Acyclic nitronic esters: Generation and utilization in the synthesis of <Emphasis Type="Italic">N</Emphasis>-and <Emphasis Type="Italic">O</Emphasis>-heterocycles

The literature concerning the chemistry of acyclic nitronic esters is surveyed and analyzed with special reference to tandem heterocyclization of unsaturated compounds with acyclic nitronic esters as 1,3-dipoles.     

Reaction of haloadamantanes with salts of nitro compounds

Silver and mercury salts of nitro compounds react with bromo- or chloroadamantanes with the formation of products of C- and/or O-alkylation, the ratio of which depends on the structure of the initial reactants. The direct experimental evidence of nitronic esters to be isomerized into nitro derivatives and of their role as intermediates in the formation of the nitro derivatives in the alkylation of nitro compounds has been presented for the first time.N. D. Zelinskii Institute of Organic Chemistry, Russian Academy of Sciences, 117913 Moscow. Translated from Izvestiva Akademii Nauk, Seriya Khimicheskaya, No. 8, pp. 1798–1803, August, 1992.     

Mechanism for exchange of heterooeganic fragment between oxygen atoms of -O-N-O group in nitronic acid esters

Conclusions The DNMR method was used to study the 1,3-(O,O) migration of the heteroorganic fragment in the diethylboron and trimethylsilyl esters of bis(carbomethoxymethane)nitronic acid, and it was shown that the migration is accomplished via a multicenter cyclic transition state.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2836–2838, December, 1977.     

Transformed steroids—part 95 : High pressure induced 1,3-dipolar cycloaddition of nitronic esters to 16-dehydro-20-oxo steroids

High pressure induced cycloaddition of Z and E nitronic esters (2) to 16-dehydro-20-oxo steroids (1) leads to regiospecific formation of steroido [16α,17α-d] tetrahydro-l',2'-oxazoles (3 and 4). It is shown that both modes (“exo-endo”) of dipolarophile approach to the dipole are realized for most steroids examined. All four possible isomers are isolated and their preferred conformations are established. It is shown that conversion of unstable stereomers to stable ones (3'→ 4; 4'→ 3) proceeds as a simultaneous nitrogen inversion and isoxazolidine cycle conformational change (_NE→E^N).     

Kinetic study of bleaching of photoinduced forms of o-methylnitrobenzothiazoles

Photochromic properties of benzothiazole derivatives containing nitro and methyl groups in the ortho positions with respect to each other were studied by flash photolysis. The rate constant of bleaching of the corresponding photoinduced nitronic acids and their anions increases as the CH₃C-CNO₂ bond becomes longer. The application of the thermodynamic approach to predict the kinetic stability of nitronic acids is limited owing to specific intramolecular interactions. The lifetime of photoinduced nitronic acid anions tends to increase with rise in the chemical shift of the methyl protons.     

Cycloadditions dipolaires 1,3. XXIX. Addition de deux esters nitroniques isomères à diverses défines. Etude de la régiosélectivitè et du mode d'approche du dipolarophile par le dipôle

1,3-Dipolar cycloaddition of two geometrical isomers of nitronic esters to some mono, di and trisubstituted olefines was studied. For each reaction only one orientation of the cycloaddition was observed which is in good agreement with theoretical predictions. The modes of approach of these dipolarophiles by the two nitronic esters were established in most cases. They are discussed in terms of secondary interactions of the frontier orbitals of the reactants.     

Heterocyclen aus α-nitroolefinen. XI . 1,2-oxazine aus α-nitroolefinen und β-dicarbonylverbindungen

Dihydro-1,2-oxazines 8 and 12-oxazines 12 are formed by the reaction of the nitronic acids of the adducts of α-nitroolefins and β-dicarbonyl compounds ( 6 ) by two ways: A ) The nitronic acids 6 are heated in a boiling solution of urea in ethanol or methanol. B ) The nitronic acids 6 are reduced with an aqueous solution of ammoniachloride and sodium sulfide. ¹H-nmr and ¹³C-nmr investigations prove the constitutions. The mechanism is discussed.     

