Novel tocopheryl compounds XXIV. Studies into the nitrosation chemistry of γ-tocopherol: preparation of 5-nitroso-γ-tocopherol via an organomercury derivative of vitamin E

5-Nitroso-γ-tocopherol (5) was not accessible by direct nitrosation of γ-tocopherol (3), but was synthesized for the first time by nitrosation of an organomercurial intermediate (11) under aprotic conditions. Under protic conditions 5 exists in equilibrium with its ortho-quinone monoxime tautomer 6, the latter being the major component with >99%. NMR and analytical data of the tautomeric couple are reported for the first time. The chemistry of the nitrosation of γ-tocopherol was studied in detail. In the presence of oxygen, 5 is readily oxidized to 5-nitro-γ-tocopherol (7), whereas at elevated temperatures an additional process, the conversion of monoxime 6 into 5,6-tocopheryldione (8) under loss of hydroxylamine sets in. The experimental results agreed with the outcome of DFT computations.     

The conversion of nickel-bound CO into an acetyl thioester: organometallic chemistry relevant to the acetyl coenzyme A synthase active site

When three become one: Within one nickel-based model system, the three reactants CO, MeI, and PhSH have been assembled to yield an acetyl thioester. The reactivity is of relevance for the functioning of the acetyl coenzyme?A synthase active site and provides insights into possible binding sequences.     

Complexes of vitamin B6—IV: Polarographic determination of the stability constants of the mixed-ligand complexes of cadmium(II) with amino acids and pyridoxamine

Ternary complexes of cadmium(II)-pyridoxamine (PM-amino acid (L) systems were studied polarographically in aqueous solution at 0.50M KNO₃ of ionic strength. The amino acids considered were glycine, dl-alanine,dl-valine and β-phenylalanine. Three complexes of the forms Cd PM L, Cd PM L₂ and Cd(PM)₂L coexist in the pH range 8.0–10.5. The tendency toward mixed 1:1:1 complex formation was expressed in terms of the two equilibria Cd PM₂+Cd L₂⇋2 Cd PM L and Cd PM⁺ + Cd L⁺⇋Cd PM L + Cd², i.e. log X and Δ log K respectively.     

Competitive electron transfers from a tyrosyl side-chain and peptide bond in the photodegradation of N-tosyl alpha-aminomethylamides: an insight into photosynthesis and photodamage in the biological oxidation of water?

Photo-excited N-tosyl derivatives of phenylalanyl- and, more particularly, O-methyltyrosylmethylamides undergo electron transfer from aryl to tosyl groups whereas the photo-degradation of aliphatic analogues is initiated by electron transfer from the peptide bond, suggesting the latter as one possible reason for the rapid turnover of the D1 protein in biological water oxidation when the essential mediating role of tyrosine 116 in the PSII complex is inhibited.     

An unexpected insertion of acetone into the silicon–carbon terminus of an sp carbon chain: syntheses and structures of model monoplatinum hexatriynyl and octatetraynyl complexes

The sequential treatment of trans-(C₆F₅)(p-tol₃P)₂PtC≡CC≡CC≡CSiEt₃ (2, acetone solution) with n-Bu₄N⁺ F^– in wet THF (to generate a PtC≡CC≡CC≡CH complex), ClSiMe₃ (fluoride ion scavenger), excess HC≡CSiEt₃, excess O₂, and CuCl/TMEDA (0.20–0.25 equiv in acetone; Hay cross-coupling conditions) gives trans-(C₆F₅)(p-tol₃P)₂PtC≡CC≡CC≡CC≡CSiEt₃ (4, 30%), as previously reported. When an analogous reaction is conducted with SiMe₄ in place of ClSiMe₃, the side-product trans-(C₆F₅)(p-tol₃P)₂PtC≡CC≡CC≡CC(Me)₂OSiEt₃ is obtained (ca 28%), the structure of which is established crystallographically. For comparison, crystal structures of three related complexes, 4, 2, and trans-(p-tol)(Ph₃P)₂PtC≡CC≡CC≡CSiEt₃, are determined.     

