Direct observation of an acyl nitroso species in solution by time-resolved IR spectrocopy

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.

Exploring the Potential of 2-(2-Nitrophenyl)ethyl-Caged N-Hydroxysulfonamides for the Photoactivated Release of Nitroxyl (HNO)

The emergence of nitroxyl (HNO) as a biological signaling molecule is attracting increasing attention. HNO-based prodrugs show considerable potential in treating congestive heart failure, with HNO reacting rapidly with metal centers and protein-bound and free thiols. A new class of 2-(2-nitrophenyl)ethyl (2-NPE)-photocaged N-hydroxysulfonamides has been developed, and the mechanisms of photodecomposition have been investigated. Three photodecomposition pathways are observed: the desired concomitant C-O/N-S bond cleavage to generate HNO, sulfinate, and 2-nitrostyrene, C-O bond cleavage to give the parent sulfohydroxamic acid and 2-nitrostyrene, and O-N bond cleavage to release a sulfonamide and 2-nitrophenylacetaldehyde. Laser flash photolysis experiments provide support for a Norrish type II mechanism involving 1,5-hydrogen atom abstraction to generate an aci-nitro species. A mechanism is proposed in which the (Z)-aci-nitro intermediate undergoes either C-O bond cleavage to release RSO₂NHO(H), concerted C-O/N-S bond cleavage to generate sulfinate and HNO, or isomerization to the (E)-isomer prior to O-N bond cleavage. The pK_a of the N(H) of the N-hydroxysulfonamide plays a key role in determining whether C-O or concerted C-O/N-S bond cleavage occurs. Deprotonating this site favors the desired C-O/N-S bond cleavage at the expense of an increased level of undesired O-N bond cleavage. Triplet state quenchers have no effect on the observed photoproducts.

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.

Mechanistic Insights into Rapid Generation of Nitroxyl from a Photocaged N-Hydroxysulfonamide Incorporating the (6-Hydroxynaphthalen-2-yl)methyl Chromophore

HNO is a highly reactive molecule that shows promise in treating heart failure. Molecules that rapidly release HNO with precise spatial and temporal control are needed to investigate the biology of this signaling molecule. (Hydroxynaphthalen-2-yl)methyl-photocaged N-hydroxysulfonamides are a new class of photoactive HNO generators. Recently, it was shown that a (6-hydroxynaphthalen-2-yl)methyl (6,2-HNM)-photocaged derivative of N-hydroxysulfonamide incorporating the trifluoromethanesulfonamidoxy group (1) quantitatively generates HNO. Mechanistic studies have now been carried out on this system and reveal that the ground state protonation state plays a key role in whether concerted heterolytic C-O/N-S bond cleavage to release HNO occurs versus undesired O-N bond cleavage. N-Deprotonation of 1 can be achieved by adding an aqueous buffer or a carboxylate salt to an aprotic solvent. Evidence is presented for C-O/N-S bond heterolysis occurring directly from the singlet excited state of the N-deprotonated parent molecule on the picosecond time scale, using femtosecond time-resolved transient absorption spectroscopy, to give a carbocation and ¹NO^-. This is consistent with the observation of significant fluorescence quenching when HNO is generated. The carbocation intermediate reacts rapidly with nucleophiles including water, MeOH, or even (H)NO in the absence of a molecule that reacts rapidly with (H)NO to give an oxime.

Nitrosocarbonyl Carbohydrate Derivatives: Hetero Diels–Alder and Ene Reaction Products for Useful Organic Synthesis

The generation and trapping of two new nitrosocarbonyl intermediates bearing carbohydrate-based chiral substituents is achieved by the mild oxidation of the corresponding nitrile oxides with tertiary amine N-oxides. Their capture with suitable dienes and alkenes afforded the corresponding hetero Diels–Alder cycloadducts and ene adducts from fair to excellent yields. The entire methodology looks highly promising by the easy conversion of aldoximes into hydroxymoyl halides, widening the access to nitrosocarbonyls, versatile tools in organic synthesis.

Stoichiometric Nitroxyl Photorelease Using the (6-Hydroxy-2-naphthalenyl)methyl Phototrigger

The design and synthesis of a photoactivatable HNO donor incorporating the (6-hydroxynaphthalen-2-yl)methyl (6,2-HNM) photocage coupled to the trifluoromethanesulfonamidoxy analogue of the well-established HNO generator Piloty's acid is described. The photoactive HNO donor stoichiometrically generates HNO (∼98%) at neutral pH conditions, and evidence for concerted C-O and N-S bond cleavage was obtained. The methanesulfonamidoxy analogue primarily undergoes undesired N-O bond cleavage.

