Generation and Trapping of Nitrosocarbonyl Intermediates

Enantioselective organocatalytic strategies to access noncanonical α-amino acids

Organocatalytic asymmetric synthesis has evolved over the years and continues to attract the interest of many researchers worldwide. Enantiopure noncanonical amino acids (ncAAs) are valuable building blocks in organic synthesis, medicinal chemistry, and chemical biology. They are employed in the elaboration of peptides and proteins with enhanced activities and/or improved properties compared to their natural counterparts, as chiral catalysts, in chiral ligand design, and as chiral building blocks for asymmetric syntheses of complex molecules, including natural products. The linkage of ncAA synthesis and enantioselective organocatalysis, the subject of this perspective, tries to imitate the natural biosynthetic process. Herein, we present contemporary and earlier developments in the field of organocatalytic activation of simple feedstock materials, providing potential ncAAs with diverse side chains, unique three-dimensional structures, and a high degree of functionality. These asymmetric organocatalytic strategies, useful for forging a wide range of C-C, C-H, and C-N bonds and/or combinations thereof, vary from classical name reactions, such as Ugi, Strecker, and Mannich reactions, to the most advanced concepts such as deracemisation, transamination, and carbene N-H insertion. Concurrently, we present some interesting mechanistic studies/models, providing information on the chirality transfer process. Finally, this perspective highlights, through the diversity of the amino acids (AAs) not selected by nature for protein incorporation, the most generic modes of activation, induction, and reactivity commonly used, such as chiral enamine, hydrogen bonding, Brønsted acids/bases, and phase-transfer organocatalysis, reflecting their increasingly important role in organic and applied chemistry.

The Development and Mechanistic Study of an Iron-Catalyzed Intramolecular Nitroso Ene Reaction of Nitroarenes

An intramolecular iron-catalyzed nitroso ene reaction was developed to afford six- or seven-membered N-heterocycles from nitroarenes using an earth abundant iron catalyst and phenylsilane as the terminal reductant. The reaction can be triggered using as little as 3 mol % of iron(II) acetate and 3 mol % of 4,7-dimethoxyphenanthroline as the ligand. The scope of the reaction is broad tolerating a range of electron-releasing or electron-withdrawing substituents on the nitroarene, and the ortho-substituent can be modified to diastereoselectively construct benzoxazines, dihydrobenzothiazines, tetrahydroquinolines, tetrahydroquinoxalines, or tetrahydrobenzooxazepines. Mechanistic investigations indicated that the reaction proceeds via a nitrosoarene intermediate, and kinetic analysis of the reaction revealed a first-order rate dependence in catalyst-, nitroarene-, and silane concentration, and an inverse kinetic order in acetate was observed. The difference in rates between PhSiH3 and PhSiD3 was found to be 1.50 ± 0.09, and investigation of the temperature dependence of the reaction rate revealed that the activation parameters to be ΔH‡ = 13.5 kcal•mol-1 and ΔS‡ = -39.1 cal•mol-1•K-1. These data were interpreted to indicate that the turnover-limiting step to be hydride transfer from iron to the coordinated nitroarene, which occurs through an ordered transition state with little Fe-H bond breaking.

Brønsted acid-catalyzed annulation reaction of hydroxamic acids: synthesis of cyclopentane-fused isoxazolidines and their benzilic amide rearrangement

Readily available hydroxamic acids were leveraged to access challenging nitrones in the presence of H3PO4 as a Brønsted acid catalyst and engaged in an intramolecular (3+2) annulation reaction to make valuable cyclopentane-fused isoxazolidines with high yields and excellent diastereoselectivity. The products were further utilized in a unique base-promoted benzilic amide rearrangement to provide cyclopentane-fused γ-lactams bearing three contiguous stereocenters as a single diastereomer.

