Strain-Promoted Copper-Free “Click” Chemistry for 18F Radiolabeling of Bombesin

18 F-Labeling of Sensitive Biomolecules for Positron Emission Tomography

Positron emission tomography (PET) imaging study of fluorine-18 labeled biomolecules is an emerging and rapidly growing area for preclinical and clinical research. The present review focuses on recent advances in radiochemical methods for incorporating fluorine-18 into biomolecules via "direct" or "indirect" bioconjugation. Recently developed prosthetic groups and pre-targeting strategies, as well as representative examples in 18 F-labeling of biomolecules in PET imaging research studies are highlighted.

Ultrafast Click Chemistry with Fluorosydnones

We report the synthesis and reactivity of 4-fluorosydnones, a unique class of mesoionic dipoles displaying exquisite reactivity towards both copper-catalyzed and strain-promoted cycloaddition reactions with alkynes. Synthetic access to these new mesoionic compounds was granted by electrophilic fluorination of σ-sydnone Pd(II) precursors in the presence of Selectfluor. Their reactions with terminal and cyclic alkynes were found to proceed very rapidly and selectively, affording 5-fluoro-1,4-pyrazoles with bimolecular rate constants up to 10(4)  m(-1)  s(-1) , surpassing those documented in the literature with cycloalkynes. Kinetic studies were carried out to unravel the mechanism of the reaction, and the value of 4-fluorosydnones was further highlighted by successful radiolabeling with [(18) F]Selectfluor.

Strain-Promoted 1,3-Dipolar Cycloaddition of Cycloalkynes and Organic Azides

A nearly forgotten reaction discovered more than 60 years ago-the cycloaddition of a cyclic alkyne and an organic azide, leading to an aromatic triazole-enjoys a remarkable popularity. Originally discovered out of pure chemical curiosity, and dusted off early this century as an efficient and clean bioconjugation tool, the usefulness of cyclooctyne-azide cycloaddition is now adopted in a wide range of fields of chemical science and beyond. Its ease of operation, broad solvent compatibility, 100 % atom efficiency, and the high stability of the resulting triazole product, just to name a few aspects, have catapulted this so-called strain-promoted azide-alkyne cycloaddition (SPAAC) right into the top-shelf of the toolbox of chemical biologists, material scientists, biotechnologists, medicinal chemists, and more. In this chapter, a brief historic overview of cycloalkynes is provided first, along with the main synthetic strategies to prepare cycloalkynes and their chemical reactivities. Core aspects of the strain-promoted reaction of cycloalkynes with azides are covered, as well as tools to achieve further reaction acceleration by means of modulation of cycloalkyne structure, nature of azide, and choice of solvent.

Ortho-Stabilized (18) F-Azido Click Agents and their Application in PET Imaging with Single-Stranded DNA Aptamers

Azido (18) F-arenes are important and versatile building blocks for the radiolabeling of biomolecules via Huisgen cycloaddition ("click chemistry") for positron emission tomography (PET). However, routine access to such clickable agents is challenged by inefficient and/or poorly defined multistep radiochemical approaches. A high-yielding direct radiofluorination for azido (18) F-arenes was achieved through the development of an ortho-oxygen-stabilized iodonium derivative (OID). This OID strategy addresses an unmet need for a reliable azido (18) F-arene clickable agent for bioconjugation reactions. A ssDNA aptamer was radiolabeled with this agent and visualized in a xenograft mouse model of human colon cancer by PET, which demonstrates that this OID approach is a convenient and highly efficient way of labeling and tracking biomolecules.

Single-step Radiosyntheses of '18F-Labeled Click Synthons' from Azide-functionalized Diaryliodonium Salts

Positron emission tomography (PET) is an increasingly important biomedical imaging technique that relies on the development of radiotracers labeled with positron-emitters to achieve biochemical specificity. Fluorine-18 (t_1/2 = 109.7 min) is an attractive positron-emitting radiolabel for organic radiotracers, primarily because of its longer half-life and greater availability relative to those for the main alternative, carbon-11 (t_1/2 = 20.4 min). Rapid simple methods are sought for labeling prospective PET radiotracers with fluorine-18 from cyclotron-produced aqueous [¹⁸F]fluoride ion, which must often be converted first into a suitably reactive labeling synthon for use in a subsequent labelling reaction. Use of ¹⁸F-labeled synthons in 'click chemistry' attracts increasing attention for labeling PE Tradiotracers. Here we describe rapid single-step radiosyntheses of azido- or azidomethyl-bearing [¹⁸F]fluoroarenes from the reactions of diaryliodonium salts with no-carrier-added [¹⁸F]fluoride ion within a microfluidic apparatus to provide previously poorly accessible ¹⁸F-labeled click synthons in radiochemical yields of 15% for [¹⁸F]4-fluorophenyl azide and about 40% for each of the [¹⁸F](azidomethyl)-fluorobenzenes. The radiosyntheses of the latter synthons was possible under 'wet conditions', so obviating the need to dry the cyclotron-produced [¹⁸F]fluoride ion and greatly enhancing the practicality of the method.

Beyond azide-alkyne click reaction: easy access to 18F-labelled compounds via nitrile oxide cycloadditions

Radiofluorinated 4-fluorobenzonitrile oxide and N-hydroxy-4-fluorobenzimidoyl chloride rapidly react with different alkenes and alkynes under mild conditions. These cycloadditions are suitable for the preparation of low-molecular weight radiopharmaceuticals and, in a strain-promoted variant, can enable easy labelling of sensitive biopolymers.