An efficient enantioselective fluorination of various beta-ketoesters catalyzed by chiral palladium complexes

Diastereoselective Synthesis of Less Accessible Fluorine-Containing Acyclic Tetrasubstituted Stereocenters via Electrophilic Fluorination of β,β-Disubstituted Metalloenamines

A stereocontrolled protocol was developed to construct less accessible fluorine-containing acyclic tetrasubstituted stereocenters bearing two sterically and electronically similar alkyl groups at the α-position of carbonyls. In this process, tBuOK-promoted stereospecific α-deprotonation of α,α-disubstituted N-tert-butanesulfinyl ketimines or NH deprotonation of β,β-disubstituted enesulfinamides generates geometry-defined multisubstituted metalloenamines, followed by stereoselective electrophilic fluorination with the N-fluoro ammonium salt of quinine, affording the acyclic α-fluorinated ketimines with excellent diastereoselectivities.

Stereoselective Synthesis of α-Fluoro Carboxylic Acids by Ireland-Claisen Rearrangement

Stereoselective synthesis of α-fluoro carboxylic acids by the Ireland-Claisen rearrangement can provide a straightforward approach to this class of compounds. We report a systematic investigation of base-dependent stereocontrol in the Ireland-Claisen rearrangement of α-fluoro esters. For substrates with various substitution patterns, the use of KN(SiMe3)2 in toluene afforded rearrangement products corresponding to the (Z)-enolate intermediate with a practically useful diastereoselectivity and yield. In contrast, lower yields and diastereoselectivity were consistently observed with the use of lithium diisopropylamide (LDA) in tetrahydrofuran (THF).

Simple purification of small-molecule-labelled peptides via palladium enolate formation from β-ketoamide tags

Palladium enolates derived from β-ketocarbonyl compounds serve as key intermediates in various catalytic asymmetric reactions. We found that the palladium enolate formed from β-ketoamide is stable in air and moisture and we applied this property to develop a peptide purification system using β-ketoamide as a small affinity tag in aqueous media. A solid-supported palladium complex successfully captured β-ketoamide-tagged molecules as palladium enolates and released them in high yield upon acid treatment. Optimum conditions for the catch and release of tagged peptides from a mixture of untagged peptides were established. To demonstrate the value of this methodology in identifying the binding site of a ligand to its target protein, we purified and identified a peptide containing the ligand-binding site from the tryptic digest of cathepsin B labelled with a covalent cathepsin B inhibitor containing a β-ketoamide tag.

Enantioselective 1,6-Addition of β-Ketoester Enolates to In Situ Generated para-Quinone Methides Enabled by Cooperative Palladium and Brønsted Acid Catalysis

We report herein an asymmetric cooperative process for the enantioselective 1,6-addition of β-ketoesters to in situ generated para-quinone methides with chiral Pd-aqua complexes as mixed Brønsted acid-base catalysts. Excellent yields, outstanding enantiocontrol, and good diastereoselectivity across a broad substrate range are highlights of this transformation. The utility of this reaction is further demonstrated by a facile scale up and subsequent complexity-increasing modifications.

Fast, highly enantioselective, and sustainable fluorination of 4-substituted pyrazolones catalyzed by amide-based phase-transfer catalysts

Highly enantioselective and sustainable fluorination of 4-substituted pyrazolones has been developed by amide-based phase-transfer catalysts.

Stereodivergent dehydrative allylation of β-keto esters using a Ru/Pd synergistic catalyst

α-Alkylation of a β-keto ester is a frequently used reaction for carbon–carbon bond formation. However, extension to a stereoselective reaction remains a significant challenge, because the product easily racemizes under acidic or basic conditions. Here, we report a hybrid system consisting of Pd and Ru complexes that catalyzes the asymmetric dehydrative condensation between cinnamyl-type allylic alcohols and β-keto esters. α-Non-substituted β-keto ester can be allylated to afford an α-mono-substituted product with high regio-, diastereo-, and enantioselectivity. No epimerization occurs owing to the nearly neutral conditions, which is achieved by a rapid proton transfer from Pd-enolate formation to Ru π-allyl complex formation. Four diastereomers can be synthesized on demand by changing the stereochemistry of the Pd or Ru complex. Eight stereoisomers with three adjacent stereogenic centers can be synthesized by employing diastereoselective reduction of the ketone in the products. The formal synthesis of (+)-pancratistatin demonstrates the utility of the reaction.

