A stepwise, zwitterionic mechanism for the 1,3-dipolar cycloaddition between (Z)-C-4-methoxyphenyl-N-phenylnitrone and gem-chloronitroethene catalysed by 1-butyl-3-methylimidazolium ionic liquid cations

Mechanism and stereoselectivity of a [3 + 2] cycloaddition involving a glucosyl nitrone: a MEDT study

In this study, Molecular Electron Density Theory (MEDT) is employed to investigate the reaction mechanism and the experimentally observed stereoselectivity of the [3 + 2] cycloaddition (32CA) between C-(D-glucoso)-N-methyl nitrone 1 and 1H-pyrrole-2,5-dione 2. Within the Conceptual DFT (CDFT) framework, 2 behaves as an electrophile, while 1 acts as the nucleophile. Bonding Evolution Theory (BET) analysis reveals that this 32CA reaction proceeds through a one-step asynchronous mechanism, where the formation of the C3-C4 bond occurs before that of the O1-C5 bond along both the endo and exo pathways. This asynchronous nature is further supported by analyses based on the Electron Localization Function (ELF) and the Quantum Theory of Atoms in Molecules (QTAIM), which confirm the absence of new disynaptic basins (V(C3,C4) and V(O1,C5)) and of new covalent bonds, respectively. Overall, the endo pathway is found to be both thermodynamically and kinetically more favorable than the exo pathway. Finally, using the distortion/interaction-activation strain model, we demonstrate that the observed endo/exo selectivity is primarily governed by differences in interaction energies. Moreover, molecular docking analyses revealed that the endo product has considerable binding affinity to the 1CIN protease, indicating its potential as a therapeutic inhibitor. Moreover, drug-likeness evaluations verified that the compounds adhere to Lipinski's rule of five, signifying advantageous pharmacokinetic characteristics.

A Comprehensive Study of the Synthesis, Spectral Characteristics, Quantum-Chemical Molecular Electron Density Theory, and In Silico Future Perspective of Novel CBr3-Functionalyzed Nitro-2-Isoxazolines Obtained via (3 + 2) Cycloaddition of (E)-3,3,3-Tribromo-1-Nitroprop-1-ene

The search for new heterocyclic compounds with biological potential is one of the current challenges in modern chemistry. Therefore, the comprehensive study of (3 + 2) cycloaddition (32CA) reactions between a series of aryl-substituted nitrile N-oxides (NOs) and (E)-3,3,3-tribromo-1-nitroprop-1-ene (TBNP) is carried out. According to the experimental research, in all tested 32CAs, the proper (4RS,5RS)-3-aryl-4-nitro-5-tribromomethyl-2-isoxazolines are obtained as only one reaction product. In turn, the quantum-chemical MEDT study shows that the creation of heterocycles occur via the polar attack of zwitterionic moderate-nucleophilic NOs to strong electrophilic TBNP. The reactions are realized according to a two-stage, one-step asynchronous mechanism, in which the formation of the O-C(CBr3) bond takes place once the C-C(NO2) bond is already formed. What is more, the computational analysis confirmed the experimental results. At the end, the obtained 2-isoxazolines were docked to three proteins: gelatinase B, cyclooxygenase COX-1, and Caspase-7. We hope that the presented study will be helpful for searching for the future direction of application for this class of organic compounds.

How Different Are Nitrile Oxides from Nitrones in zw-type [3 + 2] Cycloaddition Reactions? A Molecular Electron Density Theory Study

Using the molecular electron density theory, the zw-type [3 + 2] cycloaddition (32CA) reactions of three benzonitrile oxides (BNOs) with several ethylenes, exhibiting different nucleophilic/electrophilic activation, have been studied to establish their poor reactivity. ELF topological analysis of the three BNOs reveals that the para substitution on the benzene ring does not affect the electron density distribution on the CNO frameworks. According to a DFT-based reactivity indices analysis, the nonsubstituted BNO is a moderate electrophile and a moderate nucleophile, which accounts for its poor reactivity in polar reactions. The activation energy analysis of the 32CA reactions of the nonsubstituted BNO allows us to establish that it reacts more effectively with strongly nucleophilic ethylenes than strongly electrophilic ones. Additionally, while the 32CA reactions with nucleophilic ethylenes are entirely ortho regioselective, those with electrophilic ethylenes are poor meta regioselective. A relative interacting atomic energy analysis of these 32CA reactions confirms the establishment of participation of these BNOs in zw-type 32CA reactions. The electrophilic BNO framework stabilization in 32CA reactions with supernucleophilic ethylenes is stronger than that of the ethylene framework in reactions with strong electrophilic ethylenes, which explains the higher reactivity of BNOs toward supernucleophilic ethylenes.