Inhibition of the RNA-Dependent RNA-Polymerase from SARS-CoV-2 by 6-Chloropurine Isoxazoline-Carbocyclic Monophosphate Nucleotides

Isoxazoline-carbocyclic monophosphate nucleotides were designed and synthesized through the chemistry of nitrosocarbonyl intermediates and stable anthracenenitrile oxide. Docking and molecular dynamics studies were first conducted for determining the best candidate for polymerase SARS-CoV-2 inhibition. The setup phosphorylation protocol afforded the nucleotides available for the biological tests. Preliminary inhibition and cytotoxicity assays were then performed, and the results showed a moderate activity of the nucleotides accompanied by cytotoxicity.

Total Synthesis of Immunosuppressive Mycestericin E and G Enabled by a Highly Stereoselective Nitroso-Ene Cyclization

This report describes a streamlined synthesis of immunosuppressive mycestericin E and G through a highly stereoselective nitroso-ene cyclization in 11-12 steps using readily available materials. The stereochemical outcome in the formation of a Nα-quaternary stereogenic center is rationalized by a trajectory based on the polar diradical intermediate and subsequent hydrogen transfer. Julia olefination offers a facile chain elongation method that presents a viable strategy for structural derivatization in future medicinal applications.

Development of Nitroso-Based Probes for Labeling and Regulation of RAS Proteins in Cancer Cells via Sequential Ene-Ligation and Oxime Condensation

Prenyl functionalities have been widely discovered in natural products, nucleic acids, and proteins with significant biological roles in both healthy and diseased cells. In this work, we develop a series of new nitroso-based probes for the labeling, enrichment, and regulation of prenylated RAS protein, which is highly associated with ∼20% of human cancers and used to be regarded as an "undruggable" target via a sequential ene-ligation and oxime condensation (SELOC) process. We found that these nitroso species can rapidly react with prenyl-containing molecules through ene-ligation and install a molecular tag for functional applications under physiological conditions. We first investigated this ligation process on two peptide models and demonstrated its labeling efficiency on various proteins such as myoglobin, lysozyme, RNase A, BSA, and HSP40. We further coupled this reactive platform with proteolysis-targeting chimera technology targeting to increase its efficiency and accuracy, as well as to expand its application range. Using the prenylated RAS protein as the model, we demonstrated that RAS could be efficiently decorated with our nitroso probes, which further condensate with oxime and rapidly react with a pomalidomide-containing hydroxylamine probe for protein degradation. As a result, the RAS protein in both HeLa and A549 cell lines has been determined to be efficiently degraded both in vitro and in vivo. This is the first case targeting post-translational modification other than ligand-protein interaction to degrade and regulate RAS proteins. We envision that our SELOC strategy will have great potential in studying the fundamental structures and functions of prenylated biomolecules and developing new drugs based on these unique cellular pathways.

Three-Component Cycloaddition of Nitriles: Construction of Bicyclic 4-Aminopyrimidines and Their Photophysical Studies

A base-induced synthesis of bicyclic 4-aminopyrimidines by the cycloaddition of three types of nitriles is reported. The scope of the method is demonstrated with 44 examples. Products are found to have luminescence properties and show potential applications as organic luminescent layer materials.

Photolytic insertion of carbon monoxide into nitrosyl chloride: formation of nitrosoformyl chloride

Nitrosocarbonyls are exotic intermediates that remain scarcely characterized. By UV photolysis (365 nm) of nitrosyl chloride (ClNO) embedded in solid CO ice at 20 K, the elusive nitrosoformyl chloride (ClC(O)NO) has been synthesized via CO-insertion into the Cl-N bond in ClNO. The characterization of ClC(O)NO with matrix-isolation IR spectroscopy is supported by ¹³C and ¹⁵N isotope labeling and quantum chemical calculations at the B3LYP/6-311+G(3df) level of theory. Upon subsequent laser irradiation at 266 nm, CO-elimination in ClC(O)NO occurs by reformation of ClNO. In line with the calculated potential energy surface for ClC(O)NO at the CCSD(T)-F12a/aug-cc-pVTZ//B3LYP/6-311+G(3df) level, the observed IR frequencies and the corresponding isotopic shifts coincide with the calculated values for the lowest-energy planar conformer, in which the CO and NO moities adopt trans configuration with respect to the C-N bond. Furthermore, the CO-insertion in ClNO involves a stepwise pathway by first homolytic cleavage of the Cl-N bond in ClNO (→ Cl˙ + ˙NO), followed by successive CO-trapping (CO + Cl˙ → ClCO˙) and radical combination (ClCO˙ + ˙NO → ClC(O)NO) inside the solid CO-matrix cages.

