Effect of adsorption of vapours on the electrical conductivity of some polyene semiconductors: adsorption and desorption kinetics

The change in semiconductive properties of β-apo-8′-carotenal, astacene and methyl bixin on adsorption of various vapours on the crystallite surfaces has been studied at a constant sample temperature. The adsorption of vapours enhances the semiconductivity of the polyenes appreciably. This enhancement depends on the chemical nature and also on the pressure of the adsorbed vapour. The adsorption and desorption kinetics follow the modified Roginsky-Zeldovich relation. A two stage desorption process, the first stage of which gives a Lennard-Jones potential energy curve and is followed by a rate-determining transition over a potential energy barrier to the second stage of adsorption forming weakly bound complexes between the vapour molecules and the polyene crystallites, can explain satisfactorily the experimentally observed kinetic data.     

Convenient route to enantiopure aryl cyclopentanes via Diels-Alder reaction of asymmetric dienes. Total synthesis of (+)-herbertene and (+)-cuparene

A general route for the synthesis of highly substituted aryl cyclopentanes has been developed involving Diels-Alder reaction of asymmetric dienes prepared from (+)-camphoric acid followed by aromatization of the resulting cyclohexene derivatives. Employing this protocol enantiospecific synthesis of (+)-herbertene and (+)-cuparene has been accomplished.     

Synthesis,Conformation, and Metabolism of a Selenium Bilirubin

Summary. A symmetrical C(10)-selena-bilirubin analog, 8,12-bis(2-carboxyethyl)-7,13-dimethyl-2,3,17,18-tetraethyl-10-selenabiladiene-ac-1,19(21H,24H)-dione was synthesized from 8-(2-carboxyethyl)-2,3-diethyl-7-methyl-(10H)-dipyrrin-1-one in one step by reaction with diselenyl dichloride. The selena-rubin exhibited UV-vis and NMR spectroscopic properties similar to those of the parent mesobilirubin, and like bilirubin and mesobilirubin, it adopts an intramolecularly hydrogen-bonded conformation, shaped like a ridge-tile but with a steeper pitch. The longer C–Se bond lengths (2.2Å) and smaller bond angles at C–Se–C (88°), as compared to C–CH₂–C (1.5Å, 106°), lead to an interplanar angle between the two dipyrrinones of only 72°, which is considerably less than that of bilirubin (100°) and close to that (74°) of its 10-thia-rubin analog. Despite the conformational distortion, the sensitivity of Se toward oxidation and the typically weak C–Se bond, the selena-rubin is metabolized in normal rats, like bilirubin, to acyl glucuronides, which are secreted into bile. In mutant (Gunn) rats lacking bilirubin glucuronosyl transferase (UGT1A1), glucuronide or other metabolites of the selena-rubin were not detected in bile, demonstrating the importance of hepatic glucuronidation for its biliary excretion.     

Intramolecular [2 + 2] photocycloaddition of alkenes incorporated in a carbohydrate template. Synthesis of enantiopure bicyclo[3.2.0]heptanes and -[6.3.0]undecanes

Intramolecular [2 + 2] photocycloaddition of alkenes with a furano sugar placed between them have been investigated under both copper(I)-catalyzed and sensitized conditions. The copper(I)-catalyzed photocycloaddition of the dienes 4a, 4b, and 4c led to unexpected formation of the thermodynamically less stable cis-syn-cis 4-5-5 tricyclic adducts 5a, 5b, and 5c, respectively. The sensitized photocycloaddition of the diene 14 also gave the cis-syn-cis adduct 15 showing that the copper(I) catalyst does not have any influence on the stereochemical course through coordination with the anomeric ring oxygen of the furano sugar. The identical stereochemical course observed under both catalyzed and sensitized photoaddition reactions have been attributed to be of steric origin. Bis(dienes) 25a and 25b, which gave an intractable mixture on copper(I)-catalyzed irradiation, underwent smooth photocycloaddition in the presence of benzophenone, and the resulting 1,2-divinyl cyclobutanes underwent spontaneous [3.3]-rearrangement at room temperature to produce bicyclo[6.3.0]undecanes 30a and 30b, respectively. This investigation provides an approach for the construction of enantiopure bicyclo[3.2.0]heptanes and -[6.3.0]undecanes.     

