Selectivite de la reduction d'α-enones polycycliques par les borohydrures: effet de l'addition de tetramethyl-ethylenediamine au borohydrure de tetrabutylammonium

3^1,9octalone, 3^1,9-10-methyl octalone and testosterone were reduced by NBu₄BH₄. alone or in the presence of tetramethylethylenediamine (TMEDA), in THF and in toluene. With TMEDA, the first step of the reduction is the regioselective 1,4 attack by BH^?₄ which leads either to the saturated ketones or to the corresponding saturated alcohols. The results observed under different conditions were interpreted by the intervention of various reductive species: diborane, enoxyborohydrides in the absence of TMEDA, amine-borane in its presence.     

Influence de la solvatation par le tert-butanol sur la structure et la reactivite de l'enolate de potassium de l'acetylacetate d'ethyle en presence de couronne ou de cryptand. etude par spectrometrie infra-rouge et cinetique

The structure and the nucleophilic reactivity of crowned (18-crown-6) or cryptated {cryptand (2.2.2)} potassium ethyl acetoacetate enolate have been compared in tert-butanol and in DME (or THF). In the protic solvent tert-butanol, the crowned and the cryptated potassium enolate species both exist as loose ion pairs in which the enolate anion, strongly hydrogen-bonded to the solvent, is in a “transoid” (non chelating) conformation. Both species show similar reactivities towards alkylating agents but completely different reactivities are observed in aprotic weakly dissociating media (THF, DME). In contrast to what is observed in tert-butanol, the cryptated species and the crowned species have very different nucleophilic reactivities in THF or DME; in those solvents only the cryptated species retains a loose ion pair structure; the crowned species is a contact ion pair in which the enolate anion chelates the potassium cation. The solvation of this crowned chelate species by tert-butanol has been demonstrated in binary mixtures of solvents (C₆D₆-t-BuOH, THF-t-BuOH). The oxygen basicity of the enolate anion is very different in the crowned chelated ion pair compared with the cryptand separated ion pair.     

Reactions de wittig,wittig-horner et knoevenagel par activation anionique avec l'alumine ou le fluorure de potassium depose sur l'alumine,sans solvant

Wittig, Wittig-Horner and Knoevenagel reactions are achieved in the presence of alumina or potassium fluoride supported on alumina, without organic solvent. The presence of a small amount of water increases the rate of the Wittig and Wittig-Horner reactions. The reaction of the diethyl cyanomethylphosphonate with an aldehyde in the presence of dry alumina is oriented towards the Knoevenagel reaction.     

Polymerisation cationique de l'indene amorcee par le tetrachlorure de titane-I Influence des impuretes du solvant,le chlorure de methylene,et effets cocatalytiques de l'eau et de l'acide chlorhydrique

The polymerization of the system indene-titanium tetrachloride-methylene chloride gave irreproducible kinetics which have been attributed to variable concentrations of impurities in the solvent. Incomplete yields sometimes observed at ?70° are also caused by impurities, but introduction of water and hydrochloric acid then gives complete yields. A new method of solvent purification has been used giving reproducible yields and rates. In such cases, monomer consumption at ?70° follows the rate law log [M]_o/[M] = kt.     

Synthese et cyclisation electrophile de la bromhydrine de l'acide homogeranique

A new synthesis of 5-bromohexahydro-4,4,7a-trimethyl-2($\text{3H}$)-Benzofuranone (3aα,5β,7aβ)($\text{1}$) was achieved by electrophilic cyclization of the bromohydrin issued from pure homogeranic acid.     

Heterocyclisation intramoleculaire d'alcools possedant un squelette cedranique. Oxydation par le tetraacetate de plomb et par l'oxyde de mercure et le brome

Oxidation with lead tetra-acetate of neoisocedranol yields the corresponding ether in good yield. Under the same conditions, cedrol remains unchanged; however, when cedrol is reacted with mercuric oxide and bromine, the corresponding ether is obtained as major product. The structure of the substrates is linked with the mechanism of the two reactions and with the nature of the intermediates in the heterocyclisation reaction. The difference in behaviour between the two substrates towards the two oxidising agents is rationalized.     

Mecanismes de l'oxydation des alcools ethyleniques par le tetracetate de plomb,mise en evidence par rpe de l'influence des conditions experimentales sur le cours homolytique ou heterolytique de la reaction

A quantitative comparison of lead tetraacetate oxidation of ethylenic alcohols (4-penten-l-ol, 5-hexen-1-ol, 2-allyloxyethanol) with photolysis of the corresponding nitrites suggests that either free radical or ionic (or even concerted) pathways are involved in the former reactions, depending on experimental conditions. An ESR study of these oxidation reactions supports the preceding conclusions in each case.     

