Stereoselective 1,3-dipolar cycloadditions of a chiral nitrone derived from erythrulose. An experimental and DFT theoretical study

Design and synthesis of novel enantiopure Bis(5-Isoxazolidine) derivatives: insights into their antioxidant and antimicrobial potential via in silico drug-likeness, pharmacokinetic, medicinal chemistry properties, and molecular docking studies

A series of novel compounds, mono-5-isoxazolidines, and bis (5-isoxazolidines) derivatives, were prepared as bicycloadducts. The new series of isoxazolidines were designed and synthesized via 1,3-dipolar cycloaddition reaction of nitrones with 3,9-Divinyl-2,4,8,10-tetra oxaspiro (5-5) undecane in the context of new antimicrobial and antioxidant drugs discovery and were fully characterized by FT-IR, ¹³C-NMR, and ¹H-NMR spectroscopy. The physicochemical properties of all the novel cycloadducts, like bioactivity score and lipophilicity, were predicted using calculative methods. Similarly, the pharmacokinetic properties such as metabolism, absorption, distribution, and excretion (ADME) were also predicted. Most of the tested compounds exhibited antimicrobial properties to varying degrees against various bacterial species, including the Gram-negative bacteria Pseudomonas aeruginosa and Escherichia coli, and the Gram-positive bacteria Streptococcus pyogenus and Staphylococcus aureus, Antifungal properties were also observed against the tested fungi like Candida albicans, Aspergillus niger, and Aspergillus clavatus. The activity data exhibited that most compounds have high activity as compared to the standard drugs. In the range of graded doses, the results of some selected compounds revealed that some are high antioxidants while others are moderate or weak antioxidants. As evidenced by the molecular docking studies, the synthesized compounds showed good binding mode better than a standard drug, against the protein of a Pantothenate Synthetase enzyme (PDB-2X3F).

Theoretical analysis of the regio- and stereoselective synthesis of spiroisoxazolines

The [3+2] cycloaddition (32CA) reaction of benzonitrile oxide to α-methylene cyclopentanone and propionitrile oxide to γ-methyl-α-methylene-γ-butyrolactone, yielding regio- and stereochemically defined spiroisoxazolines, has been studied at the MPWB1K/6-311G(d,p) computational level. These processes proceed by a one-step mechanism through asynchronous transition states. Ortho regioselectivity and anti diastereofacial selectivity are predicted in complete agreement with the experimental outcomes.

Copper(I)-Catalyzed 1,3-Dipolar Cycloaddition of Ketonitrones to Dialkylcyanamides: A Step toward Sustainable Generation of 2,3-Dihydro-1,2,4-oxadiazoles

Cu^I-catalyzed cycloaddition (CA) of the ketonitrones, Ph₂C=N⁺(R')O^- (R' = Me, CH₂Ph), to the disubstituted cyanamides, NCNR₂ (R = Me₂, Et₂, (CH₂)₄, (CH₂)₅, (CH₂)₄O, C₉H₁₀, (CH₂Ph)₂, Ph(Me)), gives the corresponding 5-amino-substituted 2,3-dihydro-1,2,4-oxadiazoles (15 examples) in good to moderate yields. The reaction proceeds under mild conditions (CH₂Cl₂, RT or 45 °C) and requires 10 mol % of [Cu(NCMe)₄](BF₄) as the catalyst. The somewhat reduced yields are due to the individual properties of 2,3-dihydro-1,2,4-oxadiazoles, which easily undergo ring opening via N-O bond splitting. Results of density functional theory calculations reveal that the CA of ketonitrones to Cu^I-bound cyanamides is a concerted process, and the copper-catalyzed reaction is controlled by the predominant contribution of the HOMO_dipole-LUMO_{dipolarophile} interaction (group I by Sustmann's classification). The metal-involving process is much more asynchronous and profitable from both kinetic and thermodynamic viewpoints than the hypothetical metal-free reaction.

On the outside looking in: rethinking the molecular mechanism of 1,3-dipolar cycloadditions from the perspective of bonding evolution theory. The reaction between cyclic nitrones and ethyl acrylate

Understanding the molecular mechanism of 1,3-dipolar cycloadditions using the bonding evolution theory.

