On the reactivity of V(η-C6H3Me3-1,3,5)2I

V(η-C₆H₃Me₃-1,3,5)₂I reacts with reducing agents such as MeLi, Na[AlH₂(OCH₂CH₂OCH₃)₂] or Na to yield the neutral complex V(η-C₆H₃Me₃-1,3,5)₂ in 70–75% yield. Reaction of V(η-C₆H₃Me₃-1,3,5)₂I with compounds containing suitable donor atoms such as THF, P(OMe)₃, CH₃CN or Py yields V(η-C₆H₃Me₃-1,3,5)₂ in 40–45% yield via disproportionation. The complex V(η-C₅H₅)₂I was isolated in low yield, together with V(η-C₆H₃Me₃-1,3,5)₂ from the reaction of V(η-C₆H₃Me₃-1,3,5)₂I with Cp₂Mg in toluene. Reaction of V(η-C₆H₃Me₃1,3,5)₂I or V(η-C₆H₃Me₃-1,3,5)₂ with allyl chloride leads to oxidation of the metal and loss of both mesitylene ligands.     

A novel enantiospecific route to 10-hydroxyfenchone: a convenient intermediate for C(10)-O-substituted fenchones

A novel, highly efficient enantiospecific preparation of 10-hydroxyfenchone from commercially available fenchone is described. The three-step synthetic route is based on two consecutive Wagner–Meerwein rearrangements of the fenchone skeleton, taking place with a high overall yield. This straightforward access to optically active 10-hydroxyfenchone, together with the scope for further functionalisation of the hydroxyl group, makes 10-hydroxyfenchone a convenient key intermediate to other optically active C(10)-O-substituted fenchones, which are analogues of well-known C(10)-O-substituted camphor-derived chiral sources.     

Voltammetric determination of 2-nitrophenol at a bentonite-modified carbon paste electrode

A bentonite-modified carbon paste electrode has been applied to the determination of 2-nitrophenol by differential pulse voltammmetry. The electrochemical reduction of 2-nitrophenol at –0.8 V is carried out in an artificial sea water-formic acid/sodium formate medium at pH 4. The peak height was found to be dependent on the pH over the range 2–11; the presence of a secondary process was observed in the pH range 8–11. The peak potential showed a dependence on pH, with two linear regions with different slopes. A linear relationship between peak intensity and concentration was obtained in the range 0.07–10 mgl^–1, with a detection limit of 0.03 mg 1^–1 and a coefficient of variation of 1.3% at 5 mg 1^–1. The effects of organic and inorganic species on the 2-nitrophenol determination were studied with a view to testing the resolution of the voltammetric technique. The proposed method has been applied to sea water samples with good results.     

Kinetic study on the synthesis of some benzo[g]quinoxalin-2(1H)-one derivatives

Some novel 3-substituted benzoquinoxalinones [R = H, CH₃, C₆H₅, (CH₂)₂COOH] were synthesized by the Hinsberg reaction between 2,3-diaminonaphthalene and several α-dicarbonyl compounds. The course of the reactions was followed by the second uv/visible Derivative Spectroscopy Method at different pH values (-0.89 to 9.0) and also in organic solvents at 25°. The compound non-substituted at C-3 was the only one that could be obtained in every media in good yields (80%), having pseudo first-order anelation rate constants of relative high values (1 × 10^?1 — 1 × 10^?2 min^?1). On the other hand, only methanol could be used as the organic solvent for the synthesis of all of the other compounds; aqueous media always provided better results. In the 3-methyl derivative, as well as in the 3-phenyl derivative the change of the reaction pH medium modified the stoichiometry of the anelation, turning a non-quantitative reaction into a quantitative one. This is explained by a change in the mechanism of the reaction on going to lower hydrogen concentrations, a fact that was supported by complementary quantitative hptlc experiments. In general, pseudo first-order rate constants for the anelation were one or two logarithmic units lower than those of the non-substituted compound (R_C-3 = H), but yields were above 60% in every case. A reaction scheme is presented which includes a probable mechanism.     

