Cycloadditions dipolaires-1,3—XXIV: Addition d'esters nitroniques aux olefines monoactivees

The orientation of the cycloaddition of the geometrical isomers of nitronic esters to monoactivated olefines is unique. Depending on the nature of the olefinic substituent, a competition between two modes of approach of the dipolarophile by the 1,3-dipole is observed. All these cycloadditions lead to stable invertomers of N-alkoxyisoxazolidines.     

Regioselectivite sous controle d'interactions de charges ou d'interactions orbitalaires : Reaction d'anions α-chlores a charge localisee ou delocalisee sur des α-enones

Benzalacetophenone reacts with anions derived from methyl chloroacetate and chloroacetonitrile (which are of the charge-localized type) at the carbonyl group to give α-epoxyalkenes. In contrast, the same ketone reacts with the anions derived from methyl phenylchloroacetate and phenylchloroacetonitrile (the negative charge of which is delocalized) to give cyclopropanes by attack at the carbon-carbon double bond. These results support the hypothesis that the reaction is charge-controlled with the former two reagents and orbital-controlled in the case of the latter two. Only one cyclopropane stereoisomer is formed in HMPA.     

Etude theorique des effets de substituants dans les cycloadditions dipolaires-1,3

A theoretical study of substituent effects on 1,3-dipolar cycloadditions has been carried out by the LCAO-SCF-MO method. The potential energy hypersurfaces for ten reactions and the electronic and energetic properties of the transition states have been investigated.     

Stereochimie-LV : Etude cinetique de l'hydrocyanation d'enones cycliques

Kinetics of hydrocyanation of various enones are reported. The different behaviour of testosterone and 19-nor-testosterone towards hydrocyanation is explained by a slower partial rate factor on the α-face of testosterone.     

Formation d'enolates cetoniques par action d'organomagnesiens mixtes dans l'hexamethylphosphorotriamide sur des esters. : Direction des enolisations,limites, utilisation a la preparation de cetones

Alkylmagnesium halides in HMPA react with aliphatic esters to form predominantly the less substituted ketonic enolates. The direction of these enolizations is more selective than that of intermediate ketones. Aliphatic esters are only slightly or not at all enolized under these conditions. Hydrolysis, deuterolysis and alkylation of the ketonic enolates give the corresponding ketones. Benzoic acid derivatives and α-β unsaturated aliphatic and aromatic esters give only low yields of ketonic enolates. The low enolization of the intermediate ketones by these alkylmagnesium halides can explain this result.     

Cycloadditions dipolaires-1,3 II. Addition des diarylnitrilimines au N-methyl-indole. Etude expérimentale et essai d'interprétation

The reactivity of N-methylindole towards diarylnitrilimines has been studies. The cycloadducts are 8-methyl-3a,8a-dihydro-pyrazolo[3,4-b] indole derivatives. The structure reported was assigned on the bases of satisfactory analytical, spectroscopic and chemical data. An interpretation based on the second order perturbation method of frontier orbitals is in agreement with the observed regiospecificity.     

Reductions par le zinc en presence d'acides—II : Formation de cyclopropanes lors de la reduction des aldehydes aromatiques par le zinc en presence de trifluorure de bore et d'olefines

The reduction of several p-substituted aromatic aldehydes by zinc in the presence of boron trifluoride and cyclic or open chain olefins, vinyl or allyl ethers or acetates, leads to the corresponding substituted arylcyclopropanes by way of carbene intermediates.     

Effets cinetiques de sels lors de la methanolyse de l'acetate de phenyle par catalyse basique en milieu methanolique: Etude perturbationnelle des effets de sels lors de l'hydrolyse ou de la méthanolyse d'esters aliphatiques ou aromatiques

Methanolysis of phenyl acetate shows a slight rate increase in the presence of NaClO₄ or NaBr or a rather large rate decrease in the presence of LiClO₄ or LiBr. These results are different from those obtained for alkaline hydrolysis of an aliphatic ester. They are interpreted in terms of a frontier orbital controlled reaction. For aliphatic esters, complexation phenomena

is predominant; for aromatic esters, due to a stabilisation of the π^* CO orbital energy level, two opposite effects are possible:

complexation inducing a rate increase, and association of the nucleophile HO^- or CH₃O^- with M⁺ leading to a rate decrease. The relative importance of these two opposite effects depends on the nature of the ester.     