Syn-Propanethial S-Oxide as an Available Natural Building Block for the Preparation of Nitro-Functionalized, Sulfur-Containing Five-Membered Heterocycles: An MEDT Study

The regio- and stereoselectivity and the molecular mechanisms of the [3 + 2] cycloaddition reactions between Syn-propanethial S-oxide and selected conjugated nitroalkenes were explored theoretically in the framework of the Molecular Electron Density Theory. It was found that cycloadditions with the participation of nitroethene as well as its methyl- and chloro-substituted analogs can be realized via a single-step mechanism. On the other hand, [3 + 2] cycloaddition reactions between Syn-propanethial S-oxide and 1,1-dinitroethene can proceed according to a stepwise mechanism with a zwitterionic intermediate. Finally, we evaluated the affinity of model reaction products for several target proteins: cytochrome P450 14α-sterol demethylase CYP51 (RSCB Database PDB ID: 1EA1), metalloproteinase gelatinase B (MMP-9; PDB ID: 4XCT), and the inhibitors of cyclooxygenase COX-1 (PDB:3KK6) and COX-2 (PDB:5KIR).

Phthalazine as a Diene in Diels-Alder Reactions With P- and As-Containing Anionic Dienophiles: Comparison of Possible Reaction Channels

Phthalazine can behave as a diene in Diels-Alder (DA) cycloadditions, typically at the pyridazine ring, however, its application is somewhat limited because these reactions usually require harsh conditions or sophisticated catalysts. As an unconventional example, phthalazine was reported to undergo cycloaddition with the [PCO]- anion without any catalyst. In this computational study, we scrutinise the mechanism of the DA reactions between phthalazine and the so far known [ECX]- (E: P, As; X: O, S, Se) anions as dienophiles. In principle, the attack of an [ECX]- anion may occur at two different sites of phthalazine, either at the benzene or the pyridazine ring, and both of these possible reaction channels were juxtaposed on the basis of energetic aspects. In all of the investigated cases, the analysis of the energy profiles reveals a clear regioselectivity that favours the attack at the pyridazine ring. As a result, so far unprecedented 2-pnictanaphth-3-olate analogues seem achievable as final products. Comparing the characteristics of these pathways allowed us to clarify the source of this regioselectivity: The pyridazine ring of phthalazine exhibits lower aromaticity than the benzene subring; therefore, in the DA step, the former ring shows a higher affinity toward a dienophile than the latter, leading to lower activation barriers. To further map the electronic and structural features of the cycloaddition steps, the local interactions evolving in the transition states were analysed and compared using global and local descriptors. In most aspects, the characteristics of both pathways were found to be rather similar, in contrast to the markedly differing activation barriers on the two routes.

Energetic Aspects and Molecular Mechanism of 3-Nitro-substituted 2-Isoxazolines Formation via Nitrile N-Oxide [3+2] Cycloaddition: An MEDT Computational Study

Regioselectivity and the molecular mechanism of the [3+2] cycloaddition reaction between nitro-substituted formonitrile N-oxide 1 and electron-rich alkenes were explored on the basis of the wb97xd/6-311+G(d) (PCM) quantum chemical calculations. It was established that the thermodynamic factors allow for the formation of stable cycloadducts along all considered models. The analysis of the kinetic parameters of the main processes show that all [3+2] cycloadditions should be realized with full regioselectivity. In all cases, the formation of 5-substituted 3-nitro-2-isoxazolidines is clearly preferred. It is interesting that regiodirection is not determined by the local electrophile/nucleophile interactions but by steric effects. From a mechanistic point of view, all considered reactions should be treated as polar, one-step reactions. All attempts to locate the hypothetical zwitterionic intermediates along the cycloaddition paths were, however, not successful.

