Decatungstate-Mediated C(sp3 )-H Heteroarylation via Radical-Polar Crossover in Batch and Flow

Photocatalytic hydrogen atom transfer is a very powerful strategy for the regioselective C(sp3 )-H functionalization of organic molecules. Herein, we report on the unprecedented combination of decatungstate hydrogen atom transfer photocatalysis with the oxidative radical-polar crossover concept to access the direct net-oxidative C(sp3 )-H heteroarylation. The present methodology demonstrates a high functional group tolerance (40 examples) and is scalable when using continuous-flow reactor technology. The developed protocol is also amenable to the late-stage functionalization of biologically relevant molecules such as stanozolol, (-)-ambroxide, podophyllotoxin, and dideoxyribose.

Recent Advances in Photo-mediated Radiofluorination

Carbon-fluorine bond formations have received a lot of attention because organofluorine compounds are widely used in pharmaceutical, agricultural, and materials science applications. In particular, the incorporation of fluorine-18, which is a commonly used radioisotope for radiopharmaceuticals for positron emission tomography (PET), a molecular imaging tool for the visualization of biochemical events, human metabolism processes, and the measurement and diagnosis of diseases in humans, plays a crucial role in clinical and preclinical studies. Several synthetic methodologies for carbon-fluorine-18 bond formation have been developed. However, conventional fluorination methods have some remaining drawbacks such as the high temperature and basic environment. Photo-induced catalysis is an emerging technique that allow chemists to achieve the synthesis of target molecular architectures under mild conditions. Moreover, several radiofluorination strategies have been developed via photocatalysis. In this review, we focused on describing recent advances in the field of light-mediated radiofluorination.

Radical-Based Synthesis and Modification of Amino Acids

Amino acids (AAs) are key structural motifs with widespread applications in organic synthesis, biochemistry, and material sciences. Recently, with the development of milder and more versatile radical-based procedures, the use of strategies relying on radical chemistry for the synthesis and modification of AAs has gained increased attention, as they allow rapid access to libraries of novel unnatural AAs containing a wide range of structural motifs. In this Minireview, we provide a broad overview of the advancements made in this field during the last decade, focusing on methods for the de novo synthesis of α-, β-, and γ-AAs, as well as for the selective derivatisation of canonical and non-canonical α-AAs.

Late-Stage 18 F-Difluoromethyl Labeling of N-Heteroaromatics with High Molar Activity for PET Imaging

Despite a growing interest in CHF2 in medicinal chemistry, there is a lack of efficient methods for the insertion of CHF18 F into druglike compounds. Herein described is a photoredox flow reaction for 18 F-difluoromethylation of N-heteroaromatics that are widely used in medicinal chemistry. Following the two-step synthesis for a new 18 F-difluoromethylation reagent, the photoredox reaction is completed within two minutes and proceeds by C-H activation, circumventing the need for pre-functionalization of the substrate. The method is operationally simple and affords straightforward access to radiolabeled N-heteroaromatics with high molar activity suitable for biological in vivo studies and clinical application.

The Fluorination of C-H Bonds: Developments and Perspectives

This Review summarizes advances in fluorination by C(sp² )-H and C(sp³ )-H activation. Transition-metal-catalyzed approaches championed by palladium have allowed the installation of a fluorine substituent at C(sp² ) and C(sp³ ) sites, exploiting the reactivity of high-oxidation-state transition-metal fluoride complexes combined with the use of directing groups (some transient) to control site and stereoselectivity. The large majority of known methods employ electrophilic fluorination reagents, but methods combining a nucleophilic fluoride source with an oxidant have appeared. External ligands have proven to be effective for C(sp³ )-H fluorination directed by weakly coordinating auxiliaries, thereby enabling control over reactivity. Methods relying on the formation of radical intermediates are complementary to transition-metal-catalyzed processes as they allow for undirected C(sp³ )-H fluorination. To date, radical C-H fluorinations mainly employ electrophilic N-F fluorination reagents but a unique Mn^III -catalyzed oxidative C-H fluorination using fluoride has been developed. Overall, the field of late-stage nucleophilic C-H fluorination has progressed much more slowly, a state of play explaining why C-H ¹⁸ F-fluorination is still in its infancy.

Copper-Mediated Aminoquinoline-Directed Radiofluorination of Aromatic C-H Bonds with K18 F

A Cu-mediated ortho-C-H radiofluorination of aromatic carboxylic acids that are protected as 8-aminoquinoline benzamides is described. The method uses K¹⁸ F and is compatible with a wide range of functional groups. The reaction is showcased in the high specific activity automated synthesis of the RARβ2 agonist [¹⁸ F]AC261066.

Site-Specific Deoxyfluorination of Small Peptides with [18 F]Fluoride

Radiolabeled receptor-binding peptides are an important class of positron emission tomography tracers owing to achievable high binding affinities and their rapid blood clearance. Herein, a method to introduce a 4-[¹⁸ F]fluoro-phenylalanine residue into peptide sequences is reported, by chemoselective radio-deoxyfluorination of a tyrosine residue using a traceless activating group. The replacement of only one hydrogen atom with [¹⁸ F]fluoride results in minimal structural perturbation of the peptide, which is desirable in the labeling of tracer candidates.