SYNTHESIS OF META-SUBSTITUTED [18F]-3-FLUORO-4-AMINOPYRIDINES BY DIRECT RADIOFLUORINATION OF PYRIDINE N-OXIDES

Abstract

Disclosed herein are methods for the fluorination aromatic N-heterocyclic N-oxides that comprise at least one leaving group. The N-oxides may be reduced to the fluorinated aromatic N-heterocyclic amine analogs. This novel fluorination approach may be successfully applied for synthesizing aromatic N-heterocyclic compounds labeled with .sup.18F.

Claims

1. A method for synthesizing a fluorinated aromatic N-heterocyclic N-oxide compound, said method comprising reacting an aromatic N-heterocyclic N-oxide compound with a fluoride source; wherein the aromatic N-heterocyclic N-oxide compound comprises a leaving group.

2. The method of claim 1, wherein the leaving group is a halide, nitro, protonated ammonium, protonated alkylammonium, trialkylammonium, mesylate, tosylate, phenoxide, carboxylate, thiolate, or diazonium.

3. The method of claim 1, wherein the fluoride source is a nucleophilic fluoride source.

4. The method of claim 2, wherein the nucleophilic fluoride source is selected from the group consisting of tetrabutylammonium fluoride (TBAF), tetramethylammonium fluoride (TMAF), (diethylamino)sulfur trifluoride (DAST), bis(2-methoxyethyl)aminosulfur trifluoride (deoxofluor), 4-morpholinosulfur trifluoride (MOST), CsF, NaF, and KF.

5. The method of claim 1, wherein the aromatic N-heterocyclic N-oxide compound comprises a leaving group that is ortho-, meta-, or para- to the N-oxide.

6. The method of claim 1, wherein the reducing composition comprises hydrogen and a transition metal absorbed onto a solid support, a boron reagent, an aluminum reagent, a tin reagent, or a silane.

7. The method of claim 6, wherein the transition metal is palladium, platinum, nickel, or rhodium.

8. The method of claim 6, wherein the solid support is carbon, alumina, calcium carbonate, or silica.

9. The method of claim 6, wherein the boron reagent is sodium borohydride, sodium cyanoborohydride, zinc borohydrode, a tetraorganylammonium tetrahydroborate, lithium triethylborohydride, lithium tri-sec-butyl borohydride, L-selectride, sodium triacetoxyborohydride, or sulfurated sodium borohydride.

10. The method of claim 6, wherein the aluminum reagent is aluminum hydride, lithium aluminum hydride, lithium tri-tert-butoxy aluminum hydride, DIBAL-H, or sodium bis(2-methoxyethoxy)aluminumhydride.

11. The method of claim 6, wherein the tin reagent is tributyltin hydride.

12. The method of claim 6, wherein the silane reagent is an alkylsilane, an alkylsiloxane, a phenylsilane, or a halosilane.

13. The method of claim 1, wherein the aromatic N-heterocyclic N-oxide compound comprising a leaving group is substituted or unsubstituted.

14. A method for using the fluorinated aromatic N-heterocyclic N-oxide compound of claim 1.

15. Using a fluorinated aromatic N-heterocyclic N-oxide compound manufactured by a method comprising reacting an aromatic N-heterocyclic N-oxide compound with a fluoride source; wherein the aromatic N-heterocyclic N-oxide compound comprises a leaving group.

16. A method for synthesizing a fluorinated aromatic N-heterocyclic compound, said method comprising reacting an aromatic N-oxide compound with a fluoride source, wherein the aromatic N-oxide compound comprises a leaving group; and reducing the N-oxide group with a reducing composition to give a fluorinated aromatic N-heterocyclic compound.

17. The method of claim 16, wherein the leaving group is a halide, nitro, protonated ammonium, protonated alkylammonium, trialkylammonium, mesylate, tosylate, phenoxide, carboxylate, thiolate, or diazonium.

18. The method of claim 16, wherein the fluoride source is a nucleophilic fluoride source.

19. The method of claim 18, wherein the nucleophilic fluoride source is selected from the group consisting of tetrabutylammonium fluoride (TBAF), tetramethylammonium fluoride (TMAF), (diethylamino)sulfur trifluoride (DAST), bis(2-methoxyethyl)aminosulfur trifluoride (deoxofluor), 4-morpholinosulfur trifluoride (MOST), CsF, NaF, KF, and KF-kryptofix.

20. The method of claim 16, wherein the aromatic N-oxide compound comprises a leaving group that is ortho-, meta-, or para- to the N-oxide.

21.-48. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The following drawings illustrate by way of example and not limitation. For the sake of brevity and clarity, every feature of a given structure may not be labeled in every figure in which that structure appears. Identical reference numbers do not necessarily indicate an identical structure. Rather, the same reference number may be used to indicate a similar feature or a feature with similar functionality, as may non-identical reference numbers.

[0024] FIGS. 1A-1C. Fluorination strategies for 3-fluoro-4-aminopyridine. FIG. 1A Nucleophilic aromatic substitution of 3-bromo-4-(boc-amino)pyridine with tetrabutyl ammonium fluoride in DMSO at 125° C. followed by acid deprotection. Fluorination did not proceed under the experimental conditions. FIG. 1B Nucleophilic aromatic substitution of 3-bromo-4-nitropyridine and 3-iodo-4-nitropyridine with tetrabutyl ammonium fluoride in DMSO at 125° C. followed by reduction. The products formed were 3-bromo-4-fluoropyridine or 3-iodo-4-fluoropyridine instead of the desired products. FIG. 1C Nucleophilic aromatic substitution of 3-bromo-4-nitropyridine N-oxide with tetrabutyl ammonium fluoride in DMSO at 125° C. followed by reduction. Fluorination produced 3-fluoro-4-nitropyridine N-oxide (9) in 37% yield. Hydrogenation of 9 produced 3-fluoro-4-aminopyridine (10) in quantitative yield.

