Two-step regio- and stereoselective syntheses of [19F]- and [18F]-2-deoxy-2-(R)-fluoro-β-d-allose

Drug Discovery Based on Fluorine-Containing Glycomimetics

Glycomimetics, which are synthetic molecules designed to mimic the structures and functions of natural carbohydrates, have been developed to overcome the limitations associated with natural carbohydrates. The fluorination of carbohydrates has emerged as a promising solution to dramatically enhance the metabolic stability, bioavailability, and protein-binding affinity of natural carbohydrates. In this review, the fluorination methods used to prepare the fluorinated carbohydrates, the effects of fluorination on the physical, chemical, and biological characteristics of natural sugars, and the biological activities of fluorinated sugars are presented.

The Synthesis and Glycoside Formation of Polyfluorinated Carbohydrates

Fluorinated carbohydrates have found many applications in the glycosciences. Typically, these contain fluorination at a single position. There are not many applications involving polyfluorinated carbohydrates, here defined as monosaccharides in which more than one carbon has at least one fluorine substituent directly attached to it, with the notable exception of their use as mechanism-based inhibitors. The increasing attention to carbohydrate physical properties, especially around lipophilicity, has resulted in a surge of interest for this class of compounds. This review covers the considerable body of work toward the synthesis of polyfluorinated hexoses, pentoses, ketosugars, and aminosugars including sialic acids and nucleosides. An overview of the current state of the art of their glycosidation is also provided.

Radiosynthesis of 18F-labeled d-allose

Rare sugars are defined as monosaccharides that exist in nature but are only present in limited quantities. d-Allose is a rare sugar that has been reported to have some unique physiological effects. The present study describes suitable synthetic procedures for novel rare sugars of d-allose that are ¹⁸F-labeled at the C-3 and C-6 positions and the preparation of the appropriate labeling precursors. The goal is to facilitate in vivo, noninvasive positron emission tomography (PET) investigation of the behavior of rare sugar analogs of d-allose in organs. We found five precursors that were practical for labeling, three for 3-deoxy-3-[¹⁸F]fluoro-d-allose ([¹⁸F]3FDA) and two for 6-deoxy-6-[¹⁸F]fluoro-d-allose ([¹⁸F]6FDA). With manual operation synthesis, the highest radiochemical conversion rates were 75% for [¹⁸F]3FDA with a precursor of 1,2,4,6-tetra-O-acetyl-3-O-trifluoromethanesulfonyl-β-d-glucopyranose and 69% for [¹⁸F]6FDA with a precursor of 1,2,3,4-tetra-O-acetyl-6-O-trifluoromethanesulfonyl-β-d-allopyranose. Furthermore, the practical yields of [¹⁸F]3FDA and [¹⁸F]6FDA using an automated synthesizer were also investigated. Radiochemical yields of 67% and 49% were obtained for [¹⁸F]3FDA and [¹⁸F]6FDA, respectively, in an automated synthesizer. As basic assessment of stability for use in PET scanning, high performance liquid chromatography analysis showed no decomposition of [¹⁸F]3FDA and [¹⁸F]6FDA after up to 6 h in rabbit blood plasma.

Searching for diagnostic properties of novel fluorine-18-labeled D-allose

OBJECTIVE

Two fluorine-18-labeled analogues, 3-deoxy-3-[¹⁸F]fluoro-D-allose (3-[¹⁸F]FDA) and 6-deoxy-6-[¹⁸F]fluoro-D-allose (6-[¹⁸F]FDA), were synthesized and their potentials of diagnostic property were characterized.

METHODS

In vitro rat red blood cell (RBC) transport and phosphorylation by yeast hexokinase were evaluated in comparison with 2-deoxy-2-[¹⁸F]fluoro-D-glucose ([¹⁸F]FDG). The rate of protein binding in pooled human serum was measured by an ultrafiltration method. In vivo metabolite analysis in mice was also performed. Biodistribution, urine excretion, and in vivo renal kinetics in mice were compared with 2-deoxy-2-[¹⁸F]fluorosorbitol ([¹⁸F]FDS).

RESULTS

Rat RBC uptake of 3- and 6-[¹⁸F]FDA (7.8 ± 2.5%ID and 10.2 ± 4.8%ID, respectively) was significantly lower than that of [¹⁸F]FDG (44.7 ± 8.7%ID). RBC uptake of 3-[¹⁸F]FDA was inhibited by D-glucose (30%) and cytochalasin B (40%), indicating the involvement of GLUT1-dependent transport. In contrast, 6-[¹⁸F]FDA transport was not inhibited by D-glucose and cytochalasin B. 3- and 6-[¹⁸F]FDA were not phosphorylated by yeast hexokinase under the conditions that result in 60% conversion of [¹⁸F]FDG into [¹⁸F]FDG-6-phosphate within 30 min. Serum protein binding of 3- and 6-[¹⁸F]FDA was negligible. Metabolic transformation of both tracers was not detected in plasma and urine at 30 min after injection. The highest tissue uptake of both tracers was observed in kidneys. Heart and brain uptake of both tracers was below blood levels throughout the biodistribution studies (until 120 min after injection). No significant uptake in the bone was observed, indicating the absence of de-fluorination in mice. In vivo PET imaging visualized rapid excretion of the administered 3- and 6-[¹⁸F]FDA from the kidneys, with minimal tracer accumulation in other organs. The urine excretion rate of 3-[¹⁸F]FDA was much lower than that of 6-[¹⁸F]FDA and [¹⁸F]FDS.

CONCLUSIONS

3- and 6-[¹⁸F]FDA might be unsatisfactory for tumor imaging. In contrast, these tracers demonstrated high levels of kidney uptake and excretion, low serum protein binding, and high metabolic stability as preferable properties for renal imaging. Notably, the urine excretion rate and kidney uptake kinetics of 6-[¹⁸F]FDA were comparable with those of the potential renal imaging agent [¹⁸F]FDS. Further validation studies in animal models are required to confirm the feasibility of 6-[¹⁸F]FDA as a functional renal imaging agent.

Fluorination of aryl boronic acids using acetyl hypofluorite made directly from diluted fluorine

Aryl boronic acids or pinacol esters containing EDG were converted in good yields and fast reactions to the corresponding aryl fluorides using the readily obtainable solutions of AcOF. In reactions with aryl boronic acids containing EWG at the para position, there are two competing forces: one directing the fluorination to take place ortho to the boronic acid and the other, toward an ipso substitution. With EWG meta to the boronic acid, substitution ipso to the boron moiety takes place in good yields.