across all three oxidation states. Due to these fascinating properties, nitroxides find extensive applications in diverse fields such as biochemistry, medicinal chemistry, materials science, and organic synthesis. This review focuses on the versatile applications of nitroxides in organic synthesis. For their use in other important fields, we will refer to several review articles. The introductory part provides a brief overview of the history of nitroxide chemistry. Subsequently, the key methods for preparing nitroxides are discussed, followed by an examination of their structural diversity and physical properties. The main portion of this review is dedicated to oxidation reactions, wherein parent nitroxides or their corresponding oxoammonium salts serve as active species. It will be demonstrated that various functional groups (such as alcohols, amines, enolates, and alkanes among others) can be efficiently oxidized. These oxidations can be carried out using nitroxides as catalysts in combination with various stoichiometric terminal oxidants. By reducing nitroxides to their corresponding anions, they become effective reducing reagents with intriguing applications in organic synthesis. Nitroxides possess the ability to selectively react with transient radicals, making them useful for terminating radical cascade reactions by forming alkoxyamines. Depending on their structure, alkoxyamines exhibit weak C−O bonds, allowing for the thermal generation of C radicals through reversible C−O bond cleavage. Such thermally generated C radicals can participate in various radical transformations, as discussed toward the end of this review. Furthermore, the application of this strategy in natural product synthesis will be presented.

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Section: Oxidation Reactionsmentioning

confidence: 99%

Section: Oxidations Of Enol(ate)s Enolethers Enamines Ynamides and Ke...mentioning

confidence: 99%

Section: Oxidations Of Enol(ate)s Enolethers Enamines Ynamides and Ke...mentioning

confidence: 99%

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Organic Synthesis Using Nitroxides

Leifert

Studer

2023

Chem. Rev.

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“…Note that other reactive peptidyl radicals and related (amino)(amido) C-radicals B are rare in nature, 1 c , d but are commonly involved in synthetic radical peptidic chemistry. 2…”

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confidence: 99%

An air-stable radical with a redox-chameleonic amide

et al. 2023

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Photo‐Induced Generation of Oxygenated Quaternary Centers via EnT Enabled Singlet O₂ Addition to C3‐Maleimidated Quinoxaline: A Reagent‐Less Approach

Ghosh,

Khandelia,

Mahadevan

et al. 2024

Chemistry A European J

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Demonstrated here is an external photo‐sensitizer‐free (auto‐sensitized) singlet oxygen‐enabled solvent‐dependent tertiary hydroxylation and aryl‐alkyl spiro‐etherification of C3‐maleimidated quinoxalines. Such “reagent‐less” photo‐oxygenation at Csp3−H and etherification involving Csp3−H/Csp2−H are unparalleled. Possibly, the highly π‐conjugated N−H tautomer allows the substrate to get excited by irradiation, and subsequently, it attains the triplet state via ISC. This excited triplet‐state sensitized molecule then transfers its energy to a triplet‐state oxygen (3O2) generating reactive singlet oxygen (1O2) for hydroxylation and spirocyclization depending on the solvent used. In HFIP, the generated alkoxy radical accepts a proton via HAT giving hydroxylated product. In contrast, in an aprotic PhCl it underwent a radical addition at the ortho‐position of the C2 aryl to provide spiro‐ether. An unprecedented orthogonal spiro‐etherification was observed via the displacement of o‐substitutents for ortho‐substituted substrates. The order of ipso substitution follows the trend–OMe > –OEt > –F > –H > –Cl > –Br. Both these oxygenation reactions can be carried out with nearly equal ease using direct sunlight without the requirement of any elaborate reaction setup. Demonstration of large‐scale synthesis and a few interesting transformations have also been realized. Furthermore, several insightful control experiments and quantum chemical computations were performed to unravel the mechanism.

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Single Electron Transfer‐Induced Selective α‐Oxygenation of Glycine Derivatives

Cited by 3 publications

References 49 publications

Organic Synthesis Using Nitroxides

Organic Synthesis Using Nitroxides

An air-stable radical with a redox-chameleonic amide

Photo‐Induced Generation of Oxygenated Quaternary Centers via EnT Enabled Singlet O₂ Addition to C3‐Maleimidated Quinoxaline: A Reagent‐Less Approach

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Single Electron Transfer‐Induced Selective α‐Oxygenation of Glycine Derivatives

Cited by 3 publications

References 49 publications

Organic Synthesis Using Nitroxides

Organic Synthesis Using Nitroxides

An air-stable radical with a redox-chameleonic amide

Photo‐Induced Generation of Oxygenated Quaternary Centers via EnT Enabled Singlet O2 Addition to C3‐Maleimidated Quinoxaline: A Reagent‐Less Approach

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Product

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Photo‐Induced Generation of Oxygenated Quaternary Centers via EnT Enabled Singlet O₂ Addition to C3‐Maleimidated Quinoxaline: A Reagent‐Less Approach