Transposition des oxirannes-ethanols par l'intermediaire d'alcoxyetains. Influence de la configuration de l'oxiranne

The transposition of oxirane-ethanols, through alkoxytin compounds, into oxetane-2-methanols and/or oxolan-3-ols (tetrahydrofuran-3-ols) is dependent upon the oxirane configuration. Cis configuration is more suitable for the formation of the smallest ring. Steric hindrance is not sufficient enough to explain the results.     

Transposition des oxirannes-ethanols par l'intermediaire d'alcoxyetains

Oxiraneethoxytributyltins prepared from the corresponding oxiraneethanols, on heating at ～ 200° gave, after demetalation with isophthallic acid, 2-oxetanemethanols and/or 3-oxolanols. As appears from about thirty rearrangements the choice between oxetane and oxolane formation is dependent on: (1) the relative degree of substitution of the oxirane ring; cyclization occuring predominantly at the more substituted carbon; and (2) the configuration of the oxirane ring, when both its ends are equally substituted; cis form being more suitable for generation of the smaller ring. The reaction is shown to proceed with inversion of configuration at the site of oxygen attack. The results of attempts to perform the rearrangement in dilute-phase or through alkaline metal alkoxides in various media support the conclusion that there is a large contribution by electrophilic assistance to the oxirane opening. Such assistance can be efficiently provided by a tin atom in a push-pull mechanism which accomodates all the facts. The present method of oxiraneethanol rearrangement may offer a convenient route to functional oxetanes.     

Etude de la structure des oligomeres de l'isoprene desactives par les halogenures d'alcoyle

The deactivation of oligoisoprenyllithium RM_n^?, Li⁺ by an alkyl halide R′X yields two kinds of products: RM_nR′ and RM_nR; the microstructure of the isoprenic units in the oligomer is mainly 4,1 and 4,3 and it is determined by the addition to the active chain of an isoprene molecule or of the alkyl halide molecule. The microstructures of RM_nR′ and of RM_nR are different. The results indicate two kinds of halogen-lithium reactions leading to the products RM_nR: between the excess of initiator RLi and the alkyl halide or between oligoisoprenyllithium and the alkyl halide.     

Etude d'un schema de synthese peptidique intramoleculaire a l'aide de derives du phosphore tetraedrique

A new scheme of repetitive and controlled peptide synthesis offers two advantages compared to the usual methods of synthesis : two steps only are used for the prolongation of a peptide chain with an aminoacid residue and the risk of racemisation is minimized. The two postulated steps are verified particularly by the rearrangement, in an alkaline alcoholic medium, of phosphordiamides 1 incorporating one amino-acid residue, leading to the formation of the peptide derivatives 6 and 8. The severe limitations of this method are discussed.     

Etude cinetique de la polymerisation cationique du cyclopentadiene amorcee par l'hexachloroantimoniate de triphenylmethyle

A study of cyclopentadiene polymerization, initiated by φ₃C⁺SbCl₆^? in methylene chloride solution, has been carried out at temperatures between ?70 and +20° using a dilatometric method. An overall external second order with respect to monomer has been found. At very low temperature (?70°), the concentration of active centres remains low and roughly constant, in agreement with a quasi-stationary state assumption. Between ?50 and + 10°, experimental determination of (k_p. M^*), obtained from variation of v_p and [M] with time, shows that the concentration of centres goes through a maximum, sharper and more rapidly reached as the temperature is raised. Initiation is slower than propagation and active centres are rapidly destroyed when termination becomes faster than initiation. This explains the partial conversions and the observed maximum for concentration of active centres. Propagation and unimolecular termination rate constants have been determined at each temperature: activation energies are E_p = ?8 ± 0·5 kcal mole^?1 and E_p = ?0·3 ± 0·1 kcal mole^?1. These negative values can be explained by an exothermic process of solvation of active centres, leading to more reactive propagating species.     

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.     

