Synthesis of 2-fluorobenzoic acids by nucleophilic fluorination of 1-arylbenziodoxolones

We report a facile, transition metal-free synthesis of fluorobenzoic acids by nucleophilic fluorination of readily available 1-arylbenziodoxolones using fluoride salts in polar aprotic solvents. This protocol was applied for the preparation of 2-[ 18 F]-fluoro-5-nitrobenzoic acid, which is a potentially important radioligand for Positron Emission Tomography (PET)


Introduction
Positron emission tomography (PET) is a powerful nuclear imaging technique that is used to study and visualize human physiology by the detection of positron-emitting radiopharmaceuticals (PET radiotracers). 1 Fluorine-18 is the most commonly used radioisotope in PET because of its favorable physical and nuclear characteristics, such as, a short but manageable half-life (t1/2 = 109.7 min), which allows sufficient time for multistep synthetic labeling reactions. 2,3Nucleophilic 18 F-anion is produced with a cyclotron by the nuclear reaction from enriched [ 18 O]-water.][6] The introduction of the fluorine-18 radioactive label into a peptide molecule is an important area of PET.Derivatives of fluorobenzoic acid are the most common reagents used for peptide radiolabeling via addition to the lysine amino group in the appropriate peptide fragment. 7In particular, the N-succinimidyl derivative of 4-[ 18 F]fluorobenzoic acid (SFB, 3) represents an important example of the peptide radiolabeling reagent.[9] Scheme 1. Radiosynthesis of 4-[ 18 F]fluorobenzoic acid 2 and [ 18 F]SFB 3.
The overall time of these multistep procedures (Scheme 1) exceeds half-life decay of the 18 F-isotope, which is a significant disadvantage of the approaches utilizing quaternary ammonium salts 1 or 4 as starting materials.Carroll and coworkers have demonstrated the possibility of the synthesis of [ 18 F]-fluorobenzoic acids by nucleophilic fluorination of iodonium salts. 10However, this method requires the use of an uncommon precursor, (4-((2,5-dioxopyrrolidin-1-yloxy)carbonyl)phenyl)(thiophen-2-yl)iodonium trifluoroacetate, which can be obtained by a multistep synthesis in a low overal yield.
In the present paper, we report a convenient and fast procedure for a single-step preparation of 2fluorobenzoic acids 6 by nucleophilic fluorination of the readily available 1-arylbenziodoxolones 5 (Scheme 2).We have investigated the role of solvent and substituents in compounds 5 on the yields of 2-fluorobenzoic acids 6 and found that the 5-nitro-substituted benziodoxole (5, R = NO2, Ar = Mes) is the most efficient precursor to the corresponding 2-fluoro-5-nitrobenzoic acid (89% yield).Furthermore, we have demonstrated that our protocol can be applied to the preparation of 2-[ 18 F]-fluoro-5-nitrobenzoic acid, which is a potentially important PET-tracer for the incorporation of 18 F into peptides, proteins or antibodies.

