Substantial porgress in F-Labeling fluorination methodology has been made in recent years. Both two artificial and natural  isotopes of fluorine ( [18]F and [19]F) have been found wide applications as  tracers in the area of molecular imaging for clinical and preclinical uses, especially due to their super nuclear properties and NMR sensitivities. For examples, the isotope [18]F is the most preferred radionuclide for positron emission tomography(PET), which is widely recognized as a clinical tool for cancer diagnosis and other applications emerging in the healthcare, personalized medicine and drug discovery settings. Recently, Contrast agents (medical contrast medias) bearing the isotope [19]F have been introduced as an attrative and emerging alternative to purely hydrogenated compounds for magnetic resonance imaging, beacuse of their high signal to noise ratio as result of their unique spectroscopic signature.

1. [18]F-Labelling Precursors:

The most predominate isotope for PET imaging is undoubtedly [18]F with a half-life of 109.77 min htat allows for complex multi-step radiolabeling synthesis of one to three half-lives that is not possible with [11]C, [13]N and [15]O. Its distinct advantage of [18]F's half-life can also permit the option of preparation of the radiopharmaceutical at a location remote from the on-site of use, or application in studies of slow physiological processes, which may  require scanning times of up to half a day.

Conventional fluorination methods have limited applicatios in radiochemistry, beacuse they typically have low functional group tolerance and can't afford complex, biomedically relevant molecules as PET tracers in a relatively short synthesis time. The time used to synthesize ,purify and formulate radiotracers for injection should be less than two half-lives of the radionulcide for a good imaging performance, which is normally requiring synthesis of radiotracers containing [18]F occur at a late-stage of radiofluorination by avoiding unproductive decay.

In principle, general pathways for the fluorination of radiopharmaceuticals (most representative example is [18]F FDG([18]F fluoro-2-deoxyglucose used as PET radiotracer) can be divided into two categories: electrophilic radiofluorination and nucleophilic radiofluorination. Many historically relevant radiopharmaceuticals were prepared by electrophilic radiofluorination reactions, and certain radiopharmaceuticals are still best prepared or can only be prepared by using electrophilic radiofluorination techniques, but such reactions have predominantly been replaced by newer nucleophilic radiofluorination reactions over time. The main reason is that nucleophilic [18]F fluoride can be produced without carrier added as an aqueous solution in high specific activity(can be considered as a high ratio of Fluorine-18 to natural, PET inactive isotope Fluorine-19) by a cyclotron, whilst electrophilic radiofluorinating reagents are generally derived from carrier-added [18]F fluorine gas([18]F F2), which have a low [18]F/[19]F ratio, resulting in a lower specific activity compared with[18]F fluoride. The theoretical achievable maximum Radiochemical Yield(RCY) in electrophilic radiofluorination is limited to 50%, due to the necessary carrier addition in the [18]F F2 production and the fact that every [18]F F2 Molecule carries only one fluorine-18 atom.  High specific activity is often critical for imaging biological targets with low concentration, such as neurotransmitter receptors in the brain. [18]F F2 gas is also less practical to handle as compared to [18]F fluoride, due to its high reactivity and toxicity.

Utilized by the above two general strategies of [18]F-Labeling fluorination, radiosynthetic methods for the introduction of [18]F fluorine or fluorinated moities into organic molecules can be divided into two groups, namely, direct(true labeling) and indirect(prosthetic labeling). The direct method entails incorporation of [18]F fluorine without changing the carbon skeleton structure of the molecule of interest. The indirect  method inovlves two procedures: build-up syntheses and prosthetic groups. The former procedure means a multi-step syntheses, that is changing the carbon skeleton structure and starting from small molecules which themselves can be easily [18]F fluorinated by nucleophilic substitution. Such small [18]F-labeled intermediates (or click synthons), which bear typically reactive functional groups for further transformation reactions, are used to synthesize more complex biological molecules that can't be labelled with fluorine-18 due to mechanistic reasons or are not stable enough to tolerate direct [18]F-fluorination conditions.  The latter procedure, also called prosthetic labelling, is to provide the molecule of interest with a prosthesis that can accommodate the fluorine-18 atiom. This radiofluorination technique  is first of all applied in the labelling of macromolecules(e.g. proteins, peptides, oligonucleotides), but also in the derivation of small molecules and in particular by the replacement of a methoxy- group by a [18]F fluoroalkoxy group.

2. [19]F-Labelling Precursors

The high sensitivity of fluorine-19 as an NMR-active nucleus and the large [19]F-[1]H or [19]F-[19]F dipolar coupling strengths have found tremendous application as fluorinated contrast agent in the field of magnetic resoance imaging(MRI), which is an extremely versatile anatomical and functional imaging technique which excels at deep,soft tissue imaging.

Whilst [1]H MRI has become an indispensable tool for the imaging of disease states, it frequently suffers from low contrast owing to background signal from intrinsic [1]H. As a result, increasing attention is being directed at compounds containing [19]F nuclei, which has a similar NMR sensitivity to [1]H and , importantly, intrinsic [19]F signals are virtually undetectable in vivo.  Perfluorinated molecules and highly fluorous gases such as SF6 have traditionally been used for preparing [19]F-MRI contrast agents or probes by the conventional fluorination techniques. Recently, more and more [19]F-Labeled compounds,  such as fluorinated responsive(smart)agents, micelles, dendrimers and hyperbranched polymers,  are starting to emerge as new potential contrast agents for [19]F-MRI application.

Considering the characteristics and stringent requirements of F-labelled compounds, especially the radioactivities and short life of [18]F nuclide,  We are focusing on offering F-labeled precusors( only fluorinated by Fluorine-19) for synthesis of radiotracers and contrast agents in the area of medical imaging. such  [19]F-fluorianted precusors could be in any form of  starting materials , click synthons and advanced intermidates( already exist complex functionality) for final production of imaging modalities.

3. SF-based Reactive Probe Buiding Blocks

Sulfonyl fluoride(SF)-based activity probes that mimic a pharmacophore,ligand or lead compound are of growing interest for target engagement, protein profiling,imaging or other applications in chemical biology and molecular pharmacology. Sulfonyl fluorides are capable of modifying most amino acide side chains but generally only react with proteins in a an active site or binding pocket.  Sulfonyl fluoride building blocks have therefore become important reactive groups for context--specific labeling by forming a covalent bond with the Protein of interest(POI) and allowing enven weak-binding events to be captured.

F-labelled Precursors and Synthons Overview

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