Nitrile Oxides Research Articles

Quantum chemical analysis of the molecular mechanism and selectivity of the 32CA reaction of nitrile oxides with 5-(ethylthio) furan-2(5H)-ones and N-substituted-2-azanorborn-5-ene

Isoxazolines are well-known five-membered heterocyclic scaffolds that exhibit a variety of biological activities. These compounds can be synthesized via a 32CA reaction, but the stereo and the regiochemical factors affecting the formation of a targeted isoxazoline are not well understood despite the importance of this scaffolds. The mechanism and the selectivities of the 32CA reaction of acetonitrile oxides (1) with 5-(ethylthio) furan-2(5H)-ones (2) and benzonitrile oxides (5) with N-substituted-2-azanorborn-5-enes (4) for the formation of isoxazolines have been investigated at the M06-2X, in conjunction with the 6–311++G(d,p) basis set. The reaction between 1 and 2 proceeds with a clear-cut regio and diastereoselectivity. Substituting 4 with varying electronic and steric effects have minimal influence on the regio and stereoselectivity between 4 and 5. The exo cycloadducts are favored over the endo cycloadduct. Analysis of the electrophilic Pk+ and nucleophilic Pk− Parr functions at the different reaction sites in the 5-(ethylthio) furan-2(5H)-one indicates that the acetonitrile oxide adds across the atomic centers with the highest Mulliken atomic spin densities. Results from the global electron density transfer (GEDT) reveal the polar nature of the reactions.

Enantioselective 1,3-Dipolar Cycloadditions of α-Methylene Lactams to Construct Spirocycles.

Spirocyclic scaffolds are important motifs due to their potential to bestow favorable effects on pharmaceutical compounds. However, there is a need for efficient methods for their enantioselective construction. We report a method for the asymmetric 1,3-dipolar cycloaddition of diazoacetates or nitrile oxides with α-methylene lactams to prepare chiral spirocyclic heterocycles. The methodology is high yielding (up to 91% yield) and enantioselective (up to 89% ee) for a wide range of N-substituents and 6- and 7-membered ring lactam substrates.

Regioselective synthesis of some new pyrazole and isoxazole derivatives incorporating benzothiazole moiety: Experimental and theoretical studies

Nitrile imine derivative 3 added regioselectively to α-(benzothiazol-2-yl)cinnamonitrile derivatives 1a–c to afford 5-(benzothiazol-2-yl)-5-cyano-4,5-dihydro-1H-pyrazole derivatives 4a–c. Compound 4a underwent thermal elimination of hydrogen cyanide in sodium ethoxide solution to give the pyrazole derivative 6. The isoxazole derivative 10 was synthesized by reacting nitrile oxide 8 with cinnamonitrile derivative 1d. The structures of the synthesized compounds were characterized by FT-IR, NMR, and MS spectroscopic techniques. The reaction mechanism was studied via density functional theory (DFT) quantum chemical calculations at the B3LYP/6-31G (d) level of theory.

Synthesis and Antimicrobial Activity of some Ester Functionalized Isoxazoles incorporating Anthracene Moieties via Nucleophilic Substitution Reaction

Background and objective: Five-membered heterocycle compounds having single oxygen and nitrogen atom at adjacent positions are known as isoxazoles. Isoxazole compounds have a broad range of biological activities and therapeutic value. In view of a strategic design of antimicrobial compounds, several new ester-functionalized isoxazoles were synthesized and characterized. Methods: A regioselective isoxazole incorporating an anthracene moiety was adducted via an effective 1,3-dipolar cycloaddition between anthracene nitrile oxide and propargyl bromide as a dipolarophile. Results: Synthesized isoxazole 4 underwent nucleophilic substitution reaction to produce unprecedented ester-functionalized isoxazoles 6a-j, by condensation with equimolar amounts of different generated in situ sodium carboxylate upon dissolving in acetonitrile with refluxing. The chemical structure of all target compounds was proved by (FT-IR, 1H-NMR, and APT13C-NMR) techniques and their antibacterial and antifungal activities was evaluated. Conclusion: Allnewly synthesizedcompounds6 a-j have been obtained in good yields after purification by column chromatography. They showed significant antibacterial and antifungal activity after screening against two bacterial strains, Escherichia coli and Staphylococcus aureus and a fungi strain, Candida albicans, using disc diffusion method.

