WO2010135180A1

WO2010135180A1 - Catalysis of fluorine addition to double bonds

Info

Publication number: WO2010135180A1
Application number: PCT/US2010/034865
Authority: WO
Inventors: Cameron J. Koch; Alexander V. Kachur; William R. Dolbier
Original assignee: The Trustees Of The University Of Pennsylvania; University Of Florida Research Foundation, Inc.
Priority date: 2009-05-19
Filing date: 2010-05-14
Publication date: 2010-11-25

Abstract

Catalyzed methods for adding fluorine, F₂, across carbon-carbon double bonds are disclosed. Use of these methods for labeling by radioactive isotope ¹⁸F and methods for improving yields of novel isotopically enriched ¹⁸F-compounds are provided.

Description

CATALYSIS OF FLUORINE ADDITION TO DOUBLE BONDS

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This patent application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/179,492, filed May 19, 2009, which is incorporated by reference in its entirety

TECHNICAL FIELD

[0002] The present invention is directed to improved methods for selectively incorporating enriched radioactive isotope ¹⁸F into organic molecules.

BACKGROUND

[0003] One of the most important goals in oncology is the identification and elimination of treatment resistant cells; hypoxic cells are the most familiar examples of this type of cell. See, Kennedy, et al., Biochem. Pharm. 1980, 29, 1 ; Moulder, et al., Int. J. Radioat. Oncol. Biol. Phys. 1984, 10, 695; Adams, Cancer, 1981 , 48, 696, all of which are incorporated herein by reference in their entirety. Hypoxic cells are seldom found in normal tissues, and are generally found only in conjunction with certain tumors, vascular diseases, wounded tissue, or after a stroke.

[0004] Nitroheterocyclic drugs have been under extensive investigation as hypoxia markers. For further discussion as to the reason for this, see U.S. Patent Nos. 7,230,115 and 7,432,295 both of which are incorporated herein by reference in their entirety. [0005] In particular, Misonidazole (MISO) 3-methoxy-1 -(2-nitroimidazol-1 -yl)-2- propanol, and certain of its derivatives have been under extensive investigation as indicators of hypoxia in mammalian tissue.

Chapman et al., Int. J. Radiat. Oncol. Biol. Phys., 1989, 16, 911 ; Taylor, et al., Cancer Res., 1978, 38, 2745; Varghese, et al., Cancer Res., 1980, 40, 2165. The ability of certain misonidazole derivatives to form adducts with cellular macromolecules, referred to as binding throughout this application, has formed the basis of various detection methods.

[0006] For example, ³H or ¹⁴C labeled misonidazole has been used in vitro and in vivo, with binding analyzed by liquid scintillation counting or autoradiography. Chapman, 1984 supra; Urtasun, 1986, supra; Franko, et al., Cancer Res., 1987, 47, 5367. A monofluorinated derivative of misonidazole has utilized the positron emitting isotope ¹⁸F for imaging bound drug in vivo, Rasey, et al, Radiat. Res., 1987, 111 , 292. The method of the preparation of the PET derivative of ethanidazole was described in Tewson T. J., Nuclear Medicine & Biology, 1997 24(8):755-60. An iodine isotope has been incorporated into another azomycin derivative, azomycin arabinoside, allowing radiology techniques of detection Parliament, et al., Br. J. Cancer, 1992, 65, 90. [0007] A hexafluorinated derivative of misonidazole 1 -(2-hydroxy-3-hexafluoro- isopropoxy-propyl)-2-nitroimidazole has been assayed directly (no radioactive isotopes) via nuclear magnetic resonance spectroscopy (NMR or MRI) techniques. Raleigh, et al., Int. J. Radiat. Oncol. Biol. Phys. 1984, 10, 1337.

[0008] Polyclonal antibodies to this same derivative have allowed immunolistochemical identification of drug adducts. Raleigh, et al., Br. J. Cancer, 1987, 56, 395. Unfortunately, the hexafluorinated misonidazole derivative described above had a high degree of insolubility.

