Patent Publication Number: US-2018030074-A1

Title: Phosphonium compound and production method therefor

Description:
TECHNICAL FIELD 
     The present invention relates to a method for producing a quaternary phosphonium compound labeled with a radionuclide, in particular a positron emitting nuclide such as  18 F. This invention also relates to a quaternary phosphonium compound labeled with a radionuclide, in particular a positron emitting nuclide such as  18 F, and to an imaging agent comprising said compound. 
     BACKGROUND ART 
     There is formed a proton concentration gradient across the inner mitochondrial membrane, and due to the concentration gradient, a membrane potential from −140 mV to −180 mV is generated in the inner mitochondrial membrane. Delocalized lipophilic cations (DLCs) are molecules having a structure that permits intramolecular delocalization of positive charge by resonance stabilization. DLCs have sufficient lipophilicity to easily pass through the lipid bilayer of a cell membrane, and have the characteristic of accumulating in mitochondria of living organisms utilizing a mitochondrial membrane electrochemical potential (Non-Patent Literatures 1 and 2). 
     Research has been made on the usage of radionuclide-labeled DLCs in various applications including myocardial perfusion imaging, apoptosis imaging, and imaging of brown adipose tissue (BAT) activity. In particular, there have been many reports concerning myocardial perfusion imaging. 
     At present, ischemic heart disease is the most common cause of death in the world. According to the survey made by the World Health Organization, seven million people died of this disease in 2011. In Japan, this disease ranks the second leading cause of death behind malignant neoplasm. Ischemic heart disease is a generic name for angina pectoris, heart infarction, and the like, and is mainly caused by the progression of arteriosclerosis of the coronary artery. For the purpose of effective treatment, early detection of myocardial ischemia is essential. Therefore, the development of a highly accurate diagnostic method which leads to early detection of ischemic heart disease may well be said to be one of the important challenges in modern medicine. 
     Examples of common detection methods for ischemic heart disease include not only electrocardiographic examination, ultrasonographic examination, blood examination, and cardiac catheter examination, but also myocardial perfusion imaging using a nuclear medicine examination method such as PET or SPECT. In patients with mild myocardial ischemia, the blood flow over the whole myocardium is maintained during rest, but when a stress is imposed on the myocardium due to exercise, etc., the myocardium requires large amounts of oxygen and nutrients and thus circulatory blood flow increases. While the blood flow increases in normal sites, coronary blood flow does not increase in arterioles distal to the site of narrowing, since blood vessels are dilated almost to the maximum during rest to maintain the blood flow. Thus, when imaging is performed under myocardial stress, the site of narrowing appears as an image of loss of radioactivity. Based on this principle, diagnosis of myocardial blood flow is done by imposing a stress on the myocardium of patients through exercise or the use of a medication like adenosine and comparing images taken during rest and stress. 
     In clinical settings, SPECT agents—e.g [ 201 Tl]TlCl, and  99m Tc-labeled formulations like [ 99m Tc]sestamibi (MIBI) and [ 99m Tc]tetrofosmin—are now mainly used as myocardial perfusion imaging agents. However, SPECT has some limitations: for example, since there is no standardized radiation attenuation correction method available, the influence of radiation attenuation occurs in stout patients or large-breasted female patients; and also this method is unfit for quantitative analysis. So, there is an expectation to use a PET method characterized by high spatial resolution and sensitivity. 
     Apoptosis is a programmed form of cell death induced by the activation of a specific protease (caspase) present in cells. It is reported that the activation of caspase is mediated by cytochrome c released from mitochondria, and that the release of cytochrome c takes place in association with change in mitochondrial membrane potential. Mitochondria play an important role in the control of apoptosis (Non-Patent Literature 3). 
     Unlike white adipose responsible for energy storage, brown adipose tissue produces heat through energy consumption, and thus is one of the targets for the treatment of obesity. Brown adipose produces heat for body-temperature maintenance in cold environments or through consumption of high fat foods, and this heat production takes place in mitochondria abundant in brown adipose cells. 
     PET agents reported include [ 15 O]H 2 O, [ 13 N]NH 3 , and [ 82 Rb]RbCl. However, since  15 O abd  13 N have a short half-life of 2 and 10 minutes, respectively, the use of [ 15 O]H 2 O and [ 13 N]NH 3  is limited to clinical facilities equipped with a cyclotron. Further,  82 Rb is produced from  82 Sr in a generator, but since the generator is expensive, there is a limitation on the use of the generator for high-throughput applications. For these reasons, active research and development have in recent years been conducted on  18 F-labeled PET imaging agents having a longer half-life (110 minutes) than the aforementioned nuclides (Patent Literatures 1 to 3, and Non-Patent Literatures 5 to 11). 
     CITATION LIST 
     Patent Literatures 
     Patent Literature 1: Japanese Patent Domestic Publication No. JP 2005-532262 
     Patent Literature 2: Japanese Patent Application Publication No. JP 2012-52196 
     Patent Literature 3: Japanese Patent Domestic Publication No. JP 2013-523702 
     Non-Patent Literatures 
     Non-Patent Literature 1: Ross, M. F., et al.,  Biochemistry  (Moscow) 2005, 70, 222; 
     Non-Patent Literature 2: Yousif, L. F., et al.,  ChemBioChem  2009, 10, 1939; 
     Non-Patent Literature 3: Tatton, W. G., et al.,  Biochim. Biophys. Acta,  1999, 1410, 195; 
     Non-Patent Literature 4: Madar, I., et al.,  J. Nucl. Med.  2011, 52, 808; 
     Non-Patent Literature 5: Ravert, H. T., et al.,  J. Label. Compd. Radiopharm.  2004, 47, 469; 
     Non-Patent Literature 6: Madar, I., et al.,  J. Nucl. Med.  2006, 47, 1359; 
     Non-Patent Literature 7: Madar, I., et al.,  J. Nucl. Med.  2007, 48, 1021; 
     Non-Patent Literature 8: Higuchi, T., et al.,  J. Nucl. Med.  2011, 52, 965; 
     Non-Patent Literature 9: Shoup, T. M., et al.,  J. Mol. Imaging Biol.  2011, 13, 511; 
     Non-Patent Literature 10: Kim, D. Y., et al.,  J. Bioconjug. Chem.  2012, 23, 431; 
     Non-Patent Literature 11: Tominaga, et al.,  Abstracts of Special Programs and General Presentations at the  53 rd Annual Scientific Meeting of the Japanese Society of Nuclear Medicine,  197. 
     SUMMARY OF INVENTION 
     Technical Problem 
     In 2004, Ravert, et al. reported the synthesis of 4-[ 18 F]fluorobenzyl-triphenylphosphonium bromide ([ 18 F]FBnTP) (Non-Patent Literature 5). Such phosphonium compounds having three phenyl groups have characteristics as delocalized lipophilic cations (DLCs) which permit intramolecular delocalization of positive charge by resonance stabilization. Due to their lipophilicity, DLCs are capable of easily passing through the lipid bilayer of a cell membrane and preferentially accumulating in the inner mitochondrial membrane which has a very low membrane potential. There are reports on myocardial perfusion imaging performed with [ 18 F]FBnTP, targeting mitochondria abundant in myocardial cells (Non-Patent Literatures 6 to 8). 
     However, since  18 F has a half-life of 110 minutes, there is a need for a synthesis method that enables efficient and short-time production of a compound for use as a PET imaging agent. In the process of synthesis of [ 18 F]FBnTP, the use of a highly corrosive brominating agent is needed at the step of synthesizing benzyl bromide by bromination—this use has caused a problem with automatic synthesis using a synthesizer. Also, [ 18 F]FBnTP has the property of accumulating not only in the myocardium but also in the liver adjacent to the myocardium to the same extent, and so there is concern that [ 18 F]FBnTP has a negative effect on myocardial perfusion imaging. 
     Shoup, et al. also reported PET myocardial imaging using (4-[ 18 F]fluorophenyOtriphenylphosphonium (FTPP) (Non-Patent Literature 9). However, the synthesis of FTPP has some problems in that a high temperature of 200° C. is required for labeling with  18 F, and the radiochemical yield of [ 18 F]FTPP is as low as about 10%. Likewise, (6-[ 18 F]fluorohexyl)triphenylphosphonium reported by Kim, et al. (Non-Patent Literature 10) is problematic in practical use because the temperature condition of 220° C. is used for its synthesis. 
     An object of the present invention is to provide a production method that enables effective and short-time production of a quaternary phosphonium compound labeled with a radionuclide, in particular a positron emitting nuclide such as  18 F. Another object of this invention is to provide a quaternary phosphonium compound labeled with a radionuclide, in particular a positron emitting nuclide such as  18 F, and especially to provide a quaternary phosphonium compound that is suitable as a PET imaging agent useful for imaging of the myocardium and the like. 
     Solution to Problem 
     The present inventors have made intensive studies to achieve the aforementioned objects, and found a highly effective production method for a quaternary phosphonium compound labeled with a radionuclide. The inventors also found that the quaternary phosphonium compounds that can be produced by the aforementioned method have favorable characteristics as PET imaging agents. Thus, the inventors have completed the present invention. The disclosure of the present application includes the invention set forth below in (1) to (24). 
     (1) A phosphonium compound of formula (II): 
     
       
         
         
             
             
         
       
     
