Features of the Tautomerism of Some α-Substituted Pyridines

The AM1 quantum-chemical semi-empirical method was used to calculate the values of the enthalpy of activation (H^#), the heats of reaction (H), and the tautomeric equilibrium constants (K_T) for several -substituted pyridines. It was found that the keto-enol tautomeric conversion in -pyridone occurs more readily than the amino-imino conversion in 2-aminopyridine but in 2-methylpyridine the tautomeric equilibrium does not exist at all.     

Selective Cleavage of β-Keto and Vinylogous β-Keto Esters by 3-Quinuclidinol

As a result of a continuing study utilizing nitrogenous bases^2–6, we now wish to report that the base 3-quinuclidinol (13) is useful for the cleavage of β-keto and vinylogous β-keto esters.     

Dihalogenation of β-Keto Sulfides. Synthesis of α-Keto Methyl Esters and α-Keto S-Phenyl Thioesters

α-Keto esters and α-Keto S-Phenyl Thioesters are synthesized from β-keto sulfides by dihalogenation with sulfuryl chloride followed by solvolysis with methanol and acetone/water respectively.     

Theoretical Study of the Sensitivity-Improvement Effect of Phosphoryl Group in Mass Spectrometry of Small Peptides

Mass spectrometry plays an important role in peptide sequencing since the invention of soft ionization techniques, such as fast atom bombardment (FAB), electrospray (ESI) and matrix-assisted laser desorption/ionization (MALDI). In our previous work, it was found that phosphorylation on the N-terminal of small peptides could enormously improve their sensitivity in FAB-MS1.To understand the sensitivity-improvement effect of phosphoryl group, the energies for the protonation of glycylglycin…     

Electronic Structure of π Systems: XIX. Keto-Enol Tautomerism of Dihydrofurocoumarinones

Tautomeric transformations of 4-methyldihydrofuro[2,3-h]coumarin-9-one and its 8-substituted derivatives were studied by ¹H NMR, electronic absorption spectroscopy, and quantum chemistry. The ¹H NMR spectra of these compounds in CDCl₃ show that they exist in the ketone form, and in more polar sol- vents they can pass into the enol form. By electronic absorption spectroscopy it was established that the derivatives containing electron-acceptor substituents in the 8 position of the furanone ring undergo tautomeric transformations as the composition of the solvent is varied from 100% methanol to 100% CCl₄. At the same time, the derivatives with electron-donor substituents in the same position do not show any specific alterations in the absorption spectra with solvent. Analogous pattern was observed in the enolization of substituted di- hydrofurocoumarinones by acetylation: In presence of electron-donor substituents in the 8 position, no acetyla- tion occurred, while with the compounds containing electron-acceptor substituents, the corresponding 9-acet- oxy-4-methylangelicins were prepared in high yields. Calculations by the PPP/CI method of the electronic absorption spectra 4-methyldihydrofuro[2,3-h]coumarin-9-one showed that in polar solvents (methanol) it prefers the enol form. Data of spectral measurements were compared with results of semiempirical (MNDO, AM1, and PM3) and nonempirical quantum-chemical calculations (with 3-21G, 6-31G^*, and 31G^** basis sets).     

Condensation of 5-Aminopyrazoles Unsubstituted in Position 1 with β-Keto Esters

Condensation of -keto esters with 5-aminopyrazoles unsubstituted in position 1 gives 7-oxopyrazolo[1,5-a]pyrimidines.     

Solvent-Free Microwave-Assisted Organic Reactions Preparation of β-Keto Esters

Microwave technique has been utilised in the preparation of β-keto esters. Two different procedures are described: transesterification of β-keto esters and ring opening of 2,2,6-trimethyl-1,3-dioxin-4-one.     

Theoretical Study on Enol-keto Tautomerism of α-Fluorine-β-diketones

1 INTRODUCTION β-diketones are a class of high functional com- pounds with outstanding optical, electric and magne- tic properties[1], and the negative ion may act as an excellent chelating agent. Hall[2] have studied β-di- ketone/metal ion complexes and indicated that the ability of combination correlates with the keto-enol distribution in solution. The enol-keto equilibrium existing in β-diketones has been extensively studied over several decades[3~9]. Burdett[10] have investiga- ted…     

Study on the Tautomerism of an Indoline Spiropyran in Compressed CO_2 and Ethanol Mixture

Thesolubilityofcompressedgasesinliquidscanbeveryhigh,whichresultsintheexpansionoftheliquids.Thes0lubilitiesofsolutesinliquidscanbereducedconsiderablybydissolutionofthegases'i.e.,thegasesactasantisolvents.Recently,somecompressedgaseshavebeenusedinmanygasantisolventpr0cesses.Understandingofthepropertiesofthecompressedgas-liquidmixtUresisofgreatimportancetoboththeoreticalstUdyandindustrialprocesses.ThetautomerismofspiropyransandtheirderivativeshasbeenstUdiedbydifferentauthors"',whichindicatedth…     

Theoretical Study on Enol-keto Tautomerism of α-Fluorine-β-diketones

Density Functional Theory method is applied to investigate the enol-keto tautome- rism of both acyclic and cyclic α-fluorine-β-diketones. It is shown that, for acyclic cases, α-fluorine could improve the relative stability of keto tautomer by lessening intramolecular hydrogen bond of enol form, whereas the relative stability of cyclic enol could be attributed to two factors: destabi- lization of keto and stabilization of enol. Furthermore, the relative stabilities of all enol tautomers are improved in THF to different extents.     

