Reactions of acyl nitroso compounds with amines: Production of nitroxyl (HNO) with the preparation of amides

Azobenzene-based colorimetric and fluorometric chemosensor for nitroxyl releasing

The precise release and characterization of nitroxyl (HNO) gas signaling molecule remain a challenge due to its short lifetime to date. To solve this issue, an azobenzene-based HNO donor (Azo-D1) was proposed as a colorimetric and fluorometric chemosensor for HNO releasing, to release both HNO and an azobenzene fluorescent reporter together. Specifically, the Azo-D1 has an HNO release half-life of ∼68 min under physiological conditions. The characteristic color change from the original orange to the yellow color indicated the decomposition of the donor molecule. In addition, the stoichiometry release of HNO was qualitatively and quantitatively verified through the classical phosphine compound trap. As compared with the donor molecule by itself, the decomposed product demonstrates a maximum fluorescence emission at 424 nm, where the increase of fluorescence intensity by 6.8 times can be applied to infer the real-time concentration of HNO. Moreover, cellular imaging can also be achieved using this Azo-D1 HNO donor through photoexcitation at 405 and 488 nm, where the real-time monitoring of HNO release was achieved without consuming the HNO source. Finally, the Azo-D1 HNO donor would open a new platform in the exploration of the biochemistry and the biology of HNO.

Synthesis and nitroxyl (HNO) donating properties of benzoxadiazole-based Piloty's acids

Developing functional nitroxyl (HNO) donors play a significant role in the further exploration of endogenous HNO in biochemistry and pharmacology. In this work, two novel Piloty's acids (SBD-D1 and SBD-D2) were proposed by incorporating benzoxadiazole-based fluorophores, in order to achieve the dual-function of releasing both HNO and a fluorophore in situ. Under physiological conditions, both SBD-D1 and SBD-D2 efficiently donated HNO (t_1/2 = 10.96 and 8.18 min, respectively). The stoichiometric generation of HNO was determined by both vitamin B₁₂ and phosphine compound traps. Interestingly, due to the different substitution groups on the aromatic ring, SBD-D1 with the chlorine showed no fluorescence emission, but SBD-D2 was strongly fluorescent due to the presence of the dimethylamine group. Specifically, the fluorescent signal would decrease during the release process of HNO. Moreover, theoretical calculations were performed to understand the emission difference. A strong radiation derived from benzoxadiazole with dimethylamine group due to the large transition dipole moment (∼4.3 Debye), while the presence of intramolecular charge transfer process in the donor with chlorine group caused a small transition dipole moment (<0.1 Debye). Finally, these studies would contribute to the future design and application of novel functional HNO donors for the exploration of HNO biochemistry and pharmacology.

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.

Development and cellular application of visible-light-controllable HNO releasers based on caged Piloty's acid

Novel visible-light-controllable HNO releasers was developed based on a caged form of Piloty's acid, and applied for cellular systems.

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.

Amphiphilic Copolymers Capable of Concomitant Release of HNO and Small Molecule Organics

We demonstrate concomitant release of HNO and small molecule organics from amphiphilic poly(norbornene)-based copolymers. This key function was achieved by incorporation of thermally labile oxazine units within random and block copolymer architectures. Upon thermolysis, we observed generation of HNO and release of a small molecule conjugate. Importantly, the release kinetics of HNO and a UV-active small molecule (4-nitroaniline) were found to be 1:1, signifying an ability to monitor HNO production indirectly, or to simultaneously release organic therapeutics (e.g., nonsteroidal anti-inflammatory drugs) along with HNO. To our knowledge, these are the first reported polymeric materials demonstrating HNO release from covalently attached HNO donors.

Development of N-Substituted Hydroxamic Acids with Pyrazolone Leaving Groups as Nitrosocarbonyl Precursors

A novel class of nitrosocarbonyl precursors, N-substituted hydroxamic acids with pyrazolone leaving groups (NHPY), has been synthesized. Under physiological conditions, these compounds generate nitrosocarbonyl intermediates, which upon hydrolysis release nitroxyl (azanone, HNO) in excellent yields. The amount and rate of nitrosocarbonyl generation are dependent on the nature of the pyrazolone leaving groups and significantly on the structural properties of the NHPY donors. Pyrazolones have been found to be efficient nitrosocarbonyl traps, undergoing an N-selective nitrosocarbonyl aldol reaction. This trapping reaction has been used to confirm the involvement of nitrosocarbonyl intermediates in NHPY aqueous decomposition. In addition, NHPY compounds are shown to generate nitrosocarbonyls efficiently under mild basic conditions in organic solvent and may therefore also enjoy synthetic utility.

Investigation of the dynamic nature of 1,2-oxazines derived from peralkylcyclopentadiene and nitrosocarbonyl species

We have investigated the reversible hetero-Diels-Alder reaction of 1,2-oxazines derived from a peralkylcyclopentadiene and a series of nitrosocarbonyl dienophiles. The nature of the dienophile was found to impart broad tunability to the dynamic character of the oxazine adducts. The reversibility was also observed in polymeric systems. The fidelity of the reaction and tunable sensitivity toward elevated temperature and water signify potential applications in the development of dynamic covalent materials or delivery systems for small molecule payloads.

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.

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.

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.

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.

Photoactivatable HNO-releasing compounds using the retro-Diels-Alder reaction

We synthesized hetero-Diels-Alder cycloadducts from acyl nitroso derivatives and 9,10-dimethylanthracene, to be photo-inducible HNO-releasing agents and found that introduction of conjugated nitroaromatic groups effectively enhanced the responsiveness of HNO release to UV-A irradiation; we confirmed photoinduced HNO formation by EPR and GCMS analysis.

The pharmacology of nitroxyl (HNO) and its therapeutic potential: not just the Janus face of NO

Nitroxyl (HNO), the 1-electron reduced and protonated congener of nitric oxide (NO), has received recent attention as a potential pharmacological agent for the treatment of heart failure and as a preconditioning agent for the mitigation of ischemia-reperfusion injury. Interest in the pharmacology and biology of HNO has prompted examination, or in some instances reexamination, of many of its chemical properties. Such studies have provided insight into the chemical basis for the biological effects of HNO, although the biochemical mechanisms for many of these effects remain to be established. In this review, a brief description of the biologically relevant chemistry of HNO is given, followed by a more detailed discussion of the pharmacology and potential toxicology of HNO.