Three decades of unveiling the complex chemistry of C-nitroso species with computational chemistry

This review revisits the complex reactivity of C-nitroso derivatives through the synergistic combination of computational and synthetic organic chemistry, with an emphasis on the rationalization of mechanisms and selectivities. 

Alternating Current Electrolysis as Efficient Tool for the Direct Electrochemical Oxidation of Hydroxamic Acids for Acyl Nitroso Diels-Alder Reactions

The acyl nitroso Diels-Alder reaction of 1,3-dienes with electrochemically oxidised hydroxamic acids is described. By using alternating current electrolysis, their typical electro-induced decomposition could be suppressed in favour of the 1,2-oxazine cycloaddition products. The reaction was optimised using Design of Experiments (DoE) and a sensitivity test was conducted. A mixture of triethylamine/hexafluoroisopropanol served as supporting electrolyte in dichloromethane, thus giving products of high purity after evaporation of the volatiles without further purification. The optimised reaction conditions were applied to various 1,3-dienes and hydroxamic acids, giving up to 96 % isolated yield.

A Bridge to Alkaloid Synthesis

The construction of a structurally rigid architecture with chiral complexity, necessary to enhance the interaction with binding sites of drug targets, has been adapted as an intriguing approach in drug development. In the past few years, we have been interested in the synthesis of biologically significant and bridged alkaloids via novel synthetic methods and strategies based on recognition of the privileged pattern. Therefore, nitroso-ene and aza-Wacker cyclizations were elevated for the first time to construct bridged alkaloids, such as hosieine A, kopsone, melinonine-E and strychnoxanthine. Mechanistic investigations, including computational calculations for nitroso-ene reaction and deuterated experiments for aza-Wacker reaction, enable us to gain more insights into the chemical reactivity and selectivity of specific functional groups in developing viable synthetic methods.

Turn-folding in fluorescent anthracene-substituted cyclopenta[d]isoxazoline short peptides

Cyclopenta[d]isoxazoline aminols were used for the synthesis of β-turn mimics. The peptide chain choice ascertained the influence of their structural features on the applicability/reliability/robustness of these scaffolds as β-turn inducers and their limitations. The amino acid selection as well as steric demands can favor or disfavor the structure folding and the correct design of the peptide chains deeply influences the potential use of these nitrosocarbonyl-based compounds as turn-inducers.

Davis-Beirut Reaction Inspired Nitroso Diels-Alder Reaction

A Davis-Beirut reaction inspired nitroso Diels-Alder protocol is reported. The starting material for the procedure is a nitrophenyl moiety with the para position appropriately substituted with a 2°-amine (see 5) or 2°-alcohol (see 6). Deprotonation at the benzylic position followed by concomitant oxidation of the benzylic position and reduction of the nitro moiety delivers a nitrosophenyl intermediate, which subsequently undergoes a nitroso Diels-Alder reaction. This one-pot procedure delivers aryldihydro-1,2-oxazines in moderate yields.

Electrochemically Tuned Oxidative [4+2] Annulation and Dioxygenation of Olefins with Hydroxamic Acids

This work represents the first [4+2] annulation of hydroxamic acids with olefins for the synthesis of benzo[c][1,2]oxazines scaffold via anode-selective electrochemical oxidation. This protocol features mild conditions, is oxidant free, shows high regioselectivity and stereoselectivity, broad substrate scope of both alkenes and hydroxamic acids, and is compatible with terpenes, peptides, and steroids. Significantly, the dioxygenation of olefins employing hydroxamic acid is also successfully achieved by switching the anode material under the same reaction conditions. The study not only reveals a new reactivity of hydroxamic acids and its first application in electrosynthesis but also provides a successful example of anode material-tuned product selectivity.