     wherein Ar 1 , Ar 2 , and Ar 3  are each independently an aryl optionally substituted with one or more substituents selected from B 2 , wherein at least one of Ar 1 , Ar 2 , and Ar 3  is substituted with one or more substituents selected from radionuclide-labeled C 1-6 alkyl, radionuclide-labeled C 2-6 alkoxy, radionuclide-labeled C 2-6 alkoxyC 1-6 alkyl, radionuclide-labeled C 2-6 alkoxyC 2-6 alkoxy, radionuclide-labeled C 2-6 alkoxyC 2-6 alkoxyC 1-6 alkyl, and radionuclide-labeled C 2-6 alkoxyC 2-6 alkoxyC 2-6 alkoxy; 
     A 1  is a hydrogen atom, C 1-10  alkyl optionally substituted with one or more substituents selected from B 1 , C 2-10 alkenyl optionally substituted with one or more substituents selected from B 1 , or an aryl optionally substituted with one or more substituents selected from B 2 ; 
     each B 1 , is independently a halogen atom, C 1-6 alkoxy, phenyl, or naphthyl, wherein the phenyl and naphthyl are optionally substituted with one or more substituents selected from C 1-6 alkyl, C 1-6 alkoxy, and halogen atoms; 
     each B 2  is independently a halogen atom, C 1-6 alkyl, C 1-6 alkoxy, C 1-6 alkylthio, cyano, amino, C 1-6 alkylamino, di(C 1-6 alkyl)amino, nitro, hydroxy, or (C 1-3 alkoxy)carbonyl, wherein the alkyl, alkoxy and alkylthio are optionally substituted with one or more substituents selected from halogen atoms; 
     Ar 1 , Ar 2 , Ar 3  and A 1 , and the substituents contained therein, optionally form acid addition salts; 
     X − is an anion with a total charge of −1. 
     (2) The phosphonium compound according to (1), wherein the radionuclide is a positron emitting nuclide. 
     (3) The phosphonium compound according to (2), wherein the positron emitting nuclide is  18 F. 
     (4) The phosphonium compound according to any of (1) to (3), wherein A 1  is a hydrogen atom, C 1-6 alkyl optionally substituted with one or more halogen atoms, phenyl, or phenylC 1-4 alkyl, wherein the phenyl or a phenyl moiety of phenylC 1-4 alkyl is optionally substituted with one or more substituents selected from one or more halogen atoms, C 1-6 alkyl, C 1-6 alkoxy, C 1-6 alkylthio, cyano, amino, C 1-6 alkylamino, di(C 1-6 alkyl)amino, nitro, hydroxy, and (C 1-3 alkoxy)carbonyl. 
     (5) The phosphonium compound according to (1), wherein the phosphonium compound comprises a phosphonium selected from: 
     benzyl-[4-(2-[ 18 F]fluoroethoxy)phenyl]-diphenylphosphonium; 
     benzyl-(4-[2-{2-(2-[ 18 F]fluoroethoxy)ethoxy}ethoxy]phenyl)-diphenylphosphonium; 
     [4-(2-[ 18 F]fluoroethoxy)phenyl]-(4-methoxycarbonylphenyl)methyl-diphenylphosphonium; 
     [4-(2-[ 18 F]fluoroethoxy)phenyl]-(4-fluorophenyl)methyl-diphenylphosphonium; 
     [4-(2-[ 18 F]fluoroethoxy)phenyl]-(3,4,5-trifluorophenyl)methyl-diphenylphosphonium; 
     (4-chlorophenyl)methyl-[4-(2-[ 18 F]fluoroethoxy)phenyl]-diphenylphosphonium; 
     [4-(2-[ 18 F]fluoroethoxy)phenyl]-(4-methoxyphenyl)methyl-diphenylphosphonium; 
     [4-(2-[ 18 F]fluoroethoxy)phenyl]-(n-pentyl)-diphenylphosphonium; 
     [4-(2-[ 18 F]fluoroethoxy)phenyl]-(3-phenylpropyl)-diphenylphosphonium; 
     (4-n-butylphenyl)methyl-[4-(2-[ 18 F]fluoroethoxy)phenyl]-diphenylphosphonium; 
     (3-fluorophenyl)methyl-[4-(2-[ 18 F]fluoroethoxy)phenyl]-diphenylphosphonium; 
     [4-(2-[ 18 F]fluoroethoxy)phenyl]-[4-(trifluoromethylthio)phenyl]methyl-diphenylphosphonium; 
     [4-(2-[ 18 F]fluoroethoxy)phenyl]-(4-methylphenyl)methyl-diphenylphosphonium; 
     (3-cyanophenyl)methyl-[4-(2-[ 18 F]fluoroethoxy)phenyl]-diphenylphosphonium; 
     [4-(2-[ 18 F]fluoroethoxy)phenyl]-methyl-diphenylphosphonium; 
     allyl-[4-(2-[ 18 F]fluoroethoxy)phenyl]-diphenylphosphonium; 
     benzyl-[3-(2-[ 18 F]fluoroethoxy)phenyl]-diphenylphosphonium; 
     [3-(2-[ 18 F]fluoroethoxy)phenyl]-(4-methoxyphenyl)methyl-diphenylphosphonium; 
     [3-(2-[ 18 F]fluoroethoxy)phenyl]-(4-methoxycarbonylphenyl)methyl-diphenylphosphonium; 
     [3-(2-[ 18 F]fluoroethoxy)phenyl]-(4-fluorophenyl)methyl-diphenylphosphonium; 
     [3-(2-[ 18 F]fluoroethoxy)phenyl]-(3-phenylpropyl)-diphenylphosphonium; 
     (4-chlorophenyl)methyl-[3-(2-[ 18 F]fluoroethoxy)phenyl]-diphenylphosphonium; 
     [3-(2-[ 18 F]fluoroethoxy)phenyl]-(3,4,5-trifluorophenyl)methyl-diphenylphosphonium; 
     benzyl-[3-(3-[ 18 F]fluoropropoxy)phenyl]-diphenylphosphonium; 
     benzyl-[3-(4-[ 18 F]fluorobutoxy)phenyl]-diphenylphosphonium; 
     [3-(2-[ 18 F]fluoroethoxy)phenyl]-methyl-diphenylphosphonium; and 
     benzyl-[2-(2-[ 18 F]fluoroethoxy)phenyl]-diphenylphosphonium. 
     (6) The phosphonium compound according to (1), wherein the phosphonium compound is selected from: 
     benzyl-[4-(2-[ 18 F]fluoroethoxy)phenyl]-diphenylphosphonium bromide; 
     benzyl-[3-(2-[ 18 F]fluoroethoxy)phenyl]-diphenylphosphonium bromide; 
     [3-(2-[ 18 F]fluoroethoxy)phenyl]-(4-methoxyphenyl)methyl-diphenylphosphonium bromide; and 
     [3-(2-[ 18 F]fluoroethoxy)phenyl]-(3-phenylpropyl)-diphenylphosphonium bromide. 
     (7) A radiopharmaceutical for use in imaging, comprising the phosphonium compound according to any of (1) to (6). 
     (8) A radiopharmaceutical for use in PET imaging, comprising the phosphonium compound according to any of (2) to (6). 
     (9) The PET radiopharmaceutical according to (8), wherein the PET radiopharmaceutical is intended for use in mitochondrion imaging. 
     (10) The PET radiopharmaceutical according to (8) or (9), wherein the PET radiopharmaceutical is intended for use in imaging of myocardium, tumor, or brown adipose tissue. 
     (11) A disposable product comprising the radiopharmaceutical according to any of (7) to (10). 
     (12) A method for producing a quaternary phosphonium compound labeled with a radionuclide, the method comprising the step of: 
     reacting an electrophile of formula (I): X 1 -CH 2 -A 1  with triphenylphosphine having one or more radionuclide-labeled substituents on the benzene ring to give a quaternary phosphonium salt; 
     wherein X 1  is a leaving group; 
     A 1  is a hydrogen atom, C 1-10 alkyl optionally substituted with one or more substituents selected from B 1 , C 2-10 alkenyl optionally substituted with one or more substituents selected from B 1 , or an aryl optionally substituted with one or more substituents selected from B 2 ; 
     each B 1  is independently a halogen atom, C 1-6 alkoxy, phenyl, or naphthyl, wherein the phenyl and naphthyl are optionally substituted with one or more substituents selected from C 1-6 alkyl, C 1-6 alkoxy, and halogen atoms; 
     each B 2  is independently a halogen atom, C 1-6 alkyl, C 1-6 alkoxy, C 1-6 alkylthio, cyano, amino, C 1-6 alkylamino, di(C 1-6 alkyl)amino, nitro, hydroxy, or (C 1-3 alkoxy)carbonyl, wherein the alkyl, alkoxy and alkylthio are optionally substituted with one or more substituents selected from halogen atoms; 
     wherein the one or more radionuclide-labeled substituents are selected from radionuclide-labeled C 1-6 alkyl, radionuclide-labeled C 2-6 alkoxy, radionuclide-labeled C 2-6 alkoxyC 1-6 alkyl, radionuclide-labeled C 2-6 alkoxyC 2-6 alkoxy, radionuclide-labeled C 2-6 alkoxyC 2-6 alkoxyC 1-6 alkyl, and radionuclide-labeled C 2-6 alkoxyC 2-6 alkoxyC 2-6 alkoxy; 
     wherein the triphenylphosphine optionally further has one or more substituents selected from B 2  on the benzene ring. 
     (13) The method according to (12), wherein the radionuclide is a positron emitting nuclide. 
     (14) The method according to (13), wherein the positron emitting nuclide is  18 F. 
     (15) The method according to any of (12) to (14), wherein X 1  is a halogen atom, optionally substituted C 1-6 alkylsulfonyloxy, or optionally substituted phenylsulfonyloxy. 
     (16) The method according to any of (12) to (15), wherein the method is performed in an automatic synthesizer. 
     (17) A phosphine compound of formula (III): 
     
       
         
         
             
             
         
       
     
     wherein Ar 1 , Ar 2 , and Ar 3  are as defined in (1). 
     (18) The phosphine compound according to (17), wherein the radionuclide is  18 F. 
     (19) The phosphonium compound according to any of (1) to (6), wherein the radionuclide is replaced with a non-radioactive identical element. 
     (20) A phosphonium compound obtained by replacing  18 F in the  18 F-labeled phosphonium compound according to any of (3) to (6) with  19 F. 
     (21) A phosphine compound of formula (III): 
     
       
         
         
             
             
         
       
     