Replacement of the Cobalt Center of Vitamin B12 by Nickel: Nibalamin and Nibyric Acid Prepared from Metal-Free B12 Ligands Hydrogenobalamin and Hydrogenobyric Acid

The (formal) replacement of Co in cobalamin ( Cbl ) by Ni^II generates nibalamin ( Nibl ), a new transition-metal analogue of vitamin B₁₂. Described here is Nibl , synthesized by incorporation of a Ni^II ion into the metal-free B₁₂ ligand hydrogenobalamin ( Hbl ), itself prepared from hydrogenobyric acid ( Hby ). The related Ni^II corrin nibyric acid ( Niby ) was similarly synthesized from Hby , the metal-free cobyric acid ligand. The solution structures of Hbl , and Niby and Nibl , were characterized by spectroscopic studies. Hbl features two inner protons bound at N2 and N4 of the corrin ligand, as discovered in Hby . X-ray analysis of Niby shows the structural adaptation of the corrin ligand to Ni^II ions and the coordination behavior of Ni^II. The diamagnetic Niby and Nibl , and corresponding isoelectronic Co^I corrins, were deduced to be isostructural. Nibl is a structural mimic of four-coordinate base-off Cbls , as verified by its ability to act as a strong inhibitor of bacterial adenosyltransferase.     

N,N-Dimethylphosphoramidic dichloride: a convenient reagent for the preparation of β-lactams from acetic acids and imines

A convenient reagent for the preparation of β-lactams from acetic acids and imines is described. A new route to α-keto-β-lactams from 3-bis(ethylthio)β-lactams is also reported. Reaction of 4-acethyl-β-lactams with diazomethane is also made.     

Synthesis and incorporation into cyclic peptides of tolan amino acids and their hydrogenated congeners: construction of an array of A-B-loop mimetics of the Cε3 domain of human IgE

The disruption of the human immunolobulin E-high affinity receptor I (IgE-FcεRI) protein-protein interaction (PPI) is a validated strategy for the development of anti asthma therapeutics. Here, we describe the synthesis of an array of conformationally constrained cyclic peptides based on an epitope of the A-B loop within the Cε3 domain of IgE. The peptides contain various tolan (i.e., 1,2-biarylethyne) amino acids and their fully and partially hydrogenated congeners as conformational constraints. Modest antagonist activity (IC(50) ～660 μM) is displayed by the peptide containing a 2,2'-tolan, which is the one predicted by molecular modeling to best mimic the conformation of the native A-B loop epitope in IgE.     

Tandem insertion of halocarbenoids and lithium acetylides into zirconacycles: a novel rearrangement to zirconium alkenylidenates by β-addition to an alkynyl zirconocene

Tandem insertion of 1,1-dihalo-1-lithio species (halocarbenoids) and lithium alkynides into zirconacyclopentenes and zirconcyclopentanes affords carbocyclic products in high yields via an unusual rearrangement that probably involves addition of an organolithium species to the β-position of a zirconium-alkyne complex to give an alkenylidene-zirconate species. A wide variety of cyclopentanoid organic structures are rapidly assembled in good yield using this multicomponent coupling. The main side reaction, which becomes exclusive in some cases, is β-hydride elimination of an intermediate cyclopentyl- or cyclopentenyl zirconocene.     

Preparation of aminomethyl functionalised silanes via an α-lithiated amine: from their synthesis, stability and crystal structures to stereochemical issues

The preparation of aminomethyl functionalised silanes based on the α-lithiated amine, (1R,2R)-N,N,N',N'-tetramethylcyclohexane-1,2-diamine [(R,R)-TMCDA] is reported. This methodology can be applied for the synthesis of mono-aminomethyl substituted systems, but most remarkably also for di- and trifunctionalised compounds. The trapping of the lithiated amine is accompanied by transmetallation reactions resulting in the formation of (silylmethyl)silanes depending on the reaction temperature. The zinc(II) halide complexes of the mono-functionalised systems show the formation of exclusively one configuration of the stereogenic nitrogen atom, in which the spatially more demanding substituent exhibits the pseudo-equatorial position. The di- and trifunctionalised systems feature high sensitivity towards Si-C bond cleavage under re-formation of the (R,R)-TMCDA fragment.     