Enantioselective amination of 4-alkylisoquinoline-1,3(2H,4H)-dione derivatives

A mild and efficient enantioselective amination of 4-alkylisoquinoline-1,3(2H,4H)-dione derivatives was established, which is compatible with a broad range of substrates and delivers the final products in excellent yields (up to 99%) and ee values (up to 99%) with low catalyst loading (down to 1 mol%). The synthetic potential of this methodology was also demonstrated in the gram scale level.

N,O-Nucleoside Analogues: Metabolic and Apoptotic Activity

Two new families of N,O-nucleoside analogues containing the anthracene moiety introduced through the nitrosocarbonyl ene reaction with allylic alcohols were prepared. The core structure is an isoxazolidine heterocycle that introduces either atom either a phenyl ring or dimethyl moiety at the C3 carbon. Different heterobases were inserted at the position 5 of the heterocyclic ring. One of the synthesized compounds demonstrated a good capacity to induce cell death and an appreciable nuclear fragmentation was evidenced in treated cells.

Catalyst-Controlled Regioselective Nitrosocarbonyl Aldol Reaction of Deconjugated Butenolides

An unprecedented regiodivergent nitrosocarbonyl aldol reaction of γ-substituted deconjugated butenolides is described. While Lewis base catalyst quinidine leveraged O-selective nitrosocarbonyl aldol reaction exclusively at the γ-position of deconjugated butenolides to produce γ-aminoxylation products, Lewis acid catalyst Cu(OTf)₂ steered the competitive N-selective nitrosocarbonyl aldol reaction at the β-position, resulting in heterodifunctionalized butenolides. Both processes were amenable to a broad range of substrates and scalable, while the latter one represents a rare example of one-pot hetero-β,γ-difunctionalization of butenolide scaffolds.

Nitroso Diels-Alder Cycloadducts Derived From N-Acyl-1,2-dihydropyridines as a New Platform to Molecular Diversity

This review focuses upon the use of nitroso Diels–Alder reactions as a structural complexity generating reaction that has been so far a quite scarcely treated topic, despite its potential. In particular, the use of N-acyl-1,2-dihydropyridines as a non-symmetrical diene component in nitroso Diels–Alder reactions encompasses an initial diversification of pathways giving rise to different cycloadducts (direct and inverse). Selective elaborations of these cycloadducts, basically using a reagent-based approach, deliver a discrete number of structurally diverse compounds, including some original heterobicyclic scaffolds and functionalized heterocycles. This forward synthetic planning allowed the individuation of a new biologically active compound based on a novel oxadiaza-bicyclic-[3.3.1]-nonene scaffold which is still under preclinical evaluation.

Cycloaddition reactions for antiviral compounds

Prominent in the current stage of drug development, antiviral compounds can be efficiently prepared through cycloaddition reactions. The chapter reports the use of classical Diels–Alder and their hetero version for the design and synthesis of compounds that were tested for their antiviral activities against a variety of viruses. Furthermore, 1,3-dipolar cycloaddition reactions of selected 1,3-dipoles, such as azides, nitrones, and nitrile oxides, are reviewed in the light of their application in the preparation of key intermediates for antiviral synthesis. A few examples of [2+2] cycloaddition reactions are also presented. The products obtained from these pericyclic reaction approaches were all tested for their activities in terms of blocking the virus replication, and the relevant biological data are highlighted.

