WO1999031101A1 - Adenosine receptor antagonists with improved bioactivity - Google Patents

Abstract

Xanthine A1AR antagonist having halogenated N-1 and/or N-3 side chains are provided. The methods for the syntheses of such antagonists are also provided. The methods for using such antagonist labeled with carbon-11, fluorine-18 or isotopes of iodine such as iodine-123 for medical diagnostic imaging of the A1AR in patients are provided. Methods for improving the potency and duration of action of xanthine A1AR antagonist by halogenation of N-1 and N-3 propyl substituents is provided.

Description

ADENOSINE RECEPTOR ANTAGONISTS WITH IMPROVED

BIOACTIVITY

TECHNICAL FIELD

The present invention relates to adenosine A₁ receptor antagonist analogs and their use in the field of positron and single-photon emission tomography, and, more particularly, to such analogs used in radioligands binding assays .

BACKGROUND OF THE INVENTION

The adenosine A₁ receptor (A_XAR) is involved in central nervous system neuromodulation and in the pathogenesis of several diseases of the heart, kidney and central nervous system. Ligands and/or antagonists directed to the A_XAR are needed as therapeutic agents. See also United States Patents 5,668,139; 5,631,260; 5,565,566; 5,446,046; 4,980,379 incorporated by reference in their entirety. These patents describe adenosine and xanthine derivatives and compositions comprising them as potent, selective agonists and antagonists of adenosine receptors. Its clinical importance makes the A_jAR an important target for radionuclide imaging such a single-photon emission tomograph (SPET) , positron emission tomography (PET) and other methods of organ imaging. Single-photon emission tomography (SPET) and positron emission tomography (PET) are widely used techniques for medical imaging. PET also serves as a means for assessing tissue metabolism in vivo (Deussen, et al., 1992) and for studying receptor density and function (Merlet, et al . , 1993).

Imaging a pharmacological receptor requires a ligand that has (a) selectivity and high affinity for the receptor, ideally a K_D < 1 nM, so that imaging can distinguish specific binding to the receptor from unspecific binding to "background" structures; (b) pharmacodynamic properties that enable it to traverse the boundaries between compartments, for example, the blood-brain barrier, and thereby to achieve effective concentrations at the receptors of interest; and (c) metabolism that is either negligible or generates metabolites that clear rapidly and do not interfere with measurements of specifically bound ligand. The design of ligands must also take into account (d) the requirements of no-carrier-added (nca) radiosynthesis that ensures the high specific radioactivity on which the imaging of bound ligand depends and, (e) since the medically useful positron-emitting radionuclides have short half-lives, the possibility of incorporating longer-lived gamma-emitting isotopes, such as those of iodine, for studies of tissue distribution and metabolism as well as for SPET. The clinical importance of the A_x adenosine receptor (AiAR) makes it an attractive target for radionuclide imaging because the A_XAR is neuromodulatory . By neuromodulatory it is meant that A_jA inhibits synaptic transmission (Fredholm, et al . , 1988) .

In the brain those tonic inhibitory influences may prevent seizures (Dragunow, 1986) . Additionally, brief periods of hypoxia can reduce receptor density greatly

(Nagasawa, et al . , 1994), thereby increasing the risk of undesirable side effects such as convulsions.

Recent work indicates that the AiAR plays an essential role in cerebral protection by ischemic preconditioning (Heurteaux, eta 1., 1995) . In the heart A_xARs coupled to muscarinic potassium channels mediate the bradycardia and heart block caused by ischemia

(Belardinelli, et al . , 1995; Bertolet, et al . , 1995) and perhaps the bradyarrhythmias that occur in a substantial number of patients after heart transplantation (Haught, et al . , 1994). A_jARs in the renal juxtaglomerular apparatus inhibit renin release,

(Spielman, et al . , 1982) and receptors coupled to bicarbonate channels in the epithelium of the proximal tubule promote sodium reabsorption (Takeda, et al . , 1993) .

Accordingly, it would be beneficial to develop compounds which are A_XAR antagonists to be used as therapeutic agents . Such an antagonist must also be able to be radiolabeled in order to function for medical imaging.

It would be useful to have a method of making modifications to compounds that would improve the potency of a drug. It would also be useful to modify drugs in ways that improve resistant to metabolic degradation, thereby prolonging the duration of action.

SUMMARY OF THE INVENTION

According to the present invention, an A_XAR antagonist xanthine containing a haloalkyl side chain at N-l and/or N-3 is provided. A method of making the antagonist by halogenating the A_XAR antagonist is also provided. Also provided is a method of medical imaging by administering a halogenated A_jAR antagonist to a patien . A method is also provided for improving drug potency and duration of action by adding a halogenated substituent . DETAILED DESCRIPTION OF THE INVENTION

Generally, the present invention provides an AiAR antagonist and a method of making that A^AR antagonist wherein the antagonist is more potent and is more resistant to metabolic degradation and accordingly has a longer duration of action.

More specifically, the present invention provides an A_XAR antagonist xanthine containing haloalkyl side chain at the N-l and/or N-3 position. Examples of such haloalkyl side chains are 3-fluoropropyl, (±) 2- fluoropropyl, and the individual R- and S-enantiomers of 2-fluoropropyl and 3-iodo-2-propen-l-yl .

The fluorine substituent of the haloalkyl side chain can be a radioisotope such as fluorine-18 and the iodine substituent is a radioisotope such as iodine 123.

The N-l and N-3 side chain of the antagonist can be methyl or 2-methoxyethyl . The side chains can be radioactively labeled in the methyl moiety with a carbon-11 methyl group.

The xanthine C-8 substituents can be selected from the group including cycloalkyl, bicycloalkyl, cycloalkenyl, hetero (poly) cycloalkyl and aralkyl . The preferred A_XAR antagonist is 8-cyclopentyl-l, 3- dipropylaxanthine (hereinafter CPX) and analogues thereof. Radiohalogenating CPX at certain positions in the molecule makes it suitable for medical imaging. The halogenated CPX may be up to twice as potent as CPX.

In order to increase the effectiveness of CPX, according to the present invention, CPX is halogenated. The preferred method of halogenating CPX includes utilizing a combination of at least two protective groups. The choice of the particular protecting groups insures that halogenation occurs at the proper place in the molecule. More specifically, 8-cyclopentyl-l , 3- dipropylxanthine , CPX, must be radiolabeled in order to be used for imaging. As a preliminary to preparing a radiolabeled compound, studies, shown in Example 1, were undertaken to establish the activity of the compound by substitution of either fluorine or iodine. It was unexpectedly found that one of the substituted compounds (compound 8c, Example 1) was twice as potent as CPX. In Example 2, the activity of this compound in vivo in a PET scan is provided.

The discovery that fluorination preserves or even enhances affinity for the A_XAR indicates that such modification can improve the potency of drugs based on the xanthine pharmacophore . This includes drugs currently in use, under development and in clinical trials for (1) treating the abnormal fluid retention that occurs in heart, liver or kidney failure; (2) for the prophylaxis of acute renal failure caused by shock, infections or toxic substances; (3) for the treatment of cognitive disorders such as senile dementia; and (4) for treating a variety of cardiac diseases, including the bradycardia that occurs in patients with transplanted hearts or the high-grade heart block that complicates the resuscitation of victims of myocardial infarction or cardiac arrest [Suzuki et al, J. Med. Chem. 35:3066-75, 1992; Suzuki et al, J. Med. Chem. 36:2508-18, 1993; Mizumoto et al, J. Pharmacol. Exp. Ther. 266:200-206, 1993; and references 8 and 9 from Example 1] The compound of the present invention is administered and dosed in accordance with good medical practice, taking into account the clinical condition of the individual patient, the site and method of administration, scheduling of administration, patient age, sex, body weight and other factors known to medical practitioners. The pharmaceutically "effective amount" for purposes herein is thus determined by such considerations as are known in the art . In the method of the present invention, the compound of the present invention can be administered in various ways, for example, intravenously to patients undergoing PET scan. Owing to the strength of the fluorine-carbon bond, the fluoroalkylxanthines can advantageously be more resistant to metabolic degradation and thus have a longer duration of action.

Further, this substitution that leads to compound 8c described herein can also be used to modify other similar compounds that can also be used as diuretics providing improved method of synthesis and bioactivity.

(See U.S. Patents 5,525,607; 5,447,933; 5,290,782;

5,068,236) . The biological effects of substituents at N-l, N-3 and N-7 on the xanthine base are often additive with those at C-8. Accordingly, the combination of, for example, a 3- (2-fluoropropyl) group with a C-8 substituent other than a cyclopentyl group can be advantageous. C-8 substituents can include but are not limited to cycloalkyl, cycloalkenyl, bicycloalkyl, heterocycloalkyl, aralkyl .

In many instances, the synthesis differs from Example 1 herein not only with regard to the exocyclic substituents but also in the nature and sequence of the individual steps. An example is the preparation of the 3- (2-fluoropropyl) analogue of the diuretic CVT-124, which is 1, 3-dipropyl-8- (5, 6-exo-epoxy-2-ert o- norbornanyl) xanthine . The synthesis begins with the regliselective deprotection of N-3 of the novel xanthine 3-benzyl-8-

( endo-norborn-5-en-2 -yl) -7-pivaloyloxymethyl-1- propylxanthine . Removal of the benzyl group is not by catalytic hydrogen transfer as in Example 1 because that would reduce the alkene moiety of the C-8 substituent and prevent the direct epoxidation required in a subsequent step. Instead, removal of the group is by A1C1₃ in hot toluene or similar procedure as known in the art. The next step is alkylation at N-3 with a derivative of 2-fluoropropane (either the 2 -R, S- racemate or one of the enantiomers) that has a suitable leaving group at C-l (for example: halo, alkylsulfonoxy or arylsulfonoxy) , followed by alkaline hydrolysis of the pivaloyloxymethyl group protecting N-7. Epoxidation of the norbornenyl double bond could then employ any of several oxidants, either monoperphthalic or 27i-chloroperbenzoic acid as examples.

A detailed description of the synthesis and activity of the compounds is set forth in Examples included herewith.

The above discussion provides a factual basis for the use of halogenated AiAR antagonists. The methods used with and the utility of the present invention can be shown by the following non-limiting examples.