     wherein Ar 1 , Ar 2 , and Ar 3  are each independently an aryl optionally substituted with one or more substituents selected from B 2 ; 
     wherein at least one of Ar 1 , Ar 2 , and Ar 3  is substituted with one or more substituents selected from: C 1-6 alkyl optionally substituted with one or more substituents selected from L, C 2-6 alkoxy optionally substituted with one or more substituents selected from L, C 2-6 alkoxyC 1-6 alkyl optionally substituted with one or more substituents selected from L, C 2-6 alkoxyC 2-6 alkoxy optionally substituted with one or more substituents selected from L, C 2-6 alkoxyC 2-6 alkoxyC 1-6 alkyl optionally substituted with one or more substituents selected from L, and C 2-6 alkoxyC 2-6 alkoxyC 2-6 alkoxy optionally substituted with one or more substituents selected from L; 
     B 2  is a halogen atom, C 1-6 alkyl, C 1-6 alkoxy, C 1-6 alkylthio, cyano, amino, C 1-6 alkylamino, di(C 1-6 alkyl)amino, nitro, hydroxy, or (C 1-3 alkoxy)carbonyl, wherein the alkyl, alkoxy, and alkylthio are optionally substituted with one or more substituents selected from halogen atoms; 
     L is bromine, iodine, p-toluenesulfonyloxy, methanesulfonyloxy, chloromethanesulfonyloxy, or trifluoromethanesulfonyloxy. 
     (22) A method for producing the phosphine compound as set forth in (17) or (18) from the compound as set forth in (21) and a radionuclide. 
     (23) A kit for producing the phosphonium compound as set forth in any of (1) to (6), the kit comprising the compound as set forth in (21) as a reagent. 
     (24) A kit for producing the phosphonium compound as set forth in either (17) or (18), the kit comprising the compound as set forth in (21) as a reagent. 
     Advantageous Effects of Invention 
     According to the present invention, there is provided a production method that enables efficient and short-time production of quaternary phosphonium compounds labeled with a radionuclide such as  18 F. Also provided according to this invention are quaternary phosphonium compounds labeled with a radionuclide such as  18 F, in particular quaternary phosphonium compounds that are suitable as PET imaging agents useful for imaging of the myocardium and the like. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  shows the results of preparative HPLC of [ 18 F]TAP-001 synthesized in Example 1. 
         FIG. 2  shows the mouse PET/CT images taken with the labeled compounds of the present invention. 
         FIG. 3  shows the graphs of the results of the biodistribution study conducted with the labeled compounds of the present invention. 
         FIG. 4  shows the graph of the results of the biodistribution study conducted with the labeled compound of the present invention. 
         FIG. 5  shows the PET images taken at time points from 0 to 60 minutes after administration for the purpose of conducting myocardial imaging analysis in rats using the labeled compound of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereunder, the present invention will be more specifically described. 
     According to one aspect of the present invention, there is provided a method for producing a quaternary phosphonium compound labeled with a positron emitting nuclide, the method comprising the step of reacting an electrophile of formula (I) given above with triphenylphosphine substituted with a positron emitting nuclide-labeled substituent. 
     As referred to herein, the term “aryl” refers to an aromatic hydrocarbon ring group having 6 to 14 carbon atoms. Examples of aryl include, but are not limited to, phenyl, 1-naphthyl, and 2-naphthyl. 
     As referred to herein, the term “C 1-10 alkyl” refers to a straight-chain, branched-chain, cyclic or partially cyclic alkyl group having 1 to 10 carbon atoms. Examples of C 1-10 alkyl include, but are not limited to, methyl, ethyl, n-propyl, i-propyl, n-butyl, s-butyl, i-butyl, t-butyl, n-pentyl, 3-methylbutyl, 2-methylbutyl, 1-methylbutyl, 1-ethylpropyl, n-hexyl, 4-methylpentyl, 3-methylpentyl, 2-methylpentyl, 1-methylpentyl, 3-ethylbutyl, 2-ethylbutyl, n-heptyl, n-octyl, n-nonyl, n-decyl, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, and cyclopropylmethyl, and other examples include, but are not limited to, C 1-6 alkyl, C 1-4 alkyl and C 1-3 alkyl. 
     As referred to herein, the term “C 1-6 alkyl” refers to a straight-chain, branched-chain, cyclic or partially cyclic alkyl group having 1 to 6 carbon atoms. Examples of C 1-6 alkyl include, but are not limited to, methyl, ethyl, n-propyl, i-propyl, n-butyl, s-butyl, i-butyl, t-butyl, n-pentyl, 3-methylbutyl, 2-methylbutyl, 1-methylbutyl, 1-ethylpropyl, n-hexyl, 4-methylpentyl, 3-methylpentyl, 2-methylpentyl, 1-methylpentyl, 3-ethylbutyl, and 2-ethylbutyl, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, and cyclopropylmethyl, and other examples include, but are not limited to, C 1-4 alkyl and C 1-3 alkyl. 
     As referred to herein, the term “C 2-10 alkenyl” refers to a straight-chain, branched-chain, cyclic or partially cyclic alkenyl group having 2 to 10 carbon atoms, and this alkenyl group may contain one double bond or may contain two or more double bonds. Examples of this alkenyl group include, but are not limited to, vinyl, 1-propenyl, 2-propenyl(allyl), 1-butenyl, 2-butenyl, and 3-butenyl, and other examples include, but are not limited to, 
     C 2-6 alkenyl, C 24 alkenyl, and C 2-3 alkenyl. 
     As referred to herein, the term “C 1-6 alkoxy” refers to the alkyloxy group [—O—(C 1-6 alkyl)] having, as an alkyl moiety, the above-defined alkyl group with 1 to 6 carbon atoms. Examples of C 1-6 alkoxy include, but are not limited to, methoxy, ethoxy, n-propoxy, i-propoxy, n-butoxy, s-butoxy, i-butoxy, t-butoxy, n-pentoxy, 3-methylbutoxy, 2-methylbutoxy, 1-methylbutoxy, 1-ethylpropoxy, n-hexyloxy, 4-methylpentoxy, 3-methylpentoxy, 2-methylpentoxy, 1-methylpentoxy, 3-ethylbutoxy, cyclopentyloxy, cyclohexyloxy, and cyclopropylmethyloxy, and other examples include, but are not limited to, C 1-4 alkoxy and C 1-3 alkoxy. As referred to herein, the term “C 1-4 alkoxy” includes, but is not limited to, C 1-3 alkoxy. 
     As referred to herein, the term “C 2-6 alkoxy” refers to the alkyloxy group [—O—(C 2-6 alkyl)] having, as an alkyl moiety, an alkyl group with 2 to 6 carbon atoms. Examples of C 2-6 alkoxy include, but are not limited to, ethoxy, n-propoxy, i-propoxy, n-butoxy, s-butoxy, i-butoxy, t-butoxy, n-pentoxy, 3-methylbutoxy, 2-methylbutoxy, 1-methylbutoxy, 1-ethylpropoxy, n-hexyloxy, 4-methylpentoxy, 3-methylpentoxy, 2-methylpentoxy, 1-methylpentoxy, 3-ethylbutoxy, cyclopentyloxy, cyclohexyloxy, and cyclopropylmethyloxy, and other examples include, but are not limited to, C 1-4 alkoxy and C 1-3 alkoxy. 
     As referred to herein, the term “C 1-6 alkylthio” refers to the alkylthio group [—S—(C 1-6 alkyl)] having, as an alkyl moiety, the above-defined alkyl group with 1 to 6 carbon atoms. Examples of C 1-6 alkylthio include, but are not limited to, methylthio, ethylthio, n-propylthio, i-propylthio, n-butylthio, s-butylthio, i-butylthio, t-butylthio, n-pentylthio, 3-methylbutylthio, 2-methylbutylthio, 1-methylbutylthio, 1-ethylpropylthio, n-hexylthio, 4-methylpentylthio, 3-methylpentylthio, 2-methylpentylthio, 1-methylpentylthio, 3-ethylbutylthio, cyclopentylthio, cyclohexylthio, and cyclopropylmethylthio, and other examples include, but are not limited to, C 1-4 alkylthio and C 1-3 alkylthio. As referred to herein, the term “C 1-4 alkylthio” includes, but is not limited to, C 1-3 alkylthio. 
     As referred to herein, the term “amino” refers to —NH 2 , and may form an acid addition salt in the molecule. 
     As referred to herein, the term “C 1-6 alkylamino” refers to the alkylamino group [—NH—(C 1-6 alkyl)] having, as an alkyl moiety, the above-defined alkyl group with 1 to 6 carbon atoms. Examples of C 1-6 alkylamino include, but are not limited to, methylamino, ethylamino, n-propylamino, i-propylamino, n-butylamino, s-butylamino, i-butylamino, t-butylamino, n-pentylamino, 3-methylbutylamino, 2-methylbutylamino, 1-methylbutylamino, 1-ethylpropylamino, n-hexylamino, 4-methylpentylamino, 3-methylpentylamino, 2-methylpentylamino, 1-methylpentylamino, 3-ethylbutylamino, cyclopentylamino, cyclohexylamino, and cyclopropylmethylamino, and other examples include, but are not limited to, C 1-4 alkylamino and C 1-3 alkylamino. As referred to herein, the term “C 1-4 alkylamino” includes, but is not limited to, C 1-3 alkylamino. The C 1-6 alkylamino group may form an acid addition salt in the molecule. 
     As referred to herein, the term “di(C 1-6 alkyl)amino” refers to the alkylamino group [—N(C 1-6 alkyl) 2 ] having, as an alkyl moiety, the above-defined alkyl group with 1 to 6 carbon atoms. The two alkyl groups can be the same or different. Examples of di(C 1-6 alkyl)amino include, but are not limited to, dimethylamino, diethylamino, ethyl(methyl)amino, methyl(n-propyl)amino, ethyl(n-propyl)amino, methyl(i-propyl)amino, ethyl(i-propyl)amino, di(n-propyl)amino, and di(i-propyl)amino, and other examples include, but are not limited to, di(C 1-4 alkyl)amino and di(C 1-3 alkyl)amino. As referred to herein, the term “di(C 1-4 alkyl)amino” includes, but is not limited to, di(C 1-3 alkyl)amino. The di(C 1-6 alkyl)amino group may form an acid addition salt in the molecule. 
     As referred to herein, the term “C 1-6 alkoxycarbonyl” refers to an alkoxycarbonyl group having, as an alkoxy moiety, the above-defined C 1-6 alkoxy group. Examples of C 1-6 alkoxycarbonyl include, but are not limited to, methoxycarbonyl, ethoxycarbonyl, and tert-butoxycarbonyl, as well as C 1-3 alkoxycarbonyl. 
     As referred to herein, the term “C 1-3 alkoxycarbonyl” refers to an alkoxycarbonyl group having, as an alkoxy moiety, the above-defined C 1-3 alkoxy group. Examples of C 1-3 alkoxycarbonyl include, but are not limited to, methoxycarbonyl, ethoxycarbonyl, n-propoxycarbonyl, and isopropoxycarbonyl. 
     As referred to herein, the term “C 2-6 alkoxyC 1-6 alkyl” refers to a group of the formula —(C 1-6 alkylene)—O—(C 2-6 alkyl), and examples include, but are not limited to, ethoxymethyl, 2-ethoxyethyl, 2-ethoxypropyl, and 3-ethoxypropyl. Examples of  18 F-labeled C 2-6 alkoxyC 1-6 alkyl include, but are not limited to, —CH 2 OCH 2 CH 2 — 18 F, and —CH 2 CH 2 OCH 2 CH 2 — 18 F. 
     As referred to herein, the term “C 2-6 alkoxyC 2-6 alkoxy” refers to a group of the formula —O—(C 2-6 alkylene)—O—(C 2-6 alkyl), and examples include, but are not limited to, 2-ethoxyethoxy, 2-ethoxypropoxy, and 3-ethoxypropoxy. Examples of  18 F-labeled C 2-6 alkoxyC 2-6 alkoxy include, but are not limited to, —OCH 2 CH 2 OCH 2 CH 2 — 18 F. 
     As referred to herein, the term “C 2-6 alkoxyC 2-6 alkoxyC 1-6 alkyl” refers to a group of the formula —(C 1-6 alkylene)—O—(C 2-6 alkylene)—O—(C 2-6 alkyl), and examples include, but are not limited to, (2-ethoxyethoxy)methyl, 2-(2-ethoxyethoxy)ethyl, 2-(2-ethoxyethoxy)propyl, and 3-(2-ethoxyethoxy)propyl. Examples of  18 F-labeled C 2-6 alkoxyC 2-6 alkoxyC 1-6 alkyl include, but are not limited to, —CH 2 OCH 2 CH 2 OCH 2 CH 2 — 18 F, and —CH 2 CH 2 OCH 2 CH 2 OCH 2 CH 2 — 18 F. 
     As referred to herein, the term “C 2-6 alkoxyC 2-6 alkoxyC 2-6 alkoxy” refers to a group of the formula —O—(C 2-6 alkylene)—O—(C 2-6 alkylene)—O—(C 2-6 alkyl), and examples include, but are not limited to, 2-(2-ethoxyethoxy)ethoxy, 2-(2-ethoxyethoxy)propoxy, and 3-(2-ethoxyethoxy)propoxy. Examples of  18 F-labeled C 2-6 alkoxy C 2-6 alkoxyC 2-6 alkoxy include, but are not limited to, —OCH 2 CH 2 OCH 2 CH 2 OCH 2 CH 2 — 18 F. 
     As referred to herein, the term “optionally substituted C 1-6 alkylsulfonyloxy” can be exemplified by methanesulfonyloxy, and trifluoromethanesulfonyloxy. 
     As referred to herein, the term “optionally substituted phenylsulfonyloxy” can be exemplified by benzenesulfonyloxy, and toluenesulfonyloxy. 
     Examples of halogen atoms include, but are not limited to, fluorine atom, chlorine atom, bromine atom, and iodine atom. 
     When the compound of the present invention can form a solvate such as hydrate, this invention can be practiced as a solvate. Further, the compound of the present invention can be practiced, as appropriate, in the form of mixture, solution, polymorph, or the like. 
     Examples of X 1  as defined herein include, but are not limited to, chlorine atom, bromine atom, iodine atom, methanesulfonyloxy, trifluoromethanesulfonyloxy, benzenesulfonyloxy, and toluenesulfonyloxy. 
     In one mode of the phosphonium compound of formula (II), Ar 1 , Ar 2  and Ar 3  are each independently a phenyl optionally substituted with one or more substituents selected from B 2 . 
     In one mode of the phosphonium compound of formula (II), Ar 1  and Ar 2  are each a phenyl, and Ar 3  is a phenyl optionally substituted with one or more substituents selected from B 2 . 
     In one mode of the phosphonium compound of formula (II), A 1  is a hydrogen atom, C 1-10 alkyl optionally substituted with one or more substituents selected from B 1 , C 2-10 alkenyl optionally substituted with one or more substituents selected from B 1 , or a phenyl optionally substituted with one or more substituents selected from B 2 . 
     The anion (X) contained in the phosphonium compound is for example an anion that forms a pharmaceutically acceptable salt, and specific examples include, but are not limited to, Cl − , Br − , I − , PhSO 2 O − , CH 3 C 6 H 4 SO 2 O − , and CH 3 SO 2 O − . In one mode, X − may be an anion that is generated by the elimination of a leaving group X 1  during reaction. 
     Examples of the leaving group as defined herein include, but are not limited to, halogen atoms, optionally substituted C 1-6 alkylsulfonyloxy, or optionally substituted phenylsulfonyloxy, and preferred examples include, but are not limited to, bromine atom, iodine atom, p-toluenesulfonyloxy, methanesulfonyloxy, chloromethanesulfonyloxy, and trifluoromethanesulfonyloxy. 
     Examples of the leaving group defined herein as L or X 1  include, but are not limited to, halogen atoms, optionally substituted C 1-6 alkylsulfonyloxy, or optionally substituted phenylsulfonyloxy, and preferred examples include, but are not limited to, bromine atom, iodine atom, p-toluenesulfonyloxy, methanesulfonyloxy, chloromethanesulfonyloxy, and trifluoromethanesulfonyloxy. 
     In formula (II), Ar 1 , Ar 2 , Ar 3  and A 1 , and the substituents contained therein, when containing an amino group, an alkylamino group or a dialkylamino group, may form acid addition salts. 
     As used herein, the phrase “substituted with one or more substituents” means substitution with, for example, 1 to 3 substituents. 
     The phosphonium compound of the present invention can be contained in a variety of solutions such as aqueous solution, a variety of solvents such as hydrate, polymorphs, and the like. 
     The compound of the present invention can be labeled with a radionuclide by a known method. Examples of the radionuclide include, but are not limited to:  3 H,  14 C,  35 S, and  131 I; γray emitting nuclides such as  99m Tc,  111 In,  67 Ga,  201 Tl,  123 I and  133 Xe; and positron emitting nuclides such as  11 C,  13 N,  15 O,  18 F,  62 Cu,  64 Cu,  68 Ga and  76 Br. Compounds labeled with a γ-ray emitting nuclide can be used as imaging agents for computed tomography (Single photon emission computed tomography: SPECT)—for example, compounds labeled with  99m Tc and  123 I are commonly used for SPECT. Compounds labeled with a positron emitting nuclide can be used as imaging agents for positron emission tomography (PET). Among different positron emitting nuclides,  11 C,  13 N,  15 O and  18 F are preferred, with  18 F and  11 C being more preferred, and  18 F being particularly preferred, from various viewpoints including appropriate half-life and ease of labeling. 
     The phosphonium moiety of the phosphonium compound of formula (II) can be exemplified by the following. 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     The reaction of an electrophile of formula (I) with triphenylphosphine having one or more positron emitting nuclide-labeled substituents on the benzene ring can be performed by, for example, following the step represented by the scheme given below. 
     
