Superbase-Promoted Acylation of Hindered Alcohols

The commercially available nonionic superbase P(MeNCH(2)CH(2))(3)N (1a) is very useful for the acylation of unreactive hindered alcohols as well as acid-sensitive alcohols. The reactions proceed in high yields using an acid anhydride, and 1a can be regenerated in a single step. The relative rates for benzoylation of (+/-)-menthol in C(6)D(6) using conventional acylation reagents and strong nonionic bases are compared. In general, acetylation with 1a is accelerated in the polar solvent CH(3)CN whereas benzoylation is faster in the nonpolar solvent C(6)H(6). The benzoylation intermediate RC(O)P(MeNCH(2)CH(2))(3)N(+) was found to be in equilibrium with 1a, with lower temperatures favoring the intermediate. The relative stabilities of several known acylating intermediates are compared.     

Nonresonant third-order susceptibility measurements in the nematic liquid crystal K18

We have observed a large nonresonant third-order nonlinear susceptibility, X ⁽³⁾ (-; , 0, 0) in the isotropic phase of a nematic liquid crystal 4-n-hexyl-4-cyanobiphenyl (K18). The highest value of X ⁽³⁾ obtained at 632.8 nm is 1.16274×10^-18 m²V^-2 corresponding to a temperature 29.3°C. The observed second-order pretransitional temperature T ^* from our measurements is 1.2°C below the first-order nematic to isotropic transition temperature. The dependence of the Kerr constant on (T-T ^*)^-1 is found to be in good agreement with the predictions of the Landau-de Gennes model.     

Chiral environments for asymmetric hydrogenation of model didehydro-amino acid residues

Dedicated to Professor John C. Sheehan on the occasion of his sixty-fifth birthday Nmr and glc analysis of diastereoisomeric mixtures of dipeptides has been used to study the asymmetric hydrogenation of model benzoyldidehydro- and trifluoroacetyldidehydro-dipeptide methyl esters. Chiral enhancement of one isomeric form appears to be independent of the N-terminal acyl group, but is significantly influenced by the choice of amino-acid in the C-terminal position. C-Terminal aromatic amino-acids and their derivatives give the best chiral enhancement during hydrogenation of a neighbouring dehydroamino-acid residue.     

Synthesis, structure, theoretical studies, and Ligand exchange reactions of monomeric, T-shaped arylpalladium(II) halide complexes with an additional, weak agostic interaction

A series of monomeric arylpalladium(II) complexes LPd(Ph)X (L = 1-AdPtBu2, PtBu3, or Ph5FcPtBu2 (Q-phos); X = Br, I, OTf) containing a single phosphine ligand have been prepared. Oxidative addition of aryl bromide or aryl iodide to bis-ligated palladium(0) complexes of bulky, trialkylphosphines or to Pd(dba)2 (dba = dibenzylidene acetone) in the presence of 1 equiv of phosphine produced the corresponding arylpalladium(II) complexes in good yields. In contrast, oxidative addition of phenyl chloride to the bis-ligated palladium(0) complexes did not produce arylpalladium(II) complexes. The oxidative addition of phenyl triflate to PdL2 (L = 1-AdPtBu2, PtBu3, or Q-phos) also did not form arylpalladium(II) complexes. The reaction of silver triflate with (1-AdPtBu2)Pd(Ph)Br furnished the corresponding arylpalladium(II) triflate in good yield. The oxidative addition of phenyl bromide and iodide to Pd(Q-phos)2 was faster than oxidative addition to Pd(1-AdPtBu2)2 or Pd(PtBu3)2. Several of the arylpalladium complexes were characterized by X-ray diffraction. All of the arylpalladium(II) complexes are T-shaped monomers. The phenyl ligand, which has the largest trans influence, is located trans to the open coordination site. The complexes appear to be stabilized by a weak agostic interaction of the metal with a ligand C-H bond positioned at the fourth-coordination site of the palladium center. The strength of the Pd.H bond, as assessed by tools of density functional theory, depended upon the donating properties of the ancillary ligands on palladium.     

Electronic and vibrational second-order nonlinear optical properties of protein secondary structural motifs

A perturbation theory approach was developed for predicting the vibrational and electronic second-order nonlinear optical (NLO) polarizabilities of materials and macromolecules comprised of many coupled chromophores, with an emphasis on common protein secondary structural motifs. The polarization-dependent NLO properties of electronic and vibrational transitions in assemblies of amide chromophores comprising the polypeptide backbones of proteins were found to be accurately recovered in quantum chemical calculations by treating the coupling between adjacent oscillators perturbatively. A novel diagrammatic approach was developed to provide an intuitive visual means of interpreting the results of the perturbation theory calculations. Using this approach, the chiral and achiral polarization-dependent electronic SHG, isotropic SFG, and vibrational SFG nonlinear optical activities of protein structures were predicted and interpreted within the context of simple orientational models.     

