Chiral Aldehyde Catalysis-Enabled Asymmetric α-Functionalization of Activated Primary Amines

Direct Enantioselective Allylic Alkylation of α-Amino Esters to Quaternary Glutamates via Strategic Pyridoxal Catalyst Design

The difficulty for N-unprotected α-substituted glycinates as α-C nucleophiles originates from both competing N nucleophilic interference and steric hindrance of the α substituent, which makes direct asymmetric α-C alkylation of N-unprotected α-substituted glycinates with Morita-Baylis-Hillman (MBH) adducts especially challenging. Given the wide utilization of α-quaternary chiral glutamic acid derivatives in therapeutic studies, we took advantage of biomimetic carbonyl catalysis to achieve their efficient synthesis. A bifunctional centrally chiral pyridoxal, featured with an expanded catalytic cavity and reduced steric hindrance around the aldehyde group, was designed and synthesized. In this work, we describe the novel centrally chiral pyridoxal enabled direct asymmetric α-C alkylation of N-unprotected α-substituted glycinates with MBH acetates. A broad range of α-quaternary chiral glutamic acid derivatives with multiple modifications were produced with high reactivity and excellent stereocontrol.

Ternary Aldehyde-Copper-Iridium Catalysis Enables Stereodivergent Allylation via α-C-H Functionalization of Primary Amines

α-Chiral primary amines are recognized as one of the most valuable and versatile synthetic intermediates, widely utilized in the construction of diverse amine-containing natural products, pharmaceuticals, and agrochemicals. The direct asymmetric α-C-H functionalization of unprotected primary amines is the most straightforward method for creating these motifs. However, this transformation remains underdeveloped, particularly in stereodivergent synthesis of primary amines with multiple stereocenters. Herein, we report an aldehyde/copper/iridium ternary catalytic system, which was successfully employed for the direct enantio- and diastereodivergent α-allylation of primary α-amino-chromanone without requiring additional protection or activation of the NH2 group. A wide range of α-tertiary primary amines bearing vicinal stereocenters were prepared in high yields with excellent enantio- and diastereoselectivities (generally >20:1 dr and >99% ee). Notably, all four stereoisomers of the α-tertiary amines can be readily prepared by simply switching the configuration combinations of the two chiral metal catalysts. Furthermore, the asymmetric induction model for the α-C-H functionalization of primary amines was meticulously elucidated through comprehensive density functional theory (DFT) calculations.

Pyridoxal-Inspired Photo-Decarboxylase Catalysis: Photochemical Decarboxylation of Unprotected Amino Acids

A photochemical organocatalytic method for the protodecarboxylation of unprotected amino acids is reported. Inspired by pyridoxal 5'-phosphate-dependent decarboxylase enzymes, the catalytic activation of amino acid substrates by 3-hydroxyisonicotinaldehyde enables a photochemical decarboxylation event, which can be leveraged in combination with a thiol co-catalyst. The necessary and sufficient structural features of the pyridoxal-like framework for photoactivity are determined using ultraviolet-visible absorption spectroscopy. A broad scope of unprotected amino acids can be decarboxylated in this system, with selectivity between multiple carboxylates realized on the possible basis of hyperconjugation. The ability to engage simple amino acids in decarboxylative functionalization at ambient conditions using a pyridoxal-mimicking organocatalyst enables new possibilities for the translation of biogenic amino acids into medicinally valuable amines.

Construction of Axially Chiral Dialdehydes via Rhodium-Catalyzed Enantioselective C-H Amidation

Achieving axially chiral biaryl dialdehydes through asymmetric catalysis remains significantly challenging due to the lack of efficient strategies. In this report, we developed a rhodium-catalyzed enantioselective C-H amidation through chiral transient directing group strategy. With this new approach, a series of axially chiral amido dialdehydes were achieved in up to 86 % yields with 99.5 : 0.5 er. Furthermore, detailed mechanistic studies indicated that both the imine formation and C-H bond cleavage steps were reversible. More interestingly, the X-ray crystallographic analysis of Int-2 showed probable C-H/π interaction between biaryl group and chiral amine moiety. This process offered a convenient route to access axially chiral dialdehyde derivatives. More broadly, it demonstrated a new tool through transient and C-H/π synergistic interactions, which would stimulate further development of asymmetric catalytic system in enantioselective C-H functionalization.

