Regio- and Stereoselective Modification of Chiral α-Amino Ketones by Pd-Catalyzed Allylic Alkylation

The Allylic Alkylation of Ketone Enolates

The palladium-catalyzed allylic alkylation of non-stabilized ketone enolates was thought for a long time to be not as efficient as the analogous reactions of stabilized enolates, e. g. of malonates and β-ketoesters. The field has experienced a rapid development during the last two decades, with a range of new, highly efficient protocols evolved. In this review, the early developments as well as current methods and applications of palladium-catalyzed ketone enolate allylations will be discussed.

Stereoselective Allylic Alkylations of Amino Ketones and Their Application in the Synthesis of Highly Functionalized Piperidines

Chelated ketone enolates are excellent nucleophiles for allylic alkylations. Electron-withdrawing groups on the allyl moiety allow subsequent intramolecular Michael additions giving rise to piperidines with up to five stereogenic centers.

Diastereoselective Synthesis of Unnatural Amino Acids by Alkylation of α- tert-Butanesulfinamide Auxiliary-Bound Enolates

A new chiral auxiliary for the diastereoselective alkylation of amino ester enolates that takes advantage of chiral information stored on the enolate side of the amino ester substrate has been developed. Chiral α-sulfinamido esters were alkylated under basic conditions in good yields (up to 90%) and good to high diastereoselectivities (generally >6:1) to provide unnatural mono- and α,α-disubstituted amino acid derivatives. This auxiliary allowed for the ready conversion of ester functionality without the need for esoteric reagents. Furthermore, the auxiliary is easily removed to provide enantiopure amino acids. Computational studies revealed that a chelated transition state governs electrophile addition from the convex face of a transient bicyclic intermediate. This method allows ready access to enantioenriched natural and unnatural amino acids.

Iridium-Catalyzed Enantioselective Allylic Substitution of Aliphatic Esters with Silyl Ketene Acetals as the Ester Enolates

Enantioselective allylic substitution with enolates derived from aliphatic esters under mild conditions remains challenging. Herein we report iridium-catalyzed enantioselective allylation reactions of silyl ketene acetals, the silicon enolates of esters, to form products containing a quaternary carbon atom at the nucleophile moiety and a tertiary carbon atom at the electrophile moiety. Under relatively neutral conditions, the allylated aliphatic esters were obtained with excellent regioselectivity and enantioselectivity. These products were readily converted into primary alcohols, carboxylic acids, amides, isocyanates, and carbamates, as well as tetrahydrofuran and γ-butyrolactone derivatives, without erosion of enantiomeric purity.

Catalytic Enantioselective and Diastereoselective Allylic Alkylation with Fluoroenolates: Efficient Access to C3-Fluorinated and All-Carbon Quaternary Oxindoles

Synthetically versatile 3,3-disubstituted fluorooxindoles exhibiting vicinal chirality centers were obtained in high yields and with excellent enantio-, diastereo-, and regioselectivity through catalytic asymmetric fluoroenolate alkylation with allylic acetates. The reaction proceeds under mild conditions and can be scaled up without compromising the asymmetric induction. The unique synthetic usefulness of the products is highlighted by the incorporation of additional functionalities and the formation of 3-fluorinated oxindoles exhibiting an array of four adjacent centers of chirality. A new C-F bond functionalization path that provides unprecedented possibilities for the stereoselective generation of a chiral quaternary carbon center in the alkaloid scaffold is introduced.

Iridium-Catalyzed Diastereoselective and Enantioselective Allylic Substitutions with Acyclic α-Alkoxy Ketones

The asymmetric alkylation of acyclic ketones is a longstanding challenge in organic synthesis. Reported herein are diastereoselective and enantioselective allylic substitutions with acyclic α-alkoxy ketones catalyzed by a metallacyclic iridium complex to form products with contiguous stereogenic centers derived from the nucleophile and electrophile. These reactions occur between allyl methyl carbonates and unstabilized copper(I) enolates generated in situ from acyclic α-alkoxy ketones. The resulting products can be readily converted into enantioenriched tertiary alcohols and tetrahydrofuran derivatives without erosion of enantiomeric purity.

Prochiral alkyl-aminomethyl ketones as convenient precursors for efficient synthesis of chiral (2,3,5)-tri-substituted pyrrolidines via an organo-catalysed tandem reaction

2-Alkanoyl-3-aryl-5-hydroxy pyrrolidines were prepared in high yields and ees via a secondary amine catalyzed tandem process.