[0009] Incorporation of ¹⁸F into 2-nithmidazole compounds provides an opportunity to use these agents for the detection of hypoxia by positron emission tomography (PET). See, Jerabek, et al., Applied Radiation & Isotopes, 1986 37 (7), 599-605; see, Mathias et al., "Radiolabeled hypoxic cell sensitizers: tracers for assessment of ischemia," Life Sciences, 1987 41 (2), 199-206. Radioactive tracers, generally, and ¹⁸F short-lived isotopes (half-life of ¹⁸F is 109.771 minutes), in particular, linked to chemical compounds which permit specific physiological processes to be scrutinized. They can be given by injection, inhalation or orally.

[0010] Positron Emission Tomography (PET) is a precise and sophisticated, noninvasive technique using isotopes produced in a cyclotron. A positron-emitting radionuclide is introduced, usually by injection, and accumulates in the target tissue. As it decays it emits a positron, which promptly combines with a nearby electron resulting in the simultaneous emission of two identifiable gamma rays in opposite directions. These are detected by a PET camera and give very precise indication of their origin.

[0011] Several groups have developed ¹⁸F-labeled nitroimidazole-based PET assays. The first described and most investigated compound of this type is [¹⁸F]- fluoromisonidazole:

See, Rasey et al., Radiation Research, 1987 111 , (2), 292-304; Rasey et al. Int'l J. of Rad. One, Bio., Phys., 1996 36(2), 417-428; Gherson, Journal of Nuclear Medicine, 1989 30 (3), 343-50; Koh, et al., International Journal of Radiation Oncology, Biology, Physics, 1992 22 (1 ), 199-212; [¹⁸F]-fluoroerythronitroimidazole, See, Yang, et al., Radiology, 1995 194 (3), 795-800; and, [¹⁸F]-fluoroetanidazole, See, Tewson, Nuclear Medicine & Biology, 1997 24(8), 755-60.

[0012] [¹⁸F]-Fluoromisonidazole has been studied in several anatomic sites in humans including gliomas, see, VaIk, et al. Journal of Nuclear Medicine, 1992 33 (12), 2133-7; lung cancer, see, Koh, et al., Acta Oncologica, 1994 33 (3), 323-7; and nasopharyngeal carcinoma, see, Yeh, et al., European Journal of Nuclear Medicine, 1996 23 (10), 1378-83. However, despite the extensive investigations, none of these currently developed compounds is accepted clinically as a PET marker of hypoxia. For example, it has been shown that [¹⁸F]-fluoromisonidazole is not stable in vivo, and produces multiple radioactive products distinct from the parent drug following renal clearance. See, Rasey, et al., Journal of Nuclear Medicine, 1999 40(6), 1072-9. Our goal, therefore, has been to employ all the other beneficial aspects of hypoxia detection by EF5, including high drug stability in vivo, ability to cross blood-brain barrier, etc., with non-invasive detection of ¹⁸F incorporated into its molecular structure.

[0013] We have focused our previous study on several 2-nitroimidazoles which has superior properties to misonidazole for the purpose of hypoxia detection. These drugs are 2-(2-nitro-1 H-imidazol-1 -yl)-N-(2,2,3,3,3-pentafluoropropyl)acetamide (hereinafter referred to as EF5), and 2-(2-nitro-1 H-imidazol-1 -yl)-N-(3,3,3-trifluoropropyl)acetamide (hereinafter referred to as EF3), see, U.S. Pat. No. 5,540,908, issued to Koch et al., the disclosure of which is herein incorporated by reference in its entity, as well as (N-(3- fluoropropyl)-2-(2-nitroimidazol-1 [H]-yl))-acetamide (EF1 ), see U.S. Ser. No. 09/123,300, also incorporated herein by reference and assigned to the same entity.

[0014] More recently, [¹⁸F]-EFI compounds have been developed as PET hypoxia markers. This compound was synthesized using nucleophilic substitution of the bromine atom of a precursor-2-(2-nitroimidazol-1 [H]-yl)-N-(3-bromopropyl)-acetamide by [¹⁸F]-F-. See, Kachur et al., Journal of Applied Radiation and Isotopes, 1999, 51 (6), 643-650.

[¹⁸F]-EFI has shown good potential for labeling of hypoxic tumors and a relatively uniform biodistribution limited by slow equilibration with brain tissue Evans, et al. Journal of Nuclear Medicine, 2000 Vol. 41 , 327-336. As EF5 has been shown to predict radiotherapy resistance in individual rodent tumors with well documented pharmacological properties, attempts were made to label this compound with ¹⁸F for use in non-invasive imaging techniques.

