Dynamic Kinetic Resolution of Azlactones by a Chiral N,N-Dimethyl-4-aminopyridine Derivative Containing a 1,1′-Binaphthyl Unit: Importance of Amide Groups

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.

ArPNO-Catalyzed Acylative Dynamic Kinetic Resolution of 3-Hydroxyphthalides: Access to Enantioenriched Phthalidyl Esters

A chiral 4-aryl-pyridine-N-oxide nucleophilic organocatalyst was used to synthesize chiral phthalidyl ester prodrugs by the acylative dynamic kinetic resolution process. By using the 3,5-dimethylphenyl-derived ArPNO catalyst, the phthalidyl esters were obtained in up to 97% yield with 97% ee at room temperature. Two phthalidyl esters of prodrugs, talosalate and talmetacin, were generated. By control experiments and density functional theory calculations, an acyl transfer mechanism was proposed.

Pd(II) Complexes of Chiral Proline-Derived Ligands: Application for Dynamic Thermodynamic Resolution of α-Amino Acids and Their Antibacterial Activities

Novel type of Pd(II) complexes have been synthesized under operationally simple and convenient conditions and applied in the dynamic thermodynamic resolution of racemic N,C-unprotected α-amino acids. After rapid hydrolysis, these Pd(II) complexes produced the corresponding α-amino acids in satisfactory yields and enantioselectivities, accompanied by the recyclable proline-derived ligand. In addition, the method can be readily applied for S/R interconversion to obtain unnatural (R)-α-amino acids from readily available (S)-α-amino acids. Furthermore, biological assays showed that Pd(II) complexes (S,S)-3i and (S,S)-3m exhibited significant antibacterial activities similar to vancomycin, which may represent promising lead structures for further development of antibacterial agents.

A Facile Synthesis of 2-Aminopropane-1,2,3-tricarboxylic Acid and Its Symmetrical Dimethyl Ester

A new convenient synthetic route to 2-aminopropane-1,2,3-tricarboxylic acid is described. The first two stages of the three-step synthesis are performed in a one-pot procedure and include the cyclization of hippuric acid with DCC followed by treatment with methyl bromoacetate to yield an alkylated oxazolone. Its hydrolysis with HCl provides 2-aminopropane-1,2,3-tricarboxylic acid as its HCl salt. Esterification of the resulting acid with methanol in the presence of thionyl chloride leads selectively to its symmetrical diester.

Origin of Enantioselectivity in Chiral Phosphoric-Acid-Catalyzed Azlactone Dynamic Kinetic Resolution

Theoretical calculations, associated with control experiments, were carried out to gain insights into the mechanism and origin of enantioselectivity in the phosphoric-acid-catalyzed dynamic kinetic resolution of azlactones. The results revealed a Münchnone intermediate as the key species involved in the isomerization of azlactone rings. The developed model was successfully employed in the comprehension and prediction of enantioselectivity under diverse of reaction conditions, including alcoholysis and aminolysis protocols.

The kinetic resolution of oxazinones by alcoholysis: access to orthogonally protected β-amino acids

The catalytic, alcoholytic kinetic resolution of oxazinones is reported. A novel, stereochemically dense cinchona alkaloid-based catalyst can facilitate the highly enantiodiscriminatory (S up to 101) ring-opening of oxazinones equipped with electrophilic aryl units to generate orthogonally protected β-amino acids for the first time.

Rational Design of 2-Substituted DMAP-N-oxides as Acyl Transfer Catalysts: Dynamic Kinetic Resolution of Azlactones

A novel concept that conversion of chiral 2-substituted DMAP into its DMAP-N-oxide could significantly enhance the catalytic activity and still be used as an acyl transfer catalyst is presented. A new type of chiral 2-substituted DMAP-N-oxides, derived from l-prolinamides, has been rationally designed, facilely synthesized, and applied in the dynamic kinetic resolution of azlactones. Using simple MeOH as the nucleophile, various l-amino acid derivatives were produced in high yields (up to 98% yield) and enantioselectivities (up to 96% ee). Furthermore, α-deuterium labeled l-phenylalanine derivative was also obtained. Experiments and DFT calculations revealed that in 2-substituted DMAP-N-oxide, the oxygen atom acted as the nucleophilic site and the N-H bond functioned as the H-bond donor. High enantioselectivity of the reaction was governed by steric factors, and the addition of benzoic acid reduced the activation energy by participating in the construction of a H-bond bridge. The theoretical chemical study indicated that only when attack directions of the chiral catalyst were fully considered could the correct calculation results be obtained. This work paves the way for the utilization of the C2 position of the pyridine ring and the development of chiral 2-substituted DMAP-N-oxides as efficient acyl transfer catalysts.

Catalytic Direct α-Amination of Arylacetic Acid Synthons with Anilines

A unique α-amination approach using various anilines has been developed for arylacetic acids via adaptation as benzazoles. The reaction proceeds through a single electron transfer mechanism utilizing an iron-based catalyst system to access α-(N-arylamino)acetic acid equivalents. Modification of approved drugs, facile cleavage of the benzazole auxiliary, and tolerance of amide linkage forming conditions constitute the potential applicability of this strategy.

Axial chirality in biaryl N,N-dialkylaminopyridine derivatives bearing an internal carboxy group

Axial chirality in N,N-dimethylaminopyridines as well as N,N-dipropylaminopyridines bearing an internal carboxy group were evaluated based on their racemization barriers and circular dichroism spectra. The half-life of racemization of N,N-dipropylaminopyridine derivative 2 was estimated to be 19.7 days at 20°C. Its enantiomers isolated as optically active forms showed positive-negative and negative-positive Cotton effects for (+)-2 and (-)-2, respectively, from 310 to 210 nm. Furthermore, (-)-2 was applied as a chiral nucleophilic catalyst and exhibited asymmetric induction in acylative kinetic resolution of 1-(1-naphthyl)ethane-1-ol.