EXAMPLES GENERAL METHODS:

Delivery of the compound of the present invention:

The compound is administered and dosed in accordance with good medical practice, taking into account the clinical condition of the individual patient, the site and method of administration, scheduling of administration, patient age, sex, body weight and other factors known to medical practitioners.

The pharmaceutically "effective amount" for purposes herein is thus determined by such considerations as are known in the art. The amount must be effective to achieve improvement including but not limited to improvement and to improved survival rate or more rapid recovery, or improvement or elimination of symptoms and other indicators as are selected as appropriate measures by those skilled in the art.

In the method of the present invention, the compound can be administered in various ways. It should be noted that the compound can be administered as the compound or as a pharmaceutically acceptable salt and can be administered alone or as an active ingredient in combination with pharmaceutically acceptable carriers, diluents, adjuvants and vehicles. The compounds can be administered orally or parenterally, including the subcutaneous, intravenous, intraarterial, intramuscular, intraperitoneal and intranasal routes. The patient being treated is a warm-blooded animal and, in particular, mammals including man. The pharmaceutically accetable carriers, diluents, adjuvants and vehicles as well as implant carriers generally refere to inert, non- toxic solid or liquid fillers, diluents or encapsulating material not reacting with the active ingredients of the invention.

It is noted that humans are treated generally longer than the mice exemplified herein which treatment has a length proportional to the length of the disease process and drug effectiveness. The doses may be single doses or multiple doses over a period of several days, but single doses are preferred in PET scans .

When administering the compound parenterally, the compound is generally formulated in a unit dosage injectable form (solution, suspension, emulsion) . The pharmaceutical formulations suitable for injection include sterile aqueous solutions or dispersions and sterile powders for reconstitution into sterile injectable solutions or dispersions. The carrier can be a solvent or dispersing medium containing, for example, water, ethanol, ethylene diamine polyol (for example, glycerol, propylene glycol, liquid polyethylene glycol, and the like) , suitable mixtures thereof, and vegetable oils.

Proper fluidity is maintained, for example, by the use of a coating such as lecithin, by the maintenance of the required particle size in the case of dispersion and by the use of surfactants. Nonaqueous vehicles such a cottonseed oil, sesame oil, olive oil, soybean oil, corn oil, sunflower oil, or peanut oil and esters, such as isopropyl myristate, are also used as solvent systems for compound compositions. Additionally, various additives which enhance the stability, sterility, and isotonicity of the compositions, including antimicrobial preservatives, antioxidants, chelating agents, and buffers, can be added. Prevention of the action of microorganisms can be ensured by various antibacterial and antifungal agents, for example, parabens, chlorobutanol , phenol, sorbic acid, and the like. In many cases, it will be desirable to include isotonic agents, for example, sugars, sodium chloride, and the like. Prolonged absorption of the injectable pharmaceutical form can be brought about by the use of agents delaying absorption, for example, aluminum monostearate and gelatin. According to the present invention, however, any vehicle, diluent, or additive used would have to be compatible with the compounds.

Sterile injectable solutions can be prepared by incorporating the compounds utilized in practicing the present invention in the required amount of the appropriate solvent with various of the other ingredients, as desired.

A pharmacological formulation of the compound can be administered to the patient in an injectable formulation containing any compatible carrier, such as various vehicle, adjuvants, additives, and diluents; or the compounds utilized in the present invention can be administered parenterally to the patient in the form of slow-release subcutaneous implants or targeted delivery systems such as iontophoretic, polymer matrices, liposomes, and microspheres. Examples of delivery systems useful in the present invention include: U.S. Patent Numbers 5,225,182; 5,169,383; 5,167,616; 4,959,217; 4,925,678; 4,487,603; 4,486,194; 4,447,233; 4,447,224; 4,439,196; and 4,475,196. Many other such implants, delivery systems, and modules are well known to those skilled in the art. A pharmacological formulation of the compound utilized in the present invention can be administered orally to the patient. Conventional methods such as administering the compounds in tablets, suspensions, solutions, emulsions, capsules, powders, syrups and the like are usable.

Known techniques which deliver the compound orally, subcutaneously or parenterally including intravenous, intraarterial, intramuscular, intraperitoneally, and intranasal administration as well as intrathecal and infusion techniques and retain the biological activity are preferred.

In one embodiment, the compound is administered initially by intravenous injection to bring blood levels of compound to a suitable level. The patient's compound levels are then maintained by an oral dosage form, although other forms of administration, dependent upon the patient's condition and as indicated above, can be used. The quantity of compound to be administered will vary from nanograms to milligrams according to the diagnostic or therapeutic indication.

The amount must be effective to achieve improvement including but not limited to improved survival rate or more rapid recovery, or improvement or elimination of symptoms and other indicators as are selected as appropriate measures by those skilled in the art . Example 1 :

The high affinity of 8-cyclopentyl-l, 3- dipropylxanthine (CPX) for the A_x adenosine receptor (A_XAR) provides a good lead for developing radioligands suitable for positron emission tomography (PET) and single-photon emission tomography (SPET) . This example tests the hypothesis that the kinds of chemical modifications made in the synthesis of CPX analogues containing carbon-11, fluorine-18, or radioiodine will not alter affinity for the AiAR. This example describes the synthesis and radioligand binding assays of unlabeled CPX analogues having methyl, 2-methoxyethyl,

2-fluoropropyl, or 3 -fluoropropyl substituents, respectively, at either N-l (13a-d) or N-3 (8a-d) or an

(E) -3-iodoprop-2-en-l-yl substituent at N-3 (8f) . Compounds 8d,f and 13b,d antagonized the binding of

[³H] CPX to the A_XAR of rat brain with affinities similar to those of CPX; compound 8c was twice as potent as

CPX. Analogues 8a,b and 13a were less potent than CPX, but for each, the Ki of antagonism was > 0.5 nM. Attempts to iodinate the 8- (4-hydroxyphenyl) analogue of CPX failed, probably because the xanthine substituent strongly deactivated the phenol toward electrophilic iodination. In summary, several of the modifications of the propyl groups of CPX needed to produce ligands for imaging by PET and SPET preserve or enhance affinity for the A_XAR.

Single-photon emission tomography (SPET) and positron emission tomography (PET) are widely used techniques for medical imaging. PET also serves as a means for assessing tissue metabolism in vivo (Deussen, et al . , 1992) and for studying receptor density and function (Merlet, et al . , 1993) . Imaging a pharmacological receptor requires a ligand that has (a) selectivity and high affinity for the receptor, ideally a K_D < 1 nM, so that imaging can distinguish specific binding to the receptor from unspecific binding to "background" structures; (b) pharmacodynamic properties that enable it to traverse the boundaries between compartments, for example, the blood-brain barrier, and thereby to achieve effective concentrations at the receptors of interest; and (c) metabolism that is either negligible or generates metabolites that clear rapidly and do not interfere with measurements of specifically bound ligand. The design of ligands must also take into account (d) the requirements of no- carrier-added (nca) radiosynthesis that ensures the high specific radioactivity on which the imaging of bound ligand depends and, (e) since the medically useful positron-emitting radionuclides have short half- lives, the provisions for incorporating longer-lived gamma-emitting isotopes such as those of iodine for studies of tissue distribution and metabolism as well as for SPET.

This example describes the synthesis and structure-activity relationship of some A_XAR antagonists that, if radiolabeled, are of interest as potential ligands of SPET and PET. The ligands are analogues of

8-cyclopentyl-l, 3-dipropylxanthine (CPX), which, owing to its high selectivity and affinity for the A_XAR

(Bruns, et al . , 1987; Lohse, et al . , 1987), is the prototypical A_XAR antagonist (Watson, 1996) . The ligands for PET include some methyl analogues of CPX obtainable by carbon-11 methylation of a suitable precursor, as well as radioligands labeled with fluorine-18. The single ligand suitable only for SPET has an N-3 substituent that contains iodine. The intermediate steps described here evaluate only the pharmacological impact of the chemical modifications caused by radiolabeling and do so with unlabeled compounds. The present example obtains target ligands as directly as possible, and so some of the syntheses are inappropriate for nca syntheses of radioligands.

However, in those instances some of the precursors of radiolabeling were prepared.

Pharmacodynamic studies in rats support the candidacy of CPX as a brain-imaging agent. [³H] CPX administered intravenously penetrated into the brain to the extent of 0.8% of the injected dose/g at five minutes. That level remained stable for 15 minutes. Unlabeled CPX displaced 45-70% of bound [³H] CPX (Bisserbe, et al . , 1992). Ishikawa et al . (Ishiwata, et al., 1995) synthesized [^X1C] KF15372 , an 8- dicyclopropylmethyl analogue of CPX, and demonstrated specific uptake in mouse brain. Chemistry The syntheses of the 1- and 3-substituted 8- cyclopentylxanthines follows the route of condensation of a urea with cyanoacetic acid, cyclization in alkali to form a uracil, nitrosation and reduction of the nitroso group to form a diaminouracil that is acylated, and, finally, cyclization of the amidouracil to form a xanthine (Papesch, et al . , 1951; Speer, et al . , 1953; Blicke, et al . , 1954). The syntheses had two features in common. First, they proceeded from unsymmetrical ureas, either N-benzylurea or N-benzyl-N' -propylurea.

Second, both syntheses called for alkylations at N-3 that required prior protection of N-7 to ensure that the alkylations at N-3 would be regioselective .

Proceeding from benzylureas served two purposes . First, the benzyl group exerted steric control of the condensation of the urea with cyanoacetic acid. The condensation of cyanoacetic acid with an unsymmetrical urea occurs at either of the nitrogens but favors the nitrogen bearing the smallest (least hindering) substituent. (Papesch, et al . , 1951) Because a benzyl group is so much larger than a hydrogen or propyl substituent, the nitrogen forming the cyanoacetamide is more likely to be N-3 of the intermediate uracil and, ultimately, N-l of the xanthine. Proceeding from N-benzyl-N' -propyluracil appeared by TLC to give only one xanthine, but reserve-phase HPLC showed that the product was a 4:1 mixture of two xanthines. Reference to an authentic sample established that 3-benzyl-8-cyclopentyl-l- propylxanthine , 1, was the more polar major product. Presumably, the other isomer was l-benzyl-8- cyclopentyl-3 -propylxanthine. Fractional crystallization of the isomers was not successful, but the 7-POM derivatives (see below) separated easily on crystallization from methanol . By contrast, benzylurea gave one product, 3-benzyl-8-cyclopentylxanthine, 2. Second, the benzyl group also served as a protecting group that is easily removable by catalytic transfer hydrogenation (CTH) . (Ram, et al . , 1987) .