α-Alkylation of β-keto esters is a frequently used reaction for carbon–carbon bond formation, but a general, stereoselective version of this reaction is challenging to realize. Here, the authors report a combined ruthenium and palladium catalytic system for the asymmetric dehydrative condensation between cinnamyl-type allylic alcohols and β-keto esters.

Molecular field analysis for data-driven molecular design in asymmetric catalysis

This review highlights the recent advances (2019-present) in the use of MFA (molecular field analysis) for data-driven catalyst design, enabling to improve selectivities/reaction outcomes in asymmetric catalysis. Successful examples of MFA-based molecular design and how to design molecules by MFA are described, including how to generate and evaluate MFA-based regression models, and future challenges in MFA-based molecular design in molecular catalysis.

Enantioselective Annulation of α,β-Unsaturated N-Acyliminium Ions with β-Keto Ester Enolates via Cooperative Palladium and Brønsted Acid Catalysis

We herein report a cooperative palladium- and Brønsted acid-catalyzed strategy toward the first enantioselective annulation of in situ generated α,β-unsaturated N-acyliminium ions with chiral metal enolates. Novel polycyclic oxoisoindoles featuring three contiguous stereogenic centers have been obtained with typically good yields, outstanding enantiocontrol, and moderate to good diastereoselectivity. The utility of the process was further demonstrated by their conversion to synthetically valuable scaffolds.

Enantioselective Construction of Tertiary α-Alkyl Fluoride via BTM-Catalyzed Fluorination of α-Alkynyl-Substituted Acetic Acids

The isothiourea-catalyzed enantioselective construction of tertiary α-fluoro stereogenic centers has been demonstrated by using branched alkynyl-substituted acetic acids as starting materials, providing a broad range of optically active tertiary α-alkyl fluorides in high enantioselectivity (up to 97% ee). Furthermore, this methodology was proven to be scaled up to a Gram scale without loss of enantioselectivity.

Enantioenriched γ-Aminoalcohols, β-Amino Acids, β-Lactams, and Azetidines Featuring Tetrasubstituted Fluorinated Stereocenters via Palladacycle-Catalyzed Asymmetric Fluorination of Isoxazolinones

Enantiopure fluorine containing β-amino acids are of large biological and pharmaceutical interest. Strategies to prepare β-amino acid derivatives possessing a F-containing tetrasubstituted stereocenter at the α-C atom in a catalytic asymmetric sense are rare, in particular using an enantioselective electrophilic C-F bond formation. In the present study, a highly enantioselective palladacycle-catalyzed fluorination of isoxazolinones was developed. It is demonstrated that isoxazolinones are useful precursors toward enantiopure β-amino acid derivatives by diastereo- and chemoselective reduction. The formed γ-aminoalcohols served as valuable precursors toward β-amino acids, β-amino acid esters, and β-lactams, all featuring tetrasubstituted fluorinated stereocenters. In addition, by this work, enantioenriched fluorinated azetidines were accessible for the first time.

Enantioselective fluorination of 3-indolinone-2-carboxylates with NFSI catalyzed by chiral bisoxazolines

A chiral Cu(i)-bisoxazoline complex catalyzed enantioselective electrophilic fluorination of 3-indolinone-2-carboxylates with NFSI has been accomplished to produce chiral 2-fluoro-3-indolinone-2-carboxylates in good yields with excellent enantioselectivities. This is the first report on the enantioselective fluorination of 2-substituted 3-indolinones using a chiral Cu(i)-bisoxazoline complex.

[2.2]Paracyclophane-Based Isothiourea-Catalyzed Highly Enantioselective α-Fluorination of Carboxylic Acids

Planar chiral [2.2]paracyclophane-based isothiourea catalysts have been prepared over a few simple steps in high yields. In the presence of these catalysts, highly efficient catalytic enantioselective fluorination of carboxylic acids has been accomplished, providing a broad range of optically active α-fluoroesters in high yield and excellent enantioselectivity.

Enantiospecific deoxyfluorination of cyclic α-OH-β-ketoesters

We herein report the deoxyfluorination of cyclic α-hydroxy-β-ketoesters using diethylaminosulfur trifluoride (DAST). The reaction proceeds with excellent levels of stereospecificity, giving the configurationally inverted α-fluoro-β-ketoesters in high yields under operationally simple conditions.

Stereoselective α-Chlorination of β-Keto Esters in the Presence of Hybrid Amide-Based Cinchona Alkaloids as Catalysts

We report an enantioselective phase transfer α-chlorination of β-keto esters catalyzed by hybrid amide-based Cinchona derivatives. The chlorination process proceeds with proper quantitative yields (up to <99%) and high asymmetric induction (up to 97% ee). We show that the use of only 0.5 mol % hybrid catalyst based on a Cinchona core allows the chlorination reaction to be conducted in a highly enantioselective manner with various indanone and tetralone carboxylate esters.