[0025] FIGS. 2A-2B. FIG. 2A Fluorination of 3-bromopyridine with tetrabutyl ammonium fluoride in DMSO at 120° C. for 12 hours did not produce the fluoride-substituted product. Fluorination of 3-bromopyridine N-oxide with tetrabutyl ammonium fluoride in DMSO at 120° C. produced the desired product after only 30 minutes of reaction time. FIG. 2B .sup.18F fluorination of 3-bromo-4-nitropyridine N-oxide with tetrabutyl ammonium fluoride in DMSO at 25° C. for 15 minutes to produce the .sup.18F fluorinated product 9. Reduction of the nitro group with H.sub.2 Pd/C produced the desired .sup.18F 3-fluoro-4-aminopyridine 10.

[0026] FIGS. 3A-3C Radioactive and UV HPLC traces. FIG. 3A Co-injection of crude mixture from depicted reaction and reference standard. Unreacted [.sup.18F]-TBAF, reaction product (9) and precursor (8) elute at 2.3, 9.65 and 18.25 min, respectively. FIG. 3B Injection of reaction crude of .sup.19F/.sup.18F reaction, unreacted [.sup.18F]-TBAF and product elute at 2.3 and 5.3 min, respectively. FIG. 3C Co-injection of crude from depicted reaction and reference standard. Reaction product (10) elutes at 7.05 min, starting material (9) (minor peak) elutes at 5.80 min.

[0027] FIGS. 4A-4D Fluorination of 3-bromo-4-nitropyridine. FIG. 4A UV HPLC traces at 254 nm and 313 nm. 3-bromo-4-nitropyridine (3) elutes at 10.85 min, and absorbs at 254 and 313 nm. 3-bromo-4-fluoropyridine (6) elutes at 11.88 min, and absorbs at 254 nm only. FIG. 4B .sup.1H NMR. FIG. 4C .sup.13C NMR. FIG. 4D .sup.19F NMR.

[0028] FIGS. 5A-5C Fluorination of 3-iodo-4-nitropyridine. FIG. 5A UV HPLC traces at 254 nm and 313 nm. 3-iodo-4-nitropyridine (4) elutes at 10.98 min, and absorbs at 254 and 313 nm. 3-iodo-4-fluoropyridine (7) elutes at 13.38 min, and absorbs at 254 nm only. FIG. 5B .sup.1H NMR. FIG. 5C 19F NMR.

[0029] FIGS. 6A-6D Fluorination of 3-bromo-4-nitropyridine N-oxide. FIG. 6A UV HPLC traces at 254 nm and 313 nm. 3-bromo-4-nitropyridine N-oxide (8) elutes at 11.83 min, and absorbs at 254 and 313 nm. 3-fluoro-4-nitropyridine N-oxide (9) elutes at 7.95 min, and absorbs only at 254 and 313 nm. FIG. 6B .sup.1H NMR. FIG. 6C .sup.13C NMR. FIG. 6D .sup.19F NMR.

[0030] FIGS. 7A-7C Fluorination attempt of 3-bromopyridine. FIG. 7A UV HPLC traces of desired product (top), starting bromide (middle), and reaction mixture (bottom). FIG. 7B .sup.1H NMR. FIG. 7C .sup.19F NMR.

[0031] FIGS. 8A-8C Fluorination of 3-bromopyridine N-oxide. FIG. 8A UV HPLC traces of product (top), starting material (middle), and reaction mixture (bottom). 8B .sup.1H NMR. FIG. 8C .sup.19F NMR.

[0032] FIGS. 9A-9B Before and after UV HPLC traces of hydrogenation of 3-fluoro-4-nitropyridine N-oxide at 254 nm (top) and 313 nm (bottom). Starting material elutes at 9.6 min, product elutes at 10.1 min. FIG. 9A HPLC traces before hydrogenation. FIG. 9B HPLC trace after hydrogenation.

[0033] FIG. 10 Radioactive and UV HPLC trace of radiofluorination of 3-bromo-4-nitropyridine N-oxide (not spiked with reference standard). Starting material elutes at 17.2 min. and product elutes at 11.6 min.

[0034] FIG. 11 Non-limiting examples of fluorinated aromatic N-heterocyclic compounds that may be synthesized by the inventive methods.

DETAILED DESCRIPTION

[0035] Various features and advantageous details are explained more fully with reference to the non-limiting embodiments that are illustrated in the accompanying drawings and detailed in the following description. It should be understood, however, that the detailed description and the specific examples, while indicating embodiments of the invention, are given by way of illustration only, and not by way of limitation. Various substitutions, modifications, additions, and/or rearrangements will become apparent to those of ordinary skill in the art from this disclosure.

[0036] In the following description, numerous specific details are provided to provide a thorough understanding of the disclosed embodiments. One of ordinary skill in the relevant art will recognize, however, that the invention may be practiced without one or more of the specific details, or with other methods, components, materials, and so forth. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention.

