Organic Synthesis Using Nitroxides

Nitroxides, also known as nitroxyl radicals, are long-lived or stable radicals with the general structure R1R2N-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 R1 and R2 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 (R1R2N═O+) or reduced to anions (R1R2N-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.

Beyond the approved: target sites and inhibitors of bacterial RNA polymerase from bacteria and fungi

Covering: 2016 to 2022RNA polymerase (RNAP) is the central enzyme in bacterial gene expression representing an attractive and validated target for antibiotics. Two well-known and clinically approved classes of natural product RNAP inhibitors are the rifamycins and the fidaxomycins. Rifampicin (Rif), a semi-synthetic derivative of rifamycin, plays a crucial role as a first line antibiotic in the treatment of tuberculosis and a broad range of bacterial infections. However, more and more pathogens such as Mycobacterium tuberculosis develop resistance, not only against Rif and other RNAP inhibitors. To overcome this problem, novel RNAP inhibitors exhibiting different target sites are urgently needed. This review includes recent developments published between 2016 and today. Particular focus is placed on novel findings concerning already known bacterial RNAP inhibitors, the characterization and development of new compounds isolated from bacteria and fungi, and providing brief insights into promising new synthetic compounds.

Chiral 1,2,3-Triazolium Salt Catalyzed Asymmetric Mono- and Dialkylation of 2,5-Diketopiperazines with the Construction of Tetrasubstituted Carbon Centers

Novel asymmetric mono- and dialkylation reactions of α-substituted 2,5-diketopiperazines catalyzed by new chiral spirocyclic-amide-derived triazolium organocatalysts have been developed, resulting in a range of enantioenriched 2,5-diketopiperazine derivatives containing one or two tetrasubstituted carbon stereocenters. The reactions feature high yields (up to 98%), and excellent cis-diastereo- and enantioselectivities (up to >20:1 dr, >99 % ee), and they provide a new asymmetric synthetic approach to important functionalized 2,5-diketopiperazine skeletons. Furthermore, a possible reaction mechanism was proposed based on both control experiments and extensive DFT calculations.

Chemoselektive γ-Oxidation von β,γ-ungesättigten Amiden mit TEMPO

Ein chemoselektives und robustes Protokoll zur γ‐Oxidation von β,γ‐ungesättigten Amiden wird dargelegt. Bei dieser Methode ermöglicht elektrophile Amidaktivierung eine bei ungesättigten Amiden bisher selten angewendete regioselektive Reaktion mit TEMPO, die zu γ‐aminoxylierten α,β‐ungesättigten Amiden führt. Radikalische Zyklisierungen und Oxidationen der synthetisierten Produkte untermauern die Nützlichkeit der hergestellten Verbindungen.

Chemoselective γ-Oxidation of β,γ-Unsaturated Amides with TEMPO

A chemoselective and robust protocol for the γ‐oxidation of β,γ‐unsaturated amides is reported. In this method, electrophilic amide activation, in a rare application to unsaturated amides, enables a regioselective reaction with TEMPO resulting in the title products. Radical cyclisation reactions and oxidation of the synthesised products highlight the synthetic utility of the products obtained.

Cyclodipeptide synthases: a promising biotechnological tool for the synthesis of diverse 2,5-diketopiperazines

Covering: Up to mid-2019 Cyclodipeptide synthases (CDPSs) catalyse the formation of cyclodipeptides using aminoacylated-tRNA as substrates. The recent characterization of large sets of CDPSs has revealed that they can produce highly diverse products, and therefore have great potential for use in the production of different 2,5-diketopiperazines (2,5-DKPs). Sequence similarity networks (SSNs) are presented as a new, efficient way of classifying CDPSs by specificity and identifying new CDPS likely to display novel specificities. Several strategies for further increasing the diversity accessible with these enzymes are discussed here, including the incorporation of non-canonical amino acids by CDPSs and use of the remarkable diversity of 2,5-DKP-tailoring enzymes discovered in recent years.