Etude dans la serie des organosilylazoles : I. Action des halogenures d'alkyle,des chlorures d'acide et des cetones halogenees

The reactivity of organosilylamines of imidazole, pyrazole, 1,2,4-triazole and benzotriazole towards alkyl halides, acidic chlorides and halogenated ketones has been studied. Except for 1-trimethylsilylimidazole, which gives a mixed quaternary salt, reactions with primary halides lead to the corresponding 1-alkylated heterocyclic compounds in high yields; in each case only one isomer is obtained (1-alkyl-1,2,4-triazole and 1-alkylbenzotriazole). Similarly acidic chlorides give 1-acylated derivatives in quantitative yields. Finally α- or β-halogenated ketones show different behaviour and give the addition or substitution products.     

Catalyse asymetrique avec des complexes chiraux de rhodium-diop—V: Effets des substituants lors de la reduction d'acides N-acylamino cinnamiques

Asymmetric synthesis of phenylalanine and its derivatives can be accomplished by asymmetric reduction of N-acylamino-cinnamic acid derivatives with CIRh(+)DIOP² as catalyst. The optical yield is influenced by the stereochemistry of the double bond, the para-substituent on the N-benzoyl group and the esterification of the carboxy group. The mechanism of the reduction is discussed.     

Etude des aldimines lithiees : III. Action des epoxydes et synthese d'amino-2 et d'hydroxy-2 tetrahydrofurannes

The reaction of lithiated aldimines, prepared from activated amides, with epoxides leads to good yields of 2-aminotetrahydrofurans (neutral hydrolysis) and 2-hydroxytetrahydrofurans (acid hydrolysis). Various epoxides were examined.     

Contribution a l'etude des metaux asymetriques—II : Voie d'acces aux premiers complexes pseudo tetraedriques du chrome (0) enantiomeriquement stables

Modes of access to several pseudo tetrahedral complexes having an asymmetric chromium atom are presented. The chirality at the metal atom is demonstrated by NMR. The first enantiomers of chiral chromium (0) are obtained in optically pure forms.     

Synthese d'azides tertiaires—V: Formation d'azido- et d'amino-8α et 8β dans la serie du labdan

HN₃/BF₃-Et₂O was used in the synthesis of tertiary azides from sclareol and dihydrosclareol. An ion-pair mechanism was observed. Under appropriate conditions, stereoselective introduction an azido-group at C-8 was observed. Manoyl-oxide and 13-epimanoyl oxide provided azido-ketones by CC vinylic fragmentation with loss of C₁₄ and C₁₅.     

Catalyse par les acides de Lewis—I : Etude cinetique de la reaction d'esterification des olefines catalysee par SnCl4

Lewis acids, such as SnCl₄, TiCl₄, FeCl₃ AlCl₃ and ZnCl₂ catalyze the esterification of olefins in CH₃CO₂H or CH₃CH₂CO₂H. SnCl₄ reacts with carboxylic acids to give the acid complex SnCl₄.2(RCO₂H), which raises the level of acidity. The following relation applies: log k_ex = αH_o + β (α≠?1), activation energy (7·5 kcal/mole) and activation entropy (?28 cal/M°K). Solvent isotope effects are measured (k_H/k_D>1). The mechanism implies a slow proton transfer.     

Protonation en milieu anhydre d'aziridines,ylures d'azomethine potentiels : Nature des entites formees et etude des divers equilibres

Protonation of aziridines, which are potential azomethine ylides when the equilibrium aziridine?azomethine ylide is established, generally leads to functional iminium salts. A competition between N-protonation of the aziridine and C-protonation of the azomethine ylide may be observed. Factors governing the different acido-basic equilibriums are discussed.     

Stereochimie de la des benzoates d'argent sur des diesters iododesoxy ns: I- Action des benzoates d'argent sur des di-o-aroyl-1,2 iododesoxy-3 ns-glycerols

The stereochemistry of the reaction of silver benzoates on chiral di-O-aroyl-1,2 iododeoxy-3-sn-glycerols has been reinvestigated. The reaction proceeds either with or without neighbouring group participation, depending on : - possible electrophilic assistance by the silver ion to the ionisation of the substrate (occuring only with dissociated silver salts), - ability of substituants R¹ and R² on the substrates to stabilize a positive charge. Rearrangements are at minimum with R¹ and R² electronreleasing groups in the substrate, silver salts un-or weakly dissociated and bearing a good nucleophile. For one case, a regio- and stereo specific reaction is observed.