Copper/Ruthenium Relay Catalysis for Stereodivergent Access to δ-Hydroxy α-Amino Acids and Small Peptides

An atom- and step-economical and redox-neutral cascade reaction enabled by asymmetric bimetallic relay catalysis by merging a ruthenium-catalyzed asymmetric borrowing-hydrogen reaction with copper-catalyzed asymmetric Michael addition has been realized. A variety of highly functionalized 2-amino-5-hydroxyvaleric acid esters or peptides bearing 1,4-non-adjacent stereogenic centers have been prepared in high yields with excellent enantio- and diastereoselectivity. Judicious selection and rational modification of the Ru catalysts with careful tuning of the reaction conditions played a pivotal role in stereoselectivity control as well as attenuating undesired α-epimerization, thus enabling a full complement of all four stereoisomers that were otherwise inaccessible in previous work. Concise asymmetric stereodivergent synthesis of the key intermediates for biologically important chiral molecules further showcases the synthetic utility of this methodology.     

（E）-3-（2-呋喃基）丙烯酸的合成与晶体结构

以2-呋喃甲醛和乙酸酐为原料,在乙酸钾存在下,用相转移催化法合成了（E）-3-（2-呋喃基）丙烯酸,其结构经UV、IR、X-射线单晶衍射仪进行了表征．合成的目标化合物分子式为C7H6O3,分子量为138．12,晶体属于单斜晶系,C2／c空间群,晶胞系数为：a=18．993（6）A,b=3．8474（12）A,c=20．095（6）A,α=90．00°β=114．054°（4）,γ=90．00°,V=1341．0（7）nm^3,Z=8,Dr=1．368Mg／cm^3,F（000）=576,μ（MOK＼a）=0．71073．结构由直接法解出,最终偏离因子为R1=0．0645,wR2=0．1360,分子间通过弱的相互作用形成层状化合物．     

The Interactions of Aqueous Solutions of Chlorine with Citric Acid. A Source of Mutagens. (A Preliminary Report)

Abstract

Citric acid, an additive to an important acrylamide based polymeric flocculant, was found to react with an aqueous solution of chlorine to give several chlorinated propanones, di- and trichloroacetic acid, chloroform, and a number of as yet unidentified chlorine containing compounds. Formation of these alledged carcinogens and mutagens also takes place by treating diluted aqueous solutions of frozen orange juice concentrate with such chlorine solutions. The formation and ratios of the detected chlorinated compounds is pH-dependent. A tentative scheme which could account for these findings is presented.     

The Study of UV and VIS Absorption Spectra of the Complexes of Amino Acids with Ninhydrin

Abstract

The analysis of amino acids with post column derivatization using ninhydrin is usually carried out with automatic analyzers at wavelengths of 570 nm (for the quantitation of α-amino acids) and 440 nm (for the imino acids), or at only one wavelength, 520 nm, for the total acid content. The α-amino acids are reported to give an absorption maximum at 570 nm, after complex formation with ninhydrin; and α-imino acids have a γ at 440 nm, 520 nm is a compromise wavelength.

This paper presents a study of the absorption of the complexes of amino acids with ninhydrin between 275 and 700 nm. Two new wavelengths are suggested for use, 290 and 404 nm.

In the near UV (290 nm) all the tested acids show a common absorption maximum and good sensitivities. In the visible region at 404 nm, the sensitivities of the α-amino acids are very similar to those observed at 570 nm. At 520 nm the αamino acids have the lowest sensitivities.

The α-imino acids (proline and oxyproline) show absorption maxima only at 290 nm; after that their sensitivities decrease with increasing wavelength.

The paper offers some practical hints on changing the detector wavelength, and to adapt the HPLC's detector to work at one of these two, newly suggested, bands (290 or 404 nm).     

由对称二异丙基脲制异丙胺磺酰氯的试验研究

本文研究了由对称二异丙基脲制异丙胺磺酰氯的问题。给出了反应条件、产率和某些限制,同时也简单讨论了反应机理及其它一些问题。     

High-performance liquid chromatographic-electrospray mass spectrometric analysis of phenolic acids and aldehydes

The present work describes the development of an HPLC-electrospray mass spectrometric method for the analysis of phenolic acids and aldehydes. These compounds are important for the quality of foods and feeds, such as dietary fiber supplements, wine and lignicellulose by-products. Good separation was obtained with a phenyl column (3 μm particle size, 150 mm×3.9 mm I.D.), using McOH-H₂O (30:70, v/v) as the mobile phase with 0.01% CH,COOH and 0.2 mM tetraethyl ammonium iodide as the ion pairing agent, at a flow-rate of 0.3 ml/min. This system permits post column splitting of the eluate for analysis by electrospray mass spectrometry with a flow-rate of 11 μl/min. This new method is extremely sensitive and less than 6 pg/inj of the studied phenols can be identified and quantified. This method was applied to standard compounds as well as to components of high-fiber dietary supplements (primarily wheat bran), cornmeal, and oat bran.     

The Concept of Hard and Soft Acids and Bases as Applied to Multi-Center Chemical Reactions

Many chemical bonds of differing types and strengths have recently been regarded by Pearson^[1] as representing partnerships between (Lewis) acids and (Lewis) bases. Most acceptor molecules or ions (acids) can be placed in one or other of two categories, graphically termed “Hard” and “Soft”. There are also two broad categories of donor molecules or ions (bases) which can also be termed Hard and Soft. On the whole, strong chemical bonds are partnerships between either a Hard base and a Hard acid or a Soft base and Soft acid, whereas weaker bond types most usually result in cases of either Hard base-Soft acid or Soft base-Hard acid interactions. The present paper shows how this concept of acidity and basicity can be applied in the interpretation of multi-center chemical reactions involving interconnected acid-base relationships. In particular, fourcenter substitutions and additions involving cooperative attack by nucleophiles and electrophiles at various chemical bonds have been examined, and a conclusion is reached that especially reactive patterns of reactants can be developed if the substrates contain bonds between either a hard acid and a soft base, or a soft acid and a hard base. Indeed, the arguments can be elaborated to provide two distinct Rules which should be of interest in the interpretation of metal-ion assisted reactions and in the design of novel syntheses.     

Ionic equilibria in aqueous organic solvent mixtures the dissociation constants of acids and salts in tetrahydrofuran/water mixtures

The dissociation constants of several acids (perchloric, hydrochloric, phosphoric, acetic and benzoic acids) and of some sodium salts (chloride, acetate and benzoate) have been conductometrically determined in tetrahydrofuran/water mixtures up to a 90% of tetrahydrofuran in volume. The results demonstrate that conductometry can be successfully applied to determine the dissociation constants of salts and moderately weak and strong acids in the studied mixtures. The dissociation constants of the acids and some bases taken from the literature have been fitted to solvent composition through a previously derived equation, which is based on a preferential solvation model. The fitting parameters obtained allow calculation of the dissociation constant for any solvent composition inside the applicability solvent composition range. From the pK value, the pH of any buffered solution, such as those used in liquid chromatography, can be calculated for the particular tetrahydrofuran/ water composition of interest. Appreciable ion-pairing for sodium salts and strong acids has been observed for tetrahydrofuran contents higher than 60% in volume. Therefore, the accurate calculation of the pH values of buffers in tetrahydrofuran-rich solutions must take into account the pK values of the acid and salt.