Synthesis of functionalized 4,5-dihydroisoxazoles decorated with the dimethylphosphinoyl group

Aim. To synthesize a hybrid molecular platform incorporating dimethylphosphinoyl and 4,5-dihydroisoxazole moieties suitable for the creation of focused combinatorial libraries of compounds.Results and discussion. The base-promoted interaction of halogenoxides with dimethyl(vinyl)phosphine oxide under mild conditions allowed us to obtain 11 isoxazoline–dimethylphosphine oxide hybrids in moderate yields. The reaction was found to be regio- though non-stereoselective. Furoxans were identified as possible side products of the reaction.Experimental part. The one-pot interaction with dimethyl(vinyl)phosphine oxide was used for the synthesis of the target compounds. Nitrile oxides were obtained in situ from the corresponding halogenoximes by base-promoted generation. The ADME parameters for a synthesized 5-P(O)Me2-isoxazoline compared to its isosters with the same core structure were predicted using a SwissADME Web Tool. The compounds obtained were characterized by 1H, 13C, 19F, 31P NMR spectroscopy and HPLC-MS spectrometry methods, as well as the elemental analysis.Conclusions. A practical approach to the isoxazoline platform decorated with a 5-P(O)Me2 “magic” group and containing 3-substituent with an easy-to-modify functionality has been developed. On example of the piperidine derivative, the effect of the dimethylphosphinoyl group on physicochemical properties and ADME parameters compared to its isosters has been determined.

Front Cover: Protein Modification via Nitrile Oxide−Dehydroalanine Cycloaddition: Formation of Isoxazoline Ring on the Protein Backbone (ChemBioChem 20/2023)

Front Cover: Protein Modification via Nitrile Oxide−Dehydroalanine Cycloaddition: Formation of Isoxazoline Ring on the Protein Backbone (ChemBioChem 20/2023)

Protein Modification via Nitrile Oxide-Dehydroalanine Cycloaddition: Formation of Isoxazoline Ring on the Protein Backbone.

Here we describe a novel catalyst-free 1,3-dipolar cycloaddition bioconjugation approach for chemical modification of proteins. The dehydroalanine (Dha)-containing protein reacts with nitrile oxides generated in situ through 1,3-dipolar cycloaddition in fully aqueous-buffered systems. This leads to the formation of a new isoxazoline ring at a pre-defined site (Dha) of the protein. Furthermore, the 1-pyrene isoxazoline-installed annexin V acts as a fluorescent probe, which successfully labels the outer cellular membranes of human cholangiocarcinoma (HuCCA-1) cells for detection of apoptosis.

PhI(OAc)2‐Mediated One‐Pot Oxidative [3+2] Cycloaddition/ Heterocyclization Reactions of Aldoximes with N‐Arylmaleimides: A Facile Access to N‐Substituted Pyrrolo‐Isoxazoles

AbstractA convenient and efficient metal‐free oxidative [3+2] cycloaddition reaction of N‐arylmaleimides with nitrile oxides generated in situ from the corresponding aldoximes using diacetoxyiodobenzene (DIB) as hypervalent iodine reagent under mild conditions has been established. The one‐pot oxidative cycloaddition afforded corresponding bioinspired bicyclic isoxazole/pyrrolo‐isoxazole derivatives in up to 91 % yield at room temperature. To demonstrate the efficacy of this methodology, the reaction was scaled up to the gram‐scale showing synthetic potential in industrial application. The one‐pot fashion makes the strategy highly time‐economic, atom‐economic and minimizes chemical waste by circumventing several purification steps.

Altered methionine metabolism impacts phenylpropanoid production and plant development in Arabidopsis thaliana.

