"Designer acids": combined acid catalysis for asymmetric synthesis

Lewis and Brønsted acids can be utilized as more‐effective tools for chemical reactions by sophisticated engineering (“designer acids”). The ultimate goal of such “designer acids” is to form a combination of acids with higher reactivity, selectivity, and versatility than the individual acid catalysts. One possible way to take advantage of such abilities may be to apply a “combined acids system” to the catalyst design. The concept of combined acids, which can be classified into Brønsted acid assisted Lewis acid (BLA), Lewis acid assisted Lewis acid (LLA), Lewis acid assisted Brønsted acid (LBA), and Brønsted acid assisted Brønsted acid (BBA), can be a particularly useful tool for the design of asymmetric catalysis, because combining such acids will bring out their inherent reactivity by associative interaction, and also provide more‐organized structures that allow an effective asymmetric environment.     

Mass spectra of some 2-, 3-, and 4-pyridinecarboxylic acids. Further evidence of a N-H interaction for loss of carbon dioxide

Mass spectra of 3-acetamido-, 3-methoxy-, and 4-methoxy-2-pyridinecarboxylic acids, 2,6-pyridinedicarboxylic acid, 4-nitro-2-pyridinecarboxylic acid N-oxide, 2-chloro- and 2-nitro-x-pyridinecarboxylic acids (X = 3 and 5), 2-chloro- and 2-nitro-4-pyridinecarboxylic acids, and 4-pyridinecarboxylic acid are reported. The 2-pyridinecarboxylic acids lost carbon dioxide (M-44) as has been reported. The 3-pyridinecarboxylic acids showed no definite trend in fragmentation; however, the 4-pyridinecarboxylic acids lost OH (M-17) first. This change in fragmentation pat-tern is due to an interaction of the ring nitrogen and carboxyl group in the 2-pyridinecarboxylic acids which is not present in the 4-pyridinecarboxylic acids.     

OH-radical induced degradation of hydroxybenzoic- and hydroxycinnamic acids and formation of aromatic products—A gamma radiolysis study

The OH-radical induced degradation of hydroxybenzoic acids (HBA), hydroxycinnamic acids (HCiA) and methoxylated derivatives, as well as of chlorogenic acid and rosmarinic acid was studied by gamma radiolysis in aerated aqueous solutions. Primary aromatic products resulting from an OH-radical attachment to the ring (hydroxylation), to the position occupied by the methoxyl group (replacement –OCH₃ by ?OH) as well as to the propenoic acid side chain of the cinnamic acids (benzaldehyde formations) were analysed by HPLC–UV and LC–ESI–MS. A comparison of the extent of these processes is given for 3,4-dihydroxybenzoic acid, vanillic acid, isovanillic acid, syringic acid, cinnamic acid, 4-hydroxycinnamic acid, caffeic acid, ferulic acid, isoferulic acid, chlorogenic acid, and rosmarinic acid. For all cinnamic acids and derivatives benzaldehydes were significant oxidation products. With the release of caffeic acid from chlorogenic acid the cleavage of a phenolic glycoside could be demonstrated. Reaction mechanisms are discussed.     

Enzymatic fluorimetric determination of cholic acid and chenodeoxycholic acid in aqueous solutions and blood serum using a differential kinetic method

An enzymatic fluorimetric method is described for the determination of chenodeoxycholic acid and its conjugates and of cholic acid and its conjugates in aqueous solutions and serum. The method is based on the oxidation of 7 α-hydroxy bile acids by β-NAD⁺ in the presence of 7 α-hydroxysteroid dehydrogenase; the NADH produced is monitored fluorimetrically. Chenodeoxycholic acid is determined in the presence of cholic acid by a differential kinetic procedure; the sum of the two acids (primary bile acids) is determined by an equilibrium procedure, and cholic acid is calculated by difference. The r.s.d. was ca. 3% and 10% for aqueous solutions and sera, respectively. Recoveries of chenodeoxycholic acid, cholic acid and primary bile acids added to serum samples averaged 100.5, 105.1, and 102.9%, respectively. Ten samples can be analyzed per working day.     

