Recent developments in heterocycle labeling with carbon isotopes

[11C]CO2 BOP fixation with amines to access 11C-labeled ureas for PET imaging

Carbon-11 (11C) is a widely used radionuclide for positron emission tomography (PET) owing to the omnipresence of carbon atoms in organic molecules. While its half-life of 20.4 min is ideal for imaging and dosimetry, it also limits the synthetic possibilities. As such, the development of fast and easy, high-yielding synthesis methods is crucial for the application of 11C-labeled tracers in humans. In this study, we present a novel and efficient method for the reaction of [11C]CO2 with amine precursors using benzotriazole-1-yl-oxy-tris-(dimethylamino)-phosphonium hexafluorophosphate (BOP) to access 11C-labeled ureas. Our method is extremely fast as it only requires transfer of [11C]CO2 into a solution with precursor and BOP at room temperature, where it reacts momentary into the desired 11C-labeled urea. This simple procedure makes it possible to radiolabel urea directly from [11C]CO2 without the need for advanced equipment, making the method applicable for all laboratories where [11C]CO2 is available. We synthesized a small series of aliphatic symmetrical and non-symmetrical 11C-labeled ureas using this method, and achieved good to excellent yields. The novelty of our study lies in the fact that peptide coupling reagent BOP is used for the first time in radiochemistry to activate [11C]CO2, facilitating its reaction with amines to obtain 11C-labeled ureas.

Direct Decarboxylative C-2 Alkylation of Azoles through Minisci-Type Coupling

This note discusses the application of a Minisci-type reaction for the direct alkylation of azoles with carboxylic acids as radical precursors. Different reaction conditions were investigated to achieve high yield of the desired products, focusing on acid strength and solvent screening. Moreover, the reactivity of imidazoles with various carboxylic acids was investigated, showing good yield for most cases. The study reveals the potential of this approach for late-stage functionalization in drug discovery.

Modern Strategies for Carbon Isotope Exchange

In contrast to stable and natural abundant carbon-12, the synthesis of organic molecules with carbon (radio)isotopes must be conceived and optimized in order to navigate through the hurdles of radiochemical requirements, such as high costs of the starting materials, harsh conditions and radioactive waste generation. In addition, it must initiate from the small cohort of available C-labeled building blocks. For long time, multi-step approaches have represented the sole available patterns. On the other side, the development of chemical reactions based on the reversible cleavage of C-C bonds might offer new opportunities and reshape retrosynthetic analysis in radiosynthesis. This review aims to provide a short survey on the recently emerged carbon isotope exchange technologies that provide effective opportunity for late-stage labeling. At present, such strategies have relied on the use of primary and easily accessible radiolabeled C1-building blocks, such as carbon dioxide, carbon monoxide and cyanides, while the activation principles have been based on thermal, photocatalytic, metal-catalyzed and biocatalytic processes.

Late-Stage Carbon-14 Labeling and Isotope Exchange: Emerging Opportunities and Future Challenges

Carbon-14 (¹⁴C) is a gold standard technology routinely utilized in pharmaceutical and agrochemical industries for tracking synthetic organic molecules and providing their metabolic and safety profiles. While the state of the art has been dominated for decades by traditional multistep synthetic approaches, the recent emergence of late-stage carbon isotope labeling has provided new avenues to rapidly access carbon-14-labeled biologically relevant compounds. In particular, the development of carbon isotope exchange has represented a fundamental paradigm change, opening the way to unexplored synthetic transformations. In this Perspective, we discuss the recent developments in the field with a critical assessment of the literature. We subsequently discuss research directions and future challenges within this rapidly evolving field.

A general procedure for carbon isotope labeling of linear urea derivatives with carbon dioxide

Carbon isotope labeling is a traceless technology, which allows tracking the fate of organic compounds either in the environment or in living organisms. This article reports on a general approach to label urea derivatives with all carbon isotopes, including 14C and 11C, based on a Staudinger aza-Wittig sequence. It provides access to all aliphatic/aromatic urea combinations.

The development of 11C-carbonylation chemistry: A systematic view

The prospects for using carbon-11 labelled compounds in molecular imaging has improved with the development of diverse synthesis methods, including 11C-carbonylations and refined techniques to handle [11C]carbon monoxide at a nanomole scale. Facilitating biological research and molecular imaging was the driving force when [11C]carbon monoxide was used in the first in vivo application with carbon-11 in human (1945) and when [11C]carbon monoxide was used for the first time as a chemical reagent in the synthesis of [11C]phosgene (1978). This review examines a rich plethora of labelled compounds synthesized from [11C]carbon monoxide, their chemistry and use in molecular imaging. While the strong development of the 11C-carbonylation chemistry has expanded the carbon-11 domain considerably, it could be argued that the number of 11C-carbonyl compounds entering biological investigations should be higher. The reason for this may partly be the lack of commercially available synthesis instruments designed for 11C-carbonylations. But as this review shows, novel and greatly simplified methods to handle [11C]carbon monoxide have been developed. The next important challenge is to make full use of these technologies and synthesis methods in PET research. When there is a PET-tracer that meets a more general need, the incentive to implement 11C-carbonylation protocols will increase.

Contemporary Progress in the Synthetic Strategies of Imidazole and its Biological Activities

Heterocyclic compounds are pervasive in many areas of life and one of the heterocycles, imidazole is a unique heterocyclic five-membered aromatic compound having two sp2 hybridized nitrogen atoms. Its integral name is 1, 3 diazole and previously, it was known as glyoxalin. This moiety has achieved a considerable place among scientists in recent years by reason of its divergent synthetic strategies and uncommon biological and pharmacological activities, for example, anti-convulsant, anti-microbial, anti-cancer, anti-inflammatory, anti-tumor, anti-viral, anti-ulcer, analgesic, etc. Due to distinct therapeutic actions, it is still an engrossed area of research. Researchers currently are inventing new greener methods to synthesize its derivatives and to improve its pharmacological activities. The purpose of this review is to study the literature that can help researchers to explore this area, its prevailing program for synthesis in environmentally friendly conditions and biological profile throughout past decades.

Direct Access to Isotopically Labeled Aliphatic Ketones Mediated by Nickel(I) Activation

An extensive range of functionalized aliphatic ketones with good functional-group tolerance has been prepared by a Ni^I -promoted coupling of either primary or secondary alkyl iodides with NN₂ pincer Ni^II -acyl complexes. The latter were easily accessed from the corresponding Ni^II -alkyl complexes with stoichiometric CO. This Ni-mediated carbonylative coupling is adaptable to late-stage carbon isotope labeling, as illustrated by the preparation of isotopically labelled pharmaceuticals. Preliminary investigations suggest the intermediacy of carbon-centered radicals.