Nouveaux exemples d'acétonides d'aldéhydosucres furanniques

Further examples of O-isopropylidenaldehydosugars in the furanose series Formyl-bearing furanose derivatives 1–3 (α-D -xylo or α-D -erythro configurations) and 5–8 (configurations α-D -ribo, α-D -lyxo, β-L -threo and D -arabino) have been prepared by classical synthetic steps and their properties mainly spectroscopic, reported. The coupling constant between the formyl and the vicinal proton is always small. Like other members of the series, these new aldehydosugars constitute very useful and flexible synthetic intermediates.     

Preparation of new monomers aza-β-aminoacids for solid-phase syntheses of aza-β-peptides

The preparation of new N^β-Fmoc-protected aza-β³-amino acids (aza-β³-aa) with proteinogenic side chains as well as their N^β-Fmoc, N^β-Cbz or N^β-Boc aza-β³-amino esters (from Pro, Asn, Asp, Glu, Gln) by successive nucleophilic substitutions will be described.     

Enantioselective Diels-Alder reactions with N-hydroxy-N-phenylacrylamide

[reaction: see text] The use of hydroxamic acids as templates for Lewis acid catalyzed enantioselective Diels-Alder reactions has been examined. A very simple chiral Lewis acid, prepared by mixing optically pure binaphthol with 3 equiv of trimethylaluminum, catalyzes the [4 + 2] cycloaddition of N-hydroxy-N-phenylacrylamide with cyclopentadiene at 0 degrees C in high yield (>96%) and with a fairly good level of enantioselectivity (91% ee). Facile conversion of the products to the corresponding alcohols or aldehydes makes the hydroxamic acid intermediates particularly useful.     

Synthesis of models of metabolites: Oxidation of variously substituted chromenes including acronycine,by a porphyrin catalytic system

The influence of chemical neighbouring on oxidation of substituted 2,2‐dimethylchromenes derivatives 5‐8 by a biomimetic catalytic system was first studied. It was then applied to acronycine an anti‐cancer drug in order to obtain in one single step oxidized products resulting from the reactivity of the 1,2‐double bond in the pyranic D‐ring. These 2,2‐dimethylchromenes constitute the structural moiety responsible for the activity of acronycine. This oxidation showed the sensitivity of the ethylenic bond, leading to the formation of the corresponding epoxides, diols and/or ketoalcohol. In the case of 5‐dimethylamino‐2,2‐dimethylchromene 8 , the double bond was not sensitive to oxidation, but the N‐methyl groups reacted to lead to the formamide derivative 16 and an imino‐alcohol 17 . This methodology applied to acronycine molecule 1 , allowed to obtain in one step, two oxidized compounds, a trans‐diol 3 and a ketoalcohol 4 under preparative conditions.     

Novel "Potentially antiaromatic", acidichromic quinonediimines with tunable delocalization of their 6 pi-electron subunits

We present a novel family of "potentially antiaromatic" alkyl-substituted p-benzoquinonediimine pH-dependent chromophores. It appears from the structural data that these overall 12 pi-electron molecules should be better considered as constituted by two chemically connected but electronically not conjugated 6 pi-electron subunits. Molecule 5 appears to be the first example of two separated, conjugated, and localized 6 pi-electron systems that can be tuned by reversible protonation to become delocalized. The mono- and diprotonated derivatives have been characterized by spectroscopic methods and X-ray diffraction. These systems develop supramolecular interactions in the solid state that clearly reflect the degree of protonation and depend on the nature of the counterion. These compounds constitute new chromophores for which the color can be tuned depending on the degree of protonation, going in solution from yellow for 5 to red for 5.HCl and blue for 5.2HCl. Theoretical calculations have provided a deeper insight into the electronic structure of these molecules and allowed an assignment of the experimental UV-vis spectra. The visible and near-UV spectrum of the neutral and protonated benzoquinonediimines can be classically assigned from the coupling of two 6 pi-electron polymethine units. TD-DFT calculations confirm the observed red shift of the two lowest pi --> pi* transitions of the benzoquinonediimines upon protonation and relate it to the moderate energy lowering of the HOMO --> LUMO transition induced by the delocalization of the polymethine pi system.     