Tandem Anionic oxy-Cope Rearrangement/Oxygenation Reactions as a Versatile Method for Approaching Diverse Scaffolds

Tandem anionic oxy-Cope rearrangement/radical oxygenation reactions provide δ,ϵ-unsaturated α-(aminoxy) carbonyl compounds, which serve as convenient precursors to diverse compound classes. Functionalized carbocycles are accessible by very rare all-carbon 5-endo-trig cyclizations, but also common 5-exo-trig radical cyclizations, based on the persistent radical effect. The tandem reactions can be further extended by highly diastereoselective allylation or reduction steps to give complex scaffolds.

Mechanistic Aspects and Synthetic Applications of Radical Additions to Allenes

More than 50 years have passed since Haszeldine reported the first addition of a trifluoromethyl radical to an allene; in the intervening years, both the chemistry of allenes and the reactivity of single-electron species have become topics of intense interest. In this Review, we provide an overview of the fundamentals of radical additions to allenes and highlight the emergence of theoretical and experimental evidence that reveals unique reactivity patterns for radical additions to allenes as compared with other unsaturated compounds. Factors capable of exerting control over the chemo-, regio-, and stereoselectivities of the attack of carbon- and heteroatom-based radicals at each of the three potential reactive sites in an allene substrate are described. These include reaction conditions, the nature of the attacking radical, the substitution pattern of the allene, and the length of the linker between the radical center and the proximal allene carbon in the substrate. Cycloaddition reactions between allenes and partners containing π-bonds, which are likely to proceed through radical pathways, are presented to highlight their ability to rapidly access complex polycyclic scaffolds. Finally, the synthetic utility of the products arising from these chemistries is described, including their applications to the construction of complex molecules.

First total synthesis of ent-asperparaline C and assignment of the absolute configuration of asperparaline C

The first asymmetric total synthesis of a member of the asperparaline family was accomplished and the unknown absolute configuration of asperparaline C has been determined to be all-(S).

Radicals in natural product synthesis

Free radical intermediates have intrigued chemists since their discovery, and an ever-increasing appreciation for their unique reactivity has resulted in the widespread utilization of these species throughout the field of chemical synthesis. This is most evident from the recent surge in the application of intermolecular radical reactions that feature in complex molecule syntheses. This tutorial review will discuss the diverse methods utilized for radical generation and reactivity to form critical bonds in natural product total synthesis. In particular, stabilized (e.g. benzyl) and persistent (e.g. TEMPO) radicals will be the primary focus.

Unique Stereoselective Homolytic C-O Bond Activation in Diketopiperazine-Derived Alkoxyamines by Adjacent Amide Pyramidalization

Simple monocyclic diketopiperazine (DKP)-derived alkoxyamines exhibit unprecedented activation of a remote C-O bond for homolysis by amide distortion. The combination of strain-release-driven amide planarization and the persistent radical effect (PRE) enables a unique, irreversible, and quantitative trans→cis isomerization under much milder conditions than typically observed for such homolysis-limited reactions. This isomerization is shown to be general and independent of the steric and electronic nature of both the amino acid side chains and the substituents at the DKP nitrogen atoms. Homolysis rate constants are determined, and they significantly differ for both the labile trans diastereomers and the stable cis diastereomers. To reveal the factors influencing this unusual process, structural features of the kinetic trans diastereomers and thermodynamic cis diastereomers are investigated in the solid state and in solution. X-ray crystallographic analysis and computational studies indicate substantial distortion of the amide bond from planarity in the trans-alkoxyamines, and this is believed to be the cause for the facile and quantitative isomerization. Thus, these amino-acid-derived alkoxyamines are the first examples that exhibit a large thermodynamic preference for one diastereomer over the other upon thermal homolysis, and this allows controlled switching of configurations and configurational cycling.

Evidence of single electron transfer from the enolate anion of an N,N'-dialkyldiketopiperazine additive in BHAS coupling reactions

A designed N,N'-dialkyldiketopiperazine (DKP) provides evidence for the role of DKP additives as initiators that act by electron transfer in base-induced homolytic aromatic substitution reactions, involving coupling of haloarenes to arenes.

