Preparation of enantiopure butane-2,3-diacetals of glycolic acid and alkylation reactions leading to alpha-hydroxyacid and amide derivatives

The preparation of butane-2,3-diacetal protected glycolic acid and related systems is described together with highly selective alkylation reactions of (R,R) and (S,S) butanediacetal protected glycolic acid. These compounds are readily deprotected to give enantiopure alpha-hydroxyacids, alpha-hydroxyesters or alpha-hydroxyamides by suitable choice of conditions.     

Stereoselective routes to chiral 2-hydroxy-4-oxo acids and substituted 2-hydroxybutyrolactones using lactate dehydrogenases

The enantioselective reduction 2,4-dioxo acids catalysed by lactate dehydrogenases provided access to 2-hydroxy-4-oxo acids of both S and R configuration. Subsequent diastereoselective chemical reduction affords 4-substituted 2-hydroxybutyrolactones.     

Pseudohalogen chemistry—VII: Addition of thiocyanogen to alkynes

Thiocyanogen does not react with simple alkynes under heterolytic conditions in benzene at 20-25°. Under homolytic conditions, addition occurs readily giving mixtures of E- and Z- dithiocyanatoalkenes with high E:Z ratios; prolonged treatment with excess reagent also leads to mixtures of the dithiocyanatoalkenes but usually with lower E:Z ratios. A radical-chain mechanism, involving preferential anti-addition of thiocyanogen in an initial kinetically-controlled reaction and subsequent thermodynamically-controlled isomerisation of the adducts, is proposed. The influence of substituents on the reaction rates and product ratios is discussed in terms of their steric effects.     

Synthesis of 1,3,4-thiadiazin-2-one and 1,3,4-selenadiazin-2-one derivatives as new cardiotonic drugs

The synthesis of the new heterocycle 1,3,4-selenadiazin-2-one is described, based on the preparation of the analogue heterocycle 1,3,4-thiadiazin-2-one. The related cyclization between an α-haloketone and a thiocarbazate cannot readily be employed within the selenium series. Therefore a new route was developed based upon a diazotization/hydrolysis sequence of the related 2-amino-1,3,4-selenadiazine.     

Solvation of the potassium ion complexed with dibenzo-30-crown-10. Transfer activity coefficients between methanol and various solvents

The stability constants of the potassium complex with dibenzo-30-crown-10 have been determined from potentiometric or solubility measurements in the solvents: methanol, iso-propanol, n-butanol, propylene carbonate, acetonitrile and dimethylsulfoxide. The solubility of the ligand in these solvents has also been determined and the transfer activity coefficients of the potassium complex for transfer from methanol to solvent (S), _SM(KL⁺), have been computed. Although solid state studies indicate that dibenzo-30-crown-10 completely surrounds the potassium ion and shields it from water, the transfer activity coefficient of the potassium complex is found to be highly solvent dependent. Dibenzo-30-crown-10 is thus less effective for the removal of the solvation sphere of the potassium ion than previously estimated.     

Absolute asymmetric photoreactions of aliphatic amino acids by circularly polarized synchrotron radiation: critically pH-dependent photobehavior

The absolute asymmetric photoreaction (AAP) of racemic aliphatic amino acids, such as alanine (Ala) and leucine (Leu), by left- and right-handed circularly polarized light (l- and r-CPL) irradiation was investigated in aqueous solutions at various pHs, by using the Onuki-type polarizing undulator installed in an electron storage ring. The magnitude of the optical purity (op) generated and the enantiomer-enriching mechanism operative in the AAP were found to be entirely dependent on the ionic state (and thus pH) of the amino/carboxylic acid moieties. At pH 1, the op of Ala and Leu determined by circular dichroism (CD) spectral measurement gradually developed with CPL irradiation, according to Kagan's equation. In contrast, irradiation at pH 7 gave op's much smaller than the theoretical values predicted by Kagan's equation. However, it turned out that the photodecomposition at pH 7 produces the corresponding alpha-hydroxycarboxylic acids stereoselectively, the CD sign of which is just opposite to that of the remaining amino acid, thus affording the apparently small op. It is concluded that, irrespective of solution pH, the AAP of amino acid proceeds upon CPL irradiation. At pH 1, the photodecomposition of valine, Leu, and isoleucine occurs via a Norrish type II mechanism, which is also applicable to other amino acids possessing a gamma-hydrogen. In the case of amino acids lacking a gamma-hydrogen, such as glycine and Ala, the photodecomposition mechanism is a photodeamination/hydroxylation and a Norrish type I reaction. At pH 7, the main photoproducts were ammonia and alpha-hydroxycarboxylic acids that were produced via photodeamination.     

