Stereochimie d'halogenation d'enols en serie exocyclique du cyclohexane

The steric course of the halogenation of exocyclic cyclohexyl enols E is not affected by the nature of halogenating species, medium, or substituent. In every case, equal amounts of axial and equatorial attack are formed. The results may be interpreted by a cyclic halogenation mechanism.     

Phénylation d'alcools allyliques acycliques : II. Regioselectivite du transfert d'hydrogene

PdCl₂, HNaCO₃-catalysed phenylation of deuterium-labelled allylic butenols, a Heck type reaction, proceeds by a highly regioselective 1,2-hydrogen shift $\text{via}$ a Wacker type intermediate. No decomplexed enol was formed during this reaction.     

Reactivite d'hydrocarbures methylenecyclo-hexaniques—I : Stereochimie de la reaction d'oxymercuration-demercuration

Methylenecyclohexanes, prepared in excellent yield by Corey and Chaykovsky then Cornforth reactions, give on oxymercuration followed by reductive demercuration tertiary alcohols resulting from axial attack by solvent. The attack takes place trans to the substiuent when the latter is at the 2 position. α-Hydroxy or methoxy compounds behave similarly to each other. For acetoxy compounds, the cis alcohols are obtained preferentially, by intramolecular attack of acetoxy group on the mercurinium ion intermediate. The results can be explained satisfactorily on the assumption that the factors inducing stereochemistry are short distance interactions.     

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.     

Stereochimie d'heterocycles du groupe IVB—II : Syntheses stereoselectives d'hydrogeno-1 et d'alkyl-1 sila-1 cyclobutanes

The reaction between n-butyllithium and 1,2 (or 1,3)-dimethyl-1-t-butoxy-1-silacyclobutanes proceeds with retention of configuration at silicon, and yields 10/90 (or 80/20) Z/E mixture of 1,2 (or 1,3)-dimethyl-1-n-butyl-1-silacyclobutanes. Methylation of 2 (or 3)-methyl-1-t-butoxy-1-silacyclobutanes (Z/E = 15/85 or 65/35) by methylmagnesium iodide gives 15/85 (or 65/35) Z/E mixture of 1,2 (or 1,3)-dimethyl-1-silacyclobutanes. The proposed configurations for substituted silacyclobutanes are supported by spectroscopic data (RMN) and chemical results (correlation of configuration and stereomutation of hydrogenosilacyclobutanes).     

Quelques aspects du mecanisme de l'oxydation anodique de quelques halogenures d'adamantyl-2

For the anodic oxidation in acetonitrile of some 2-adamantyl halides, the distribution of reaction products is dependent upon the halogen and substitution at the bridgehead positions. Iodides gave exclusively 2-adamantyl acetamides. 2-Chloroadamantane yields to chloroadamantyl acetamides. From bromides both 2-adamantyl acetamides and bromoadamantyl acetamides are formed. Part of the reaction products obtained with 2-fluoroadamantane are formed from the cleavage of C₂F bond; this result is explained by analogy with the mass spectrometric fragmentation behavior of this compound. A general reaction mechanism covering all our experimental results is proposed.     

Condensation du cyanoacetate d'éthyle sodé sur les sels d'iminoethers cycliques synthese directe de dérivés d'oxazepine-1,4 one-7

Sodium ethylcyanoacetate was found to react exclusively at the 2-position of trifluoromethanesulfonate 2-alkyl(aryl) 1,3-oxazolinium salts 2. In most cases, the reaction leads specifically to a new series of 5-alkyl(aryl) 6-cyano 2,3-dihydro 1,4-oxazepine 7-ones 3. This efficient one-step ring-enlargement process occurs at the oxazolidine stage. Structural parameters affecting the scope and limitations of this condensation are discussed.     