Bioinspired total synthesis of tetrahydrofuran lignans by tandem nucleophilic addition/redox isomerization/oxidative coupling and cycloetherification reactions as key steps

Three steps suffice to complete a bioinspired total synthesis of tetrahydrofuran lignans using tandem addition/isomerization/dimerization and cycloetherification reactions.

Further Investigation of the Intermolecular Diels-Alder Cycloaddition for the Synthesis of Bicyclo[2.2.2]diazaoctane Alkaloids

The convergent synthesis of bicyclo[2.2.2]diazaoctane structures using an intermolecular Diels-Alder cycloaddition between a pyrazinone and commercially available fumarate or maleate precursors is reported. High reactivity and stereoselection is observed with both dienophile substrates. Structure validation was achieved by conversion of cycloadducts into known [2.2.2]diazabicyclic compounds or into crystalline derivatives suitable for X-ray analysis. The cycloadduct derived from reaction of pyrazinone and maleic anhydride underwent selective anhydride ring opening and intersected an established precursor in the synthesis of brevianamide B.

Sequential Oxidative and Reductive Radical Cyclization Approach toward Asperparaline C and Synthesis of Its 8-Oxo Analogue

The most advanced approach, so far, to the asperparalines is developed. Consecutive oxidative and reductive radical cyclizations serve as the key steps to stereoselectively access the complex fully elaborated skeleton containing the cyclopentane and spiro-succinimide units.

Oxidative radical cyclizations of diketopiperazines bearing an amidomalonate unit. Heterointermediate reaction sequences toward the asperparalines and stephacidins

A novel approach to the diazabicyclo[2.2.2]octane core of prenylated bridged diketopiperazine alkaloids is described by direct oxidative cyclizations of functionalized diketopiperazines mediated by ferrocenium hexafluorophosphate or the Mn(OAc)₃•2H₂O/Cu(OTf)₂ system. Divergent reaction pathways take place depending on the substitution pattern of the substrates and the oxidation conditions such as temperature or the presence or absence of persistent radical TEMPO. For ester-substituted diketopiperazines, the ester group exerts a significant influence on the reaction outcome and stereochemistry of the radical cyclizations.

COST Action CM1201 "Biomimetic Radical Chemistry": free radical chemistry successfully meets many disciplines

The COST Action CM1201 "Biomimetic Radical Chemistry" has been active since December 2012 for 4 years, developing research topics organized into four working groups: WG1 - Radical Enzymes, WG2 - Models of DNA damage and consequences, WG3 - Membrane stress, signalling and defenses, and WG4 - Bio-inspired synthetic strategies. International collaborations have been established among the participating 80 research groups with brilliant interdisciplinary achievements. Free radical research with a biomimetic approach has been realized in the COST Action and are summarized in this overview by the four WG leaders.

Nitroxyl-Radical-Catalyzed Oxidative Coupling of Amides with Silylated Nucleophiles through N-Halogenation

A nitroxyl-radical-catalyzed oxidative coupling reaction between amines with an N-protecting electron-withdrawing group (EWG) and silylated nucleophiles was developed to furnish coupling products in high yields, thus opening up new frontiers in organocatalyzed reactions. This reaction proceeded through the activation of N-halogenated amides by a nitroxyl-radical catalyst, followed by carbon-carbon coupling with silylated nucleophiles. Studies of the reaction mechanism indicated that the nitroxyl radical activates N-halogenated amides, which are generated from N-EWG-protected amides and a halogenation reagent, to give the corresponding imines.

Intermolecular Diels-Alder Cycloaddition for the Construction of Bicyclo[2.2.2]diazaoctane Structures: Formal Synthesis of Brevianamide B and Premalbrancheamide

A stereoselective intermolecular Diels-Alder cycloaddition of an intermediate pyrazinone with both achiral and chiral acrylate-derived dienophiles provides rapid access to the bicyclo[2.2.2]diazaoctane core shared among several prenylated indole alkaloids. The product derived from cycloaddition with 2-nitroacrylate required an additional five to six synthetic operations to intercept established precursors to premalbrancheamide and brevianamide B. The chemistry detailed in this manuscript constitutes a formal total synthesis (12 steps each) of these [2.2.2]diazabicyclic natural products from proline methyl ester.