WO2007096193A2 - Radiolabelled carbamates - Google Patents

Abstract

The present invention provides a process for the preparation of a 11C of 18F- labelled carbamate comprising reacting a primary or secondary amine or an anion thereof with carbon dioxide and a compound of formula RX or R1X, wherein X represents a leaving group; R represents an alkyl or alkylene containing group; R1 represents an alkyl or alkylene containing group which group comprises a 18F-label; and said group R and/or the carbon dixoide has a 11C-labelled carbon atom.

Description

90271 /01 Radiolabeled carbamates

This invention relates to a process for the production of ¹⁸F or ^πC-labelled carbamates, to their uses in radiopharmaceutical evaluation and investigation, to their use in clinical studies in humans as well as to certain new ¹⁸F or ^πC-labelled carbamates themselves.

The carbamation of amines has been utilized in the synthesis of organic carbamates for many years. The literature comprises many different ways of forming carbamates from amines although many of these processes involve phosgene, a noxious gas whose use is now discouraged. The vast majority of carbamate syntheses described in the literature concern the formation of non radioactive species and the chemistry involved is therefore quite simple and process requirements are undemanding compared to synthesis of radioactive compounds. Where, however, radioactive carbamates are being synthesised, especially with radionuclides of short half-life such as ' ¹C and ¹⁸F, syntheses are much more complicated and the synthetic chemist is faced with a number of major challenges.

¹¹C and ^l8F-labelled carbamates can be used in positron emission tomography (PET) to allow the non-invasive imaging of, inter alia, biological structures, particularly those of mammals. This can involve depiction and quantification of receptors and transporters. To be generally useful, a

1 1 152 radiopharmaceutical based on short lived radionuclides such as C and F, needs to be obtainable rapidly in high radiochemical yield and with high specific radioactivity. ¹¹C and ¹⁸F have relatively short physical half lives of 20.3 min and 109.7 min respectively, which means that any process for the manufacture of ' ¹C or ¹⁸F-labelled compounds needs to occur in a short time period and be capable of providing the desired yields and specific activity of the compounds in question.

High radiochemical yield is obviously desirable in the same way that high yield is desirable in any organic synthesis. If more ¹¹C is incorporated into the final product then the amount of active compound formed is higher and there is less wasted expensive starting material.

High specific radioactivity is an important issue in PET studies where amount of tracer should be minimized to keep the biological system unperturbed. Especially for compounds intended for use in quantifying receptors in the central nervous system (CNS), which often present at a relatively low concentration, these requirements are often limiting. Thus, these requirements impose significant limitations on the chemical-synthetical procedures used for the preparation of the

1 1 IR C or F-labelled compounds relative to non-radioactive compounds.

A further problem faced during the synthesis of ¹¹C or ¹⁸F labelled compounds is the possibility of side reactions. In conventional non radioactive synthesis, stoichiometric amounts of reactants would be used. This is often not possible in ¹¹C or ¹⁸F compound synthesis and there is typically considerable molar excess of reagents and reactants other than the ¹¹C or ¹⁸F -containing species. The difference in the molar ratio of the ¹¹C or ^l8F-containing reagent relative to other reagents present in the reaction mixture (including the molecule or species to be radiolabeled, called the precursor for radiolabelling) can represent an important difference to procedures in which the stoichiometry of reagents is in the range normally encountered in the synthesis of non-radioactive compounds. In particular, due to the relatively low amount of ¹¹C or ¹⁸F -containing reactant present, impurities in the reaction mixture can, if they possess a comparable or higher reactivity than the precursor for radiolabelling, be an important loss of yield. Thus, side-reactions that are of low importance for a high yield of a given product in a non-radioactive syntheses, can be a limiting factor for ¹¹C and ¹⁸F-radiolabelling reactions.

A further issue with ¹¹C and ¹⁸F syntheses is the potential exposure of the chemist to radiation. ¹¹C and ¹⁸F syntheses of radiopharmaceuticals for use in PET are typically performed in lead shielded boxes (called hot-cells) and the manipulation of reagents, reactants and equipment is performed via a remote controlled apparatus (called the syntheses module). Under such conditions, the number of synthetic steps performed with the radioactivity present greatly affects the performance, including reliability and reproducibility, of the procedure. It is therefore advantageous to have as few steps in the procedure as possible in order to ensure a high performance of the radiosynthesis and reduce the complexity of the apparatus required. C and F-labelled carbamates may have utility in the evaluation of several

1 1 1 δ biological targets. One example is the use of a C or F -labelled carbamates for the investigation of kappa-opioid receptors (κ-0R). The opioid receptors (ORs) belong to the superfamily of G protein-coupled receptors (GPCRs) that produce their effects by activation of intracellular G proteins. The kappa-opioid receptor (κ-OR) is one of three well-defined classes of ORs. The central opioid system plays a role in several physiological and pathophysiological phenomena. Different drugs of abuse, such as heroine, cocaine and amphetamine stimulate dopamine release in the ventral striatum, which includes the nucleus accumbens. Striatal dopamine release is stimulated by κ-OR-activation, the center of origin of mesolimbic dopaminergic projections to the ventral striatum, and inhibited by striatal K-OR. Moreover, dynorphin-mediated κ-OR-activation may represent a homeostatic mechanism that limits cocaine-induced dopamine release in the ventral striatum. It is also likely that during cocaine withdrawal, κ-OR-activation contribute to dysphoric mood states and thus increase the relapse risk.