Cyclopenta[d]isoxazoline β-Turn Mimics: Synthetic Approach, Turn Driving Force, Scope, and Limitations

Model β-turn inducers were prepared from constrained oxazanorbornene aminols. Taking advantage of the starting materials geometry, new diastereoisomeric compounds were synthesized, introducing different amino acidic residues. The products were spectroscopically characterized (VT and NMR titration). Temperature coefficients in dimethyl sulfoxide denote the existence of an intramolecular hydrogen bond. Chiroptical properties disclosed a β-turn arrangement of the synthesized compounds. The fused isoxazoline ring constraints the cyclopentane moiety, stabilizing a boatlike conformation that ensures the turn efficiency but limiting the accessibility to hindered amino acids.

Ene Reaction of Nitrosocarbonyl Mesitylene with the Cinnamyl Alcohol: Metabolic Activity and Apoptosis of the Synthetized 6-Chloropurine N,O-Nucleoside Analogues

Nitrosocarbonyl mesitylene intermediate undergoes an ene reaction with cinnamyl alcohol affording the corresponding 5-hydroxy-isoxazolidine in fair yields. The synthesized 5-acetoxy-isoxazolidine serves as synthon for the preparation of 6-chloropurine N,O-nucleoside analogues, according to the Vorbrüggen reaction. The compounds were evaluated for their metabolic and apoptotic activity, and their structure-activity relationship is discussed.

Nitrogen Dioxide Reaction with Nitroxide Radical Derived from Hydroxamic Acids: The Intermediacy of Acyl Nitroso and Nitroxyl (HNO)

Hydroxamic acids (RC(O)NHOH) form a class of compounds that display interesting chemical and biological properties The chemistry of RC(O)NHOH) is associated with one- and two-electron oxidations forming the respective nitroxide radical (RC(O)NHO^•) and acyl nitroso (RC(O)N═O), respectively, which are relatively unstable species. In the present study, the kinetics and mechanism of the ^•NO₂ reaction with nitroxide radicals derived from acetohydroxamic acid, suberohydroxamic acid, benzohydroxamic acid, and suberoylanilide hydroxamic acid have been studied in alkaline solutions. Ionizing radiation was used to generate about equal yields of these radicals, demonstrating that the oxidation of the transient nitroxide radical by ^•NO₂ produces HNO and nitrite at about equal yields. The rate constant of ^•NO₂ reaction with the nitroxide radical derived from acetohydroxamic acid has been determined to be (2.5 ± 0.5) × 10⁹ M^-1 s^-1. This reaction forms a transient intermediate absorbing at 314 nm, which decays via a first-order reaction whose rate increases upon increasing the pH or the hydroxamic acid concentration. Transient intermediates absorbing around 314 nm are also formed during the oxidation of hydroxamic acids by H₂O₂ catalyzed by horseradish peroxidase. It is shown that HNO is formed during the decomposition of these intermediates, and therefore, they are assigned to acyl nitroso compounds. This study provides for the first time a direct spectrophotometric detection of acyl nitroso compounds in aqueous solutions allowing the study of their chemistry and reaction kinetics.

Ene Reactions of Nitrosocarbonyl Intermediates with Trisubstituted Cycloalkenes: "Cis Effect" and Steric and Conformational Factors Drive the Selectivity

Nitrosocarbonyl intermediates, generated at room temperature by oxidation of nitrile oxides, undergo clean ene reactions with trisubstituted cycloalkenes. Nitrosocarbonyl benzene follows a Markovnikov orientation and abstracts preferentially the twix hydrogens over the lone ones. With the more sterically demanding nitrosocarbonyl mesitylene in the presence of 5- and 6-membered ring olefins, the Markovnikov directing effect is relieved, and twix and lone abstractions are observed. Endocyclic allylic hydrogens on the more congested side of the alkene are exclusively abstracted (the "cis effect") resembling the singlet oxygen behavior. The balance between steric and conformational factors, as well as the acylnitroso generation conditions, dictates the regioselectivity in some cases. Larger ring olefins undergo selective twix allylic hydrogen abstraction. The photochemical generation of nitrosocarbonyl is totally selective according to the Markovnikov orientation. The synthetic utility of the ene compounds is also accounted.