       
         
         
             
             
         
       
     
     [As used in this scheme, Ar 1 , Ar 2 , Ar 3 , X 1 , A 1 , X −  are as defined hereinabove.] 
     The aforementioned reaction can be effected by performing heating (at a temperature of, for example, 50-150° C., specifically 90-120° C., more specifically 105-115° C.) in the presence of an appropriate solvent (e.g., acetonitrile, DMSO, DMF, toluene, xylene). The reaction period can be set to be in the range of, for example, 5-60 minutes. This reaction can be performed, for example, under pressure by heating in a sealed container. 
     The production method of the present invention, together with preparation of a labeled precursor, can be performed according to the scheme given below. 
     
       
         
         
             
             
         
       
     
     [As used in this scheme, Ar 2 , Ar 3 , X 1 , A 1  and X −  are as defined hereinabove; n is an integer selected from 1 to 4; R 1  is a group selected from B 2  defined above; A 2  is selected from —(C 1-6 alkylene)-, —(C 1-6 alkylene)—O—(C 2-6 alkylene)-, —(C 1-6 alkylene)—O—(C 2-6 alkylene)—O—(C 2-6 alkylene)-, —O—(C 2-6 alkylene)-, —O—(C 2-6 alkylene)—O—(C 2-6 alkylene)-, and —O—(C 2-6 alkylene)—O—(C 2-6 alkylene)—O—(C 2-6 alkylene)-; X 2  is a leaving group.] 
     As [ 18 F]KF used at the first step of fluorination, there can be used an aqueous solution of [ 18 F]KF/K 2 CO 3  which is obtained by, for example, irradiating [ 18 O]H 2 O with a proton beam using a cyclotron to thereby generate  18 F −  and reacting  18 F −  with K 2 CO 3 . The [ 18 F] fluorination reaction can be effected by performing heating (at a temperature of, for example, 80-150° C., specifically 100-120° C., more specifically 110° C.) in the presence of an appropriate solvent (e.g., acetonitrile, DMSO, DMF, DMA). The reaction period can be set to be in the range of, for example, 5-60 minutes, specifically 5-20 minutes, and more specifically set to 10 minutes. This reaction can be performed, for example, under pressure by heating in a sealed container. At the fluorination step, an additive like [2,2,2] cryptand (1,10-diaza-4,7,13,16,24,24-hexaoxabicyclo[8.8.8]hexacosane, Kryptofix® [2,2,2]) can be used to improve reaction efficiency. The aforementioned fluorination step can be performed by, for example, the following procedure: an aqueous solution of [ 18 F]KF/K 2 CO 3  and [2,2,2]cryptand are azeotropically dried to give solids, a solution of phosphine serving as a substrate is added to the solids, and then the contents are heated under sealing. 
     The leaving group (X 2 ) used at the aforementioned step is a halogen atom, optionally substituted C 1-6 alkylsulfonyloxy, or optionally substituted phenylsulfonyloxy, and preferred examples include, but are not limited to, bromine atom, iodine atom, p-toluenesulfonyloxy, methanesulfonyloxy, chloromethanesulfonyloxy, and trifluoromethanesulfonyloxy. 
     The second step can be performed by the same procedure as scheme A. The two steps of scheme B can be performed in a one-pot reaction: for example, after completion of the fluorination reaction, an electrophilic reagent is added to the reaction system without posttreatment and the contents are heated under sealing to obtain a product of interest. 
     Purification of a product of interest can be conducted by a common method: for example, purification can be done using preparative HPLC based on radiation intensity. Alternatively, purification can also be done easily using a small disposable column cartridge (Sep-Pak®) or the like. 
     The imaging agent of the present invention can be prepared by dissolving or suspending the positron emitting nuclide-labeled quaternary phosphonium compound into physiological saline or the like. The imaging agent may contain any additives such as pH adjustor, resolvent, dispersant, solubilizer, and radical scavenger (radiolysis inhibitor) depending on the need. 
     The dose of the imaging agent of this invention can be selected, as appropriate, depending on various factors including the organ to be imaged and the body shape of a subject (patient). The imaging agent of this invention can comprise a therapeutically and/or prophylactically effective amount of the compound of formula (I) given above. In this invention, the compound of formula (I) can be generally used at a dose of 0.001 mg/body kg. 
     The imaging agent of the present invention can be used for detection of tumor cells by PET and monitoring of the circulatory system, especially the myocardium. Therefore, the inventive imaging agent can be used for the purposes of: examination of tumors such as brain tumor, head and neck cancer, lung cancer and liver cancer; and diagnosis of ischemic heart diseases such as angina pectoris, myocardial infarction, arteriosclerosis of coronary artery, etc., and myocardial ischemia. 
     EXAMPLES 
     Hereunder, the present invention will be described in more detail by way of working examples, but this invention is not limited to these examples. Additionally, in the Hi-flash silica column chromatography conducted for purification of the compounds synthesized in the working examples given below, the flash chromatography system YFLC-AI-700 and the Hi-flash columns (silica gel) produced by Yamazen Corporation were used. Mass spectroscopy was done using JMS-700 produced by JEOL Ltd. or LCMS-2020 produced by Shimadzu Corporation. High performance liquid chromatography (HPLC) was done using the LC-2000Plus system produced by JASCO Corporation. 
     Synthesis Example 1 
     4-Diphenylphosphanyl Phenol 
     
       
         
         
             
             
         
       
     
