α-Amino Acids: An Emerging Versatile Synthon in Visible Light-Driven Decarboxylative Transformations

Enantioselective Desymmetrization of Biaryls via Cooperative Photoredox/Brønsted Acid Catalysis and Its Application to the Total Synthesis of Ancistrobrevolines

Photoredox catalysis has emerged as a powerful tool for forming and breaking chemical bonds, further taking hold with its integration with asymmetric catalysis. While the dual-catalytic approach has led to successful examples of the control of stereogenic centers, the control of stereogenic axes has remained underexplored. In this study, an acylimine intermediate was generated through photoredox catalysis, and a symmetric substrate, 2-arylresorcinol, was desymmetrized with the aid of chiral phosphoric acid catalysis. Using this approach, a stereogenic center and stereogenic axis were successfully controlled to provide a natural-product-driven compound. The origins of enantioselectivity and diastereoselectivity were investigated through a density functional theory study of four possible enantiodetermining transition states. Consequently, the first total syntheses of the ring-contracted naphthylisoquinoline alkaloid ancistrobrevolines A and B were accomplished concisely. This approach provides not only a novel methodology and strategy to synthesize naphthylisoquinoline alkaloids but also a direction to advance catalytic research and total synthesis studies.

Reflexive Chirality Transfer (RCT): Asymmetric 1,3-Dipolar Cycloaddition of α-Amino Acid Schiff Base with Nonchiral Copper Catalyst

Although optically pure α-amino acids are ubiquitous, their chirality is usually lost during the α-C-H deprotonation. Consequently, precious chiral catalysis has been necessary to synthesize optically active α-tetrasubstituted unnatural α-amino acid derivatives, even when starting with optically pure α-amino acids. However, here, we report a catalytic asymmetric 1,3-dipolar cycloaddition that preserves the α-carbon chirality of α-amino acid derivatives. This process directly converts readily available optically active α-amino acid Schiff bases into optically active α-tetrasubstituted pyrrolidine derivatives without external chiral additives, despite the temporary loss of α-carbon chirality through the formation of planar 1,3-dipole intermediates. Mechanistic studies revealed that the α-carbon chirality of the α-amino acid Schiff base is transiently transferred to metal-centered chirality in enolates and subsequently restored as the carbon-centered chirality of the products. This conceptually novel "reflexive chirality transfer (RCT)" strategy offers a simple and cost-effective approach to optically active unnatural α-amino acid derivatives, addressing the current limitations of chiral pool synthesis.

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.

Mechanistic Insight into the Photoinduced Decarboxylative Radical Addition of Carboxylic Acid to Alkenes in a Two-Molecule Photoredox System

The detailed mechanism of photoinduced decarboxylative radical addition to alkenes using both the effect of an electron donor (ED)/electron acceptor (EA) and laser flash photolysis in a two-molecule photoredox system was investigated. The concentration of EA•- played an important role in the photoreaction and could be controlled by varying the concentrations of ED/EA and their identity, which influenced ΔGPET. Higher concentrations of ED/EA and a larger negative ΔGPET led to a higher concentration of EA•-, thereby increasing the yield of the adduct; however, the large negative ΔGPET for the generation of the EDA complex hindered decarboxylation. The two-molecule photoredox system is simple and can be easily tuned by adjusting the concentrations and type (ΔGPET) of ED/EA through a simple replacement of ED/EA, which is easier than modulating the oxidation/reduction potential in one-molecule photoredox systems.

Visible-light-induced decarboxylative cyclization

The application of visible light as an energy source provides a new avenue in organic transformation due to its mildness, efficiency and selectivity. In fact, recent years have witnessed remarkable advances in photoinduced decarboxylative coupling reactions involving carboxylic acids and their derivatives. Under appropriate photoredox conditions they undergo single electron transfer (SET), resulting in reactive radicals which can assemble with suitable reaction partners. Many types of carboxylic acid derivatives, such as amino acids, N-hydroxy phthalimide (NHPI) esters, α-keto acids, aliphatic/aromatic carboxylic acids, and [bis(difluoroacetoxy)iodo]benzene, can couple with a wide variety of substrates to build structurally complex molecules. The present review summarizes the last five years of progress (2020-2024) in the decarboxylative cyclization of carboxylic acids for constructing carbo-/heterocycles under visible-light irradiation. Annulation could be attained via organophotocatalysis (4CzIPN, g-C3N4, Eosin Y, methylene blue, etc.), metallaphotocatalysis or photocatalyst-free approaches. With an emphasis on the mechanistic rationales and scope of the reactions, this review focuses on recent trends in this emerging area.