Thermodynamic properties of alkali halides. IV. Apparent molal volumes,expansibilities, compressibilities,and heat capacities in urea-water mixtures

The apparent molal volume φ_v, expansibility φ_E, compressibility φ_K, and heat capacity φ_c of NaCl were measured in urea-water mixtures, as a function of salt (<1.5m) and urea (<13m) concentrations at 25°C. At a fixed urea concentration, the transfer functions from H₂O to 3m urea are linear functions of the NaCl aquamolality. At a fixed salt aquamolality, (0.1m), the sign of the transfer functions is in the direction of a decrease in the structure-breaking effect, and the absolute values of the transfer functions tend to level off at high urea concentrations (13m). The functions φ_v, φ_E, φ_K, φ_c, and (?φ_v/?T)_p were measured for the sodium halides and alkali, bromides (chlorides in the case of φ_K) at a fixed salt aquamolality of 0.1m and fixed urea molality of 3m. The corresponding transfer functions from H₂O to 3m urea are opposite those from H₂O to D₂O and similarly are relatively independent of ionic size. This suggests that urea, shows no specific interaction affinity for ions and that the overall number of water molecules influenced by the ions is relatively constant for all alkali halides. The lithium halides are an exception in that Li⁺ seems to have hardly any structure-breaking effect.     

Substituted pyridines

5-Methyl-4-phenyl-2-styryipyridine has been oxidized to 5-methyl-4-phenylpyridinie-2-carboxylic acid. The decarboxylation of this acid has given 3-methyl-4-phenylpyrldine. To prove the structure of -(5-methyl-4-phenyl-2-pyridyl)acetophenone, it has been converted into 2, 5-dimethyl-4-phenylpyridine. The reaction of benzyl chloride with the lithium derivative of 2, 5-dimethyl-4-phenylpyridine has been studied. 3,7-Dimethyl-2-azafluorene has been obtained from 2,5-dimethyl-4-p-tolylpyridine.     

Electrospray tandem mass spectrometric analysis of zeaxanthin and its oxidation products

Carotenoids have been implicated in protection of the eye from light-mediated photo-toxicity caused by free radicals. Under conditions of normal oxidative stress the carotenoids serve as protective antioxidants; however, when the oxidative stress exceeds the antioxidant capacity, carotenoids can be oxidized into numerous cleavage products. The determination and identification of oxidized carotenoids in biological samples remains a major challenge due to the small sample size and low stability of these compounds. We investigated the reaction of various zeaxanthin cleavage products with O-ethyl hydroxylamine to evaluate their levels in a biological sample. For this, a sensitive and specific electrospray tandem mass spectrometry (ESI-MS/MS) was developed, avoiding the classical lower sensitive and specific HPLC-UV and fluorescence absorption methods. Protonated molecules [M + H](+) of carotenoids upon collision-induced dissociation produced a number of structurally characteristic product ions. A series of complicated clusters of product ions differing in 14 (CH(2))and 26 (C(2)H(2))Da was characteristic of the polyene chain of intact carotenoids. All carotenoid ethyl oximes of zeaxanthin cleavage products were characterized by the losses of 60 and 61 Da in their MS/MS spectra. Through the application of the LC/MS/MS method, we identified two oxime derivatives of 3-hydroxy-beta-ionone and 3-hydroxy-14'-apocarotenal with protonated molecules at m/z 252 and m/z 370 respectively, in a human eye sample.     

Determination of the pesticides Chlorpropham,Propham, Propoxur,and Terbucarb by low temperature phosphorimetry

Summary Simple, direct, and sensitive low temperature phosphorimetric methods for the determination of Chlorpropham, Propham, Propoxur, and Terbucarb in EPA solvent are described. The minimum detectable amounts are 0.01 p. p. m. for Chlorpropham and Propham and 0.02 and 0.3 p. p. m. for Propoxur and Terbucarb respectively. The ranges of linearity of the analytical calibration graphs are 0.05-7.5, 0.05–10, 0.5–50, and 1–50 p. p. m. for Chlorpropham, Propham, Propoxur, and Terbucarb respectively. The procedure for Chlorpropham has been applied to the analysis of a solid commercial formulation.

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Bestimmung der Pesticide Chlorpropham, Propham, Propoxur und Terbucarb bei Tief-Temperatur-Phosphorimetrie

Zusammenfassung Einfache, direkte und empfindliche Methoden für die Bestimmung der genannten Pesticide in dem Lösungsmittel Diethylether-i-Pentan-abs. Ethanol wurden beschrieben. Die geringsten nachweisbaren Mengen sind 0,01 ppm für Chlorpropham und Propham sowie 0,02 und 0,3 ppm für Propoxur und Terbucarb. Die Eichkurven verlaufen zwischen 0,05 und 7,5, 0,05 und 10, 0,5 und 50 sowie zwischen 1 und 50 ppm für Chlorpropham, Propham, Propoxur bzw. Terbucarb linear. Das Verfahren für Chlorpropham wurde zur Analyse eines festen Handelsproduktes angewendet.