Chemical transformations of the condensation products of pyridoxal with L-α-alanine and D-α-alanine

The kinetics and mechanism of the reactions of pyridoxal with L- and D-α-alanine were studied. Under comparable conditions, the condensation of L- and D-α-alanines with pyridoxal includes three kinetically different steps. The first fast step is addition of the amino acid to pyridoxal with formation of the corresponding amino alcohol, the second (slower) step is dehydration of the amino alcohol to give Schiff base, and the third (very slow) step is elimination of α-hydrogen atom from the L-α-amino acid fragment or decarboxylation of the D-α-amino acid fragment, followed by isomerization of the Schiff base to quinoid structure whose subsequent hydrolysis yields pyridoxamine and pyruvic acid or acetaldehyde, respectively. A scheme was proposed for chemical transformations of the pyridoxal condensation products with L- and D-α-alanines.     

Synthesis and Properties of L-α-aming-γ-nitroguanidinobutyric acid

We wish to report the synthesis of L-α-amino-γ-nitroguanidinobutyric acid, a hitherto unkonwn amino acid derivative. This derivative can be the base for preparing peptides (for instance hormonse) contairing Lα-amono-γ-guanidinobutyric acid, the lower homologue of L-arginine. The higher homologue, L-homoarginine, has already been built in peptides through its nitroderivative, as was the case with angiotensin IIanalogues¹and Har-Bradykinin².     

Some reactions of fluorosulfonyldifluoroacetic acid with N-heterocyclic compounds

Difluorocarbene generated from the decomposition of fluorosulfonyldifluoroacetic acid (2)reacted with various sodium salts of N-heterocyclic compounds(1) giving the corresponding difluoro-methylated products in acetonitrile at 10—40℃.Benzotriazole(1a),benzimidazole(1b) and imidazole(1c) were converted into 1-(difluoromethyl)benzotriazole(3a),1-(difluoromethyl)benzimidazole(3b) and1-(difluoromethyl)imidazole(3c)respectively.Indole(1d)reacted with 2 to give -(fluorosulfonyldifluoro-acetate)indole(2d) rather than the expected difluoromethylated derivatives.     

Kinetic and mechanism of oxidation of thiocarbohydrazide and its metal complex by acid bromate in aqueous and partial aqueous media

The kinetics of oxidation of thiocarbohydrazide in the free and zinc(Ⅱ)-bound states byacid bromate have been studied in aqueous and water-acetic acid(1:1,V/V)media under varyingconditions,both in the absence and presence of added bromide ion.The rates of oxidations show firstorder kinetics in[bromate]in all the cases,but exhibit different kinetic behaviour in[substrate]and[H~+].Oxidation of TCH in aqueous medium shows zero order in[TCH]and nearly second order in[H~+],while oxidation in aqueous acetic acid shows two ranges in[H~+].The rate shows first and fractionalorder kinetics in[TCH]in the first and second acid ranges.Kinetics observed in the presence of Br~-are similar to those observed for oxidation of TCH in second acid range.In addition,the reactionshows fractional order in[Br~-].Oxidation of TCH in Zn(Ⅱ)-bound state exhibits first order kinetics in[substrate]and second order in[H~+].Increase in ionic strength of the medium decreases the rate in allthe cases.Increase in acetic acid composition of the solvent increases the rate.Mechanisms consistentwith the observed results have been considered and the rate laws deduced.The rate limiting steps havebeen identified and the coefficients of these steps have been calculated at different temperatures.Therelated activation parameters have also been computed.The validity of the deduced rate laws has alsobeen tested by recalculating the rate constants from them as[TCH]and[H~+]are varied.     

Synthesis of polyfunctional β-Ketosulfides and their reactions with m-chloroperbenzoic acid and chlorine dioxide

The reaction of bromoacetophenone with various thiols furnished β-ketosulfides: 2-(1-methyl-1H-imidazol-2-ylsulfanyl)-1-phenylethanone, 2-(1H-imidazol-2-ylsulfanyl)-1-phenylethanone, 2-(2-hydroxyethylsulfanyl)-1-phenylethanone. β-Ketosulfoxides were obtained by the oxidation of β-ketosulfi des with m-chloroperbenzoic acid and chlorine dioxide.     