Mono Halogenation of β-Keto Sulfides. Synthesis of β-Keto-α-phenylthio p-Tolyl Sulfones

β-Keto-α-Phenylthio p-Tolyl sulfones are obtained from β-keto-alkyl phenyl sulfides by halogenation with NCS followed by treatment with sodium p-toluene sulphinate.     

pH-Independent Transfer Hydrogenation in Water: Catalytic, Enantioselective Reduction of β-Keto Esters

A pH-independent asymmetric transfer hydrogenation of β-keto esters in water with formic acid/sodium formate is described. The reaction is conducted open to air and gives access to β-hydroxy esters in excellent yields and selectivities.     

DSC Study of the Postdenaturated Structures in Biopolymer–water Systems

The temperature dependences of heat capacity for water–denaturated biopolymer (globular proteins, collagen and DNA) were measured in a wide range of temperatures (0–140°C) and water content of the systems. It has been shown that thermally denaturated globular proteins (lysozyme, myoglobin and catalase) are able to form the thermoreversible heat-set structures under the certain conditions studied. The additional endothermal maximum observed is the calorimetric manifestation of the phase transition related to the melting of these thermotropic non-native structures. The melting gels are completely formed just after denaturation during relatively short time and only their prolonged state at T>T _d leads to their transformation to thermoirreversible non-melting ones. The post denaturated structures from water-denaturated protein (Mb, Lys and RN-ase) systems with a different amount of free water were also studied. The thermoreversible cold-set gels are formed from both water-denaturated DNA and water-denaturated collagen systems. These thermotropic structures are metastable. A spatial gel network of both collagen and DNA is formed from the native-like renaturated structures. This revised version was published online in July 2006 with corrections to the Cover Date.     

A Practical Synthesis of β-Keto Phosphonates from Triethyl Phosphonoacetate

An efficient and practical preparation of β-keto phosphonates, via acylation reaction of triethyl phosphonoacetate with carboxylic acid chlorides in the presence of magnesium chloride-triethylamine followed by decarbethoxylation, is described.     

Photocatalytic Synthesis of β-Keto Primary Chlorides by Selective Chlorocarbonylation of Olefins

Functionalized primary alkyl chlorides are precursors to a plethora of scaffolds but their access from chemical feedstocks remains challenging. Herein, we report a concise dual Ni/photoredox catalytic protocol for regioselective chlorocarbonylation of unactivated alkenes that enables rapid access to β-keto primary chlorides. The catalytic process features an extensive substrate scope, scalability and functional group tolerance. The Ni/photocatalytic Cl⋅ generation and subsequent cross-coupling is implicated for the process based on the control experiments and DFT study. The synthetic utility of the protocol has been further corroborated through functionalization of complex substrates and modifications of the product.     

Synthesis and Reactivity of β-Carboline-Hydantoin Systems

The synthesis of 2,5-disubstituted 1,3-dioxo-6H-1,2,3,5,11,11a-hexahydroimidazo[1, 5-b]-β-3a-y and the reduction of carbolines 3m-r with different amounts of lithium aluminium hydride is described.     

Analytical Study of β-Irradiated Antibiotics in the Solid State

This paper reports results of the study on the influence of β-irradiation on the physical and chemical properties of selected salts of β-lactam antibiotics in solid state (sodium salt: ampicillin, azlocillin, benzylpenicillin, carbenicillin and piperacillin; potassium salt of benzylpenicillin, ampicillin anhydricum, ampicillin trihydricum, amoxicillin trihydricum and bacampicillin hydrochloride). The source of irradiation was a linear accelerator of electrons, and the irradiation effects were checked on the basis of the following: determination of mass, melting point and water contrent, and spectrophotometric (UV, IR) chromatographic and thermal (DTG, DSC) studies. This revised version was published online in August 2006 with corrections to the Cover Date.     

Photoinduced Reaction of α-Bromoacetophenone with Thiosulfonate: Synthesis of β-Keto Thiosulfone

A photoinduced synthesis of β-keto thiosulfone/β-keto selenosulfone by the reaction of α-bromoacetophenone with thiosulfonate/selenosulfonate under metal-free and visible light irradiation conditions is developed. Two C−S bonds or one C−S bond and one C−Se bond were constructed simultaneously.