Reduction asymetrique de cetones par des alcoxyaluminohydrures chiraux prepares a partir de l'ephedrine et divers alcools ou phenols achiraux

The effect of the nature of the achiral component R′OH on the stereoselectivity of the asymmetric reduction of acetophenone by [LiAlH(OR′)₂(N-methylephedrine)] is examined. 3,5-Dimethylphenol as achiral component proved to be very suitable for making optically active secondary alcohols in high optical yield.     

Comparaison entre le role de l'eau et celui d'un ether couronne dans le cadre de la reaction de wittig realisee en milieu heterogene : liquide-solide

The similar results issued from the use of water and the 18-crown 6 ether for the Wittig reaction (in liquid-solid two-phase systems) allowed us to obtain very high yields in olefin starting from aromatic as well as aliphatic aldehydes.     

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.     

Chloration d'un polyethylene ramifie influence de la temperature et du solvant

The chlorination of branched high-pressure polyethylenes, promoted by u.v. at various temperatures in carbon tetrachloride and 1,1–2,2 tetrachloroethane, has been studied. It has been possible to elucidate the influence of the temperature and the nature of the solvent on the characteristics of the chlorinated polymers.The chlorination is more efficient in carbon tetrachloride, where the efficiency of the u.v. is not affected by the medium.At the same degree of chlorination, the chain-breaking mechanisms are more important in carbon tetrachloride than in tetrachloroethane; they increase when the temperature of the medium increases. The chlorine-saturated polyethylene obtained in carbon tetrachloride is richer in 1,2 dichloroethylene sequences.Below 60°, the yield of chlorinated polymer is the same in the two solvents. Chlorination at higher temperature in tetrachloroethane does not improve the structural regularity or the yield of the chlorine-saturated polyethylenes.     

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.     

Effet de longueur d'onde et controle par la geometrie de l'etat fondamental de la photoisomerisation de derives de l'hexatriene-1,3,5

Irradiations at different wavelengths (313, 254, 229 and 214 nm) of the 4 geometric isomers of 5-phenyl-1,3,5-heptatriene show that the photochemical behaviour of these trienes is mainly controlled by their ground state equilibrium.     

Dosage de l'oxygene dans l'uranium et ses composes par la methode au monochlorure de soufre

A sulfur monochloride method is proposed for the determination of oxygen in uranium compounds. Sulfur monochloride reacts with oxygenated compounds at temperatures depending upon their nature; the sulfur dioxide produced is titrated by iodometry, after the excess reagent has been eliminated by a selective adsorption-desorption process using activated charcoal. This method has been successfully applied to uranium oxides (UO₂, U₃O₈), to mixtures of uranium dioxide with uranium, uranium nitride, and uranium carbide, and to substituted carbides (UC_1-xO_x). The results are generally satisfactory for oxygen contents higher than 500 p.p.m. However, in the presence of free or combined carbon, this limit is considerably higher. A loss of oxygen as carbon monoxide is also possible, and a simultaneous determination of carbon monoxide must be carried out. The relative error is of the order of a few per cent.     

Effets de complexation de l'ester lors de la catalyse par l'eau ou les sels de l'hydrolyse alcaline des esters maloniques: etude cinetique,spectroscop

When water or salts are added, catalytic effects on alkaline hydrolysis of malonic esters in methanol are observed; they are definitely greater than those observed with monoesters. Kinetic data in the presence of salts suggest that ester-M⁺ complexation phenomena is predominant and prevails over HO^-, M⁺ ionic association. Infrared and Raman spectra of esters in water-alcohol mixtures, with or without added salts, indicate the formation of 1:1 and 1:2 complexes with each carbonyl of the diester and one proton donor molecule as well as with Li⁺.Ab initio calculations of these diesters and of their complexes allow the estimation of the positive charge density at the carbonyl group carbon as well as the LUMO energy levels; they lead to an interpretation of these entities reactivities according to perturbation theory. Only orbital control of alkaline hydrolysis of esters gives an account of the relative reactivities of mono- and diesters with or without added salts. The only consideration of charges at the carbonyl carbon is, on the other hand, inadequate for justifying all the observed phenomena.     

Selectivite de la reduction de cyclohexen-2 ones par NBu4BH4 dans divers solvants et par NaBH4 dans des conditions de transfert de phase

The 2-cyclohexenone 1 and isophorone 2 reductions with NBu₄BH₄ in aprotic solvents lead to a highly preferential 1–4 attack; i.e. 85% with 1 and 96% with 2 in THF. These regioselectivities are nearly the same as those observed with LiBH₄ in the presence of[2.1.1) cryptand confirming thus the cation influence. This method which is inexpensive and easy to work up, seems to constitute a general way to reduction of α-enones to saturate alcohols while other reagents such as K(sec Bu)₃BH are not able to reduce the carbon-carbon double bond of isophoron.Phase transfer catalysis conditions are not useful for selective reduction: large amounts of allylic alcohols are formed in liquid-liquid phase transfer conditions (60% in toluene-water); a good regioselectivity is only obtained when a cryptand is used as a transfer agent in solid-liquid conditions.