Ca at C82 isomers: computed temperature dependency of relative concentrations

Relative concentrations of nine isomers of Ca at C82 derived from the C82 isolated-pentagon-rule satisfying cages are computed in a wide temperature interval. The computations are based on the Gibbs energy constructed from partition functions supplied with molecular parameters from density functional theory calculations. Five structures show significant populations at higher temperatures: C2v > Cs > C2 > C3v > Cs. The computed relative stabilities agree well with available observations.     

Terpolymerization of sulfur dioxide or maleic anhydride with unsaturated compounds under ultraviolet irradiation

The effect of ultraviolet irradiation on the terpolymerization was investigated. In the terpolymerizations of sulfur dioxide–butene-1–acrylonitrile, sulfur dioxide–butene-1–n-butyl acrylate, and maleic anhydride–allyl chloride–acrylonitrile systems, the composition of the terpolymers prepared under ultraviolet irradiation was different from those prepared in the dark. The unit content of sulfur dioxide and butene-1 or of maleic anhydride and allyl chloride in the terpolymer increased under ultraviolet irradiation. The nature of the growing end under ultraviolet irradiation is supposed to be the same as that of the dark polymerization on the basis of the same solvent effect on the terpolymer composition, the rate of polymerization and the molecular weight of polymer. The experimental results suggest that the complex between sulfur dioxide and butene-1 or maleic anhydride and allyl chloride might be excited by ultraviolet light and the excited complex may participate in the terpolymerization.     

Vascular dilatory action of the Chinese crude drug. II. Effects of scoparone on calcium mobilization

Further experiments were conducted to examine the effect of scoparone (6,7-dimethoxycoumarin), a coumarin derivative found in the Chinese crude drug "Capillaris Flos," on calcium mobilization. Scoparone does not affect Ca2+ influx through the voltage-dependent channel due to membrane depolarization. Its inhibitory action may be dependent not only on the inhibition of Ca2+ release from sarcoplasmic reticulum but also on the inhibition of extracellular Ca2+ influx through the receptor-operated channel.     

Catalytic,asymmetric Mannich-type reactions of alpha-imino esters bearing readily removable substituents on nitrogen

[reaction: see text] Catalytic, enantioselective Mannich-type reactions of alpha-imino esters bearing readily removable substituents on nitrogen are described. Several N-carbamate-protected alpha-imino esters, which are readily prepared from 2-bromoglycine esters using a polymer-supported amine, reacted with silicon enolates to afford the desired adducts in high yields with high enantioselectivity using a copper(II)-diamine complex. Easy deprotection of the product amine and transformation to free alpha-amino acid derivatives have also been demonstrated.     

Transition metal salts-catalyzed aza-Michael reactions of enones with carbamates

Several transition metal salts were found to catalyze aza-Michael reactions of enones with carbamates efficiently. The catalytic activity was strongly dependent on the nature of the metal salts. While conventional Lewis acids such as BF(3).OEt(2), AlCl(3), or TiCl(4) showed lower activity, group 7-11 transition metal salts in higher oxidation states such as ReCl(5), Fe(ClO(4))(3).9H(2)O, RuCl(3).nH(2)O, OsCl(3).3H(2)O, RhCl(3).nH(2)O, PtCl(4).5H(2)O, or AuCl(3).H(2)O exhibited higher catalytic activity. [reaction: see text]     

Polymerization of β-cyanopropionaldehyde. IV. Polymerization by the adducts of β-cyanopropionaldehyde with triethylaluminum

Adducts (X, Y, and Z) between triethylaluminum and β-cyanopropionaldehyde (CPA) have been prepared and characterized. It was found that an equimolar amount of triethylaluminum undergoes Grignard type addition reaction with aldehyde group of CPA to give aluminum alkoxide and that another equimolar quantity of triethylaluminum undergoes coordination with the nitrile group of CPA (adduct X, in which the molar ratio of CPA to aluminum is 1:2). The coordinated triethylaluminum in adduct X may be changed to aluminum alkoxide by the addition of further equimolar amount of CPA (adduct Y, molar ratio = 1:1); on the other hand, heating at 130°C affords mixtures of aluminum aldimine and aluminum ketenimine structures (adduct Z, molar ratio = 1:2). From the cryoscopic measurement, adduct Z may be regarded as a coordinated polymer joined through bridged structures I and II. In the polymerization of CPA at ?78°C, the stereoregularity of the resulting poly-(cyanoethyl)oxymethylene was found to increase in the order: X < triethylaluminum < Y < Z. The polymerizations with triethylaluminum, X, and Y are considered to be initiated by NCCH₂CH₂CH(C₂H₅)? O? Al(C₂H₅)₂. The degree of association of the species may influence the stereoregularity of the polymer.     

Convenient synthesis of 3,3,3-trifluoropropenyl compounds from aromatic aldehydes by means of the TBAF-mediated Horner reaction

A simple synthesis of 3,3,3-trifluoropropenyl compounds by means of the TBAF-mediated Horner reaction is described. The reagent, 2,2,2-trifluoroethyldiphenylphosphine oxide, was readily prepared either by Arbuzov reaction of ethyl diphenylphosphinite with 2,2,2-trifluoroethyl iodide or by treating chlorodiphenylphosphine with trifluoroacetic acid and water. Treatment of the phosphine oxide with aromatic aldehydes in the presence of TBAF at room temperature afforded the corresponding 3,3,3-trifluoropropenyl compounds in good yields. The present method is very convenient for preparing 3,3,3-trifluoropropenyl compounds from aromatic aldehydes in terms of availability of the reagent, operational simplicity, and good yields of the products.     

Microdetermination of fluorine in organic compounds by oxygen flask combustion and conductometric titration

For the conductometric titration of fluoride with calcium acetate, solvent system in sample solution and titrant, concentration, and acidity of sample solution were examined to establish the titration conditions. Results of these examinations were transferred to the microdetermination of fluorine in organic compounds using oxygen flask combustion method.Comparative examination between quartz and Pyrex flasks for the combustion of fluorine samples indicated that good results were obtained by the use of the former flask, whereas the use of the latter one gave negative values of 1.0–1.6% due to the formation of boron fluoride during the combustion.     

Absolute stereochemistry of amphidinolide C

[structure in text] The absolute configurations at 12 chiral centers in amphidinolide C (1), a potent cytotoxic 25-membered macrolide isolated from a marine dinoflagellate Amphidinium sp., were determined to be 3S, 4R, 6R, 7R, 8R, 12R, 13S, 16S, 20R, 23R, 24R, and 29S by combination of NMR analyses, degradation experiments, and synthesis of the C-1-C-7 segment.     

Absolute stereochemistry of amphidinolides G and H

[structure: see text] Absolute stereochemistry of amphidinolides G (1) and H (2), potent cytotoxic 27- and 26-membered macrolides, respectively, isolated from a marine dinoflagellate Amphidinium sp., was determined by X-ray diffraction analysis, synthesis of a degradation product (3) of 2, and interconversion between 1 and 2.