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 . 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 . the term “ alkyl ” includes straight - chain alkyl , branched - chain alkyl , cycloalkyl ( alicyclic ), cyclic alkyl , heteroatom - unsubstituted alkyl , heteroatom - substituted alkyl , heteroatom - unsubstituted c 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 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 1 - c 10 - alkyl has 1 to 10 carbon atoms . the groups , — ch 3 ( me ), — ch 2 ch 3 ( et ), — ch 2 ch 2 ch 3 ( n - pr ), — ch ( ch 3 ) 2 ( iso - pr ), — ch ( ch 2 ) 2 ( cyclopropyl ), — ch 2 ch 2 ch 2 ch 3 ( n - bu ), — ch ( ch 3 ) ch 2 ch 3 ( sec - butyl ), — ch 2 ch ( ch 3 ) 2 ( iso - butyl ), — c ( ch 3 ) 3 ( tert - butyl ), — ch 2 c ( ch 3 ) 3 ( neo - pentyl ), cyclobutyl , cyclopentyl , and cyclohexyl , are all non - limiting examples of heteroatom - unsubstituted alkyl groups . the term “ heteroatom - substituted c 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 1 - c 10 - alkyl has 1 to 10 carbon atoms . the following groups are all non - limiting examples of heteroatom - substituted alkyl groups : trifluoromethyl , — ch 2 f , — ch 2 cl , — ch 2 br , — ch 2 oh , — ch 2 och 3 , — ch 2 och 2 cf 3 , — ch 2 oc ( o ) ch 3 , — ch 2 nh 2 , — ch 2 nhch 3 , — ch 2 n ( ch 3 ) 2 , — ch 2 ch 2 cl , — ch 2 ch 2 oh , ch 2 ch 2 oc ( o ) ch 3 , — ch 2 ch 2 nhco 2 c ( ch 3 ) 3 , and — ch 2 si ( ch 3 ) 3 . the term “ aryl ” or “ aromatic ” includes heteroatom - unsubstituted aryl , heteroatom - substituted aryl , heteroatom - unsubstituted c n - aryl , heteroatom - substituted c 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 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 6 - c 10 - aryl has 6 to 10 carbon atoms . non - limiting examples of heteroatom - unsubstituted aryl groups include phenyl ( ph ), methylphenyl , ( dimethyl ) phenyl , — c 6 h 4 ch 2 ch 3 , — c 6 h 4 ch 2 ch 2 ch 3 , — c 6 h 4 ch ( ch 3 ) 2 , — c 6 h 4 ch ( ch 2 ) 2 , — c 6 h 3 ( ch 3 ) ch 2 ch 3 , — c 6 h 4 ch ═ ch 2 , — c 6 h 4 ch ═ chch 3 , — c 6 h 4 c ≡ ch , — c 6 h 4 c ≡ cch 3 , naphthyl , and the radical derived from biphenyl . the term “ heteroatom - substituted c 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 1 - c 10 - heteroaryl has 1 to 10 carbon atoms . non - limiting examples of heteroatom - substituted aryl groups include the groups : — c 6 h 4 f , — c 6 h 4 cl , — c 6 h 4 br , — c 6 h 4 i , — c 6 h 4 oh , — c 6 h 4 och 3 , — c 6 h 4 och 2 ch 3 , — c 6 h 4 oc ( o ) ch 3 , — c 6 h 4 nh 2 , — c 6 h 4 nhch 3 , — c 6 h 4 n ( ch 3 ) 2 , — c 6 h 4 ch 2 oh , — c 6 h 4 ch 2 oc ( o ) ch 3 , — c 6 h 4 ch 2 nh 2 , — c 6 h 4 cf 3 , — c 6 h 4 cn , — c 6 h 4 cho , — c 6 h 4 cho , — c 6 h 4 c ( o ) ch 3 , — c 6 h 4 c ( o ) c 6 h 5 , — c 6 h 4 co 2 h , — c 6 h 4 co 2 ch 3 , — c 6 h 4 conh 2 , — c 6 h 4 conhch 3 , — c 6 h 4 con ( ch 3 ) 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 . the term “ alkylamino ” includes straight - chain alkylamino , branched - chain alkylamino , cycloalkylamino , cyclic alkylamino , heteroatom - unsubstituted alkylamino , heteroatom - substituted alkylamino , heteroatom - unsubstituted c n - alkylamino , and heteroatom - substituted c n - alkylamino . the term “ heteroatom - unsubstituted c 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 1 - c 10 - alkylamino has 1 to 10 carbon atoms . the term “ heteroatom - unsubstituted c n - alkylamino ” includes groups , having the structure — nhr , in which r is a heteroatom - unsubstituted c n - alkyl , as that term is defined above . a heteroatom - unsubstituted alkylamino group would include — nhch 3 , — nhch 2 ch 3 , — nhch 2 ch 2 ch 3 , — nhch ( ch 3 ) 2 , — nhch ( ch 2 ) 2 , — nhch 2 ch 2 ch 2 ch 3 , — nhch ( ch 3 ) ch 2 ch 3 , — nhch 2 ch ( ch 3 ) 2 , — nhc ( ch 3 ) 3 , — n ( ch 