[0015] More recently, we have disclosed methods of preparing ¹⁸F labeled compounds, including fluorinated derivatives of EF5, using the addition of molecular fluorine across double bond precursors. See U.S. Patent Nos. 7,230,115 and 7,432,295 both of which are incorporated herein by reference in their entirety. While these methods proved useful for preparing such compounds, yields were low, typically ca. 10%. Because of the inherently low specific activity of ¹⁸F-F₂ gas, and the relatively short half-life of ¹⁸F, an improvement in efficiency was desirable for a more practical application of the process.

SUMMARY

[0016] This invention presents novel methods for improving the yields and selectivity of incorporating ¹⁸F into compounds that are useful in non-invasive imaging techniques, such as PET. In particular, the methods are improvements over the methods previously disclosed in U.S. Patent 7,230,115, demonstrating 50% higher yields of fluorinated product and approximately double the degree of fluorine incorporation into the product.

[0017] In one embodiment of the present invention, a method for fluorinating a

carbon-carbon double bond that comprises reacting a compound having a structure of formula (I):

wherein R₂, R₃, and R₄ are independently selected from the group consisting of H, halogen, alkyl, heteroalkyl, cycloalkyl, hetero cycloalkyl, aryl, heteroaryl, alkoxy, aminoalkyl, hydroxyalkyl, ether, amide, keto, and carboxyl;

with F₂ in the presence of an organic solvent for a time and under conditions effective to form a compound having a structure of formula (II):

is improved by adding a catalytic amount of a source of molecular iodine, bromine, or BF₃ to the reaction mixture.

[0018] This invention also teaches an improvement of the method for preparing a

compound having a structure of formula (II) wherein at least one of R₂, R3, and R₄ is an aryl or heteroaryl, wherein said aryl or heteroaryl is substituted with at least one nitro group, and the remaining R₂, R₃, and R₄ moieties are independently hydrogen or

fluorine.

[0019] In some embodiments, R₃ and R₄ are fluorine. [0020] In other embodiments, the improvements provide that said organic solvent is an organic acid, which may be additionally selected from the group consisting of HCOOH, CH₃COOH, CFH₂COOH, CHF₂COOH, and CF₃COOH.

[0021] The invention also teaches an improvement of the method for preparing a compound having a structure of formula (II) wherein R₂ has the formula:

[0022] In still other embodiments, the improvement provide for the conversion of the compound having the structure of formula (I) is:

X, Y, and Z are independently H or F.

One, two, or all three of X, Y, and Z may be F.

[0023] In other embodiments, improved methods are provided for incorporating ¹⁸F into compounds of formula Il by contacting precursors of Formula I with [¹⁸F]-F₂ in the presence of an organic solvent for a time and under conditions effective to produce the [¹⁸F]-labeled compounds.

[0024] These methods provide compounds having formula IV:

(IV), wherein Ri is selected from the group consisting of -CH₂-CHF —

CH₂F, -CH₂CHFCHF₂, -CH₂-CF₂-CH₂, -CH₂CHFCF₃, -CH₂CF₂CHF₂, and - CH₂CF₂CF₃ One or more F may be present as an ¹⁸F isotope.

[0025] The use of a source of molecular iodine, such as I₂, as a catalyst is described, including the conditions in which it is present at concentrations in the range of about 0.1 to about 3 mole percent, relative to the total amount of the compound having a structure of formula (I). Still other embodiments describe the use of iodine at concentrations in the range of about 0.5 to about 1 mole percent, relative to the total amount of the compound having a structure of formula (I). Still other embodiments describe the use of iodine at concentrations of about 0.7 mole percent, relative to the total amount of the compound having a structure of formula (I). Other sources of iodine may also be used.

[0026] The invention also teaches that a source of bromine may also be an effective catalyst to the fluorination reactions described herein. In one embodiment, the source of bromine is HBr. The HBr provides good catalytic activity when present at concentrations in the range of about 0.3 to about 3 mole percent, relative to the total amount of the compound having a structure of formula (I). Other embodiments describe even higher activity at concentrations of HBr in the range of about 0.5 to about 2 mole percent, relative to the total amount of the compound having a structure of formula (I). [0027] The catalyst may also comprise a source of BF₃ , when present in concentrations of at least about 0.5 mole percent, relative to the total amount of the compound having a structure of formula (I). Other strong Lewis acids may also provide this level of enhanced activity.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] FIGURE 1 shows the effects of catalytic amounts of iodine (mole percent of precursor) on the EF5 yield (upper curves) and the degree of fluorine incorporation (lower curve) as described in Example 2.