[0037] The term “alkyl” includes straight-chain alkyl, branched-chain alkyl, cycloalkyl (alicyclic), cyclic alkyl, heteroatom-unsubstituted alkyl, heteroatom-substituted alkyl, heteroatom-unsubstituted C.sub.n-alkyl, and heteroatom-substituted C-alkyl. In certain embodiments, lower alkyls are contemplated. The term “lower alkyl” refers to alkyls of 1-6 carbon atoms (that is, 1, 2, 3, 4, 5 or 6 carbon atoms). The term “heteroatom-unsubstituted C.sub.n-alkyl” refers to a radical, having a linear or branched, cyclic or acyclic structure, further having no carbon-carbon double or triple bonds, further having a total of n carbon atoms, all of which are nonaromatic, 3 or more hydrogen atoms, and no heteroatoms. For example, a heteroatom-unsubstituted C.sub.1-C.sub.10-alkyl has 1 to 10 carbon atoms. The groups, —CH.sub.3 (Me), —CH.sub.2CH.sub.3 (Et), —CH.sub.2CH.sub.2CH.sub.3 (n-Pr), —CH(CH.sub.3).sub.2 (iso-Pr), —CH(CH.sub.2).sub.2 (cyclopropyl), —CH.sub.2CH.sub.2CH.sub.2CH.sub.3 (n-Bu), —CH(CH.sub.3)CH.sub.2CH.sub.3 (sec-butyl), —CH.sub.2CH(CH.sub.3).sub.2 (iso-butyl), —C(CH.sub.3).sub.3 (tert-butyl), —CH.sub.2C(CH.sub.3).sub.3 (neo-pentyl), cyclobutyl, cyclopentyl, and cyclohexyl, are all non-limiting examples of heteroatom-unsubstituted alkyl groups. The term “heteroatom-substituted C.sub.n-alkyl” refers to a radical, having a single saturated carbon atom as the point of attachment, no carbon-carbon double or triple bonds, further having a linear or branched, cyclic or acyclic structure, further having a total of n carbon atoms, all of which are nonaromatic, 0, 1, or more than one hydrogen atom, at least one heteroatom, wherein each heteroatom is independently selected from the group consisting of N, O, F, Cl, Br, I, Si, P, and S. For example, a heteroatom-substituted C.sub.1-C.sub.10-alkyl has 1 to 10 carbon atoms. The following groups are all non-limiting examples of heteroatom-substituted alkyl groups: trifluoromethyl, —CH.sub.2F, —CH.sub.2Cl, —CH.sub.2Br, —CH.sub.2OH, —CH.sub.2OCH.sub.3, —CH.sub.2OCH.sub.2CF.sub.3, —CH.sub.2OC(O)CH.sub.3, —CH.sub.2NH.sub.2, —CH.sub.2NHCH.sub.3, —CH.sub.2N(CH.sub.3).sub.2, —CH.sub.2CH.sub.2Cl, —CH.sub.2CH.sub.2OH, CH.sub.2CH.sub.2OC(O)CH.sub.3, —CH.sub.2CH.sub.2NHCO.sub.2C(CH.sub.3).sub.3, and —CH.sub.2Si(CH.sub.3).sub.3.

[0038] The term “aryl” or “aromatic” includes heteroatom-unsubstituted aryl, heteroatom-substituted aryl, heteroatom-unsubstituted C.sub.n-aryl, heteroatom-substituted C.sub.n-aryl, heteroaryl, heterocyclic aryl groups, carbocyclic aryl groups, biaryl groups, and single-valent radicals derived from polycyclic fused hydrocarbons (PAHs). The term “heteroatom-unsubstituted C.sub.n-aryl” refers to a radical, having a single carbon atom as a point of attachment, wherein the carbon atom is part of an aromatic ring structure containing only carbon atoms, further having a total of n carbon atoms, 5 or more hydrogen atoms, and no heteroatoms. For example, a heteroatom-unsubstituted C.sub.6-C.sub.10-aryl has 6 to 10 carbon atoms. Non-limiting examples of heteroatom-unsubstituted aryl groups include phenyl (Ph), methylphenyl, (dimethyl)phenyl, —C.sub.6H.sub.4CH.sub.2CH.sub.3, —C.sub.6H.sub.4CH.sub.2CH.sub.2CH.sub.3, —C.sub.6H.sub.4CH(CH.sub.3).sub.2, —C.sub.6H.sub.4CH(CH.sub.2).sub.2, —C.sub.6H.sub.3(CH.sub.3)CH.sub.2CH.sub.3, —C.sub.6H.sub.4CH═CH.sub.2, —C.sub.6H.sub.4CH═CHCH.sub.3, —C.sub.6H.sub.4C≡CH, —C.sub.6H.sub.4C≡CCH.sub.3, naphthyl, and the radical derived from biphenyl. The term “heteroatom-substituted C.sub.n-aryl” refers to a radical, having either a single aromatic carbon atom or a single aromatic heteroatom as the point of attachment, further having a total of n carbon atoms, at least one hydrogen atom, and at least one heteroatom, further wherein each heteroatom is independently selected from the group consisting of N, O, F, Cl, Br, I, Si, P, and S. For example, a heteroatom-unsubstituted C.sub.1-C.sub.10-heteroaryl has 1 to 10 carbon atoms. Non-limiting examples of heteroatom-substituted aryl groups include the groups: —C.sub.6H.sub.4F, —C.sub.6H.sub.4Cl, —C.sub.6H.sub.4Br, —C.sub.6H.sub.4I, —C.sub.6H.sub.4OH, —C.sub.6H.sub.4OCH.sub.3, —C.sub.6H.sub.4OCH.sub.2CH.sub.3, —C.sub.6H.sub.4OC(O)CH.sub.3, —C.sub.6H.sub.4NH.sub.2, —C.sub.6H.sub.4NHCH.sub.3, —C.sub.6H.sub.4N(CH.sub.3).sub.2, —C.sub.6H.sub.4CH.sub.2OH, —C.sub.6H.sub.4CH.sub.2OC(O)CH.sub.3, —C.sub.6H.sub.4CH.sub.2NH.sub.2, —C.sub.6H.sub.4CF.sub.3, —C.sub.6H.sub.4CN, —C.sub.6H.sub.4CHO, —C.sub.6H.sub.4CHO, —C.sub.6H.sub.4C(O)CH.sub.3, —C.sub.6H.sub.4C(O)C.sub.6H.sub.5, —C.sub.6H.sub.4CO.sub.2H, —C.sub.6H.sub.4CO.sub.2CH.sub.3, —C.sub.6H.sub.4CONH.sub.2, —C.sub.6H.sub.4CONHCH.sub.3, —C.sub.6H.sub.4CON(CH.sub.3).sub.2, furanyl, thienyl, pyridyl, pyrrolyl, pyrimidyl, pyrazinyl, quinolyl, indolyl, and imidazoyl. In certain embodiments, heteroatom-substituted aryl groups are contemplated. In certain embodiments, heteroatom-unsubstituted aryl groups are contemplate. In certain embodiments, an aryl group may be mono-, di-, tri-, tetra- or penta-substituted with one or more heteroatom-containing substitutents.

