An effective enantioselective approach to the securinega alkaloids: total synthesis of (-)-norsecurinine

[reaction: see text] A highly versatile approach to the enantioselective synthesis of securinega alkaloids is presented. Crucial steps are a palladium-catalyzed enantioselective imide alkylation, a vinylogous Mannich reaction, and a ring-closing metathesis process. Through this strategy, the synthesis of (-)-norsecurinine has been accomplished in nine steps and 11% overall yield.     

Functional polythiiranes 7: Surface activity of polythiiranes with PEO side chains

 Some comb-like polythiiranes with PEO side chains were prepared from the corresponding macromonomers. These new materials are amphiphiles and act as surfactants. Their surface tension and interfacial tension are measured and studied in this paper on account of their structures. The lowering of surface tension measured in polymers bearing methyl terminal group in PEO side chains, are in the same range as these observed with polymers of identical structures but different main chains. An increase of the hydrophobic units in the main chain, obtained in copolymers with methylthiirane does not significantly modified the surface tensions. Better lowering is afforded with structures bearing large alkyl groups as terminal group of PEO side chains. On the contrary, some of these macro-molecules with an optimized EO content largely lower the water/xylene surface tension. The main interest of these new materials is their very low cmc and the stabilization of L₁-type microemulsions. Received: 20 May 1997 Accepted: 25 November 1997     

Trifluoromethoxy substituted anilines: metalation as the key step for structural elaboration

Trifluoromethoxy-substituted anilines undergo hydrogen/lithium permutation ("metalation") with optional site selectivity depending on the N-protective group employed. N-tert-Butoxycarbonyl-2- and -4-(trifluoromethoxy)aniline react with tert-butyllithium at the nitrogen-adjacent 6- and 2-position affording, after electrophilic trapping, products 1-6. In contrast, deprotonation of the para isomer occurs at the oxygen-neighboring 3-position, giving rise to the acid 12, when the amino group is carrying two trimethylsilyl groups. sec-Butyllithium attacks 3-trifluoromethoxy-N-mono(trimethylsilyl)aniline at the 2-position, but 3-trifluoromethoxy-N,N-bis(trimethylsilyl)aniline at the 4-position to provide respectively the acids 10 and 11 after carboxylation. The synthesis of two new benzodiazepines illustrates (19 and 22) the preparative potential of the aniline functionalization mediated by organometallic reagents.     

ECE reaction pathways in the electrochemical reduction of dicyanocobalamin: Kinetics of ligand substitution in vitamin B12r cyanocob(II)alamin

The reduction of dicyanocob(III)alamin leads in a first stage to monocyanocob(II)alamin which can be partially converted into the base-off and base-on Co(II) complexes (B12r). The latter species are easier to reduce than the starting Co(III) complex leading to a single two-electron wave at low cyanide concentrations and/or low diffusion rates. Upon raising one of these two parameters two successive one-electron waves tend to be obtained corresponding to the Co(III)/Co(II) and Co(II)/Co(I) conversion respectively. The kinetics of the reduction process is investigated using potential-dependent potentiostatic chronoamperometry which allows a simpler analysis than cyclic voltammetry for systems involving a slow initial charge-transfer step. It is seen that the second electron, at the level of the first wave, comes from the electrode and not from the cyano-Co(II) complex in the solution. The reduction thus follows an ECE rather than a DISP-type mechanism in conditions where they can be distinguished by the usual electrochemical kinetic techniques. This contrasts with that which occurs in organic electrochemistry where the electron transfers are generally fast, while in the present case they are slow. The analysis of the reduction kinetics as a function of cyanide concentration gives some insight into the mechanism of the ligand substitution reaction at the Co(II). The kinetic data are discussed in terms of SN1-, SN2- and SNAr-like mechanisms.     

Regiospecifically controlled formation of tetrasubstituted tributyltin ketone enolates catalyzed by a palladium complex

The palladium-catalyzed hydrostannolysis of α-disubstituted allyl β-keto esters yields the corresponding tributyltin β-keto carboxylates which loose carbon dioxide at very moderate temperature, thus leading to the regiospecific formation of tributyltin tetrasubstituted enolates.     

Chiral induction in quinoline-derived oligoamide foldamers: assignment of helical handedness and role of steric effects

Chiral groups attached to the end of quinoline-derived oligoamide foldamers give rise to chiral helical induction in solution. Using various chiral groups, diastereomeric excesses ranging from 9% to 83% could be measured by NMR and circular dichroism. Despite these relatively weak values and the fact that diastereomeric helices coexist and interconvert in solution, the right-handed or left-handed helical sense favored by the terminal chiral group could be determined unambiguously using X-ray crystallography. Assignment of chiral induction was performed in an original way using the strong tendency of racemates to cocrystallize, and taking advantage of slow helix inversion rates, which allowed one to establish that the stereomers observed in the crystals do correspond to the major stereomers in solution. The sense of chiral helical induction was rationalized on the basis of sterics. Upon assigning an Rs or Ss chirality to the stereogenic center using a nomenclature where the four substituents are ranked according to decreasing sizes, it is observed that Rs chirality always favors left-handed helicity and Ss chirality favors right-handed helicity (P). X-ray structures shed some light on the role of sterics in the mechanism of chiral induction. The preferred conformation at the stereocenter is apparently one where the bulkiest group should preferentially point away from the helix, the second largest group should be aligned with the helix backbone, and the smallest should point to the helix.     

