Radiodifluoromethylation of well-functionalized molecules

Abstract

The strategic installation of a [¹⁸F]fluorine atom at the specific position of the lead molecule is a never-ending challenge for radiochemists in their endeavour to develop novel positron emission tomography (PET) imaging applications. Although the radiosynthesis of [¹⁸F]CF₂H-containing molecules has been explored in the past decade, more methods need to be explored for various well-functionalized compounds. Recently, two novel strategies of radiodifluoromethylation were reported, namely the utilization of [¹⁸F]difluorocarbene building block and frustrated Lewis pair-mediated C-¹⁸F bond formation, respectively. These methods provide an efficient radiofunctionalization of complex CF₂H-containing molecules for drug discovery and PET ligand development.

Introduction

PET has evolved as a functional and noninvasive imaging modality, playing a pivotal role in diagnosing diseases, monitoring disease progression, and aiding drug development. The cornerstone of this technique lies in the development of PET tracers - biologically targeted small or large molecules labeled with positron-emitting radionuclides [1]. Among the PET nuclides, fluorine-18 (¹⁸F) stands out as the preferred radioisotope due to its advantageous attributes, including a practical half-life (t_1/2 = 109.8 min), a favorable positron emission profile (97%), relatively low positron energy (E = 0.635 keV) and high availability. Within the realm of various fluorinated moieties, the difluoromethyl (CF₂H) group, considered a lipophilic bioisostere of conventional hydrogen-bond donor groups such as hydroxy (-OH) and thiol (-SH) groups [2], has garnered significant attention in drug discovery [3]. A plethora of bioactive molecules bearing the CF₂H motif have been discovered, including thiazopyr, pantoprazole, and ZSTK474 etc. (Figure 1).

Figure 1.

Representative CF₂H-containing bioactive molecules.

As various methodologies for the synthesis of CF₂H-containing molecules have emerged, there have been continuous efforts in the exploration of radiosynthesis methods involving [¹⁸F]CF₂H (Figure 2). Gouverneur and coworkers developed a silver(I)-mediated [¹⁸F]fluorodecarboxylation method, using electrophilic [¹⁸F]fluorinating reagents to convert α-fluorarylacetic acids (Figure 2A) [4]. They also introduced an ¹⁸F/Cl exchange reaction involving Ar-CHFCl and nucleophilic [¹⁸F]fluoride (Figure 2A) [5]. Similarly, Shen, Gouverneur and coworkers disclosed the radiosynthesis of [¹⁸F]Ar-SCF₂H by an Ag-mediated ¹⁸F/Cl exchange strategy [6]. Furthermore, this strategy was implemented for the radiolabeling of ethyl α-bromo-α-fluoro(aryloxy)acetates to generate [¹⁸F]α,α-difluoroaryloxy acetic acid, which could be further transformed into [¹⁸F]Ar-OCF₂H [7]. In 2019, Gouverneur’s work established a Mn-mediated [¹⁸F]fluorodecarboxylation process, enabling access to [¹⁸F]Ar-CF₂H and [¹⁸F]ArO-CF₂H moieties (Figure 2A) [8]. Ritter, Vasdev, Liang and coworkers disclosed a cascade method for preparing [¹⁸F](het)Ar-CF₂H, which involved the activation of the benzoyl auxiliary, followed by sequential benzylic bromination and ¹⁸F/Br exchange (Figure 2A) [9]. Subsequently, the Liang group reported a two-step strategy that included the nucleophilic ¹⁸F-fluorination of benzyl (pseudo)halides, followed by oxidative benzylic C-H fluorination (Figure 2B) [10]. Genicot and coworkers described a photoredox flow approach for direct radical C-H ¹⁸F-difluoromethylation of N-heteroarenes (Figure 2C) [11]. In addition, some multi-step syntheses were reported for obtaining [¹⁸F]CF₂H-containing compounds. For example, these syntheses involve Ag-mediated desulfurative [¹⁸F]fluorination [12] and [¹⁸F]fluorination of iodonium salt [13]. In spite of these outstanding results, there is still an unmet need to develop new methods to introduce [¹⁸F]CF₂H group on well functionalized drug-like scaffolds. In this highlight, we focus on two recent advances of radiodifluoromethylation using a new type of [¹⁸F]difluorocarbene building block and frustrated Lewis pair-activated C-¹⁸F bond formation, respectively. These new methodologies may provide a new strategy and ‘retro-radiosynthetic’ approach to design ¹⁸F-labeled difluoromethyl groups beyond conventional methods and enable novel functionalities to be explored in PET pharmaceuticals.

Figure 2.

Previous approaches to access [¹⁸F]CF₂H-containing molecules.

Literature highlight

Direct [¹⁸F]difluoromethylation from [¹⁸F]difluorocarbene

The Gouverneur group introduced a novel method for [¹⁸F]difluoromethylation using [¹⁸F]difluorocarbene ([¹⁸F]DFC) to generate a variety of (hetero)arenes bearing [¹⁸F]CF₂H motif [14]. The work entailed [¹⁸F]DFC insertion into X-H (X = O, S, and N) bonds, and cross-coupling reactions with palladium-DFC complexes.