On the Question of Zwitterionic Intermediates in the [3+2] Cycloaddition Reactions between Aryl Azides and Ethyl Propiolate

The molecular mechanism of the [3+2] cycloaddition reactions between aryl azides and ethyl propiolate was evaluated in the framework of the Molecular Electron Density Theory. It was found that independently of the nature of the substituent within the azide molecule, the cycloaddition process is realized via a polar but single-step mechanism. All attempts of localization as postulated earlier by Abu-Orabi and coworkers' zwitterionic intermediates were not successful. At the same time, the formation of zwitterions with an "extended" conformation is possible on parallel reaction paths. The ELF analysis shows that the studied cycloaddition reaction leading to the 1,4-triazole proceeds by a two-stage one-step mechanism. It also revealed that both zwitterions are created by the donation of the nitrogen atom's nonbonding electron densities to carbon atoms of ethyl propiolate.

A molecular electron density theory study of asymmetric Diels-Alder [4 + 2] reaction's mechanism of furan with three substituted alkynes (5-R substituted-3-(3-(phenylsulfonyl)-propioloyl)-oxazolidin-2-one)

CONTEXT

The [4 +2 ] cycloaddition reactions between furan and three substituted alkynes (5-R-substituted-3-(3-(phenylsulfonyl)-propioloyl)-oxazolidin-2-one) have been investigated using the MEDT approach. Reactivity indices, reaction pathways, and activation energies are calculated. In an investigation of conceptual DFT indices, furan acts as a nucleophile, while the three substituted alkynes (5-R-substituted-3-(3-(phenylsulfonyl)-propioloyl)-oxazolidin-2-one) function as electrophiles in this reaction. The cycloaddition is regioiselective, as demonstrated by the activation and reaction energies, in clear agreement with the experiment's results. Hetero Diels-Alder [4 + 2] cycloadditions occur following a non-concerted two stages one-step molecular mechanism.

METHODS

For the purpose of this study, all calculations were performed using the Gaussian 09 software. Optimization was achieved through Berny's computational gradient optimization method, employing the B3LYP functional and the 6-31G(d) basis set. Analysis of both local and global reactivity indices provided insights into the reactivity tendencies of the reactants, distinguishing between electrophilic and nucleophilic characteristics via Parr functions. Frequency calculations were employed to identify and characterize stationary points, with transition states indicated by a single imaginary frequency and positive values of all frequencies for reactants and product. The electron localization function (ELF) was investigated using the Multiwfn software within the context of topological analyses.

Full Regio- and Stereoselective Protocol for the Synthesis of New Nicotinoids via Cycloaddition Processes with the Participation of Trans-Substituted Nitroethenes: Comprehensive Experimental and MEDT Study

[3 + 2] Cycloaddition reactions with the participation of Z-C-(3-pyridyl)-N-methylnitrone and series of E-2-R-nitroethenes were both experimentally and theoretically explored in the framework of Molecular Electron Density Theory. It was found that all considered processes are realized under mild conditions and in full regio- and stereocontrol. The ELF analysis additionally showed that the studied reaction proceeds by a two-stage, one-step mechanism.

DFT Study of Regio- and Stereoselective 13DC Reaction between Diazopropane and Substituted Chalcone Derivatives: Molecular Docking of Novel Pyrazole Derivatives as Anti-Alzheimer's Agents

In the present work, a combination of experimental and density functional theory (DFT) investigation of the (3+2) cycloaddition reactions of diazopropane with chalcone derivatives was reported. All calculations were performed using several DFT approaches (B3LYP, M06, M06-2X) and 6-311+G(d, p) basis set. Based on the NMR, MS analyses and IRC calculations, the pyrazole derivatives are the kinetic adducts over the oxadiazoles. The use of two equivalents of diazopropane leads to thermodynamical products. A molecular docking analysis was performed to investigate the efficiency of the obtained products against selected drug targets in anti-Alzheimer ligand-receptor interactions. We revealed that the ligands selected were bound mainly to the catalytic (CAS) and peripheral (PAS) anionic sites of acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) inhibitors, respectively. The selected ligands 1, 3, 4 and P14 may act as the best inhibitors against Alzheimer's disease (AD).

Understanding the Regioselectivity and the Molecular Mechanism of [3 + 2] Cycloaddition Reactions between Nitrous Oxide and Conjugated Nitroalkenes: A DFT Computational Study

Regiochemical aspects and the molecular mechanism of the [3 + 2] cycloaddition between nitrous oxide and conjugated nitroalkenes were evaluated on the basis of the wb97xd/6-311 + G(d) (PCM) computational study. It was found that, independently of the nature of the nitroalkene, all considered processes are realized via polar, single-step mechanisms. All attempts at the localization of hypothetical zwitterionic intermediates were unsuccessful. Additionally, the DFT computational study suggested that, in the course of the reaction, the formation of respective Δ²-4-nitro-4-R₁-5-R₂-1-oxa-2,3-diazolines was preferred from the kinetic point of view.