3 ) 2 , — n ( ch 3 ) ch 2 ch 3 , — n ( ch 2 ch 3 ) 2 , n - pyrrolidinyl , and n - piperidinyl . the term “ heteroatom - substituted c 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 1 - c 10 - alkylamino has 1 to 10 carbon atoms . the term “ heteroatom - substituted c n - alkylamino ” includes groups , having the structure — nhr , in which r is a heteroatom - substituted c n - alkyl , as that term is defined above . the term “ alkylsilyl ” includes straight - chain alkylsilyl , branched - chain alkylsilyl , cycloalkylsilyl , cyclic alkylsilyl , heteroatom - unsubstituted alkylsilyl , heteroatom - substituted alkylsilyl , heteroatom - unsubstituted c n - alkylsilyl , and heteroatom - substituted c n - alkylsilyl . the term “ heteroatom - unsubstituted c 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 1 - c 10 - alkylsilyl has 1 to 10 carbon atoms . an alkylsilyl group includes dialkylamino groups . the groups , — si ( ch 3 ) 3 and — si ( ch 3 ) 2 c ( ch 3 ) 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 1 - c 10 - alkylsilyl has 1 to 10 carbon atoms . 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 18 f 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 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 . 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 . 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 . for the synthesis of [ 18 f ]- 3 - fluoro - 4 - aminopyridine , three possible routes from commercially available reagents were considered ( fig1 ). 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 . 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 ( fig3 b 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 ) ( fig5 a - 5b ). this type of reactivity is expected from previously published data on nitro - substituted pyridines 2 , 13 and consistent with the results from abrahim et al . 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 . 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 ( fig1 , 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 ) ( fig6 a - 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 . 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 ( fig2 a - 2b ). 3 - bromopyridine did not undergo reaction with 1 . 2 eq . of tbaf , despite reaction conditions of 120 ° c . for 12 hours ( fig7 a - 7b ). by contrast , over 25 % conversion of 3 - bromopyridine n - oxide was detected after 30 minutes at 120 ° c . ( fig8 a - 8b ). this result indicates that the para - nitro group &# 39 ; 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 & gt ; 4 & gt ;& gt ; 3 position 14 . this difference may be due to the different nature of the nucleophile and solvation effects . after obtaining compound 9 , it was then reduced using standard hydrogenation conditions to generate the desired final product , 3 - fluoro - 4 - aminopyridine ( 10 ) quantitatively ( fig9 ). pyridine n - oxides have not previously been used as precursors for radiofluorination . this approach was further examined for the production of [ 18 f ]- 3 - fluoro - 4 - aminopyridine . radiochemical synthesis of [ 18 f ]- 3 - fluoro - 4 - aminopyridine was conducted as shown in fig3 c . the procedure was similar to the non - radioactive synthesis described above except that [ 18 f ]- tbaf was prepared immediately before the reaction by trapping [ 18 f ]- fluoride in a strong anion exchange cartridge and eluting it with tba - hco 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 [ 18 f ]- tbaf and a main peak corresponding to the desired product ( fig1 ). 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 ) ( fig3 a ). this led to the hypothesis that the non - radioactive 3 - fluoro - 4 - nitropyridine n - oxide ( 9 ) could be contributing to the radiolabeling yield by 19 f / 18 f exchange . labeling of 3 - fluoro - 4 - nitropyridine n - oxide was examined with [ 18 f ]- tbaf in the absence of the bromo precursor we obtained 33 . 