A wide range of electronically and/or sterically diverse (hetero)aryl phenols, thiophenols, and N-heteroarenes were investigated, yielding the desired products featuring [¹⁸F]OCF₂H, [¹⁸F]SCF₂H, and [¹⁸F]NCF₂H moieties (Figure 3).

Figure 3.

[¹⁸F]DFC insertion to form [¹⁸F]CF₂H-X-Arene motifs (X = O, S and N). ^a[¹⁸F]P1 was used. ^b[¹⁸F]P2 was used.

Notably, [¹⁸F]DFC selectively inserted into the phenolic O-H bond of 4-hydroxybenzaldehyde ([¹⁸F]1, 51% radiochemical yield, RCY). Selective ¹⁸F-difluoromethylation at the nitrogen atom was achieved in the case of (1H-benzo[d]imidazol-2-yl)methanol, yielding [¹⁸F]4 with 52% RCY. Subsequently, late-stage ¹⁸F-difluoromethylation of complex biologically active molecules was successfully demonstrated, including analogues of roflumilast ([¹⁸F]5), MPC-6827 ([¹⁸F]6) and DPA-714 ([¹⁸F]7). Additionally, [²H¹⁸F]7 was obtained using deuterated solvents, such as CD₃CN and D₂O, which is significant because deuterium (D) incorporation is a common strategy to enhance the metabolic stability of drug molecules and radiotracers. An in vivo study was conducted with [¹⁸F]7 in the application of imaging microglial activation of Huntington’s disease. The authors also reported a one-pot tandem procedure, involving borylation, oxidation, and sequential ¹⁸F-difluoromethylation, for producing labeled analogues of drugs and bioactive molecules (for example [¹⁸F]8 and [¹⁸F]9). One plausible reaction mechanism involves a series of cascade steps, namely the deprotonation of DFC precursor, DFC expulsion and addition, and protonation. The free DFC is favored as the predominant pathway for this DFC insertion transformation (Figure 4A). By contrast, the concerted SN2-like pathway is of higher energy barrier (Figure 4B).

Figure 4.

[¹⁸F]DFC insertion transformation (X = O, S and N).

Pd-catalyzed [¹⁸F]difluoromethylation of aryl boronic acids was also successfully achieved through the capture and transfer of [¹⁸F]DFC (Figure 5). This method was applied to a diverse array of aryl boronic acids, including those featuring heterocycles and vinyl groups. Importantly, [¹⁸F]CF₂H analogues of various drugs and bioactive molecules, such as Fenofibrate and protected phenylalanine, were successfully obtained under these conditions.

Figure 5.

[¹⁸F]DFC-mediated cross coupling reactions to form [¹⁸F]CF₂H-Arene motifs. ^a[¹⁸F]P1 was used. ^b[¹⁸F]P2 was used.

Frustrated Lewis pair-mediated C-¹⁸F bond formation

Young and coworkers have previously reported that the frustrated Lewis pair (FLP) can selectively activate C-F bonds within CF₃- and CF₂H-containing compounds [15]. This activation leads to the formation of pyridinium and phosphonium salts in high yield. These resulting salts can be employed in a diverse range of transformations, involving Wittig coupling [15a], nucleophilic substitution, photoredox alkylation, nucleophilic transfer, and hydrogenation reactions [15b], to incorporate a wide array of functional groups into parent molecules. Building upon their pioneering work, Young and his team have now successfully applied this two-step approach to ¹⁸F labeling of difluoromethyl and trifluoromethyl groups [16].

This FLP-mediated transformation process tolerates a variety of well functionalized groups (Figure 6). Difluoromethylarenes with electron-withdrawing nitro, bromo, and ester groups can be isotopically labeled, to generate the corresponding products in 31-59% radiochemical conversions. Furthermore, difluoromethylated heteroarenes, difluoromethoxide and difluoromethylsulfide substrates ([¹⁸F]19-[¹⁸F]21) were also obtained in 19-93% conversions. As a proof-of-concept, a biologically relevant molecule [¹⁸F]22, as a derivative of pantoprazole C, was successfully isolated with non-decay corrected yields of ca. 10%. Control experiments indicated that the cationic fragment of the salts likely served as a source of ¹⁹F fluoride, which is unfortunately detrimental to obtain higher molar activity in the [¹⁸F]fluorination process.

Figure 6.

[¹⁸F]Difluoromethylation using frustrated Lewis pair method.

Conclusion

Undoubtably, these recent advances present novel opportunities and stimulate innovative ideas for the development of potential [¹⁸F]CF₂H-containing PET imaging probes. It is firmly anticipated that an expanding array of strategies for synthesizing compounds featuring the [¹⁸F]CF₂H group will be unveiled, thus contributing to the ongoing advancement of drug discovery and PET applications across diverse scientific disciplines.

Disclosure of conflict of interest

None.