Asymmetric catalytic construction of fully substituted carbon stereocenters using acyclic α-branched β-ketocarbonyls: the “Methyl Rule” widely exists

This review illustrates the recent advances in catalytic asymmetric α-functionalization of acyclic β-ketocarbonyls. A thorough survey of all these reactions indicates the existance of a general principle which is called the “Methyl Rule”.

Chiral Discrimination by a Binuclear Pd Complex Sensor Using 31P{1H} NMR

An axially chiral binuclear μ-hydroxo Pd complex (BPHP) first served as an excellent chiral sensor for discriminating a variety of analytes including amino alcohol, amino amide, amino acid, mandelic acid, diol, diamine, and monoamine by ³¹P{¹H} NMR. A detailed recognition mechanism was proposed based on the single crystal and mass spectrum of Pd-complexes. In general, BPHP sensor, through extracting the acidic hydrogen of an analyte by its Pd-OH group, forms stable diastereomeric complexes with two enantiomers of the analyte giving well distinguishable split ³¹P{¹H} NMR signals for chiral discrimination.

Stereoselective Synthesis of 2-Fluoro-1,3-Diols via Lithium Binaphtholate-Catalyzed Aldol-Tishchenko Reaction

Lithium binaphtholate, readily prepared from ( R)-3,3'-I₂-BINOL and lithium tert-butoxide, efficaciously catalyzed the enantioselective aldol-Tishchenko tandem reaction of α-fluoroketones with aldehydes, achieving the enantioselective synthesis of 2-fluoro-1,3-diols with three contiguous stereogenic centers. Kinetic studies revealed that the aldol reaction and the subsequent hemiacetal formation are in equilibrium under the reaction conditions and that the lithium binaphtholate catalyst selectively promotes hydride shift of one of the eight stereoisomers to produce 2-fluoro-1,3-diols containing a tetrasubstituted fluorinated carbon center.

Catalytic Asymmetric Fluorination of Alkyl 1-indanone-2-carboxylates Ruled by Pybox-Eu(III) Combination

A highly enantioselective catalytic method for the synthesis of quaternary α-fluoro derivatives of 3-oxo esters is described. The reaction uses europium (III) triflate and commercially available chiral pybox-type C2-symmetric ligand. Excellent results in terms of yields and enantioselectivities were assured using the electrophilic NFSI reagent under mild reaction conditions.

[RhIII(Cp*)]-catalyzed arylfluorination of α-diazoketoesters for facile synthesis of α-aryl-α-fluoroketoesters

Here we describe the Cp*Rh(iii)-catalyzed cascade arylfluorination reactions of α-diazoketoesters with arylboronic acids and N-fluorobenzenesulfonimide for one-pot C(sp3)-C(aryl) and C(sp3)-F bond formation. The arylfluorination reaction can be accomplished with remarkable chemo- and regioselectivity. Our mechanistic investigation showed that the Rh-catalyzed arylfluorination of diazoacetates occurred by (1) transmetalation of arylboronic acids to form an arylrhodium(iii) complex, (2) coupling of diazomalonates with the arylrhodium(iii) complex to form carbene-rhodium, (3) migratory carbene insertion to form a diketonato-rhodium(iii) complex - probably via rearrangement of the putative σ-alkylrhodium(iii) complex, and (4) electrophilic fluorination of the diketonato-rhodium to form the α-aryl-α-fluoromalonates.

Diastereoselective α-Fluorination of N- tert-Butanesulfinyl Imidates

A diastereoselective α-fluorination of N- tert-butanesulfinyl imidates was developed. Deprotonation of N- tert-butanesulfinyl imidates with lithium hexamethyldisilazide generates aza-enolates that can be intercepted, with excellent diastereocontrol, by the inexpensive electrophilic fluorinating agent NFSI. This protocol was applied to the preparation of synthetically useful trans-2-fluoro-cyclohexamine with high enantiomeric purity (99.5% ee).

Asymmetric fluorination of indanone-2-carboxylates using a polystyrene-supported diphenylamine-linked bis(oxazoline) complex

A highly enantioselective fluorination of indanone-2-carboxylates catalyzed by a polystyrene-supported diphenylamine-linked bis(oxazoline) (PS-box)-Cu(OTf)₂ complex has been developed in a continuous flow system. The supported complex exhibited extremely efficient catalytic performance with high activity, affording the corresponding products in excellent yields (up to 99% yield) with excellent enantioselectivities (up to 99% ee) and more than 4000 turnover number (TON).