Alkylation at N-7 of 1 with di-tert-butyl pyrocarbonate , (boc)₂0, chloromethyl methyl ether, MOM-

Scheme 1

1 4

68-C

6a 6b

Bzi: Q _f° -

Cl, or chloromethyl pivalate, POM-Cl, generated the tert-butoxycarbonyl (BOC) , methoxymethyl (MOM) , and pivaloyloxymethyl (POM) derivatives 3a-c, respectively, for evaluation of the suitability of those protecting groups. In a subsequent step hydrogenolysis cleaved the t-BOC and MOM groups, and thus, they were not useful. The POM group resisted hydrogenolysis and, just as importantly, underwent removal by alkali.

The initial plan to synthesize the 3 -substituted 8-cyclopentyl-l-propylxanthines called for debenzylation of 1 to form 8-cyclopentyl-l- propylxanthine, 4, which was then to undergo protection of N-7 followed by alkylation at N-3 (Scheme 1) . The reaction of P0M-C1 with 4 could, in principle, produce three products: the desired 7-POM derivative 5a as well as the 3-POM derivative 5b and the 3,7-bis-POM derivative 5c. The original description (Hu, et al . , 1980) of the protection of xanthines by the POM group indicated that the reaction of 1-methylxanthine with POM-Cl gives a mixture of 5a, c. On the basis of that information, 4, the monosubstituted product of the reaction with POM-Cl, underwent alkylation with iodomethane to form 6a, which was then deprotected. Unexpectedly, that product, 6b, was 300-fold less potent than CPX. ^XH NMR analysis of the products of the reaction of POM-Cl with 4 showed that the monosubstituted product was the 3 -POM derivative 5b rather than the expected 7-POM derivative 5a. That structural assignment was based on the following evidence: (a) The resonance for the proton on the unsubstituted nitrogen was at 13.5 ppm, which is characteristic of an N-7 rather than an N-3 proton;

(Muller, et al . , 1993) (b) the unambiguous synthesis of 8-cyclopentyl-l-propyl-7-POM-xanthine, 5a, by the debenzylation of 3c gave a product that had an NH resonance at 11.7 ppm, characteristic of a 3-H xanthine; (Muller, et al . , 1993) (c) the alkylation of the mono-POM derivative with methyl iodide gave a product, 6a, having a methyl proton resonance at 3.84 ppm, that expected of a 7-methyl rather than a 3 -methyl substituent, which would have a resonance at about 3.4 ppm; (Muller, et al . , 1993; Erickson, et al . , 1991) (d) the methylation of authentic 5a gave a product, 8- cyclopentyl-l-propyl-3-methyl-7-POM-xanthine, 7a, that had the expected methyl resonance at 3.38 ppm; and (e) in parallel with the shifts of the methyl resonance, the resonance of the methylene moiety of the POM group of 5a at 6.25 ppm was further downfield than that resonance of 5b, which was at 6.15 ppm. Thus, proceeding from 5b had yielded 8-cyclopentyl-7-methyl- 3-POM-1-propylxanthine, 6a, and, after deprotection, 8- cyclopentyl-7-methyl-1-propylxanthine, 6b. Further, that result showed that to use the POM group for the protection of N-7, it was essential to first protect N- 3 with the benzyl group.

6a-| 7ιM

• (i) NH^CO_jH, PD/C, MeOH, reflux; (u) RX, K2CO3, D F; (iii⁾ NAOH, reflux. The synthesis of the 3 -substituted CPX analogues

(Scheme 2) began with the fractional crystallization of 3c to remove the unwanted 1-benzyl-3 -propyl isomer. The ¹H NMR spectrum of 3c was a convenient way to follow that separation. Two crystallizations usually sufficed to remove the unwanted isomer, as shown by the disappearance of its benzylic resonance at 5.25 ppm. Removal of the 3-benzyl group by CTH (Hu, et al . , 1980) gave 8-cyclopentyl-7- (pivaloyloxymethyl) -1- propylxanthine, 5a. CTH required elevated temperatures (bath temperatures of 140 °C) and anhydrous conditions, specifically, anhydrous methanol as the solvent and excess ammonium formate dried over P₄O₁₀ as the hydrogen donor. Regioselective alkylation with either methyl iodide, 2-chloroethyl methyl ether, l-bromo-2- fluoropropane, or l-bromo-3-fluoropropane provided 7a- d, respectively. Because the scope of the project included iodinated ligands and because vmylic iodides tend to resist metabolism, (Coenen, et al . , 1983 ) we

Scheme S^α

!^■

β 7f

« (i) (15)-Bu₃Sn-CH-Ch-CH₂-OTos, DMF, K2CO3, rt; (ii) h, DCM, rt; (ϋi) NaOH, reflux. alkylated 5a with (E) -3 - (tri-n-butyl-stannyl) prop-2-en- 1-yl p-toluenesulfonate, (Jung, et al . , 1984; Musachio, et al . , 1992), obtaining the intermediate (E)-3-[3- tributylstannyl) -l-prop-2-en-l-yl] xanthine, 7e, which was then reacted with iodine to give the (E)-3-(3- iodoprop-2 -en- 1-yl) xanthine, 7f (Scheme 3). Alkaline cleavage of the POM groups of 7a-f completed the syntheses of 8a-f.

The reactions in Scheme 2 include the preparations of precursors for the nca synthesis of carbon-11- labeled 8a, and fluorine-18-labeled 8c, d. The precursor for carbon-11-labeled 8a is 5a. The alkylation of 5a with chloroethanol followed by deprotection of the -intermediate 7g gave 8g, the precursor of carbon-11-labeled 8b.

The synthesis of 8c, d labeled with fluorine-18 proceeds from the 2- and 3-hydroxypropyl analogues, 8h,i. Since l-halo-2-propanols cyclize to the oxiranes under the conditions of alkylation, 5a was alkylated with chloroacetone and then reduced by CTH in methanol to yield 7h as a mixture of isomers. The alkylation of 5a with 3-bromopropanol yielded 7i. Deprotection of 7h,i in alkali gave 8h,i. Alkylation of 5a by chloroethanol gave intermediate 7g and, following deprotection, 8g. Compound 7g serves as a precursor 5 for carbon-11-labeled 8b, the precursor for carbon- 11- labeled 8a is 5a. An alternative approach suited to the nca synthesis of 8f began with the alkylation of 5a with propargyl chloride to give the 3 - (prop-2-yn-l-yl) derivative 7j . However, 7j did not react with 10 tributyltin hydride, and so we abandoned that approach. The synthesis of the 1-substituted CPX analogues (Scheme 4) from 3-benzyl-8-cyclopentylxanthine, 2, began with the protection of N-7 by a POM group to give

128-fl «»fl

13«-fl

T C _l, POM-CL K.CO, DMF. (u) RX, K.CO, DMF. ⁽uⁱ⁾ I JW∞," Pd/C. °OH. rVnux. (,») CH.<CHΛI. K,C0, DMF, ⁽v⁾ 2 N N.OH reflux.

9. Alkylation of 9 at N-l with either iodomethane, 2- chloroethyl methyl ether, l-bromo-2-fluoropropane, or

30 l-bromo-3-fluoropropane formed lOa-d. Successive steps included debenzylation to the 3-H xanthines, lla-d, alkylation with 1-iodopropane to form the 3-propyl- xanthines, 12a-d, and removal of the POM groups to form the 1-methyl, 1-ethoxymethyl, 1- (2 -fluoropropyl ) , and

35 1- (3 -fluoropropyl) analogues of CPX, 13a-d. The syntheses of the 1- (2-hydroxyethyl) and l-(3- hydroxypropyl) analogues of CPX, 13e, g, precursors of carbon-11-labeled 13b and fluorine-18-labeled 13d, followed those of 8g,i. Alkylation of 9 with chloroacetone began with the synthesis of the 1- (2- hydroxypropyl) analogue of CPX, 13f, but differed from the synthesis of 8h in that CTH simultaneously reduced the 2-keto group during debenzylation rather than in a separate step. Resul ts and Discussion Table 1 lists the chemical characteristics of the CPX analogues .

Table 2 summarizes the results of the radioligand binding assays. Modifications of the 3-propyl group (analogues 8a-d,f) were well-tolerated. All of the analogues had values of K_x < 0.5 nM. (+) -8-Cyclopentyl-

3- (2 -fluoropropyl) -1-propylxanthine, 8c, was twice as potent as CPX, and the least active, 8-cyclopentyl-3-

(2-methoxyethyl) -1-propylxanthine, 8b, was only 3 -fold less potent than CPX. Because 8c is a racemate, the possibility remains that one isomer might be even more potent. The iodinated analogue, 8f, was only 1.4-fold less potent than CPX. Generally, the modifications of the 1-propyl group (analogues 13a-d) reduced affinity, though only modestly. The least potent analogue, 13c, had a K₁ slightly above 1 nM and was only 8-fold less potent than CPX. The other 1-substituted analogues had values of K_x in the subnanomolar range .

In summary, several chemical modifications of the 1- and 3-propyl groups of CPX have little impact on affinity of the Ai adenosine receptor. Since the CPX analogues studied here are surrogates for ligands labeled with carbon-11, fluorine-18, or radioiodine, such radioligands could be useful agents for imaging by PET and SPET. CPX analogues radiolabeled in the C-8 substituent are not promising because the present experiments show that 1, 3-dipropyl-8- (4- hydroxyphenyl) xanthine is refractory to iodination. The low potency of 1, 3-dipropyl-8- (3- fluorocyclopentyl) xanthine (Jacobson, et al . , 1988) suggests that fluorination of the cyclopentane moiety affects potency.