Reactivity of amino acids in nitrosation reactions and its relation to the alkylating potential of their products

Nitrosation reactions of amino acids with an -NH(2) group [namely, six alpha-amino acids (glycine, alanine, alpha-aminobutyric acid, alpha-aminoisobutyric acid, valine, and norvaline); two beta-amino acids (beta-alanine and beta-aminobutyric acid), and one gamma-amino acid (gamma-aminobutyric acid)] were studied. Nitrosation was carried out in aqueous acid media, mimicking the conditions of the stomach lumen. The rate equation was r = k(3)(exp)[amino acid][nitrite](2), with a maximum k(3)(exp) value in the 2.3-2.7 pH range. The existence of an isokinetic relationship supports the argument that all the reactions share a common mechanism. A nitrosation mechanism is proposed, and the following conclusions are drawn: (i) Nitrosation reactions of amino acids with a primary amino group in acid media occur with dinitrogen trioxide as the main nitrosating agent. The finding that the nitrosation rate is proportional to the square of the nitrite concentration suggests that the yield of nitrosation products in the stomach would increase sharply with higher nitrate/nitrite intakes. (ii) Stomach hypochlorhydria could be a potential enhancer of in vivo amino acid nitrosation. (iii) The reactivity (k(3)()(exp)) [alpha-amino acids > beta-amino acids > gamma-amino acids] is the same as that found in a previous work for the alkylating potential of lactones formed from nitrosation products of the same amino acids. This implies that the nitrosation reactions of the most common natural amino acids are the most efficient precursors of the most powerful alkylating agents. (iv) The order of magnitude (10(7)-10(8) M(-1) s(-1)) of the bimolecular rate constants of nitrosation shows that such reactions occur through an encounter process.     

Micropore to macropore structure-designed silicas with regulated condensation of silicic acid nanoparticles

A new preparation method for porous silica particles was developed using activated silica sols which are called nano-silica solutions in this paper. Several kinds of organic and inorganic acids are employed to neutralize diluted sodium silicate solutions to form the nano-silica solutions: formic acid, acetic acid, propionic acid, oxalic acid, succinic acid, dl-malic acid, citric acid, and tricarballylic acid as carboxylic acids, and sulfuric acid and hydrochloric acid as inorganic acids. The effect of salts in the nano-silica solution is also studied. The products were investigated using a field emission scanning electron microscope, an X-ray diffractometer, the nitrogen adsorption technique, and a mercury porosimeter. Microporous silicas were produced when carboxylic acids were applied; the formation of micropores was influenced by the pH of the nano-silica solutions and molecular sizes of the carboxylic acids. Addition of a salt in a citric acid solution increased the mesopore volume. Macropores were formed when inorganic acids including salts were applied; the salt nanoparticles which were crystallized in silica spheres acted as templates. The anion types and salt concentrations in the nano-silica solutions affected the aggregation condition of silica nanoparticles, following the Schulze-Hardy rule.     

Lamellar zirconium phosphonates containing pendant sulphonic acid groups

The versatile sulphonic acid group has been introduced into the family of interlamellar anchored materials. Zirconium bis-3-sulphopropylphosphonate is an example of an aliphatic acid. Zirconium bis-2-(sulphophenyl)ethylphosphonate is an example with an aromatic sulphonic acid group. In general, the sulphonic acids are not as crystalline as the carboxylic acid analogs. This is probably due to the relatively large size of the sulphonic acid group compared to the available cross sectional area of the layer face. The aliphatic compounds are more crystalline than the aromatics, as is expected from size considerations. The sulphonic acid group in both crystalline and semi-crystalline examples is accessible to reaction with bases. A few preliminary experiments have demonstrated the utility of these compounds as both strong acid ion exchangers and Bronsted acid catalysts. The layered sulphonic acid—zirconium 3-sulphopropylphosphonate—is thermally stable to well over 200°C. This indicates good potential for applications in Bronsted catalysis. This stability compares favorably with organic resin based sulphonic acids.The sulphonic acid class of interlamellar anchored materials have now been established. Both aliphatic and aromatic examples have been prepared. The utility of the sulphonic acids has been demonstrated with the typical reactions of this functional group. Specifically, the acids have been shown to be strong acid cation exchangers and a Bronsted acid catalyst. In addition, we have begun to develop an insight into the structural ramifications of these compounds. The size constraints of the zirconium phosphate type backbone are evident. Further, the thermal stability of this group of compounds is encouraging relative to its applications potential.     