Cu-Catalyzed Oxidation of C2 and C3 Alkyl-Substituted Indole via Acyl Nitroso Reagents

The selective oxidation of C2-alkyl-substituted indoles to 3-oxindole and the selective C-H oxygenation or amination of C2,C3-dialkyl-substituted indoles at C2 are reported under mild conditions. The position of the alkyl substitution on the indole directs the reaction to different pathways under similar conditions.

Direct Observation of Acyl Nitroso Compounds in Aqueous Solution and the Kinetics of Their Reactions with Amines, Thiols, and Hydroxamic Acids

Acyl nitroso compounds or nitrosocarhonyls (RC(O)N═O) are reactive short-lived electrophiles, and their hydrolysis and reactions with nucleophiles produce HNO. Previously, direct detection of acyl nitroso species in nonaqueous media has been provided by time-resolved infrared spectroscopy demonstrating that its half-life is about 1 ms. In the present study hydroxamic acids (RC(O)NHOH) are oxidized electrochemically in buffered aqueous solutions (pH 5.9-10.2) yielding transient species characterized by their maximal absorption at 314-330 nm. These transient species decompose via a first-order reaction yielding mainly HNO and the respective carboxylic acid and therefore are ascribed to RC(O)N═O. The sufficiently long half-life of RC(O)N═O in aqueous solution allows for the first time the study of the kinetics of its reactions with various nucleophiles demonstrating that the nucleophilic reactivity follows the order thiolate > hydroxamate > amine. Metal chelates of CH₃C(O)NHOH catalyze the hydrolysis of CH₃C(O)N═O at the efficacy order of Cu^II > Zn^II > Ni^II > Co^II where only Cu^II catalyzes the hydrolysis also in the absence of the hydroxamate. Finally, oxidation of hydroxamic acids generates HNO, and the rate of this process is determined by the half-life of the respective acyl nitroso compound.

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.

Highly Functionalized Tricyclic Oxazinanones via Pairwise Oxidative Dearomatization and N-Hydroxycarbamate Dehydrogenation: Molecular Diversity Inspired by Tetrodotoxin

Benzenoids in principle represent attractive and abundant starting materials for the preparation of substituted cyclohexanes; however, the synthetic tools available for overcoming the considerable aromatic energies inherent to these building blocks limit the available product types. In this paper, we demonstrate access to heretofore unknown heterotricyclic structures by leveraging oxidative dearomatization of 2-hydroxymethyl phenols with concurrent N-hydroxycarbamate dehydrogenation using a common oxidant. The pairwise-generated, mutually reactive species then participate in a second stage acylnitroso Diels-Alder cycloaddition. The reaction chemistry of the derived [2.2.2]-oxazabicycles, bearing four orthogonal functional groups and three stereogenic centers, is shown to yield considerable diversity in downstream products. The methodology allows for the expeditious synthesis of a functionalized intermediate bearing structural and stereochemical features in common with the complex alkaloid tetrodotoxin.

Generation and Trapping of Nitrosocarbonyl Intermediates

The nitrosocarbonyls (R-CONO) are highly reactive species and remarkable intermediates toward different synthetic targets. This review will cover a research area whose impact in current organic synthesis is constantly increasing in the chemical community. This review represents the first and comprehensive picture on the generation and trapping of nitrosocarbonyls and is solidly built on more than 380 papers. Six different classes of key starting materials such as hydroxamic acids, N-hydroxy carbamates, N-hydroxyureas, nitrile oxides, and 1,2,4-oxadiazole-4-oxides were highlighted. The content of the review surveys all the methods to generate the nitrosocarbonyls through different approaches (oxidative, thermal, photochemical, catalytic, aerobic, and the less common ones) in the light of efficiency, yields, and mildness. The most successful trapping agents employed to catch these fleeting intermediates are reviewed, exploiting their superior dienophilic, enophilic, and electrophilic power. The work is completed by paragraphs dedicated to the detection of the intermediates, theoretical studies, and insights about the challenges and future directions for the field.

Continuous flow of nitroso Diels-Alder reaction

Our flow reaction systems have provided quantitative yields of nitroso Diels-Alder products with no byproducts in cases of cyclic dienes without temperature and pressure controls. Additionally, the reaction times were significantly shortened by using homogeneous catalyst (CuCl) or heterogeneous reagent (MnO2) in comparison with batch reaction.

Biological signaling by small inorganic molecules

Small redox active molecules such as reactive nitrogen and oxygen species and hydrogen sulfide have emerged as important biological mediators that are involved in various physiological and pathophysiological processes. Advancement in understanding of cellular mechanisms that tightly regulate both generation and reactivity of these molecules is central to improved management of various disease states including cancer and cardiovascular dysfunction. Imbalance in the production of redox active molecules can lead to damage of critical cellular components such as cell membranes, proteins and DNA and thus may trigger the onset of disease. These small inorganic molecules react independently as well as in a concerted manner to mediate physiological responses. This review provides a general overview of the redox biology of these key molecules, their diverse chemistry relevant to physiological processes and their interrelated nature in cellular signaling.