Acyclic and cyclic nitrone cycloadditions to 1-cinnamoyl-1-piperidine: A DFT study

DFT studies have been carried out for the cycloaddition reactions of a cyclic nitrone, 1-pyrroline-1-oxide and an acyclic nitrone, C,N-diphenyl nitrone to an unsymmetrically disubstituted dipolarophile, 1-cinnamoyl-1-piperidine. These reactions proved to be opposite to each other with respect to the electron demand character predicted by the electronic chemical potentials, electrophilicities and charge transfer at the transition states. The regio- and stereoselectivities have been predicted from DFT based reactivity indices, interaction energy calculations using a perturbative orbital independent theoretical model and the activation parameters of the located transition states. Two different concepts have been used for the evaluation of interaction energies. The selectivities were found to be in conformity with the experimental findings. The time gaps between the formations of C – C and C – O bonds were evaluated from single trajectory simulations. The asynchronicity of bond formation process was analyzed from the wiberg bond indices, atom–atom overlap weighted NAO bond orders and the calculated asymmetry indices of the transition states.

Distortion–interaction analysis along the reaction pathway to reveal the reactivity of the Alder-ene reaction of enes

The reactivity of uncatalyzed Alder-ene type reactions of hetero-substituted propylene is interpreted by distortion–interaction analysis of both the transition states and the complete reaction pathways.

Mechanism and stereoselectivity of the stepwise 1,3-dipolar cycloadditions between a thiocarbonyl ylide and electron-deficient dipolarophiles: a computational investigation

The 1,3-dipolar cycloadditions of a thiocarbonyl ylide to dimethyl 2,3-dicyanofumarate and 2,3-dicyanomaleate have been studied with DFT computations. The first C-C bond is formed via an anti attack to produce a very polar, zwitterionic diradical. The low stereoselectivity of the reaction was found to arise from rotations about single bonds in the intermediates that compete with cyclization. A distortion-interaction energy analysis was performed to compare the stepwise and concerted mechanisms, and to explain why the stepwise mechanism is favored in this unusual case.

Predictions of substituent effects in thermal azide 1,3-dipolar cycloadditions: implications for dynamic combinatorial (reversible) and click (irreversible) chemistry

Substituent effects in 1,3-dipolar cycloadditions of azides with alkenes and alkynes were investigated with the high-accuracy CBS-QB3 method. The possibilities for noncatalytic activation and the reversibility or irreversibility of these reactions was explored; the possibilities for uses in dynamic combinatorial chemistry (DCC) or click chemistry were explored. The activation enthalpies for reactions of ethylene and acetylene with hydrazoic acid, formyl, phenyl-, methyl-, and methanesulfonylazides exhibit modest variation, with Delta H++ ranging from 17 to 20 kcal/mol. A detailed study of formylazide cycloadditions with various alkenes and alkynes reveals a narrow range of activation enthalpies (17-21 kcal/mol). The activation enthalpies for the reactions of azides with alkenes and alkynes are similar. FMO theory and distortion/interaction energy control have been used to rationalize the rates and regiochemistries of cycloadditions involving alkene dipolarophiles. Significantly, triazoles, formed from alkynes, are 30-40 kcal/mol more stable than tetrazolines formed from alkenes. On the basis of initial reactant concentrations, kinetic and thermodynamic values are suggested for the identification of reversible reactions that approach equilibrium over 24 h, as well as for fast irreversible reactions. Although azide cycloadditions are suitable for irreversible chemistry and are typically unsuitable for reversible applications, theoretical procedures established by these studies have provided guidelines for the prediction of useful reversible libraries.

(1R,4R,5R)-1,3,4-Triphenyl-7-[(R)-1-phenyl-ethyl]-2-oxa-3,7-diaza-spiro-[4.5]decan-10-one

In the title compound, C₃₃H₃₂N₂O₂, the polysubstituted piperidine ring adopts a chair conformation. The isoxazolidine ring is in an envelope conformation. In the crystal structure, intra- and inter­molecular C—H⋯π inter­actions involving the phenyl rings are observed.

