Enantio- and diastereoselective, stereospecific mannich-type reactions in water

Catalytic asymmetric Mannich-type reactions in water proceeded in high yields and selectivities using a combination of ZnF2 and a chiral diamine that has the methoxy groups on the aromatic rings. In these reactions, either syn- or anti-Mannich adducts were stereospecifically obtained from (E)- or (Z)-silicon enolate, in contrast with most asymmetric Mannich-type reactions.     

Gas chromatographic analysis of estrogens. II

Summary A simple method is described for the introduction of samples to a gas Chromatograph by means of a septumless port. The reproducibility and accuracy of the method have been shown by the analysis of ng quantities of estrone, estradiol, and estriol as the heptafluorobutyrates with an electron capture detector.

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Zusammenfassung Die Einbringung von Proben in einen Gaschromatographen durch einen membranlosen Einspritzblock wurde beschrieben. Die Reproduzierbarkeit und Genauigkeit der so erhaltenen Ergebnisse wurde durch die Bestimmung von Nanogrammengen Östron, Östradiol und Östriol als Heptafluorobutyrate mit einem Elektroneneinfangdetektor erwiesen.

     

A regio- and stereoselective alpha-methylation of gamma,delta-epoxy-alpha,beta-unsaturated esters with a Me2Zn-CuCN reagent

A highly regio- and stereoselective alpha-methylation reaction of gamma,delta-epoxy-alpha,beta-unsaturated esters was achieved by using a Me2Zn-CuCN reagent.     

Gamma-p-toluenesulfonyl-alpha,beta-epoxysilane: a new and practical acrolein beta-anion equivalent

[reaction: see text] Reaction of gamma-p-toluenesulfonyl-alpha,beta-epoxysilane with alkyl halides and aldehydes followed by treatment with n-Bu4NF affords alpha,beta-unsaturated aldehydes via a Brook rearrangement-mediated tandem process under extremely mild conditions.     

Role of electron-transfer processes in reactions of diarylcarbenium ions and related quinone methides with nucleophiles

Second-harmonic alternating current voltammetry has been used to determine one-electron reduction potentials of 15 diarylcarbenium ions and 5 structurally analogous quinone methides, which have been employed as reference electrophiles for the development of nucleophilicity scales. A linear correlation (r(2) = 0.993) between the empirical electrophilicity parameters E and the reduction potentials in acetonitrile (E = 14.091E degrees (red) - 0.279) covering a range of 1.64 V (or 158 kJ mol(-)(1)) has been observed. For a large number of nucleophiles, it has been demonstrated that the observed activation free energies of the electrophile-nucleophile combinations are 61-195 kJ mol(-)(1) smaller than the free energy change of electron transfer from nucleophile to electrophile, which definitely excludes outer-sphere electron transfer occurring during these reactions.     

Electron-transfer oxidation of coenzyme B12 model compounds and facile cleavage of the cobalt(IV)-carbon bond via charge-transfer complexes with bases. A negative temperature dependence of the rates

The electron-transfer oxidation and subsequent cobalt-carbon bond cleavage of vitamin B12 model complexes were investigated using cobaloximes, (DH)2Co(III)(R)(L), where DH- = the anion of dimethylglyoxime, R = Me, Et, Ph, PhCH2, and PhCH(CH3), and L = a substituted pyridine, as coenzyme B12 model complexes and [Fe(bpy)3](PF6)3 or [Ru(bpy)3](PF6)3 (bpy = 2,2'-bipyridine) as a one-electron oxidant. The rapid one-electron oxidation of (DH)2Co(III)(Me)(py) (py = pyridine) with the oxidant gives the corresponding Co(IV) complexes, [(DH)2Co(IV)(Me)(py)]+, which were well identified by the ESR spectra. The reorganization energy (lambda) for the electron-transfer oxidation of (DH)2Co(Me)(py) was determined from the ESR line broadening of [(DH)2Co(Me)(py)]+ caused by the electron exchange with (DH)2Co(Me)(py). The lambda value is applied to evaluate the rate constants of photoinduced electron transfer from (DH)2Co(Me)(py) to photosensitizers in light of the Marcus theory of electron transfer. The Co(IV)-C bond cleavage of [(DH)2Co(Me)(py)]+ is accelerated significantly by the reaction with a base. The overall activation energy for the second-order rate constants of Co(IV)-C bond cleavage of [(DH)2Co(IV)(Me)(py)]+ in the presence of a base is decreased by charge-transfer complex formation with a base, which leads to a negative activation energy for the Co(IV)-C cleavage when either 2-methoxypyridine or 2,6-dimethoxypyridine is used as the base.     

Photo-induced cellulose radicals capable of initiating graft copolymerization

The decay behavior of cellulose radicals produced by photo-irradiation at room temperature and the characteristics of photo-irradiated cellulose samples to initiate graft copolymerization of methyl methacrylate (MMA) were investigated. ESR spectra of such untreated, swollen, oximated, and ferric ion-sensitized samples irradiated at room temperature were constructed mainly of a single absorption line with a line width of 20 to 22 gauss and a g value of 2.003, and it is surely conceivable that the radicals showing a singlet spectrum should agree with those of alkoxy end produced at either the C₁ or C₄ position of the glucose unit by the scission of glucosidic bonds. The decay of radicals was accelerated by contact of various solvents with the samples, the activity decreasing in the order, water ≈ methanol ? acetone > dioxane. On the other hand, the decay of radicals by vinyl monomers became smaller in the order, methacrylic acid > MMA ≈ styrene. Graft copolymerization of MMA by a photo-irradiated sample was effectively initiated with the use of a certain amount of water or methanol, but not with acetone and dioxane. As no initiation can occur with the unirradiated sample, it is concluded that the initiation of graft copolymerization on the photo-irradiated sample is attributable to cellulose radicals showing a singlet spectrum which are formed in photo-irradiation at room temperature.     

Langevin model of the temperature and hydration dependence of protein vibrational dynamics

The modification of internal vibrational modes in a protein due to intraprotein anharmonicity and solvation effects is determined by performing molecular dynamics (MD) simulations of myoglobin, analyzing them using a Langevin model of the vibrational dynamics and comparing the Langevin results to a harmonic, normal mode model of the protein in vacuum. The diagonal and off-diagonal Langevin friction matrix elements, which model the roughness of the vibrational potential energy surfaces, are determined together with the vibrational potentials of mean force from the MD trajectories at 120 K and 300 K in vacuum and in solution. The frictional properties are found to be describable using simple phenomenological functions of the mode frequency, the accessible surface area, and the intraprotein interaction (the displacement vector overlap of any given mode with the other modes in the protein). The frictional damping of a vibrational mode in vacuum is found to be directly proportional to the intraprotein interaction of the mode, whereas in solution, the friction is proportional to the accessible surface area of the mode. In vacuum, the MD frequencies are lower than those of the normal modes, indicating intramolecular anharmonic broadening of the associated potential energy surfaces. Solvation has the opposite effect, increasing the large-amplitude vibrational frequencies relative to in vacuum and thus vibrationally confining the protein atoms. Frictional damping of the low-frequency modes is highly frequency dependent. In contrast to the damping effect of the solvent, the vibrational frequency increase due to solvation is relatively temperature independent, indicating that it is primarily a structural effect. The MD-derived vibrational dynamic structure factor and density of states are well reproduced by a model in which the Langevin friction and potential of mean force parameters are applied to the harmonic normal modes.