Phenylpropanoids are specialized metabolites derived from phenylalanine. Glucosinolates are defense compounds derived mainly from methionine and tryptophan in Arabidopsis. It was previously shown that the phenylpropanoid pathway and glucosinolate production are metabolically linked. The accumulation of indole-3-acetaldoxime (IAOx), the precursor of tryptophan-derived glucosinolates, represses phenylpropanoid biosynthesis through accelerated degradation of phenylalanine ammonia lyase (PAL). As PAL functions at the entry point of the phenylpropanoid pathway, which produces indispensable specialized metabolites such as lignin, aldoxime-mediated phenylpropanoid repression is detrimental to plant survival. Although methionine-derived glucosinolates in Arabidopsis are abundant, any impact of aliphatic aldoximes (AAOx) derived from aliphatic amino acids such as methionine on phenylpropanoid production remains unclear. Here, we investigate the impact of AAOx accumulation on phenylpropanoid production using Arabidopsis aldoxime mutants, ref2 and ref5. REF2 and REF5 metabolize aldoximes to respective nitrile oxides redundantly, but with different substrate specificities. ref2 and ref5 mutants have decreased phenylpropanoid contents due to the accumulation of aldoximes. As REF2 and REF5 have high substrate specificity toward AAOx and IAOx, respectively, it was assumed that ref2 accumulates AAOx, not IAOx. Our study indicates that ref2 accumulates both AAOx and IAOx. Removing IAOx partially restored phenylpropanoid content in ref2, but not to the wild-type level. However, when AAOx biosynthesis was silenced, phenylpropanoid production and PAL activity in ref2 were completely restored, suggesting an inhibitory effect of AAOx on phenylpropanoid production. Further feeding studies revealed that the abnormal growth phenotype commonly observed in Arabidopsis mutants lacking AAOx production is a consequence of methionine accumulation.

Synthesis of 5-ethynylisoxazoles based on 1,3-dipolar cycloaddition reactions of nitrile oxides with conjugated diynes

Synthesis of 5-ethynylisoxazoles based on 1,3-dipolar cycloaddition reactions of nitrile oxides with conjugated diynes

Mechanochemical Desymmetrization of Unbiased Bis‐ and Tris‐alkynes to Access 3,5‐Isoxazoles‐Alkyne Adducts and Unsymmetrical Bis‐3,5‐isoxazoles**

AbstractA mechanochemical desymmetrization of symmetrical bis‐ and tris‐alkynes by a controlled 1,3‐dipolar cycloaddition reaction using nitrile oxide dipoles (NOs). This operationally simple protocol allows access to 3,5‐isoxazole‐alkyne adducts from easily prepared or commercially available symmetrical bis‐ and tris‐alkynes in moderate to excellent yields. In addition, we have highlighted the synthetic utility of 3,5‐isoxazole‐alkyne by developing a route to access, for the first time, β‐ketoenamine‐alkyne derivatives, and unsymmetrical bis‐3,5‐isoxazoles.

Synthesis, and discovery of a potential anti-HepG2 agents: Coumarin derivatives containing 3-Aryl isoxazolyl moiety

In here, a series of coumarin-isoxazole derivatives were synthesized, characterized by spectroscopy technologies and evaluated for their cytotoxicity against HepG2 cell lines. Meanwhile, eight bioactive compounds were also evaluated for their toxicity on normal Vero cell lines. MC-16, a monoclinic crystal with P21/c space group, was further confirmed by X-ray and 2D-NMR. Results demonstrated that b5 was reacted with 7 equivalents NCS to given c5, and then MC-16 was obtained by the reaction of nitrile oxides derivatized from c5 with e1, passing intermediate state TS1. Bioactive results indicated that eight compounds (MC-2, MC-4, MC-5, MC-9, MC-10, MC-14, MC-19 and MC-20) exhibited weak to strong cytotoxicity against HepG2 cell lines, and their toxicity to Vero cells was much lower than that of Cisplatin (IC50 (HpeG2) = 8.44 μM/ L, IC50 (Vero) = 28.63 μM/ L, SI= 3.39). Among them, MC-14 (IC50 (HpeG2) = 12.85 μM/ L, IC50 (Vero) = 144.32 μM/ L, SI= 11.23) owned the strongest anti- HepG2 activity and obviously induced apoptosis under 48 μM. Structure-relationship analysis displayed that substituent group R = Cl or Br was the key factors to enhance the anti-HepG2 activity. The docking results revealed that bioactive compounds can effectively interacted with protein (PDB ID: 6O5N).