Stability of lyophobic sols

The coagulation of positively charged sol of hydrous ferric oxide of different average sizes for potassium sulphate has been studied in the presence of glucose and urea which stabilise the sol. The degree of stabilisation increases with the fineness of dispersed particles. It has been concluded that the adhesive tendency of the surface of dispersed particles decreases in the presence of said non-electrolytes leading to a decrease in the frequency of collisions resulting in stabilisation. The stability ratioW increases in the presence of glucose and urea for the same amount of the coagulant.

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Zusammenfassung Die Koagulation positiv geladener Sole von wäßrigem Ferrioxid verschiedener mittlerer Partikelgrößen mit Kaliumsulfat wird in Gegenwart von Glykose und Harnstoff, die beide das Sol stabilisieren, untersucht. Der Grad der Stabilisierung hängt von der Teilchengröße ab. Es wird geschlossen, daß die Adhäsionstendenz der Oberflächen der dispergierten Partikel in Gegenwart besagter Nichtelektrolyte abnimmt und so zur Abnahme der Kollisionshäufigkeit und damit zur Stabilisierung führt. Das StabilisierungsverhältnisW wächst in Anwesenheit von Glykose und Harnstoff für gleiche Beträge des Koagulators.

     

A density-functional calculation of dynamic dipole polarizabilities of noble gas atoms

Using a very simple trial function and unperturbed electron densities calculated by a new procedure, the frequency-dependent dipole polarizability () of Ne, Ar, Kr and Xe has been calculated in the range 0 0.45 a.u., by a Karplus-Kolker-type variation-perturbation method. Results progressively worsen for larger systems so that, for Xe, (0) is only 75% of the experimental value. Probable reasons for this are discussed.     

Ionic Interactions of Calcium Sulfate Dihydrate in Aqueous Sodium Chloride Solutions: Solubilities,Densities, Viscosities,Electrical Conductivities,and Surface Tensions at 35 <Superscript>∘</Superscript>C

The thermodynamic, volumetric, transport, and surface properties, solubilities, densities, viscosities, electrical conductivities, and surface tensions of calcium sulfate dihydrate in aqueous sodium chloride solutions have been measured at 35 C, with a view to determine the ionic interactions that occur in these solutions. The experimental density values have been used to calculate the mean apparent molar volumes of the ternary mixtures. Viscosity values have been analyzed using different empirical equations and the experimental values of the viscosity were combined with conductivity to yield the Walden product. Molar surface energies have been computed using experimental surface tension data. The experimental data have been fitted to polynomial equations by a least-squares analysis to obtain the coefficients and their standard errors. Results have been examined in the light of structure making or structure breaking effects of the various ions present in the solutions.     

Enantioselective total synthesis of (+)-amphidinolide t1

An enantioselective first total syntheis of amphidinolide T1 (1) is described. Amphidinolide T1 (1), a 19-membered macrolide isolated from Amphidinium sp., has shown potent antitumor properties against a variety of NCI tumor cell lines. The synthesis is convergent and involves the assembly of C1-C10 segment 2 and C11-C21 segment 3 by an oxocarbenium ion-mediated alkylation and Yamaguchi macrolactonization sequence. The synthesis of fragment 2 involves an efficient cross metathesis and hydrogenation sequence between the terminal olefins of 5 and 6 to form the C4-C5 carbon-carbon bond. Enol ether 4 is designed to be the surrogate of fragment 3 where the sensitive C16-exo-methylene and the C13-hydroxyl group were protected as the bromoether derivative during the Lewis acid-catalyzed alkylation process. Both stereocenters in fragment 5 as well as the C2 and C3 stereocenters in fragment 4 are accessed by a highly diastereoselective ester-derived titanium enolate-mediated syn-aldol reaction. The bromoether derivative 24 was unraveled at the final stage of the synthesis, providing (+)-amphidinolide T1.     

Total synthesis of antitumor depsipeptide (-)-doliculide

[reaction: see text] (-)-Doliculide, a potent antitumor agent, is synthesized stereoselectively in a convergent manner. The key strategy involves a stereoselective synthesis of the polyketide unit and synthesis of the D-tyrosine derivative, followed by assembly of the fragments by an esterification and cycloamidation reaction sequence. The synthesis of the polyketide fragment was achieved by an iterative asymmetric synthesis to install stereoselectively both 1,3-dimethyl groups and the 1,3-diol unit by utilizing asymmetric cyclopropanations and Sharpless asymmetric epoxidations as the key steps.