Syntheses of isoxazoline-carbocyclic nucleosides and their antiviral evaluation: a standard protocol

The current synthesis of racemic purine and pyrimidine isoxazoline-carbocyclic nucleosides is reported, detailing the key-steps for standard and reliable preparations. Improved yields were obtained by the proper tuning of the single synthetic steps, opening the way for the preparation of a variety of novel compounds. Some of the obtained compounds were also evaluated against a wide variety of DNA and RNA viruses including HIV. No specific antiviral activity was observed in the cases at hand. Novel compounds were prepared for future biological tests.

Nitroso Diels-Alder (NDA) reaction as an efficient tool for the functionalization of diene-containing natural products

This review describes the use of nitroso Diels-Alder reactions for the functionalization of complex diene-containing natural products in order to generate libraries of compounds with potential biological activity. The application of this methodology to the structural modification of a series of natural products (thebaine, steroidal dienes, rapamycin, leucomycin, colchicine, isocolchicine and piperine) is discussed using relevant examples from the literature from 1973 onwards. The biological activity of the resulting compounds is also discussed. Additional comments are provided that evaluate the methodology as a useful tool in organic, bioorganic and medicinal chemistry.

Analysis of the HNO and NO donating properties of alicyclic amine diazeniumdiolates

Nitroxyl (HNO) donors have been shown to elicit a variety of pharmacological responses, ranging from tumoricidal effects to treatment of heart failure. Isopropylamine-based diazeniumdiolates have been shown to produce HNO on decomposition under physiological conditions. Herein, we report the synthesis and HNO release profiles of primary alicyclic amine-based diazeniumdiolates. These compounds extend the range of known diazeniumdiolate-based HNO donors. Acetoxymethyl ester-protected diazeniumdiolates were also synthesized to improve purification and cellular uptake. The acetoxymethyl derivative of cyclopentylamine diazeniumdiolate not only showed higher cytotoxicity toward cancer cells as compared to the parent anion but was also effective in combination with tamoxifen for targeting estrogen receptor α-negative breast cancer cells.

Quantitative detection of nitroxyl upon trapping with glutathione and labeling with a specific fluorogenic reagent

Donors of nitroxyl (HNO) have shown promise for treatment of stroke, heart failure, alcoholism and cancer. However, comparing the pharmacological capacities of various donors is difficult without first quantifying the amount of HNO released from each donor. Detection and quantitation of HNO has been complicated by the rapid self-consumption of HNO through irreversible dimerization, poor selectivity of trapping agents against other nitrogen oxides, and/or low sensitivity towards HNO. Here, an assay is described for the trapping of HNO by glutathione (GSH) followed by labeling of GSH with the fluorogenic agent, naphthalene-2,3-dicarboxaldehyde (NDA), and subsequent quantitation by fluorescence difference. The newly developed assay was used to validate the pH-dependence of HNO release from isopropylamine NONOate (IPA/NO), which is a dual donor of HNO and NO at physiological pH. Furthermore, varied assay conditions were utilized to suggest the ratios of the products of the reaction of GSH with HNO. At intracellular concentrations of GSH, the disulfide (GSSG) was the major product, but significant concentrations of glutathione sulfinamide (GS(O)NH₂) were also detected. This suggests that GS(O)NH₂, which is a selective biomarker of HNO, may be produced in concentrations that are amenable to in vivo analysis.

Development of N-substituted hydroxylamines as efficient nitroxyl (HNO) donors

Due to its inherent reactivity, nitroxyl (HNO), must be generated in situ through the use of donor compounds, but very few physiologically useful HNO donors exist. Novel N-substituted hydroxylamines with carbon-based leaving groups have been synthesized, and their structures confirmed by X-ray crystallography. These compounds generate HNO under nonenzymatic, physiological conditions, with the rate and amount of HNO released being dependent mainly on the nature of the leaving group. A barbituric acid and a pyrazolone derivative have been developed as efficient HNO donors with half-lives at pH 7.4, 37 °C of 0.7 and 9.5 min, respectively.

Optimization of HNO production from N,O-bis-acylated hydroxylamine derivatives

A wide range of N,O-bis-acylated hydroxylamine derivatives with chloro or arenesulfonyl leaving groups, and a related set of N-hydroxy-N-acylsulfonamides, have been synthesized and evaluated for nitroxyl (HNO) production. Mechanistic studies have revealed that the observed aqueous chemistry is more complicated than originally anticipated, and have been used to develop a new series of efficient HNO precursors (4u-4x, 7c-7d) with tunable half-lives.