Reactions of sodium tetrahydroborate with alkyl and aryl halides: A new approach to the synthesis of B3H 8 − and B12H 12 2− anions

A new method for the synthesis of the B₃H ₈ ^? anion based on the reactions of alkyl and aryl halides (C₂H₄Cl₂, C₆H₅CH₂Cl, C₄H₉Br, (C₆H₅)₃CCl, C₁₀H₇CH₂Cl, CH₂I₂, C₂HCl₃) with sodium tetrahydroborate in diglyme was proposed. The method is characterized by high (up to quantitative) yields and easy isolation of the target products, (n-C₄H₉)₄N)[B₃H₈] and Cs[B₃H₈], and also easy preparation of a salt of higher hydroborate anion, B₁₂H ₁₂ ^2? , without multistage purification from impurities of other anions of this series.     

The chemistry of localized singlet 1,3-diradicals (biradicals): from putative intermediates to persistent species and unusual molecules with a π-single bonded character

Localized singlet diradicals (biradicals) are key intermediates in chemical reactions involving homolytic bond-cleavage and formation processes. The molecular structure and electronic structure had been historically elusive due to the short-lived character of the reactive intermediates. In the last 15 years, a significant development of singlet diradical chemistry was achieved after the pioneering findings of long-lived singlet diradicals. In this tutorial review, the recent development of localized singlet diradical chemistry is summarized and discussed. The following subjects are included (a) the mechanism by which the ground state spin-multiplicity of localized 1,3-diradicals is controlled; (b) the substituent and heteroatom effect on the most stable electronic configuration of the singlet 1,3-diradicals, type-1 versus type-2; (c) the molecular design for the long-lived singlet ground state diradicals; (d) the generation and characterization of the singlet diradicals; and (e) the future prospects.     

Local optical emission spectroscopy of excited species effused from an evaporation cell and a sputter source into dense plasmas – Basic studies for the deposition of thin gradient films

Space resolved optical emission spectroscopy has been applied to determine the distribution of excited species in dense plasmas which are used for the deposition of thin coatings. Typical electron densities and electron temperatures in the plasma facility PETRA ( Plasma Engineering and Technology Research Assembly) are in the range of n(e) = 10(12) cm(-3) and T(e) = 10 eV. During the deposition process material (Al) is evaporated from a vapour cell under controlled conditions. The vapour stream is guided into a dense plasma which is composed of inert gas, Ar or He, and hydrocarbon species produced from the dissociation of C(2)H(2). The evaporated Al-stream which travels with thermal velocity into a plasma of high electron density, is nearly completely ionized due to the short mean free path for electron impact ionization in the above mentioned parameter range. Optical emission spectroscopy has been applied to investigate the interaction processes between the vapour stream and the plasma as well as the transport of the ionized Al along the applied magnetic field. For the measurements space resolved optical emission spectroscopy with an in-situ translation mechanism of the optical fibre has been used to measure the local concentrations of excited Al neutrals and ions as well as the concentration of the background plasma species.     

Preparation of an A-ring building block for the total synthesis of 1α,25-dihydroxy vitamin D3 and structurally related congeners: lipase-catalyzed stereoselective esterification of a suitable epoxyalcohol

An useful A-ring building block for the total synthesis of vitamin D₃ congeners, compound 7, has been prepared starting from vitamin D₂ by a chemo-enzymatic approach that relies on lipase-catalyzed acylation in an organic solvent for the stereoselective step.     

Preparation of trans Diequatorial 2,3-Pyruvate Acetals in a Di- and a Trisaccharide Related to the K-Antigen from E. Coli 0101:K103:H−

Abstract

Using methyl 2,2-bis(ethylthio)propionate as acetalating agent and triflic acid-sulfuryl chloride as catalyst, synthesis of 2,3-trans diequatorial pyruvate ketal was achieved. Starting from D-galactose and L-rhamnose derivatives, methyl 2,3,4,6-tetra-O-benzyl-α-D-galactopyranosyl-(1→4)-6-O-benzyl-2,3-O-(1-methoxycarbonyl)ethylidene- α-D-galactopyranosyl-(1→3)-2,4-di-O-benzyl-α-L-rhamnopyranoside and methyl 4,6-di-O-benzyl-2,3-O-(1-methoxy-carbonyl)ethylidene-α-D-galactopyranosyl-(1→3)-2,4-di-O-benzyl-α-L-rhamnopyranoside were synthesized. Removal of the protecting groups from the former, afforded the trisaccharide repeating unit of the K-antigen from E.coli O101:K103:H^? in the form of its methyl glycoside methyl ester.     

