Synthesis and reactivity of potassium 2,2,2-trifluoroethoxideNucleophilic addition of fluoroform to aldehydes to produce carbinols is well-known40. However, the reaction of fluoroform with the simplest aldehyde, formaldehyde, has not been explored. We considered that this reaction should produce a very useful reagent, 2,2,2-trifluoroethoxide. Because of the toxicity and volatility of formaldehyde41, we decided to explore solid paraformaldehyde as an alternative for producing 2,2,2-trifluoroethoxide. Treatment of fluoroform as limiting reagent with a mixture of paraformaldehyde and t-BuOK in DMF produced 2,2,2-trifluoroethanol in 41% yield after water quench (Supplementary Table 1, entry 1). Under optimal conditions, fluoroform was converted into 2,2,2-trifluoroethanol in 88% yield within 30 min (Supplementary Table 1, entry 13). Increasing the reaction time did not improve the conversion appreciably (Supplementary Table 1, entries 11 and 12). Other bases or solvents adversely affected yield (Supplementary Table 1, entries 14–18). We next tested the reactivity of the putative intermediate, potassium 2,2,2-trifluoroethoxide, by treatment with 2-chloropyrimidine. To our delight, reaction proceeded smoothly at room temperature to afford 2-(2,2,2-trifluoroethoxy)pyrimidine1 in 71% yield within 2 h (Fig. 2A). However, treatment of potassium 2,2,2-trifluoroethoxide with 1-fluoronaphthalene gave only trace 1-(2,2,2-trifluoroethoxy)naphthalene2, even at elevated temperature. We considered that an aryl hypervalent iodine center in an aryliodonium ylide might show more reactivity. Indeed, treatment of potassium 2,2,2-trifluoroethoxide with naphthalen-1-yl iodonium-(2,2-dimethyl-1,3-dioxane-4,6-dione)ylide at 60 °C produced 2 in 63% yield (Fig. 2A). Thus, gratifyingly, we had succeeded in converting stoichiometric amounts of fluoroform into a useful synthon, potassium 2,2,2-trifluoroethoxide, in high yield and in showing the utility of this synthon for trifluoroethoxylation of homoarene and heteroarene under mild conditions. We anticipate that this transition metal-free transformation can find wide applications in fluorine chemistry. Indeed, we readily produced several trifluoroethoxy compounds in high yields by this chemistry as standards for use in the remainder of this study (see Supporting Information, Supplementary methods).Fig. 2: Reactions of fluoroform and [11C]fluoroform.A One-pot two-stage trifluoroethoxylations with fluoroform to give 2,2,2-trifluoroethyl ethers. Yields were determined with 19F NMR spectroscopy. (For more detail, see Supplementary Table 1 and Supplementary Fig. 1). B Optimization of conversion of [11C]fluoroform with paraformaldehyde into [2-11C]2,2,2-trifluoroethanol (left panel) and HPLC chromatogram of the crude reaction mixture from entry 5 (right panel). C Example of one-pot, two-stage synthesis of 2-[11C](2,2,2-trifluoroethoxy)pyrimidine.Synthesis of [11C]potassium 2,2,2-trifluoroethoxideWe next aimed to explore this synthetic methodology for robust broad-scope radio-trifluoroethoxylations as a potential route to PET tracers. For this purpose, we routinely produce [11C]fluoroform by CoF3-mediated fluorination of cyclotron-produced [11C]methane22. Treatment of [11C]fluoroform (37–296 MBq) with a mixture of t-BuOK (50 μmol) and paraformaldehyde (17 μmol) in DMF for only 3 min at room temperature gave an excellent yield (88%) of 11CF3CH2OH upon hydrolysis of the putative intermediate (Fig. 2B, entry 1). Increasing the reaction time to 5 min only slightly increased the yield. A larger quantity of paraformaldehyde (50 µmol) afforded 11CF3CH2OH in 97% yield. The yield of 11CF3CH2OH was quantitative when [11C]fluoroform was treated with 1: 2 molar mixture of paraformaldehyde and t-BuOK for 4 min (Fig. 2B, entry 5). These reaction conditions were therefore deemed optimal. This success encouraged us to test the efficacy