Syntheses of novelN-([18F]fluoroalkyl)-N-nitroso-4-methyl-benzenesulfonamides and decomposition studies of corresponding19F- and bromo-analogues: potential new compounds for the18F-labelling of radiopharmaceuticals

Strained Ammonium Precursors for Radiofluorinations

The increasing application of positron emission tomography (PET) in nuclear medicine has stimulated the extensive development of a multitude of novel and versatile techniques to introduce fluorine‐18, especially for the radiolabelling of biologically or pharmacologically active molecules. Taking into consideration that the introduction of fluorine‐18 (t_1/2=109.8 min) mostly proceeds under harsh conditions, radiolabelling of such molecules represents a challenge and is of enormous interest. Ideally, it should proceed in a regioselective manner under mild physiological conditions, in an acceptable time span, with high yields and high specific activities. Special attention has been drawn to 2‐fluoroethyl and 3‐fluoropropyl groups, which are often the active sites of radiofluorinated compounds. Precursors containing an ammonium leaving group – such as a strained azetidinium or aziridinium moiety – can help to overcome these obstacles leading to a convenient and mild introduction of [¹⁸F]fluoride with high radiochemical yields.

Determining the Ionization Constants of Organic Acids Using Fluorine Gauche Effects

Using NMR spectroscopy, the conformational studies of two fluoroethylsulfonamides (N-(2-fluoroethyl)-p-tolylsulfonamide (1) and N-(2-fluoroethyl)trifluoromethanesulfonamide (2)) revealed that fluorine gauche effects are a function of ionization. While acids 1 and 2 exhibited gauche effects (with gauche populations of 87% and 92% in DMSO-d₆, respectively), their anions, on the other hand, preferred the anti conformer (with gauche populations of 35% and 55%, respectively). The ability of these compounds to undergo conformational changes as a function of ionization enabled their application as molecular probes (standards) for determining the acidity (pK_a) of organic compounds in DMSO, which was achieved with the aid of the equation K_rel = [(³J_AH - ³J_obs)/(³J_obs - ³J_A)]², where K_rel is the ratio of ionization constants of two acids (standard and test acids), ³J_AH and ³J_A are the proton-fluorine vicinal coupling constants of the standard acid and its anion, respectively, and ³J_obs represents the proton-fluorine vicinal coupling constant observed at the midpoint of an acid-base equilibrium. As a means of demonstrating its utility, this equation accurately calculated the ionization constants (K_a) of several organic compounds in DMSO. Taking advantage of fluorine's unique gauche effect as a strategy for molecular design has the potential to open a new frontier in structural chemistry.

Aza-Wacker-type reaction between electron-deficient olefins and N-alkylsulfonamides

A palladium-catalyzed oxidative amination protocol of electron-deficient olefins by aza-Wacker-type reaction with N-alkylsulfonamides was developed. The presence of the stoichiometric amount of methanesulfonic acid was crucial for the success of this transformation. The reactions were conducted in green solvent under mild conditions and scalable with excellent E-type stereoselectivity. In addition, a Pd(II)/Pd(0) catalytic cycle with the existence of a very strong oxidant (Selectfluor) was proposed.

1-(3-[18F]fluoropropyl)piperazines as model compounds for the radiofluorination of pyrido[2,3-d]pyrimidines

The visualization of cyclin-dependent kinases (CDKs), which are overexpressed in multiple tumor types, with radiolabeled CDK inhibitors by means of positron emission tomography in vivo is a promising approach for tumor imaging. Pyrido[2,3-d]pyrimidines belong to a class of inhibitors, which bind with high affinity to CDK4 and CDK6. 1-(3-[18F]Fluoropropyl)-4-(4-nitrophenyl)piperazine and 1-(3-[18F]fluoropropyl)-4-(6-nitropyridin-3-yl)piperazine represent structural elements of the appropriate CDK inhibitors and were therefore chosen as model compounds for the incorporation of fluorine-18 into pyrido[2,3-d]pyrimidines. Three methods are known for the preparation of tertiary 3-[18F]fluoropropyl-amines: 1) the direct substitution of a good leaving group, 2) the two-step reaction synthesizing a 3-[18F]fluoropropyl intermediate, and 3) the utilization of aziridinium or azetidinium salts. In general, radiofluorinations using azetidinium salts lead to excellent radiochemical yields in short periods of time. For these reasons, we developed a synthesis route to tosylated piperazine precursors and established a radiolabeling approach based on the incorporation of fluorine-18 into open-chained tosylates as well as the respective azetidinium spiro compounds to yield the desired radiofluorinated piperazines successfully.

Synthesis and in vitro evaluation of (S)-2-([11C]methoxy)-4-[3-methyl-1-(2-piperidine-1-yl-phenyl)-butyl-carbamoyl]-benzoic acid ([11C]methoxy-repaglinide): a potential beta-cell imaging agent

The 11C-labeled sulfonylurea receptor 1 (SUR1) ligand (S)-2-([11C]methoxy)-4-[3-methyl-1-(2-piperidine-1-yl-phenyl)-butyl-carbamoyl]-benzoic acid ([11C]methoxy-repaglinide) was synthesized in an overall radiochemical yield of 35% after 55 min with a radiochemical purity higher than 99%. This compound is considered for the noninvasive investigation of the SUR1 receptor status of pancreatic beta-cells by positron emission tomography (PET) in the context of type 1 and type 2 diabetes. The specific activity was 40-70 GBq/micromol. In vitro testing of the nonradioactive methoxy-repaglinide was performed to characterize the affinity for binding to the human SUR1 isoform. Methoxy-repaglinide induced a complete monophasic inhibition curve with a Hill coefficient close to 1 (1.03) yielding a dissociation constant (KD) of 83 nM and an IC50 of 163 nM. Insulin secretion experiments on isolated rat islets were performed to prove biological activity, which was determined to be in the same range as that of original repaglinide.