Temperature effects on the stereospecificity of nucleophilic fluorination: Formation of trans-[¹⁸F]4-fluoro-l-proline during the synthesis of cis-[¹⁸F]4-fluoro-l-proline

Fluorine-18 labeled (2S,4S)-4-fluoro-l-proline (cis-[¹⁸F]4-FPro) has been reported to be a potential positron emission tomography tracer to study abnormal collagen synthesis occurring in pulmonary fibrosis, osteosarcomas, mammary and colon carcinomas. In this paper, we report the stereospecific radiofluorination of (2S,4R)-N-tert-butoxycarbonyl-4-(p- toluenesulfonyloxy) proline methyl ester (at 110°C) to produce diastereomerically pure cis-[¹⁸F]4-FPro in 38% radiochemical yield at the end of a 90-min synthesis. Investigation of the effect of temperature on the stereospecificity of nucleophilic fluorination showed that diasteriomerically pure cis-[¹⁸F]4-FPro or trans-[ ¹⁸F]4-FPro was produced at lower temperatures (85°C-110°C) during the fluorination of (2S,4R) or (2S,4S) precursors, respectively. However, at higher temperatures (130°C-145°C), fluorination of (2S,4R) precursor produced a mixture of cis-[¹⁸F]4-FPro and trans-[ ¹⁸F]4-FPro diastereomers with cis-[¹⁸F]4-FPro as the predominant isomer. Hydrolysis of the purified fluorinated intermediate was carried out either in one step, using 2m triflic acid at 145°C for 10min, or in two steps where the intermediate was heated in 1m HCl at 110°C for 10min followed by stirring at room temperature in 1 N NaOH for 5min. The aqueous hydrolysis mixture was loaded onto an anion exchange column (acetate form for one-step hydrolysis) or an ion retardation column (two-step hydrolysis) followed by a C₁₈ Sep-Pak (Waters Corporation, Milford, MA, USA). Pure cis-[¹⁸F]4-FPro was then eluted with sterile water. We also report that epimerization of cis-[¹⁸F]4-FPro occurs during the two-step hydrolysis (H⁺ followed by OH^-) of the intermediate, resulting in 5±3% trans-[¹⁸F]4-FPro, whereas the one-step acid hydrolysis yielded pure cis-[¹⁸F]4-FPro in the final product.