of 11CF3CH2OK for introducing the 11C-trifluoromethyl moiety into a wide range of compounds.Synthesis of 11C-2,2,2-trifluoroethoxy arenesFirst, we examined the reactivity of 11CF3CH2OK towards heteroarenes. To our delight, 11CF3CH2OK reacted at room temperature to produce a wide range of desired 11C-2,2,2-trifluoroethoxy heteroarenes in moderate to excellent yields within just 1 min (Fig. 3A). Attractive features to emerge from this labeling protocol were: (1) the 11CF3CH2OK, can be used without isolation; (2) reaction conditions are mild and rapid; (3) substrate scope is broad, and encompasses pyridines, pyrimidines, pyrazine, thiazoles, triazines, quinolines, and isoquinolines; (4) in addition to halides (F, Cl, and Br), leaving groups such as nitro ([11C]4) and methyl sulfone ([11C]9) are highly effective; (5) functional group tolerance is high, with aldehyde ([11C]3), bromo ([11C]4, [11C]6), nitrile ([11C]5), methoxy ([11C]7), and Boc protection ([11C]9) all well tolerated. Heteroarenes having 1 to 3 nitrogen atoms ([11C]1, [11C]3–[11C]14) were compatible with the reaction conditions and afforded the desired 11C-labeled products in acceptable yields (21–94%). Furthermore, the late-stage 11C-trifluoroethoxylation of complex biomolecules was highly effective as shown by the labeling of analogs of several drug-like compounds [Imiquimod ([11C]15), Milrinone ([11C]18)], drug precursors [Erlotinib ([11C]19), Canagliflozin ([11C]20), Pazopanib ([11C]21), Palbociclib ([11C]22)], the herbicide Clopyralid ([11C]17), and a purine derivative ([11C]16)] in moderate to very good yields. Notably, 11C-trifluoroethoxylation occurred preferentially at the more electron-deficient site (e.g., the ortho- vs meta-pyridinyl site for [11C]17) or aryl ring (e.g., the pyridinyl vs homoarene ring in [11C]6 and [11C]20) as expected for aromatic nucleophilic substitution reactions.Fig. 3: Scope for 11C-2,2,2-trifluoroethoxylation of arenes.LG = leaving group. Yields are based on HPLC analyses of reaction mixtures. All yields are based on [11C]fluoroform conversion into the products, decay-corrected and expressed as mean ± SD (n = 3). Radioactive products were collected at least once for each substrate to check that HPLC yields matched isolated yields. A Substrate scope for heteroarenes using reaction condition (A). B Substrate scope for homoarenes using reaction condition (B). Auxiliary derived from Meldrum’s acid.We found that homoarene precursors with common leaving groups were more challenging for 11C-trifluoroethoxylation than the reactive heteroarenes42. We anticipated that the use of a more powerful hypervalent aryliodonium leaving group43,44 could alleviate this issue. We opted to explore this possibility for the one-pot 11C-trifluoroethoxylation of homoarene substrates. First, we screened conditions for the reaction of [11C]potassium 2,2,2-trifluoroethoxide with an iodonium ylide derived from Meldrum’s acid (naphthalen-1-yl(2,4,6-trimethoxyphenyl)iodonium tosylate) (Supplementary Table 3). We found that treating a mixture of [11C]CF3CH2OK with the iodonium salt precursor (55 µmol) in DMF at 60 °C for 3 min gave a high and optimal yield of the desired [11C]2 (87%; Supplementary Table 3, entry 2). The 2,4,6-trimethoxyphenyl group served as an effective aryl spectator ring; no [11C]1,3,5-trimethoxy-(2-(2,2,2-trifluoroethoxy))benzene, was produced. An increase in temperature did not improve the yield of [11C]2. Reduction in precursor amount reduced yield. Given the high yield obtained for [11C]2 with this approach under optimal conditions, we proceeded to explore substrate scope. Substituent electronics had substantial influence on reaction yields ([11C]23–[11C]32). Electron-withdrawing groups in ortho- and para-position gave high yields for the 11C-trifluoroethoxylation (Fig. 3B). 