Results and Discussion
1-Arylbenziodoxolones 5 were prepared by the previously reported one-pot procedure starting from commercially available substituted iodobenzoic acids. 11Previously, we have demonstrated that compounds 5 can react with the azide anion as a nucleophile producing respective 2-azidobenzoic acids in moderate to high yields. 11,12In the present study, we investigated reactions of 1-arylbenziodoxolones 5 with the fluoride anion in polar aprotic solvents under anhydrous conditions.In the first step of our study, we have checked the effect of the aryl group, Ar, on the reaction of unsubstituted benziodoxoles 7a-c with CsF.We have found that unactivated benziodoxoles 7 do not react with CsF in dimethylformamide under reflux conditions.Activation of compounds 7 by addition of trifluoroacetic acid improves reactivity, probably due to protonation leading to the formation of the more reactive noncyclic intermediates 8. Reaction of the activated reagents with CsF in DMF in the presence of TEMPO (required to supress radical side reactions) resulted in 70% conversion after 4 h heating at 150 o C (Scheme 3).According to GC-MS data, the main product of these reactions was benzoic acid 10, while the product of nucleophilic fluorination, 2-fluorobenzoic acid 9, was observed in low yields with the highest yield of 8% in the reaction the mesityl derivative 7c.Therefore, reactions of unsubstituted benziodoxoles 7 are unsuitable for efficient preparation of 2-fluorobenzoic acids because of the low reactivity of the reagents.The yields of the products of nucleophilic fluorination in these reactions are low; however, the use of the more bulky Ar groups leads to noticeable improvement.Scheme 3. Reactions of unsubstituted benziodoxoles 7a-c with CsF after activation with CF3CO2H.
Our previous study of the reactions of 1-arylbenziodoxolones with the azide anion as a nucleophile has demonstrated that compounds 5 bearing methyl substituent in the benziodoxole ring ortho to the iodine atom have a significantly higher reactivity than unsubstituted benziodoxoles. 11,12The dramatic enhancement of reactivity observed in the reactions of 1-aryl-7-methylbenziodoxolones with nucleophiles is explained by steric effect of the bulky methyl substituent on the structure of the reaction intermediate. 11At the next step, we have investigated reactions of 7-methyl-substituted 1-arylbenziodoxolones 11 with CsF in DMF without activation by trifluoroacetic acid (Scheme 4).As expected, reagents 11 were more reactive and 100% conversion was achieved without pre-activation by addition of trifluoroacetic acid.However, the yields of the desired fluorobenzoic acid 12 remained low (6-18% after aqueous work-up) and the main product of these reactions was 3-methylbenzoic acid 13.Same as in the reaction of unsubstituted benziodoxoles 7, the use of the more bulky Ar groups resulted in noticeable improvement of the yield up to 18%.Pre-activation of reagents 11 by trifluoroacetic acid via the formation of the more reactive noncyclic intermediates 14 led to further improvement of the yield (Scheme 5).The yield of 2-fluoro-3-methylbenzoic acid 12 in the reaction of 1-mesytyl-7-methylbenziodoxolone 11c has reached 78% ( 1 H NMR data); however, the formation of 3-methylbenzoic acid 13 (the product of radical decomposition of benziodoxolones 11) was not completely supressed under these conditions.Because of the difficult separation of fluorobenzoic acids (9 and 12) from benzoic acids (10 and 13) it was important to find more selective reaction conditions for nucleophilic fluorination.Scheme 5. Reactions of 1-aryl-7-methylbenziodoxolones 11a-c with CsF after activation with CF3CO2H.
In a search for optimized reaction conditions for the nucleophilic fluorination of 1-mesytyl-7methylbenziodoxolone 11c, we have investigated reactions of different sources of fluoride anions in different solvents.In particular, NMR experiments have demonstrated that the reactions of 11c with tetramethylammonium fluoride in acetonitrile, benzene, toluene, or DMF resulted in low yelds (under 23%) of 2-fluoro-3-methylbenzoic acid 12 with 3-methylbenzoic acid 13 as major product.Therefore, we came to a conclusion that 1-mesytyl-7-methylbenziodoxolone 11c is not the optimal reagent for selective nucleophilic fluorination that can be utilized in PET.
Previously, we have reported that 5-nitro-substituted 1-arylbenziodoxolones have increased reactivity in reactions with nucleophiles due to the electron-withdrawing effect of the nitro group. 11Also, the presence of the highly polar nitro-group suppresses radical decomposition of benziodoxole preventing formation of the non-fluorinated benzoic acids.Therefore, we decided to investigate the reactions of 1-aryl-5nitrobenziodoxolones 14 with different sources of fluoride anion in different solvents.We have found that the greatest selectivity and highest isolated yields of the products of nucleophilic fluorination could be achieved in the reactions of reagents 14 with CsF in anhydrous DMSO (Scheme 6).Scheme 6.Reactions of 1-aryl-5-nitrobenziodoxolones 14a-c with CsF.