[0039] The term “alkylamino” includes straight-chain alkylamino, branched-chain alkylamino, cycloalkylamino, cyclic alkylamino, heteroatom-unsubstituted alkylamino, heteroatom-substituted alkylamino, heteroatom-unsubstituted C.sub.n-alkylamino, and heteroatom-substituted C.sub.n-alkylamino. The term “heteroatom-unsubstituted C.sub.n-alkylamino” refers to a radical, having a single nitrogen atom as the point of attachment, further having one or two saturated carbon atoms attached to the nitrogen atom, further having a linear or branched, cyclic or acyclic structure, containing a total of n carbon atoms, all of which are nonaromatic, 4 or more hydrogen atoms, a total of 1 nitrogen atom, and no additional heteroatoms. For example, a heteroatom-unsubstituted C.sub.1-C.sub.10-alkylamino has 1 to 10 carbon atoms. The term “heteroatom-unsubstituted C.sub.n-alkylamino” includes groups, having the structure —NHR, in which R is a heteroatom-unsubstituted C.sub.n-alkyl, as that term is defined above. A heteroatom-unsubstituted alkylamino group would include —NHCH.sub.3, —NHCH.sub.2CH.sub.3, —NHCH.sub.2CH.sub.2CH.sub.3, —NHCH(CH.sub.3).sub.2, —NHCH(CH.sub.2).sub.2, —NHCH.sub.2CH.sub.2CH.sub.2CH.sub.3, —NHCH(CH.sub.3)CH.sub.2CH.sub.3, —NHCH.sub.2CH(CH.sub.3).sub.2, —NHC(CH.sub.3).sub.3, —N(CH.sub.3).sub.2, —N(CH.sub.3)CH.sub.2CH.sub.3, —N(CH.sub.2CH.sub.3).sub.2, N-pyrrolidinyl, and N-piperidinyl.

[0040] The term “heteroatom-substituted C.sub.n-alkylamino” refers to a radical, having a single nitrogen atom as the point of attachment, further having one or two saturated carbon atoms attached to the nitrogen atom, no carbon-carbon double or triple bonds, further having a linear or branched, cyclic or acyclic structure, further having a total of n carbon atoms, all of which are nonaromatic, 0, 1, or more than one hydrogen atom, and at least one additional heteroatom, that is, in addition to the nitrogen atom at the point of attachment, wherein each additional heteroatom is independently selected from the group consisting of N, O, F, Cl, Br, I, Si, P, and S. For example, a heteroatom-substituted C.sub.1-C.sub.10-alkylamino has 1 to 10 carbon atoms. The term “heteroatom-substituted C.sub.n-alkylamino” includes groups, having the structure —NHR, in which R is a heteroatom-substituted C.sub.n-alkyl, as that term is defined above.

[0041] The term “alkylsilyl” includes straight-chain alkylsilyl, branched-chain alkylsilyl, cycloalkylsilyl, cyclic alkylsilyl, heteroatom-unsubstituted alkylsilyl, heteroatom-substituted alkylsilyl, heteroatom-unsubstituted C.sub.n-alkylsilyl, and heteroatom-substituted C.sub.n-alkylsilyl. The term “heteroatom-unsubstituted C.sub.n-alkylsilyl” refers to a radical, having a single silicon atom as the point of attachment, further having one, two, or three saturated carbon atoms attached to the silicon atom, further having a linear or branched, cyclic or acyclic structure, containing a total of n carbon atoms, all of which are nonaromatic, 5 or more hydrogen atoms, a total of 1 silicon atom, and no additional heteroatoms. For example, a heteroatom-unsubstituted C.sub.1-C.sub.10-alkylsilyl has 1 to 10 carbon atoms. An alkylsilyl group includes dialkylamino groups. The groups, —Si(CH.sub.3).sub.3 and —Si(CH.sub.3).sub.2C(CH.sub.3).sub.3, are non-limiting examples of heteroatom-unsubstituted alkylsilyl groups. The term “heteroatom-substituted C-alkylsilyl” refers to a radical, having a single silicon atom as the point of attachment, further having at least one, two, or three saturated carbon atoms attached to the silicon atom, no carbon-carbon double or triple bonds, further having a linear or branched, cyclic or acyclic structure, further having a total of n carbon atoms, all of which are nonaromatic, 0, 1, or more than one hydrogen atom, and at least one additional heteroatom, that is, in addition to the silicon atom at the point of attachment, wherein each additional heteroatom is independently selected from the group consisting of N, O, F, Cl, Br, I, Si, P, and S. For example, a heteroatom-substituted C.sub.1-C.sub.10-alkylsilyl has 1 to 10 carbon atoms.

[0042] The present disclosure provides a method for synthesizing fluorinated aromatic N-heterocyclic compounds. In particular embodiments, the present disclosure provides a novel method for the synthesis of meta-substituted .sup.18F 3-fluoro-4-aminopyridines by direct radiofluorination of pyridine N-oxides. 3-fluoro-4-aminopyridine is a fluorine-containing derivative of 4-aminopyridine, a clinically-approved drug for multiple sclerosis that binds to potassium channels in the central nervous system, currently under investigation for imaging demyelination.sup.10-12. The use of pyridine N-oxides for the preparation of fluoropyridines is unprecedented in the chemical literature and provides a novel approach for the synthesis of these important structures in pharmaceuticals and radiopharmaceuticals.

[0043] Derivatives of compounds of the present invention are also contemplated. In certain aspects, “derivative” refers to a chemically modified compound that still retains the desired effects of the compound prior to the chemical modification. Such derivatives may have the addition, removal, or substitution of one or more chemical moieties on the parent molecule. Non-limiting examples of the types modifications that can be made to the compounds and structures disclosed herein include the addition or removal of lower alkanes such as methyl, ethyl, propyl, or substituted lower alkanes such as hydroxymethyl or aminomethyl groups; carboxyl groups and carbonyl groups; hydroxyls; nitro, amino, amide, and azo groups; sulfate, sulfonate, sulfono, sulfhydryl, sulfonyl, sulfoxido, phosphate, phosphono, phosphoryl groups, and halide substituents. Additional modifications can include an addition or a deletion of one or more atoms of the atomic framework, for example, substitution of an ethyl by a propyl; substitution of a phenyl by a larger or smaller aromatic group. Alternatively, in a cyclic or bicyclic structure, heteroatoms such as N, S, or O can be substituted into the structure instead of a carbon atom.