Keto<==>enol, imine<==>enamine, and nitro<==>aci-nitro tautomerism and their interrelationship in substituted nitroethylenes. Keto, imine, nitro, and vinyl substituent effects and the importance of H-bonding

Tautomeric isomers and conformers of 2-nitrovinyl alcohol (1), 2-nitrovinylamine (2), and 1-nitro-propene (3) are reported at the MP2 and B3LYP levels of theory, using the 6-31G* basis set, with energy evaluation at B3LYP/6-311+G** and G2MP2. The nitroalkenes are the global minima on their respective potential energy surfaces. The barriers for the concerted 1,5-H transfer to the corresponding nitronic acids amount to only 5.0 kcal/mol for 1, 13.2 kcal/mol for 2, and a sizable 37.8 kcal/mol for 3. Whereas the aci-nitro tautomer of 2-nitrovinyl alcohol is easily accessible, beta-iminonitronic acid has little kinetic stability. H-bonding is a strong stabilizing factor in these nitroalkenes, estimated at 7.0 and 3.7 kcal/mol for the OH and NH2 derivatives, respectively, while its stabilization in their nitronic acids amounts to as much as 13 kcal/mol. The H-bonds are evident from the very short O...H and N...H distances and are characterized by bond critical points. The NO2 substituent effect of about 11.4 kcal/mol at G2MP2 on both the classical keto <==> enol and imine <==> enamine tautomeric processes stabilizes the nitroethylene derivatives. The keto, imine, and vinyl substituent effects at G2MP2 on the nitro <==> aci-nitro tautomeric process are also determined as are their pi-resonance components. The substituents have a large influence on the ionization energies of the nitroethylene derivatives.     

Esters of nitromethane nitronic acid

Russian Chemical Bulletin - For the first time, O-methyl and O-ethyl esters of dinitromethane were isolated in an individual state. Their structures were confirmed by spectral methods, elemental...     

Electrochemical activation and carboxylation of fluorine-containing aromatic imines

On the basis of a study of the processes involved in the electrochemical activation of a series of fluorine-containing aromatic imines with and without the presence of carbon dioxide it was shown that the initial stage of such processes is the formation of imine radical-anions, which can both undergo spontaneous transformations and react with CO₂ with the formation of fluorine-containing amino acids. A correlation was established between the effects of the electronic structure of the imines on the electrochemical characteristics of their reduction and on the stability of the radical-anions. A relationship was established between the parameters of electrochemical activation of the imines in the presence of CO₂ and the yields of the amino acids. Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 44, No. 5, pp. 265–271, September–October, 2008.     

Nanosecond and picosecond laser flash photolysis studies of 2,2′-dinitrodiphenylmethanes

The transient absorption spectra of the 2,2′-dinitrodiphenylmethanes 1a–1c in solution were examined in the picosecond and nanosecond time ranges. The absorption bands, observed at 420–450 nm on 355 nm (18 ps) excitation of these compounds, were attributed to the nitronic acids 2a–2c, formed through the singlet-state- mediated intramolecular hydrogen abstraction and the triplet states of 1a–1c. Biradical intermediates, formed through the intramolecular abstraction of the benzylic hydrogen by the triplet excited state, were detected for 1a and 1b. Transient species produced following 355 nm (approximately 6 ns) pulse excitation of 1a–1c result in the formation of the corresponding nitronic acids 2a–2c, with absorption maxima around 415–430 nm. These nitronic acids are the precursors of the various products formed in the steady state irradiation of 1a–1c     

Transmission of electronic effects of substituents in vinyl esters

To elucidate the transmission mechanism of electronic effects of substituents in vinyl esters on the optical parameters of the absorption bands in the IR spectra, a comparison was made of the frequencies of corresponding absorption bands in vinyl ethers, vinyl esters, alkyl esters of carboxylic acids, ketones, and acid anhydrides. Calculation of the - and -electron structure of the molecules by the Hückel and Del Re approximations was made. The results were compared with the spectral characteristics. The conclusion was reached that the ester oxygen atom in vinyl esters reduces interaction of the carbonyl and vinyl groups, although it is not removed completely. In vinyl esters of aromatic acids the effect of benzene ring substituents is displayed by a change of the -electron structure of the CH₂=CH-O-CO- fragment. In vinyl esters of aliphatic acids the electronic influence of radicals of the acid portion is transmitted by the ester oxygen, mainly inductively, which, in its turn, shows a change in the ability of this atom to conjugate with the vinyl group.The authors express their thanks to Yu. A. Kruglyak for allowing use of the programs for the computation and for helpful advice.     

Structure of some organic compounds containing tin and phosphorus

Summary On the basis of Malatesta's results and of our confirmatory experiments, it must be considered that the organic compounds of tine and phosphorus under consideration are esters of phosphorus acids in which tine is linked to phosphorus through oxygen.     