11C-Trifluorethoxylation yields were lower for substrates with para-electron-donating or meta-electron-withdrawing groups. Novel cross-coupling synthons, [11C]24–[11C]27, were obtained in useful yields. We draw attention to the syntheses of [11C]26 and [11C]28, where mesityl was used as the partner aryl ring in the iodonium salt precursor11.C-Trifluoroethoxylation was directed to the other aryl ring. This is an interesting observation because here the ring chemoselectivity is opposite to that seen for the non-copper mediated radiofluorination of aryl(mesityl)iodonium salts45.We also explored aryliodonium ylides as precursors. 11C-Trifluoroethoxylation of three model ylides gave [11C]33–[11C]35, in moderate to high yields. Moreover, [11C]36, an analog of the antidiabetic drug emplagliflozin (®Jardiance) was also obtained in moderate yield (47%) from an ylide. This exemplifies how iodonium ylides can serve as precursors for trifluoroethoxylation reactions. In addition, two activated fluoroarenes, with ortho-electron-deficient aryl rings, gave [11C]37 and [11C]38 in moderate to good yields where fluoride was the leaving group.
11C-2,2,2-Trifluoroethoxylation of aliphatic substratesWe were further interested in whether 11C-2,2,2-trifluoroethoxylation would occur on aliphatic substrates as well as arenes. This consideration prompted us to investigate the reactivity of 11CF3CH2OK with aliphatic substrates. We started with a model compound, a precursor to Posaconazole (®Noxafil) with a tosylate leaving group to optimize precursor amount and reaction temperature (Supplementary Table 4). 11C-Trifluoroethoxylation produced excellent yields of [11C]45 (89%) under conditions found to be optimal for hypervalent iodonium precursors (Supplementary Table 4, entry 3). Again, yield did not increase with temperature (Supplementary Table 4, entry 4). Reduction in precursor amount drastically diminished yield (Supplementary Table 4, entries 5 and 6). We next tested reaction scope by attempting to prepare a range of 11C-labeled alkyl-2,2,2-trifluoroethyl ethers from aliphatic precursors, including fourteen 11C-labeled biomolecules (Fig. 4). [11C]41 was obtained from a long chain iodoalkyl precursor in acceptable yield (45%). Benzyl halide and α-chloroacetyl precursors were readily converted into their analogous 11C-2,2,2-trifluoroethoxy ethers ([11C]42, [11C]47, [11C]48, [11C]51, [11C]52) in high yields (53–89%) with good tolerance of other functional groups. Aliphatic tosylates derived from a variety of commercially available drug-like molecules MCPA (2-methyl-4-chlorophenoxyacetic acid), Helional, Ketoconazole, Bendazac, an α-D-glucopyranoside derivative, Oxaprozin, Ospemifene, Pterostilbene, and Cyhalofop-butyl reacted readily with 11CF3CH2OK to provide the desired 11C-2,2,2-trifluoroethoxy ethers, [11C]39, [11C]40, [11C]43–[11C]46, [11C]49, [11C]50, and [11C]53–[11C]56, in moderate to excellent yields (34–93%). Precursors with leaving groups attached to an ethylene glycol linker gave excellent yields of 11C-2,2,2-trifluoroethoxy ethers. Notably, aliphatic 11C-trifluoroethoxylation occurred in preference to reaction at aromatic sites.Fig. 4: Scope for 11C-2,2,2-trifluoroethoxylation of aliphatic substrates.LG = leaving group, Decay-corrected yields are based on HPLC analyses of crude reaction mixtures. All yields are based on [11C]fluoroform conversion into the products and expressed as mean ± SD (n = 3). Radioactive products were collected at least once for each substrate to check that HPLC yields matched isolated yields.Determination of molar activity of [11C]1 as a model compoundWe measured the molar activity for a model product [11C]1, produced by the 11C-trifluoroethoxylation of 2-chloropyrimidine, to verify that this labeling technique is no-carrier-added (NCA) and gives high molar activity. Starting with about 10 GBq of cyclotron-produced [11C]methane that has been produced from a 10 µA × 10 min cyclotron