Complexation in Benzo-15-Crown-5–Chloroform–Substituted Acetic Acid Extraction Systems

To study the complexing processes occurring in the extraction systems, the extraction of acetic, propionic (methylacetic), monochloroacetic, dichloroacetic and trichloroacetic acids using benzo-15- crown-5 as an extractant is considered. The extraction isotherms of acids with the pure solvent and 1 M benzo-15-crown-5 in chloroform are determined. The dependences of the distribution ratios of the considered acids on the extractant concentration in the organic phase are presented. Based on the experimental data and the “blank” extraction isotherm equations, the acid: benzo-15-crown-5 ratio in the complexes formed in the extract have been calculated (minus the contribution of extraction with the pure solvent). No complexation of methylacetic acid with benzo-15-crown-5 occurs, and the stability of the complexes of chloro-substituted acetic acids rises in the order acetic acid < monochloroacetic acid < dichloroacetic acid < trichloroacetic acid.     

Higher N-acyl-L-amino acid derivatives

A preparative method is proposed for obtaining higher N-acylamino acids by reaction of free amino acids with fatty acid nitrophenyl esters. It was shown that these acids can transport positive ions through a liquid lipophilic medium. A direct method is proposed for obtaining fatty acid 4-nitro-phenyl esters by boiling 4-nitrophenol and the fatty acid in xylene in a Soxhlet apparatus in the presence of an acid catalyst.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2555–2560, November, 1990.     

Analysis of a mixture of a known and an unknown weak acid by titration to a preset pH

The preset-pH titration method has been used to indicate the presence of a second acid when the titration curve (pH vs. volume of added titrant) seems to indicate only one acid. By use of the method even small amounts of propionic acid can be detected in an acetic acid solution despite the small value of Δ log K^H_HA = 0·18. Binary mixtures of acids may bs analysed when one acid is known, and log k^H_HA for the unknown acid may be found. Acetic acid, as the known acid, has been determined together with hydrochloric, mandelic, hydroxyacetic or boric acid or ammonium ion, with an error of about 1%. The method can be used in some cases for titration of ternary mixtures of one known and two unknown acids. Only the sum of the unknown acids can then be determined together with the known acid.     

N-Boc-Protected α-Amino Acids by 1,3-Migratory Nitrene C(sp3)−H Insertion

N-Boc-protected α-amino acids are synthesized in two steps from linear or branched carboxylic acid feedstocks. In the first step, the carboxylic acid is coupled with tert-butyl aminocarbonate (BocNHOH) to generate azanyl ester (acyloxycarbamate) RCO₂NHBoc. In the second step, this azanyl ester undergoes a stereocontrolled iron-catalyzed 1,3-nitrogen migration to generate the N-Boc-protected non-racemic α-amino acid. This straightforward protocol is applicable to the catalytic asymmetric synthesis of α-monosubstituted α-amino acids with aryl, alkenyl, and alkyl side chains. Furthermore, α,α-disubstituted α-amino acids are accessible in an enantioconvergent fashion from racemic carboxylic acids. The new method is also advantageous for the synthesis of α-deuterated α-amino acids. N-Boc-protected α-amino acids synthesized using this two-step protocol are ready-to-use building blocks.     

DNA-bound lipids: computer modeling of DNA interaction with stearic acid and unsaturated fatty acids

It was shown for the first time by computer experiments that fatty acids are strongly bound to DNA. This is consistent with the presence of free fatty acids in the specimens of DNA-bound lipids isolated from various cells. Binding of all fatty acids to the DNA minor groove is stronger than to the major groove, which is correlated with the presence of two pools of free fatty acids isolated from DNA specimens by biochemical methods. Since DNA polymerase is also bound to the DNA minor groove, fatty acids can play an important role in the regulation mechanism of DNA replication and signal transmission. The energy of interaction of fatty acids with DNA depends on both the number of double bonds and the geometric configuration of the fatty acid and the nucleotide composition of DNA. Dependence on the bond energy in the DNA—fatty acid complex on the nucleotide composition attests to the possibility of site-specific binding of lipids to DNA. On passing from a saturated fatty acid to unsaturated acids containing one, two, or three double trans-bonds, the bond energy of DNA with the fatty acid gradually decreases. The presence of one or three double cis-bonds results in weakening of the strength of the DNA—fatty acid complexes compared to those with the saturated acid. The strongest binding between DNA and fatty acid was found for the unsaturated acid with two double cis-bonds (linoleic). This can be explained by the fact that the bent (boomerang) shape of the molecule of this acid follows the curve of the DNA helix. The pattern of variation of the energy of DNA complexes with stearic, linoleic, oleic, and linolenic acids correlates with experimental data on the melting points of these complexes: the more stable the DNA—fatty acid complex, the lower the melting point of DNA.     

