Syntheses stereoselectives des isomeres cinetiques et thermodynamiques d'alcoxy-spirocetals et de trioxa-bis-spirocetals

The photolysis of tetrahydropyranic ketoacetals gives a mixture of diastereoisomericspiroketals or bis-spiroketals. The kinetic diastereoisomers are preferentially formed at low temperature ; acid isomerisation produces the thermodynamic diastereoisomers.     

Etude de la reactivite d'ylures de sulfonium et de pyridinium stabilises par un groupement cyanoformyle

The sulfonium and pyridinium ylides stabilized by a cyanoformyl group, in an acidic medium, lead to sulfonium and pyridinium salts which are equivalents of pyridinium or sulfonium ketene salts. This originality has been used to prepare new stabilized ylides, or derivatives of substituted arylacetic acids or 4-hydroxy thiazoles.     

Spectres de vibration et structure d'un cristal d'amidogallate de sodium

Raman spectra (4000-150 cm^?1) of a single crystal of NaGa(NH₂)₄ and infrared spectra (4000-200 cm^?1 ) of a polycrystalline sample have been studied at different temperatures. An assignment of the bands is given. The spectra are discussed assuming S₄ and T_d point group symmetry of the Ga(NH₂)^?₄ ion at low temperature and at room temperature respectively. Metal-ligand and N-H stretching frequencies are compared to those of some other amido metalates.     

Comportement sous impact electronique d'allenes ω-fonctionnels et des acetyleniques isomeres. Mise en evidence du transfert d'un chlore et d'un hydroxyle par un rearrangement de type mclafferty a l'aide de la spectrometrie cid-mike.

This paper describes the use of collision induced dissociation spectra in structural elucidation of C₄H₅X^+. ions, formed by chlorine or hydroxyl group migration in a McLafferty type rearrangement of the molecular ion of (ω-functionnalized allenes and their isomeric acetylenes.     

Reactivite du tetraallyletain et d'allyltrialkyletains vis a vis d'aldehydes,de cetones et d'epoxydes

Tetraallytin reacts readily with non activated ketones and exothermically with aldehydes contrary to other allylic organotins such as allytributyltin. Homoallylic alcohols are obtained after acidolysis of the adducts. Upon extended heating, allylic organotins and epoxides form products which correspond to additon products of isomeric carbonyl compounds. However, starting from cis- and trans-1-phenyl-1,2-epoxypropanes, direct regiospecific but non-stereospecific ring opening is observed with poor yields.     

Diverses methodes d'analyse des alliages d'etain et de samarium

Several methods of analysis for tin-samarium alloys are studied: chemical analysis, X-ray fluorescence and arc spectrography. Their characteristics and the results obtained are compared. The chemical methods are most precise but are insensitive and tedious. X-Ray fluorescence is suitable for routine work and for control of alloy homogeneity. Arc spectrography is by far the most sensitive method The methods can be applied to alloys of other rare earths with tin.     

Transitions dans une serie de clathrates de thiourée et d'urée avec encages cycliques

Crystallographic and thermal studies have been made on the adducts of thiourea with cyclohexane, cycloheptane, cyclooctane, and tetrahydropyran over the temperature range ?170 to 25°C. All the samples exhibit several transitions, which have been classified. Results from previous work are recalled in order to discuss the possible nature of the transitions.     

Synthese d'alkenyl et de cycloalkenyl polyflones

A method of synthesis of alkenyl and cycloalkenyl polyflones is described. It involves the reaction of a perfluoro or polyfluoro organometallic compound with an α-chloro alkyl or cycloalkyl thiocyanate. The obtained sulfide is oxidized into sulfone with m-chloro-perbenzoïc acid. Then this sulfone is transformed into vinylpolyflone by dehydrochloration.     

Enolates de l'acetylacetate d'ethyle. Structure et reactivite du sel de potassium en presence d'ether couronne et de cryptand

It is shown that, through the addition of 18-crown-6 or cryptand (2.2.2) to potassium ethyl acetoacetate enolate solutions in tetrahydrofuran, 1:1 complexes are formed. A single crystal of the 1:1 potassium énolate-18-crown-6 complex has been obtained. Its structure has been determined by X-ray diffraction. The crystal includes entities formed from an enolate anion chelating a potassium cation externally complexed by the crown-ether. The vibrational spectrometry shows that the structure of the entity is kept in solution. In the species formed through the addition of cryptand (2.2.2), the enolate anion has a structure (IR spectroscopy) and a reactivity very close to that of the free anion, observed in a dissociating solvent (DMSO, HMPA). When crown ether is added, a contact ion pair is formed, in which the cation is externally solvated by the crown. On the other hand, the cation encapsulation by the cryptand leads to a released anion with an “ S-trans” or “W” structure. The reactivity and the orientation of the alkylation reactions of these entities have been measured in THF solutions. They are discussed in relation to the structure of the species present in the reaction medium.     

Action comparée de l'eau oxygénée et d'un peracide sur un sel d'immonium pyrrolinique stéroïdique et sur l'énamine correspondante. Formation et propriétés d'un sel d'oxaziridinium

The reaction of a pyrrolinium salt with a peracid affords an oxaziridinium salt. The corresponding enamine reacts similarly to give products oxidized at the α-position to the nitrogen atom. These results are rationalized in terms of the formation of an intermediate oxaziridinium ion. The oxaziridinium salt reacts with nucleophiles to give either an carbinol iminium salt or products resulting from oxygen transfer.