Modern Approaches for Asymmetric Construction of Carbon-Fluorine Quaternary Stereogenic Centers: Synthetic Challenges and Pharmaceutical Needs

New methods for preparation of tailor-made fluorine-containing compounds are in extremely high demand in nearly every sector of chemical industry. The asymmetric construction of quaternary C-F stereogenic centers is the most synthetically challenging and, consequently, the least developed area of research. As a reflection of this apparent methodological deficit, pharmaceutical drugs featuring C-F stereogenic centers constitute less than 1% of all fluorine-containing medicines currently on the market or in clinical development. Here we provide a comprehensive review of current research activity in this area, including such general directions as asymmetric electrophilic fluorination via organocatalytic and transition-metal catalyzed reactions, asymmetric elaboration of fluorine-containing substrates via alkylations, Mannich, Michael, and aldol additions, cross-coupling reactions, and biocatalytic approaches.

A diastereoselective Mannich-type reaction of α-fluorinated carboxylate esters: synthesis of β-amino acids containing α-quaternary fluorinated carbon centers

We report a diastereoselective Mannich-type reaction of α-alkyl, α-aryl, and α-vinyl fluoroacetates with N-tert-butylsulfinyl imines. This method provides a powerful means to access a broad range of highly functionalized β-amino acids containing α-fluorinated quaternary stereogenic carbon centers. We also show that the stereochemical outcome of the present reaction is highly dependent on the steric and electronic properties of the fluorocarbon nucleophiles. This protocol uses readily available starting materials, tolerates a variety of functional groups, and is operationally simple.

Enantioselective Di-/Perfluoroalkylation of β-Ketoesters Enabled by Cooperative Photoredox/Nickel Catalysis

Efficient and enantioselective radical difluoroalkylation and perfluoroalkylation reactions of β-ketoesters were successfully developed through an asymmetric photoredox and nickel catalysis cascade. This protocol provides R_f-containing quaternary stereocenters in up to 67% yield and 95:5 er with ethyl iododifluoroacetate and perfluoroalkyl iodides (C₃F₇I and C₄F₉I) as radical sources under extremely mild conditions.

Electrophilic fluorination of stereodefined disubstituted silyl ketene hemiaminals en route to tertiary α-fluorinated carbonyl derivatives

A highly diastereoselective synthesis of tertiary α-fluoro carbonyl compounds is reported in only two chemical steps from a simple alkyne through the reaction of stereodefined fully substituted silyl ketene hemiaminal derivatives with Selectfluor.

Forging Fluorine-Containing Quaternary Stereocenters by a Light-Driven Organocatalytic Aldol Desymmetrization Process

Reported herein is a light-triggered organocatalytic strategy for the desymmetrization of achiral 2-fluoro-substituted cyclopentane-1,3-diketones. The chemistry is based on an intermolecular aldol reaction of photochemically generated hydroxy-o-quinodimethanes and simultaneously forges two adjacent fully substituted carbon stereocenters, with one bearing a stereogenic carbon-fluorine unit. The method uses readily available substrates, a simple chiral organocatalyst, and mild reaction conditions to afford an array of highly functionalized chiral 2-fluoro-3-hydroxycyclopentanones.

Chiral phase-transfer catalysis in the asymmetric α-heterofunctionalization of prochiral nucleophiles

Chiral phase-transfer catalysis is one of the major catalytic principles in asymmetric catalysis. A broad variety of different catalysts and their use for challenging applications have been reported over the last decades. Besides asymmetric C–C bond forming reactions the use of chiral phase-transfer catalysts for enantioselective α-heterofunctionalization reactions of prochiral nucleophiles became one of the most important field of application of this catalytic principle. Based on several highly spectacular recent reports, we thus wish to discuss some of the most important achievements in this field within the context of this review.

Catalytic Enantioselective [3 + 2] Cycloaddition of α-Keto Ester Enolates and Nitrile Oxides

An enantioselective [3 + 2] cycloaddition reaction between nitrile oxides and transiently generated enolates of α-keto esters has been developed. The catalyst system was found to be compatible with in situ nitrile oxide-generation conditions. A versatile array of nitrile oxides and α-keto esters could participate in the cycloaddition, providing novel 5-hydroxy-2-isoxazolines in high chemical yield with high levels of diastereo- and enantioselectivity. Notably, the optimal reaction conditions circumvented concurrent reactions via O-imidoylation and hetero-[3 + 2] pathways.