Specific Methods Used

Melting points were measured on an Electrothermal apparatus and are uncorrected. Elemental analyses were performed by MHW Laboratories, Tucson, AZ, and by the Zentralabteilung fur chemische Analysen at the Forschungszentrum Julich, and are within ±0.4% of calculated composition. Thin-layer chromatogrpahy employed precoated silica gel sheets (Polygram, Macherey-Nagel, Duren, Germany) developed with ethyl acetate-hexane, 1:1 (solvent A) or 3:1 (solvent B) , or chloroform-methanol, 95:5 (solvent C) . A ^XH Bruker WP- 80 spectrometer provided 80-MH_zNMR spectra. Resonances are reported as chemical shifts (δ) downfield from a TMS internal standard. Refluxing over Mg turnings and distillation dried methanol for CTH. Crying of ammonium formate was over P₄O₁₀ at room temperature and atmospheric pressure. CTH employed equal weights of substrate and 10% Pd-C and a 10-fold molar excess of ammonium formate for times and temperatures indicated in individual experiments. Purification of dimethylformamide (DMF) consisted of distillation and storage in a light-proof container over 4A molecular sieves. Storage over 4A molecular sieves dried CH₂C1₂ and toluene. Other solvents and reagents were used as supplied by the vendors. Analytical HPLC used a 250- x 4 -mm column of C-18 silica gel, eluted with methanol- water (64:36, v/v) . Table 1 lists the solvents used for purification by crystallization or liquid chromatography . 3 -Benzyl - 8 -cyclopentyl -l -propylxanthine (1 ) . A solution of N-benzyl-N' -propylurea (96.1 g, 0.5 mol) and cyanoacetic acid (40.2 g, 0.55 mol) in acetic anhydride (200 mL) was stirred for three hours at 70° C and evaporated. The syrupy residue was mixed with water (500 mL) and then basified with 5 N NaOH. Refrigeration overnight deposited a solid that was filtered off and crystallized from methanol-water . A yield of 123 g was obtained, 95%, of the 6-aminouracil as silky white needles . A solution of the aminouracil (30 g, 116 mmol) in 80% acetic acid (200 mL) and ethanol (400 mL) was stirred at 40° C during the dropwise addition of a solution of NaN0₂ (12 g, 174 mmol) in water (100 mL) over a two hour period. Stirring continued for an additional 30 minutes, at which time TLC (solvent C) showed the reaction was complete. Cooling on ice and filtration collected the nitrosouracil, which was washed with water and dried. A yield of 28.5 g, 85% was obtained. The nitrosouracil was suspended in water (100 mL) , heated to 85° C and, stirred during the addition of solid sodium dithionite in portions until the red color disappeared. The cooled solution was extracted with ethyl acetate (5 x 50 mL) , and the organic phases were pooled, dried over MgS0₄, and evaporated to a thick syrup that was then dried at high vacuum to remove traces of moisture . Yield of the diaminouracil was 25.9 g, 95%. A solution of cyclopentanecarbonyl chloride, freshly prepared from cyclopentanecarboxylic acid (11.3 g, 99 mmol) and SOCl₂(8.0 mL, 110 mmol) and dissolved in dry CH₂C1₂ (50 mL) , was added dropwise to a stirred solution of the diaminouracil in dry CH₂C1₂ (50 mL) . After 15 minutes HPLC showed that a more polar product had completely replaced the starting material. The gummy residue after evaporation was dissolved in ethanol (100 mL) and 1 N NaOH (100 mL) and refluxed overnight. Cooling and acidification with HC1 precipitated essentially pure product that was filtered off and crystallized. ^XH NMR (CDCl₃)δ: 0.85 (t, 3H, CH₂CH₂CH₃) , 1.2-2.3 (m, 10H, CH₂CH₂CH₃ and cyclopentyl CH₂) , 2.85-3.45 (m, 1H, cyclopentyl H-l) , 3.85 (m, 2H, CH₂CH₂CH₃) , 5.25 (s, 2H, CH₂C₆H_S) , 7.2-7.5 (m, 5H, C₆H₅) , 13.08 (br s, 1H, N⁷H) .

3 -Benzyl -8-cyclopentylxanthine (2) . A solution of 6-amino-l-benzyluracil (Papesch and Schroeder, 1951) (34.2 g, 157 mmol) in glacial acetic acid (100 mL) , ethanol (100 mL) , and water (200 mL) was cooled in an ice bath and stirred during the dropwise addition of a solution of NaN0₂ (14 g, 200 mmol) in water (50 mL) . Stirring continued for an additional 30 minutes. The nitrosouracil was filtered off, washed with water, suspended in water (250 mL) , and heated to 85° C. The addition of solid sodium dithionite discharged the purple color and precipitated a bright-green solid that was filtered off and freeze-dried. A yield of 27.0 g, 74% was obtained.

A suspension of the green diamincuracil in dry CH₂C1₂ was treated with cyclopentanecarbonyl chloride, freshly prepared from cyclopentanecarboxylic acid (15.4 g, 124 mmol) and S0C1₂ (16.2 g, 136 mmol). The addition of the acid chloride turned the suspension a deep red.

The mixture was stirred overnight; a white solid was filtered off and boiled for 12 hours in 2 N NaOH (100 mL) . The clear solution was brought to pH 5 with concentrated HC1, which precipitated a solid that was filtered off, washed with water, and crystallized. ¹H NMR (DMSO-d₆)δ: 1.35-2.20 (m, 8H, cyclopentyl CH₂) , 2.85-3.5 (br m, 1H, cyclopentyl H-l), 5.08 (s, 2H, C₆H_SCH₂) , 7.30 (m, 5H, C₆H₅₎ , 11.00 (s, 1H, N^XH) , 13.08 (s, 1H, N⁷H) .

3 -Benzyl - 7 - ( tert-bu toxycarbonyl ) - 8 - cyclopen tyl -1 - propylxanthine (3a) . A solution of 1 (14.0 g, 39.3 mmol) and triethylamine (4.0 g, 39.5 mmol) in dry CH₂C1₂ (50 mL) was stirred during the portionwise addition of di-tert-butyl pyrocarbonate (8.6 g, 39.4 mmol). When gas evolution ceased, the clear solution was filtered through a 1- x 3 -cm column of silica gel to remove a polar impurity, and the filtrate was evaporated. A solution of the residue in CH₂C1₂ was washed with water

(100 mL) , 1 N NaOH (3 x 100 mL) , and water (100 mL) . Drying over MgS0₄ and evaporation gave 18 g (100%) of product as an off-white solid. H NMR (CDCl₃)δ: 0.91

(t, 3H, CH₂CH₂CH₃) , 1.40-2.25 (m, 19H, CH₂CH₂CH₃, cyclopentyl CH₂ and C(CH₃)₃), 3.55 (t, 1H, cyclopentyl H-l), 3.95 (m, 2H, CH₂CH₂CH₃) , 5.25 (s, 2H, PhCH₂) , 7.30

(m, 5H, C_eH₅) . 3 -Benzyl - 8 - cycl open tyl - 7- (me thoxyme thyl ) - 1 - propylxanthine (3b) . A suspension of 1 (103 mg, 0.4 mmol) in hexamethyldisilazane (10 mL) containing a catalytic amount of ammonium sulfate was refluxed for 90 minutes. During that time the mixture became homogeneous. Evaporation gave a liquid residue that was treated with methyl bromomethyl ether (MOM-Br; 350 μL, 4 mmol) in dry THF (4 mL) . After stirring for 20 hours at room temperature the reaction mixture was treated with methanol (20 mL) and stirred for 30 minutes. The solution was concentrated nearly to dryness and was treated with 33% ammonia (2 mL) and acetone (25 mL) . Crystals of NH₄Br were filtered off, and the filtrate was evaporated to dryness . The residue was taken up in ethyl acetate, washed with water, dried over Na₂S0₄, and evaporated to yield a yellow oil. Crystallization from petroleum ether gave fluffy white needles. ^XH NMR (CDCl₃)δ: 0.95 (t, 3H, CH₂CH₂CH₃) , 1.45-2.27 (m, 10H, CH₂CH₂CH₃ and cyclopentyl CH₂) , 3.0-3.3 (m, 1H, cyclopentyl H-l), 3.40 (s, 3H, OCH₃) , 3.95 (t, 2H, CH₂CH₂CH₃) , 5.75 (s, 2H, N⁷CH₂0) .

Compounds 3a and 3b were not useful in the synthesis of the target compounds because the protecting groups were not stable in a subsequent step.

This data demonstrates that the POM group is unique among all protecting groups.

3 -Benzyl -8- cycl open tyl -7- (pi val oyl oxyme thyl ) -1 - propylxanthine (3c) . A mixture of 1 (20.6 g, 58.5 mmol), chloromethyl pivalate (POM-Cl; 17.7 mL, 122.6 mmol), and anhydrous Na₂C0₃ (13.0 g, 122.6 mmol) in anhydrous DMF (150 mL) was stirred overnight at 50° C.

TLC (solvent A) showed complete conversion of the starting material (R_f=0.68) to a less polar product

(R_f=0.91). Evaporation in vacuo gave a gummy residue that was taken up in CH₂C1₂, washed with water, dried over Na₂S0₄ and evaporated. Crystallization gave 25.1 g

(92%) of product. *H NMR (CDCl₃)δ: 0.95 (t, 3H, CH₂CH₂CH₃) , 1.20 (s, 9H, C(CH₃)₃), 1.40-2.20 (m, 10H, CH₂CH₂CH₃ and cyclopentyl CH₂) , 3.05-3.45 (m, 1H, cyclopentyl H-l), 3.97 (t, 2H, CH₂CH₂CH₃) , 5.25 (s, 2H, C₆H₅CH₂) , 6.25 (s, 2H, N⁷CH₂OPOM) , 7.2-7.7 (m, 5H, C₆H₅) .

8 -Cyclopentyl -1 -propylxanthine (4) . For CTH a 500-mL flask was flushed with argon and charged with 2 (5.2 g, 16.8 mmol), 10% Pd-C (5.2 g) , dry ammonium formate (10.6 g, 168 mmol), and absolute methanol (250 mL) . The contents were stirred in an oil bath heated to 140° C. After 4 hours TLC (solvent A) showed the reaction was complete. Filtration through sea sand removed the catalyst, and methanol was evaporated. The filter cake was washed three times with 1 N NaOH (3 x 80 mL) , and the washings and residue from the evaporation were combined. Adding crushed ice to the alkaline solution followed by careful acidification to pH 3-4 with dilute HC1 precipitated crude product.