irradiation, [11C]1 was obtained with a molar activity of 60 GBq/µmol, corrected to the end of radionuclide production (ERP). Such a high molar activity from a relatively limited cyclotron irradiation shows that the labeling reaction is invulnerable to carrier addition and dilution of molar activity15,39.Synthesis of [18F]potassium 2,2,2-trifluoroethoxideFluorine-18 labeling of PET tracers at aliphatic carbon by nucleophilic substitution of a good leaving group with [18F]fluoride19 can often lead to an [18F]fluoroalkyl group that is vulnerable to radiodefluorination in vivo and to accumulation of [18F]fluoride ion in the bone including skull. This can hamper accurate quantification of tracer uptake, especially in brain20,46,47.18F-Labeling in a 2,2,2-trifluoroethoxy group instead of an 18F-fluoroalkyl group could be a strategy to circumvent this issue. Having established an efficient route for 11C-trifluoroethoxylation, we next focused on translation of this labeling method from carbon-11 to fluorine-18 with a few representative substrates. For this purpose, [18F]fluoroform was produced from no-carrier-added [18F]fluoride and difluoroiodomethane48. CF218FCH2OK was generated by treatment of the [18F]fluoroform with paraformaldehyde and t-BuOK in DMF in >95% yield. The reactivity of CF218FCH2OK was assessed under the optimal conditions found for 11C-trifluoroethoxylations (Fig. 5).18F-Trifluoroethoxylations of aryl and aliphatic precursors proceeded smoothly and provided corresponding products in moderate to excellent yields similar to those from 11C-trifluoroethoxylation. Heteroarenes, such as pyridine, quinoline, pyrimidine, isothiazole, and 1,3,5-triazine, with halogen leaving groups were converted into the corresponding 18F-2,2,2-trifluoroethoxy ethers in high yields (82–96%; Fig. 5A). Dependency of labeling position on aryl ring position of the leaving group (e.g., ortho- vs meta- as in [18F]4 and [18F]17) or on the nature of the aryl ring (e.g., [18F]20) was as seen for 11C-labeling. Heteroaryl rings with more structural complexity and diverse functionality were conveniently labeled at room temperature within 5 min and produced the corresponding 18F-labeled compounds in excellent yields ([18F]15, [18F]17–[18F]22; 56–77%; Fig. 5A). These results indicate high potential for application of this labeling method to prospective structurally complex PET tracers. Homoarene precursors, including diaryliodonium salts ([18F]27 and [18F]31), aryliodonium ylides ([18F]35 and [18F]36), and fluoro precursors ([18F]37 and [18F]38), afforded useful yields of 18F-labeled products (27–69%; Fig. 5B), as for the11C-trifluoroethoxylations. Remarkably, the unprotected hydroxyl group in the Ataluren precursor was well tolerated ([18F]38) showing compatibility of this labeling protocol to sensitive functionality.Fig. 5: Scope for the 18F-2,2,2-trifluoroethoxylation of aromatic and aliphatic substrates.LG = leaving group. Yields are calculated from HPLC analyses of crude reaction mixtures and based are [18F]fluoroform conversion into the products. Radioactive products were collected at least once for each substrate to confirm that HPLC yields match isolated yields. All yields are decay-corrected and reported as mean ± SD for n = 3. A Substrate scope for homoarenes using reaction condition (B). B Substrate scope for heteroarenes using reaction condition (B). C Substrate scope for aliphatic compounds using reaction condition (B).Furthermore, we were keen to know whether potassium 18F-2,2,2-trifluoroethoxide could be useful for labeling at aliphatic carbon, given the limited availability of methods for constructing stable alkyl-CF218F bonds49. In this regard, we tested identical reaction conditions to those used for 11C-trifluoroethoxylation on diverse aliphatic substrates, prepared from drugs, herbicides, and other biomolecules. To our delight, this protocol