Dual Reactivity Pattern of Allenolates “On Water”: The Chemical Basis for Efficient Allenolate-Driven Organocatalytic Systems

A study of the reactivity pattern associated with zwitterionic allenolates "on water" is reported. This study establishes the chemical basis for two organocatalyzed allenolate-driven reaction networks operating "on water". The first one is a chemodifferentiating three building block (ABB') three-component reaction (ABB' 3CR) manifold comprising terminal alkynoates and aldehydes. The manifold produces propargylic enol ethers 3 with higher average efficiency than their homologues in organic solvents. The second one is a novel organocatalytic system elicited by the reaction of alkynoates and nitrones in the presence of tertiary amines or phosphines. While terminal alkynoates afford 2,3,5-trisubstituted 2,3-dihydroisoxazoles 5 and propargylic N-hydroxylamines 6, internal alkynoates selectively afford the 2,3,4,5-tetrasusbstituted 2,3-dihydroisoxazaole 10. Importantly, in both cases, the 2,3-dihydroisoxazole ring is obtained as a sole regioisomer.

Theoretical Study of Chemo-, Regio-, and Stereoselectivity in 1,3-Dipolar Cycloadditions of Nitrones and Nitrile Oxides to Free and Pt-Bound Bifunctional Dipolarophiles

1,3-Dipolar cycloadditions of nitrones RCH=N(CH3)O and the nitrile oxide CH3CNO to the bifunctional cyanoalkynes NC-CCR' and cyanoalkenes E-NCCH=CHR' (R=H, Ph; R'=H, Ph)--both free and ligated to PtII and PtIV--were investigated by theoretical methods at B3LYP and, for some reactions, CCSD(T) and CBS-Q levels of theory. Chemo-, regio-, and stereoselectivity of the processes and factors, which affect the reactivity and selectivity, were analyzed, and verified trends are discussed in details. Coordination of dipolarophiles to PtII and, particularly, to PtIV facilitates the CN relative to the CC additions of nitrones due to higher activation of the CN group in comparison to the CC group. The bonding of the ligands to platinum also favors the meta versus ortho pathways and endo versus exo pathways that sometimes lead to a switch of the reaction direction. Introduction of Ph groups into the reactant(s) molecules also leads to the promotion of the CN versus CC routes, and this effect is especially strong when both reactants are Ph-substituted. The substituent effect is accounted for by steric repulsions imposed by the Ph groups in transition states (TSs) and by the loss of a conjugation in phenylnitrone and phenylcyanoalkene molecules upon the TS formation. Solvation inhibits the CN and meta-CC additions and, hence, generally favors the CC versus CN pathway, the ortho versus meta pathway, and the exo versus endo pathway. All reactions except one proceed concertedly via a nearly synchronous mechanism for the CN and meta-CC additions to free ligands and asynchronous mechanism for the other processes. For the reaction CH2=N(CH3)O + PtIV-1, a stepwise route is realized.

Experimental and Theoretical Studies on Stereo- and Regioselectivity in Intramolecular Nitrone−Alkene Cycloaddition of Hept-6-enoses Derived from Carbohydrates

The effect of blocking groups and stereochemistry of the substituents on the regio- and stereoselectivity in intramolecular nitrone-alkene cycloaddition (INAC) of hept-6-enoses are reported. L-ribo-Hept-6-enose 12 and D-lyxo-hept-6-enose 15, both containing a 2,3-O-isopropylidene blocking group, and L-xylo-hept-6-enose 23 and D-arabino-hept-6-enose 30, both with a 2,3-O-trans-diacetal blocking group, were prepared from D-ribose and D-arabinose, respectively. With N-alkyl hydroxylamine, lactols 12 and 15 underwent an INAC reaction to give cis-fused isoxazolidines exclusively whereas lactols 23 and 30 gave a mixture of cis-, trans-fused isoxazolidines (cyclohexanols) and bridged isoxazolidines (cycloheptanols). With the 2,3-O-trans-diacetal protecting group, the bridged bicyclo[4.2.1]isoxazolidines (cycloheptanols), via the endo mode of INAC cyclization, were synthesized for the first time from unbranched sugar derivatives 23 and 30. The stereochemical outcomes of these reactions were rationalized on the basis of transition state energies obtained by computation. The present INAC showed trivial temperature, but significant solvent dependence. For lactols 23 and 30, performing the INAC in 2-propanol gave the best yields of fused isoxazolidines (cyclohexanols) whereas in dichloromethane afforded the best yields of bridged isoxazolidines (cycloheptanols).