These reactions (Scheme 6) selectively afforded 2-fluoro-5-nitrobenzoic acid 16 in excellent preparative yields after aqueous workup.Additional 19 F NMR study of this reaction in NMR tube indicated that the interaction of benziodoxole 14 with fluoride anion proceeds via initial formation of diaryliodonium fluoride 15 ( 19 F NMR signal at -12.96 ppm), which is in agreement with literature data on nucleophilic fluorination of iodonium salts. 13The final product 16 can be conveniently separated from iodoarene (ArI) by treatment with NaHCO3 followed by extraction and acidification of the aqueous solution.Therefore, the study using nonradioactive fluoride anion indicates that 1-aryl-5-nitrobenziodoxolones 14 are better substrates than 1aryl-7-methylbenziodoxolones 11 for the selective nucleophilic fluorination with fluoride anion.
Next, we have investigated the radiofluorination of nitro-substituted benziodoxoles 14 using [ 18 F]KF•K2.2.2 in the Synthra RN Plus synthesizer (Synthra GmbH, Germany) adapted to the reaction conditions (Scheme 7).We used benziodoxoles 14b and 14c as the precursors based of the high yields of the products of fluorination and high solubility in DMSO and acetonitrile.Nucleophilic 18 F-anion was produced with a cyclotron from enriched [ 18 O]-water by the standard 18 O(p,n) 18 F nuclear reaction.The produced [ 18 F]F - was eluted by a mixture of solutions of Kriptofix 2.2.2 in acetonitrile and K2CO3 in water for conversion into [ 18 F]KF•K2.2.2.Solutions of benziodoxoles 14b and 14c in anhydrous acetonitrile or DMSO were added to [ 18 F]KF•K2.2.2 after azeotropic drying.The reaction mixture was heated at 150 o C for 30 min, then cooled, acidified with 0.01 M HCl, and purified using Sep-Pak C18 cartridge washing with water and finally eluting 2-[ 18 F]-fluoro-5-nitrobenzoic acid 18 with 2 mL of acetonitrile.© AUTHOR(S) Scheme 7. Radiofluorination of 1-aryl-5-nitrobenziodoxolones 14.
We have investigated the influence of solvent, temperature, and reaction time on the isolated radiochemical yield (RCY) of 2-[ 18 F]-fluoro-5-nitrobenzoic acid 18 in the reactions of benziodoxoles 14b and 14c (Scheme 7).We have used the most common solvents for radiofluorination, acetonitrile and DMSO.In order to identify the optimal solvent, we have performed reactions of 14b and 14c in these two solvents under standard conditions (Table 1).The results shown in Table 1 indicate that RCY of 18 was significantly higher in the reaction of precursor 14b when acetonitrile was used as a solvent.Lower yield of 18 in DMSO can be explained by lower solubility of 14b in DMSO compared to acetonitrile.A potentially important factor could be the excessive pressure developed at 150 o C the closed reaction vessel, resulting in even better solubility of benziodoxole 14b.In the case of precursor 14c, the RCY of 18 is practically independent on solvent because of the excellent solubility of the n-butyl derivative 14c in acetonitrile and DMSO.
The effect of temperature on RCY of 2-[ 18 F]-fluoro-5-nitrobenzoic acid 18 was investigated in the range from 80 to 200 o C in acetonitrile (Table 2).The best yields of product 18 were achieved at 150 o C. The lower RCY at 200 o C can be explained by side processes via benzyne intermediates.A study of the reaction time on RCY has demonstrated that 30 min is the optimal time (Table 3).Increased reaction time did not lead to improved yields.In order to achieve high levels of radiochemical purity (RCP) of product 18, it is required to remove the unreacted [ 18 F]KF•K2.2.2 by washing with no less than 40 mL of water followed by elution of product 18 with 2 mL of acetonitrile.Using smaller volumes of water and acetonitrile results in lower RCP and RCY.In the reaction of precursor 14b, we observed the presence of two products in the radio-TLC chromatogram (Figure 1).Based on the studies with the non-radioactive fluorine derivatives (Scheme 6), we suggest that the second peak belongs to the intermediate benziodoxole [ 18 F]fluoride 17.The formation of the analogous fluoride intermediates has been reported in the literature on nucleophilic fluorination of iodonium salts. 13After purification, RCP of 2-[ 18 F]-fluoro-5-nitrobenzoic acid increases from 67% to 75%; however, we were unable to achieve the higher levels of RCP required for PET.In contrast to benziodoxole 14b, the reaction of precursor 14c results in the formation of a single product of radiofluorination.A similar purification of crude product obtained from the reaction of 14c affords 2-[ 18 F]fluoro-5-nitrobenzoic acid 18 with RCP above 98% (Figure 2).Therefore, we have achieved the goal of efficient preparation of a substituted [ 18 F]-fluorobenzoic acid, which is a potentially important PET-tracer for the incorporation of 18 F into peptides, proteins, or antibodies.