Half-Sandwich Ru(II) Complexes Featuring Metal-Centered Chirality: Configurational Stabilization by Ligand Design, Preparation via Kinetic Resolution, and Application in Asymmetric Catalysis

While it is well established that half-sandwich Ru(II) complexes possess metal-centered chirality, effective strategies to leverage their metal-centered chirality toward asymmetric synthesis have been challenging due to the configurational lability of the metal stereocenters. The metal-centered chirality is typically mediated by enantiopure ligands, which occupy a relatively narrow chemical space compared to their achiral counterparts. We demonstrate that achiral ligands can be used to access chiral-at-ruthenium half-sandwich complexes with exceptionally high configurational stability. Key to success is the introduction of a rigid bidentate ligand with a minimized dihedral angle between the pyridyl and phenolic moieties, which proved effective for preventing racemization. Computational studies revealed the energetic factors contributing to the exceptional configurational stability enabled by the rigid and planar structure of the successful ligand design. These optically active chiral-at-ruthenium complexes incorporating aldehyde moieties were obtained by NHC-catalyzed kinetic resolution with excellent selectivities (s-factor up to >200, ee up to 99%). We further demonstrate that they are highly effective chiral aldehyde catalysts for the asymmetric 1,6-conjugate addition of glycine ester and para-quinone methide.

Enantioselective Three-Component α-Allylic Alkylation of α-Amino Esters by Synergistic Photoinduced Pd/Carbonyl Catalysis

Photoinduced excited-state Pd catalysis has emerged as an intriguing strategy for unlocking new reactivity potential of simple substrates. However, the related transformations are still limited and the enantiocontrol remains challenging. Organocatalysis displays unique capability in substrate activation and stereocontrol. Combination of organocatalysis and photoinduced excited-state Pd catalysis may provide opportunities to develop new enantioselective reactions from simple substrates. By applying cooperative triple catalysis including excited-state Pd catalysis, ground-state Pd catalysis, and carbonyl catalysis, we have successfully realized enantioselective α-allylic alkylation of α-amino esters with simple styrene and alkyl halide starting materials. The reaction allows rapid modular assembly of the three reaction partners into a variety of chiral quaternary α-amino esters in good yields with 90-99 % ee, without protecting group manipulations at the active NH2 group. The cooperation of the chiral pyridoxal catalyst and the chiral phosphine ligand accounts for the excellent chirality induction.

Chiral Aldehyde/Palladium Catalysis Enables Asymmetric Branched-Selective Ring-Opening Functionalization of Methylenecyclopropanes with Amino Acid Esters

Achieving catalytic asymmetric functionalization of methylenecyclopropanes (MCPs) by selective C-C bond cleavage is a notable challenge due to the intricate reaction partners involved. In this work, we report that chiral aldehyde/palladium combined catalysis enables the asymmetric functionalization of MCPs with NH2-unprotected amino acid esters. This reaction proceeds through a regiospecific branched ring-opening mechanism, resulting in optically active α,α-disubstituted α-amino acid esters bearing nonconjugated terminal alkene units. Mechanism studies indicate that the ring-opening pathways are irreversible and the ultimate regioselectivity is governed by palladium catalysis. The products can be utilized in the construction of chiral dihydropyrazoles, α-methyl aspartic acid derivatives, and analogues of VPC01091 and BMS-986104.

Direct Enantioselective α-C-H Conjugate Addition of Propargylamines to α,β-Unsaturated Ketones via Carbonyl Catalysis

Direct asymmetric α-C-H conjugate addition of propargylamines to α,β-unsaturated ketones remains a great challenge due to the low α-amino C-H acidity of propargylamines and the nucleophilic interference of the NH2 group. Utilizing a new type of pyridoxals featuring a benzene-pyridine biaryl skeleton and a bulky amide side chain as carbonyl catalyst, we have accomplished direct asymmetric α-C-H conjugate addition of NH2-unprotected propargylamines to α,β-unsaturated ketones. The adducts undergo subsequent in situ intramolecular cyclization, delivering a wide range of chiral polysubstituted 1-pyrrolines in high yields (up to 92%) with excellent diastereo- and enatioelectivities (up to >20:1 dr and 99% ee). This work has demonstrated a straightforward approach to access pharmaceutically important chiral 1-pyrrolines, and it has also provided an impressive instance of direct asymmetric functionalization of inert C-H bonds enabled by biomimetic organocatalysts.

