A Universal Procedure for the [18F]Trifluoromethylation of Aryl Iodides and Aryl Boronic Acids with Highly Improved Specific Activity

[11 C]Fluoroform, a Breakthrough for Versatile Labeling of PET Radiotracer Trifluoromethyl Groups in High Molar Activity

Positron-emission tomography (PET) is an immensely important imaging modality in biomedical research and drug development but must use selective radiotracers to achieve biochemical specificity. Such radiotracers are usually labeled with carbon-11 (t1/2 =20 min) or fluorine-18 (t1/2 =110 min), but these are only available from cyclotrons in a few simple chemical forms. [18 F]Fluoroform has emerged for labeling tracers in trifluoromethyl groups but is severely limited in utility by low radioactivity per mass (low molar activity). Here, the synthesis of [11 C]fluoroform is described, based on CoF3 -mediated fluorination of cyclotron-produced [11 C]methane. This process is efficient and repetitively reliable. [11 C]Fluoroform shows versatility for labeling small molecules in very high molar activity (>200 GBq μmol-1 ), far exceeding that possible by using [18 F]fluoroform. Therefore, [11 C]fluoroform represents a major breakthrough for labeling prospective PET tracers in trifluoromethyl groups at high molar activity.

Fluorine-18 labelled building blocks for PET tracer synthesis

This review presents a comprehensive overview of the synthesis and application of fluorine-18 labelled building blocks since 2010.

Pd-Catalysed Suzuki coupling of α-bromoethenylphosphonates with organotrifluoroborates: a general protocol for the synthesis of terminal α-substituted vinylphosphonates

No concern will arise in the preparation of terminal α-substituted vinylphosphonates employing our general and practical protocol.

Efficient DBU accelerated synthesis of 18F-labelled trifluoroacetamides

Nucleophilic ¹⁸F-fluorination of bromodifluoromethyl derivatives was performed using [¹⁸F]Bu₄NF in the presence of DBU (1,8-diazabicyclo[5.4.0]undec-7-ene). This novel procedure provided a diverse set of [¹⁸F]trifluoroacetamides in good to excellent radiochemical conversions. A mechanism where DBU acts as organomediator in this transformation is proposed.

Fluorine-18 patents (2009-2015). Part 2: new radiochemistry

Fluorine-18 ((18)F) is one of the most common positron-emitting radionuclides used in the synthesis of positron emission tomography radiotracers due to its ready availability, convenient half-life and outstanding imaging properties. In Part 1 of this review, we presented the first analysis of patents issued for novel radiotracers labeled with fluorine-18. In Part 2, we follow-up with a focus on patents issued for new radiochemistry methodology using fluorine-18 issued between January 2009 and December 2015.

Synthesis of (18) F-Difluoromethylarenes from Aryl (Pseudo) Halides

A general method for the synthesis of [(18) F]difluoromethylarenes from [(18) F]fluoride for radiopharmaceutical discovery is reported. The method is practical, operationally simple, tolerates a wide scope of functional groups, and enables the labeling of a variety of arenes and heteroarenes with radiochemical yields (RCYs, not decay-corrected) from 10 to 60 %. The (18) F-fluorination precursors are readily prepared from aryl chlorides, bromides, iodides, and triflates. Seven (18) F-difluoromethylarene drug analogues and radiopharmaceuticals including Claritin, fluoxetine (Prozac), and [(18) F]DAA1106 were synthesized to show the potential of the method for applications in PET radiopharmaceutical design.

(18)F-Labeling of Arenes and Heteroarenes for Applications in Positron Emission Tomography

Diverse radiochemistry is an essential component of nuclear medicine; this includes imaging techniques such as positron emission tomography (PET). As such, PET can track diseases at an early stage of development, help patient care planning through personalized medicine and support drug discovery programs. Fluorine-18 is the most frequently used radioisotope in PET radiopharmaceuticals for both clinical and preclinical research. Its physical and nuclear characteristics (97% β(+) decay, 109.8 min half-life, 635 keV positron energy) and high specific activity make it an attractive nuclide for labeling and molecular imaging. Arenes and heteroarenes are privileged candidates for (18)F-incorporation as they are metabolically robust and therefore widely used by medicinal chemists and radiochemists alike. For many years, the range of (hetero)arenes amenable to (18)F-fluorination was limited by the lack of chemically diverse precursors, and of radiochemical methods allowing (18)F-incorporation in high selectivity and efficiency (radiochemical yield and purity, specific activity, and radio-scalability). The appearance of late-stage fluorination reactions catalyzed by transition metal or small organic molecules (organocatalysis) has encouraged much research on the use of these activation manifolds for (18)F-fluorination. In this piece, we review all of the reactions known to date to install the (18)F substituent and other key (18)F-motifs (e.g., CF3, CHF2, OCF3, SCF3, OCHF2) of medicinal relevance onto (hetero)arenes. The field has changed significantly in the past five years, and the current trend suggests that the radiochemical space available for PET applications will expand rapidly in the near future.

N'-3-(Trifluoromethyl)phenyl Derivatives of N-Aryl-N'-methylguanidines as Prospective PET Radioligands for the Open Channel of the N-Methyl-d-aspartate (NMDA) Receptor: Synthesis and Structure-Affinity Relationships

N-Methyl-d-aspartate (NMDA) receptor dysfunction has been linked to several neuropsychiatric disorders, including Alzheimer's disease, epilepsy, drug addiction, and schizophrenia. A radioligand that could be used with PET to image and quantify human brain NMDA receptors in the activated "open channel" state would be useful for research on such disorders and for the development of novel therapies. To date, no radioligands have shown well-validated efficacy for imaging NMDA receptors in human subjects. In order to discover improved radioligands for PET imaging, we explored structure-affinity relationships in N'-3-(trifluoromethyl)phenyl derivatives of N-aryl-N'-methylguanidines, seeking high affinity and moderate lipophilicity, plus necessary amenability for labeling with a positron-emitter, either carbon-11 or fluorine-18. Among a diverse set of 80 prepared N'-3-(trifluoromethyl)phenyl derivatives, four of these compounds (13, 19, 20, and 36) displayed desirable low nanomolar affinity for inhibition of [(3)H](+)-MK801 at the PCP binding site and are of interest for candidate PET radioligand development.

