Proximal regioselectivity of the O,O'-dialkylation of tetrahydroxyketocalix[4]arene

The only dialkylated products obtained in the reaction of tetrahydroxyketocalixarene 2a with MeI/K(2)CO(3) or PhCH(2)Br/K(2)CO(3) are the corresponding proximal (i.e., 1,2) O,O'-dialkyl ethers, in contrast to the parent tetrahydroxycalix[4]arene 1a which affords the distal (1,3) dialkyl ether derivatives. Pairs of geminally alkylated phenoxy groups in the conformationally rigid dibenzylated and tetrabenzylated derivatives are oriented in an anti fashion. These results can be rationalized assuming that the 1,3-alternate arrangement of the rings preferred by 2a is adopted during all the intermediate stages of the alkylation. The NMR spectra (in CDCl(3)) of the monomethyl, dimethyl, trimethyl, and tetramethyl ether derivatives of 2a are in agreement with a 1,3-alternate conformation.     

Precision measurement of the 7Be(p,gamma)8B cross section with an implanted 7Be target

The 7Be(p,gamma)8B reaction plays a central role in the evaluation of solar neutrino fluxes. We report on a new precision measurement of the cross section of this reaction, following our previous experiment with an implanted 7Be target, a raster-scanned beam, and the elimination of the backscattering loss. The new measurement incorporates a more abundant 7Be target and a number of improvements in design and procedure. The point at E(lab)=991 keV was measured several times under varying experimental conditions, yielding a value of S17(E(c.m.)=850 keV)=24.0+/-0.5 eV b. Measurements were carried out at lower energies as well. Because of the precise knowledge of the implanted 7Be density profile, it was possible to reconstitute both the off- and on-resonance parts of the cross section and to obtain from the entire set of measurements an extrapolated value of S17(0)=21.2+/-0.7 eV b.     

The magnetic moment of the first excited state of 19O

The magnetic moment of the first excited 96 keV, $\text{3}\text{2}{}^{\mathrm{+}}$ state of ¹⁹O has been determined by an integral perturbed angular correlation measurement in a transverse external magnetic field. The value obtained for the g-factor, ?0.48(6), is discussed in terms of shell-model configurations. The wave function of the $\text{3}\text{2}{}^{\mathrm{+}}$ level is shown to consist mainly of the $\text{d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}{}^3$ configuration.     

Liquid chromatographic determination of multiple sulfonamides, nitrofurans, and chloramphenicol residues in pasteurized milk

A rapid and selective liquid chromatographic method was developed to detect 6 sulfonamides, 3 nitrofurans, and chloramphenicol residues in pasteurized milk. The 10 drugs were extracted with chloroform-acetone and the organic phase was evaporated; the residues were dissolved in an aqueous sodium acetate buffer solution 0.02M (pH = 4.8), and the fat was removed by washing with hexane. The aqueous layer was collected, filtered, and injected. The 6 sulfonamides and chloramphenicol were detected at 275 nm ultraviolet (UV) using a gradient system starting with sodium acetate buffer solution-acetonitrile (95 + 5) and finishing with sodium acetate buffer solution-acetonitrile (80 + 20). Nitrofurans were detected at 375 nm (UV) isocratically with sodium acetate buffer solution-acetonitrile (80 + 20). For 50 ppb fortified milk, the average recoveries were (sulfathiazole) 65.52%; (sulfamerazine) 75.36%; (sulfamethazine) 93.94%; (sulfachlorpyridazine) 75.94%; (sulfamethoxazole) 85.18%; (sulfamonomethoxine) 83.45%; (chloramphenicol) 104.17%; (nitrofurazone) 91.81%; (furazolidone) 100.76%; and (furaltadone) 72.38%. Method detection limits ranged from 4 ppb (nitrofurazone) to 16 ppb (sulfamethazine). Some matrix interferences (3-7 ppb) were observed only with sulfonamides.     

Ozonolysis of 1,4-cyclohexadienes in the presence of methanol and acid. Mechanism and intermediates in the conversion of 1,4-cyclohexadiene derivatives to beta-keto esters

Conditions for the preparation of beta-keto esters directly from 1,4-cyclohexadiene derivatives are described. This procedure is a further step in the application of the synthetic methodology, which consists of the combination of Birch reduction of available benzene derivatives followed by ozonolysis. In this work, the syntheses of derivatives of dimethyl gamma-keto-alpha-aminoadipate and dimethyl beta-keto glutamate from the corresponding 1,4-cyclohexadiene derivatives are described. The latter compounds are prepared from phenylalanine and phenylglycine, respectively. The study of the ozonolysis of simple alkyl derivatives of 1,4-cyclohexadiene in the presence of methanol, both in the presence and absence of acid, helped to establish the mechanism of this reaction. The proximity of the two double bonds, which are cleaved, leads to the intermediate formation of 1,2-dioxolane derivatives that could be identified by NMR spectroscopy. It is shown that regardless of the regioselectivity of the cleavage of the primary ozonide, which is formed, all 1,2-dioxolane derivatives can lead to beta-keto esters. This is due to the equilibrium between these dioxolanes in the presence of methanol and acid.     