In vivo brain imaging with κ-0R selective radioligands has the potential to allow assessment of the opioidergic system in drug-dependent humans, to examine the psychologic correlates of its functional state, and to develop new strategies to target drug effects and alleviate drug addiction. The κ-0R has also been implied in Alzheimer's disease, Tourette's syndrome, and epilepsy. It is thus extremely interesting for commercial and academic researchers to compare the neurologic and psychologic correlates of K-OR function across these different neurological and psychiatric disorders and to explore potential neuroprotective effects with molecular imaging. Several opioid receptor (OR) radiotracers are currently available for PET in humans, including [' 'CJcarfentanil, [¹⁸F]diprenorphine, [^πC]buprenorphine, [¹⁸F]fluorocyclofoxy and [' ^l C-methy Ijnaltήndole but none is selective for the kappa- opioid receptors (κ-OR).

(±)-4-Methoxycarbonyl-2-[(l -pyrrolidinylmethyl]- 1 -[(3,4- dichlorophenyl)acetyl]-piperidine (GR89696)

is a highly potent and selective κ-0R agonist (J Med Chem 36, 2075-83, 1993 Naylor et al). The (-)-isomer, (-)-4-methoxycarbonyl-2-[(l-pyrrolidinylmethyl]-l- [(3,4-dichlorophenyl)acetyl]-piperidine, also known as GR 103545, is the more potent optical isomer of GR89696. In Nucl. Med Biol 1999, 26:737-741, Ravert et al. reported the radiolabelling of the racemic [ⁿC]GR89696 using the reaction of ¹¹C methyl chloroformate with the normethylcarbamoyl GR 89696 (i.e. GR89696 without the COOCH₃ group).

The synthetic procedure is based on reacting the amine precursor in question with ["CJmethyl chloroformate obtained via reduction of cyclotron produced [ⁿC]carbon dioxide to [^uC]methanol followed by reaction with phosgene. A recent report of the synthesis of [^πC]GR103545 indicated the method to give only a very low radiochemical yield of the product (~2 %) and low specific activity of 5.55 to 18.31GBq/μmol (150-490mCi/μmol) in a syntheses time of -50 min after end-of- bombardment. (Talbot et al. J Nucl Med 2005; 46:484-^94).

Both these factors severely limit the applicability of the compound for preclinical and clinical use. Moreover, for the preparation of radiopharmaceuticals for use in PET-investigations using remote controlled syntheses, a high operational complexity is limiting and disadvantageous. This methodology for ^uC-carbamate formation is limited by the need for complex procedures and the reaction also relies on the use of toxic phosgene.

1 1 I R

It has now been surprisingly found that C or F-labelled carbamates can be prepared in high yield and rapidly by the conversion of an amine to a carbamate in a one-pot, two-step reaction sequence comprising reacting carbon dioxide with a primary or secondary amine followed by reaction of the intermediate with an alkylating agent. The reaction can be performed in two ways: i) using radioactive [¹¹C]CO₂ in the first step and a non-radioactive alkylating agent in the second or ii) using non-radioactive CO₂ in the first step and a radioactive alkylating agent in the second, for example a ¹¹C or ¹⁸F-labelled alkylating agent. The reaction is particularly effective when the amine reacts in the presence of a carbonate and/or an ammonium salt, especially in the presence of excess carbon dioxide. The amine can be deprotonated prior to reaction to form the corresponding anion although it is preferably used in its uncharged form.

Thus, viewed from one aspect the invention provides a process for the preparation of a ' ¹C of ¹⁸F-labelled carbamate comprising reacting a primary or secondary amine or an anion thereof with carbon dioxide and a compound of formula RX or R¹X, preferably in the presence of a metal carbonate and an ammonium salt, wherein X represents a leaving group; R represents an alkyl or alkylene containing group;

R¹ represents an alkyl or alkylene containing group which group comprises a ¹⁸F-label; and said group R and/or the carbon dioxide has a ' ^-labelled carbon atom.