Chemical transformations of pyridoxal and pyridoxal 5′-phosphate condensation products with amino acids

The mechanism of chemical transformations of pyridoxal and pyridoxal 5′-phosphate condensation products with amino acids is studied by kinetic measurements. The Schiff bases are shown to be fairly stable in neutral media. In acid media, the Schiff bases are hydrolyzed into the initial components. In alkaline media, cleavage of α-hydrogen from the amino acid fragment and structural rearrangement into the quinoid form followed by hydrolysis of the latter with elimination of pyridoxamine and keto acid take place. The rate constants of the chemical transformations of the Schiff bases are found to depend on the pH of the medium. It is shown for the first time that the phosphate group in the pyridoxal 5′-phosphate fragment catalyzes the α-hydrogen cleavage and strongly accelerates alkaline decomposition of the Schiff bases.     

Kinetics and mechanism of reaction of cis-α,β-dinitrostilbene with morpholine

Reaction of cis-,-dinitrostilbene (substrate) with morpholine (reagent) in n-hexane leads to cis--nitro--morpholinostilbene (end product). The process is of first order with respect to the substrate and second order with respect to the reagent. Possible reaction mechanisms are analyzed, and it is established that the following are most probable on the basis of kinetic patterns and stereochemistry: development of a charge transfer complex having a hydrogen bond between the substrate nitro group and reagent amino group; reaction of the complex with a second reagent molecule and formation of a carbanion (this stage determines the overall reaction rate); and detachment of a nitrite ion from the nitrocarbanion and its protonation to form the end product.N. N. Semenov Institute of Chemical Physics, Russian Academy of Sciences, 117977 Moscow. A. N. Nesmeyanov Institute of Heteroorganic Compound, Russian Academy of Sciences, 117813 Moscow. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 1, pp. 78–83, January, 1992.     

α-nitroacrylates in reactions with malonic and acetylaminomalonic esters

Reactions of α-nitroacrylates of aromatic series with malonic and acetylaminomalonic esters proceed in the presence of sodium alcoholates and lead to the formation in high yields of Michael adducts, precursors of modified analogs of γ-aminobutyric and glutamic acids.     

Kinetic characteristics of gas–liquid ozone reactions

An experimental chemical method for determining the kinetic characteristics (volumetric mass transfer coefficient and rate constant of a second-order reaction) of gas–liquid ozone reactions in a bubble column reactor is described. The calculation formulas are substantiated, and the ranges of values of the experimental factors that determine the method’s limits of applicability are found. The conditions under which the boundary-value problem of a gas–liquid ozone reaction of the second order can be reduced to a problem of a pseudo-first order reaction allowing an analytical solution are revealed.     

Specific features of the reaction of l-cysteine with pyridoxal and N-pyridoxylidene-β-alanine

The mechanisms of condensation of l-cysteine, l-methionine, and l-serine with pyridoxal and of transaldimination of N-pyridoxylidene-??-alanine with l-cysteine were studied by the kinetic method. Unlike methionine and serine, the condensation of cysteine with pyridoxal and transaldimination with Npyridoxylidene-??-alanine involves intermediate formation of stable product having a thiazolidine ring. Its structure was determined by elemental analysis, UV, and IR spectroscopy, and quantum-chemical calculations. The thiazolidine fragment in the pyridoxal condensation product with l-cysteine is turned through an angle of ??90° with respect to the pyridine ring plane due to mutual repulsion of the negatively charged oxygen atom in the ortho position of the pyridine ring and sulfur and nitrogen atoms in the thiazolidine ring.     

