Emmanuel Vrancken

Gold-catalyzed propargylic substitutions: Scope and synthetic developments

Summary This personal account summarizes our recent developments in gold-catalyzed direct substitutions on propargylic (allylic, benzylic) alcohols, with various nucleophiles (and bi-nucleophiles) based on the σ- and/or π-acidity of gold(III) complexes. Synthetic developments are also briefly described.

Stereoselective and catalytic access to β-enaminones: an entry to pyrimidines.

We describe herein a highly stereoselective access to Cbz-protected β-enaminones 2 based on the NaOH catalyzed rearrangement of propargylic hydroxylamines 1. The synthetic potential of these β-enaminones is illustrated in an original synthesis of pyrimidines.

Transition-Metal-Catalyzed Uninterrupted Four-Step Sequence to Access Trisubstituted Isoxazoles

We describe herein a novel uninterrupted four-step sequence to access trisubstituted isoxazoles from readily available propargylic alcohols using sequentially iron and palladium catalytic systems. The advantages of such a strategy are illustrated by the high overall yields and the time-saving procedure that are reported.

Highly diastereoselective Baldwin rearrangement of isoxazolines into cis-acylaziridines.

An atom-economical and practical synthesis of cis-N-benzenesulfonamide acylaziridines through the Baldwin rearrangement of various N-benzenesulfonamide isoxazolines has been reported. A detailed experimental study revealed the beneficial effect of microwaves and pointed out the crucial role of the temperature in the reaction course. Moderate to good yields and excellent cis stereoselectivities were achieved for 13 examples.

A practical and efficient synthesis of chiral N,N-disubstituted C2 symmetric diamines derived from (R,R)-1,2-diaminocyclohexane

An improved synthesis of chiral diamine ligands derived from (R,R)-1,2-diaminocyclohexane is described, providing N-substituted diamines. The synthesis of other new ligands based on this methodology is also reported.

Tandem reactions involving 1-silyl-3-boryl-2-alkenes. New access to (Z)-1-fluoro-1-alkenes, allyl fluorides, and diversely α-substituted allylboronates.

The reactions of 1-silyl-3-boryl-2-alkenes with various electrophilic reagents (Selectfluor, N-halosuccinimides, benzhydryl, and propargylic alcohols in the presence of a Lewis acid, N-alkoxycarbonyliminium ion) have been investigated as new routes to α-substituted allylboronates. Further functional transformations, including allylboration, Suzuki coupling, protodeboronation, and cycloisomerization, have been carried out to illustrate the synthetic potential of these γ-borylallylsilanes.

Palladium-Catalyzed Cyclization of Unsaturated β-Amino Alcohols: A New Access to Enantiopure Bicyclic Oxazolidines

Chiral unsaturated beta-amino alcohols possessing a dialkylamino function cyclize in the presence of Pd(II) catalysts and reoxidants to afford enantiopure bicyclic oxazolidines with total regio- and stereocontrol. The scope and limitations of this transformation have been studied.

Diastereodivergent Behavior of Alkyl versus Cyano Allenylcuprates toward Aldehydes: A Key Role for Lithium

The stereodivergent behavior of allenyl(cyano)- and allenyl(alkyl)cuprates toward aldehydes, providing a selective preparation of both syn- and anti-homopropargylic alcohols, is described. This study, which combines both experimental and theoretical support, shows that the copper nontransferred dummy ligand controls the localization of the lithium cation with respect to the allenylcuprate moiety. As a consequence, Li(+) acts as a Lewis acid activator but also controls the diastereoselectivity during the addition of allenylcuprates onto aldehydes. The combined high selectivity, efficiency, and versatility of these cuprate compounds opens the way to new one-pot synthetic procedures, as illustrated by the combined Klein rearrangement/transmetalation methodology described herein.

Short enantioselective syntheses of trans-5-alkylprolines from new functionalized amino alcohols.

Amino alcohols, having an enol ether function, cyclized in acidic medium to give quantitatively diastereosomerically pure bicyclic compounds that were transformed in five steps in enantiopure trans-5-alkylproline derivatives.

Regioselective SN2 opening of α,β-ethylenic epoxides by RLi–BF3 combination

Organolithium reagents effect a regioselective SN2 nucleophilic cleavage of α,β-ethylenic epoxides only when BF3·Et2O is added. The reaction works with a variety of RLi reagents and with cyclic as well as acyclic epoxides.