[0044] Compounds employed in methods of the invention may contain one or more asymmetrically-substituted carbon or nitrogen atoms, and may be isolated in optically active or racemic form. Thus, all chiral, diastereomeric, racemic form, epimeric form, and all geometric isomeric forms of a structure are intended, unless the specific stereochemistry or isomeric form is specifically indicated. Compounds may occur as racemates and racemic mixtures, single enantiomers, diastereomeric mixtures and individual diastereomers. In some embodiments, a single diastereomer is obtained. The chiral centers of the compounds of the present invention can have the S- or the R-configuration, as defined by the IUPAC 1974 Recommendations. Compounds may be of the D- or L-form, for example. It is well known in the art how to prepare and isolate such optically active forms. For example, mixtures of stereoisomers may be separated by standard techniques including, but not limited to, resolution of racemic form, normal, reverse-phase, and chiral chromatography, preferential salt formation, recrystallization, and the like, or by chiral synthesis either from chiral starting materials or by deliberate synthesis of target chiral centers.

[0045] For the synthesis of [.sup.18F]-3-fluoro-4-aminopyridine, three possible routes from commercially available reagents were considered (FIG. 1). The first route A.1 entailed fluorination of Boc-protected 3-bromo-4aminopyridine (1) followed by acid deprotection. Treatment of 1 with tetrabutylammonium fluoride (TBAF) did not produce the desired product even after several hours at high temperature.

[0046] The second route A.2 involved fluorination of 3-bromo-4-nitropyridine (3) followed by reduction of the nitro group. Treatment of 3 with 0.5 equivalents of TBAF at room temperature produced the para-substituted produce 3-bromo-4-fluoropyridine (6) in 71.1+3.6% yield (relative to TBAF, n=4) as determined by HPLC and NMR (FIGS. 3B and 4A). Under these conditions, less than 0.2% of 3-fluoro-4-nitropyridine (5) was produced demonstrating a clear preference for the substitution to occur at the para- or nitro-position. Similar results were obtained with 3-iodo-4-nitropyridine (4) (FIGS. 5A-5B). This type of reactivity is expected from previously published data on nitro-substituted pyridines.sup.2, 13 and consistent with the results from Abrahim et al..sup.5 who tested fluorination of a pyridine containing an ortho-nitro group and a meta-bromo group, and found exclusive substitution at the ortho-nitro position.

[0047] Fluorination of 3-bromo-4-nitropyridine N-oxide (8) was attempted, given the supposition that the electron withdrawing N-oxide would further increase the reactivity of the pyridine and result in fluorination at the meta-position (FIG. 1, route A3). In this case, treatment of 8 with 0.5 equivalents TBAF at room temperature produced the meta-fluorinated compound, 3-fluoro-4-nitropyridine N-oxide (9), as the main product in 20.7±2.7% yield (relative to TBAF, n=4) (FIGS. 6A-6B). Under these conditions, less than 2% fluorination at the para position was detected, indicating that the presence of the N-oxide group favors meta fluorination.

[0048] To examine the potential use of pyridine N-oxides as fluorination precursors, the reactivity of 3-bromopyridine and 3-bromopyridine N-oxide with TBAF was compared (FIGS. 2A-2B). 3-bromopyridine did not undergo reaction with 1.2 eq. of TBAF, despite reaction conditions of 120° C. for 12 hours (FIGS. 7A-7B). By contrast, over 25% conversion of 3-bromopyridine N-oxide was detected after 30 minutes at 120° C. (FIGS. 8A-8B). This result indicates that the para-nitro group's enhancement of pyridine reactivity (likely by lowering its activation energy), it is not sufficient to explain the reactivity and regioselectivity of the N-oxide towards fluorination. This regioselectivity is particularly surprising given early studies on the nucleophilic displacement of monosubstituted pyridine N-oxides by sodium ethoxide in ethanol, which round that the rate of displacement of nitro was 1,100-3,100 times larger than the bromo, and that the order of reactivity was 2>4>>3 position.sup.14. This difference may be due to the different nature of the nucleophile and solvation effects.

[0049] After obtaining compound 9, it was then reduced using standard hydrogenation conditions to generate the desired final product, 3-fluoro-4-aminopyridine (10) quantitatively (FIG. 9). Pyridine N-oxides have not previously been used as precursors for radiofluorination. This approach was further examined for the production of [.sup.18F]-3-fluoro-4-aminopyridine.

[0050] Radiochemical synthesis of [.sup.18F]-3-fluoro-4-aminopyridine was conducted as shown in FIG. 3C. The procedure was similar to the non-radioactive synthesis described above except that [.sup.18F]-TBAF was prepared immediately before the reaction by trapping [.sup.18F]-fluoride in a strong anion exchange cartridge and eluting it with TBA-HCO.sub.3. The reaction was carried out in DMSO at room temperature. After 15 min, HPLC analysis of the reaction crude showed an early peak that elutes with the solvent front corresponding to unreacted [.sup.18F]-TBAF and a main peak corresponding to the desired product (FIG. 10). Under the test conditions, the isolated decay-corrected yield for the desired product was 10.4±1.8% (n=8). Upon further characterization, co-injection of the reaction crude with a small amount of reference standard consistently gave higher yields (25±4%, decay corrected, n=8) (FIG. 3A). This led to the hypothesis that the non-radioactive 3-fluoro-4-nitropyridine N-oxide (9) could be contributing to the radiolabeling yield by .sup.19F/.sup.18F exchange.

[0051] Labeling of 3-fluoro-4-nitropyridine N-oxide was examined with [.sup.18F]-TBAF in the absence of the bromo precursor we obtained 33.1+5.4% (n=4) decay-corrected isolated yield (FIG. 2A). This reaction reaches equilibrium within seconds. Fluorine exchange in organic compounds has been reported before.sup.15, however, this is the first example of fluoride exchange of a C—F bond in a heterocyclic compound.