Development of Practical Synthetic Methods Utilizing O-X or N-X Species

In chemical species containing of oxygen-halogens and nitrogen-halogens bonds, the halogen substituents show versatile reactivities, because oxygen and nitrogen atoms have higher electronegativity than halogen elements except for fluorine. I have focused on such monovalent halogen species and developed various synthetic methods utilizing O-X and N-X bonds which involves ionic and radical reactions. In this account, I describe such practical synthetic methodologies, which are CO2 fixation to unsaturated alcohols and amines, oxidative homo- and cross-coupling reactions of aromatic amines, novel generation of nitrile oxides from aldoximes, synthesis of oxazole-fused C60, iodoamination of alkenes in water, diaziridination of secondary amines, diastereodivergent diamination of unactivated alkenes, practical aziridination of electron-deficient alkenes, selective synthesis of nitrogen-bridged fullerenes, and benzylic C-H amidation.

A computational study of 1,3-dipolar cycloadditions of nitrile oxides with dienes

Density functional theory calculations at the M06-2X level of theory were employed to study the reactivity and regioselectivity of 1,3-dipolar cycloaddition reactions of nitrile oxides with cyclodienes. Calculated relative activation free energies reproduce the experimentally observed product ratios. The electronic energies of activation are found to be mainly controlled by distortion energies required to achieve the transition states. Both electronic and steric effects influence regioselectivities. Theoretical predictions were performed on previous experimental data and analyzed with the use of the distortion/interaction model. The obtained results will help in obtaining a better understanding of the factors that affect the relative cycloaddition rate.

Glycosyl Isoxazoles for Targeting of Tumor Microenvironment and Cancer Cells: Highly Selective Inhibitors of Carbonic Anhydrases IX and XII Showing Cytotoxic Activity

AbstractWe report the synthesis of twelve novel O‐glycosylmethyl isoxazoles incorporating the phenoxyaryl moiety, that were designed to selectively target the cancer‐related human carbonic anhydrase (CA, EC 4.2.1.1) isoforms. The O‐glycosides have been prepared by cycloaddition of nitrile oxides to O‐propargyl glycosides. Compounds were assessed as inhibitors of the physiologically dominant isozymes hCA I and II and the tumor‐associated isozyme hCA IX and hCA XII. In this study, several glycosides have been identified as highly selective inhibitors of hCA IX and XII. Glycoconjugation enhances the anticancer activity against human osteosarcoma cell line. Thus, these glycosides may prove interesting lead candidates for a targeted cancer therapy.

Nitrile Oxide, Alkenes, Dipolar Cycloaddition, Isomerization and Metathesis Involved in the Syntheses of 2-Isoxazolines.

The involvement of 1,3-dipolar cycloaddition (1,3-DP), double bond migration, metathesis, and nitrile oxide (including in situ-generated nitrile oxide) as dipoles, together with the C=C bond containing dipolarophiles, in the syntheses of 2-isoxazolines is presented. Methods for synthesizing isoxazolines (other than 1,3-DP cycloaddition) were also presented briefly. Various methods of nitrile oxide preparation, especially in situ-generated procedures, are presented. Special attention was paid to the application of various combinations of 1,3-DP cycloaddition with double bond migration (DBM) and with alkene metathesis (AM) in the syntheses of trisubstituted isoxazolines. Allyl compounds of the type QCH2CH=CH2 (Q = ArO, ArS, Ar, and others) play the role of dipolarophile precursors in the combinations of DPC mentioned, DBM and AM. Mechanistic aspects of cycloadditions, i.e., concerted or stepwise reaction mechanism and their regio- and stereoselectivity are also discussed from experimental and theoretical points of view. Side reactions accompanying cycloaddition, especially nitrile oxide dimerization, are considered. 2-Isoxazoline applications in organic synthesis and their biological activity, broad utility in medicine, agriculture, and other fields were also raised. Some remaining challenges in the field of 1,3-DP cycloaddition in the syntheses of isoxazolines are finally discussed.

