Nucleophilic ¹⁸F-fluorination Of Phosphorofluoridates （Via Imidazole-activated Precursors）

Phosphate is one of the essential phosphorus-containing compounds in body and pharmaceutical industry.And positron emission tomography(PET)is an important in vivo imaging technology for diagnosing diseases.Thus,developing novel tracers based on phosphate and its derivatives has great significance for PET imaging not only to diagnose diseases,but to study physiological processes and advance new drug discoveries.Fluorine-18(18F)is the most widely employed positron emission radionuclide,however,18F-labeled phosphate derivatives as tracers are barely reported.The currently reported strategies involve complex synthesis of the precursors,multi-step radiosynthesis and products with low molar activities,limiting the wide application in PET imaging.Due to analogous van der Waals radius and valence electron numbers between fluoro and hydroxyl group,fluorination on phosphate group can produce a chemically stable phosphorofluoridate or phosphonofluoridic acid which is isostructural and isoelectronic to the unmodified substrate.Thus,the phosphate group is an ideal fluorination site for 18F-fluorination on phosphate derivatives.Herein,18F/19F isotopic exchange reaction and Zn(Ⅱ)-mediated nucleophilic 18F-fluorination reaction via imidazole-activated precursor are both explored for 18F-fluorination on phosphate derivatives.Firstly,the mechanism and activation energy of 18F/19F isotopic exchange reaction for benzyl phosphonofluoridic acid were simulated by density functional theory(DFT)calculations.Then,five imidazole-activated phosphate derivatives and their fluorinated products were designed and synthesized.And the fluorination efficiency of 18F-Iabeled benzyl phosphonofluoridic acid under different conditions by isotopic exchange strategy and the feasibility of DFT calculation were discussed.On the other hand,a novel Zn(Ⅱ)-mediated one-step 18F-fluorination strategy was developed via imidazole-activated precursors.The optimal conditions were screened based on reaction time,temperature,amounts of precursor and Zn(Ⅱ)salt,and types of Zn(Ⅱ)salts.In addition,the in vitro and in vivo stabilities of the 18F-labeled benzyl phosphorofluoridate,phenyl phosphorofluoridate and benzyl phosphonofluoridic acid as model compounds were evaluated.The mechanism of 18F/19F isotopic exchange reaction for benzyl phosphonofluoridic acid was simulated as addition-elimination process,and its high activation energy and low reaction rate at room temperature was calculated.[18F]KF/K222 and CH3CN were screened as the best efficient combination for 18F-fluorination of benzyl phosphonofluoridic acid,but only 6.6 ± 4.1%RCY were obtained.And five imidazole-activated phosphate derivatives with 35-82%yields were obtained by one-step coupling.The>50%and 87.0±1.6%RCYs of 18F-labeled phosphorofluoridates and benzyl phosphonofluoridic acid were respectively achieved via imidazole-activated precursors,and 63.1 ± 1.2%RCY was obtained for 18F-labeled benzyl phosphonofluoridic acid within only 5 min at room temperature.The molar activities of 18F-labeled benzyl phosphorofluoridate and benzyl phosphonofluoridic acid were analyzed as 7.5 GBq/μmol and 2.2 GBq/μunol,respectively.In addition,the18F-labeled phosphorofluoridates and benzyl phosphonofluoridic acid show sufficient in vitro stabilities,and the latter exhibits better in vivo metabolic stability.Compared with18F/19F isotopic exchange reaction,Zn(Ⅱ)-mediated one-step method for 18F-fluorination on phosphate via imidazole-activated precursors is a better strategy,providing an effective method for 18F-labeling on phosphate and its derivatives.And 18F-labed phosphorofluoridate derivatives can be used as potential PET tracers and prosthetic groups to 18F-labeling of peptides and other biological molecules,broadening the scope of phosphate derivatives in PET imaging.