Acyl Nitroso Research Articles

Diastereoselective acyl nitroso Diels-Alder reactions using 1,3-dienes derived from (R)-4-tert-butyldimethylsilyloxy-2-cyclohexen-1-one

1,3-Dienes derived from (R)-4-t-butyldimethylsilyloxy-2-cyclohexen-1-one react with acyl nitroso dienophiles to form exclusively anti [4 + 2] cycloaddition products. These regio and diastereoselective acyl nitroso Diels-Alder reactions increase the asymmetric complexity from one chiral center in the starting material to three chiral centers in the products in a single step and provide a powerful approach for the asymmetric synthesis of compounds containing 3,6-dihydro-1,2-oxazine structural feature.

An Oxidative Dearomatization Approach to Tetrodotoxin via a Masked ortho-Benzoquinone.

Progress toward a stereoselective synthesis of tetrodotoxin (TTX) is presented. Oxidative dearomatization of a tetrasubstituted guaiacol arene yielded a masked ortho-benzoquinone that intercepted an acyl nitroso species generated in situ by the copper-catalyzed aerobic oxidation of an acyl hydroxylamine. The subsequent alkene dihydroxylation and reduction of a bis-neopentylic ketone proceeded with perfect diastereoselectivity to reveal advanced intermediates toward the synthesis of TTX.

The Application of 1,2-Oxazinanes as Chiral Cyclic Weinreb Amide-Type Auxiliaries Leading to a Three-Component, One-Pot Reaction

Abstract1,2-Oxazines were synthesised via a copper-catalysed aerobic acyl nitroso Diels–Alder reaction from 1,4-disubstituted 1,3-dienes and N-Boc-hydroxylamine. From this, 1,2-oxazinanes were obtained in a novel follow-up reaction path. The stability of several 1,2-oxazines and 1,2-oxazinanes towards organometallic compounds was tested to rate their operability as cyclic chiral Weinreb amide auxiliaries. 3,6-Di-tert-butyl-1,2-oxazinane gave the best results and was introduced as a chiral Weinreb amide-type auxiliary to yield chiral α-substituted ketones in a diastereomeric ratio of up to 98:2. The removal of the auxiliary can be performed with BuLi to form unsymmetrical α-chiral ketones. Thereafter, the chiral auxiliary can be re-isolated and purified by sublimation under vacuum.

π-Facial Stereoselectivity in Acyl Nitroso Cycloadditions to 5,5-Unsymmetrically Substituted Cyclopentadienes: Computational Exploration of Origins of Selectivity and the Role of Substituent Conformations on Selectivity.

The π-facial selectivity of Diels-Alder cycloadditions of 5-monosubstituted cyclopentadienes is known experimentally and has been extensively studied computationally. Previous studies on 5-monosubstituted cyclopentadienes by the Burnell and Houk groups showed that facial selectivity arises principally from hyperconjugative aromaticity or antiaromaticity of polar groups that cause distortion of the cyclopentadiene; steric effects of nonpolar groups can also be important. We have now explored the stereoselective cycloaddition of 5,5-unsymmetrically substituted cyclopentadienes to an acyl nitroso dienophile reported by Kan and co-workers. Computational studies with M06-2X/6-311+G(d,p) indicate that the stereoselectivity in the cycloadditions of 5,5-unsymmetrically substituted cyclopentadienes is not just a simple combination of effects found for monosubstituted counterparts. Substituent conformations and diene-dienophile steric and electronic interaction effects all influence stereoselectivity. Predictions are made about several as-yet-unstudied cyclopentadiene cycloadditions.

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.

Wechselstromelektrolyse als effizientes Instrument für die direkte elektrochemische Oxidation von Hydroxamsäuren für die Acyl‐Nitroso Diels‐Alder‐Reaktion

AbstractBeschrieben wird die Acyl‐Nitroso Diels‐Alder‐Reaktion von 1,3‐Dienen mit elektrochemisch oxidierten Hydroxamsäuren. Durch die Verwendung von Wechselstrom konnte ihr elektro‐induzierter Zerfall zugunsten der 1,2‐Oxazin‐Cycloadditionsprodukte unterdrückt werden. Die Reaktion wurde mittels statistischer Versuchsplanung (Design of Experiment, DoE) optimiert und durch einen Sensitivitätstest weiter untersucht. Ein Gemisch aus Triethylamin und Hexafluorisopropanol dient als Leitsalz in Dichlormethan, wodurch hochreine Produkte nach Verdampfen der flüchtigen Komponenten gewonnen werden können. Mit den optimierten Bedingungen wurden verschiedene 1,3‐Diene und Hydroxamsäuren umgesetzt, deren Produkte in bis zu 96 % Ausbeute isoliert wurden.

Synthetic Studies on Pactamycin: A Synthesis of Johnson's Intermediate.

A formal total synthesis of pactamycin (1) has been accomplished by face-selective and regioselective nitroso Diels-Alder (NDA) reaction of acyl nitroso compound 14, which contains a camphorsultam chiral auxiliary, and chiral cyclopentadiene 12. Construction of the chiral secondary alcohol of 12 was performed by (S,S)-Ts-DENEB catalyst-mediated reduction, and the NDA adduct 15a was readily converted to Johnson's intermediate 21.

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 CuII > ZnII > NiII > CoII where only CuII 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.

Photochemical locking and unlocking of an acyl nitroso dienophile in the Diels–Alder reaction

Photochromic Diels–Alder cycloadducts consisting of acyl nitroso dienophiles, which are known nitroxyl (HNO) donors, and dithienyldienes are presented. The dithienylethene-type photochromic cycloadducts were found to exhibit reversible electrocyclic ring closing and ring opening reactions to ‘lock’ or ‘unlock’ the retro Diels–Alder reaction, respectively. The release of an acyl nitroso dienophile via a retro Diels–Alder reaction at 92°C was shown to occur only from the open or ‘unlocked’ form of a photochromic dithienylcyclopentene and not from the closed or ‘locked’ isomer.