Stereochimie d'heterocycles du groupe IVB—I : Syntheses stereoselectives d'alcoxy-1 et d'amino-1 sila-1 cyclobutanes

Stereoselective synthéses (60 to 90%) of some silacyclobutanes were performed by alcoholysis or aminolysis of 2 (or 3)-methyl-1-chloro-1-silacyclobutanes. Relative configurations of 2 (or 3)- methyl-1-alkoxy (or 1-amino)-1-silacyclobutanes thus prepared have been assigned from their NMR spectra. The mechanism of these stereoselective reactions, based on the rapid stereomutation of 1-chloro-1-silacyclobutanes, is discussed.     

Conformation et reactivite de derives (4.n.0) bicycliques a jonction trans—XXIV: Trifluoroéthanolyse du phényl-4a et du méthyl-4a tosyloxy-3a bicyclo (4.2.0) octane trans. mise en évidence d'une réaction d'élimination à partir d'union phénonium

Rate constants and reaction products for trifluoroethanolysis of transfused 4a-phenyl 3a-tosyloxy bicyclo (4.2.0) octane 1 and 4a-methyl 3a-tosyloxy bicyclo(4.2.0) octane 1' are reported. As for methanolysis, this reaction occurs through the E₁(ip) process. Nevertheless, first-formed secondary cations are rearranged before reacting, by hydride migration for 1 and 1', and phenyl migration for 1. Reaction products result from solvent or leaving group basic and nucleophilic attack on tertiary ions, and phenonium ion for 1. Elimination via phenonium ion has been established by the characterisation of a rearranged olefin (with equatorial phenyl). A mechanism for this elimination reaction is proposed.     

Heterocyclisation des α-acylaminoamides—IV : Mecanismes d'ouverture et de formation du cycle amino-5 oxazole

It was established by using ¹⁸O enriched water that the 5-aimno-oxazole cycle is opened via a nucleophilic attack of water on the 5 position. The mechanism of the heterocyclisation of ¹⁸O enriched α-acylaminoamides by trifluoracetic anhydride has been studied. Two cyclisation reactions occur, involving the attack of the anhydride by the amidic O atoms.     

Conformational equilibrium isotope effects in glucose by (13)C NMR spectroscopy and computational studies.

Anomeric equilibrium isotope effects for dissolved sugars are required preludes to understanding isotope effects for these molecules bound to enzymes. This paper presents a full molecule study of the alpha- and beta-anomeric forms of D-glucopyranose in water using deuterium conformational equilibrium isotope effects (CEIE). Using 1D (13)C NMR, we have found deuterium isotope effects of 1.043 +/- 0.004, 1.027 +/- 0.005, 1.027 +/- 0.004, 1.001 +/- 0.003, 1.036 +/- 0.004, and 0.998 +/- 0.004 on the equilibrium constant, (H/D)K(beta/alpha), in [1-(2)H]-, [2-(2)H]-, [3-(2)H]-, [4-(2)H]-, [5-(2)H]-, and [6,6'-(2)H(2)]-labeled sugars, respectively. A computational study of the anomeric equilibrium in glucose using semiempirical and ab initio methods yields values that correlate well with experiment. Natural bond orbital (NBO) analysis of glucose and dihedral rotational equilibrium isotope effects in 2-propanol strongly imply a hyperconjugative mechanism for the isotope effects at H1 and H2. We conclude that the isotope effect at H1 is due to n(p) --> sigma* hyperconjugative transfer from O5 to the axial C1--H1 bond in beta-glucose, while this transfer makes no contribution to the isotope effect at H5. The isotope effect at H2 is due to rotational restriction of OH2 at 160 degrees in the alpha form and 60 degrees in the beta-sugar, with concomitant differences in n --> sigma* hyperconjugative transfer from O2 to CH2. The isotope effects on H3 and H5 result primarily from syn-diaxial steric repulsion between these and the axial anomeric hydroxyl oxygen in alpha-glucose. Therefore, intramolecular effects play an important role in isotopic perturbation of the anomeric equilibrium. The possible role of intermolecular effects is discussed in the context of recent molecular dynamics studies on aqueous glucose.     

Les organocadmiens benzyliques : obtention exclusive d'alcools: Secondaires d'addition avec l'acetaldehyde et le benzaldehyde

In contrast to the Grignard reaction, the reaction of various substituted benzylic cadmium reagents with aldehydes gives specifically secondary benzylic alcohols with good yields.     