4+2]-Cycloaddition to 5-Methylidene-Hydantoins and 5-Methylidene-2-Thiohydantoins in the Synthesis of Spiro-2-Chalcogenimidazolones.

Novel hydantion and thiohydantoin-based spiro-compounds were prepared via theDiels-Alder reactions between 5-methylidene-hydantoins or 5-methylidene-2-thiohydantoins and 1,3-dienes (cyclopentadiene, cyclohexadiene, 2,3-dimethylbutadiene, isoprene). It was shown that the cycloaddition reactions proceed regioselectively and stereoselectively with the formation of exo-isomers in the reactions with cyclic dienes andthe less sterically hindered products in the reactions with isoprene. Reactions of methylideneimidazolones with cyclopentadiene proceed viaco-heating the reactants; reactions with cyclohexadiene, 2,3-dimethylbutadiene, and isoprene require catalysis by Lewis acids. It was demonstrated that ZnI2 is an effective catalyst in the Diels-Alder reactions of methylidenethiohydantoins with non-activated dienes. The possibility of alkylation and acylation of the obtained spiro-hydantoinsat the N(1)nitrogen atoms with PhCH2Cl or Boc2O and the alkylation of the spiro-thiohydantoinsat the S atoms with MeI or PhCH2Cl in high yields have been demonstrated. The preparativetransformation of spiro-thiohydantoins into corresponding spiro-hydantoinsin mild conditions by treating with 35% aqueous H2O2 or nitrile oxide has been carried out. The obtained compounds show moderate cytotoxicity in the MTT test on MCF7, A549, HEK293T, and VA13 cell lines. Some of the tested compounds demonstrated some antibacterial effect against Escherichia coli (E. coli) BW25113 DTC-pDualrep2 but were almost inactive against E. coli BW25113 LPTD-pDualrep2.

Unveiling the exclusive stereo and site selectivity in [3+2] cycloaddition reactions of a tricyclic strained alkene with nitrile oxides from the molecular electron density theory perspective

Unveiling the exclusive stereo and site selectivity in [3+2] cycloaddition reactions of a tricyclic strained alkene with nitrile oxides from the molecular electron density theory perspective

Koser’s Reagent Mediated Oxidation of Aldoximes: Synthesis of Isoxazolines by 1,3-Dipolar Cycloadditions

AbstractA metal- and base-free, robust, and convenient approach for the synthesis of isoxazoline derivatives is reported. This protocol involves 1,3-dipolar cycloaddition between in situ generated nitrile oxides from the corresponding aldoximes using [hydroxy(tosyloxy)iodo]benzene (HTIB, Koser’s reagent) and maleimides, styrene and acrylonitrile. The described methodology is very attractive as it is operationally simple, has broad scope, and does not require any base, metal, or other additives.

Tert-Butyl Nitrite Initiated C-N Bond Cleavage of 1-Nitromethyl-N-aryltetrahydroisoquinolines: Synthesis of Furoxans with N-NO Skeleton.

A series of furoxan derivatives with N-nitroso groups were synthesized in good yields by TBN initiated radical sp3 C-N bond cleavage of 1-nitromethyl-N-aryltetrahydroisoquinolines. This reaction grafts the biologically important furoxan skeleton and N-nitroso group into on molecule, greatly improving the molecular complexity in one step transformation. The mechanistic study shows that this reaction is mediated by the in situ generated α-carbonyl nitrile oxide, which is afforded by TBN promoted C-N bond cleavage.