Theoretical Study of Reactant Activation in 1,3-Dipolar Cycloadditions of Cyclic Nitrones to Free and Pt-Bound Nitriles

[reaction: see text] 1,3-Dipolar cycloaddition of the cyclic nitrones CH2(CH2)2CH=NO (N1), CH2CH2OCH=NO (N2), CH2-OCH2CH=NO (N3), and O(CH2)2CH=NO (N4) to organonitriles, RCN-both free (R = CH(3), CF(3)) and ligated to Pt(II) and Pt(IV) (in the complexes trans-[PtCl(2)(NCCH(3))(2)] (1) and trans-[PtCl(4)(NCCH(3))(2)] (2))-was investigated extensively by theoretical methods at different levels of theory. The effectiveness of two types of dipolarophile activation (by introducing a strong electron-acceptor group R and by coordination to a metal center) was analyzed and compared. The influence of factors such as the nature of the cyclic nitrone and the nature of the solvent on the reactions was also studied. The reactivity of dipoles and dipolarophiles increases along the series N4 < N1 approximately N3 < N2 and CH(3)CN < CF(3)CN < 1 < 2; the latter demonstrates that the coordination of RCN to a Pt center provides an even higher activation effect upon cycloaddition in comparison with the introduction of a strong electron-acceptor group R such as CF(3). A higher reactivity of the cyclic dipole N1 in comparison with acyclic nitrones (e.g., CH(3)CH=N(CH(3))O) is interpreted to be a result of its exclusive existence in a more strained and hence more reactive E- rather than Z-configuration. The activation and reaction energies have been calculated at different basis sets and levels of theory, up to MP4(SDTQ), CCSD(T), and CBS-Q. The activation energies are weakly sensitive to a change of the correlated methods. The consideration of the solvent effects results in the increase of the activation barriers, and such enhancement is less pronounced for the nonpolar or low polar solvents. The cycloadditions to CH(3)CN and CF(3)CN were found to be nearly synchronous, but these reactions involving 1 and 2 are clearly asynchronous. Moreover, the reaction of N2 with 2 proceeds via a very early acyclic transition state, while for all other reactions the transition states have a cyclic nature.

The mechanism of 1,3-dipolar cycloaddition reactions of cyclopropanes and nitrones — A theoretical study

The 1,3-dipolar cycloaddition reaction of cyclopropanes and nitrones to give tetrahydro-1,2-oxazine has been studied with density functional theory calculations at the B3LYP/6-31+G(d,p) level of theory. Realistic substituents were modelled including those at the 2-, 3-, 4-, and 6-positions of the final oxazine ring product. The strained σ bond of the cyclopropane was found to play the role of an alkene in a conventional [3+2] dipolar cycloaddition. Two distinct, but similar, reaction mechanisms were found — an asymmetric concerted pathway and a stepwise zwitterionic pathway. The reaction barriers of the two pathways were nearly identical, differing by less than ~1 kcal/mol, no matter what the substituents were. The effect of a Lewis acid catalyst was examined and found to have a very large effect on the calculated barriers through coordination to the carbonyl oxygen atoms of the diester substituents on the cyclopropane. The reaction barrier was found to decrease by as much as ~19 kcal/mol when using a BF3 molecule as a model for the Lewis acid catalyst. Solvent effects and the nature of the regiospecificity of the reaction were also examined. Trends in the calculated barriers for the reaction were in good agreement with available trends in the reaction rates measured experimentally. Key words: 1,3-dipolar cycloaddition, cyclopropane, nitrone, tetrahydro-1,2-oxazines, ab initio quantum chemistry, mechanism.

Asymmetric Synthesis of the Azabicyclic Core of the Stemona Alkaloids

A general strategy for the construction of the 1-azabicyclo[5.3.0]decane core of Stemona alkaloids is developed. Our diversity-oriented approach exploits 1,3-dipolar cycloaddition of five-membered cyclic nitrones to C(6) olefins, followed by N-O reductive cleavage and azepine closure. The use of various enantiopure pyrroline N-oxides allows for a practical, stereoselective preparation of several putative precursors of different Stemona alkaloids.