On the Question of the Formation of Nitro-Functionalized 2,4-Pyrazole Analogs on the Basis of Nitrylimine Molecular Systems and 3,3,3-Trichloro-1-Nitroprop-1-Ene

Experimental and theoretical studies on the reaction between (E)-3,3,3-trichloro-1-nitroprop-1-ene and N-(4-bromophenyl)-C-arylnitrylimine were performed. It was found that the title process unexpectedly led to 1-(4-bromophenyl)-3-phenyl-5-nitropyrazole instead of the expected Δ²-pyrazoline molecular system. This was the result of a unique CHCl₃ elimination process. The observed mechanism of transformation was explained in the framework of the molecular electron density theory (MEDT). The theoretical results showed that both of the possible channels of [3 + 2] cycloaddition were favorable from a kinetic point of view, due to which the creation of 1-(4-bromophenyl)-3-aryl-4-tricholomethyl-5-nitro-Δ²-pyrazoline was more probable. On the other hand, according to the experimental data, the presented reactions occurred with full regioselectivity.

The First Examples of [3+2] Cycloadditions with the Participation of (E)-3,3,3-Tribromo-1-Nitroprop-1-Ene

The first examples of [3+2] cycloaddition reactions between 3,3,3-tribromo-1-nitroprop-1-ene (TBMN) were explored on the basis of experimental and theoretical approaches. It was found that reactions involving TBMN and diarylnitrones realized with full regio- and stereoselectivity lead to respective 3,4-cis-4,5-trans-4-nitroisoxazolidines. The regioselecticity and the molecular mechanism of title processes was analyzed on the basis of the advanced DFT computational study.

On the Mechanism of the Synthesis of Nitrofunctionalised Δ2-Pyrazolines via [3+2] Cycloaddition Reactions between α-EWG-Activated Nitroethenes and Nitrylimine TAC Systems

We investigated the reactivity of different substituted nitrylimine-type three atom components (TACs) in [3+2] cycloaddition (32CAs) reactions with electrophilically activated nitroethenes within molecular electron density theory (MEDT). In parallel research, the molecular mechanism of the considered transformation was examined through analysis of all possible reaction channels and full optimization of all critical structures. In particular, the existence of zwitterionic intermediates on reaction paths was verified. On the basis of the bonding evolution theory (BET), the mechanism of the 32CA reaction between C,N-diphenylnitrylimine and (E)-2-phenyl-1-cyano-1-nitroethene should be treated as a one-step two-stage mechanism.

Unveiling the intramolecular [3 + 2] cycloaddition reactions of C,N-disubstituted nitrones from the molecular electron density theory perspective

The zwitterionic type intramolecular [3 + 2] cycloaddition reactions of nitrones show remarkable substituent effects and varying reactivities in the cases of cyclic and acyclic alkenes.

Discovery of 4-nitro-3-phenylisoxazole derivatives as potent antibacterial agents derived from the studies of [3 + 2] cycloaddition

Polysubstituted phenylisoxazoles were designed and synthesized to discover new antibacterial agents via [3 + 2] cycloaddition. Thirty-five compounds with a phenylisoxazole scaffold were characterized by NMR, HRMS, and X-ray techniques. After being evaluated against Xanthomonas oryzae (Xoo), Pseudomonas syringae (Psa), and Xanthomonas axonopodis (Xac), 4-nitro-3-phenylisoxazole derivatives were found to better antibacterial activities. Further studies have shown that the EC₅₀ values of these compounds were much better than that of the positive control, bismerthiazol.

Thirty-five compounds with phenylisoxazole scaffold were synthesized via [3+2] cycloaddition. After being evaluated against Xoo, Psa and Xac, 4-nitro-3-phenylisoxazole derivatives were found well antibacterial activities.