Reactivite des carbanions benzyliques : II. Stereochimie d'alcoylation des ethyl et isopropyl-9 lithio-10 dihydro-9,10 anthracenes

A detailed study of the stereochemistry of alkylation of 9-ethyl- and 9-isopropyl-10-lithio-9,10-dihydroanthracenes by alkyl halides (R′ = Me, Et, i-Pr) is reported. The reaction is under kinetic control. cis-9-Ethyl-10-isopropyl-9,10-dihydroanthracene has been found more stable than the trans isomer. If we take into account the stereoelectronic factor, which favors axial attack, and the main steric interactions in the transition state, stereochemistry seems to be mainly controlled by the competition between axial-axial and peri interactions.     

Voie d'acces aux benzylidene-1 cyclohexadiene-2,4 fer tri-carbonyle et autres composes apparentes. Stabilisation singuliere d'une forme tautomere d'un aromatique

2,4-Cyclohexadien-1-yldimethylphenylphosphonium tetrafluoroborate obtained from the reaction of 2,4-cyclohexadien-1-yltricarbonyliron with PMe₂Ph in 96% yield, reacts readily with benzaldehyde in the presence of base to give 1-benzylidiene-2,4-cyclohexadienetricarbonyliron in 90% yield; this and other related reactions provide the first examples of the Wittig reaction adjacent to a tricarbonyl moiety.     

Dosage du platine par spectrométrie d'absorption atomique: Partie I. Mise au point d'une méthode d'analyse dans les complexes organométalliques et de coordination

A simple fast method for the determination of platinum in complexes in aqueous solution by atomic absorption spectrometry, without previous destruction of complexes, is reported. Several parameters affecting absorbance measurements are discussed. The use of some spectrochemical buffers was necessary in order to prevent interference effects. Lanthanum (20,000 p.p.m. in 0.5 M hydrochloric acid)and phosphate (30,000 p.p.m. in water) were used as buffers. The method requires only standard equipment without scale expansion. The concentration range is 1–100 p.p.m. of platinum.     

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.     

Action des halogenures d'alkylmagnesium sur les pyrones-2—VI : Etude de la regioselectivite de l'ion enolate intermediaire de la reaction

Grignard reagents normally react with 2H-pyrones to yield unsaturated ketols, dihydropyranols, dienones and 2H-pyrans. In some cases, products are obtained by attack on the most sterically crowded position of the enolate ion. Since steric hindrance is an unsatisfactory basis for predicting the reactivity, we have applied a theoretical study to this problem. The model used is based on an enlarged electronic delocalization in the transition state, and allows the interpretation of the regioselectivity of the reaction.     

Reaction d'aminomercuration—xv: Aminomercuration d'olefines par les amines aromatiques en milieu aqueux

Aminomercuration of double bond by aromatic amines is easily performed by using an aqueous solvent. No oxymercuratio occurs. The reaction is general, regiospeciflc with terminal olefins. This specific feature of aromatic amines is due to a preliminary ring mercuration whose effect is to destroy the preformed amine/mercuric salt complex preventing the reaction.     

Synthese d'antibiotiques triterpeniques a partir d'acides biliaires-II : Introduction du methyle 14α associe á Δ-8,9

The synthesis of methyl 14α-methyl-15-oxo-5β-chol-8-en-24-oate derivatives from cholic acid is described, as are the spectroscopic data of the various intermediates. A route to tetracyclic triterpene acids related to polyporenic acid is thus opened.     

Substitutions electrophiles sur les heterocycles pentagonaux—II : Role des facteurs steriques dans les reactions d'acylation du phenyl-2 methyl-5 thiophene

Antagonism of steric and electronic effects is shown by the orientation on the β and β′ reaction sites of 2-phenyl 5-methyl thiophene in acylation reactions. The regiospecificity of the reaction can be controled by the appropriate choice of catalyst and solvent which allows selective synthesis of the expected ketones.Mechanism and spectroscopic properties are discussed.