Mecanismes de Fragmentation de l'Oxazole

The fragmentation mechanisms of oxazole have been studied in detail on using various experimental techniques (refocusing of metastable ions, deuteration, AP measurements) and by theoretical calculations.     

Structure (x-ray analysis) of Manicoline B,a mixture of two diastereoisomers of a new alkaloid from dulacia guianensis (olacaceae)

Manicoline B, isolated from the root bark of $\text{Dulacia guianensis}$ (Olacaceae), was shown by X-ray analysis to be an equimolecular mixture of two diastereoisomers of a new alkaloid and to have structure (3).     

Reactivity of stable trifluoroacetaldehyde hemiaminals. 2. Generation and synthetic potentialities of fluorinated iminiums

Under Lewis acid activation, hemiaminals of trifluoroacetaldehyde and related (fluoroalkyl)aldehydes generate iminium species that can react with various nucleophiles to provide fluorinated amines.     

Naphthopyranquinone Antibiotics: Novel enantioselective syntheses of frenolicin B and some of its stereoisomers

Two new enantioselective syntheses of the naphthopyranquinone antibiotic frenolicin B ( 1 ), of its enantiomer 2 , and of its diastereoisomers 3 and 4 were accomplished using two different routes from optically active β-Hydroxy esters (R)- and (S)- 11 and 18. β-Hydroxy esters (R)- and (S)- 11 were prepared stereoselectively from optically active sulfenylacetates (S)- and (R)- 10 , respectively (Scheme 2, Method A). Alternatively, compound 18 was obtained in excellent yield by enantioselective hydrogenation of the corresponding β-keto ester 17 , using a chiral ruthenium-complex catalyst (Scheme 3, Method B). Subsequently, compounds (S)- 11 and 18 were transformed into frenolicin B (1). In analogy, Stereoisomers 2–4 were prepared from (S)- and (R)- 11 in good yields.     

A dinuclear uranium(IV) complex of the chelating ligand 1,2,3,4‐tetra­methyl‐5‐(2‐pyridyl)­cyclo­penta­diene

The ligand 1,2,3,4‐tetra­methyl‐5‐(2‐pyridyl)­cyclo­penta­diene (cp*py) forms a dinuclear complex with U^IV, i.e. di‐μ‐oxo‐bis­{chloro­(diethyl ether‐κO)[(η⁵,κN)‐1,2,3,4‐tetra­methyl‐5‐(2‐pyridyl)­cyclo­penta­dienyl]uranium(IV)}, [U₂Cl₂O₂(C₁₄H₁₆N)₂(C₄H₁₀O)₂], in which cp*py acts as a chelating ligand, being bound to the metal atom by the cyclo­penta­dienyl unit and also by the N atom of the pyridyl ring.     

Optimization of a nitrogen analyser based on the Dumas method

The objectives of this work was to obtain a total nitrogen concentration in milk with the Dumas method for industrial and research applications. This method was faster than the Kjheldal method (5 min against 2 h or 3 h) but less precise. A three factor experimental design was performed to optimize the instrument, a FP-2000 supplied by LECO. The first interpretation of the experimental design was disappointing and no conclusion could be done. It was not a failure of experimental design but a lack of thinking on the physical and chemical aspect of the problem. Selection and construction of composite responses based on chemical and physical consideration were the keys to optimize the Dumas method. This method is now as precise as the Kjheldal method, but considerably faster.     

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.     

Bismuth triflate catalyzed Claisen rearrangement of allyl naphthyl ethers

Bismuth triflate was found to be an efficient catalyst for the Claisen rearrangement of allyl naphthyl ethers. The reaction proceeds smoothly with a catalytic amount of bismuth triflate (20 mol %) to afford the corresponding ortho-allyl naphthol in moderate to good yields in most cases.     

An electron transfer approach to the preparation of highly functionalized anthraquinones

A series of highly functionalized quinones was prepared by an original reaction of 2,3-bis(chloromethyl)-1,4-dimethoxyanthraquinone (6) with various nitronate anions under electron transfer reaction conditions.     

Disorder effects in Mn12–acetate at 83 K

The structure of hexadeca‐μ‐acetato‐tetra­aqua­dodeca‐μ₃‐oxo‐dodecamanganese bis(acetic acid) tetrahydrate, [Mn₁₂O₁₂(CH₃COO)₁₆(H₂O)₄]·2CH₃COOH·4H₂O, known as Mn₁₂–acetate, has been determined at 83 (2) K by X‐ray diffraction methods. The fourfold (S₄) molecular symmetry is disrupted by a strong hydrogen‐bonding interaction with the disordered acetic acid mol­ecule of solvation, which displaces one of the acetate ligands in the cluster. Up to six Mn₁₂ isomers are potentially present in the crystal lattice, which differ in the number and arrangement of hydrogen‐bonded acetic acid mol­ecules. These results considerably improve the structural information available on this molecular nanomagnet, which was first synthesized and characterized by Lis [Acta Cryst. (1980), B 36 , 2042–2046].