[0029] FIGURE 2 shows the dependence of fluorine incorporation on the relative ratio of F₂ / precursor, and the impact of iodine catalyst thereon. Lower curve represents data without catalyst (one point of 5% at ratio of 6:1 is not shown); the upper curve was obtained in the presence of 0.7 mol% iodine concentration.

[0030] FIGURE 3 shows the catalytic effect of HBr on the EF5 yield (upper curve) and the degree of fluorine incorporation (lower curve) as described in Example 4.

[0031] FIGURE 4 shows the catalytic effect of BF₃ on the EF5 yield (upper curve) and the degree of fluorine incorporation (lower curve) as described in Example 5.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0032] The present invention may be understood more readily by reference to the following detailed description taken in connection with the accompanying Figures and Examples, which form a part of this disclosure. It is to be understood that this invention is not limited to the specific products, methods, conditions or parameters described and / or shown herein, and that the terminology used herein is for the purpose of describing particular embodiments by way of example only and is not intended to be limiting of any claimed invention. Similarly, any description as to a possible mechanism or mode of action or reason for improvement is meant to be illustrative only, and the invention herein is not to be constrained by the correctness or incorrectness of any such suggested mechanism or mode of action or reason for improvement. Throughout this text, it is recognized that the descriptions refer both to the method of preparing such devices and to the resulting, corresponding physical devices themselves.

[0033] In the present disclosure the singular forms "a," "an," and "the" include the plural reference. Thus, for example, a reference to "a source of iodine" is a reference to one or more of such sources and equivalents thereof known to those skilled in the art, and so forth. Conversely, various features of the invention that are, for brevity, described in the context of a single embodiment, may also be provided separately or in any subcombination. Further, reference to values stated in ranges include each and every value within that range. When values are expressed as approximations, by use of the antecedent "about," it will be understood that the particular value forms another embodiment. As used herein, "about B" (where B is a numerical value) refers to ± 10% of the recited value, inclusive. For example, the phrase "about 8" refers to a value of 7.2 to 8.8, inclusive. Where present, all ranges are inclusive and combinable.

[0034] This invention presents novel methods for improving the yields and selectivity of incorporating ¹⁸F into compounds that are useful in non-invasive imaging techniques, such as PET. In particular, the methods are improvements over the methods previously disclosed in U.S. Patent 7,230,115, the present invention demonstrating upwards of 50% higher yields of fluorinate product and approximately double the degree of fluorine incorporation into the product. For example, laboratory yields for the incorporation of fluorine increased from 11 to 22%.

[0035] In each case herein, fluohnated vinylic olefins are used. In one embodiment of the present invention, a method for fluorinating a carbon-carbon double bond that comprises reacting a compound having a structure of formula (I):

wherein R₂, R3, and R₄ are independently selected from the group consisting of H, halogen, alkyl, heteroalkyl, cycloalkyl, hetero cycloalkyl, aryl, heteroaryl, alkoxy, aminoalkyl, hydroxyalkyl, ether, amide, keto, and carboxyl; with F₂ in the presence of an organic solvent for a time and under conditions effective to form a compound having a structure of formula (II):

[0036] This invention also teaches an improvement of the method for preparing a compound having a structure of formula (II) wherein at least one of R₂, R3, and R₄ is an aryl or heteroaryl, wherein said aryl or heteroaryl is substituted with at least one nitro group, and the remaining R₂, R3, and R₄ moieties are independently hydrogen or fluorine.

[0037] In some embodiments, both R₃ and R₄ are fluorine.

[0038] Generally, the compounds of the invention can be synthesized using various reaction conditions depending on the starting material and ultimate requirements. Precursors are provided and fluohnated with F₂ or [¹⁸JF-F₂. Making of PET isotope- containing derivatives requires rapid addition of the ¹⁸F moiety followed by immediate purification and use because of the half-life of ¹⁸F, 110 minutes.