Dissolution in alkali and reprecipitation gave 3.7 g

(83%) of essentially pure product. Crystallization from methanol gave an analytical sample. H NMR (DMSO- d₆) δ: 0.88 (t, 3H, CH₂CH₂CH₃) , 1.2-2.3 (m, 10H, CH₂CH₂CH₃ and cyclopentyl CH₂) , 2.90-3.35 (m, 1H, cyclopentyl H- 1), 3.85 (t, 2H, CH₂CH₂CH₃) , 12.51 (s, 1H, N³H) , 13.54 (s, 1H, N⁷H) .

8 - Cycl open tyl - 7- (pi val oyl oxyme thyl ) -1 - propylxanthine (5a) . The debenzylation of 3c (10 g, 2.15 mmol) followed the general method for CTH. Workup consisted of removal of catalyst by filtration, washing the filter cake with CHC1₃ (3 x 100 mL) , and evaporation of the filtrate and washings to a yellowish oil that solidified on standing. Two crystallizations from methanol gave 7.4 g (92%) of white crystals, mp 127° C. ^XH NMR (CDC1₃) δ: 0.95 (t, 3H, CH₂CH₂CH₃) , 1.20 (s, 9H,

C(CH₃)₃), 1.4-2.2 (m, 10H, CH₂CH₂CH₃ and cyclopentyl CH₂) ,

3.05-3.45 (m, 1H, cyclopentyl H-l), 3.98 (t, 2H,

CH₂CH₂CH₃) , 6.25 (s, 2H, N⁷CH₂OPOM) , 11.7 (br s, 1H, N³H) .

Reaction of 4 wi th Pivaloyloxymethyl Chloride . 8- Cycl open tyl - 3 - (pi val oyl - oxyme thyl ) - 1 -propylxan thine

(5b) and 3 , 7 -Bis (pi val oyl oxyme thyl ) - 8 - cycl open tyl -1 - propylxanthine (5c) . A suspension of 4 (1.56 g, 6.0 mmol) and anhydrous Na₂C0₃ (630 mg, 5.94 mmol) in anhydrous DMF (40 mL) was stirred during the dropwise addition of POM-Cl (1.0 g, 6.6 mmol) in dry DMF (10 mL) . Stirring continued for 16 hours at room temperature. The suspension was filtered, and filtrate was evaporated, and the residue was taken up in acetone. The insoluble educt was filtered off, and the filtrate was taken to dryness. MPLC on a 1.2- x 23- cm column of silica gel eluted with solvent A gave two products. The fraction eluting between 30 and 50 mL contained 5c (1 g, 35%). H NMR (CDC1₃) δ: 0.99 (m, 3H,

CH₂CH₂CH₃) , 1.2 (br s, 18H, 2 x C(CH₃)₃), 1.5-2.10 (m, 10H, CH₂CH₂CH₃ and cyclopentyl CH₂) , 3.05-3.42 (m, 1H, cyclopentyl H-l), 3.97 (t, 2H, CH₂CH₂CH₃) , 6.10 (s, 2H, NCH₂OPOM) , 6.23 (s, 2H, N⁷CH₂OPOM) . The second fraction eluting between 60 and 95 mL contained 5b (0.27 g,

12%). ^XH NMR (CDCI₃) δ: 1.0 (t, 3H, CH₂CH₂CH₃) , 1.20 (s, 9H, C(CH₃)₃), 1.48-2.25 (m, 10H, CH₂CH₂CH₃ and cyclopentyl CH₂) , 3.05-3.46 (m, 1H, cyclopentyl H-l), 4.05 (t, 2H, CH₂CH₂CH₃) , 6.15 (s, 2H, N³CH₂OPOM) , 13.5 (br, s, 1H, N⁷H) .

8 - Cycl open tyl - 7-methyl -3 - (pi val oyl oxyme thyl ) -1 - propylxanthine (6a) . General Method A. A mixture of 5b (376 mg, 1 mmol) , Na₂C0₃ (106 mg, 1 mmol) , and iodomethane (125 μL, 2 mmol) in dry DMF (4 mL) was stirred for one hour at room temperature. Workup began with dilution with water (10 mL) , acidification with 2 N HC1, and extraction with CH₂C1₂ (4 x 25 mL) . The combined extracts were washed with brine, dried over Na₂S0₄, and evaporated to give a semisolid residue that was crystallized from an appropriate solvent (Table 1) . Yield 355 mg, 91%. ^XH NMR (CDC1₃) δ: 0.98 (t, 3H, CH₂CH₂CH₃) , 1.20 (s, 9H, C(CH₃)₃), 1.48-2.31 (m, 10H, CH₂CH₂CH₃ and cyclopentyl CH₂) , 3.05-3.49 (m, 1H, cyclopentyl H-l), 3.84 (s, 3H, N⁷CH₃) , 4.07 (t, 2H, CH₂CH₂CH₃) , 6.13 (s, 2H, N³CH₂OPOM) .

8 -Cyclopentyl - 7 -methyl -1 -propylxanthine (6b) . A suspension of 6a in 4 N aqueous NaOH (4 mL, 16 mmol) was stirred at reflux for six hours. Cooling and acidification to pH 3 precipitated a yellowish solid that was dried over P₄O₁₀. Crystallization gave white crystals. Yield 107 mg, 77%. ^XH NMR (CDC1₃) δ: 0.93 (t, 3H, CH₂CH₂CH₃) , 1.50-2.36 (m, 10H, CH₂CH₂CH₃ and cyclopentyl CH₂) , 3.06-3.50 (m, 1H, cyclopentyl H-l), 3.81 (s, 3H N⁷CH₃) . 4.02 (t, 2H, CH₂CH₂CH₃) , 9.79 (s, 1H, N³H) .

8 - Cycl open tyl - 3 -me thyl -7- (pi val oyl oxyme thyl ) -1 - propylxanthine (7a) . Alkylation by general method A employed iodomethane as a alkylating reagent and a reaction time of 24 hours at 80° C. The yield was 363 mg, 93%, white crystals.

8- Cycl open tyl -3 - (me thoxye thyl ) - 7 -

(pivaloyloxymethyl) -1 -propylxanthine (7b) . General method A employed 2-chloroethyl methyl ether (182 μL, 2 mmol) as the alkylation reagent and a reaction time of

24 hours at 80° C. The yield was 338 mg, 78%, colorless oil.

8 - Cycl open tyl -3 - (2 -fl uoropropyl ) - 7 - (pivaloyloxymethyl ) -1 -propylxanthine (7c) . General method A employed l -bromo-2 -f luoropropane (282 mg, 2 mmol) as the alkylation reagent and a reaction time of

24 hours at 80° C. The yield was 170 mg, 39%, a colorless oil.

8 - Cycl open tyl -3 - (3 -fl uoropropyl ) - 7 - (pivaloyloxymethyl ) -1 -propylxanthine (7d) . General method A employed l-bromo-3-fluoropropane (282 mg, 2 mmol) as the alkylation reagent and a reaction time of 24 hours at 60° C. The yield was 201 mg, 46%, white crystals . 8 -Cyclopentyl -3 - [ (E) -3 - (tri -n-butyls tannyl )prop-2 - en-l -yl] - 7- (pivaloyloxymethyl ) - 1 -propylxanthine (7e) .

A suspension of 5a (545 mg, 1.5 mmol) and K₂C0₃ (249 mg,

1.8 mmol) in dry DMF (15 mL) was stirred for 5 minutes at room temperature before the addition of (E)-3-(tri- n-butylstannyl) -prop-2-en-l-yl tosylate (Erickson, et al., 1991; Coenen, et al . , 1983) (867 mg, 1.8 mmol).

Stirring continued at room temperature for three hours. Acidification of the reaction mixture with 2 N HCl, extraction with CH₂C1₂ (3 x 25 mL) , drying of the combined organic phases over Na₂S0₄, and evaporation of the solvent yielded 1.12 g (95%) of 7e as a colorless oil. *H NMR (CDC1₃) δ: 0.5-1.0 (m, 12H, CH₃ of POM and CH₂CH₂CH₃) , 1.1-1.75 (m, 28H, CH₂_Bu and cyclopentyl CH₂ and CH₂CH₂CH₃) , 1.20 (s, 9H, C(CH₃)₃), 3.0-3.50 (m, 1H, cyclopentyl H-l), 3.88-4.15 (q, 2H, CH₃CH₂CH₂) , 4.62- 4.75 (q, 2H, CH₂-propenyl) , 5.97-6.17 (m, 2H, CH-CH) , 6.25 (s, 2H, N⁷CH₂OPOM) .

8 - Cyclopentyl -3 - [ (E) -3 -iodoprop-2 -enyl] - 7 -

(pivaloyloxymethyl ) -1 -propylxanthine (7f) . A solution of I₂ (127 mg, 0.5 mmol) in CH₂C1₂(7.5 mL) was added dropwise to a solution of 7e (350 mg, 0.5 mmol) in CH₂C1₂ (5 mL) . After the addition of a stochiometric amount of iodine the color of iodine persisted. Evaporation of the solvent left an oily residue for purification by MPLC (silica gel 60, 25- x 1.5-cm, eluted with hexanes-ethyl acetate, 80:20; flow 15 mL/min; UV detection at 254 nm) . Evaporation of effluent collected between 6:40 and 9:25 min yielded a colorless oil (246 mg, 92%) . ^XH NMR(CDC1₃) δ: 0.95 (t,

3H, CH₂CH₂CH₃) , 1.20 (s, 9H, C(CH₃)₃) , 1.28-2.15 (m, 12H, cyclopentyl CH₂ and CH₃CH₂CH₂) , 3.05-3.47 (m, 1H, cyclopentyl H-l) , 3.85-4.15 (q, 2H, CH₃CH₂CH₂ , 4.5-4.65

(t, 2H, CH₂-propenyl) , 6.25 (s, 2H, N⁷CH₂OPOM) , 6.35-

6.83 (m, 2H, CH-CH) .