The coordination chemistry of cyclohexanepolycarboxylate ligands. Structures,conformation and functions

This review gives an overview on the coordination chemistry of eight cyclohexanepolycarboxylic acids, i.e. cyclohexanecarboxylic acid, 1,2-, 1,3- and 1,4-cyclohexanedicarboxylic acid, 1,3,5- and 1,2,4-cyclohexanetricarboxylic acid, 1,2,4,5-cyclohexanetetracarboxylic acid and 1,2,3,4,5,6-cyclohexanehexacarboxylic acid, and explores their possible applications in materials science, especially as magnetic materials. The conformational transformation of cyclohexanepolycarboxylic acids in the presence of various metal ions under hydrothermal conditions is included and the α-proton removal mechanism is discussed as well.     

The removal of Cu,Ni, and Zn in industrial soil by washing with EDTA-organic acids

In order to improve the heavy metal removal ability of traditional single washing agents and explore the removal mechanism of heavy metals. Then, the washing reagents that mixed by low-molecular weight organic acids (citric acid, oxalic acid, and tartaric acid) and artificial chelating compound ethylenediaminetetraacetic acid disodium (EDTA) were selected. Furthermore, the effect of soil washing parameters, the variation of leaching toxicity, mobility, stability and speciation of heavy metals were also considered. The results of soil washing experiments showed that mixing an equal volume of 0.05 M EDTA and 0.2 M organic acids (citric acid, oxalic acid, and tartaric acid) could remove more than about 80% heavy metals from soil under the optimal conditions. In addition, the soil leaching toxicity was decreased and the stability of remaining heavy metals was increased, indicating that EDTA-organic acid washing reagents could effectively reduce the ecological risk of contaminated soil. EDTA had a stronger chelating ability with heavy metals than the organic acids, and the organic acids could not only chelate heavy metals but also decrease the pH of the mixture for promoting the desorption of heavy metals. Thus, mixing EDTA and organic acids was advisable method to improve soil washing technology.     

Local acid environment in poly(ethylene‐ran‐methacrylic acid) ionomers

The local environment of unneutralized carboxylic acid groups in poly(ethylene‐ran‐methacrylic acid) (E/MAA) ionomers neutralized with monovalent (Li and Na) and divalent (Ca and Zn) ions has been investigated with Fourier transform infrared spectroscopy. These unneutralized acid groups interact with one another to form acid dimers, and they associate with existing neutralized complexes. At room temperature, no free acids can be detected for any system, not even for pure E/MAA. With the acid dimer peak (1700 cm^?1) and a known unneutralized acid concentration, the concentration of acids associated with a neutralized complex can be determined. This concentration of associated acids increases with increasing neutralization, reaches a maximum below 50% neutralization, and then decreases toward zero near 80% neutralization. This behavior is perhaps due to the increased driving force for aggregation of the neutralization acids. Although Li, Na, and Ca contain similar concentrations of associated acids over the range of neutralizations, the Zn system contains far fewer associated acids (i.e., more acid dimers) at any particular neutralization level. These results are confirmed by an analysis of the absorbance in the neutralized region (1650–1500 cm^?1). © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2833–2841, 2002     

Radiation Chemical Studies of Protein Reactions: Dependence of Restorative Amino Acids on Radiation Dose

Abstract

Changes of specific rotation of protein irradiated with Co-60 gamma-rays were restored by some amino acids, such as mono-sodium 1-glutamate. Loss of this recovery effect in some amino acids by irradiation was studied by polarimetry and infrared spectometry. Optical rotation of protein-amino acid solutions increased with an increasing radiation dose given to the amino acids. Changes in optical rotation with irradiation may be due to the loss of the recovery effect of amino acids for the irradiated protein molecule. Changes in infrared spectra increased with an increasing radiation dose given to the amino acids. Such changes in infrared spectra with irradiation may be due to destruction of some radicals in the amino acid molecule.     