[0039] In certain embodiments of the present invention, the preparation of unlabeled PET analogue compounds generally requires that the precursors be dissolved in a suitable organic solvent at a temperature ranging from about -15° to about 100° C, depending on the solvent employed. Typically, lower temperatures are preferred to minimize or prevent side reactions with F₂. Preferred solvents include one or more organic acids including, but not limited to, carboxylic acids such as HCOOH, CH₃COOH, CFH₂COOH, CF₂HCOOH, CF₃COOH. Typically, the precursors are dissolved in CF₃COOH at a temperature ranging from about -15° C to about 5° C. Owing to the low level of catalysts required, it is most convenient that they are pre- dissolved in the solvent or co-solvent, which may include HCOOH, CH₃COOH, CFH₂COOH, CF₂HCOOH, CF₃COOH, so as to control the quantity of catalyst added. After the pre-dissolved catalysts are added, F₂ gas is bubbled through the solution to affect an electrophilic fluorination across the double bond. It should be noted that the amount of fluorine gas should be controlled carefully, and the reaction should be stopped once the starting material is consumed to prevent the further reaction of other parts of the molecule with fluorine. In a simple embodiment, the solvent is evaporated and the residue is dissolved in a suitable solvent, such as methanol :water (1 :1 ). The mixture is filtered and the organic solvent evaporated to obtain the residue. After the organic acid is evaporated, the residue is purified, preferably by HPLC. These methods are similar to those described in Dolbier, et al., "[18F]-EF5, a marker for PET detection of hypoxia: synthesis of precursor and a new fluorination procedure," Applied Radiation & Isotopes, 54(1 ): 73-80 (2001 ).

[0040] The reaction mixture may also comprise or yield a slurry from which the product must be recovered. Methods of recovering the sample include any filtration or separation techniques known in the art. Such methods include, but are not limited to, vacuum filtration, separatory extraction, or distillation. A preferred method is filtration using air or liquid, but other methods will be apparent to those skilled in the art.

[0041] The filtration solid may further require washing with organic solvents to separate out impurities or other reaction intermediates or byproducts. Organic solvents include, but are not limited to, ether, methanol ethanol ethyl acetate, or hexanes. Ethyl acetate is a preferred solvent, but other types of solvents will be apparent to those skilled in the art. Any organic solvent should be evaporated using methods known in the art. Evaporation methods may be accomplished at room temperature, by vacuum, aspiration, or by using latent heat. The evaporation methods are not limited to these techniques and other techniques will be apparent to those skilled in the art.

[0042] The reaction product is then purified using purification techniques known in the art. These techniques include, but are not limited to, column chromatography, flash chromatography, recrystallization, or gel chromatography. When using chromatographic purification methods, gradient elution is preferred. Combinations of organic solvents include, but are not limited to, methanol acetonitrile, hexanes

[0043] This improved method is particularly suited for the practical preparation of ¹⁸F-labeled PET agents, such as ¹⁸F-E5. In certain embodiments, the preparation of [¹⁸F]-labeled compounds generally requires a procedure similar to that described above, using isotopically enriched [¹⁸F]-F₂ gas.

[0044] The present invention provides improvements of the methods for preparing a compound having a structure of formula (II) wherein R₂ has the formula

[0045] In still other embodiments, the improvements provide for the conversion of

the compound having the structure of formula (I) is

wherein

X, Y, and Z are independently H or F. One, two, or all three of X, Y, and Z may be F.

[0046] In other embodiments, improved methods are provided for incorporating ¹⁸F into compounds of formula Il by contacting precursors of Formula I with [¹⁸]-F₂ in the presence of an organic solvent for a time and under conditions effective to produce the [¹⁸F]-labeled compounds.

[0047] These methods provide compounds having formula IV:

(IV), wherein Ri is selected from the group consisting of -CH₂-CHF —

[0048] The fluorination reactions are catalyzed by relatively small quantities (on the order of a few mole percent or less) of sources of iodine, bromine, or BF₃. In fact, in the case of iodine and bromine, the catalytic activity is greatest at levels less than about 1 -2 mole percent.

