Effects of allylic and homoallylic substituents on the ring closing metathesis reaction used to synthesise simplified eleuthesides

During the course of our synthetic studies towards simplified eleuthesides, we have found that p-methoxyphenyl (PMP) protected allylic alcohols are compatible with the RCM reaction and can give better yields than the corresponding free allylic alcohols.     

Co60 γ-radiation-induced copolymerization of ethylene and carbon monoxide

The γ-radiation-induced free-radical copolymerization of ethylene and CO has been investigated over a wide range of pressure, initial gas composition, radiation intensity, and temperature. At 20°C., concentrations of CO up to 1% retard the polymerization of ethylene. Above this concentration the rate reaches a maximum between 27.5 and 39.2% CO and then decreases. The copolymer composition increases only from 40 to 50% CO when the gas mixture is varied from 5 to 90% CO. A relatively constant reactivity ratio is obtained at 20°C., indicating that CO adds 23.6 times as fast as an ethylene monomer to an ethylene free-radical chain end. For a 50% CO gas mixture, the above value of 23.6 and the copolymerization rate decrease with increasing temperature to 200°C. The kinetic data indicate a temperature-dependent depropagation reaction. Infrared examination of copolymers indicates a polyketone structure containing ? CH₂? CH₂? and ? CO? units. The crystalline melting point increases rapidly from 111 to 242°C., as the CO concentration in the copolymer increases from 27 to 50%. Molecular weight of copolymer formed at 20°C. increased with increasing CO, indicating M?_n values >20,000. Increasing reaction temperature results in decreasing molecular weight. Onset of decomposition for a 50% CO copolymer was measured at ≈250°C.     

Estimation of precision and efficiency mass transfer steps for the determination of pesticides in vegetables aiming at the expression of results with reliable uncertainty

A 'bottom-up' approach for the expression of results obtained from analytical methods that include analytical steps with recovery inherently different from 100% [mass transfer steps (MTS): extraction, evaporation, clean-up procedures, digestion, etc.] is presented. The estimation of the combination of all MTS uncertainty involves the comparison of the experimental dispersion of replicated analyses of spiked samples with the estimation of the uncertainty obtained for the combination of all uncertainty sources except MTS ones ('incomplete' estimation). The estimation of MTS uncertainty by difference is performed after evaluating the statistical difference between the 'incomplete' estimation and the experimental dispersion (F-test). When the two estimations are statistically equivalent, the MTS uncertainty is considered to be negligible in relation to the other sources budget. The assumption of constancy of MTS performance within the analytical range is tested through single analyses at several concentration levels and is evaluated by the inclusion of the expected values at the intervals resulting from the combination of the MTS uncertainty estimation performed at one concentration level and the 'incomplete' estimation. The developed methodology can also be useful for method optimisation and validation and for the detection of small trends in results. The determination of pesticides in sweet peppers by GC-NPD was used to explore the above concepts.     

Synthesis, structure and conformation of partially-modified retro- and retro-inverso psi[NHCH(CF3)]Gly peptides

Partially modified retro- (PMR) and retro-inverso (PMRI) psi[NHCH(CF(3))]Gly peptides, a conceptually new class of peptidomimetics, have been synthesized in wide structural diversity and variable length by aza-Michael reaction of enantiomerically pure alpha-amino esters and peptides with enantiomerically and geometrically pure N-4,4,4-trifluorocrotonoyl-oxazolidin-2-ones. The factors underlying the observed moderate to good diastereocontrol have been investigated. The conformations of model PMR-psi[NHCH(CF(3))]Gly tripeptides have been studied in solution by (1)H NMR spectroscopy supported by MD calculations, as well as in the solid-state by X-ray diffraction. Remarkable stability of turn-like conformations, comparable to that of parent malonyl-based retropeptides, was evidenced, as a likely consequence of two main factors: 1) severe torsional restrictions about sp(3) bonds in the [CO-CH(2)-CH(CF(3))-NH-CH(R)-CO] module, which is biased by the stereoelectronically demanding CF(3) group and the R side chain; 2) formation of nine-membered intramolecularly hydrogen-bonded rings, which have been clearly detected both in CHCl(3) solution and in some crystal structures. The former factor seems to be more important, as turn-like conformations were found in the solid-state even in the absence of intramolecular hydrogen bonding. The relative configuration of the -C*H(CF(3))NHC*H(R)- stereogenic centers has a major effect on the stability of the turn-like conformation, which seems to require a syn stereochemistry. X-ray diffraction and ab initio computational studies showed that the [-CH(CF(3))NH-] group can be seen as a sort of hybrid between a peptide bond mimic and a proteolytic transition state analogue, as it combines some of the properties of a peptidyl -CONH- group (low NH basicity, CH(CF(3))-NH-CH backbone angle close to 120 degrees, C-CF(3) bond substantially isopolar with the C=O) with some others of the tetrahedral intermediate [-C(OX)(O(-))NH-] involved in the protease-mediated hydrolysis reaction of a peptide bond (high electron density on the CF(3) group, tetrahedral backbone carbon).     

Rheology of a lyotropic mesophase through a stress-relaxation experiment

We report on the first rheological study of the structural relaxations in a nematic liquid crystalline phase. Linear dynamic and transient shear experiments were applied to a polydomain nematic phase of the CTAB/water system: a liquid crystalline mixture composed of 28 wt% CTAB at 35 degrees C. The decay of the shear modulus, G(t), was analyzed using the CONTIN inverse Laplace transform to obtain a distribution of relaxation times which were compared with ones from the usual fitting procedure based on a generalized Maxwell model. The behavior of the nematic lyotropic structure of the CTAB/water system is characterized by the presence of both slow and fast relaxation times. These were interpreted as being due to a progressive loss of the lyotropic domain orientation and to the breaking/reforming process of the cylindrical aggregates, respectively.     

Enantioselective synthesis of secondary alcohols in the presence of chiral ligands

The synthesis of two S-proline derivatives is described together with their use as chiral ligands for lithium in reactions of n-butyllithium with benzaldehyde to give 1-phenyl-1-pentanol with moderate optical purity. The presence of lithium salts in the reaction mixture causes a decrease in asymmetric induction.     

Y-shaped two-photon absorbing molecules with an imidazole-thiazole core

Two new classes of two-photon absorbing Y-shaped molecules have been developed to possess an imidazole-thiazole core and a stilbene-type conjugation pathway with either nitro or sulfonyl as terminal electron-accepting group.     

Anisotropic quantum spin fluid and quantum spin glass in La cuprate-based catalyst

Evidence of quantum spin fluid and of quantum spin glass behaviour has been detected by extended EPR analysis in some La-cuprate catalysts of general formula La_2−xEu_xCuO_{4 + δ}. Some features of the EPR spectrum, appearing in a few cases, are also discussed and attributed to a pseudo Jahn-Teller effect due to a particular coordination of up to five oxygen ions around surface copper ions at low temperature.     

A borel-weil-bott approach to representations of sl q (2, ℂ)

We use a quite concrete and simple realization of sl _q (2, ) involving finite difference operators. We interpret them as derivations (in the noncommutative sense) on a suitable graded algebra, which gives rise to the noncommutative scheme ¹ II ^1* as the counterpart of the standard ¹ = Sl(2, )/B.