Solid state radiolysis of non-proteinaceous amino acids in vacuum: astrochemical implications

The analysis of the amino acids present in Murchison meteorite and in other carbonaceous chondrites has revealed the presence of 66 different amino acids. Only eight of these 66 amino acids are proteinaceous amino acids used by the present terrestrial biochemistry in protein synthesis, the other 58 amino acids are somewhat “rare” or unusual or even “unknown” for the current terrestrial biochemistry. For this reason in the present work a series of “uncommon” non-proteinaceous amino acids, namely, l-2-aminobutyric acid, R(?)-2-aminobutyric acid, 2-aminoisobutyric acid (or α-aminoisobutyric acid), l-norleucine, l-norvaline, l-β-leucine, l-β-homoalanine, l-β-homoglutamic acid, S(?)-α-methylvaline and dl-3-aminoisobutyric acid were radiolyzed in vacuum at 3.2 MGy a dose equivalent to that emitted in 1.05 × 10⁹ years from the radionuclide decay in the bulk of asteroids or comets. The residual amount of each amino acid under study remained after radiolysis was determined by differential scanning calorimetry in comparison to pristine samples. For optically active amino acids, the residual amount of each amino acid remained after radiolysis was also determined by optical rotatory dispersion spectroscopy and by polarimetry. With these analytical techniques it was possible to measure also the degree of radioracemization undergone by each amino acid after radiolysis. It was found that the non-proteinaceous amino acids in general do not show a higher radiation and radioracemization resistance in comparison to the common 20 proteinaceous amino acids studied previously. The unique exception is represented by α-aminoisobutyric acid which shows an extraordinary resistance to radiolysis since 96.6 % is recovered unchanged after 3.2 MGy. Curiously α-aminoisobutyric acid is the most abundant amino acid found in carbonaceous chondrites. In Murchison meteorite α-aminoisobutyric acid represents more than 20 % of the total 66 amino acids found in this meteorite.     

Chemistry and biology of the acylneuraminic acids

Acylneuraminic acids occupy an outstanding position both sterically and with respect to biological functions in gangliosides and in many glycoprotein molecules. Acylneuraminic acids are N-acylated and partly O-acylated derivatives of neuraminic acid, a polyhydroxyamino keto acid that has the properties of a carbohydrate and is therefore capable of forming a glycoside linkage. The present report provides a survey of our knowledge of the chemistry and biology of the acylneuraminic acids, which occur widely in particular in the animal kingdom.     

Synthesis of α-N-Ethylamino Acids and their Derivatives

 A new synthesis of α-N-ethylamino acids starting from α-amino acids using hexafluoroacetone as protecting and activating agent is described. The hexafluoroacetone-protected N-ethylamino acid derivatives obtained are activated lactons. Therefore, they can be directly transformed without the need of an additional activation step with various nucleophiles into the corresponding carboxylic acid derivatives.     

A new technique to determine organic and inorganic acid contamination

A new acid indicator pad was developed for the detection of acid breakthrough of gloves and chemical protective clothing. The pad carries a reagent which responds to acid contaminant by producing a color change. The pad was used to detect both organic and inorganic acids permeating through glove materials using the modified ASTM F-739 and direct permeability testing procedures. Breakthrough times for each type of glove were determined, and found to range from 4 min to > 4 h for propionic acid, from 3 min to > 4 h for acrylic acid, and from 26 min to > 4 h for HCl. A quantification was performed for propionic and acrylic acids following solvent desorption and gas chromatography. Both acids exhibited > 99% adsorption [the acid and its reactivity (the acid reacted with an indicator to contribute the color change)] on the pads at a spiking level of 1.8 microL for each acid. Acid recovery during quantification was calculated for each acid, ranging from 52-72% (RSD < or = 4.0%) for both acids over the spiking range 0.2-1.8 microL. The quantitative mass of the acids on the pads at the time of breakthrough detection ranged from 260-282 and 270-296 microg cm(-2) for propionic acid and acrylic acid, respectively. The new colorimetric indicator pad should be useful in detecting and collecting acid permeation samples through gloves and chemical protective clothing in both laboratory and field studies, for quantitative analysis.