8- Cycl open tyl -3- (2 -hydroxye thyl ) -7 -

(pivaloyloxymethyl) -1 -propylxanthine (7g) . General method A employed 2-chloroethanol (134 μL, 2 mmol) as the alkylation reagent and a reaction time of 24 hours at 80° C. The yield was 310 mg, 72%, a colorless oil.

8 - Cycl open tyl -3- (2 -hydroxypropyl ) - 7 - (pi val oyl oxyme thyl ) - 1 -propylxan thine (7h) . General method A employed chloroacetone (160 μL, 2 mmol) as the alkylation reagent and a reaction of time of 24 hours at 60° C. Workup yielded a yellowish solid (415 mg, 96%, >95% pure as shown by TLC) , which underwent CTH in dry methanol (5 mL) at a bath temperature of 140° C for ten minutes. Filtration of the catalyst, washing the filter cake three times with hot methanol (3 x 10 mL) , and evaporation of the solvent gave a solid residue which was recrystallized from methanol. A yield of 382 mg, 88%, white crystal was obtained. 8 - Cycl open tyl -3- (3 -hydroxypropyl ) -7-

(pivaloyloxymethyl) -1 -propylxanthine (7i) . General method A employed 3-bromo-l-propanol (278 mg, 2 mmol) as the alkylation reagent and a reaction time of 24 hours at 80° C. The yield was 291 mg, 67%, white crystals.

8 - Cycl open tyl - 7- (pi valoyl oxyme thyl ) -3- (1 -propyn -3- yl) xanthine (7j) . A solution of 5a (720 mg, 2 mmol) in dry DMF (10 mL) containing Na₂C0₃ (212 mg, 2 mmol) was stirred for 5 minutes at room temperature. Propargyl chloride (145 mg, 2 mmol) was added via syringe, and the reaction mixture was stirred for six hours at 85° C. After cooling and addition of water (30 mL) , the mixture was extracted with chloroform (3 x 50 mL) . The combined organic phases were washed with water (2 x 25 mL) , dried over Na₂S0₄, filtered, and evaporated, and the yellowish solid was recrystallized from methanol to yield 583 mg (73%) of 7j as white crystals. ^XH NMR

(CDC1₃) δ: 0.95 (t, 3H, CH₂CH₂CH₃) , 1.20 (s, 9H, C(CH₃)₃), 1.45-2.18 (m, 10H, CH₂CH₂CH₃ and cyclopentyl CH₂) , 2.47 (t, 1H, CH₂C-CH) , 3.0-3.50 (m, 1H, cyclopentyl H-l), 3.97 (t, 2H, CH₂CH₂CH₃) , 4.85 (d, 2H, CH₂C-CH) , 6.25 (s, 2H, N⁷CH₂OPOM) .

8 -Cyclopentyl -3 -methyl -1 -propylxanthine (8a) .

General Method B. To 1 mmole of 7a-d dissolved in DMSO (5-10mL/mmol) , was added 4 N aqueous NaOH (4 mL, 16 mmol) . The solution was stirred at room temperature for 30 minutes. Slow dilution with water precipitated the 7-H xanthine. Recrystallization from an appropriate solvent yielded the analytically pure compound. Yield 254 mg, 92%. ^XH NMR (CDC1₃) δ: 1.00 (t, 3H, CH₂CH₂CH₃) , 1.50-2.38 (m, 10H, CH₂CH₂CH₃ and cyclopentyl CH₂) , 3.09-3.53 (m, 1H, cyclopentyl H-l), 3.61 (s, 3H, N³CH₃) , 4.05 (t, 2H, CH₂CH₂CH₃) , 12.76 (s, 1H, N⁷H) .

8- Cycl open tyl -3 - (me thoxye thyl ) - 1 -propylxan thine (8b) . Yield 282 mg, 88%; mp 173-175° C (MeOH-hexane) , white crystals .

8 - Cycl open tyl -3 - (2-fl uoropropyl ) - 1 -propylxan thine

(8c) . Yield 300 mg, 93%; mp 206-208° C (hexane) , white crystals . 8 - Cycl open tyl -3 - (3 -fl uoropropyl ) -1 -propylxan thine

(8d) . Yield 264 mg, 82%; mp 190-193° C (hexane), white crystals .

8-Cyclopentyl -3 - [ (E) -3 - ( tri -n-butylstannyl ) prop-2 - en- 1 -yl] -1 -propylxanthine (8e) . A solution of the POM tin compound 7e (175 mg, 0.25 mmol) in DMSO (10 mL) containing 4 H NaOH (3 mL) was stirred for five minutes at room temperature. After dilution with water (100 mL) the aqueous phase was extracted with ethyl acetate (3 x 50 mL) and the combined organic phases were dried over Na₂S0₄. The resulting oily residue was purified by MPLC, on a 1.5- x 25-cm column of silica gel 60, eluted with hexanes-ethyl acetate, 60: 40, at 20 mL/min. UV detection at 254 nm. Evaporation of the fraction eluted from 2:5 to 4:1 min yielded 544 mg (92%) of a colorless oil which solidified upon standing, mp 69-71° C. H NMR (CDC1₃) δ: 0.68-0.98 (m, 12H, C(CH₃)₃), and

CH₃.CH₂CH₂) , 1.0-1.53 (M, 18H, CH₂-C (CH₃) ₃) , 1.54-2.36 (m,

10H, cyclopentyl CH₂ and CH₃CH₂-CH₂) , 3.05-3.48 (m, 1H, cyclopentyl H-l), 3.85-4.26 (t, 2H, CH₂CH₂CH₃) , 4.75 (d,

2H, CH₂-propenyl) , 5.97-6.17 (m, 2H, CH-CH) , 12.63 (br s, 1H, N⁷H) .

8 -Cyclopentyl -3 - [ (E) -3 -iodoprop-2-enyl] -1 - propylxanthine (8f) . Compound 7f (108 mg, 0.2 mmol) was stirred in 2 N NaOH (5 mL) for 12 hours at 100° C.

Acidification with 6 N HCl, extraction with CH₂C1₂ (3 x 20 mL) , drying of the combined organic phases over

Na₂S0₄, and rotary evaporation of the solvent yielded a solid residue which was recrystallized from methanol- hexane. Yield 80 mg, 91%. H NMR(CDC1₃) δ: 0.95 (t, 3H, CH₃CH₂CH₂) , 1.25-2.3 (m, 10H, cyclopentyl CH₂ and CH₃CH₂CH₂) , 3.05-3.45 (m, 1H, cyclopentyl H-l), 3.75- 4.15 (t, 2H, CH₂CH₂CH₃) , 4.55-4.75 (d, 2H, CH₂-propenyl) , 6.45-6.95 (m, 2H, CH-CH), 12.6 (br s, 1H, N⁷H) .

8 - Cycl open tyl -3 - (2 -hydroxye thyl ) - 1 -propylxan thine

(8g) . Yield 239 mg, 78%, white crystals. 8 - Cycl open tyl -3 - (2 -hydroxypropyl ) - 1 -propylxan thine

(8h) . Yield 272 mg, 85%, white crystals.

8 - Cycl open tyl -3 - (3 -hydroxypropyl ) -1 -propylxan thine (8i) . Yield 285 mg, 89%, white crystals. 3 -Benzyl -8 - cycl open tyl -7- (pivaloyloxymethyl ) xanthine (9) . A mixture of 2 (15.5 g, 50 mmol), anhydrous Na₂-C0₃ (10.6 g, 100 mmol), and pivaloyl chloromethyl ester (15 g, 100 mmol) in dry DMF (100 mL) was stirred overnight at room temperature, diluted with water (200 mL) , and extracted with CH₂C1₂ (3 x 50 mL) . The combined extracts were washed with water and brine, dried over Na₂S0₄, and evaporated.

Product crystallized from a solution of the residue in methanol. Yield 20 g, 94%. ^XH NMR (DMSO-d₆) δ: 1.10 (s, 9H, C(CH₃)₃), 1.40-2.20 (m, 8H, cyclopentyl CH₂) ,

3.40 (br s, 1H, cyclopentyl H-l), 5.1 (s, 2H, CH₂C₆H₅) , 6.20 (s, 2H, N⁷CH₂OPOM) , 7.30 (m, 5H, C₆H₅) , 11.2 (s, 1H,

N^XH) .

3 -Benzyl - 8- cycl open tyl - 1 -me thyl -7-

(pivaloyloxymethyl ) -xanthine (10a) . General Method C.

A solution of 9 (412 mg, 1 mmol) in dry DMF (5 mL) was treated with K₂C0₃ (138 mg, 1 mmol) and iodomethane (125 μL, 2 mmol) . After stirring for four hours at room temperature, the mixture was concentrated in vacuo (bath temperature < 70° C) , the residue taken up in ethyl acetate (50 mL) , the organic phase washed with water (6 x 30 mL) , dried over Na₂S0₄ and filtered, and the solvent evaporated. The residue was recrystallized from an appropriate solvent. Yield 384 mg, 91%, white crystals. Η NMR (CDC1₃) δ: 1.16 (s, 9H, C(CH₃)₃), 1.50- 2.27 (m, 8H, cyclopentyl CH₂) , 3.05-3.47 (m, 1H, cyclopentyl H-l), 3.35 (s, 3H, CH₃) , 5.5 (s, 2H, CH₂C₆H₅) , 6.25 (s, 2H, N⁷CH₂OPOM) , 7.17-7.65 (m, 5H, C₆H₅) .

3 -Benzyl -8- cycl open tyl -1 - (2 -me thoxye thyl ) - 7 -

(pivaloyloxymethyl ) xanthine (10b) . Alkylation with 1- chloroethyl methyl ether proceeded for 24 hours at 80°

C. Yield 364 mg, 78% mp 121° C (MeOH-H₂0) .

3 -Benzyl -8 - cycl open tyl -1 - (2 -fl uoropropyl ) - 7 - (pivaloyloxymethyl ) xanthine (10c) . Alkylation with 1- bromo-2-fluoropropane proceeded for 48 hours at 80° C. Yield 394 mg, 84%, white crystals; mp 116° C (meOH) . 3 -Benzyl -8- cycl open tyl -1- (3 -fl uoropropyl ) -7 - (pivaloyloxymethyl ) xanthine (lOd) . Alkylation with 1- bromo-3-fluoropropane proceeded for 24 hours at 80° C.