[0049] The specific use of iodine as a catalyst is described. The results shown in FIGURE 1 show that iodine is effective at low concentrations - i.e., under conditions in which it is present at concentrations in the range of about 0.1 to about 3 mole percent, relative to the total amount of the precursor compound. Still other embodiments describe the use of iodine at concentrations in the range of about 0.5 to about 1 mole percent, relative to the total amount of the compound having a structure of formula (I). Still other embodiments describe the use of iodine at concentrations of about 0.7 mole percent, relative to the total amount of the precursor used. The data in FIGURE 1 show that EF5 production can be increased about 50% (accounted vs. amount of precursor, upper curves) and the level of fluorine incorporation (represented as a yield vs. F₂, lower curve) can be nearly doubled, relative to the uncatalyzed reaction. At higher iodine concentrations, the product yield decreased, approaching the levels of the uncatalyzed reactions. Without being bound to any specific theory, is possible that the iodine loses its catalytic effect because of its involvement in side reactions.

[0050] The result of additional attempts to increase the incorporation of ¹⁸F into the final product are shown in FIGURE 2. Varying the ratio of F₂ / precursor, such that the precursor was in excess allowed for some increase in yield vs. fluorine, but provided problems with product separation. Nevertheless, the presence of 0.7 mole percent iodine (upper curve) showed dramatic improvements relative to the uncatalyzed reaction (lower curve). The degree of fluorine incorporation in the presence of catalyst (32%) was comparable to the maximal yield of the reaction achieved with excess fluorine, but without catalyst.

[0051] Iodine may be added directly to the reaction mixture as I₂ or generated in situ, such that other sources iodine or iodide may also be used. For example, given the highly oxidative character of molecular fluorine, mineral acids or inorganic salts of iodide, triiodide, or polyiodides generate iodine in situ, and may be used. If a mineral acid or inorganic salt of iodide, triiodide, or polyiodide is used as the source of iodine, corrections for stoichiomethes (i.e., correcting for the actual number of moles of iodine produced), as would be routine for those of ordinary skill in the art, may need to be made for optimal catalytic activity.

[0052] The invention also teaches that a source of bromine may also be an effective catalyst to the fluorination reactions described herein. As with iodine, bromine may be added directly or generated in situ, using mineral acids or inorganic salts of bromide, the generation in situ preferred. Pure Br₂ is difficult to handle and reacts with the precursor prior to the addition of fluorine. Instead, in one embodiment, the source of bromine as HBr is described. The results of experiments with HBr are shown in FIGURE 4. The catalytic effect of HBr on the reaction is similar to that shown with iodine - i.e., exhibiting an optimal concentration, decreasing in activity at higher concentrations. The HBr provides good catalytic activity when present at concentrations in the range of about 0.3 to about 3 mole percent, relative to the total amount of the precursor compound. Other embodiments describe even higher activity at concentrations of HBr in the range of about 0.5 to about 2 mole percent, relative to the total amount of the precursor. Without wishing to be bound by any theory as to the mechanism or mode of interaction of the catalyst with the reaction system, we note that the maximum activity occurs at HBr concentrations about twice those seen for molecular iodine, consistent with the idea that HBr is converted to Br₂ under the reaction conditions, and the Br₂ is the catalytically active species (i.e., 2HBr oxidized to form Br₂).

[0053] The catalyst may also comprise Lewis acids. For example, the invention teaches that a source of BF₃, when present in concentrations of at least about 0.5 mole percent, relative to the total amount of the compound having a structure of formula (I), provides enhanced reactivity. BF3 is most conveniently added by use of Lewis base adducts, for example complexes of carboxylic acids. In the case of BF₃, however, in contrast with the halogens, the data shown in FIGURE 5 shows there to be no loss of catalytic activity at higher concentrations.

[0054] Preferred aspects of the invention are discussed in the following examples. While the present invention has been described with specificity in accordance with certain of its preferred embodiments, the invention is not so limited. Each reference provided herein is incorporated by reference in its entirety.