Total synthesis of (+/-)-phomactin G, a platelet activating factor antagonist from the marine fungus Phoma sp

A total synthesis of phomactin G (), which is a central intermediate in the biosynthesis of phomactin A () in Phoma sp. is described. The synthesis is based on a Cr(ii)/Ni(ii) macrocyclisation from the aldehyde vinyl iodide, leading to, followed by sequential conversion of into the [small beta]-epoxide and the ketone which, on deprotection, led to (+/-)-phomactin G. Phomactin G () shares an interesting structural homology with phomactin D (), the most potent PAF-antagonist metabolite in Phoma sp. It is most likely converted into phomactin A (), by initial allylic oxidation to the transient [small alpha]-alcohol 'phomactin' structure, known as Sch 49028, followed by spontaneous pyran ring formation.     

Cytotoxic alkaloids motuporamines A-C: synthesis and structural verification

[formula: see text] The unusual structure and biological properties of the marine alkaloids motuporamines A-C, as well as the uncertainty as to the position of the olefin within the ring of motuporamine C, led us to synthesize these compounds. The strategy utilized the ring-closing metathesis reaction to form the 14- and 15-membered rings and Michael addition and amidation chemistry to introduce the spermine-like unit. The syntheses, structure assignment verifications, and also the determination of the position of the olefin in motuporamine C are described.     

Temperature-dependent and friction-controlled electrochemically induced shuttling along molecular strings associated with electrodes.

The temperature and solvent composition dependence of the electrochemically stimulated rate of shuttling of the redox-active cyclophane, cyclobis(paraquat-p-phenylene), on a molecular string has been studied. The molecular string includes a pi-donor diiminebenzene-site that is associated on one side with an electrode, and stoppered on the other side with an adamantane unit. The cyclophane rests on the pi-donor site, owing to stabilizing pi-donor-acceptor interactions. Electrochemical reduction of the cyclophane units, to the bis-radical cation cyclophane, results in the shuttling of the reduced cyclophane towards the electrode, a process that is driven by the removal of the stabilizing donor-acceptor interactions, and the electrostatic attraction of the reduced product by the electrode. The latter process is energetically downhill, and is temperature-independent. Upon oxidation of the reduced cyclophane that is associated with the electrode, the energetically uphill shuttling of the oxidized cyclophane to the pi-donor site proceeds. The rate of this translocation process has been found to be temperature-dependent, and controlled by the solvent composition. The experimental results have been theoretically analyzed in terms of Kramers' molecular friction model. The theoretical fitting of the experimental results, using solutions of variable composition, reveals that the rate-constants for the uphill reaction in a pure aqueous solution follow the temperature-dependence of the viscosity of water. The results demonstrate the significance of friction phenomena in shuttling processes within molecular machines.     

Stability of sulfonamides,nitrofurans, and chloramphenicol residues in preserved raw milk samples measured by liquid chromatography

A stability study was made of 10 antimicrobials: 6 sulfonamides, 3 nitrofurans, and chloramphenicol residues in raw milk samples preserved with 0.1 % potassium dichromate (K2Cr2O7) and 0.05% mercuric bichloride (HgCl2) during cold storage for 7 days. Preserved milk samples fortified with 50 ppb of each antimicrobial were analyzed by liquid chromatography (modified AOAC Method 993.32). Drugs were extracted with chloroform-acetone after solvent evaporation residues were dissolved with aqueous sodium acetate buffer solution (0.02M, pH 4.8), and fat was removed with hexane. Sulfonamides and chloramphenicol were detected at 275 nm (UV) by using a gradient system of sodium acetate buffer solution-acetonitrile starting at 95 + 5 (v/v) and finishing at 80 + 20 (v/v). Nitrofurans were detected at 375 nm (UV) isocratically with sodium acetate buffer solution-acetonitrile (80 + 20, v/v). Residues stability was measured through recovery data. Sulfamethoxazole, sulfachloropyridazine, nitrofurazone, furazolidone, and furaltadone residues remained stable in the presence of either preservative for 7 days. Sulfamethazine and chloramphenicol were not affected by K2Cr2O7, but had significant losses (p <0.05) when HgCl2 was used: 26.2 and 13.4%, respectively. Average recoveries of sulfamonomethoxine, sulfamerazine, and sulfathiazole significantly decreased by Day 7, with losses of 17.1, 17.2, and 23.2% for K2Cr2O7, and 23.3, 20.7, and 48.0% for HgCl2, respectively. During 5 days of cold storage all antimicrobials tested, except sulfathiazole, remained stable in milk samples preserved with 0.1 % K2Cr2O7 or 0.05% HgCl2.     

Electronic structure of copper phthalocyanine: a comparative density functional theory study

We present a systematic density functional theory study of the electronic structure of copper phthalocyanine (CuPc) using several different (semi)local and hybrid functionals and compare the results to experimental photoemission data. We show that semilocal functionals fail qualitatively for CuPc primarily because of underbinding of localized orbitals due to self-interaction errors. We discuss an appropriate choice of functional for studies of CuPc/metal interfaces and suggest the Heyd-Scuseria-Ernzerhof screened hybrid functional as a suitable compromise functional.