Formal Transformation of Benzylic Carboxylic Acids to Phenols

Phenols play a crucial role as core structural motifs in natural products and also serve as fundamental building blocks in synthetic chemistry. Apart from the known protocols for the conversion of aryl precursors to phenols (i. e., decarboxylative oxygenation), we report here the efficient synthesis of phenols from the stable and readily available benzylic carboxylic acids under mild reaction conditions. The photocatalytic conversion of carboxylic acids to peroxides is a crucial step in this strategy, allowing the subsequent C-O bond formation via Hock rearrangement.

One-Pot Photocascade Catalysis: Access to Pyrrole Derivatives from N-Arylglycines and Morita-Baylis-Hillman (MBH) Acetates

The step-economical synthesis of pyrrole derivatives has posed a challenge in the field of N-heterocyclic chemistry. A novel photocascade catalytic radical SN2'-type reaction/radical addition/annulation sequence of MBH acetates provides a straightforward route to pyrrole derivatives by forming new C-C, C-N, and C═C bonds in one pot, using N-arylglycines as the α-arylaminomethyl radical precursors for double insertion.

Organophotocatalytic pyridination of N-arylglycines with 4-cyanopyridines by decarboxylative and decyanative radical-radical coupling

A photocatalytic decarboxylative aminoalkylation of 4-cyanopyridines with N-arylglycines is achieved, providing 4-(aminomethyl)pyridine derivatives in moderate to good yields. This organic photocatalytic reaction undergoes a radical-radical cross-coupling process under redox-neutral conditions, featuring simple operation, readily available N-arylglycines and a broad substrate scope. Mechanistic investigations indicated that a proton-coupled electron-transfer process was involved to enable the single electron transfer between the reduced photocatalyst and 4-cyanopyridine in the presence of N-arylglycines.

Photocatalytic Decarboxylative Allylation of α-Amino Acids and Peptides under Metal-Free Conditions

We developed an organophotoredox catalytic system to facilitate the decarboxylative allylation coupling process concerning α-amino acids and related C-terminal carboxylate peptides using Morita-Baylis-Hillman adducts as allylic precursors. This metal-free method operates under mild conditions and is compatible with various amino acids. The versatility of this protocol, particularly in chemical biology research, has been preliminarily demonstrated through the ligation of bioactive peptide chains.

TEMPO/O2 Synergistically Mediated BiBrO-Photocatalyzed Decarboxylative Phosphorylation of N-Arylglycines

With both TEMPO and O2 (in air) as the homogeneous redox mediators, BiBrO as the heterogeneous semiconductor photocatalyst, the first example of semi-heterogeneous photocatalytic decarboxylative phosphorylation of N-arylglycines with diarylphosphine oxides was established. A series of α-amino phosphinoxides were efficiently synthesized.

Modular synthesis of congested β2,2-amino acids via the merger of photocatalysis and oxidative functionalisations

A two-step protocol for the modular synthesis of β2- and α-quaternary β2,2-amino acid derivatives is reported. The key steps are a photocatalytic hydroalkylation reaction, followed by an oxidative functionalisation to access N-protected β-amino acids, esters, and amides. This strategy can be effectively scaled up via continuous-flow technology.

Ferrocene/air double-mediated FeTiO3-photocatalyzed semi-heterogeneous annulation of quinoxalin-2(1H)-ones in EtOH/H2O

With both ferrocene and air as the redox catalysts, for the first time, the low-cost natural ilmenite (FeTiO3) was successfully used for photocatalytic bond formations. Under the assistance of a traceless H-bond, and HCHO as the methylene reagent, a variety of imidazo[1,5-a]quinoxalinones were semi-heterogeneously photosynthesized in high yields with good functional group compatibility.