Aqueous Flavin Photoredox Catalysis Drives Exclusive C3-Alkylation of Indolyl Radical Cations in Route to Unnatural Tryptophan Mimetics

Introduction of flavin anions into photoredox catalysis: acid-base equilibria of lumichrome allow photoreductions with an anion of an elusive 10-unsubstituted isoalloxazine

Flavins have been established as effective catalysts in oxidative photoredox catalysis. Conversely, their use in reductive photocatalysis remains limited, mainly due to the relatively low stability of the transient flavin radicals (semiquinones), which are used in photoreductions. The fully reduced forms of flavins are also disadvantaged in photocatalysis because they absorb light in the UV rather than in the visible region. In this work, we present a new approach for reductive flavin photocatalysis that utilises a flavin (isoalloxazine) anion derived from the elusive 10-unsubstituted 3,7,8-trimethylisoalloxazine, an unstable tautomer of 3-methyllumichrome. We found the conditions under which this isoalloxazine anion is formed by in situ deprotonation/isomerisation from the readily available 3-methyllumichrome and we subsequently used it as a photoredox catalyst in the reductive dehalogenation of activated bromoarenes and their C-P coupling reaction with trimethyl phosphite to form an arylphosphonate. Steady-state and transient absorption spectroscopy, NMR and cyclic voltammetry investigations, together with quantum chemical calculations, showed that the anion of oxidised isoalloxazine has several advantages, compared to other forms of flavins used in photoreductions, such as high stability, even in the presence of oxygen, an absorption maximum in the visible region, thereby allowing the use of excitation light between 470 and 505 nm, and a relatively long-lived singlet excited-state.

Flavin-Catalyzed, Photochemical Conversion of Dehydroalanine into 4,5-Dihydroxynorvaline

The chemical synthesis of unnatural amino acids (UAA) is a key strategy for preparing designed peptides, including pharmaceutically active compounds. Alterations of existing amino acid residues such as dehydroalanine (Dha) are particularly important since selected positions can be addressed without the necessity of a complete de novo synthesis. The intriguing UAA 4,5-dihydroxynorvaline (Dnv) is found in a variety of naturally occurring peptides and biologically active compounds. However, no method is currently available to modify an existing peptide with this residue. We report the use of flavin catalysts and visible light irradiation for this challenge, which serves as a versatile strategy for converting Dha into Dnv. Our study shows that excited flavins are competent hydrogen atom abstraction catalysts for ethers and acetals, which allows masked 1,2-dihydroxyethylene functionalization from 2,2-dimethyl-1,3-dioxolane. The masked diol was successfully coupled to Dha residues, and a series of Dnv-containing products is reported. A mild and orthogonal protocol for deprotection of the acetal group was also identified, allowing free Dnv-modified peptides to be obtained. This method provides a straightforward strategy for Dnv functionalization, which is envisioned to be crucial for accessing natural products and synthetic analogues with pharmaceutical activity.

Flavin-Mediated Photocatalysis Provides a General Platform for Sulfide C-H Functionalization

Functionalized sulfides are important in many areas of science, ranging from chemical biology through drug discovery to organic materials chemistry. Sulfides bearing pendant reactive groups in the α-position are particularly useful; however, methods for the selective valorization of simple sulfides or the late-stage functionalization of complex sulfides by the convenient addition of valuable functionality are underexplored. Here we exemplify a general reaction platform for sulfide functionalization by showcasing three modes of α-sulfur C-H functionalization; cyanation, alkenylation, and alkynylation. Using inexpensive and commercially available riboflavin tetraacetate and visible light, decoration of both feedstock and complex sulfides proceeds in a good yield and with high selectivity. Methionine-containing peptides can also be selectively functionalized and a tolerance screen using amino-acid dopants suggests that the platform is compatible with most amino-acid side chains and thus is a potential tool for bioconjugation.

Electrochemical 1,3-Alkyloxylimidation of Arylcyclopropane Radical Cations: Four-Component Access to Imide Derivatives

Herein, a general electrochemical radical-cation-mediated four-component ring-opening 1,3-alkyloxylimidation of arylcyclopropanes, acetonitrile, carboxylic acids, and alcohols is described, providing a facile and sustainable approach to quickly construct structurally diverse imide derivatives from easily available raw materials in an operationally simple undivided cell. This metal-catalyst- and oxidant-free single-electron oxidation strategy offers a green alternative for the formation of highly reactive cyclopropane-derived radical cations, and this protocol features a broad functional group tolerance.