Visible-light-mediated catalyst-free synthesis of unnatural α-amino acids and peptide macrocycles

Peptides, crucial bioactive components in various cells, are attractive therapeutic agents due to their high affinity and selectivity for diverse biological targets. However, limitations such as poor membrane permeability, low metabolic stability, and bioavailability hinder their therapeutic potential. To overcome these challenges, strategies like incorporating unnatural amino acids (UAAs) and macrocyclization of linear peptides have been explored. Recent advancements in photochemistry, particularly visible-light-promoted radical coupling reactions, offer mild reaction conditions and excellent functional group tolerance, making them ideal for chemoselective biomolecule modification. Visible light photoredox catalysis has been widely applied in modifying amino acids, peptides, and proteins. Dehydroalanine (Dha), a naturally occurring amino acid, serves as a versatile backbone for UAA synthesis. While methods exist for UAA synthesis, access to β-alkyl-substituted UAAs remains limited, often requiring transition metal catalysts or stoichiometric metal reagents. Compatible methods for modifying Dha units within peptides are also scarce. Traditional peptide cyclization methods rely on lactamization and disulfide bond formation, but the development of new peptide pharmaceuticals necessitates varying ring-forming linkages. Transition-metal-catalyzed macrocyclization strategies, including C-H activation, oxidative cross-couplings, and radical reactions, are gaining popularity. This work aims to develop a visible light-induced, ionic compound-promoted method for C-N bond cleavage of Katritzky salts to address the limitations of existing methods for preparing β-alkyl-substituted UAAs and peptide macrocycles. This approach is expected to offer a milder, more efficient, and catalyst-free alternative.

Previous studies have demonstrated visible light induced, photocatalyst or photoabsorbing EDA complex mediated cleavage of the pyridinium C-N bond. However, these methods often require the use of transition metal catalysts or stoichiometric amounts of reagents. The existing methods for the synthesis of β-alkyl substituted UAAs are limited and often require harsh reaction conditions. Moreover, existing methods for the macrocyclization of peptides are often complex and lack broad substrate scope. The authors cite several papers detailing successful photoredox catalysis in organic chemistry and its application in modifying amino acids and peptides. They highlight the use of dehydroalanine (Dha) as a versatile building block and the challenges in synthesizing β-alkyl-substituted UAAs and efficiently macrocyclizing peptides. The literature review also discusses existing methods for peptide cyclization, such as those based on lactamization and disulfide bond formation, as well as newer transition metal-catalyzed approaches. The authors note that the use of Katritzky salts as alkyl radical precursors has emerged as an important strategy in deaminative reactions and that they are particularly interested in the use of photocatalysis in peptide synthesis.

The authors investigated a visible light-induced, ionic compound-promoted homolytic cleavage of the C-N bond in Katritzky salts. They hypothesized that the addition of ionic compounds (e.g., K2CO3, KOCH3) would enhance visible light absorption by the Katritzky salt, allowing for direct C-N bond cleavage without a photocatalyst. The reaction was explored using N-Ac-Dha methyl ester (4a) as a Michael acceptor and N-Boc-protected cyclic pyridinium salt (1) as an alkyl radical precursor. Two sets of reaction conditions were optimized: Condition A employed Et3N, K2CO3, and H2O in CH3CN, and Condition B used PPh3, KOCH3, and H2O in acetone. The optimal conditions were identified through a series of control experiments, varying the presence of ionic compounds, reductants (Et3N or PPh3), and water. The substrate scope was then explored using various alkenes (4b-4l) including dehydrobutyrine, and Katritzky salts (Fig. 2a and 2b), demonstrating the broad applicability of the methodology. The reaction's compatibility with various functional groups was tested using Dha-containing peptides with different amino acid residues (Fig. 3a). Finally, the deaminative N-terminal macrocyclization of linear peptides was investigated (Fig. 3b). Mechanistic studies, including radical trapping experiments and isotope labeling using D2O, were conducted to investigate the reaction pathway. UV-Vis absorption spectroscopy was used to analyze the effect of ionic compounds on light absorption by the Katritzky salt. A detailed mechanism involving C-N homolysis, alkyl radical formation, and subsequent addition to the Michael acceptor is proposed. The general procedures for both Condition A and Condition B are detailed, specifying reagents, solvents, irradiation conditions, and workup procedures.

The authors successfully developed a catalyst-free method for the synthesis of β-alkyl-substituted unnatural α-amino acids and peptide macrocycles using visible light. The key innovation lies in the use of ionic compounds to promote the homolytic cleavage of the pyridinium C-N bond in Katritzky salts, enabling the generation of alkyl radicals under visible light irradiation. This approach eliminates the need for traditional photocatalysts. The method demonstrates broad substrate scope, successfully employing a variety of alkenes, including naturally occurring dehydroalanine and dehydrobutyrine, as well as diverse Katritzky salts. High yields (up to 97%) were achieved under optimized reaction conditions (Condition A and Condition B). The method showed excellent functional group tolerance, as demonstrated by the successful modification of various Dha-containing peptides with different amino acid residues. Importantly, the method was also applied to the deaminative cyclization of peptide N-terminals, leading to the formation of peptide macrocycles, albeit with lower yields (28-40%). Mechanistic studies supported a radical pathway, with water acting as the hydrogen atom source and either Et3N or PPh3 functioning as single-electron reductants. The role of ionic compounds in enhancing visible light absorption by the Katritzky salt was confirmed through UV-Vis absorption studies.

The findings of this study address the limitations of existing methods for synthesizing β-alkyl-substituted UAAs and peptide macrocycles by providing a catalyst-free, visible light-mediated approach. The method’s mild reaction conditions, broad substrate scope, and functional group tolerance represent significant advantages. The successful application to peptide macrocyclization opens new avenues for peptide drug discovery. The mechanistic understanding of the reaction, particularly the role of ionic compounds in enhancing light absorption, provides valuable insights into the design of future photochemical transformations. The use of readily available and inexpensive reagents (Et3N, PPh3, H2O) makes this methodology highly practical and scalable. While the yields for macrocyclization are relatively moderate, the success of this strategy is a substantial advancement given the difficulty of peptide cyclization using conventional techniques. This work contributes to the growing field of visible-light-driven organic synthesis and offers a valuable tool for the development of novel therapeutic peptides.

This study presents a novel and efficient visible-light-mediated catalyst-free method for the synthesis of unnatural α-amino acids and peptide macrocycles. The use of ionic compounds to enhance visible light absorption allows for direct C-N bond cleavage in Katritzky salts, leading to the generation of alkyl radicals that participate in deaminative hydroalkylation reactions. The method's broad substrate scope, functional group tolerance, and practicality make it a valuable tool for peptide synthesis and drug discovery. Future research could focus on further optimization of the reaction conditions for peptide macrocyclization, exploring alternative ionic compounds, and expanding the scope of substrates.

The yields for peptide macrocyclization, while successful, were lower (28-40%) compared to the synthesis of unnatural α-amino acids. The attempt to macrocyclize Dha-containing peptides on resin during solid-phase peptide synthesis was unsuccessful. Although a detailed mechanism is proposed, complete elucidation of the mechanism, especially the role of ionic compounds in light absorption, requires further investigation. The reaction failed to give desired products when unactivated primary alkyl substrates were used. Further optimization of the reaction conditions may be necessary to improve yields and expand the scope of the methodology.