The yield was 357 mg, 76%, white crystals; mp 123.9° C

(EtOH) . 3 -Benzyl -8- cycl open tyl -1 - (2 -hydroxye thyl ) - 7 -

(pivaloyloxymethyl ) xanthine (l Oe) . Alkylation with 2- chloroethanol proceeded for 24 hours at 80° C. The yield was 351 mg, 79%, yellowish oil.

3 -Benzyl -8- cycl open tyl -1 - (2- oxopropyl ) - 7 - (pivaloyloxymethyl ) xanthine (lOf) . Alkylation with chloroacetone proceeded for 24 hours at 60° C. Yield 404 mg, 87%, white crystals; mp 126.1° C. (meOH) .

3 -Benzyl -8- cycl open tyl -1 - (3 -hydroxypropyl ) - 7 -

(pivaloyloxymethyl ) xanthine (l Og) . Alkylation with 1- bromo-3-hydroxypropane proceeded for 24 hours at 80° C.

A yield of 387 mg, 72%, colorless oil was obtained. 8 - Cycl open tyl - 1 -me thyl - 7-

(pivaloyloxymethyl) xanthine (11a) . General Method D.

A mixture of 1 mmol of lOa-g, 10% Pd-C (0.5 g/g of benzyl compound), and dry ammonium formate (0.94 g, 20 mmol) in dry methanol (10 mL) was stirred in an oil bath for four hours at 140° C. After cooling the mixture was filtered through sea sand, the filter cake was washed three times with warm methanol (3 x 25 mL) , and the combined filtrates were evaporated. The resulting residue was recrystallized from an appropriate solvent. Yield 331 mg, 95%, white crystals. 'H NMR (CDC1₃) δ: 1.20 (s, 9H, C(CH₃)₃), 1.65- 2.00 (m, 8H, cyclopentyl CH₂) , 3.00-3.60 (br s, 1H, cyclopentyl H-l), 3.63 (s, 3H, CH₃) , 6.25 (s, 2H, N⁷CH₂OPOM) , 9.20 (s, 1H, N³H) .

8- Cycl open tyl -1 - (2 -me thoxye th-l -yl) -7-

(pivaloyloxymethyl ) xanthine (lib) . Yields 349 mg, 89%, white crystals. 8 - Cycl open tyl -l -) 2-fl uoropropyl ) - 7 - (pivaloyloxymeth-yl ) xanthine (lie) . Yields 361 mg,

93%, white crystals.

8 - Cycl open tyl -1 - (3 - f luoropropyl ) - 7 -

(pivaloyloxymethyl ) xanthine (lid) . Yields 339 mg, 86%, white crystals.

8 - Cycl open tyl -1 - (2 -hydroxye thyl ) - 7 -

(pivaloyloxymethyl ) xanthine (lie) . Yields 334 mg, 91%, white crystals.

8 - Cycl open tyl -1 - (2- hydroxypropyl ) - 7 - (pivaloyloxymethyl) xanthine (llf) . Yield 351 mg, 91%, white crystals.

8- Cycl open tyl -1 - (3 -hydroxypropyl ) - 7 -

(pivaloyloxymethyl ) xanthine (llg) . Yields 338 mg, 86%, yellowish oil. 8 - Cycl open tyl - 1 -me thyl - 7- (pi val oyl oxyme thyl ) -3 - propylxanthine (12a) . General Method E. A solution of

11a (348 mg, 1 mmol) in dry DMF (5 mL) was treated with

K₂C0₃ (138 mg, 1 mmol) and 1-iodopropane (195 μL, 2 mmol) . The mixture was stirred for 12 hours at room temperature and then concentrated in vacuo (bath temperature < 70° C) . A solution of the residue in ethyl acetate (50 mL) was washed with water (6 x 30 mL) to remove DMF, dried over Na₂S0₄, and filtered and the solvent evaporated. The residue was recrystallized. Yield 336 mg, 86%, white crystals. *H NMR(CDC1₃) δ:

0.95 (t, 3H, CH₃CH₂CH₂) , 1.21 (s, 9H, C(CH₃)₃), 1.45-2.20 (m, 10H, CH₃CH₂CH₂ and cyclopentyl CH₂) , 3.00-3.50 (m, 1H, cyclopentyl H-l), 3.40 (s, 3H, CH₃) , 4.07 (t, 2H, CH₃CH₂CH₂) , 6.25 (s, 2H, N⁷CH₂OPOM) . 8 -Cyclopentyl -1 - (2 -methoxy eth-l -yl) - 7 -

(pivaloyloxymethyl ) -3 -propylxanthine (12b) . Yields 356 mg, 82%, white crystals.

8 - Cycl open tyl -1 - (2-fl uoropropyl ) - 7 -

(pivaloyloxymethyl) -3 -propylxanthine (12c) . Yields 334 mg, 76%, colorless oil.

8 - Cycl open tyl -1 - (3 -fl uoropropyl ) - 7 - (pivaloyloxymethyl ) -3 -propylxan thine (12d) . Yields 301 mg, 69% , colorless oil .

8 - Cycl open tyl -2 - (2 -hydroxye thyl ) - 7 -

(pivaloyloxymethyl ) -3 -propylxanthine (12e) . Yields 315 mg, 75% , colorless oil .

8 - Cycl open tyl -1 - (2 -hydroxypropyl ) - 7 -

(pivaloyloxymethyl ) -3 -propylxanthine (12f) . Yield 359 mg , 82 % , colorless oil .

8- Cycl open tyl -1 - (3 -hydroxypropyl ) - 7 - (pivaloyloxymethyl ) -3 -propylxanthine (12g) . Yields 330 mg, 76%, colorless oil.

8 - Cycl open tyl - 1 -me thyl - 3 -propylxan thine (13a) .

General method B served for the deprotection of 13a-g.

The yield of 13a was 260 mg, 95%, white crystals. ^XH NMR (CDC1₃) δ: 0.98 (t, 3H, CH₃CH₂CH₂) , 1.45-2.30 (m, 10H, CH₃CH₂CH₂ and cyclopentyl CH₂) , 3.05-3.60 (m, 1H, cyclopentyl H-l), 3.45 (s, 3H, CH₃) , 4.10 (t, 2H, CH₃CH₂CH₂) , 12.63 (br s, 1H, N⁷H) .

8 -Cyclopentyl -1 - (2 -methoxy ethyl ) -3 -propylxanthine (13b) . Yield 219 mg, 87%, white crystals.

8 - Cycl open tyl - 1 - (2 -fl uoropropyl ) - 3 -propylxan thine (13c) . Yield 214 mg, 67%, colorless oil.

8 - Cycl open tyl -1 - (3 -fl uoropropyl ) - 3 -propylxan thine (13d) . Yield 293 mg, 91%, white crystals. 8 - Cycl open tyl -1 - (2 -hydroxye thyl ) - 3 -propylxan thine (13e) . Yield 251 mg, 84%, white crystals.

8 - Cyclopentyl - 1 - (2 -hydroxypropyl ) -3 -propylxanthine (13f) . Yield 262 mg, 82%, colorless oil.

8 - Cycl open tyl -1 - (3 -hydroxypropyl ) - 3 -propylxan thine (13g) . Yield 285 mg, 89%, white crystals.

Radioligand Binding- Studies . Competitive radioligand binding assays were performed at room temperature in a total volume of 250 μL in 50 mM Tris-

HC1, 0.02% CHAPS, pH 7.4, containing 0.2 nM [³H] CPX (4 Gbq/μmol; New England Nuclear, Bad Homburg, Germany) , 25 μg of bovine cerebral cortex membranes, 0.2 U/mL adenosine deaminase, and a CPX analogue. Binding was initiated by the addition of the cerebral cortex membranes. Incubations were terminated after two hours by filtration of a 200-μL aliquot through Whatman GF/B glass fiber filters, and filter-bound radioactivity was determined by liquid scintillation counting. Binding in the presence of 10 μM (R) -PIA (N⁶- (1R) -methyl-2- phenethyladenosine) defined unspecific binding. Equilibrium binding data were analyzed by nonlinear curve fitting using the program SCTFIT described by De Lean et al . (De Lean, et al . , 1982) Reported values of K_t represent the geometric means and 95% confidence limits of three to five independent experiments performed on duplicate samples. Example 2 :

TN VIVO IMAGING OF THE Al ADENOSINE RECEPTOR (AiAR) IN PRIMATE BRAIN USING THE NOVEL SELECTIVE AiAR- ANTAGONIST [¹⁸F] 8 -CYCLOPENTYL-3 - (3-FLUOROPROPYL) -1- PROPYLXANTHINE ( [¹⁸F] CPFPX) .

Recent data have pointed out the physiological role of the A_λ adenosine receptor (AiA ) in central neuromodulation and its involvement in the pathogenesis of different neuropsychiatric diseases. The present study focuses on the demonstration of Ai R in the brains of healthy baboons (Papio hamadryas) , n=3 , 1 male, 2 female, 9-11 kG body weight) using no carrier added

[¹⁸F] 8-cyclopentyl-3-3-fluoropropyl) -1-propylxanthine,

(n.c.a. [¹⁸F] CPFPX) and positron emission tomography (PET) . All PET studies were done under isofluorane anesthesia. Analyses of specificity and reversibility of the ligand-receptor binding were performed using the blockade of A₁ binding sites by non- labeled A_XAR antagonists (DPCPX, 0.2 μmol/kg and 0.5 μmol/kg i.v., applied 20 minutes before [¹⁸F] CPFPX injection), N⁶- cyclopentyl-9-methyladenine (N-0840, 30 μmol/kg i.v., applied 20 minutes before [¹⁸F] CPFPX injection) and carrier added [¹⁸F] CPFPX (specific activities: no carrier added, 7500 Ci/mmol, carrier added, 3-6 Ci/mmol) . The impact of changes of cerebral blood flow due to the application of the different non-labeled AIAR- antagonists was investigated using the established PET tracer of [¹⁵0] butanol . The anatomical localization of receptor-specific binding was established by superimposition on magnetic resonance images of each individual animal . [¹⁸F] CPFPX rapidly enters the brain reaching regional maximum concentrations of about 0.015% injected dose/mL (% id/mL) during the first ten minutes after tracer application. Specific binding of [¹⁸F] CPFPX to the A_XAR can be shown in thalamus (100%), cerebellum (71%) , neocortex (mean 64%) , basal ganglia

(36%) and other subcortical regions. In all regions studied the binding of the radioligand [¹⁸F] CPFPX to the

AiAR can be antagonized by DPCPX, N-0840 as well as by the non labeled ligand CPFPX. The present data demonstrate adenosine A_x receptors in regional distribution corresponding to recent data on the localization of AiAR-coding mRNA. It is concluded that [¹⁸F] CPFPX is a suitable radioligand for the non- invasive imaging of the A_XAR in experimental and clinical PET. Example 3 :

IN VIVO BIODISTRIBUTION OF THE NOVEL Al ADENOSINE RECEPTOR ANTAGONIST (AIAR ANTAGONIST) [18F] 8- CYCLOPENTYL-l-PROPYL-3- (3-FLUOROPROPYL) XANTHINE ( [18F] CPFPX) IN HEALTHY PRIMATES.