On the Question of Stepwise [4+2] Cycloaddition Reactions and Their Stereochemical Aspects

Even at the end of the twentieth century, the view of the one-step [4+2] cycloaddition (Diels-Alder) reaction mechanism was widely accepted as the only possible one, regardless of the nature of the reaction components. Much has changed in the way these reactions are perceived since then. In particular, multi-step mechanisms with zwitterionic or diradical intermediates have been proposed for a number of processes. This review provided a critical analysis of such cases.

Regio- and stereoselectivity of the [3+2] cycloaddition of nitrones with methyl-acetophenone: A DFT investigation

A theoretical study of the regio- and stereoselectivities of the [3 + 2] cycloaddition reactions of nitrones with substituted alkene (methyl acetophenone) is investigated using density functional theory (DFT) and carried out at B3LYP/6-311+G(d,p) level. The reactivity of these cycloadditions is rationalized by FMO model, activation energy calculations, and philicity indexes. The electronic populations have been calculated from natural orbital, which based on charges by using NBO analysis, MK and CHelpG electrostatic population. The four possible pathways, fused and bridged regioisomeric modes, and the two stereoisomeric approaches endo and exo for the cycloaddition reactions are analyzed and discussed. Analysis of TS geometries and bond lengths demonstrate that these reactions follow a one-step mechanism with asynchronous transition states. The activation energy indicated a favored endo approach along the four reaction pathways.

Understanding the different reactivity of (Z)- and (E)-β-nitrostyrenes in [3+2] cycloaddition reactions. An MEDT study

The experimental reactivity of isomeric (Z)- and (E)-β-nitrostyrenes participating in [3+2] cycloaddition (32CA) reactions has been analysed on the basis of molecular electron density theory (MEDT) at the HF/6-311G(d,p), B3LYP/6-311G(d,p) and ωB97X-D/6-311G(d,p) computational levels. It was found that the polar zw-type 32CA reactions with 5,5-dimethylpyrroline-N-oxide proceed via a one-step mechanism, characterised by the attack of the nucleophilic oxygen centre of the nitrone on the electrophilically activated β-position of these nitrostyrenes. This behaviour is completely understood by means of the analysis of the conceptual DFT reactivity indices. These 32CA reactions present low activation enthalpies of 4.4 (Z) and 5.0 (E) kcal mol-1, and are exo (Z) and endo (E) stereoselective (B3LYP), as well as completely meta regioselective (ωB97X-D, B3LYP). The less stable (Z)-β-nitrostyrene is more reactive than the (E)-one (HF). ELF and AIM topological analyses of the reagents and TSs show the great similitude between their electronic structures. Finally, NCI allows explaining the exo stereoselectivity found in the reaction of (Z)-β-nitrostyrene. The present MEDT study explains the different reactivity, selectivity and competitiveness in the title reactions.

Synthesis of spirocyclic Δ4-isoxazolines via [3 + 2] cycloaddition of indanone-derived ketonitrones with alkynes

A [3 + 2] cycloaddition of indanone-derived nitrones and alkynes under mild conditions is developed, allowing facile synthesis of spirocyclicindenyl isoxazolines with structural diversity. The sequential protocol of generated in situ ketonitrone from unsaturated ketones and N-alkylhydroxylamines is also achieved successfully, affording the desired products in considerable yield with moderate to good diastereoselectivity. Moreover, the spirocyclic product can be conveniently transformed into indenyl-based allylic alcohol and enamide.

A [3 + 2] cycloaddition of indanone-derived nitrones with alkynes under mild conditions has been developed. It is a highly efficient and straightforward method for the synthesis of diverse spirocyclicindenyl isoxazolines.

On the Question of Zwitterionic Intermediates in the [3+2] Cycloaddition Reactions: A Critical Review

New discoveries require a fundamental revision of the view on the mechanism of the 32CAreaction (according to the older nomenclature defined as 1,3-dipolar cycloaddition reactions). The view of the one-step, “concerted” mechanism of such processes developed in the 20-century is very popular today, both in academic literature and among organic chemists who do not specialize in such transformations. Meanwhile, more and more reports bring examples of reactions that clearly cannot be treated as processes without intermediates. However, these examples are documented very differently. In addition to comprehensive studies using many complementary research techniques, there are also reports in which the presence of intermediates in the cycloaddition environment is postulated on the basis of very unreliable premises. This review is an attempt at a critical analysis and systematization of data in the presented area.