1 + 5 . 4 % ( n = 4 ) decay - corrected isolated yield ( fig2 a ). this reaction reaches equilibrium within seconds . fluorine exchange in organic compounds has been reported before 15 , however , this is the first example of fluoride exchange of a c — f bond in a heterocyclic compound . 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 + exp . : 174 . 9423 , calc : 174 . 9433 ) and 1 h , 13 c and 19 f nmr . product amount was calculated from the area under the curve of the hplc uv1 trace using a calibration curve . 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 + exp . : 222 . 9288 , calc : 222 . 9294 ) and 1 h , 13 c and 19 f nmr . 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 + exp . : 158 . 0141 , calc . : 158 . 0128 ) and 1 h , 13 c and 19 f nmr . product amount was calculated from the area under the curve of the hplc uv2 trace using a calibration curve . 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 + 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 . production of tetrabutyl ammonium [ 18 f ]- fluoride ([ 18 f ]- tbaf ): fifty to a hundred mci ( 1 . 85 - 3 . 7 gbq ) of cyclotron - produced 18 f was trapped in a sep - pak accell plus qma plus light cartridge ( waters corporation ) preconditioned with 5 ml of 50 mm of khco 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 [ 18 f ]- tbaf residue was dissolved in 100 - 400 μl of anhydrous dmso and used for the reactions . radiochemical synthesis of [ 18 f ]- 3 - fluoro - 4 - nitropyridine n - oxide ([ 18 f ]- 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 [ 18 f ]- 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 . radiochemical synthesis of [ 18 f ]- 3 - fluoro - 4 - nitropyridine n - oxide ([ 18 f ]- 9 ) from 3 - fluoro - 4 - nitropyridine n - oxide ( 9 ) by 19 f / 18 f exchange : 100 μl of 3 - fluoro - 4 - nitropyridine n - oxide ( 9 ) dissolved in dmso ( 1 mg / ml ) was added to 100 μl of [ 18 f ]- 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 . synthesis of [ 18 f ]- 3 - fluoro - 4 - aminopyridine ([ 18 f ]- 10 ): 1 - 10 mci ( 37 - 370 mbq ) of [ 18 f ]- 3 - fluoro - 4 - aminopyridine n - oxide ([ 18 f ]- 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 ). procurement of [ 18 f ]- fluoride : cyclotron - produced no - carrier - added aqueous 18 f fluoride was obtained from iba molecular north america , inc . 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 4 hco 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 . nucleodur 5 μm , 4 . 6 × 250 mm c18ec ( macherey - nagel ). flow 1 . 4 ml / min . solvent a : 50 mm nh 4 hco 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 4 po 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 4 hco 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 . mobile phase : 50 mm nah 4 po 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 . 3 - bromo - 4 - nitropyridine ( 3 ): 1 h - nmr ( d 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 ). 13 c - nmr ( d 6 - dmso , 125 mhz ) δ : ( ppm ) 102 . 0 ( s ), 122 . 6 ( s ), 137 . 1 ( s ), 139 . 3 ( s ), 142 . 7 ( s ). 3 - iodo - 4 - nitropyridine ( 4 ): 1 h - nmr ( d 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 ). 13 c - nmr ( d 6 - dmso , 125 mhz ) δ ( ppm ): 83 . 5 ( s ), 118 . 2 ( s ), 150 . 0 ( s ), 158 . 4 ( s ), 159 . 8 ( s ). 3 - fluoro - 4 - nitropyridine ( 5 ): 1 h - nmr ( d 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 ). 13 c - nmr ( d 6 - dmso , 125 mhz ) δ ( ppm ): 118 . 4 ( s ), 118 . 5 ( s ), 141 . 9 ( d , j = 23 . 1 ), 149 . 7 ( d , j = 269 . 5 ). 19 f - nmr ( d 6 - dmso , 470 mhz ) δ ( ppm ): − 135 . 13 ( dd , j 2 = 8 . 9 hz , j 1 = 2 . 5 hz ). 3 - bromo - 4 - fluoropyridine ( 6 ): 1 h - nmr ( d 6 - dmso , 500 mhz ) δ ( ppm ): 7 . 50 ( 1h , dd , j 2 = 9 . 