Besides its function in the central nervous system the Ai adenosine receptor (AiAR) acts as an important mediator of purinergic (neuro-) modulation in peripheral tissues and has been attributed to be involved in cardiovascular diseases. The present example investigated the biodistribution of the novel AiAR antagonist 8-cyclopentyl-3- (3-fluoropropyl) -1- propylxanthine (CPFPX) using whole body positron emission tomography (PET) in healthy primates (Papio hamadryas, n=3, 1 male, 2 female, 9-11 kg body weight) .

For this purpose CPFPX was labeled on the no carrier added (n.c.a.) level with the positron-emitting isotope fluorine-18 (t_1/2 110 min) to give n.c.a [¹⁸F] CPFPX

(specific radioactivity 7500 Ci/mmole) . All PET studies were done under isofluorane anesthesia. The regional radioactivity concentration was measured as a function of time during 60 minutes after the application of [¹⁸F] CPFPX (0.6 mCi/kg b.w. , corresponding to 26 ng/kg b.w.). Arterial blood was continuously sampled to determine the concentration of metabolically unchanged [¹⁸F] CPFPX and its putative metabolites. Regions of interest (ROIs) in whole body PET scans were identified by overlay of whole body magnetic resonance images.

Sixty minutes after the intravenous application of [¹⁸F] CPFPX, ≤0.8% of accumulated radioactivity remained in the brain, ≤0,5% in the heart, ≤11% in the liver, and ≤2.6% in the kidneys. The relative plasma fraction of unchanged [¹⁸F] CPFPX 0.5, 1, 2, 3, 5, 10, 20 and 60 minutes p.i. were 92, 91, 83, 67, 44, 31, 22 and 11%. At least two [polar metabolites are formed during the time interval studied, the same metabolites can be found in urine samples. In the heart receptor specific binding of [¹⁸F] CPFPX can be reduced due to the application of the A_XAR antagonist N-0840, 50 μmole/kg i.v., applied 20 minutes before [¹⁸F] CPFPX injection, however to a lesser extent than observed in the brain.

It is concluded that n.c.a. [¹⁸F] CPFPX is shown to be a radioligand for the central nervous system. In the periphery, the detection of the A_XAR in vivo is obscured by the formation of radioactive metabolites that bind unspecifically but with high affinity to organs of interest . Example 4 :

Fluorination of the CPX molecule at certain posi tions can increase drug potency.

The discovery that fluorination preserves or even enhances affinity for the A_XAR means such modifications improve the potency of drugs based on the xanthine pharmacophore . That discovery applies directly to drugs currently under development for (1) treating the abnormal fluid retention that occurs in heart, liver or kidney failure, (2) for the prophylaxis of acute renal failure caused by shock, infections or toxic substances, (3) for the treatment of cognitive disorders such as senile dementia and (4) for treating a variety of cardiac diseases, including the bradycardias that occur in patients with transplanted hearts or the high-grade heart block that complicates the resuscitation of victims of myocardial infarotion or cardiac arrest (See references below) .

Owing to the strength of the fluorine-carbon bond, the haloalkylxanthines are advantageously more resistant to metabolic degradation and thus have a longer duration of action. For biological reasons, the claims surpass those of the chemical modifications claimed herein (the BioGen problem) The biological effects of substituents at N-l, N-3 and N-7 on the xanthine base are often additive with those at C-8. Accordingly, the combination of, for example, a 3- (2 -fluoropropyl) group with a C-8 substituent other than a cyclopentyl group is advantageous. C-8 substituents include cycloalkyl, cycloalkenyl, bicycloalkyl, heterocycloalkyl, arelkyl and so forth. In many instances, the synthesis of such an analogue differs from that described herein, not only with regard to the exocyclic substituents (obvious) but also in the nature and sequence of the individual steps .

An example is the preparation of the 3- (2- fluoropropyl) analogue of the diuretic CVT-124, which is (±) 1, 3-dipropyl-8- (5, 6-exo-epoxy-2-endo- norbornanyl) xanthine. The synthesis begins with the regloselective deprotection of N-3 of the novel xanthine 3-benzyl-8- (endo-norborn-5-en-2-yl) -7- pivaloyloxymethyl-1-propylxanthine . Removal of the benzyl group could not be by catalytic hydrogen transfer because that would reduce the alkene moiety of the C-8 substituent and prevent the direct epoxidation required in a subsequent step . Rather one can remove the group by A1C1₃ in hot toluene (Eur J Med Chem) . The next step is alkylation at N-3 with a derivative of 2- fluoropropane (either the 2-R, S-racemate or one of the enantiomers) , that has a suitable leaving group at C-l (for example, halo, alkylsulfonoxy or arylsulfonoxy) , followed by alkaline hydrolysis of the pivaloyloxymethyl group protecting N-7. Epoxidation of the norbornenyl doubled bond could then employ any of several oxidants, either monoperphthalic or m- chloroperbenzoic acid being but two common examples . For applications that do not require radiolabeling of the xanthine N-3 substituent, one may advantageously introduce the fluoroalkyl substituent at an earlier stage of the synthesis. Thus, the alkylation of 6- amino-3-propyluracil (Muller et al . , 199X) with 3- fluoro-1-bromopropane affords 6-amino-l- (3- fluoropropyl) -3 -propyluracil, which undergoes transformations well-known in the art (Speer and Raymond, 195X) ; Scammells et al . , 1995) to form 8- substituted 3- (3 -fluoropropyl) -1-propoylxanthines . Avoiding the need for protection and deprotection of N- 3 and N-7 of the xanthine increases the efficiency of such a synthesis.

Similarly, it is possible to proceed, for example, from a precursor such as 6-amino-3- (3- fluoropropyl) uracil and, through an identical sequence of reactions well-known in the art, to arrive at 8- substituted 1,3-di- (3-fluoropropyl) xanthines . Throughout this application, various publications, including United States patents, are referenced by citation and patents by number. The disclosures of these publications and patents in their entireties are hereby incorporated by reference into^" this application in order to more fully describe the state of the art to which this invention pertains.

The invention has been described in an illustrative manner, and it is to be understood that the terminology which has been used is intended to be in the nature of words of description rather than of limitation.

Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the described invention, the invention may be practiced otherwise than as specifically described.

Table 1. Characteristics of Xanthines

and N agreed with calculated composition by ± 0.4%. ' Baaed on the 6-aminourac start ng Table 2. Antagonism of Binding of PfflCPX to Bovine Brain Cortex Membranes

no. RI-RJ-RT" UTi. nM 95% confidence limits

6 Pr-H-Me 149 132-166

8a Pr-Me-H 0.310 0.269-0.356

8b Pr-MeOEt-H 0.493 0.377-0.644

8c Pr-2FPr-H 0.094 0.085-0.103

8d Pr-3FPr-H 0.183 0.163-0.207

8f Pr-Ipren-H 0.237 0.217-0.258

CPX* Pr-Pr-H 0.170 0.161-0.180

ISa Me-Pr-H 0.526 0.420-0.659

13b MeOEt-Pr-H 0.286 0.259-J0.315

13c 2FPr-Pr-H 1.23 1.08-1.40

13d 3FPr-H 0.216 0.181-0.257

CPX' Pr-Pr-H 0.1S5 0.127-0.180

T — - Abbr rcatωns: Me, methyl; Et, ethyl; Pr, propyl; 2FPr, I 2-fluoropropyl; 3FPr, 3-fluoropropyl; Ipren, (J5}-3-iodoprop-2-en- 1-yl. * Standard for comparison of 8a-d,f. ^c Standard for comparison of 18a— d.

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Claims

CLAIMSWhat is claimed is:

1. An A_XAR antagonist xanthine containing a haloalkyl side chain at N-l and/or N-3.

2. The AiAR antagonist according to claim 1, wherein said haloakyl side chain is 3 -fluoropropyl, (┬▒) 2- fluoropropyl , the individual R- and S-enantiomers of 2- fluoropropyl and 3-iodo-2-propen-l-yl .

3. The A_XAR antagonist according to claim 2, wherein said fluorine and iodine substituents are a radioisotope.

4. The AiAR antagonist according to claim 3 , wherein the radioisotope is selected from the group consisting essentially of fluorine-18 and iodine-123.

5. The AiAR antagonist according to claim 1, wherein the N-l and N-3 side chains are methyl or 2-methoxyethyl .

6. The A_XAR antagonist according to claim 5, wherein the N-l and N-3 side chains are radioactively labeled in the methyl moiety with a carbon-11 methyl group.

7. The A_XAR antagonist according to claims 1-6, wherein the xanthine C-8 substituents are selected from the group consisting of cycloalkyl, bicycloakyl, cycloalkenyl, hetero (poly) cycloalkyl and aralkyl .

8. A method of making an A₁AR antagonist by the halogenation of another A_XAR antagonist.

9. The method according to claim 8, wherein said halogenation step includes the step of introducing an unique combination of protecting groups essential for the regioselective introduction of the halogen-containing group .

10. A method of imaging the A_XAR for medical diagnostic purposes by administering a radiofluorinated AiAR antagonist to a patient.

11. A method of improving the potency, resistance to metabolic degradation and duration of action of 1,3- dipropylxanthines by introducing 3 -fluoropropyl substituents at N-l and N-3.