Thermodynamics of interaction of L-α-phenylalanine with urea and dimethylformamide in aqueous solution

The thermal effects of solution of L-phenylalanine in aqueous solutions of urea and dimethylformamide (DMF) at 25°C were determined. The solubility of L-phenylalanine in water and aqueous DMF solutions was measured. The standard enthalpies, free energies, and entropies of solution of the amino acid in aqueous solutions of amides were calculated. The parameters of pair and ternary amino acid-amide interactions were determined within the framework of the McMillan-Mayer theory. The amino acid-amide pair interaction is accompanied by a decrease in the Gibbs free energy, controlled by the entropy term with DMF and by the enthalpy term with urea. The interaction of L-phenylalanine with two amide molecules is repulsive, which in the case of DMF leads to an increase in the standard free energies of solution of the amino acid at the amide mole fraction X ₂ > 0.05.     

Evaluation of the mechanism,regio-, and diastereoselectivity of aza-Diels–Alder reactions of 2H-azirine under a Lewis acid catalyst

Structural Chemistry - The molecular mechanism of the cycloaddition reactions of 2H-azirine with 1-methoxybutadiene and cyclohexadiene has been studied at the M06-2X/cc-pVDZ level of theory....     

Kinetics of the reactions of catechins with hypochlorite,peroxynitrite, and amino acid–derived peroxyl- radicals

Catechins, one of the class of flavonoids, are known as very efficient antioxidants. Here we investigated the kinetics of the reactions of three catechins, namely, catechin, epigallocatechin, and epigallocatechin gallate (EGCG) with some oxidants, which are formed in vivo under oxidative stress, hypochlorite, peroxynitrite, and amino acid peroxyl radicals. Stopped-flow spectrophotometry and pulse radiolysis technique with absorption detection were used to observe the formation of intermediate products of oxidized catechins. We found that catechins react with hypochlorite with the rate constant of the order of 10⁵–10⁶ M⁻¹ s⁻¹ at pH 7.4. Experimental kinetic traces of the reaction of EGCG with valine peroxyl radicals were fitted using chemical simulation, and the rate constant of this reaction was found to be 5 × 10⁵ M⁻¹ s⁻¹. The rate constants of the formation of unstable catechin quinones in the reaction with peroxynitrite were comparable to that of spontaneous peroxynitrite isomerization, which indicates that catechins are oxidized indirectly by peroxynitrite. Biological consequences of these reactions are discussed.     

Synthesis and reactions of α-fluoro-α-amino amides

N-((S)-1-Phenylethyl)halofluoroethanamides have been investigated as precursors to N-protected α-fluoro-α-amino amides by nucleophilic displacement of halide with nitrogen nucleophiles such as potassium phthalimide, sodium succinimide, sodium glutarimide, trimethylamine and sodium azide. With single diastereoisomers of the iodofluoroethanamide, clean inversion of configuration occurs at room temperature, but subsequent epimerisation may occur as a result of the liberated iodide. The α-fluoro-α-amino amides made underwent a wide variety of reactions depending on conditions, but in many cases the carbon-fluorine bond was compromised. However, reacting trimethylamine and N-((S)-1-phenylethyl)iodofluoroethanamide gave the corresponding α-fluorobetaine amide, and subsequent acidic hydrolysis led to α-fluorobetaine as the first example of an ‘unprotected’ α-fluoroamino acid.     

Pulse radiolysis studies on reactions of α-hydroxyalkyl radicals with nicotinamide and 6-methyl nicotinic acid

Reactions of α-hydroxyalkyl radicals derived from 2-propanol, ethanol and methanol with nicotinamide (NICAM) and 6-methyl nicotinic acid (6-MNA) were studied at various pHs using pulse radiolysis technique. It is found that α-hydroxyalkyl radicals react with NICAM and 6-MNA at pHs when nitrogen is in the protonated state. In these reactions, radical adducts of NICAM/6-MNA with α-hydroxyalkyl radicals are formed which have absorption maxima at about 340–350 nm which subsequently decay to give pyridinyl type of radicals of NICAM and 6-MNA having λ_max at 410 nm. Rate constants for the reactions of (CH₃)₂COH, CH₃CHOH and CH₂OH radicals with NICAM and 6-MNA were found to have linear dependence on reduction potentials of corresponding α-hydroxyalkyl radicals. Adducts formed in the reactions of CH₃CHOH and CH₂OH radicals with both NICAM and 6-MNA decayed slowly compared to the decay of adduct formed in reactions with (CH₃)₂COH radicals.