Viewed from another aspect the invention provides a process for the preparation of a ^l ^-labelled carbamate comprising reacting a primary or secondary amine or an anion thereof with carbon dioxide and a compound of formula RX, preferably in the presence of a metal carbonate and an ammonium salt, wherein X represents a leaving group; and

R represents an alkyl or alkylene containing group; said group R and/or the carbon dioxide having a ' 'C-labelled carbon atom.

Viewed from another aspect the invention provides a process for the preparation of a ¹⁸F-labelled carbamate comprising reacting a primary or secondary amine or an anion thereof with carbon dioxide and a compound of formula R¹X, preferably in the presence of a metal carbonate and an ammonium salt, wherein R¹ represents an alkyl or alkylene containing group which group comprises a F-label; and

X represents a leaving group. Viewed from another aspect the invention provides a ¹¹C or ¹⁸F-labelled carbamate prepared by a process as hereinbefore described. Viewed from another aspect, the invention provides a pharmaceutical composition containing such a carbamate, e.g. a pharmaceutically acceptable solution of said carbamate. Viewed from another aspect, the invention provides the use of a ¹¹C or ¹⁸F- labelled carbamate prepared by the process of the invention in the manufacture of a medicament /radiopharmaceutical for use in a method of diagnosis or treatment, such as a method for investigation of the status of receptors in the CNS. Investigation of κ-opioid receptors may allow the diagnosis or treatment of diseases, such as those of the central nervous system. Specific diseases/conditions which could be treated or diagnosed include drug abuse and drug addiction, pathological pain, Tourette's syndrome, epilepsy, psychosis, hallucinations, or neurodegenerative diseases such as Parkinson's and Alzheimer's disease and prion-mediated disease.

Viewed from another aspect the invention provides a method of diagnosis or treatment of a condition/disease as described above comprising administering to a patient a carbamate made by the process of the invention and detecting the presence of the carbamate in the patient (e.g. using PET).

As used herein a ^l 'C-labelled carbamate is a compound comprising the linkage -N-CO-O-R where the group R contains a ^πC-labelled carbon atom and/or the carbonyl carbon is ' ¹C labelled. It will be appreciated however that it is highly preferred if only one carbon atom is labelled. A F labelled carbamate will comprise the linkage -N-CO-O-R¹ with an ¹⁸F label in the R¹ group. Again, whilst it is possible that, for example, ¹¹C labelled CO₂ could be used to give a ¹¹C label in this linkage, preferably ¹⁸F is the only radio centre in the molecule (i.e. non radioactive carbon dioxide is employed in the synthesis when the compound R¹X is utilised).

The group X is a leaving group. The skilled chemist is aware of numerous leaving groups regularly used in organic synthesis and any of these may be used here. Preferred leaving groups are listed in standard text books such as Jerry March, Advanced Organic Chemistry and include alkoxys, halides and sulphonate esters. Preferred halides are chloride and iodide. Preferred sulphonate esters are brosylate, nosylate, mesylate, nonaflate, tresylate, triflate or tosylate, especially triflate, tosylate or mesylate. Halides and sulphonate esters are preferred in particular sulphonate esters of low molecular mass. Preferably therefore X is iodide or triflate, especially triflate (trifiuoromethane sulphonyl CFsSO₂O).

The leaving group X is preferably bound to a saturated aliphatic carbon atom in the group R or R¹. Hence the R/R¹ group may contain an alkyl group or alkylene group. Thus, the R/R¹ group may comprise at least an alkyl group such as methyl or alkylene group such as -CH₂- from which the leaving group X can be substituted. The group R can also contain many other functional groups such as aryl, cycloalkyl, heterocyclic groups as well as functionalities such as hydroxyl, and other non nucleophilic and non electrophilic groups. R can therefore vary considerably as long as other functional groups present therein do not interfere with the desired carbamate formation reaction.

Whilst it is possible for the ¹¹C label to be introduced via the carbon dioxide, better yields are obtained when non-radioactive carbon dioxide is employed and the ¹¹C label is introduced into the carbamate through the R group. Preferably therefore, the R group contains a ¹¹C carbon atom. The ⁿC-label is preferably present on a saturated aliphatic carbon atom, especially the atom which binds to the leaving group X.

Preferred groups R are Cj_-2O hydrocarbyl groups, more preferably Ci_-1O- hydrocarbyl groups. Representative groups R therefore include ' ^-labelled benzyl (e.g. ¹¹CH₂-Ph) and ^uC-labelled ethylphenyl (e.g. ¹¹CH₂CH₂-Ph).

Most preferably however, R is any ^πC-labelled alkyl group, preferably a C_1-7 alkyl group, especially "C-labelled linear alkyl group such linear C_1-7 alkyl group, e.g. a ^πC-ethyl or a ^u C-labelled methyl group. Highly preferred groups RX are therefore ¹¹C labelled Ci_-7 alkyl halides or sulphonates esters, especially methyl halides or methyl sulphonate esters. Most preferably, RX is ¹¹C labelled methyl iodide or ^π C-labelled methyl triflate.