     A reaction vessel was charged with 4-iodophenol (5.90 g, 26.9 mmol), potassium acetate (3.17 g, 32.3 mmol), palladium acetate (II) (36.0 mg, 0.6 mol %), and N,N-dimethylacetamide (27 mL), and while the contents were stirred, diphenylphosphine (4.63 mL, 26.9 mmol) was dropwise added slowly. Then, the reaction vessel was heated to 130° C., and reaction was effected for 2 hours. After the reaction, water was added to the reaction system, and the product was extracted three times with CH 2 Cl 2  into the organic layer. The resulting organic layer was washed with saturated saline and dried with MgSO 4 . After concentration under reduced pressure, the concentrate was purified by Hi-flash silica column chromatography (developing solvent, AcOEt:hexane=11:89→32:68) to yield a product (6.74 g (24.2 mmol), 90% yield, white solid). 
       1 H-NMR (600 MHz, CDCl 3 ):δ7.36-7.15 (m, 12H), 6.82 (d, J=7.8 Hz, 2H), 5.01 (s, 1H); 
     LRMS (EI) calculated for C 18 H 15 OP (M t ): 278.1, found: 278.1. 
     Synthesis Example 2 
     Toluene-4-sulfonic acid 2-(4-diphenylphosphanyl-phenoxy)-ethyl ester 
     
       
         
         
             
             
         
       
     
     A reaction vessel was charged with 4-diphenylphosphanyl phenol (434 mg, 1.56 mmol), 1,2-bis(tosyloxy)ethane (2.89 g, 7.80 mmol), potassium carbonate (431 mg, 3.12 mmol), and N,N-dimethylformamide (DMF, 5 mL). The reaction vessel was heated to 50° C., and reaction was effected for 19 hours. After the reaction, water and ethyl acetate were added to the reaction system, unreacted 1,2-bis(tosyloxy)ethane was removed by filtration, and then the product was extracted three times with ethyl acetate into the organic layer. The resulting organic layer was washed with saturated saline and dried with MgSO 4 . After concentration under reduced pressure, the concentrate was separated by Hi-flash silica column chromatography (developing solvent, ethyl acetate:hexane=22:78→43:57) to yield a product (587 mg (1.23 mmol), 79% yield, white solid). 
       1 H-NMR (600 MHz, CDCl 3 ): 67.81 (d, J=7.8 Hz, 2H), 7.34-7.31 (m, 8H), 7.29-7.21 (m, 6H), 6.77 (d, J=7.8 Hz, 2H), 4.38-4.35 (m, 2H), 4.16-4.14 (m, 2H), 2.43 (s, 3H); 
     LRMS (EI) calculated for C 27 H 25 O 4 PS (M + ): 476.1, found: 476.1. 
     Synthesis Example 3 
     2-Diphenylphosphanyl Phenol 
     
       
         
         
             
             
         
       
     
     The same synthesis procedure as in Synthesis Example 1 was performed except that 4-iodophenol was replaced by 2-iodophenol. Separation was done by Hi-flash silica column chromatography (developing solvent, ethyl acetate:hexane=18:82→39:61) to yield a product (53% yield, white solid). 
       1 H-NMR (600 MHz, CDCl 3 ):δ7.38-7.28 (m, 11H), 7.00-6.87 (m, 3H), 6.16 (d, J=6.6, Hz, 1H); 
     LRMS (EI) calculated for C 18 H 15 OP (M + ): 278.1, found: 278.1. 
     Synthesis Example 4 
     Toluene-4-sulfonic acid 2-(2-diphenylphosphanyl-phenoxy)-ethyl ester 
     
       
         
         
             
             
         
       
     
     The same synthesis procedure as in Synthesis Example 2 was performed using compound 5 as a starting material. Separation was done by Hi-flash silica column chromatography (developing solvent, ethyl acetate:hexane=22:78→43:57) to yield a product (36% yield, white solid). 
     LRMS (EI) calculated for C 27 H 25 O 4 PS (M + ): 476.1, found: 476.0. 
     Synthesis Example 5 
     3-Diphenylphosphanyl Phenol 
     
       
         
         
             
             
         
       
     
     The same synthesis procedure as in Synthesis Example 1 was performed except that 4-iodophenol was replaced by 3-iodophenol. Separation was done by Hi-flash silica column chromatography (developing solvent, ethyl acetate:hexane=12:88→33:67) to yield a product (47% yield, white solid). 
       1 H-NMR (600 MHz, CDCl 3 ):δ7.37-7.28 (m, 10H), 7.22 (td, J=7.8, 1.8 Hz, 1H), 6.90 (t, J=7.8 Hz, 1H), 6.81 (dd, J=7.8, 2.4 Hz, 1H), 6.70 (dt, J=7.2, 1.2 Hz, 1H), 4.82 (s, 1H); 
     LRMS (EI) calculated for C 18 H 15 OP (M + ): 278.1, found: 278.1. 
     Synthesis Example 6 
     Toluene-4-sulfonic acid 2-(3-diphenylphosphanyl-phenoxy)-ethyl ester 
     
       
         
         
             
             
         
       
     
     The same synthesis procedure as in Synthesis Example 2 was performed using compound 8 as a starting material. Separation was done by Hi-flash silica column chromatography (developing solvent, ethyl acetate:hexane=22:78→43:57) to yield a product (65% yield, white solid). 
       1 H-NMR (600 MHz, CDCl 3 ): δ7.78 (d, J=7.8 Hz, 2H), 7.37-7.25 (m, 13H), 6.87 (t, J=7.8 Hz, 1H), 6.75 (d, J=7.8 Hz, 1H), 6.72 (d, J=7.8 Hz, 1H), 4.34-4.29 (m, 2H), 4.06-4.03 (m, 2H), 2.42 (s, 3H); 
     LRMS (EI) calculated for C 27 H 25 O 4 PS (M + ): 476.1, found: 476.1. 
     Synthesis Example 7 
     Toluene-4-sulfonic acid 3-phenylpropyl ester 
     
       
         
         
             
             
         
       
     
     A reaction vessel was charged with 3-phenyl-1-propanol (500 pL, 3.67 mmol), p-toluenesulfonylchloride (770 mg, 4.04 mmol), and pyridine (1 mL), and reaction was effected at room temperature for 1 hour. After the reaction, water and hydrochloric acid were added to the reaction system, and the product was extracted three times with ethyl acetate into the organic layer. The resulting organic layer was washed with an aqueous solution of saturated sodium bicarbonate and dried wtih MgSO 4 . After concentration under reduced pressure, the concentrate was purified by Hi-flash silica column chromatography (developing solvent, ethyl acetate:hexane=22:78→43:57) to yield a product (898 mg (3.09 mmol), 84% yield, colorless liquid). 
       1 H-NMR (600 MHz, CDCl 3 ): δ7.79 (d, J=7.8 Hz, 2H), 7.34 (d, J=7.8 Hz, 2H), 7.24 (t, J=7.8 Hz, 2H), 7.17 (t, J=7.8 Hz, 1H), 7.06 (d, J=7.8 Hz, 2H), 4.03 (t, J=6.0 Hz, 2H), 2.65 (t, J=7.2 Hz, 2H), 2.46 (s, 3H), 1.99-1.93 (m, 2H); 
     LRMS (EI) calculated for C i6 H 18 0 3 S (M + ): 290.1, found: 290.1. 
     Synthesis Example 8 
     3-Iodopropylbenzene 
     
       
         
         
             
             
         
       
     
     A reaction vessel was charged with 3-(4-methylphenylsulfonyloxy)propylbenzene (200 mg, 0.688 mmol), sodium iodide (310 mg, 2.07 mmol), and acetone (3.4 mL), and reaction was effected at room temperature for 6 hours. After the reaction, water was added to the reaction system, and the product was extracted three times with ethyl acetate into the organic layer. The resulting organic layer was washed with saturated saline and dried with MgSO 4 . After concentration under reduced pressure, the concentrate was purified by Hi-flash silica column chromatography (developing solvent, ethyl acetate:hexane=0:100→11:89) to yield a product (140 mg (0.569 mmol), 83% yield, orange liquid). 
       1 H-NMR (600 MHz, CDCl 3 ): δ7.29 (t, J=7.8 Hz, 2H), 7.22 (d, J=7.8 Hz, 1H), 7.20 (d, J=7.8 Hz, 2H), 3.18 (t, J=7.2 Hz, 2H), 2.73 (t, J=7.2 Hz, 2H), 2.14 (quint, J=7.2 Hz, 2H); 
     LRMS (EI) calculated for C 9 H 11 I (M + ): 246.0, found: 246.0. 
     As a reference specimen for use in the identification of labeled compounds, an unlabeled compound was synthesized according to the procedure described below. 
     Synthesis Example 9 
     Toluene-4-sulfonic acid 2-fluoroethyl ester 
     
       
         
         
             
             
         
       
     
     A reaction vessel was charged with 2-fluoroethanol (1.00 mL, 16.9 mmol), p-toluenesulfonylchloride (3.85 g, 20.2 mmol), N-methylmorpholine (9.30 mL, 84.5 mmol), and CH 2 Cl 2  (50 mL), and reaction was effected at room temperature for 17 hours. After the reaction, water was added to the reaction system, and the product was extracted three times with ethyl acetate into the organic layer. The resulting organic layer was washed with saturated saline and dried with MgSO 4 . After concentration under reduced pressure, the concentrate was purified by Hi-flash silica column chromatography (developing solvent, ethyl acetate:hexane=19:81→40:60) to yield a product (3.79 g (17.2 mmol), &gt;100% yield, light yellow liquid). 
       1 14-NMR (600 MHz, CDCl 3 ): δ7.81 (d, J=8.4 Hz, 2H), 7.36 (d, J=8.4 Hz, 2H), 4.63-4.60 (m, 1H), 4.55-4.52 (m, 1H), 4.31-4.27 (m, 1H), 4.26-4.23 (m, 1H), 2.46 (s, 3H); 
     LRMS (EI) calculated for C 9 H i1 F0 3 S (M t ): 218.0, found: 218.0. 
     Synthesis Example 10 
     [4-(2-Fluoroethoxy)phenyl]-diphenylphosphine 
     
       
         
         
             
             
         
       
     
     A reaction vessel was charged with 4-diphenylphosphanyl phenol (1.00 g, 3.59 mmol), toluene-4-sulfonic acid 2-fluoroethyl ester (1.02 g, 4.67 mmol), potassium carbonate (992 mg, 7.18 mmol), and DMF (5 mL). The reaction vessel was heated to 50° C., and reaction was effected for 1 day. After the reaction, water was added to the reaction system, and the product was extracted three times with ethyl acetate into the organic layer. The resulting organic layer was washed with saturated saline and dried with MgSO 4 . After concentration under reduced pressure, the concentrate was purified by Hi-flash silica column chromatography (developing solvent, ethyl acetate:hexane=4:96→25:75) to yield a product (921 mg (2.84 mmol), 80% yield, white solid). 
       1 H-NMR (600 MHz, CDCl 3 ): δ7.69-7.43 (m, 1H), 7.36-7.24 (m, 11H), 7.01-6.89 (m, 2H), 4.75 (dt, J=48.0, 4.2 Hz, 2H), 4.21 (dt, J=28.2, 4.2 Hz, 2H); 
     LRMS (EI) calculated for C 20 H 18 FOP (M t ): 324.1, found: 324.1. 
     Synthesis Example 11 
     Benzyl-[4-(2-fluoroethoxy)phenyl]-diphenylphosphonium bromide (TAP-001) 
     
       
         
         
             
