Organocatalytic synthesis of optically active β-branched α-amino esters via asymmetric biomimetic transamination

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.

Catalytic Enantioselective Tautomerization of Metastable Enamines

The first example of catalytic enantioselective tautomerization of structurally labile but isolable enamines for accessing their chiral imine-tautomers is described. Kinetically stable enamine-based dibenzo[b,d]azepines were tautomerized by a simple chiral BINOL-phosphoric acid, providing a variety of seven-membered imine products bearing both central and axial stereogenic elements in good yields (up to 96%) with excellent enantio- and diastereoselectivities (up to 97% ee, >20:1 dr).

Phosphine-Catalyzed Asymmetric Synthesis of α-Quaternary Amine via Umpolung γ-Addition of Ketimines to Allenoates

A first phosphine-catalyzed enantioselective umpolung γ-addition of ketimines to allenoates has been developed that provides efficient access to optically active γ,δ-unsaturated α-amino esters and δ-amino esters with a chiral tertiary stereocenter under mild conditions. The salient features of this reaction include general substrate scope, mild conditions, good yields, high enantioselectivity, ease of scale-up to gram scale, and further transformations.

Synthesis of 2,3-Dialkylated Tartaric Acid Esters via Visible Light Photoredox-Catalyzed Reductive Dimerization of α-Ketoesters

A mild transition-metal-free protocol to prepare 2,3-dialkylated tartaric acid esters has been developed by taking advantage of a visible light photoredox-catalyzed reductive dimerization of α-ketoesters with a combination of an organic dye photocatalyst and a Hantzsch-type 1,4-dihydropyridine hydrogen donor. A broad range of functional groups including cyclopropane, alkene, alkyne, 4-methoxybenzyl ether, acetal, silyl ether, carbamate, cyclic ether, cyclic thioether, bromoalkane, and N-alkoxyphthalimide are well-compatible. By employing the visible light photoredox-catalyzed reductive coupling and the subsequent optical resolution, both enantioenriched diastereomers of 2,3-dialkylated tartaric acid could be acquired conveniently.

Progress in asymmetric biomimetic transamination of carbonyl compounds

Transamination of α-keto acids is an important process to form α-amino acids in biological systems. Various biomimetic transamination systems have been developed for carbonyl compounds with chiral vitamin B₆ analogues, artificial transaminase mimics, chiral nitrogen sources, and chiral catalysts. This review provides a brief summary in this area.