Zoltan Welvart

Organomagnesien reactif reducteur : VI. Comparaison de la stereochimie de la reduction des cetones et des composes a double liaison ethylenique activee par les organomagnesiens chiraux

Abstract The reduction of alkylidene cyanoacetic esters (Z- and E-ethyl 2-cyano-3-phenyl butenates) and the corresponding ketone (acetophenone) by chiral organomagnesium halides gives opposite stereoselectivities. These results are rationalized by the previously proposed cyclic mechanism of Whitmore for the ketone reduction and by a non-cyclic mechanism with polar orientation of the reagents in the reduction of the ethylenic compounds.

Surface enhanced enantiodifferentiation in reactions of chiral acetoacetates

Abstract The stereoselectivity of the alkylation of chiral acetoacetates by racemic secondary alkyl halides changes when the reaction is carried out on a solid support; the change can be interpreted in terms of a model involving surface imposed conformational restrictions.

Etude stereochimique de l'alkylation d'anions par des derives p-nitrobenzyliques optiquement actifs : Competition SN2-SNR1

Abstract Competition between S RN 1,S N 2 and S RN 1 mechanisms is discussed according to the stereochemical results in the alkylation of anions by optically active secondary p -nitrobenzyl reagents. Results from alkylation of the anions of benzylcyanide C and α-aminonitrile A by p -nitrobenzyl chloride 2 rule out S N 2 and S RN 2 mechanisms. On the other hand, the S N 2 process becomes exclusive in O -and C -alkylation of the acetoacetic ester anion B by the p -nitrobenzyl phosphonium salt, and this result shows that it is possible to obtain p -nitrobenzyl alkylation products without racemisation. C -Alkylation of the anion B by halide 2 involves an S N 2-electron transfer competition. The whole result illustrates that the stereochemical method provides precise information on the mechanism of these reactions.

Alkylation des esters acetylacetiques par des alcools chiraux, par deshydratation intermoleculaire. Correlation entre les puretes optiques des produits formes.

Abstract The formation of the C and O-alkylates involves inversion of configuration and the same weak racemization.

Organomagnesien reactif reducteur : V. Induction asymetrique dans la reduction des esters chiraux α-β ethyleniques☆

Abstract Prelogs rule, predicting the stereoselectivity of nucleophilic addition to the α-ketoesters of chiral alcohols, is also applicable to the reduction of α,β-ethylenic esters by LiAlH4, but this rule is no longer valid when organomagnesium halides are used as reducing agents. This difference between the two reactions is rationalized by the degree of complexation of the metallic ions of the two reducing agents in the transition states.

Influence de l'isomerie cis-trans des alcoylidene-cyanacetates sur leur reactivite vis-a-vis des reactifs organomagnesiens

Abstract The reactivities of Z and E isomers of ethyl-α-cyanoβ-methyl cinnamate are very different towards Grignard reagents, indicating that there is no isomerization of the ethylenic double bond during reaction. Grignard reagents prepared either from a racemic or from an optically-active halide have the same reactivity, suggesting that the species R 2 Mg does not participate significantly in the reaction.

Alkylation de l'acetylacetate de methyle par le α(p-nitrophenyl)-ethanol chiral par deshydratation intermoleculaire

Abstract Mitsunobus method of acetoacetic ester alkylation by p -nitrophenyl-ethanol avoid the SRN1 reaction and is well appropriate for the correlation of absolute configuration and ee of the O and C alkylation products.

High diastereoselectivity in the reaction of α-aminonitriles bearing an (S)-(+)-2-methoxymethylpyrrolidine group with Grignard reagents

(S)-(+)-2-Methoxymethylpyrrolidine induces high diastereoselectivities (80% diastereoisomeric excess) in the reaction of α-aminonitriles with Grignard reagents.

Stereochimie de l'alkylation de carbanions derives d'α-aminonitriles et de cyanhydrines bloquees.

Abstract Alkylations with 2-octyl iodide of carbanions generated in THF by LDA from α-aminonitriles or protected cyanohydrines occur with inversion of configuration and an important racemization; these reactions do not involve radical intermediates.

Effet des hautes pressions sur le rearrangement de wittig

Resume The pressure increase from 0,1 to 1000 MPa decreases the rate, but does not change the stereoselectivity of theWittig rearrangement of the 2-octyl benzhydryl ether in HMPT. These results rule out any contribution of a concerted process to the observed stereoselectivity (20%)