Unprecedented Regio- and Stereoselective Synthesis of Pyrene-Grafted Dispiro[indoline-3,2'-pyrrolidine-3',3″-indolines]: Expedient Experimental and Theoretical Insights into Polar [3 + 2] Cycloaddition

A series of dispiro[indoline-3,2'-pyrrolidine-3',3″-indolines] was synthesized via a multicomponent polar [3 + 2] cycloaddition (32CA) reaction of isatin derivatives, sarcosine and (E)-3-(2-oxo-2-(pyren-1-yl)ethylidene)indolin-2-one derivatives. The regio- and stereochemistries of the cycloadducts were established on the basis of one-dimensional (1D) (1H-, 13C-, 13C-CRAPT NMR) and two-dimensional (2D) homonuclear and heteronuclear correlation NMR spectrometry experiments (1H-1H gDQFCOSY, 13C-1H-HSQCAD, 13C-1H-HMBCAD, 1H-1H-ROESYAD). The molecular mechanism and regio- and stereoselectivities of the cycloaddition (CA) reaction have been investigated utilizing a density functional theory (DFT) method and were thoroughly explained based on the transition-state stabilities and global/local electrophilicity/nucleophilicity reactivity indices of the reactants.

Unravelling the regio- and stereoselective synthesis of bicyclic N,O-nucleoside analogues within the molecular electron density theory perspective

The [3 + 2] cycloaddition (32CA) reactions of 1-pyrroline-1-oxide with N-vinyl nucleobases leading to bicyclic N,O nucleoside analogues have been studied within the molecular electron density theory (MEDT) at the MPWB1K/6-311G(d,p) computational level. These non-polar zwitterionic type 32CA reactions take place through a one-step mechanism with minimal global electron density transfer (GEDT) at the TSs and the exo/ortho approach mode as the energetically favoured reaction path. The 32CA reactions of N-vinyl nucleobases with thymine and cytosine substituents respectively show the activation enthalpies of 15.2 and 12.5 kcal mol⁻¹ in toluene. The reactions are irreversible due to strong exothermic character of − 35.4–− 26.4 kcal mol⁻¹ in toluene. The bonding evolution theory (BET) study suggests that these 32CA reactions take place through the coupling of pseudoradical centres with earlier C–C bond formation and the formation of new C–C and C–O covalent bonds has not been started in the TSs. Non-covalent interactions (NCI) are predicted at the TSs from the visualization of NCI gradient isosurfaces.

Prediction of the [4 + 2]- and [5 + 4]-cycloaddition reactions in zig-zag carbon nanotubes via an ambimodal transition state: density functional theory calculations

A unique type of chemical reaction known as an ambimodal reaction has drawn tremendous attention owing to its intriguing feature of forming multiple (two or more) products from the same (single) transition state. In contrast to conventional reactions, bifurcation of the potential energy surface takes place in ambimodal reactions. Density functional theory (DFT) based calculations were performed to probe the Diels–Alder (DA) cycloaddition reactions of various carbon nanotubes (CNTs) with 1,3-butadiene. The present investigation reveals the possibility of ambimodal transition state formation on a potential energy surface (PES) corresponding to an unusual [5 + 4]-cycloadduct along with the conventional [4 + 2]-cycloadduct. The ground state of the [5 + 4]-cycloadduct obtained from butadiene and the H-terminated CNTs is a triplet (³T) state, but on the other hand the [4 + 2]-cycloadduct is a singlet (¹S) state. The [5 + 4]-adduct is energetically more stable in comparison with the [4 + 2]-adduct. The possibility of the formation of the [5 + 4]-adduct is validated using frontier molecular orbitals. The length of the nanotube significantly influences the overall kinetics and thermodynamics of the reaction.

A DFT study has been performed to unveil the ambimodal reaction in H-terminated CNTs.

Role of Water in the Reaction Mechanism and endo/exo Selectivity of 1,3-Dipolar Cycloadditions Elucidated by Quantum Chemistry and Machine Learning