An exhaustive conformational analysis of N-acetyl-L-cysteine-N-methylamide. Identification of the complete set of interconversion pathways on the ab initio and DFT potential energy hypersurface

The full conformational space of N-acetyl-l-cysteine-N-methylamide was explored by ab initio (RHF/ 6-31G(d)) and DFT (B3LYP/6-31G(d)) computations. Multidimensional conformational analysis predicts 81 structures in N-acetyl-l-cysteine-N-methylamide, but only 47 relaxed structures were previously determined at the RHF/3-21G level of theory. These structures were now optimized using RHF/6-31G(d) and B3LYP/6-31G(d) approaches. Seven conformational migrations were observed when recalculated at higher level of theory. Besides these major changes, only smaller conformational shifts were operative for the remaining stationary points. The exploration of the whole conformational space of N-acetyl-l-cysteine-N-methylamide, including the transition-state structures allowing the conformational interconversion among the low-energy forms, was analyzed in this study. Our results offer new insights into the influence of polar side chains on the conformational preferences of peptide structures.     

Unexpected reactivity of 1-amine-2-methylenenorbornane hydrochlorides with m-CPBA

Two different 1-amino-3,3-dimethyl-2-methylenenorbornane hydrochlorides, a primary ammonium chloride and a tertiary one, react unexpectedly with m-CPBA (meta-chloroperbenzoic acid) according to two different paths. The primary ammonium chloride gives place to a diastereomeric mixture of the corresponding spiranic 1-nitronorbornane-based epoxides, whereas the tertiary derivative undergoes a skeleton rearrangement giving 10-chlorocamphor. The results are interpreted in terms of competitive reaction pathways controlled by the nitrogenated group located at the C1 norbornane position.     

Probiotics Production: An Interesting Example for the Undergraduate Analytical Chemistry Laboratory

An analytical chemistry laboratory experiment for undergraduates that combines traditional analytical chemistry techniques with the interdisciplinary field of biotechnology is presented. It involves a process in which the combination of a yeast (Schizosacaromyces pombe) and a bacteria (Acetobacter xylimun), usually named Kombucha, in a very simple culture of commercial black tea and glucose produces metabolites that are sampled and quantified by students. Students determine the biomass and quantify the production of several organic acids, and the consumption of glucose. This is an interdisciplinary project where students apply many tools of the general analytical process and combine results obtained by all the participating students to learn the characteristics of a biotechnological process. Overall, the system discussed is easy to implement because the selected micro-organisms are not pathogenic, are safe and easy to manipulate, and are resistant to contamination.     

Thermal Aggregation of Methyl Cellulose in Aqueous Solution: A Thermodynamic Study and Protein Partitioning Behaviour

The formation of a biphasic system from aqueous solutions of methyl cellulose induced by temperature was studied through heating curves of the polymer solution measured by absorbance spectroscopy, differential scanning calorimetry and solution viscosimetry. The treatment of the heating curve data according to a reversible two-state transition model allows us to calculate the middle point temperature (T_m) of the formation of the two phases and the thermodynamic functions associated to the polymer aggregation. The middle point temperature was found within the range 50–70 °C. It decreased significantly in a Na₂SO₄ 0.3 M medium when the polymer concentration increased. The heat associated to the two-phase formation was positive and it increased with increases in temperature. Cosolutes that affect the water structure induced changes in the T_m values, which suggests the presence of a hydrophobic effect in the two-phase formation from the polymer solution. Hydrophilic proteins were partitioned in favour of the methyl cellulose rich phase according to their surface hydrophobicity. The partition was also influenced by the presence of salts that modify the protein hydrophobicity such as sodium sulphate.     

A chemical circular communication network at the nanoscale

In nature, coordinated communication between different entities enables a group to accomplish sophisticated functionalities that go beyond those carried out by individual agents. The possibility of programming and developing coordinated communication networks at the nanoscale—based on the exchange of chemical messengers—may open new approaches in biomedical and communication areas. Here, a stimulus-responsive circular model of communication between three nanodevices based on enzyme-functionalized Janus Au–mesoporous silica capped nanoparticles is presented. The output in the community of nanoparticles is only observed after a hierarchically programmed flow of chemical information between the members.

A community of three nanodevices communicates through a hierarchically programmed circular flow of chemical information between members.     

Synthesis of seven-membered oxacycles. Part 2: The furan approach

We describe an efficient new approach for the synthesis of seven-membered oxacycles that is based on the oxidation of a furan ring with singlet oxygen followed by an intramolecular Michael addition.