             
         
       
     
     A reaction vessel was charged with [4-(2-fluoroethoxy)phenyl]-diphenylphosphine (100 mg, 0.308 mmol), benzyl bromide (366 μL, 3.08 mmol), and MeCN (2 mL). The reaction vessel was heated to 100° C., and the contents were refluxed under heating for 45 minutes. After the reaction, the reaction solution was introduced into an open silica gel column chromatography system to elute unreacted benzyl bromide with ethyl acetate. Then, with the developing solvent being replaced by methanol:CH 2 Cl 2 =25:75, separation was done to yield a product (161 mg (0.326 mmol), 106% yield, white solid). 
       1 H-NMR (600 MHz, CDCl 3 ): δ7.78-7.70 (m, 4H), 7.69-7.59 (m, 8H), 7.25-7.07 (m, 7H), 5.26 (d, J=14.4 Hz, 2H), 4.80 (dt, J=48.0, 4.2 Hz, 2H), 4.35 (dt, J=28.2, 4.2 Hz, 2H); 
     LRMS (FAB) calculated for C 27 H 25 FOP +  ([M—Br] + ): 415.2, found: 415.2 ([M—Br] + ). 
     Likewise, following the same synthesis procedure, the compounds shown below were also synthesized by reacting [4-(2-fluoroethoxy)phenyl]-diphenylphosphine with an electrophile. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Compound 
                 Structure 
                 Yield 
                 Property 
                 MS 
               
               
                   
               
             
            
               
                 TAP- 002 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 38% 
                 White solid 
                 LRMS (FAB) calculated for C 31 H 33 FO 3 P +  ([M—OTs] + ): 503.2, found: 503.2 ([M—OTs] + ) 
               
               
                   
               
               
                 TAP- 003 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 90% 
                 Colorless solid 
                 LRMS (ESI) calculated for C 29 H 27 FO 3 P +  ([M—Br] + ): 473.1, found: 473.1 ([M—Br] + ) 
               
               
                   
               
               
                 TAP- 004 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 99% 
                 White solid 
                 LRMS (ESI) calculated for C 27 H 24 F 2 OP +  ([M—Br] + ): 433.1, found: 433.1 ([M—Br] + ) 
               
               
                   
               
               
                 TAP- 005 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 85% 
                 Colorless solid 
                 LRMS (ESI) calculated for C 27 H 22 FO 4 P +  ([M—Br] + ): 469.1, found: 469.1 ([M—Br] + ) 
               
               
                   
               
               
                 TAP- 006 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 104%  (containing MeOH) 
                 White  solid 
                 LRMS (ESI) calculated for C 27 H 24 ClFOP +  ([M—Br] + ): 449.1, found: 449.1 ([M—Br] + ) 
               
               
                   
               
               
                 TAP- 007 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 90% 
                 White solid 
                 LRMS (ESI) calculated for C 28 H 27 FO 2 P +  ([M—Cl] + ): 445.1, found: 445.1 ([M—Cl] + ) 
               
               
                   
               
               
                 TAP- 008 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 87% 
                 Colorless gum 
                 LRMS (ESI) calculated for C 25 H 29 FOP +  ([M—OTs] + ): 395.1, found: 395.1 ([M—OTs] + )  
               
               
                   
               
               
                 TAP- 010 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 56% 
                 Colorless solid 
                 LRMS (ESI) calculated for C 29 H 29 FOP +  ([M—OTs] + ): 443.1, found: 443.1 ([M—OTs] + )  
               
               
                   
               
               
                 TAP- 011 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 87% 
                 Colorless solid 
                 LRMS (ESI) calculated for C 31 H 33 FOP +  ([M—Br] + ): 471.2, found: 471.2 ([M—Br] + )  
               
               
                   
               
               
                 TAP- 012 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 85% 
                 Colorless solid 
                 LRMS (ESI) calculated for C 31 H 33 FOP +  ([M—Br] + ): 433.1, found: 433.1 ([M—Br + ) 
               
               
                   
               
               
                 TAP- 013 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 89% 
                 Colorless solid 
                 LRMS (ESI) calculated for C 31 H 33 FOP +  ([M—Br] + ): 515.1, found: 515.1 ([M—Br] + ) 
               
               
                   
               
               
                 TAP- 014 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 89% 
                 Colorless solid 
                 LRMS (ESI) calculated for C 31 H 33 FOP +  ([M—Br] + ): 429.1, found: 429.1 ([M—Br] + ) 
               
               
                   
               
               
                 TAP- 015 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 88% 
                 White solid  
                 LRMS (ESI) calculated for C 31 H 33 FOP +  ([M—Br] + ): 440.1, found: 440.1 ([M—Br] + ) 
               
               
                   
               
               
                 TAP- 016 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 77% 
                 Colorless solid 
                 LRMS (ESI) calculated for C 31 H 33 FOP +  ([M—I] + ): 339.1, found: 339.0 ([M—I] + ) 
               
               
                   
               
               
                 TAP- 017 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 85% 
                 Colorless solid 
                 LRMS (FAB) calculated for C 23 H 23 FOP +  ([M—Br] + ): 365.1, found: 365.2 ([M—Br] + ) 
               
               
                   
               
            
           
         
       
     
     Synthesis Example 12 
     [2-(2-Fluoroethoxy)phenyl]-diphenylphosphine 
     
       
         
         
             
             
         
       
     
     The same synthesis procedure as in Synthesis Example 10 was performed using 2-diphenylphosphanyl phenol as a starting material. Separation was done by Hi-flash silica column chromatography (developing solvent, ethyl acetate:hexane=3:97→21:79) to yield a product (76% yield, white solid). 
       1 H-NMR (600 MHz, CDCl 3 ): δ7.36-7.29 (m, 11H), 6.91-6.87 (m, 2H), 6.74-6.70 (m, 1H), 4.40 (dt, J=47.4, 4.8 Hz, 2H), 4.11 (dt, J=25.8, 4.8 Hz, 2H); 
     LRMS (EI) calculated for C 20 H 18 FOP (M 1 ): 324.1, found: 324.1. 
     Synthesis Example 13 
     Benzyl-[2-(2-fluoroethoxy)phenyl]-diphenylphosphonium bromide (TAP-028) 
     
       
         
         
             
             
         
       
     
     The same synthesis procedure as in Synthesis Example 11 was performed using [2-(2-fluoroethoxy)phenyl]-diphenylphosphine as a starting material. Unreacted benzyl bromide was eluted with ethyl acetate, and then, with the developing solvent being replaced by methanol:CH 2 Cl 2 =25:75, separation was done to yield a product (70% yield, white solid). 
       1 H-NMR (600 MHz, CDCl 3 ): δ7.90-7.83 (m, 1H), 7.80-7.73 (m, 2H), 7.66-7.58 (m, 4H), 7.55-7.46 (m, 5H), 7.35-7.25 (m, 2H), 7.23-7.16 (m, 3H), 7.01 (d, J=6.6 Hz, 1H), 5.03 (d, J=15.0 Hz, 2H), 4.51 (s, 2H), 4.48-4.46 (m, 1H), 4.44-4.42 (m, 1H); 
     LRMS (FAB) calculated for C 27 H 25 FOP +  ([M—Br] + ): 415.2, found: 415.2 ([M—Br] + ). 
     Synthesis Example 14 
     [3-(2-Fluoroethoxy)phenyl]-diphenylphosphine 
     
       
         
         
             
             
         
       
     
     The same synthesis procedure as in Synthesis Example 10 was performed using 3-diphenylphosphanyl phenol as a starting material. Separation was done by Hi-flash silica column chromatography (developing solvent, ethyl acetate:hexane=4:96→25:75) to yield a product (91% yield, white solid). 
       1 H-NMR (600 MHz, CDCl 3 ): δ7.41-7.24 (m, 11H), 6.94-6.84 (m, 3H), 4.77-4.71 (m, 1H), 4.69-4.63 (m, 1H), 4.24-4.08 (m, 2H); 
     LRMS (EI) calculated for C 20 H 18 FOP (M + ): 324.1, found: 324.1. 
     Synthesis Example 15 
     Benzyl-[3-(2-fluoroethoxy)phenyl]-diphenylphosphonium bromide (TAP-018) 
     
       
         
         
             
             
         
       
     
     The same synthesis procedure as in Synthesis Example 11 was performed using [3-(2-fluoroethoxy)phenyl]-diphenylphosphine as a starting material. Unreacted benzyl bromide was eluted with ethyl acetate, and then, with the developing solvent being replaced by methanol:CH 2 Cl 2 =20:80, separation was done to yield a product (88% yield, white solid). 
       1 H-NMR (600 MHz, CDCl 3 ): δ7.80-7.74 (m, 3H), 7.70-7.64 (m, 4H), 7.63-7.58 (m, 4H), 7.50-7.46 (m, 1H), 7.35-7.31 (m, 1H), 7.24-7.20 (m, 1H), 7.16-7.09 (m, 5H), 5.49 (d, J =14.4 Hz, 2H), 4.84-4.80 (m, 1H), 4.76-4.72 (m, 1H), 4.57-4.54 (m, 1H), 4.52-4.49 (m, 1H); 
     LRMS (FAB) calculated for C 27 H 25 FOP +  ([M—Br] + ): 415.2, found: 415.2 ([M—Br] + ). 
     Synthesis Example 16 
     [3-(2-Fluoroethoxy)phenyl]-(4-methoxycarbonylphenyl)methyl-diphenylphosphonium bromide (TAP-020) 
     
       
         
         
             
             
         
       
     
     A reaction vessel was charged with [3-(2-fluoroethoxy)phenyl]-diphenylphosphine (63.8 mg, 0.197 mmol), 4-(bromomethyl)benzoic acid methyl ester (54.8 mg, 0.239 mmol), and acetonitrile (2 mL). The reaction vessel was heated to 90° C., and the contents were refluxed under heating for 4 hours. After the reaction, the reaction solution was introduced into an open silica gel column chromatography system to elute an unreacted compound with ethyl acetate. Then, with the developing solvent being replaced by methanol:CH 2 Cl 2 =20:80, separation was done to yield a product (102 mg (0.184 mmol), 93% yield, white solid). 
       1 H-NMR (600 MHz, CDCl 3 ): δ7.85 (d, J=14.4 Hz, H), 7.79-7.74 (m, 4H), 7.70 (d, J=14.4 Hz, 2H), 7.68 (d, J=14.4 Hz, 2H), 7.63-7.58 (m, 4H), 7.46 (td, J=7.8, 4.2 Hz, 1H), 7.32 (d, J=7.8 Hz, 1H), 7.23-7.20 (m, 2H), 7.11 (dd, J=13.2, 7.2 Hz, 1H), 5.71 (d, J=15.0 Hz, 2H), 4.77 (dt, J=47.4, 3.6 Hz, 2H), 4.54 (dt, J=28.8, 3.6 Hz, 2H), 3.86 (s, 3H); 
     LRMS (FAB) calculated for C 29 H 27 FO 3 P +  ([M—Br]): 473.2, found: 473.2 ([M—Br]). 
     Synthesis Example 17 
     [3-(2-Fluoroethoxy)phenyl]-(4-methoxyphenyl)methyl-diphenylphosphonium bromide (TAP-019) 
     
