Search results for "Tautomer"
showing 10 items of 141 documents
An ab initio study of the tautomeric equilibria of the N-oxides of hydroxypyridines in the vapour phase
1993
Abstract An ab initio self-consistent field molecular orbital study of the N-oxides of hydroxypyridines has been carried out. The optimized structures have been obtained at the 6-31G ∗ level, and the energy values have been computed at the 6-31G ∗ and MP2-6-31G ∗ //6-31G ∗ levels. The tautomerization energies for the three couples of systems in the vapour phase have been evaluated.
Identity Double-Proton Transfer in (3Z)-3-Hydroxy-1,4-di(quinolin-2-yl)but-3-en-2-one
2003
Although there is a very fast (on the NMR timescale) double-proton transfer in (1Z,3Z)-3-hydroxy-4-quinolin-2-yl-1-quinolin-2(1H)-ylidenbut-3-en-2-one (the product of the condensation of ethyl oxalate with 2-lithiomethylquinoline), it is the only species present in chloroform solution. Comparison of the product of condensation of ethyl oxalate with 2-lithiomethyl derivatives of pyridine (recent studies) and quinoline (present studies) shows that benzoannulation considerably affects the tautomeric equilibrium. The observed changes are not only quantitative but also qualitative. Moreover, contrary to the proton transfer in the pyridine tautomers, this process is fast in the quinoline tautomer…
GIAO/DFT calculated chemical shifts of tautomeric species. 2-Phenacylpyridines and (Z)-2-(2-hydroxy-2-phenylvinyl)pyridines
2001
1H, 13C and 15N NMR chemical shifts for 28 substituted 2-phenacylpyridines (ketimine forms) and their enolimine tautomers, (Z)-2-(2-hydroxy-2-phenylvinyl)pyridines, were calculated via the GIAO/DFT approach. Among four tested methods at the B3LYP level of theory, the 6–311G, 6–311++G and 6–311G** basis sets gave acceptable result for 13C NMR chemical shifts whereas the 6–311++G** basis set was the minimum needed for reproduction of 15N NMR chemical shifts. Satisfactory reproduction of 13C and 15N NMR chemical shifts for different tautomers revealed that intramolecular hydrogen bonding could be modeled reliably by these calculations when the geometry optimizations were done with the HF/3–21G…
A molecular electron density theory (MEDT) study of the role of halogens (X2= F2, Cl2, Br2and I2) on the aza-Michael-addition reactions
2020
Using the Molecular Electron Density Theory (MEDT), a set of uncatalyzed and X2 molecule-catalyzed (X2 = F2, Cl2, Br2, or I2) aza Michael addition reactions (aza-MARs) between pyrrolidine (PYR) and methyl acrylate (MA) have been studied in the gas phase and in dichloromethane (DCM) medium from the kinetics and molecular mechanism aspects. Analysis of the conceptual DFT reactivity indices indicates that the coordination of the X2 molecules to the oxygen of the MA's carbonyl group increases both the softness and the electrophilic character of MA, thus explaining the catalytic role of the X2 molecules. The nucleophilic attack of the nitrogen of PYR on the MA's β conjugated carbon is the rate-d…
Ab initioanalysis on metal ion catalysis in the enolization reactions of some acetylheterocycles: kinetics of the enolization reactions of 3-acetyl-5…
2002
Kinetic data on the enolization reaction of 3-acetyl-5-methylisoxazole, 5-acetyl-3-methylisoxazole, 3(5)-acetylpyrazole and some previously studied acetylheterocycles have been the object of a comprehensive ab initio analysis. Enolization rate constants were measured spectrophotometrically by the halogen trapping technique at 25 °C and ionic strength of 0.3 mol dm−3 in water, in acetate buffers, in dilute hydrochloric acid, in dilute sodium hydroxide and in the presence of some metal ion salts. In the spontaneous (water) and base (acetate) catalysed reactions the ketones investigated are generally more reactive than acetophenone, according to the electron-withdrawing effect of the heterocyc…
Tautomerism and the Protonation/Deprotonation of Isocytosine in Liquid- and Solid-States Studied by NMR Spectroscopy and Theoretical Calculations
2011
Isocytosine crystallizes as a 1:1 ratio of two tautomers in a manner similar to that of the guanine and cytosine pairs in DNA. The solid-state NMR chemical shifts of crystalline neutral isocytosine, the sodium salt of isocytosine, and those of isocytosine hydrochloride were measured and related to the NMR parameters in aqueous and organic solutions. The experimental chemical shifts were compared with those calculated by three different methods: (1) calculations on isolated molecules, (2) calculations on isocytosine clusters of various sizes, and (3) infinite crystal calculations, that is, the gauge-including projector-augmented wave (GIPAW) method. The data obtained with the GIPAW method we…
1H,13C and17O NMR study of substituted nitropyridines
1991
1H, 13C and 17O NMR spectra for 22 substituted nitropyridines were measured and their 1H NMR spectra were analysed. The most significant variations in the NMR parameters are found for isomeric hydroxy derivatives, owing to the possibility of keto–enol tautomerism. The prevalence of the keto form is observed in 2- and 4-hydroxy derivatives, while the 3-hydroxy derivative exists in its enol form. Among the three nuclei studied, 17O seems to be the best nucleus for probing the keto–enol tautomerism. No correlation is observed between the torsion angle of the nitro group and its 17O NMR chemical shift. Molecular mechanics calculations were performed to clarify the torsional energetics of the ni…
Synthesis and Tautomerization of 6,7-Dihydro-(1,2,3)-triazolo[1,5-a]pyrimidines.
2007
The condensation of 5-amino-4-phenyl-1,2,3-triazole (1) with chalcones 2a-e or 3-dimethylamino-propiophenone (4f) leads to the 6,7-dihydro-(1,2,3)-triazolo[1,5-a]pyrimidines 3a-f. The equilibrium of 3 and the tautomeric 4,7-dihydro-(1,2,3)-triazolo[1,5-a]pyrimidines 3′ is described.
Dynamic formation of hybrid peptidic capsules by chiral self-sorting and self-assembly.
2014
Owing to their versatility and biocompatibility, peptide-based self-assembled structures constitute valuable targets for complex functional designs. It is now shown that artificial capsules based on β-barrel binding motifs can be obtained by means of dynamic covalent chemistry (DCC) and self-assembly. Short peptides (up to tetrapeptides) are reversibly attached to resorcinarene scaffolds. Peptidic capsules are thus selectively formed in either a heterochiral or a homochiral way by simultaneous and spontaneous processes, involving chiral sorting, tautomerization, diastereoselective induction of inherent chirality, and chiral self-assembly. Self-assembly is shown to direct the regioselectivit…
New Strategy for the Stereoselective Synthesis of Fluorinated β-Amino Acids
2002
Racemic and chiral nonracemic alpha-substituted and alpha-unsubstituted beta-fluoroalkyl beta-amino acid derivatives 6 and 9 have been synthesized in two steps starting from fluorinated imidoyl chlorides 1 and ester enolates. This approach is based on the chemical reduction of previously obtained gamma-fluorinated beta-enamino esters 4 by using ZnI(2)/NaBH(4) in a nonchelated aprotic medium (dry CH(2)Cl(2)) as the reducing agent. A metal-chelated six-membered model has been suggested to explain the stereochemical outcome of the reduction reaction. The process takes place with high yields and with moderate to good diastereoselectivity. The best results related to diastereoselective reduction…