EXAMPLES

Example 1

[0052] Non-radioactive fluorination of 3-bromo-4-nitropyridine (3): 10 μL of 1 M tetrabutylammonium fluoride (TBAF) solution in THF (10 μmol, 0.5 eq.) was added to a solution of 3-bromo-4-nitropyridine (96%, Aurum Pharmatech, LLC) (20 μmol, 1 eq.) in 500 L of anhydrous dimethylsulfoxide (DMSO) in a 2 mL HPLC vial. The reaction was analyzed by HPLC (conditions A). Retention times: 3-bromo-4-nitropyridine (3)=10.83 min, 3-fluoro-4-nitropyridine=8.38, 3-bromo-4-fluoropyridine (6)=11.76 min. Retention times for the product matched within 0.05 min the reference standard. Identity of the product was confirmed by HR-MS (m/z M.sup.+ exp.: 174.9423, calc: 174.9433) and .sup.1H, .sup.13C and .sup.19F NMR. Product amount was calculated from the area under the curve of the HPLC UV1 trace using a calibration curve.

Example 2

[0053] Non-radioactive fluorination of 3-iodo-4-nitropyridine (4): 10 μL of 1 M TBAF solution in THF (10 μmol, 0.5 eq.) was added to a solution of 3-iodo-4-nitropyridine (96%, Aurum Pharmatech, LLC) (20 μmol, 1 eq.) in 500 μL of anhydrous dimethylsulfoxide (DMSO) in a 2 mL HPLC vial. The reaction was analyzed by HPLC (conditions A). Retention times: 3-iodo-4-aminopyridine (4)=11.02 min, 3-iodo-4-fluoropyridine (7)=13.43 min. Starting material absorbs at 254 and 313 nm. Product does not absorb at 313 nm. Identity of the product was confirmed by HR-MS (m/z M.sup.+ exp.: 222.9288, calc: 222.9294) and .sup.1H, .sup.13C and .sup.19F NMR.

Example 3

[0054] Non-radioactive fluorination of 3-bromo-4-nitropyridine N-oxide (8): 10 μL of 1 M TBAF solution in THF (10 μmol, 0.5 eq.) was added to a solution of a 3-bromo-4-nitropyridine N-oxide (98+%, Alfa Aesar) (20 μmol, 1 eq.) in 500 μL of anhydrous DMSO in a 2 mL HPLC vial. The reaction was analyzed by HPLC (conditions B). Retention times: 3-bromo-4-nitropyridine N-oxide (8)=11.84 min, 3-fluoro-4-nitropyridine N-oxide (9)=7.93 min. Retention time for the product matched within 0.05 min the reference standard. Identity of the product was confirmed by HR-MS (m/z M.sup.+ exp.: 158.0141, calc.: 158.0128) and .sup.1H, .sup.13C and .sup.19F NMR. Product amount was calculated from the area under the curve of the HPLC UV2 trace using a calibration curve.

Example 4

[0055] Catalytic hydrogenation of 3-fluoro-4-nitropyridine N-oxide (9): 0.1 mg of 3-fluoro-4-aminopyridine N-oxide (9) was dissolved in 4 mL of MeOH in a 50 mL round bottom flask containing a stir bar. While stirring, 3-4 mg of 10% Pd/C (dry basis) was added and the flask sealed with a rubber septum. The vial was evacuated and backfilled with hydrogen gas from a balloon and the mixture was allowed to react for 10 min. After 10 min, the suspension was passed through a 0.4 m PTFE filter and analyzed by HPLC (conditions C). Reference retention times: 3-fluoro-4-aminopyridine N-oxide (9)=5.80 min, 3-fluoro-4-aminopyridine (10)=7.05 min. Retention time for the product matched within 0.05 min the reference standard. Identity of the product was confirmed by HR-MS (m/z M.sup.+ exp.: 112.0416, calc: 112.0437). Product amount was calculated from the area under the curve of the HPLC UV2 trace using a calibration curve.

Example 5

[0056] Production of tetrabutyl ammonium [.sup.18F]-fluoride ([.sup.18F]-TBAF): Fifty to a hundred mCi (1.85-3.7 GBq) of cyclotron-produced .sup.18F was trapped in a Sep-Pak Accell Plus QMA Plus Light Cartridge (Waters Corporation) preconditioned with 5 mL of 50 mM of KHCO.sub.3 followed by 10 mL of water and 20 mL of air. The cartridge was eluted with a solution containing of 300 μL of 50 mM TBA-HCO3 in water with 5% EtOH (ABX advanced biochemical compounds GmbH) and 600 μL of acetonitrile (MeCN). The water-MeCN solution was dried azeotropically at 85° C. under reduced pressure (20 mbar) for 7.5 min. To ensure complete dryness, two additional aliquots of MeCN (500 μL) were added, followed by evaporation (3 min). After drying, the vial was filled with argon gas and cooled down to room temperature in a water beaker. The [.sup.18F]-TBAF residue was dissolved in 100-400 μL of anhydrous DMSO and used for the reactions.

Example 6

[0057] Radiochemical synthesis of [.sup.18F]-3-fluoro-4-nitropyridine N-oxide ([.sup.18F]-9) from 3-bromo-4-nitropyridine N-oxide (8): 100 μL of 3-bromo-4-nitropyridine N-oxide (8) dissolved in DMSO (20 mg/mL) were added to 100 μL of [.sup.18F]-TBAF solution (˜10 mCi, ˜370 MBq) in 3 mL microreactor vial and allowed to react for 15 min. 100 μL of this solution with or without reference standard (20 μg) were injected into a semiprep C-18 HPLC column equipped with a variable wavelength UV-Vis detector and a radiation detector (conditions C). The radioactive peaks were collected and the radioactivity of each fraction measured using a Capintec dose calibrator. The radiochemical yield was calculated as radioactivity in the peak corrected for decay over radioactivity injected.

Example 7

[0058] Radiochemical synthesis of [.sup.18F]-3-fluoro-4-nitropyridine N-oxide ([.sup.18F]-9) from 3-fluoro-4-nitropyridine N-oxide (9) by .sup.19F/.sup.18F exchange: 100 μL of 3-fluoro-4-nitropyridine N-oxide (9) dissolved in DMSO (1 mg/mL) was added to 100 μL of [.sup.18F]-TBAF solution (˜10 mCi, ˜370 MBq) in 3 mL microreactor vial and allowed to react for 1 min. 100 μL of this solution were injected into a semiprep C-18 HPLC column equipped with a variable wavelength UV-Vis detector and a radiation detector (conditions E). The radioactive peaks were collected and the radioactivity of each fraction measured using a Capintec dose calibrator. The radiochemical yield was calculated as radioactivity in the peak corrected for decay over radioactivity injected.