[0055] EXAMPLES

[0056] General. Reagents and solvents were purchased from Aldrich Chemical Co. and used without additional purification unless otherwise noted ¹H NMR spectra were recorded on a Bruker-AMX-300 using CDCI3 or acetone-dβ as solvent and tetramethylsilane as an internal standard; ¹⁹F NMR spectra were measured on a Varian XL at 282 MHz, referenced to external CF₃COOH in D₂O. HPLC was performed on a Waters system (with Waters UV detector and radioactivity detector from IN/US Service Corp., Fairfield, N.J.) using an Phemonenex (or analogous) C-18 column (5 micron particle size, 4-10 mm x 250 mm) and ammonia-acetate buffer containing 35-40% CH₃OH (pH=4.7, final concentration 0.1 M) as a mobile phase (flow rate 1 -1.5 ml/min) with serial detection of 325 nm absorbency (specific for 2-nitroimidazole moiety) and radioactivity. The same HPLC conditions were used for the purification of [¹⁸F]-EF5.

[0057] Example 1. The same general synthetic and analytical methodologies described in U.S. Patent Nos. 7,230,115 and 7,432,295 were used to carry out the work described herein, and the methods and descriptions of both of these references are incorporated by references herein. For example, the synthesis described in U.S. Patent No. 7,230,115 was slightly modified to include the following steps:

[¹⁸J-F₂ was prepared by the ²⁰Ne(d, Ct)¹⁸F reaction using 50 mL target filled with 0.1 % F₂/Ne at 20 atm. The [¹⁸]-F₂ (0.1 % in Ne, up to 80 mCi, specific activity 0.2 Ci/mmol) was bubbled through 6-8 mL of thfluoroacetic acid (TFA) containing 2-(2-Nitro-1 H-imidazol-1 -yl)-N-(2,3,3-trifluoro-allyl)-acetamide (7 mg, 0.025 mmol) and required amount of catalyst in a 15 mL glass tube at minus 10° to O⁰C (-10° to O⁰C) for 20 minutes. The resulting mixture was transferred into a 10 mL glass flask and evaporated to dryness under reduced pressure at 100° C. This removes the solvent TFA and the major impurity [¹⁸F]-F^" in the form of HF. The residue was dissolved in 1.5 mL of water and injected through stainless-steel 0.45 micron filter into semi- preparative HPLC column. Purification conditions: Phenomenex (or analogous) C-18 column (5 micron particle size, 10 x 250 mm), 35% C₂H₅OH-water as a mobile phase (flow rate 1.5 mL/min); detection of the solution absorption at 325 nm. The fraction containing EF5 (retention time may vary between the columns from 30 to 40 min; the exact retention time has to be determined by the injection of EF5 prior to the experiment) was collected and evaporated to remove ethanol at reduced pressure at 90° C during 5 minutes. Typical time of the preparation is 1.5 - 2 hrs. The residue contains about 2 mg of [¹⁸F]-EF5 with 5-10 mCi of activity, corrected radiochemical yield 15 -25%.

[0058] See also Alexander V. Kachura, William R. Dolbier, Jr., Wei Xuc, Cameron J. Koch, "Catalysis of fluorine addition to double bond: An improvement of method for synthesis of 18F PET agents," Applied Radiation & Isotopes 68; 293-296, 2010, which is incorporated by reference in its entirety herein.

[0059] Example 2. Fluorination of the EF5-precursor was carried out in trifluoroacetic acid according to the reaction:

Using the general procedure described in Example 1 , 25-30 micromoles of the precursor were reacted with 60-70 micromoles of F₂ in 8 milliliters of thfluoroacetic acid (TFA) in the absence or presence of catalytic amounts of iodine (0 to 3 mole percent, based on precursor). The results are shown in FIGURE 1 (upper curve closed squares and lower curve). Although the overall reaction described had formally equimolar stoichiometry, significant amounts of precursor remained in solution.

[0060] To confirm the relevance of the PET experiment conditions to maximum possible yield of the reaction, the reaction was also done with an excess of fluorine gas. Iodine (2.5 mg, 0.01 mmol) was dissolved in trifluoroacetic acid (300 ml_) at room temperature and used as a reaction catalyst. A 100 ml_ three-neckedflask was equipped with a magnetic stirrer, a gas inlet tube (ID 0.5 mm) for introduction of the 1 % F₂/Ar mixture, and an outlet to a bottle trap containing saturated Kl solution. 30 ml_ of a solution of l₂-trifluoroacetic acid (containing 0.001 mmol of I₂) was added to the flask and the mixture cooled to -1 O⁰C. The EF5-precursor was added in one portion with stirring to create a solution with the required ratio of I₂ to precursor: for example, 26.4 mg (0,1 mmol) for ratio of 0.01 (1 %), Fluorine gas (1 % in Ar, obtained by dilution of 10% F₂/Ar with argon) was introduced slowly at a flow rate of 10.5 mL/min for about 20 minutes, with the excess F₂ being trapped by the Kl solution. After removal of the solvent, the residue was dissolved in d6-acetone (1 ml_),and trifluoromethylbenzene was added as an internal standard in order to determine the yields using 19F-NMR. The results are also shown in FIGURE 1 (upper curve open squares).