Difluorocarbene-Derived Trifluoromethylthiolation and [(18)F]Trifluoromethylthiolation of Aliphatic Electrophiles

The first trifluoromethylthiolation and [(18)F]trifluoromethylthiolation of alkyl electrophiles with in situ generated difluorocarbene in the presence of elemental sulfur and external (radioactive) fluoride ion is described. This transition-metal-free approach is high yielding, compatible with a variety of functional groups, and operated under mild reaction conditions. The conceptual advantage of this exogenous-fluoride-mediated transformation enables unprecedented syntheses of [(18)F]CF3S-labeled molecules from most commonly used [(18)F]fluoride ions. The rapid radiochemical reaction time (≤1 min) and high functional-group tolerance allow access to a variety of aliphatic [(18)F]CF3S compounds in high yields.

(18)F-Labeling of Aryl-SCF3, -OCF3 and -OCHF2 with [(18)F]Fluoride

We report that halogenophilic silver(I) triflate permits halogen exchange (halex) nucleophilic (18)F-fluorination of aryl-OCHFCl, -OCF2Br and -SCF2Br precursors under mild conditions. This Ag(I)-mediated process allows for the first time access to a range of (18)F-labeled aryl-OCHF2, -OCF3 and -SCF3 derivatives, inclusive of [(18)F]riluzole. The (18)F-labeling of these medicinally important motifs expands the radiochemical space available for PET applications.

Transition metal complex assisted Csp3-F bond formation

Fluoroorganic compounds have attracted significant attention in various fields, such as pharmaceutical, agricultural chemistry, and materials science, as a result of their unique physical, chemical, and physiological properties. Consequently, extensive efforts have been devoted to the site-specific synthesis of organofluorine compounds. In recent years, transition-metal-mediated C-F bond formation has emerged as a powerful method for fabrication of these compounds. this Perspective mainly focuses on the most recent advances in transition-metal-assisted synthesis of alkyl fluorides.

MS565: A SPECT Tracer for Evaluating the Brain Penetration of BAF312 (Siponimod)

BAF312 (siponimod) is a sphingosine-1-phosphate (S1P) receptor modulator in clinical development for the treatment of multiple sclerosis, with faster organ/tissue distribution and elimination kinetics than its precursor FTY720 (fingolimod). Our aim was to develop a tracer to better quantify the penetration of BAF312 in the human brain, with the potential to be labeled for positron emission tomography (PET) or single-photon emission computed tomography (SPECT). Although the PET radioisotopes (11)C and (18)F could have been introduced in BAF312 without modifying its structure, they do not have decay kinetics compatible with the time required for observing the drug's organ distribution in patients. In contrast, the SPECT radioisotope (123) I has a longer half-life and would suit this purpose. Herein we report the identification of an iodinated derivative of BAF312, (E)-1-(4-(1-(((4-cyclohexyl-3-iodobenzyl)oxy)imino)ethyl)-2-ethylbenzyl)azetidine-3-carboxylic acid (18, MS565), as a SPECT tracer candidate with affinity, S1P receptor selectivity, overall physicochemical properties, and blood pharmacokinetics similar to those of the original molecule. A whole-body autoradiography study performed with [(14)C]MS565 subsequently confirmed that its organ distribution is similar to that of BAF312. This validates the selection of MS565 for (123)I radiolabeling and for use in imaging studies to quantify the brain penetration of BAF312.

Targeted fluorination with the fluoride ion by manganese-catalyzed decarboxylation

We describe the first catalytic decarboxylative fluorination reaction based on the nucleophilic fluoride ion. The reported method allows the facile replacement of various aliphatic carboxylic acid groups with fluorine. Moreover, the potential of this method for PET imaging has been demonstrated by the successful (18) F labeling of a variety of carboxylic acids with radiochemical conversions up to 50 %, representing a targeted decarboxylative (18) F labeling method with no-carrier-added [(18) F]fluoride. Mechanistic probes suggest that the reaction proceeds through the interaction of the manganese catalyst with iodine(III) carboxylates formed in situ from iodosylbenzene and the carboxylic acid substrates.

Difluoromethylation and gem-difluorocyclopropenation with difluorocarbene generated by decarboxylation

Difluoromethylation of the activated X–H bond and aliphatic thiols, and gem-difluorocyclopropenation of alkynes with difluorocarbene generated by decarboxylation are described.

Late-stage [18F]Fluorination: New Solutions to Old Problems

The last 2-3 years have seen numerous relationships develop between organometallic chemists, fluorine chemists and PET Centers around the world. These collaborations have led to the development of many new strategies for the late-stage introduction of fluorine-18 into complex bioactive molecules. In this perspective we highlight recent developments and key milestones since 2011.

Mild silver-mediated geminal difluorination of styrenes using an air- and moisture-stable fluoroiodane reagent

An air- and moisture-stable fluoroiodane in the presence of AgBF4 is suitable for selective geminal difluorination of styrenes under mild reaction conditions. One of the CF bonds is formed by transfer of electrophilic fluorine from the hypervalent iodine reagent, while the other one arises from the tetrafluoroborate counterion of silver. Deuterium-isotope-labelling experiments and rearrangement of methyl styrene substrates suggest that the reaction proceeds through a phenonium ion intermediate.