       
         
         
             
             
         
       
     
     A reaction vessel was charged with 4-methoxybenzylchloride (431 μL, 3.19 mmol), sodium iodide (1.44 g, 9.58 mmol), and acetone (16 mL), and reaction was effected at room temperature for 18 hours. After the reaction, water was added to the reaction system, and the product was extracted three times with ethyl acetate into the organic layer. The resulting organic layer was washed with saturated saline and dried with MgSO 4 . After concentration under reduced pressure, the concentrate was purified by Hi-flash silica column chromatography (developing solvent, ethyl acetate:hexane=1:99→20:80) to yield 4-methoxybenzyl iodide (527 mg (2.12 mmol), 67% yield, yellow liquid). 
     Subsequently, the reaction vessel was charged with the resulting 4-methoxybenzyl iodide (57.3 mg, 0.231 mmol), [3-(2-fluoroethoxy)phenyl]-diphenylphosphine (50.0 mg, 0.154 mmol), and acetonitrile (2 mL). The reaction vessel was heated to 100° C., and the contents were refluxed under heating for 25 minutes. After the reaction, the reaction solution was introduced into an open silica gel column chromatography system to elute an unreacted compound with ethyl acetate. Then, with the developing solvent being replaced by methanol:CH 2 Cl 2 =10:90, separation was done to yield a product (76.3 mg (0.133 mmol), 86.6% yield, light yellow solid). 
       1 H-NMR (600 MHz, CDCl 3 ): δ7.79 (t, J=7.2 Hz, 2H), 7.70-7.61 (m, 8H), 7.52 (td, J=8.4, 4.2 Hz, 1H), 7.46 (d, J=13.2 Hz, 1H), 7.63-7.58 (m, 4H), 7.46 (td, J=7.8, 4.2 Hz, 1H), 7.32 (d, J=9.6 Hz, 1H), 7.14 (dd, J=12.3, 7.2 Hz, 1H), 7.02 (d, J=7.2 Hz, 1H), 6.67 (d, J=8.4 Hz, 2H), 5.22 (d, J=13.8 Hz, 2H), 4.77 (dt, J=46.5, 3.6 Hz, 2H), 4.46 (dt, J=30.0, 3.6 Hz, 2H), 3.73 (s, 3H); 
     LRMS (FAB) calculated for C 28 H 27 FIO 2 P +  ([M—I] + ): 445.2, found: 445.2 ([M—I] +).    
     Synthesis Example 18 
     [3-(2-Fluoroethoxy)phenyl]-diphenyl-(3-phenylpropyl)phosphonium bromide (TAP-022) 
     
       
         
         
             
             
         
       
     
     A reaction vessel was charged with [3-(2-fluoroethoxy)phenyl]-diphenylphosphine (50.0 mg, 0.154 mmol), 3-iodopropylbenzene (76.0 mg, 0.308 mmol), and acetonitrile (1 mL). The reaction vessel was heated to 100° C., and the contents were refluxed under heating for 3.5 hours. After the reaction, the reaction solution was introduced into an open silica gel column chromatography system to elute an unreacted compound with ethyl acetate. Then, with the developing solvent being replaced by methanol:CH 2 Cl 2 =20:80, separation was done to yield a product (73.5 mg (0.145 mmol), 94% yield, light yellow solid). 
       1 H-NMR (600 MHz, CDCl 3 ): δ7.81-7.77 (m, 2H), 7.74-7.69 (m, 4H), 7.68-7.64 (m, 4H), 7.54 (td, J=7.8, 4.2 Hz, 1H), 7.50-7.46 (m, 1H), 7.34-7.32 (m, 1H), 7.29-7.25 (m, 2H), 7.22-7.16 (m, 4H), 4.79 (dt, J=36.6, 4.2 Hz, 2H), 4.48 (dt, J=28.8, 4.2 Hz, 2H), 3.89-3.83 (m, 2H), 3.05 (t, J=7.2 Hz, 2H), 2.01-1.93 (m, 2H); 
     LRMS (FAB) calculated for C 29 H 29 FIOP +  ([M—I] + ): 443.2, found: 443.2 ([M—I] + ). Likewise, following the same synthesis procedure, the compounds shown below were synthesized by reacting [3-(2-fluoroethoxy)phenyl]-diphenylphosphine, [3-(3-fluoropropoxy)phenyl]-diphenylphosphine, or [3-(4-fluorobutoxy)phenyl]-diphenylphosphine with an electrophile. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Compound 
                 Structure 
                 Yield 
                 Property 
                 MS 
               
               
                   
               
             
            
               
                 TAP- 021 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 92% 
                 White solid  
                 LRMS (ESI) calculated for C 27 H 24 F 2 OP +  ([M—Br] + ): 433.1, found: 433.0 ([M—Br] + ) 
               
               
                   
               
               
                 TAP- 023 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 90% 
                 Colorless solid  
                 LRMS (ESI) calculated for C 27 H 24 ClFOP +  ([M—Br] + ): 449.1, found: 449.0 ([M—Br] + ) 
               
               
                   
               
               
                 TAP- 024 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 55% 
                 White solid 
                 LRMS (ESI) calculated for C 27 H 22 F 4 OP +  ([M—Br] + ): 469.1, found: 469.1 ([M—Br] + ) 
               
               
                   
               
               
                 TAP- 025 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 91% 
                 White solid 
                 LRMS (FAB) calculated for C 27 H 22 F 4 OP +  ([M—Br] + ): 429.1, found: 429.2 ([M—Br] + ) 
               
               
                   
               
               
                 TAP- 026 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 97% 
                 White solid 
                 LRMS (FAB) calculated for C 26 H 29 FOP +  ([M—Br] + ): 443.1, found: 443.2 ([M—Br] + ) 
               
               
                   
               
               
                 TAP- 027 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 71% 
                 White solid 
                 LRMS (FAB) calculated for C 21 H 21 FOP +  ([M—I] + ) 339.1, found: 339.2 ([M—I] + ) 
               
               
                   
               
            
           
         
       
     
     Procedure for  18 F-Labeling Reaction 
     A 12 MeV proton beam accelerated by the cyclotron Cypris HM12 (produced by Sumitomo Heavy Industries, Ltd.) was irradiated to [ 18 O]H 2 O with an isotropic purity of 97% or higher (produced by Taiyo Nippon Sanso Corporation) for 40 minutes, thereby effecting an  18 O(p,n) 18 F nuclear reaction to synthesize  18 F − . Then, the solution was passed through a cation exchange resin (AG1-X8) to trap  18 F −  on the resin. The trapped  18 F −  was eluted with a 33 mM solution of K 2 CO 3  to give an aqueous solution of [ 18 F]KF/K 2 CO 3 . An appropriate amount of radioactivity (55-65 mCi) of the aqueous solution was taken in a volume of about 150-250 μL, and added to a reaction vial containing 800 μL of a Kryptfix® 2.2.2. MeCN solution (20 mg/mL), and the contents were azeotropically dried under stream of helium in an oil bath at 100° C. Then, 600 μL each of the prepared different MeCN solutions of labeling precursor {3 mg (6.30 μmol)/600 μL} was added to the reaction vial, and an  18 F fluorination reaction was performed at 100° C. for 10 minutes. Next, to take the second step of phosphonium compound formation, 315 μmol (50 eq. relative to precursor) of the corresponding bromide (alkylating agent) was added to the vial, and a phosphonium compound formation reaction was performed at 100° C. for an appropriate time for each case. After the reaction, the vial was lifted from the oil bath, and 5 mL of water was added to the vial. The solution contained in the vial was introduced into a Sep-Pak® tC18 cartridge which had been activated in advance by introducing 5 mL of ethanol and water (10 mL), and then the Sep-Pak® tC18 cartridge was washed by introducing water (10 mL). After air was introduced to remove water in the cartridge, the solution containing an [ 18 F]-labeled compound was eluted from the cartridge using ethanol (5 mL). The eluate was diluted with water (1 mL) and the resulting solution was introduced into a preparative HPLC column to effect separation and purification (preparative column: Inertsil® ODS-4, 10 mm×250 mm; mobile phase: PBS:MeCN=52:48; flow rate: 5.0 mL/min.; detection UV wavelength: 254 nm). The desired radioactive fraction was fractionated and diluted with pure water (20 mL). The fraction solution was introduced into a Sep-Pak® tC18 cartridge which had been activated in advance by introducing ethanol (5 mL) and water (10 mL), and then the Sep-Pak® tC18 cartridge was washed by introducing water (10 mL). After air was introduced to remove water in the cartridge, the [ 18 F]-labeled compound was eluted from the cartridge into a vial using ethanol (5 mL). The eluate was azeotropically dried under stream of helium in an oil bath at 100° C. Thereafter, an appropriate amount of physiological saline was added to the reaction vial, and the resulting solution was used as a chemical solution for use in biological analysis. 
     The compounds of working examples as shown below were synthesized by the same procedure as mentioned above. 
     Example 1 
     Benzyl-[4-(2-[ 18 F]fluoroethoxy)phenyl]-diphenylphosphonium bromide ([ 18 F]TAP-001) 
     
       
         
         
             
             
         
       
     
     The total label synthesis time taken until the completion of preparing the formulation was about 1.5 hours, the radiochemical yield was 43±18% (corrected for attenuation), and the radiochemical purity was at least 98±1.8%.  FIG. 1  shows the HPLC results obtained during purification. 
     HPLC conditions: column: Inertsil ODS-4 (4.6x150 mm, particle size 5 μm); mobile phase: MeCN/PBS=50/50; UV wavelength: 254 nm; flow rate: 1.5 mL/min. 
     Retention time: 4.8 min. 
     Example 2 
     Benzyl[2-(2-[ 18 F]fluoroethoxy)phenyl]-diphenylphosphonium bromide ([ 18 F]TAP-028) 
     
       
         
         
             
             
         
       
     
     The total label synthesis time taken until the completion of preparing the formulation was about 1.5 hours, the radiochemical yield was 12±9.9% (corrected for attenuation), and the radiochemical purity was at least 92±5.8%. 
     HPLC conditions: column: InertSustain C-18 (4.6x150 mm, particle size 5 μm); mobile phase: MeCN/PBS=50/50; UV wavelength: 254 nm; flow rate: 1.5 mL/min. 
     Retention time: 4.4 min. 
     Example 3 
     Benzyl[3-(2-[ 18 F]fluoroethoxy)phenyl]-diphenylphosphonium bromide ([ 18 F]TAP-003) 
     
       
         
         
             
             
         
       
     
