Enantioselective and Diastereodivergent Access to α-Substituted α-Amino Acids via Dual Iridium and Copper Catalysis

Stereodivergent Construction of 1,3-Chiral Centers via Tandem Asymmetric Conjugate Addition and Allylic Substitution Reaction

Herein, we report a synthesis of cyclohexanones bearing multi-continuous stereocenters by combining copper-catalyzed asymmetric conjugate addition of dialkylzinc reagents to cyclic enones with iridium-catalyzed asymmetric allylic substitution reaction. Good to excellent yields, diastereoselectivity and enantioselectivity can be obtained. Unlike the stereodivergent construction of adjacent stereocenters (1,2-position) reported in the literature, the current reaction can achieve the stereodivergent construction of nonadjacent stereocenters (1,3-position) by a proper combination of two chiral catalysts with different enantiomers.

Boron-Catalyzed α-Functionalizations of Carboxylic Acids

Catalytic, chemoselective, and asymmetric α-functionalizations of carboxylic acids promise up-grading simple feedstock materials to value-added functional molecules, as well as late-stage structural diversifications of multifunctional molecules, such as drugs and their leads. In this personal account, we describe boron-catalyzed α-functionalizations of carboxylic acids developed in our group (five reaction types). The reversible boron carboxylate formation is key to the acidification of the α-protons and enolization using mild organic bases, allowing for chemoselective and asymmetric bond formations of carboxylic acids. The ligand effects on reactivity and stereoselectivity, substrate scopes, and mechanistic insights are summarized.

Copper(I)-Catalyzed Asymmetric Synthesis of Unnatural α-Amino Acid Derivatives and Related Peptides Containing γ-(aza)Aryls

Chiral α-amino acids are indispensable compounds in organic chemistry, biochemistry, and medicinal chemistry. Herein, by means of copper(I)-catalyzed asymmetric conjugate addition of derivatives of glycine, serine, cysteine, and β-amino-alanine to electron-deficient vinyl(aza)arenes, an array of novel unnatural chiral α-amino acid derivatives bearing a γ-(aza)aryl is prepared in moderate to high yields with high enantioselectivity. Various azaarenes, such as pyrimidine, 1,3,5-triazine, pyridine, pyridine-N-oxide, quinoline, quinoxaline, purine, benzo[d]imidazole, benzothiazole, and 1,2,4-oxadiazole, are well tolerated. Moreover, the electrophiles are nicely extended to (Z)/(E) mixtures of electron-deficient butadienylpyridine and benzene, which are transformed to the corresponding chiral α-amino acid derivatives in high (E)/(Z) ratio and high enantioselectivity. More importantly, the present methodology is successfully applied in the catalytic asymmetric functionalization of Schiff bases derived from peptides, which finally afforded a new chiral tripeptide bearing two electron-deficient azaaryls and one electron-deficient aryl in high total yield with high diastereo- and excellent enantioselectivities.

DFT-Based Stereochemical Rationales for the Bifunctional Brønsted Acid/Base-Catalyzed Diastereodivergent and Enantioselective aza-Henry Reactions of α-Nitro Esters

A pair of chiral bis(amidine) [BAM] proton complexes provide reagent (catalyst)-controlled, highly diastereo- and enantioselective direct aza-Henry reactions leading to α-alkyl-substituted α,β-diamino esters. A C₂-symmetric ligand provides high anti-selectivity, while a nonsymmetric congener exhibits syn-selectivity in this example of diastereodivergent, enantioselective catalysis. A detailed computational analysis is reported for the first time, one that supports distinct models for selectivity resulting from the more hindered binding cavity of the C₁-symmetric ligand. Binding in this congested pocket accommodates four hydrogen bond contacts among ligands and substrates, ultimately favoring a pre-syn arrangement highlighted by pyridinium-azomethine activation and quinolinium-nitronate activation. The complementary transition states reveal a wide range of alternatives. Comparing the C₁- and C₂-symmetric catalysts highlights distinct electrophile binding orientations despite their common hydrogen bond donor-acceptor features. Among the factors driving unusual high syn-diastereoselection are favorable dispersion forces that leverage the anthracenyl substituent of the C₁-symmetric ligand.

Stereodivergent silver-catalyzed synthesis of pyroglutamic acid esters

We report here a silver-catalyzed method for the enantio- and diastereodivergent synthesis of chiral pyroglutamic acid esters with multiple stereocenters. This process proceeds through asymmetric conjugate addition of glycine imine esters to a broad range of β-substituted α,β-unsaturated perfluorophenyl esters followed by lactamization. By leveraging catalyst control and stereospecificity of the 1,4-addition process, all four product stereoisomers containing two adjacent stereocenters are accessible with high stereoselectivity.