About SnF2 stannous fluoride. IV. Kinetics of the α → γ and β, γ → α transitions

The kinetics of the reconstructive first order phase transitions α → γ (upon heating) and β, γ → α (upon cooling) of SnF₂ were studied by quantitative X-ray powder diffraction. The main feature of the kinetics is that these transformations are incomplete over a wide temperature range. Consequently it is not possible to fit the experimental data using the usual solid state rate expressions. Although these transitions are not reversible (large hysteresis) the empirical use of a kinetic model of a reversible transformation provides a good fit. The α → γ transformation was also investigated by D.T.A.; this transition was found to be greatly influenced by particle size: the transition temperature increases with grain size, and its enthalpy decreases. The X-ray investigation revealed that the process of the α → γ transformation involves an intermediate noncrystalline phase named the “transition phase.” It is suggested that the transition proceeds through a process of fragmentation-reconstruction.     

The Choice of Internal Standards for Measuring Volatile Pollutants in Water

54 volatile pollutants have been examined by static headspace-gas chromatography-mass selective detection in order to find the proper internal standard for each of the pollutants examined. The applicability of the internal standards has been assessed mathematically. For modelling, we prepared 2 × 4 × 4 solutions using blank water with added sodium sulphate and humic acid at four different concentrations for each. These solutions were spiked with two different concentrations of dilute standard solutions. We also examined 44 real water samples for traces of the 54 volatile pollutants, spiking them with dilute standard solutions. The results of a single measurement were 54 quotients for relative extraction efficiency: the area of the pollutant divided by the corresponding area of the spiked blank water measurement. For each pair of pollutants, the Pearson correlation coefficients were calculated for both the model and real water samples. We regarded two pollutants as being the same, if their Pearson correlation coefficient was greater than 0.95. Among similar pollutants we selected candidates for being suitable internal standards based on the highest correlation coefficients. We found, that five compounds are sufficient to cover 49 pollutants. Two pollutants did not exhibit a matrix effect and for these only the external calibration method can be used. For three pollutants, special considerations apply.The measurement data generally verified that structurally similar compounds have high correlation coefficients, but there were exceptions among similar compounds and unexpected similarities were also found. Nothing was found in the literature on determining the proper internal standard using Pearson correlation coefficients.     

Interdependence of redox state,hydrogen bonding,anion recognition and charge partition in crystals of (EDT-TTF-CONHMe)6 [Re6Se8(CN)6] (CH3CN)2(CH2Cl2)2

Neutral pi-conjugated molecules and their radical cations co-exist in [(EDT-TTF-CONHMe+*)4(EDT-TTF-CONHMe0)2] [Re6Se8(CN)6]4- (CH3CN)2(CH2Cl2)2 whose crystal structure reveals that, upon one-electron oxidation, an activation of the N-H and C-H hydrogen bond donor ability is coupled to a deactivation of the hydrogen bond acceptor character of the carbonyl oxygen atom: this is expressed in the supramolecular hydrogen bond pattern and, ultimately, into charge localisation and partition in the solid state.     

Biamperometric neutralisation titrations in non-aqueous solvents applying tin electrode pair for the end-point detection

Summary Tertiary amines and salts of organic acids in acetic anhydride with 5% of acetic acid were titrated applying a biamperometric indicating system consisting of a polarised or unpolarised pair of tin electrodes by means of 0.1 N perchloric acid in glacial acetic acid. Amounts of 15.4–29.8 mg were analysed with average deviations of <0.6%, the results obtained being in a good agreement with those of potentiometry. The biamperometric end-point detection using an indicating system of polarised pair of tin electrodes can be applied for coulometric determinations of these bases with satisfying accuracy. By means of anodic generation of hydrogen ions in the presence of hydroquinone amounts of 0.5–1.0 mg were titrated with average deviations of < 1.3%. The results obtained were compared with those obtained of catalytic thermometric and photometric titrations.

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Biamperometrische Neutralisationstitrationen in nichtwärigen Lösungsmitteln unter Anwendung eines Zinn-Elektrodenpaares als Indicatorsystem

Zusammenfassung Tertiäre Amine und Salze organischer Säuren werden in Acetanhydrid mit 5% Essigsäure durch Titration mit einer 0,1 N Lösung von Perchlorsäure in Essigsäure bestimmt. Das verwendete biamperometrische Indicatorsystem besteht aus einem Paar polarisierter oder nichtpolarisierter Zinnelektroden. Mengen von 15,4–29,8 mg wurden mit einer mittleren Abweichung von <0,6% analysiert. Die Ergebnisse stimmen gut mit denen von potentiometrischen Titrationen überein. Die biamperometrische Endpunkttechnik mit polarisierten Zinnelektroden kann für coulometrische Bestimmungen dieser Basen mit zufriedenstellender Genauigkeit benutzt werden. Durch anodische Erzeugung von Wasserstoffionen in Gegenwart von Hydrochinon wurden Mengen von 0,5–1,0 mg mit einer mittleren Abweichung von < 1,3% titriert. Die Ergebnisse wurden mit denen von katalytisch-thermometrischen und photometrischen Titrationen verglichen.