Example 8

[0059] Synthesis of [.sup.18F]-3-fluoro-4-aminopyridine ([.sup.18F]-10): 1-10 mCi (37-370 MBq) of [.sup.18F]-3-fluoro-4-aminopyridine N-oxide ([.sup.18F]-9) containing 20-100 μg of cold 3-fluoro-4-aminopyridine N-oxide was dissolved in 4 mL of MeOH and the reaction was carried out as described above. The presence of cold compound facilitated obtaining reproducible yields. The product was purified by semiprep HPLC (conditions D). The final specific activity was 10-100 mCi/μmol (0.37-3.7 GBq/μmol).

[0060] Procurement of [.sup.18F]-fluoride: Cyclotron-produced no-carrier-added aqueous .sup.18F fluoride was obtained from IBA Molecular North America, Inc.

[0061] HPLC conditions (A-E): A. Nucleodur 5 μm, 4.6×250 mm C18ec (Macherey-Nagel). Flow 1.4 mL/min. Solvent A: 50 mM NH.sub.4HCO.sub.3, pH 8.0. Solvent B: 100% MeOH. Method: 0-2 min: 5% B, 2-6 min: 5-50% B, 6-12 min: 50% B, 12-12.5 min: 50-5% B, 12.5-17 min: 5% B. B.

[0062] Nucleodur 5 μm, 4.6×250 mm C18ec (Macherey-Nagel). Flow 1.4 mL/min. Solvent A: 50 mM NH.sub.4HCO.sub.3, pH 8.0. Solvent B: 100% MeOH. Method: 0-2 min: 1% B, 2-9 min: 1-20% B, 9-11 min: 20% B, 11-11.5 min: 20-1% B, 11.5-14 min: 5% B. C. Eclipse XDB 5 m, 9.4×250 mm C18 column (Agilent). Flow 4: mL/min. Mobile phase: 50 mM NaH.sub.4PO.sub.4, 10 mM triethylamine, pH 8.0, 5% EtOH. Isocratic: 0-20 min. D. Eclipse XDB 5 μm, 9.4×250 mm C18 column (Agilent). Flow: 4 mL/min. Solvent A: 50 mM NH.sub.4HCO.sub.3, pH 8.0. Solvent B: 100% MeOH. Method: 0-14 min: 5% B, 14-17 min: 5-25% B, 17-19 min: 25% B, 19-20 min: 25-5% B, 20-25 min: 5% B. E. Eclipse XDB 5 μm, 9.4×250 mm C18 column (Agilent). Flow 4: mL/min.

[0063] Mobile phase: 50 mM NaH.sub.4PO.sub.4, 10 mM triethylamine, pH 8.0, 5% EtOH. Isocratic: 0-10 min. RadioTLC analysis: The radioactive sample was spotted on a 25×75 mm TLC plate (PE SIL G, Whatman) next to the non-radioactive standard (1 mg/mL). The TLC was run in 95:5 methanol:acetic acid.

[0064] Reference NMR data of the used compounds.

[0065] 3-bromo-4-nitropyridine (3): .sup.1H-NMR (d.sub.6-DMSO, 500 MHz) δ (ppm): 8.06 (1H, t, J=1.3 Hz), 8.85 (1H, d, J=1.3 Hz), 9.11 (1H, s). .sup.13C-NMR (d.sub.6-DMSO, 125 MHz) δ: (ppm) 102.0 (s), 122.6 (s), 137.1 (s), 139.3 (s), 142.7 (s).

[0066] 3-iodo-4-nitropyridine (4): .sup.1H-NMR (d.sub.6-DMSO, 500 MHz) δ (ppm): 7.99 (1H, d, J=5.2 Hz), 8.80 (1H, d, J=5.2 Hz), 9.23 (1H, s). .sup.13C-NMR (d.sub.6-DMSO, 125 MHz) δ (ppm): 83.5 (s), 118.2 (s), 150.0 (s), 158.4 (s), 159.8 (s).

[0067] 3-fluoro-4-nitropyridine (5): .sup.1H-NMR (d.sub.6-DMSO, 500 MHz) δ (ppm): 8.13 (1H, t, J=6.2 Hz), 8.75 (1H, d, J=5.25 Hz), 9.03 (1H, d, J=2.9 Hz). .sup.13C-NMR (d.sub.6-DMSO, 125 MHz) δ (ppm): 118.4 (s), 118.5 (s), 141.9 (d, J=23.1), 149.7 (d, J=269.5). .sup.19F-NMR (d.sub.6-DMSO, 470 MHz) δ (ppm): −135.13 (dd, J.sub.2=8.9 Hz, J.sub.1=2.5 Hz).

[0068] 3-bromo-4-fluoropyridine (6): .sup.1H-NMR (d.sub.6-DMSO, 500 MHz) δ (ppm): 7.50 (1H, dd, J.sub.2=9.0 Hz, J.sub.1=5.5 Hz), 8.56 (1H, dd, J.sub.2=7.5 Hz, J.sub.1=5.5 Hz), 8.81 (1H, J: 9.5 Hz). .sup.13C-NMR (d.sub.6-DMSO, 125 MHz) δ (ppm): 106.0 (s), 112.9 (d, J: 16.2 Hz), 151.1 (d, J: 6.2 Hz), 153.5 (s), 165.0 (d, J: 275.4 Hz). .sup.19F-NMR (d.sub.6-DMSO, 470 MHz) δ (ppm): −99.14 (dd, J.sub.2=16.9 Hz, J.sub.1=9.1 Hz).

[0069] 3-bromo-4-nitropyridine N-oxide (8): .sup.1H-NMR (d.sub.6-DMSO, 500 MHz) δ (ppm): 8.15 (1H, d, J=7.1 Hz), 8.38 (1H, dd, J.sub.2=7.1 Hz, J.sub.1=2.0 Hz), 8.85 (1H, d, J=1.9 Hz). .sup.13C-NMR (d.sub.6-DMSO, 125 MHz) δ (ppm): 110.3 (s), 118.5 (s), 150.8 (s), 150.9 (s), 154.3 (s).