Conclusions
In conclusion, we have demonstrated the possibility of using 1-arylbenziodoxoles as efficient precursors for the synthesis of fluorobenzoic acids via nucleophilic fluorination using fluoride salts in polar aprotic solvents.5-nitro-substituted benziodoxole 14c was found to be an excellent reagent for the radiofuorination leading to [ 18 F]-fluorobenzoic acids in up to 39% radiochemical yield with excellent radiochemical purity above 98%.This protocol under optimized reaction conditions (30 min at 150 o C in acetonitrile) was applied for the preparation of 2-[ 18 F]-fluoro-5-nitrobenzoic acid 18, which is a potentially important radioligand for Positron Emission Tomography (PET).

Experimental Section
General.2-Iodobenzoic acid, all aromatic precursors, and other reagents and solvents were from commercial sources and used without further purification from freshly opened containers.NMR spectra were recorded at 300, 400 and 500 MHz ( 1 H NMR) and 75, 100, 125 MHz ( 13 C NMR) and 376 and 470 MHz ( 19 F NMR).Chemical shifts (δ) are reported in parts per million.
General procedure for preparation of 1-arylbenziodoxolones ( 7a-c, 11a-c). 11The finely crushed, solid 2iodobenzoic acid (2.0 mmol) was mixed with powdered Oxone (0.75−0.8 g, 1.2−1.3mmol) in a 50 mL roundbottom flask and stirred without solvent for 5 min using a magnetic stirrer until a homogeneous reaction mass was formed.Then the reaction mixture was cooled with ice to 5 °C and, under magnetic stirring, and H2SO4 (total 1.6 mL, precooled to 5 °C) was added via syringe by 0.2 mL portions to the center of the reaction mixture.After addition of each portion of H2SO4, the reaction mass was mechanically shaken to achieve better mixing; the color of the resulting mass can vary from pale yellow to brown depending on the intensity of mixing.After all H2SO4 was added, the magnetic stirring was continued for 30 min at room temperature, the mixture was cooled to 5 °C, and CH2Cl2 (3 mL) and ArH (2.0−4.0 mmol) were added.The magnetic stirring of the resulting mixture was continued at 5 °C for 1 h and then at room temperature for 2 h.The reaction mixture was recooled to 5 °C and CH2Cl2 (10mL), and then a saturated aqueous solution of NaHCO3 were added in small portions until pH 8.0.The organic layer was separated, and the aqueous layer was additionally extracted with CH2Cl2 (5 mL).The organic extracts were combined and dried with Na2SO4, the solvent was evaporated, and the crystalline product was dried in vacuum.Additional purification of the products can be performed by crystallization from water. 12The reaction of 2-iodobenzoic acid (248 mg, 1.0 mmol), Oxone (400 mg, 0.65 mmol), H2SO4 (total 0.8 mL) and benzene (0.2 mL) according to the general procedure afforded 288 mg (88%) of product 7a monohydrate, isolated as off-white crystals; mp 221-222°C (from water) (lit. 122 The reaction of 2-iodobenzoic acid (248 mg, 1.0 mmol), Oxone (400 mg, 0.65 mmol), H2SO4 (total 0.8 mL), and toluene (0.2 mL) according to the general procedure afforded 288 mg (81%) of product 7b monohydrate, isolated as off-white crystals: mp 217−219°C (from water) (lit. 12mp 217-219°C). 1 H NMR (CDCl3, 500 MHz): δ= 8.37 (dd, J 1.5, 7.5 Hz, 1H), 7.88 (d, J 8.0 Hz, 2H), 7.53 (m, 1H), 7.37 (m, 3H), 6.76 (d, J 8.5 Hz, 1H), 2.50 (s, 3H) ppm. 13

4 a
The reaction was carried out using 10 mg of 14b or 14c in solvent (1.0 mL) at 150 o C for 30 min in a closed reaction vessel.b All statistical data were obtained in three parallel experiments.

Table 1 .
Effect of solvent on radiochemical yield a,b

Table 2 .
Effect of temperature on radiochemical yield a,b b All statistical data were obtained in three parallel experiments.

Table 3 .
Effect of reaction time on radiochemical yield a,b a The reaction was carried out using 10 mg of 14b or 14c in acetonitrile (1.0 mL) at 150 o C in a closed reaction vessel.b All statistical data were obtained in three parallel experiments.