Etude de l'equilibre entre l'oxyde de nickel et l'oxygene. Essai d'interpretation des lois de variation de la conductivite electrique en fonction de la temperature

In a previous paper we have shown that at 740°C and under an oxygen pressure less than 1,3 torr the electrical conductance G of a NiO powder reaches rapidly an apparently stabilized signal G_i which sharply decreases after a sufficiently long time to a G₀ value independent of P_O2.From this observation we have studied the influence of the temperature successively on the electrical conductance G_i and G₀. The Arrhenius law is only verified for the G₀ = f(T) curve. G₀ appears as the representative value of the gas-solid equilibrium.     

Etude quantitative par absorption optique de l'homogeneite de monocristaux d'arseniure de gallium

The optical absorption coeffient of n-type gallium arsenide is measured at many places of samples with an infrared microscope (spot of 100 × 100 μm) and variable energies for the incident light. The inhomogeneities we observed are associated with free carrier density fluctuations in heavily doped single crystals and with the impurities diffusion around dislocations in lower ones.     

Etude de la protolyse du cryptate d'iode positif

The diprotonated cryptand ∣2.2.2 H₂∣²⁺ is formed by the addition in nitromethane 1 of two equivalent of trifluoroacetic acid (or hydroiodic acid) to the cryptate ∣2.2.2 I∣⁺, I^?; the reaction is accompanied by the release of molecular iodine. Upon adding only one equivalent of trifluoroacetic acid, no release of iodine is observed and the formation of complex ∣H - 2.2.2- I∣²⁺ is deduced from NMR, UV and conductance measurements.     

Etude de l'aromaticite par la theorie de hückel

Dewar theory and Breslow theory of aromaticity in contrast to Kruszewski-Krygowski theory, do not regard aromaticity as a chemical property, resulting in numerous contradictions between experimental behaviour of hydrocarbons and theoretical expectations are observed. Two new indexes can be introduced into the Kruszewski-Krygowski's theory,² one concerned with hydrocarbon stability, the other with its aromatic character.     

Etude par thermogravimetrie,analyse thermique differentielle et spectrographie d'absorption infra-rouge de la reaction du peroxyde de sodium sur l'oxyde de Beryllium

Thermogravimetric and differential thermal methods as well as chemical analysis show that sodium peroxide attacks beryllium oxide to form beryllate, apparently in the molecular ratio 1 Na₂O:1 BeO. An unidentified peroxy compound of beryllium is formed at about 165° by the action of the NaO² present in the Na₂O₂ sample; it decomposes between 280 and 310°. IR absorption spectroscopy indicates the presence of Be-O bonds in the sodium beryllate which are different from those in the oxide used.     

Synthese de la cyclo-5'-8 desoxy-5' adenosine: stereochimie et mecanisme de la cyclisation d'un derive de l'iodo-5' adenosine par le zinc

N,N'-Dibenzoyl 2', 3'-O-isopropylidene 5'-deoxy 5'-iodo adenosine can efficiently transformed into the corresponding 5',8-cyclo-7,8-dihydronucleoside by zinc in pyridine. Only one diastereoisomer is obtained. By spin decoupling and NOE techniques at 250 MHz, the R-configuration at C-8 was established. This compound showed typical dihedral angles of about 90° for the vicinal protons H-1', H-2' and H-3', H-4'. Using adenosine derivatives stereospecifically labelled at C-5', it was demonstrated that the cyclisation occurs with racemisation at that position.     

Evidence d'un intermediaire carbonium dans la methoxybromation du cyclooctene cis

The formation of transannular products in the bromination of cis-cyclooctane by NBA in MeOH is taken as direct evidence for the existence of a carbonium ion intermediate.     

Etude cinetique de la copolymerisation alternee de l'indene et du methacrylate de methyle en presence de sesquichlorure d'ethylaluminium

The copolymerization of indene with methyl methacrylate (MAM) initiated by u.v. irradiation, in the presence of ethylaluminium sesquichloride (SCEA) as a complexing agent, yields alternating copolymers when [Indene] ≥ [MAM] ≥ [SCEA]. Values of $\text{M}$_n of the copolymers range from 32,000 to 900,000, depending on the concentrations of the reagents, reaction temperature and intensity of u.v. light. The kinetics can be explained simply by assuming a statistical copolymerization of indene with a MAM-SCEA binary complex, but intervention of a ternary complex SCEA-MAM-indene cannot be discarded. At high MAM concentrations, the free MAM becomes involved in the copolymerization.