Iodine-Mediated Oxidative Coupling of Hydroxamic Acids with Amines towards a New Peptide-Bond Formation

An efficient and straightforward approach for the coupling of Nα-protected hydroxamic acids with an amino component in the presence of iodine is delineated. The reaction is mediated by the formation of unstable but reactive acyl nitroso intermediates. The peptide hydroxamic acids were found to be useful substrates in coupling reactions.

An Experimental and Computational Approach to Understanding the Reactions of Acyl Nitroso Compounds in [4 + 2] Cycloadditions.

Catalytic aerobic oxidation of phenyl hydroxycarbamate 1 and 1-hydroxy-3-phenylurea 2 using CuCl2 and 2-ethyl-2-oxazoline in methanol gave acyl nitroso species in situ, which were trapped in nitroso-Diels-Alder (NDA) reactions with various dienes to afford the corresponding cycloadducts in high yields (90-98%). Competing ene products were also present for dienes containing both alkene π-bonds and allylic σ-bonds, and the ene yields are higher with 1 than with 2. The use of the chiral hydroxamic acid, (R)-1-hydroxy-3-(1-phenylethylurea) 3 (same conditions) gave NDA cycloadducts in high yields (97-99%) with no ene product from 2,3-dimethyl-1,3-butadiene. NDA cycloadducts were not obtained from other hydroxamic acid analogues [RCONHOH (R = PhCH2 4; Ph(CH2)2 5; Ph(CH2)3 6; Ph(CH2)4 7; Ph 8; 2-pyridyl 9; 3-pyridyl 10] with various dienes using copper-oxidation but rather were obtained using sodium periodate, resulting in variable NDA yields (13-51%) from hydroxamic acids 1-10 with cyclohexa-1,3-diene and 2,3-dimethyl-1,3-butadiene (several cycloadducts characterized by X-ray crystallography). The NDA and nitroso-ene reaction pathways of nitroso intermediates with dienes were mapped by DFT computations (B3LYP/6-31G*), which showed that the acyl nitroso species are super-reactive and that activation energies in the NDA processes are lower than the isomerization barriers between some cis- and trans-butadienes.

Studies toward the synthesis of (−)-stenine

Herein is described an application of the acyl nitroso type 2 intramolecular Diels–Alder reaction as a key step in the progress toward the total synthesis of the Stemona alkaloid (−)-stenine. The chiral cycloaddition precursor is rapidly assembled from a cyclohexene intermediate synthesized through an asymmetric, organocatalyzed Robinson annulation. A key selective alkene reduction provides a tricyclic intermediate which could prove useful in the synthesis of the titular natural product.

ChemInform Abstract: Semicarbazones from N‐Hydroxyureas and Amines: A Novel Entry in the Reactivity of the Acyl Nitroso Group.

AbstractThe use of the large excess of amines is crucial to prevent formation of by‐products.

Semicarbazones from N-Hydroxyureas and Amines: A Novel Entry in the Reactivity of the Acyl Nitroso Group

The condensation of carbamoyl nitroso compounds, obtained by oxidation of N-hydroxyureas, with amines unexpectedly afforded semicarbazones (aka carbamoyl hydrazones). Although the substitution of the nitrosyl moiety might compete to afford the corresponding urea, an excess of amine led to the semicarbazone as the major product, which is presumably formed via isomerization of an initially generated acyl azo compound.

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.

Hydroxyurea (therapeutics and mechanism): Metabolism, carbamoyl nitroso, nitroxyl, radicals, cell signaling and clinical applications

During a century, hydroxyurea has received much attention in relation to its physiological properties. This review mainly deals with the metabolism, mechanism, cell signaling, therapeutic properties, bioactivity, receptors, and toxicity. Metabolism provides insight concerning the mechanism. Carbamoyl nitroso is an intermediate, based on ease of oxidation of the parent and subsequent formation of nitroxyl and nitric oxide. Carbamoyl nitroso bears structural and electrochemical similarity to acyl nitroso from hydroxamic acids, to the phenylhydroxylamine-nitrosobenzene couple, and to α-dicarbonyls. Carbamoyl nitroso may be involved in electron transfer, reactive oxygen species formation, and oxidative stress. Cell signaling plays a significant role in the biological action. The therapeutic properties are discussed with emphasis on cancer, sickle cell disease, HIV, skin, and genes. Promise as a practical medicine is indicated by clinical trials. Toxicity is also included. Carbamoyl nitroso, nitroxyl, nitric oxide, and metal complexes of the parent drug are designated the main actors in the physiological effects. The mechanistic theme is in keeping with prior reports in Medical Hypotheses.

Ru(II)- and Ir(I) Catalyzed Hydrogen Peroxide Oxidation of Hydroxamic Acids and their Subsequent Hetero Diels-Alder Cycloadditions with Chiral N-Dienyl Lactams

Asymmetric hetero Diels-Alder (HDA) reaction of highly reactive acyl nitroso intermediates are reported. Transient acyl nitroso compounds 2a-c are formed by Ru(II)- or Ir(I)-catalyzed hydrogen peroxide oxidation of hydroxamic acids 2a-c and are trapped in situ by the optically pure N-dienyl-L-pyroglutamates 1a-b to afford the corresponding diastereomeric adducts 3a-f and 4a-f up to 98% yield (70% de) and 72% de (70% yield) with complete regioselectivity at 0°C to room temperature.