     The total label synthesis time taken until the completion of preparing the formulation was about 1.5 hours, the radiochemical yield was 36±8.4% (corrected for attenuation), and the radiochemical purity was at least 99±0.1%. 
     HPLC conditions: column: InertSustain C-18 (4.6×150 mm, particle size 5 μm); mobile phase: MeCN/PBS=50/50; UV wavelength: 254 nm; flow rate: 1.5 mL/min. 
     Retention time: 4.4 min. 
     Example 4 
     [3-(2-[ 18 F]fluoroethoxy)phenyl]-(4-methoxycarbonylphenyl)methyl-diphenylphosphonium bromide ([ 18 F]TAP-020) 
     
       
         
         
             
             
         
       
     
     In the process of the synthesis of the labeled compound [ 18 F]TAP-004, 4-(bromomethyl)benzoic acid methyl ester used as a reagent for phosphonium compound formation is solid; therefore, after the first step of fluorination reaction, acetonitrile was once distilled away and then a solution of the reagent in acetonitrile (72 mg (50 eq.)/600 μL) was added again, and thereafter the second reaction step was performed. The other synthesis operations were done by the same procedure as the aforementioned procedure for  18 F-labeling reaction. The total label synthesis time taken until the completion of preparing the formulation was about 1.5 hours, the radiochemical yield was 32% (corrected for attenuation), and the radiochemical purity was at least 95%. 
     HPLC conditions: column: InertSustain C-18 (4.6×150 mm, particle size 5 μm); mobile phase: MeCN/PBS=50/50; UV wavelength: 254 nm; flow rate: 1.0 mL/min. 
     Retention time: 5.5 min. 
     Example 5 
     [3-(2- [ 18 F]fluoroethoxy)phenyl]-(4-methoxyphenyl)methyl-diphenylphosphonium bromide ([ 18 F]TAP-019) 
     
       
         
         
             
             
         
       
     
     The synthesis operations were performed by the same procedure as the aforementioned procedure for  18 F-labeling reaction, except that (4-methoxyphenyl)methylchloride was used as an alkylating agent and the reaction temperature was set to 130° C. The total label synthesis time taken until the completion of preparing the formulation was about 1.5 hours, the radiochemical yield was 42% (corrected for attenuation), and the radiochemical purity was at least 99%. 
     HPLC conditions: column: InertSustain C-18 (4.6×150 mm, particle size 5 μm); mobile phase: MeCN/PBS=50/50; UV wavelength: 254 nm; flow rate: 1.0 mL/min. 
     Retention time: 6.8 min. 
     Example 6 
     [ 18 F][3-(2-fluoroethoxy)phenyl]-(3-phenylpropyl)-diphenylphosphonium bromide ([18F]TAP-022) 
     
       
         
         
             
             
         
       
     
     In the process of the synthesis of the labeled compound [ 18 F]TAP-006, 3-iodopropylbenzene was used as an alkylating agent. Since 3-iodopropylbenzene is solid, after the first step of fluorination reaction, acetonitrile was once distilled away and then a solution of the reagent in acetonitrile (78 mg (50 eq.)/600 μL) was added again, and thereafter the second reaction step was performed. The other synthesis operations were done by the same procedure as the aforementioned procedure for  18 F-labeling reaction. The total label synthesis time taken until the completion of preparing the formulation was about 1.5 hours, the radiochemical yield was 21% (corrected for attenuation), and the radiochemical purity was at least 99%. 
     HPLC conditions: column: InertSustain C-18 (4.6×150 mm, particle size 5 μm); mobile phase: MeCN/PBS=50/50; UV wavelength: 254 nm; flow rate: 1.5 mL/min. 
     Retention time: 6.8 min. 
     Likewise, the compounds shown below were also synthesized according to the same synthesis procedure. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                   
                   
                   
                 Radiochemical yield (%)/ 
               
               
                   
                 Structural 
                 Electro- 
                 HPLC retention time 
               
               
                 Compound 
                 formula 
                 phile 
                 (HPLC conditions) 
               
               
                   
               
             
            
               
                 [ 18 F]TAP- 002 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 12% 5.8 min. (column: Inertsil ODS-4 (4.6 × 150 mm, particle size: 5 μm); mobile phase: MeCN/PBS = 50/50, UV wavelength: 254 nm; flow rate: 1.5 mL/min.) 
               
               
                   
               
               
                 [ 18 F]TAP- 004  
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 49% 6.1 min. (column: Inertsil ODS-4 (4.6 × 150 mm, particle size: 5 μm); mobile phase: MeCN/PBS = 50/50; UV wavelength: 254 nm; flow rate: 1.5 mL/min.) 
               
               
                   
               
               
                 [ 18 F]TAP- 008  
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 22% 4.9 min. (column: InertSustain C-18 (4.6 × 150 mm, particle size: 5 μm); mobile phase: MeCN/PBS = 50/50; UV wavelength: 254 nm; flow rate: 1.5 mL/min.) 
               
               
                   
               
               
                 [ 18 F]TAP- 024  
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 31% 5.2 min. (column: InertSustain C-18 (4.6 × 150 mm, particle size: 5 μm); mobile phase: MeCN/PBS = 50/50; UV wavelength: 254 nm; flow rate: 1.5 mL/min.) 
               
               
                   
               
            
           
         
       
     
     Example 16 
     Analysis of Phosphonium Compound Formation Reaction Time 
     The reaction according to the scheme shown below was performed by following the aforementioned procedure for  18 F-labeling reaction. In this process, the reaction was performed using varied times of phosphonium compound formation reaction, whereby the resulting radiochemical yields for the different cases were compared. The results are shown in the table given below. Among the cases where the reaction time at the second step was set to 20, 10 or 5 min., the highest radiochemical yield was obtained for the case of setting the reaction time to 5 min. 
     
       
         
         
             
             
         
       
     
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                 Reaction time at 
                 Radiochemical 
               
               
                   
                 2nd step (X) 
                 yield (%) 
               
               
                   
                   
               
             
            
               
                   
                 20 min (n = 4) 
                 36 ± 8.4 
               
               
                   
                 10 min (n = 3) 
                 39 ± 9.3 
               
               
                   
                  5 min (n = 13) 
                 47 ± 7.4 
               
               
                   
                   
               
            
           
         
       
     
     Test Example 1 
     Mouse PET/CT Imaging 
     The animals used in this measurement were 6-10-week-old male Slc:ICR mice (from Japan SLC, Inc.). Each of different physiological saline solutions of the [ 18 F]-labeled compounds (200 μL, 1.94-14.7 MBq) used in this measurement was administered via the caudal vein to the awake mice, and the mice were left to settle for 50 minutes. Then, the mice were fixed under isoflurane anesthesia (2%, 1.5 mL/min.), and after 60 minutes of the administration, the whole bodies of the mice were imaged by small-animal PET (ClairvivoPET produced by Shimadzu Corporation) for 10 minutes. Subsequently, while the mice were kept fixed, their whole bodies were imaged by small-animal CT (ClairvivoCT produced by Shimadzu Corporation). The PET data were reconstructed by 3D-DRAMA, and the pixel values of the obtained PET images were converted to SUV to generate SUV images. These images were fused with CT images to produce fused images. The results are shown in  FIG. 2 . 
     The accumulation of radioactivity in the myocardium was observed in all the cases of using [ 18 F]TAP-001, −018, −019, and −022 —this confirmed that these compounds are usable for myocardial imaging. In particular, high-contrast myocardial images were obtained in the cases of using [ 18 F]TAP-018, −019, and −022. 
     Test Example 2 
     Mouse Biodistribution Study 
     The animals used in this measurement were 6-8-week-old male Slc:ICR mice (from Japan SLC, Inc.) (n=4). Each of different physiological saline solutions of the [ 18 F]-labeled compounds (200 μL, 370-740 kBq) used in this measurement was administered to the awake mice via the caudal vein, and after 60 or 120 minutes of the administration, the mice were euthanized by cervical dislocation under isoflurane anesthesia. After blood was sampled from the heart immediately, the heart, lung, liver, spleen, kidney, small intestine, thigh muscle, thigh bone, and brain were excised and measured for amount of radioactivity by a γ counter. The weights of the excised organs were measured, and on that basis, the accumulation rates per gram of the organs with respect to dose (% dose/g of organ, % ID/g) were calculated as indicators for accumulation of the chemical compounds used (n=4). The results are shown in  FIG. 3 . 
     The accumulation of labeled compound in the myocardium was observed in all the cases of using [ 18 F]TAP-001, −018, −019, and −022 —this confirmed that the radioactivity more highly accumulates in the myocardium than in other adjacent organs. In particular, the ratio of radioactivity accumulations in the myocardium and liver (heart/liver ratio)—those accumulations were considered most likely to affect imaging—was calculated and found to be 2.70 for [ 18 F]TAP-001, 9.40 for [ 18 F]TAP-018, 4.56 for [ 18 F]TAP-019, or 4.98 for [ 18 F]TAP-022, at the time point of 60 minutes. This demonstrated that all the compounds accumulate in a heart-specific manner. 
     Test Example 3 
     Rat Biodistribution Study 
     The biodistribution study of [ 18 F]TAP-018 in rats was conducted by the same procedure as in Test Example 2 using 6-week-old male Wistar rats (from Japan SLC, Inc.) (n=4). The results are summarized in a bar graph ( FIG. 4 ). [ 18 F]TAP-018 highly accumulated in the heart, and the heart/liver ratio was found to be about 15.5 at the time point of 60 minutes. This value is about ten times higher than the value of 1.5 for [ 18 F]FBnTP which has been used as a gold standard, and is sufficiently higher than the value of 9.2 for [ 18 F]FTPP which is currently under clinical study. This demonstrates that [ 18 F]TAP-018 is useful as a myocardial perfusion imaging agent. 
     Test Example 4 
     Small-Animal PET Study in Rats 
     The myocardial imaging study of [ 18 F]TAP-018 in rats was conducted using 6-week-old male Wistar rats. 200 μL (7.83-9.66 MBq) of a physiological saline solution of the test compound [ 18 F]TAP-018 was administered via the caudal vein to the rats fixed under isoflurane anesthesia (2%, 1.5 mL/min.), and then the whole bodies of the rats were imaged by small-animal PET at time points from 0 to 120 minutes after the administration. Subsequently, while the rats were kept fixed, their whole bodies were imaged by small-animal CT. The PET data were reconstructed by 3D-DRAMA, and the pixel values of the obtained PET images were converted to SUV to generate SUV images. These images were fused with CT images to produce fused images. Further, the locations of the myocardium, lung and liver were identified based on the fused images, and regions of interest (ROI) were drawn to calculate respective SUVavg values.  FIG. 5A  shows the PET images taken until 60 minutes after the administration.  FIG. 5B  shows the radioactivity-time curves in the heart, liver and lung. As evident, radioactivity disappeared rapidly from the liver until 10 minutes after the administration, whereas a high level of radioactivity accumulated in the heart immediately after the administration. These results of PET imaging demonstrated that [ 18 F]TAP-018 is useful as a myocardial perfusion agent.