[0061] Example 3. The reactions described in Example 2 were repeated, except that the iodine concentration was fixed at 0.7 mole percent, based on precursor, and the ratio of F₂ to precursor was varied. The results are shown in FIGURE 2.

[0062] Example 4. The reactions described in Example 2 were repeated, except that commercially available HBr in acetic acid was used as the catalyst. The results are shown in FIGURE 3.

[0063] Example 5. The reactions described in Example 2 were repeated, except that the complex BF₃2CH₃COOH (dissolved in TFA) was used as the catalyst. The results are shown in FIGURE 4.

[0064] Those skilled in the art will appreciate that numerous changes and modifications may be made to the preferred embodiments of the present invention, and that such changes and modifications may be made without departing from the spirit of the invention. It is, therefore, intended that the spirit and scope of the appended claims should not be limited to the description of the preferred embodiments contained herein, but, that the appended claims cover all such equivalent variations as fall within the true spirit and scope of the invention.

Claims

What is Claimed:

1. In a method for fluorinating a carbon-carbon double bond that comprises reacting a compound having a structure of formula (I):

the improvement comprising adding a catalytic amount of a source of molecular iodine, molecular bromine, or BF₃ to said reaction.

2. The method of claim 1 wherein at least one of R₂, R₃, and R₄ is an aryl or heteroaryl, wherein said aryl or heteroaryl is substituted with at least one nitro group.

3. The method of claim 2 wherein R₂ is an aryl or heteroaryl, wherein said aryl or heteroaryl is substituted with at least one nitro group and R3 and R₄ are independently hydrogen or fluorine.

4. The method of claim 3 wherein R₃ and R₄ are fluorine.

5. The method of claim 1 wherein said organic solvent is an organic acid selected from the group consisting of HCOOH, CH₃COOH, CFH₂COOH, CHF₂COOH, CF₃COOH.

6. The method of claim 3 wherein said organic solvent is CF₃COOH.

7. The method of claim 3 wherein said aryl or heteroaryl substituted with at least one nitro group has the formula:

8. The method of claim 1 wherein the compound having the structure of formula (I) is

wherein X, Y, and Z are independently H or F.

9. The method of claim 8 wherein at least one of X, Y, and Z is F.

10. The method of claim 8 wherein at least two of X, Y, and Z is F.

11. The method of claim 1 wherein the compound having the structure of formula (I) is

12. The method according to any one of the preceding claims wherein the catalyst is a source of molecular iodine.

13. The method according to any one claims 1 to 11 , wherein the catalyst is a source of bromine, and is present at a concentration in the range of about 0.3 to about 3 mole percent, relative to the total amount of the compound having a structure of formula (I).

14. The method of any one of claims 1 -11 wherein the catalyst is a source of BF₃ and is present at a concentration of at least about 0.5 mole percent, relative to the total amount of the compound having a structure of formula (I).

15. The method of claim 12 wherein the catalyst is I₂ and is present at a concentration in the range of about 0.1 to about 3 mole percent, relative to the total amount of the compound having a structure of formula (I).

16. The method of claim 15 wherein the I₂ is present at a concentration in the range of about 0.5 to about 1 mole percent, relative to the total amount of the compound having a structure of formula (I).

17. The method of claim 16 wherein the I₂ is present at a concentration of about 0.7 mole percent, relative to the total amount of the compound having a structure of formula (I).

18. The method according to any one of the preceding claims wherein said F₂ is [¹⁸F]- F₂.

19. The method according to claim 18, wherein the compound having the structure of formula (I) is

and the compound having the structure of formula (II) is

wherein F' or F" is 1¹8⁰r

F.