Crystallization-induced asymmetric transformation of chiral-at-metal ruthenium(II) complexes bearing achiral ligands

Recently, we observed that the enantiopure Lambda form of the tributylammonium salt of the chiral anion tris[tetrachlorobenzene-1,2-bis(olato)]phosphate, also named Trisphat, was able to induce an efficient resolution of a Delta,Lambda racemic mixture of cis-[Ru(dmp)2(NCCH3)2](PF6)2 (dmp=2,9-dimethyl-1,10-phenanthroline) due to the spontaneous and selective precipitation of the heterochiral pair [Delta-Ru(dmp)2(CH3CN)2][Lambda-Trisphat]2. We report here that the combination of such a stereoselective precipitation process and irradiation results in the quantitative conversion of the initial [Ru(dmp)2(NCCH3)2]2+ racemate into only one of the two enantiomers. This is the first example in inorganic chemistry of an asymmetric transformation that leads to a chiral complex with no chiral ligand. Finally, three new racemic ruthenium bis(diimine) complexes, namely [Ru(dmp)2(NCCH3)Py](PF6)2 (Py=pyridine), [Ru(dmp)2(1,3-diaminopropane)](PF6)2, and [Ru(dmp)2(ethylenediamine)](PF6)2 were synthesized. For all of them, crystallization-induced asymmetric transformation proved to be an efficient way of obtaining the corresponding optically active chiral-at-metal complexes in high yields and with excellent stereoselectivities.     

Biomimetic zinc funnel complexes based on calix[6]N3ArO ligands: an acid-base switch for guest binding

The coordination chemistry of Zn in an N(3)ArOH environment has been explored. The ligands are based on calix[6]arenes that present two imidazole arms and an amino phenol moiety at the narrow rim. Three different types of complexes have been characterized. One is dicationic with Zn(2+) coordinated to the three nitrogen atoms and to the oxygen of the phenol group of the calix[6]ligand. This complex is very sensitive to exogenous coordinating molecules and exists as a 5-coordinate species due to the endo-complexation of a guest. The second species is a monocationic complex for which the phenol group has been deprotonated. The resulting N(3)ArOZn complex can also bind a guest ligand albeit with a lower affinity than the dicationic complex. The third species is neutral. It can be obtained upon reaction with a base to yield a hydroxo complex or with an anion such as a chloride that coordinates the metal center from the outside of the calixarene cavity. The simultaneous binding of two anionic donors decreases the Zn Lewis acidity, allowing an impressive conformational reorganization of the system. One imidazole arm is released by the metal center. The other one undergoes self-inclusion into the pi-basic calixarene cavity because the low affinity of the metal center for neutral ligand does not allow the endo-coordination of an exogenous guest. Hence, the calix[6]N(3)ArOH-based Zn complexes act as an acid-base switch for guest binding. Several aspects of this system appear reminiscent of Zn-peptidases of the astacin and serralisin families.     

Radical Cation States of 2,3,5,6,7,8-Hexamethylidenebicyclo [2.2.2]octane

The photoelectron spectrum of the title compound is reported and assigned by correlation with the photoelectron spectra of related molecules.     

Dinuclear Complexes with Predictable Magnetic Properties

When two paramagnetic transition metal ions are present in the same molecular entity, the magnetic properties can be totally different from the sum of the magnetic properties of each ion surrounded by its nearest neighbors. These new properties depend on the nature and the magnitude of the interaction between the metal ions through the bridging ligands. If both ions have an unpaired electron (e.g. Cu²⁺ ions), then the molecular state of lowest energy is either a spin singlet or a spin triplet. In the former case, the interaction is said to be antiferromagnetic, in the latter case ferromagnetic. The nature and the order of magnitude of the interaction can be engineered by judiciously choosing the interacting metal ions and the bridging and terminal ligands, and, thus, by the symmetry and the delocalization of the orbitals centered on the metal ions and occupied by the unpaired electrons (magnetic orbitals). The first success in this “molecular engineering” of bimetallic compounds was in the synthesis of a Cu²⁺VO²⁺ heterobimetallic complex in which the interaction is purely ferro-magnetic. The same strategy could be utilized for designing molecular ferromagnets, one of the major challenges in the area of molecular materials. Another striking result is the possibility of tuning the magnitude of the interaction through a given bridging network by modifying the nature of the terminal ligands, which, in some way, play the role of “adjusting screws”. By careful selection of the bridging and terminal ligands, a very large antiferro-magnetic interaction can be achieved, even if the metal ions are far away from each other. Some sulfur-containing bridges are especially suitable in this respect.     

The singlet—triplet accidental degeneracy versus two local doublets problem in copper(II) dimers

In a copper(II) dimer, two situations, I and II, may lead to a J = 0 singlet—triplet energy gap. In situation I, the metal ions do not interact and all the contributions to J vanish. In situation II, the metal ions interact, but the positive and negative contributions cancel. The aim of this letter is to specify the differences between these situations, to analyse how they can be distinguished from a physical point of view, particularly from EPR spectroscope, and to propose some typical geometries corresponding to both situations.