0 hz , j 1 = 5 . 5 hz ), 8 . 56 ( 1h , dd , j 2 = 7 . 5 hz , j 1 = 5 . 5 hz ), 8 . 81 ( 1h , j : 9 . 5 hz ). 13 c - nmr ( d 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 ). 19 f - nmr ( d 6 - dmso , 470 mhz ) δ ( ppm ): − 99 . 14 ( dd , j 2 = 16 . 9 hz , j 1 = 9 . 1 hz ). 3 - bromo - 4 - nitropyridine n - oxide ( 8 ): 1 h - nmr ( d 6 - dmso , 500 mhz ) δ ( ppm ): 8 . 15 ( 1h , d , j = 7 . 1 hz ), 8 . 38 ( 1h , dd , j 2 = 7 . 1 hz , j 1 = 2 . 0 hz ), 8 . 85 ( 1h , d , j = 1 . 9 hz ). 13 c - nmr ( d 6 - dmso , 125 mhz ) δ ( ppm ): 110 . 3 ( s ), 118 . 5 ( s ), 150 . 8 ( s ), 150 . 9 ( s ), 154 . 3 ( s ). 3 - fluoro - 4 - nitropyridine n - oxide ( 9 ): 1 h - nmr ( d 6 - dmso , 500 mhz ) δ ( ppm ): 8 . 24 ( 1h , dd , j 2 = 31 . 6 hz , j 1 = 0 . 8 hz ), 8 . 25 ( 1h , dd , j 2 = 35 . 1 hz , j 1 = 1 . 8 hz ), 8 . 91 ( 1h , dd , j 2 = 7 . 1 hz , j 1 = 1 . 8 hz ). 13 c - nmr ( d 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 ). 19 f - nmr ( d 6 - dmso , 470 mhz ) δ ( ppm ): − 126 . 7 ( dd , j 2 = 8 . 5 hz , j 1 = 0 . 8 hz ). 3 - bromopyridine ( 11 ): 1 h - nmr ( d 6 - dmso , 500 mhz ) δ ( ppm ): 7 . 38 ( 1h , dd , j 2 = 8 . 1 hz , j 1 = 3 . 5 hz ), 7 . 85 ( 1h , dt , j 2 = 8 . 2 hz , j 1 = 1 . 0 hz ), 8 . 56 ( 1h , m ), 8 . 69 ( 1h , d , j = 2 . 3 hz ). 3 - fluoropyridine ( 12 ): 1 h - nmr ( d 6 - dmso , 500 mhz ) δ ( ppm ): 7 . 47 ( 1h , m ), 7 . 70 ( 1h , dt , j 2 = 7 . 5 hz , j 1 = 1 . 5 hz ), 8 . 44 ( 1h , m ), 8 . 56 ( 1h , d , j = 3 . 0 hz ). 19 f - nmr ( d 6 - dmso , 470 mhz ) δ ( ppm ): − 127 . 9 ( t , j = 5 . 1 hz ). 3 - bromopyridine n - oxide ( 13 ): 1 h - nmr ( d 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 ). 3 - fluoropyridine n - oxide ( 14 ): 1 h - nmr ( d 6 - dmso , 500 mhz ) δ ( ppm ): 7 . 40 ( 1h , dt , j 2 = 55 . 4 hz , j 1 = 7 . 9 hz ), 7 . 42 ( 1h , dd , j 2 = 54 . 5 hz , j 1 = 7 . 2 hz ), 8 . 13 ( 1h , d , j = 6 . 4 hz ), 8 . 51 ( 1h , s ). 19 f - nmr ( d 6 - dmso , 470 mhz ) δ ( ppm ): − 122 . 1 ( td , j 2 = 5 . 3 hz , j 1 = 0 . 8 hz ). 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 . 1 . s . preshlock , m . tredwell and v . gouverneur , chemical reviews , 2016 , 116 , 719 - 766 . 2 . m . karramkam , f . hinnen , f . vaufrey and f . dollé , journal of labelled compounds and radiopharmaceuticals , 2003 , 46 , 979 -. 992 . 3 . j . h . chun , c . l . morse , f . t . chin and v . w . pike , chem commun ( camb ), 2013 , 49 , 2151 - 2153 4 . h . f . beer , m . haeberli , s . ametamey and p . a . schubiger , journal of labelled compounds and radiopharmaceuticals , 1995 , 36 , 933 - 945 . 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 . 6 . h . liu , s . liu , z . miao , z . deng , b . shen , x . hong and z . cheng , journal of medicinal chemistry , 2013 , 56 , 895 - 901 . 7 . m . a . carroll , j . nairne and j . l . woodcraft , journal of labelled compounds and radiopharmaceuticals , 2007 , 50 , 452 - 454 . 8 . j . h . chun and v . w . pike , chem commun ( camb ), 2012 , 48 , 9921 - 9923 . 9 . b . h . rotstein , n . a . stephenson , n . vasdev and s . h . liang , nature communications , 2014 , 5 , 4365 . 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 . 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 . 12 . p . brugarolas , j . sanchez - rodriguez , j . lacroix , c .- t . chen , f . bezanilla and b . popko , journal of nuclear medicine , 2015 , 56 , 493 . 13 . m . tredwell and v . gouverneur , angew chem int ed engl , 2012 , 51 , 11426 - 11437 . 14 . r . m . johnson , journal of the chemical society b : physical organic , 1966 , doi : 10 . 1039 / j29660001058 , 1058 - 1061 . 15 . e . blom , f . karimi and b . långström , journal of labelled compounds and radiopharmaceuticals , 2009 , 52 , 504 - 511 . 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 . 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 .