The chemistry of nitroxyl-releasing compounds

Nitroxyl (HNO) demonstrates a diverse and unique biological profile compared to nitric oxide, a redox-related compound. Although numerous studies support the use of HNO as a therapeutic agent, the inherent chemical reactivity of HNO requires the use of donor molecules. Two general chemical strategies currently exist for HNO generation from nitrogen-containing molecules: (i) the disproportionation of hydroxylamine derivatives containing good leaving groups attached to the nitrogen atom and (ii) the decomposition of nitroso compounds (X-N=O, where X represents a good leaving group). This review summarizes the synthesis and structure, the HNO-releasing mechanisms, kinetics and by-product formation, and alternative reactions of six major groups of HNO donors: Angeli's salt, Piloty's acid and its derivatives, cyanamide, diazenium diolate-derived compounds, acyl nitroso compounds, and acyloxy nitroso compounds. A large body of work exists defining these six groups of HNO donors and the overall chemistry of each donor requires consideration in light of its ability to produce HNO. The increasing interest in HNO biology and the potential of HNO-based therapeutics presents exciting opportunities to further develop HNO donors as both research tools and potential treatments.

The nitrosocarbonyl hetero-Diels-Alder reaction as a useful tool for organic syntheses

Organic transformations that result in the formation of multiple covalent bonds within the same reaction are some of the most powerful tools in synthetic organic chemistry. Nitrosocarbonyl hetero-Diels-Alder (HDA) reactions allow for the simultaneous stereospecific introduction of carbon-nitrogen and carbon-oxygen bonds in one synthetic step, and provide direct access to 3,6-dihydro-1,2-oxazines. This Review describes the development of the nitrosocarbonyl HDA reaction and the utility of the resulting oxazine ring in the synthesis of a variety of important, biologically active molecules.

Photogeneration and reactivity of acyl nitroso compounds

Acyl nitroso compounds have been generated by photolysis of several different classes of precursors including 9,10-dimethylanthracene adducts, nitrodiazo compounds, and 1,2,4-oxadiazole-4-oxides. Consideration of the nitronate-like resonance structure of nitrodiazo compounds led to an examination of the photochemistry of nitronates with α-leaving groups. Photolysis of such nitronates has been shown to generate an acyl nitroso species along with a carbene intermediate. Nanosecond time-resolved infrared (TRIR) spectroscopy has been used to detect photogenerated acyl nitroso compounds directly and to examine their reaction kinetics with amines and thiols. The mechanism of acyl nitroso aminolysis by primary amines involves general base catalysis, while the mechanism of aminolysis by secondary amines is strictly bimolecular. Thiols do not seem to be reactive with acyl nitroso compounds on the microsecond time scale, but thiolates are quite reactive. The reaction between benzoyl nitroside and an organic-soluble thiolate, tetrabutylammonium dodecanethiolate, proceeds via a proposed tetrahedral intermediate, which is observable by TRIR spectroscopy.

Dual mechanisms of HNO generation by a nitroxyl prodrug of the diazeniumdiolate (NONOate) class

Here we describe a novel caged form of the highly reactive bioeffector molecule, nitroxyl (HNO). Reacting the labile nitric oxide (NO)- and HNO-generating salt of structure iPrHN-N(O)═NO(-)Na(+) (1, IPA/NO) with BrCH(2)OAc produced a stable derivative of structure iPrHN-N(O)═NO-CH(2)OAc (2, AcOM-IPA/NO), which hydrolyzed an order of magnitude more slowly than 1 at pH 7.4 and 37 °C. Hydrolysis of 2 to generate HNO proceeded by at least two mechanisms. In the presence of esterase, straightforward dissociation to acetate, formaldehyde, and 1 was the dominant path. In the absence of enzyme, free 1 was not observed as an intermediate and the ratio of NO to HNO among the products approached zero. To account for this surprising result, we propose a mechanism in which base-induced removal of the N-H proton of 2 leads to acetyl group migration from oxygen to the neighboring nitrogen, followed by cleavage of the resulting rearrangement product to isopropanediazoate ion and the known HNO precursor, CH(3)-C(O)-NO. The trappable yield of HNO from 2 was significantly enhanced over 1 at physiological pH, in part because the slower rate of hydrolysis for 2 generated a correspondingly lower steady-state concentration of HNO, thus, minimizing self-consumption and enhancing trapping by biological targets such as metmyoglobin and glutathione. Consistent with the chemical trapping efficiency data, micromolar concentrations of prodrug 2 displayed significantly more potent sarcomere shortening effects relative to 1 on ventricular myocytes isolated from wild-type mouse hearts, suggesting that 2 may be a promising lead compound for the development of heart failure therapies.