Supramolecular chirogenesis with bis-chlorin versus bis-porphyrin hosts: peculiarities of chirality induction and modulation of optical activity

[structure: see text] The complexation behavior, chirality induction and inversion in the achiral host, a racemic mixture of ethane-bridged bis(zinc octaethylchlorin) (1), and optical activity modulation in the chiral hosts, enantiopure 1(R) and 1(S), upon interaction with chiral and achiral amine guests have been investigated by means of the UV-vis, CD, and (1)H NMR techniques and compared with the corresponding spectral data of the bis-porphyrin analogue. It was found that the chirogenesis pathway is strongly dependent upon the structures of both major components (hosts and guests) of these supramolecular systems. Particularly, the distinct orientation of electronic transitions in the chlorin chromophores arisen from the reduced pyrrole ring, which makes it radically different from that of the porphyrin chromophores, and the size of the guest's substituents lead to the remarkable phenomenon of chirality induction-inversion in racemic 1 originating from the process of asymmetry transfer from enantiopure guests of the same homologous type and absolute configuration. This surprising chirogenic behavior is found to be in a sharp contrast to that observed in the analogous porphyrin-based systems. Furthermore, these structural and electronic phenomena also account for the effective optical activity quenching of enantiopure 1(R) and 1(S) upon interaction with chiral and with achiral amines, which results in formation of supramolecular complexes of opposite chirality.     

Simultaneous structural and elemental nano-imaging of human brain tissue

Examining chemical and structural characteristics of micro-features in complex tissue matrices is essential for understanding biological systems. Advances in multimodal chemical and structural imaging using synchrotron radiation have overcome many issues in correlative imaging, enabling the characterization of distinct microfeatures at nanoscale resolution in ex vivo tissues. We present a nanoscale imaging method that pairs X-ray ptychography and X-ray fluorescence microscopy (XFM) to simultaneously examine structural features and quantify elemental content of microfeatures in complex ex vivo tissues. We examined the neuropathological microfeatures Lewy bodies, aggregations of superoxide dismutase 1 (SOD1) and neuromelanin in human post-mortem Parkinson''s disease tissue. Although biometals play essential roles in normal neuronal biochemistry, their dyshomeostasis is implicated in Parkinson''s disease aetiology. Here we show that Lewy bodies and SOD1 aggregates have distinct elemental fingerprints yet are similar in structure, whilst neuromelanin exhibits different elemental composition and a distinct, disordered structure. The unique approach we describe is applicable to the structural and chemical characterization of a wide range of complex biological tissues at previously unprecedented levels of detail.

Structural and chemical characterisation of microfeatures in unadulterated Parkinson''s disease brain tissue using synchrotron nanoscale XFM and ptychography.     

The study of chiral discrimination of organophosphonate derivatives on pirkle type chiral stationary phase by molecular modeling

Molecular modeling and molecular dynamics (MD) have been used to study the chiral discrimination and interaction energy of organophosphonate in N-(3,5-dinitrobenzoyl)-S-leucine chiral stationary phase (CSP). The elution order of the enantiomers can be predicted from the interaction energy. Quantitative structure-retention relationship (QSRR) has also been used as an alternative method to confirm the elution order of enantiomers. Molecular mechanics (MM), molecular dynamics and QSRR proved to be useful methods to study chiral discrimination.     

Degradation of epoxy polymers: 3—Photo-degradation of bisphenol-A diglycidyl ether

The principal characteristics and products of photo-degradation of a commercial epoxy resin, bisphenol-A diglycidyl ether, have been studied. Comparison with the results of thermal degradation shows that many of the products are identical, especially the phenols and derivatives of phenol. The principal differences are the appearance of hydrogen in the photo, but not in the thermal, reaction and a series of aromatic hydrocarbons, including xylenes and/or ethyl benzene, styrene, isopropyl benzene and isopropenyl benzene, which are completely absent from the products of thermal degradation. Mechanisms are outlined.