Squaric acid is a suitable building-block in 4C-Ugi reaction: access to original bivalent compounds

The 4C-Ugi reaction requires an acidic component as one of the building-blocks. We report here the first use of squaric acid for this reaction to access original symmetrical compounds. The scope and conditions of the reaction were explored.     

Supramolecular control of hetero-multinuclear polytopic binding of metal ions (Zn(II), Cu(I)) at a single calix[6]arene-based scaffold

A Calix[6]arene scaffold was functionalized to provide a tridentate binding site at the small rim and three bidentate chelate sites at the large rim of the cone to generate a heteropolytopic ligand. Its complexation to one equivalent of Zn(II) at the small rim yields a funnel complex displaying both host-guest properties and preorganization of the three chelate groups at the large rim. These two aspects allowed the full control of the binding events to regioselectively form dinuclear Zn(II) and heteropolynuclear Zn(II)/Cu(I) complexes. The heteropolynuclear systems all rely on the host-guest relationship thanks to the induced-fit behavior of the calix cavity. With the short guest MeCN, the large rim is preorganized into a trigonal tris-triazole core and accommodates a single Cu(I) ion. A long guest breaks this spatial arrangement, and three Cu(I) ions can then be bound at the tris-bidentate triazole-dimethylamine site at the large rim. In a noncoordinating solvent however, the tetranuclear complex is submitted to scrambling and the addition of exogenous π-acceptor ligands is required to control the binding of Cu(I) in a well-defined environment. Hindrance selectivity was then induced by the accessibility at the small rim site. Indeed, while CO can stabilize Cu(I) at both coordination sites, PPh(3) cannot fit into the cavity and forces Cu(I) to relocate at the large rim. The resulting well-defined symmetrical tetranuclear complex thus arises from the quite remarkable selective supramolecular assembly of nine partners (1 Zn(II), 3 Cu(I), 1 calixarene, 1 guest alkylamine, 3 PPh(3)).     

Kinetic Formulation for Systems of Two¶Conservation Laws and Elastodynamics

For scalar conservation laws, the kinetic formulation makes it possible to generate all the entropies from a simple kernel. We show how this concept replaces and simplifies greatly the concept of Young measures, avoiding the difficulties encountered when working in L ^p . The general construction of the two kinetic functions that generate the entropies of 2 × 2 strictly hyperbolic systems is also developed here. We show that it amounts to building a “universal” entropy, i.e., one that can be truncated by a “kinetic value” along Riemann invariants. For elastodynamics, this construction can be completed and specialized using the additional Galilean invariance. This allows a full characterization of convex entropies. It yields a kinetic formulation consisting of two semi-kinetic equations which, as usual, are equivalent to the infinite family of all the entropy inequalities. Accepted May 29, 2000?Published online November 16, 2000     

Hedetniemi's Conjecture and the Retracts of a Product of Graphs

We show that every core graph with a primitive automorphism group has the property that whenever it is a retract of a product of connected graphs, it is a retract of a factor. The example of Kneser graphs shows that the hypothesis that the factors are connected is essential. In the case of complete graphs, our result has already been shown in [4, 17], and it is an instance where Hedetniemi's conjecture is known to hold. In fact, our work is motivated by a reinterpretation of Hedetniemi's conjecture in terms of products and retracts.     

Development of Sustainable Chemistry in Madagascar: Example of the Valuation of CNSL and the Use of Chromones as an Attractant for Mosquitoes

This article describes a part of the results obtained from the cooperation between the University of Lyon1 (France) and the University of Antananarivo (Madagascar). It shows (among others) that useful research can be carried out in developing countries of the tropics if their social, technical, and economic conditions are taken into account. The concepts and methods associated with so-called “green chemistry” are particularly appropriated for this purpose. To illustrate this approach, two examples are shown. The first deals with industrial ecology and concerns waste transformation from the production of cashew nut into an amphiphilic product, oxyacetic derivatives. This product was obtained with a high yield and in a single step reaction. It exhibited an important surfactant property similar to those of the main fossil-based ones but with a much lower ecological impact. The second talks about chemical ecology as an alternative to insecticides and used to control dangerous mosquito populations. New substituted chromones were synthesized and showed biological activities toward Aedes albopictus mosquito species. Strong repellent properties were recorded for some alkoxylated products if others had a significant attractant effect (Kairomone) depending on their stereochemistry and the length of the alkyl chain.     

A 13C-Labeled Triarylmethyl Radical as an EPR Spin Probe Highly Sensitive to Molecular Tumbling

A stable triarylmethyl spin probe whose electron paramagnetic resonance (EPR) spectrum is highly sensitive to molecular tumbling is reported. The strong anisotropy of the hyperfine coupling tensor with the central carbon of a ¹³C₁-labeled triarylmethyl radical enables the measurement of the probe rotational correlation time with applications to measure microviscosity and molecular dynamics.     

Photoinduced Electron Transfer Involving a Naphthalimide Chromophore in Switchable and Flexible [2]Rotaxanes

The interlocking of ring and axle molecular components in rotaxanes provides a way to combine chromophoric, electron-donor and electron-acceptor moieties in the same molecular entity, in order to reproduce the features of photosynthetic reaction centers. To this aim, the photoinduced electron transfer processes involving a 1,8-naphthalimide chromophore, embedded in several rotaxane-based dyads, were investigated by steady-state and time-resolved absorption and luminescence spectroscopic experiments in the 300 fs–10 ns time window. Different rotaxanes built around the dialkylammonium/ dibenzo[24]crown-8 ether supramolecular motif were designed and synthesized to decipher the relevance of key structural factors, such as the chemical deactivation of the ammonium-crown ether recognition, the presence of a secondary site for the ring along the axle, and the covalent functionalization of the macrocycle with a phenothiazine electron donor. Indeed, the conformational freedom of these compounds gives rise to a rich dynamic behavior induced by light and may provide opportunities for investigating and understanding phenomena that take place in complex (bio)molecular architectures.     

Brillouin scattering of N(CH3)4 ZnCl4

The elastic behaviour with temperature near the commensurate incommensurate phase transition of [N (CH₃)₄ Zn Cl₄] have been investigated by Brillouin scattering and compared to ultrasonic results. Elastic and attenuation anomalies observed at 8 MHz are not found at Brillouin frequency. Photoelastic coefficients have been measured.