[0070] 3-fluoro-4-nitropyridine N-oxide (9): .sup.1H-NMR (d.sub.6-DMSO, 500 MHz) δ (ppm): 8.24 (1H, dd, J.sub.2=31.6 Hz, J.sub.1=0.8 Hz), 8.25 (1H, dd, J.sub.2=35.1 Hz, J.sub.1=1.8 Hz), 8.91 (1H, dd, J.sub.2=7.1 Hz, J.sub.1=1.8 Hz). .sup.13C-NMR (d.sub.6-DMSO, 125 MHz) δ (ppm): 122.3 (s), 131.4 (s), 131.7 (s), 137.0 (d, J=4.3), 153.3 (d, J=264.8). .sup.19F-NMR (d.sub.6-DMSO, 470 MHz) δ (ppm): −126.7 (dd, J.sub.2=8.5 Hz, J.sub.1=0.8 Hz).

[0071] 3-bromopyridine (11): .sup.1H-NMR (d.sub.6-DMSO, 500 MHz) δ (ppm): 7.38 (1H, dd, J.sub.2=8.1 Hz, J.sub.1=3.5 Hz), 7.85 (1H, dt, J.sub.2=8.2 Hz, J.sub.1=1.0 Hz), 8.56 (1H, m), 8.69 (1H, d, J=2.3 Hz).

[0072] 3-fluoropyridine (12): .sup.1H-NMR (d.sub.6-DMSO, 500 MHz) δ (ppm): 7.47 (1H, m), 7.70 (1H, dt, J.sub.2=7.5 Hz, J.sub.1=1.5 Hz), 8.44 (1H, m), 8.56 (1H, d, J=3.0 Hz). .sup.19F-NMR (d.sub.6-DMSO, 470 MHz) δ (ppm): −127.9 (t, J=5.1 Hz).

[0073] 3-bromopyridine N-oxide (13): .sup.1H-NMR (d.sub.6-DMSO, 500 MHz) δ (ppm): 7.37 (1H, t, J=7.8 Hz), 7.58 (1H, d, J=7.6 Hz), 8.25 (1H, d, J=7.1 Hz), 8.56 (1H, s).

[0074] 3-fluoropyridine N-oxide (14): .sup.1H-NMR (d.sub.6-DMSO, 500 MHz) δ (ppm): 7.40 (1H, dt, J.sub.2=55.4 Hz, J.sub.1=7.9 Hz), 7.42 (1H, dd, J.sub.2=54.5 Hz, J.sub.1=7.2 Hz), 8.13 (1H, d, J=6.4 Hz), 8.51 (1H, s). .sup.19F-NMR (d.sub.6-DMSO, 470 MHz) δ (ppm): −122.1 (td, J.sub.2=5.3 Hz, J.sub.1=0.8 Hz).

[0075] The claims are not to be interpreted as including means-plus- or step-plus-function limitations, unless such a limitation is explicitly recited in a given claim using the phrase(s) “means for” or “step for,” respectively.

REFERENCES

[0076] 1. S. Preshlock, M. Tredwell and V. Gouverneur, Chemical Reviews, 2016, 116, 719-766. [0077] 2. M. Karramkam, F. Hinnen, F. Vaufrey and F. Dollé, Journal of Labelled Compounds and Radiopharmaceuticals, 2003, 46, 979-.992. [0078] 3. J. H. Chun, C. L. Morse, F. T. Chin and V. W. Pike, Chem Commun (Camb), 2013, 49, 2151-2153 [0079] 4. H. F. Beer, M. Haeberli, S. Ametamey and P. A. Schubiger, Journal of Labelled Compounds and Radiopharmaceuticals, 1995, 36, 933-945. [0080] 5. A. Abrahim, P. Angelberger, K. Kletter, M. Müller, C. Joukhadar, T. Erker and O. Langer, Journal of Labelled Compounds and Radiopharmaceuticals, 2006, 49, 345-356. [0081] 6. H. Liu, S. Liu, Z. Miao, Z. Deng, B. Shen, X. Hong and Z. Cheng, Journal of medicinal chemistry, 2013, 56, 895-901. [0082] 7. M. A. Carroll, J. Nairne and J. L. Woodcraft, Journal of Labelled Compounds and Radiopharmaceuticals, 2007, 50, 452-454. [0083] 8. J. H. Chun and V. W. Pike, Chem Commun (Camb), 2012, 48, 9921-9923. [0084] 9. B. H. Rotstein, N. A. Stephenson, N. Vasdev and S. H. Liang, Nature communications, 2014, 5, 4365. [0085] 10. P. Brugarolas, J. Sanchez-Rodriguez, A. V. Caprariello, J. Lacroix, C.-T. Chen, D. E. Appelbaum, R. H. Miller, F. Bezanilla and B. Popko, 2014. [0086] 11. P. Brugarolas, J. Sanchez-Rodriguez, J. Lacroix, F. Bezanilla, C.-T. Chen, D. Appelbaum and B. Popko, J NUCL MED MEETING ABSTRACTS, 2014, 55, 1124. [0087] 12. P. Brugarolas, J. Sanchez-Rodriguez, J. Lacroix, C.-T. Chen, F. Bezanilla and B. Popko, Journal of Nuclear Medicine, 2015, 56, 493. [0088] 13. M. Tredwell and V. Gouverneur, Angew Chem Int Ed Engl, 2012, 51, 11426-11437. [0089] 14. R. M. Johnson, Journal of the Chemical Society B: Physical Organic, 1966, DOI: 10.1039/J29660001058, 1058-1061. [0090] 15. E. Blom, F. Karimi and B. Långström, Journal of Labelled Compounds and Radiopharmaceuticals, 2009, 52, 504-511. [0091] 16. Z. Liu, M. Pourghiasian, F. Benard, J. Pan, K. S. Lin and D. M. Perrin, Journal of nuclear medicine: official publication, Society of Nuclear Medicine, 2014, 55, 1499-1505. [0092] 17. Z. Liu, G. Amouroux, Z. Zhang, J. Pan, N. Hundal-Jabal, N. Colpo, J. Lau, D. M. Perrin, F. Benard and K. S. Lin, Mol Pharm, 2015, 12, 974-982.