László Tőke

Asymmetric C–C bond forming reactions with chiral crown catalysts derived from d-glucose and d-galactose

Abstract New chiral monoaza-15-crown-5 derivatives anellated to methyl-4,6-O-benzylidene-α- d -glucopyranoside 2a, 2e, 2g–i and to methyl-4,6-O-benzylidene-α- d -galactopyranoside 3a, 3e, 3i have been synthesized. These crown ethers showed significant asymmetric induction as phase transfer catalysts in the Michael addition of 2-nitropropane to chalcone (87% ee), in the Darzens condensation of phenacyl chloride with benzaldehyde (71% ee) and in the self-condensation of phenacyl chloride (64% ee) to give 14. The absolute configurations of (−)-(2R,3S)-epoxy-3-(4-chlorophenyl)-1-phenyl-1-propanone 12 and (−)-4-chloro-(2R,3S)-epoxy-1,3-diphenyl-1-butanone 14 have also been determined by X-ray diffraction.

Chiral azacrown ethers derived from D-glucose as catalysts for enantioselective Michael addition

Abstract Chiral monoaza-crown ethers incorporating glucose units have been synthesized and applied as phase transfer catalysts in the enantioselective Michael addition of 2-nitropropane to a chalcone to give the corresponding adduct in enantiomeric excess up to 90%.

An intermolecular 1,3-dipolar cycloaddition approach to the tricyclic core of martinelline and martinellic acid

Abstract The tricyclic core of the martinellines has been synthesised stereoselectively in two steps, using 1,3-dipolar cycloaddition of azomethine ylides as a key step.

Enantioselective Michael addition of 2-nitropropane to chalcone analogues catalyzed by chiral azacrown ethers based on α-d-glucose and d-mannitol

Abstract The chiral monoaza-15-crown-5 type lariat ethers 1 and 2 derived from α- d -glucose and from d -mannitol, respectively, have been applied as phase transfer catalysts in the enantioselective Michael addition of 2-nitropropane to aromatic 3b–c and heteroaromatic 3d–h chalcone analogues. Among the catalysts, the glucose-based 1c with a phosphinoxidobutyl side arm proved to be the most effective, it inducing 34% e.e. for 4b, 59% e.e. for 4c, 80% e.e. for 4d, 64% e.e. for 4e, 17% e.e. for 4f. Catalyst 1a having 3-hydroxypropyl substituent resulted in 81% e.e. for compound 4g. The formation of the (+)-(S)-enantiomer of 4 was preferred using crown ethers 1a–c, while the (−)-(R)-enantiomer was in excess with catalyst 2. The absolute configuration of the Michael adduct 4d was determined by single-crystal X-ray analysis.

Diastereoselective synthesis of 1,2,3,6-tetrahydrophosphinine 1-oxides with an exocyclic P-function by a Michael type addition

The anions generated from diphenylphosphine oxide or dialkyl phosphites add easily at the α,β-double-bond of 1,2-dihydrophosphinine oxides 1 to afford a single diastereomer of 3-substituted tetrahydrophosphinine oxides 2–4 existing in a twist-boat conformation.

Efficient synthesis and resolution of (±)-1-[2-carboxy-6-(trifluoromethyl)phenyl]pyrrole-2-carboxylic acid

Abstract A novel, efficient synthesis and resolution of (±)-1-[2-carboxy-6-(trifluoromethyl)-phenyl]pyrrole-2-carboxylic acid has been developed for the preparation of new members of optically active atropisomers. The e.e. values were determined by a highly sensitive 19F NMR spectroscopic method using β-cyclodextrin as chiral complexing agent. Single-crystal X-ray structures of the two diastereoisomeric salts and consequently, the absolute configurations of the enantiomers are also reported.

Tandem in situ Generation of Azomethine Ylides and Base Sensitive Nitroethylene Dipolarophiles

Abstract We have studied the behaviour of 2-acetoxy-nitroethane derivatives in the presence of triethylamine and of an azomethine ylide generated also by the action of this base in the same reaction mixture. Under these conditions the nitro-olefins are generated and react in situ with the corresponding dipoles, giving pyrrolidine derivatives in moderate to good yields.

Competition and co-operation between ortho directing groups and activating agents: Regioselective metallation of 1-(methoxyphenyl)pyrroles

Abstract The role of di- and tri-dentate tertiary amine type ligands was studied in regioselective lithiation reactions of 1-(methoxyphenyl)pyrroles with butyllithium (LIC). Clean α-metallation of the pyrrole ring occured when N,N,N′,N′,N″-pentamethyldiethylenetriamine (PMDTA) was used while the benzene ring was lithiated ortho position to the methoxy group in the presence of N,N,N′,N′-tetramethylethylenediamine (TMEDA).

Useful base promoted elaborations of oxiranyl ethers

Abstract Functionalized oxiranyl ethers can be regio- and stereoselectively converted into hydroxy oxetanes or cis -diols by treatment with organometallic bases. These two rearrangements can be conveniently carried out either using different reaction conditions starting from the oxirane or in two consecutive steps from the oxirane via the oxetane.

Phosphine-Boranes Based on the 7-Phosphanorbornene Framework: a Regioselective Approach to the Monoboranes of the Dimers of Phospholes

Abstract The reaction of dimers of phosphole oxides ( 3a – e ) with an excess of dimethylsulfide borane follows an unexpected route to give phosphine-boranes 5a – e based on the 7-phosphanorbornene framework in a regioselective manner and in good yields. In small quantities, the diboranes ( 10a – e ), that can be synthesized in acceptable yields by the reaction of the corresponding diphosphines 12a – e with borane, were also formed.