Revising Space Groups from Simulated SHELXfcf Structure Factors. More Examples of Incorrect Space Groups, an Example of a Chemically-Incorrect Structure and the Special Case of Pgl to PRevisions

1 INTRODUCTION Although the incorrect assignment of the space groups of crystal structures has been addressed in a number of reports, instances of crystal structures refined in incorrect symmetry turn up even in the recent literature[1～4]. A simple method of space group revision makes use of the published atomic coordinates and temperature factors to simulate the diffraction intensities; the structure is then 搒olved?in the correct space group from the simulated hkl-F2 data[5] and an O…     

Determination of intermolecular copper–copper distances from the EPR half-field transitions and their comparison with distances from X-ray structures: applications to copper(II) complexes with biologically important ligands

An EPR method involving measurement of half-field transitions was applied to determine the intermolecular Cu–Cu distances in copper(II)-carboxylate complexes with biologically important ligands. The experimental powder EPR spectra are composed of allowed (ΔM _S  = ±1) transitions centered at ~3,200 Gauss and of weak intensity, nominally forbidden, half-field (ΔM _S  = ±2) peaks observable at ~1,600 Gauss. Values of the average interspin distance for each complex were determined from the ratios of integrated allowed and forbidden peak areas using each of several methods. The calculated interspin distances were correlated with the copper–copper distances experimentally obtained by X-ray crystallography. The distances determined from the EPR spectra agree well with the X-ray determined values when the crystallographic value for one member of a series is used to calibrate the series. Less satisfactory agreement is found when methods based on Cu-spin-label systems are used.     

One-pot synthesis of cis-bis(2,2′-bipyridine)carbonylruthenium(II) complexes from a carbonato precursor: X-ray crystal structures and electron transfer processes of the series complexes

The reaction of Brønsted acids with cis-[Ru(bpy)₂(CO₃)] (bpy?=?2,2′-bipyridine) under CO results in cleavage of the carbonato ligand and formation of cationic cis-[Ru(bpy)₂(CO)L] ^{n +} complexes [L?=?ONO₂ (1 ⁺), OH₂ (2 ²⁺), Cl (3 ⁺), OCOH (4 ⁺), and OCOCH₃ (5 ⁺)]. The structures of 1 ⁺ and 2 ²⁺ were confirmed by single-crystal X-ray diffraction. Crystal data for 1(PF₆): monoclinic, P2₁/c, a?=?10.5242(3), b?=?15.4727(3), c?=?14.6571(3) Å, β?=?92.3219(9)°, V?=?2384.77(9) ų, Z?=?4, D _calcd?=?1.806?g cm^?3, 5460 unique reflections (R _int?=?0.032), R ₁?=?0.0540 [I?>?2σ(I)], wR ₂?=?0.1642 (all reflections); crystal data for 2(ClO₄)₂?·?H₂O: monoclinic, C2/c, a?=?20.4247(7), b?=?10.0777(3), c?=?15.6039(5) Å, β?=?127.7569(8)°, V?=?2539.31(14) ų, Z?=?4, D _calcd?=?1.769?g cm^?3, 2895 unique reflections (R _int?=?0.036), R ₁?=?0.0343 [I?>?2σ(I)], wR ₂?=?0.0907 (all reflections). Except for 2(PF₆)₂ the complexes exhibit oxidation at 1.02–1.30?V versus Fc⁺/Fc in acetonitrile. Bipyridine-centered reductions are also observed; these redox potentials depend on the nature of L. This convenient synthesis will be useful for producing cis-[Ru(bpy)₂(CO)L] ^{n +}-type complexes in high yield.