Asymmetric 1,3-dipolar cycloadditions of azomethine ylides with activated olefins are among the most important and versatile methods for the synthesis of enantioenriched pyrroline and pyrrolidine derivatives. Despite both theoretical and practical importance, the role of water molecules in the reactivity and endo/exo selectivity remains unclear. To explore how water accelerates the reactions and improves the endo/exo selectivity of the cycloadditions of 1,3-dipole phthalazinium-2-dicyanomethanide (1) and two dipolarophiles, an ab initio-quality neural network potential that overcomes the computational bottleneck of explicitly considering water molecules was used. It is demonstrated that not only the nature of both the dipolarophile and the 1,3-dipole, but also the solvent medium, can perturb or even alter the reaction mechanism. An extreme case was found for the reaction of 1,3-dipole 1 with methyl vinyl ketone, in which the reaction mechanism changes from a concerted to a stepwise mode on going from MeCN to H₂ O as solvent, with formation of a zwitterionic intermediate that is a very shallow minimum on the energy surface. Thus, high stereocontrol can still be expected despite the stepwise nature of the mechanism. The results indicate that water can induce global polarization along the reaction coordinate and highlight the role of microsolvation effects and bulk-phase effects in reproducing the experimentally observed aqueous acceleration and enhanced endo/exo selectivity.

Novel functionalized β-nitrostyrenes: Promising candidates for new antibacterial drugs

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A homogenous series of β-EWG functionalyzed β-nitrostyrenes was synthesized and characterized by IR, UV, ¹H NMR and ¹³C NMR spectra.

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The obtained compounds were screened in vitro against a panel of the reference strains of bacteria and fungi - five of them exhibited significant antimicrobial activity against all tested reference bacteria and fungi belonging to yeasts.

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Cytotoxicity of the compounds towards cultured human HepG2 and HaCaT cells was established – five of them displayed similar low pattern of activity in both cells lines.

The process of searching for new antibacterial agents is more and more challenging due to the increasing drug resistance which has become a major concern in the field of infection management. Our study presents a synthesis and characterization by IR, UV, ¹H NMR and ¹³C NMR spectra of a homogenous series of 1-EWG functionalized 2-aryl-1-nitroethenes which could prove good candidates for the replacement of traditional antibacterial drugs In vitro screening against a panel of the reference strains of bacteria and fungi and their cytotoxicity towards cultured human HepG2 and HaCaT cells was performed. Antimicrobial results indicated that four of the synthesized compounds exhibited a significant antimicrobial activity against all tested reference bacteria and fungi belonging to yeasts with a specific and strong activity towards B. subtilis ATCC 6633. Two of these compounds had no detectable cytotoxicity towards the cultured human cell lines, making them promising candidates for new antibacterial drugs.

Regiospecific formation of the nitromethyl-substituted 3-phenyl-4,5-dihydroisoxazole via [3 + 2] cycloaddition

ABSTRACT

5-(Nitromethyl)-3-phenyl-4,5-dihydroisoxazole was obtained as a product of a high-yielding [3 + 2] cycloaddition reaction of in situ-generated benzonitrile N-oxide and 3-nitroprop-1-ene. For the first time, the regiochemistry of this reaction was unambiguously proven by X-ray structural analysis. The quantum-chemical calculation performed at the M06-2X/6-31G(d) (PCM) theoretical level affords a basis for explaining the course of reaction as well as the nature of transition states. Next, further DFT calculations together with spectral data shed light on structural aspects of the product.

GRAPHICAL ABSTRACT

A Molecular Electron Density Theory Study of the Reactivity and Selectivities in [3 + 2] Cycloaddition Reactions of C,N-Dialkyl Nitrones with Ethylene Derivatives

The zw-type [3 + 2] cycloaddition (32CA) reactions of C,N-dialkyl nitrones with a series of ethylenes of increased electrophilic character have been studied within the Molecular Electron Density Theory (MEDT) at the MPWB1K/6-311G(d,p) computational level. Both, reactivity and selectivities are rationalized depending on the polar character of the reaction. Due to the strong nucleophilic character of C,N-dialkyl nitrones, the corresponding zw-type 32CA reactions are accelerated with the increased electrophilic character of the ethylene, which also plays a crucial role in the reaction mechanism, thus determining the regio- and stereoselectivities experimentally observed. While, in the 32CA reactions with nucleophilic ethylenes, the reaction begins with the formation of the C-C single bond, determining the ortho regioselectivity, in the 32CA reactions with strong electrophilic ethylenes, the reaction begins with the formation of the C-O single bond involving the β-conjugated carbon of the ethylene, determining the meta regioselectivity. The present MEDT study also provides an explanation for the unexpected ortho regioselectivity experimentally found in the 32CA reactions involving weak electrophilic ethylenes such as ethyl acrylate and acrylonitrile.