successfully enabled the installation of a 18F-2,2,2-trifluoroethoxy moiety onto aliphatic carbon in a variety of complex structures in acceptable to excellent yields ([18F]44–[18F]46, [18F]49–[18F]56; 15–95%; Fig. 5C). Taken together, these results show that this methodology, based on the transformation of fluoroform into potassium 2,2,2-trifluoroethoxide and subsequent functionalization of aliphatic carbons, is equally versatile for both carbon-11 and fluorine-18 with the same non-radioactive precursor.Determination of molar activity of [18F]1 as a model compoundWe measured the molar activity of [18F]1 to be 1.3 GBq/µmol, decay-corrected. The method of [18F]fluoroform synthesis that we used was one reported in the literature48 and known to give low molar activity. We did not observe any significant release of fluoride ion in the production of [18F]potassium 2,2,2-trifluoroethoxide under basic conditions, as evidenced by absence of [18F]fluoride at the solvent front in the HPLC analysis of derived [18F]2,2,2-trifluoroethanol (Supplementary Fig. 17). Therefore, the molar activity of the starting [18F]fluoroform determines the molar activity of 18F-labeled 2,2,2-trifluoroethoxy products.Synthesis of isotopologues of potassium 2,2,2-trifluoroethoxideIsotopologues differ only in their isotopic substitutions and play an important role in drug development50. Deuteration51 is widely practiced to improve the metabolic stability of PET tracers in 18F-fluoroalkyl positions.13C-Labeling enables investigations of drug pharmacokinetics and metabolism by 13C-NMR spectroscopy and mass spectrometry52,53. Simple methods for accessing stable isotopically labeled compounds are highly desirable. The availability of isotopically labeled fluoroforms (H13CF3, H11CF3, and HCF218F) and paraformaldehyde [(CD2O)n and (13CH2O)n] and our method for the in situ generation of CF3CH2OK from fluoroform, provide an opportunity to explore the incorporation of isotopically labeled 2,2,2-trifluoroethoxy groups into a diverse array of substrates. For demonstration, we synthesized isotopologues of 57, an analog of a well-known COX-1 PET tracer, [11C]PS1332. Compounds [11C]57 and [18F]57 were readily obtained by treating a tosylate precursor with [11C/18F]CF3CH2OK under optimized conditions. The reaction was equally effective when substituting paraformaldehyde with (CD2O)n and (13CH2O)n, leading to high yield syntheses of [2H]57, [2H/11C]57, [2H/18F]57, [13C]57, [13C/11C]57, and [13C/18F]57 (Fig. 6). Hence, this isotope labeling protocol has exceptional potential for broad application.Fig. 6: The syntheses of nine isotopologues of a model compound (an analog of the COX-1 PET tracer [11C]PS13).Yields were based on HPLC analyses of crude reaction mixtures. Radioactive products were collected at least once with identity confirmed with LC-MS. HPLC radiochemical yields (RCYs) matched isolated RCYs and are reported as mean ± SD for n = 3.In summary, based on the transformation of paraformaldehyde with fluoroform, we devised a highly effective one-pot method for appending a wide range of aryl, heteroaryl, and aliphatic organic compounds with an isotopically labeled 2,2,2-trifluoroethoxy group. Especially, reaction of paraformaldehyde with [11C]fluoroform or [18F]fluoroform efficiently provides 11CF3CH2OK and 18FF2CCH2OK, respectively, as broadly useful no-carrier-added labeling synthons with ability to produce candidate PET tracers bearing either a 11C- or 18F-labeled 2,2,2-trifluoroethoxy group. Use of paraformaldehyde and fluoroform labeled with stable isotopes (2H, or 13C) gives ready access to isotopologues of 2,2,2-trifluoroethoxy compounds. Consequently, the 2,2,2-trifluoroethoxy group may garner increasing interest for both drug and PET tracer development.
Isotopologues of potassium 2,2,2-trifluoroethoxide for applications in positron emission tomography and beyond
