An investigation on stereospecific fluorination at the 2'-arabino-position of a pyrimidine nucleoside: radiosynthesis of 2'-deoxy-2'-[(18)F]fluoro-5-methyl-1-β-D-arabinofuranosyluracil

Strategies for the Synthesis of Fluorinated Nucleosides, Nucleotides and Oligonucleotides

Fluorinated nucleosides and oligonucleotides are of specific interest as probes for studying nucleic acids interaction, structures, biological transformations, and its biomedical applications. Among various modifications of oligonucleotides, fluorination of preformed nucleoside and/or nucleotides have recently gained attention owing to the unique properties of fluorine atoms imparting medicinal properties with respect to the small size, electronegativity, lipophilicity, and ability for stereochemical control. This review deals with synthetic protocols for selective fluorination either at sugar or base moiety in a preformed nucleosides, nucleotides and nucleic acids using specific fluorinating reagents.

A novel radiochemical approach to 1-(2'-deoxy-2'-[(18) F]fluoro-β-d-arabinofuranosyl)cytosine ((18) F-FAC)

(18) F-FAC (1-(2'-deoxy-2'-[(18) F]fluoro-β-D-arabinofuranosyl)-cytosine) is an important 2'-fluoro-nucleoside-based positron emission tomography (PET) tracer that has been used for in vivo prediction of response to the widely used cancer chemotherapy drug gemcitabine. Previously reported synthetic routes to (18) F-FAC have relied on early introduction of the (18) F radiolabel prior to attachment to protected cytosine base. Considering the (18) F radiochemical half-life (110 min) and the technical challenges of multi-step syntheses on PET radiochemistry modular systems, late-stage radiofluorination is preferred for reproducible and reliable radiosynthesis with in vivo applications. Herein, we report the first late-stage radiosynthesis of (18) F-FAC. Cytidine derivatives with leaving groups at the 2'-position are particularly prone to undergo anhydro side-product formation upon heating because of their electron density at the 2-carbonyl pyrimidone oxygen. Our rationally developed fluorination precursor showed an improved reactivity-to-stability ratio at elevated temperatures. (18) F-FAC was obtained in radiochemical yields of 4.3-5.5% (n = 8, decay-corrected from end of bombardment), with purities ≥98% and specific activities ≥63 GBq/µmol. The synthesis time was 168 min.

Radiochemical synthesis of 2'-[18F]-labelled and 3'-[18F]-labelled nucleosides for positron emission tomography imaging

This review article considers 2'-labelled and 3'-labelled nucleosides, which are of great importance as positron emission tomography (PET) probes in clinical diagnostics and PET research. Although the radiochemical preparation of several [(18)F]-labelled nucleosides such as [(18)F]fluorothymidine or [(18)F](fluoroarabinofuranosyl)cytosine has been accomplished within the last two decades, a number of potentially interesting nucleoside-based biomarkers are not yet available for automated good manufacturing practice production due to the lack of fast and efficient synthetic methods for late-stage [(18)F]-introduction. In order to meet recent demands for new PET-based biomarkers in various clinical applications, appropriate precursors that can easily be fluorinated and deprotected need to be developed.