Copper/Lewis base cooperatively catalyzed asymmetric allylic alkylation of Morita-Baylis-Hillman carbonates with azomethine ylides

In this paper, an asymmetric allylic alkylation of easily available azomethine ylides with Morita-Baylis-Hillman (MBH) carbonates through a copper (i)/Lewis base cooperative catalysis strategy has been realized. The co-catalyzed asymmetric allylic alkylation provided the corresponding amino acid derivatives in up to 90% yields with up to 99% ee as well as good to excellent regioselectivity.

Synergistic Cu/Pd-catalyzed asymmetric allylation: a facile access to α-quaternary cysteine derivatives

An efficient synthetic methodology to access biologically important and synthetically useful α-quaternary cysteine derivatives via asymmetric catalytic α-allylation of readily available 2-thiazoline-4-carboxylates was successfully developed through a synergistic Cu/Pd catalytic system. A wide array of α-quaternary cysteine derivatives were obtained in moderate to high yields with good to excellent enantioselectivities (45-98% yields and 69->99% ee). Gram-scale asymmetric allylation was performed to obtain high yields maintaining the enantioselectivity. Moreover, some synthetic transformations to access chiral spirocyclic compounds proceeded smoothly, which exhibited the important utility of this methodology.

Synergistic Ir/Cu Catalysis for Asymmetric Allylic Alkylation of Oxindoles: Enantio- and Diastereoselective Construction of Quaternary and Tertiary Stereocenters

3,3-Disubstituted oxindoles bearing quaternary and tertiary stereogenic centers are privileged structural motifs, which widely exist in pharmaceutical and natural products. Herein, a highly regio-, enantio-, and diastereoselective allylic alkylation of 3-alkyl oxindoles through synergistic iridium and copper catalysis is described, which provides a series of 3,3-disubstituted oxindole derivatives containing adjacent quaternary and tertiary stereogenic centers in excellent yields, enantiomeric excess, and diastereomeric ratio (for 30 examples, up to 97 % yield, >99 % ee, and >20 : 1 dr). This method provides exclusive branched selectivity, excellent enantio- and diastereoselectivities, and good functional compatibility. Control experiments suggested that the chiral copper catalyst is required for achieving high reactivities and diastereoselectivities under mild reaction conditions.

Au(I)/(R)-BINOL-Ti(IV) Concerted Catalyzed Asymmetric Cascade Cycloaddition Reaction of Arylalkynols

An efficient catalytic asymmetric cascade cycloaddition reaction of arylalkynols with dioxopyrrolidines was developed. This reaction was achieved using Au(I) and (R)-BINOL-Ti(IV) bimetallic catalysts and exclusively delivered a series of chiral oxo-bridged bicyclic benzooxacine compounds in up to 86% yield with 96% ee as well as >33:1 dr. Meanwhile, three new σ bonds and three new stereogenic centers were formed in a one-pot process.

Stereodivergent Pd/Cu catalysis: asymmetric alkylation of racemic symmetrical 1,3-diphenyl allyl acetates

A stereodivergent Pd/Cu catalyst system was developed, which was successfully applied to the asymmetric allylic alkylation of symmetrical 1,3-disubstituted allyl acetates with prochiral imino esters in a fully stereodivergent manner.

Stereodivergent Carbon-Carbon Bond Formation between Iminium and Enolate Intermediates by Synergistic Organocatalysis

We report here a stereodivergent method for the Michael addition of aryl acetic acid esters to α,β-unsaturated aldehydes catalyzed by a combination of a chiral pyrrolidine and a chiral Lewis base. This reaction proceeds through a synergistic catalytic cycle which consists of one cycle leading to a chiral iminium electrophile and a second cycle generating a nucleophilic chiral enolate for the construction of a carbon-carbon bond. By varying the combinations of catalyst enantiomers, all four stereoisomers of the products with two vicinal stereocenters are accessible with high enantio- and diastereoselectivity. The products of the Michael addition, 1,5-aldehyde esters, can be readily transformed into a variety of other valuable enantioenriched structures, including those bearing three contiguous stereocenters in an acyclic system, thus providing an efficient route to an array of structural and stereochemical diversity.

Palladium-Catalyzed Regio- and Stereoselective Cross-Coupling of Vinylethylene Carbonates with Ketimine Esters to Generate (Z)-Tri- and Tetra-substituted Allylic Amino Acid Derivatives

Herein we report the palladium-catalyzed regio- and stereoselective cross-coupling of vinylethylene carbonates with ketimine esters to construct allylic amino acid scaffolds. This operationally simple protocol furnished (Z)-tri- and tetra-substituted allylic amino acid derivatives in good to excellent yields with distinguished geometric control under mild reaction conditions and proved to be sufficient in large-scale synthesis while retaining excellent reactivity and stereoselectivity, highlighting the practical value of this transformation.

A Pd-catalyzed asymmetric allylic substitution cascade via an asymmetric desymmetrization for the synthesis of bicyclic dihydrofurans

Chiral bicyclic dihydrofurans bearing two vicinal carbon stereocenters have been synthesized in high yields and with up to 97% ee via a Pd-catalyzed asymmetric allylic substitution cascade and an asymmetric desymmetrization process.