Cycloadditions dipolaires-1,3—XXIV: Addition d'esters nitroniques aux olefines monoactivees

The orientation of the cycloaddition of the geometrical isomers of nitronic esters to monoactivated olefines is unique. Depending on the nature of the olefinic substituent, a competition between two modes of approach of the dipolarophile by the 1,3-dipole is observed. All these cycloadditions lead to stable invertomers of N-alkoxyisoxazolidines.     

Cycloadditions dipolaires-1,3-XXV: Formation sous controle cinetique d'invertomeres stables lors des cycloadditions d'esters nitroniques

The addition of nitronic esters to α,β-diactivated olefins leads to the formation under kinetic control of invertomers of N-alkoxy isoxazolidines. These results and the conformational properties of the heterocycles are discussed in terms of the anomeric effect and ‘gauge effect”.     

Reactions of 3-acycloxysulfolanes with nucleophilic reagents

In contrast to alkyl esters of carboxylic acids, 3-acyloxysulfolanes on reaction with nucleophilic reagents readily split out a carboxylic acid to give 2-sulfolene, which then reacts with compounds containing a labile hydrogen atom. On the basis of data on the kinetics of alkaline hydrolysis of benzoates, a scheme for elimination of the acid that includes the intermediate formation of a transition state of the E₁CB type is proposed.     

Comparative studies of three- and four-ring mesogenic esters containing p-carborane, bicyclo[2.2.2]octane, cyclohexane, and benzene

Twelve esters were prepared from pentyl-substituted p-carborane, bicyclo[2.2.2]octane, cyclohexane, and benzene carboxylic acids and three substituted phenols. The mesogenic properties of the series of esters were examined using thermal analysis and optical microscopy. The relationships between structure and mesogenic properties were analysed by comparison of the series of homostructural esters. Thus, the effects of variation of the carboxylic acid structure, introduction of fluorine into the phenol part, and replacement of the central phenyl ring with the -CH₂CH₂- group on the stability of mesophases and their widths were investigated. In general, carborane derivatives exhibit broad nematic phases and narrow SmA phases, while other derivatives demonstrate rich smectic and soft crystal polymorphism.     

Resonant electron capture by some amino acids and their methyl esters

Resonant electron capture mass spectra of aliphatic and aromatic amino acids and their methyl esters show intense [M-H](-) negative ions in the low-energy range. Ion formation results from a predissociation mechanism mediated by the low-energy pi*oo resonant state. Methylation in general has little influence on the electronic structure according to quantum chemical calculations, but the corresponding ions from the methyl esters, [M-Me](-), could be ascertained to arise only at higher resonance energies. Aromatic amino acids are characterized by an additional low-energy fragmentation channel associated with the generation of negative ions with loss of the side chain. The complementary negative ions of the side chains are more efficiently produced at higher energies. The results have significant implications in biological systems as they suggest that amino acids can serve as radiation protectors since they have been found to efficiently thermalize electrons.     

Bond Dissociation Energies and Electronic Structures in a Series of Peroxy Radicals: A Theoretical Study

Quantum chemical calculations are performed to estimate the bond dissociation energies (BDEs) for 18 peroxy radicals. Since DFT methods are researched to have low basis sets sensitivity, these radicals are studied by utilizing the hybrid density functional theory (DFT) (B3LYP, B3P86, B3PW91 and PBE1PBE) in conjunction with the 6‐311G** basis set and the complete basis set (CBS‐Q) method. On the basis of comparisons of the computational results and the experimental values, we evaluate the effectiveness of above methods. It is demonstrated that CBS‐Q method is the best method for computing the reliable BDEs of C—OO bond, with the average absolute errors of 2.1 kcal/mol. So CBS‐Q method is suitable to predict accurate BDEs of C‐OO bond for peroxy compounds. The computational energy gaps between the HOMO and LUMO of studied compounds are almost identical from the point of view of stability and substantial HOMO‐LUMO gaps for all molecules suggest their electronic stability. In addition, substituent effect on the C—OO BDE of peroxy radicals is analyzed. It is noted that the effects of substitution on the C—OO BDE of peroxy radicals are significant. Our results will shed lights on future theoretical and experimental work.     

Lipase catalysed kinetic resolutions of 3-aryl alkanoic acids

Hydrolase catalysed kinetic resolutions leading to a series of 3-aryl alkanoic acids (?94% ee) are described. Hydrolysis of the ethyl esters with a series of hydrolases was undertaken to identify biocatalysts that yield the corresponding acids with excellent enantiopurity in each case. Steric and electronic effects on the efficiency and enantioselectivity of the biocatalytic transformation were also explored.