Asymmetric α-Allylation of Amino Acid Esters with Alkynes Enabled by Chiral Aldehyde/Palladium Combined Catalysis

A highly efficient, atom-economical α-allylation reaction of NH2-unprotected amino acid esters and alkynes is achieved by chiral aldehyde/palladium combined catalysis. A diverse range of α,α-disubstituted nonproteinogenic α-amino acid esters are produced in 31-92% yields and 84-97% ee values. The allylation products are utilized for the synthesis of drug molecule BMS561392 and other chiral molecules possessing complex structures. Mechanistic investigations reveal that this reaction proceeds via a chiral aldehyde-/palladium-mediated triple cascade catalytic cycle.

Catalytic Asymmetric Reductive Amination for Axially Chiral Aryl Aldehydes via Desymmetrization/Kinetic Resolution Cascade

Herein we present an efficient chiral phosphoric-acid-catalyzed atropoenantioselective asymmetric reductive amination of biaryl dialdehydes. The process involves desymmetrization and the following kinetic resolution, with a wide range of axially chiral aryl aldehydes obtained with high optical purities.

Asymmetric bifunctionalization of allenes with aryl iodides and amino acids enabled by chiral aldehyde/palladium combined catalysis

Even though catalytic asymmetric bifunctionalization of allenes has been extensively studied, almost all of the reported examples have been achieved in a two-component manner. In this study, we report a highly efficient asymmetric bifunctionalization of allenes with iodohydrocarbons and NH2-unprotected amino acid esters. The adopted chiral aldehyde/palladium combined catalytic system precisely governs the chemoselectivity, regioselectivity, and stereoselectivity of this three-component reaction. A wide range of substituted aryl iodides, allenes and amino acid esters can well participate in this reaction and deliver structurally diverse α,α-disubstituted α-amino acid esters with excellent experimental outcomes. One of the resulting products is utilized for the total synthesis of the molecule (S,R)-VPC01091.

Asymmetric three-component Tsuji-Trost allylation reaction enabled by chiral aldehyde/palladium combined catalysis

Despite the long-standing exploration of the catalytic asymmetric Tsuji-Trost allylation reaction since the mid-20th century, most reported instances have adhered to a two-component approach. Here, we present a remarkably efficient three-component asymmetric allylation reaction enabled by the collaborative action of chiral aldehyde and palladium. A diverse array of NH2-unprotected amino acid esters, aryl or alkenyl iodides, and allyl alcohol esters exhibit robust participation in this reaction, resulting in the synthesis of structurally diverse non-proteinogenic α-amino acid esters with favorable experimental outcomes. Mechanistic investigations reveal the dominance of the allylation/Heck coupling cascade in reactions involving electron-rich aryl iodides, while the Heck coupling/allylation cascade emerges as the dominant pathway in reactions involving electron-deficient aryl iodides. This chiral aldehyde/palladium combining catalytic system precisely governs the chemoselectivity of C-allylation and N-allylation, the regioselectivity of linear and branched allylation, and the enantioselectivity of C-allylation products.

Enantioselective organocatalytic strategies to access noncanonical α-amino acids

Organocatalytic asymmetric synthesis has evolved over the years and continues to attract the interest of many researchers worldwide. Enantiopure noncanonical amino acids (ncAAs) are valuable building blocks in organic synthesis, medicinal chemistry, and chemical biology. They are employed in the elaboration of peptides and proteins with enhanced activities and/or improved properties compared to their natural counterparts, as chiral catalysts, in chiral ligand design, and as chiral building blocks for asymmetric syntheses of complex molecules, including natural products. The linkage of ncAA synthesis and enantioselective organocatalysis, the subject of this perspective, tries to imitate the natural biosynthetic process. Herein, we present contemporary and earlier developments in the field of organocatalytic activation of simple feedstock materials, providing potential ncAAs with diverse side chains, unique three-dimensional structures, and a high degree of functionality. These asymmetric organocatalytic strategies, useful for forging a wide range of C-C, C-H, and C-N bonds and/or combinations thereof, vary from classical name reactions, such as Ugi, Strecker, and Mannich reactions, to the most advanced concepts such as deracemisation, transamination, and carbene N-H insertion. Concurrently, we present some interesting mechanistic studies/models, providing information on the chirality transfer process. Finally, this perspective highlights, through the diversity of the amino acids (AAs) not selected by nature for protein incorporation, the most generic modes of activation, induction, and reactivity commonly used, such as chiral enamine, hydrogen bonding, Brønsted acids/bases, and phase-transfer organocatalysis, reflecting their increasingly important role in organic and applied chemistry.