When a ¹⁸F labelled carbamate is required, it is necessary to use an alkylating agent of formula R¹X. It will be appreciated that in this reaction, it is highly preferred if the carbon dioxide used is non radioactive. The group R¹ comprises an ¹⁸F label as opposed to a ¹¹C label which can be present in the group R but can otherwise have the same structure. Preferred groups R¹ are therefore a Ci_-2Q hydrocarbyl group carrying an ¹⁸F substituent, more preferably Ci-io-hydrocarbyl group carrying an ¹⁸F substituent. Representative groups R therefore include ¹⁸F- labelled benzyl (e.g. ¹⁸FCH₂-Ph) and ¹⁸F-labelled ethylphenyl (e.g. ¹⁸FCH₂CH₂-Ph).

Most preferably however, R¹ is any ¹ F-labelled alkyl group, preferably a C_{1- 7} alkyl group carrying an F substituent, especially F-labelled linear alkyl group such linear Ci_-7 alkyl group carrying an ¹⁸F substituent. It is preferred, if possible, that the ¹⁸F substituent is attached to a different carbon atom than the leaving group X.

Especially preferred groups R¹ are [¹⁸F]fluoromethyl, 2- [¹⁸F]fluoroethyl, 3- [¹⁸F]-fluoropropyl, 4-[¹⁸F]fluorobutyl, 5- [¹⁸F]fluoroρentyl 6-[¹⁸F]-fluoroehexyl and 7-[¹⁸F]fluoroheptyl and isomer thereof.

¹⁸F is a poor leaving group so it is easily possible to avoid substitution of the radioactive atom by the use of an X group which is a more potent leaving group. Halides such as iodide, bromide and chloride as well as the sulphonate esters are all much better leaving groups than F.

It is preferred if the reaction of the carbon dioxide, amine (or its anion) and compound RX/R'X is effected in the presence of a metal carbonate. The metal carbonate can be formed from any metal in the periodic table such as a Al, a transition metal or rare earth metal. Preferably, however the metal carbonate is from group (I) or (II) especially group (I). Highly preferably, the carbonate is sodium, potassium or caesium carbonate, especially caesium carbonate.

It is preferred if the reaction of the carbon dioxide, amine (or its anion) and compound RX/R'X is effected in the presence of an ammonium salt. The ammonium salt may be of formula NR₄Y where each R' is independently hydrogen, aryl or an Ci_-I0 alkyl. Preferably, R' is an C_1-6 alkyl group such as methyl, ethyl, propyl, isopropyl, n-butyl (Bu) or tertbutyl. Most preferably all R' groups are the same. A highly preferred ammonium ion is NBu₄ ⁺.

The group Y can be any suitable counterion such as a halide ion, or sulphonate ester anion, e.g. as described above. Y is preferably iodide or triflate (OSO₂CF₃). Highly preferred ammonium salts are therefore NBu₄I and NBu₄Tf.

Ideally, both a metal carbonate and an ammonium salt will be employed. The amounts each reagent used will be readily determined by the skilled man. It will be appreciated that all reagents/reactants other than the radio labelled material will likely be in excess. A typical quantity, in terms of activity from a ¹¹C- alkylating agent, is in the range of 370MBq-37GBq (lOmCi-lCi) (although it is possible to have starting activities of less than 370GBq too). The typical specific activity of a ^πC-alkylating agent is 1000-6000mCi/μmol (37-222 GBq/μmol). Thus, the quantity of the ' ^-species used in a typical labelling would be in the range lOnmol-1 μmol.

It is preferred also if there are molar excesses of the ammonium salt and the carbonate relative to the amine (or its anion). Thus the molar ratio of amine(or its anion) to ammonium salt and/or carbonate may be 2 to 10, preferably about 2 to 5, especially around 3. Carbon dioxide is preferably present in excess (assuming it is non radioactive).

The reaction occurs very rapidly. Thus, the time between addition of the radiolabeled material to starting isolation of the target material may be less than 10 mins, preferably less than 5 minutes. High yields are obtained in the examples in as little as 2 minutes. The overall reaction time from the end of bombardment (i.e. from the formation of the radiolabeled starting material), to isolation of the desired carbamate can be less than 45 mins, preferably less than 40 mins. The reaction can be carried out at any desired temperature but it is preferred if the reaction is not heated. It has been found that the best yields of carbamate are achieved at ambient temperature (i.e. 25°C). Preferably therefore the reaction temperature is in the range 15 to 40°C, preferably 20 to 35°C.

Typically, the reaction will take place in a non protic solvent. The skilled chemist can select any known non-protic solvent but preferred solvents include DMF, THF, DMSO or acetamide. DMF is preferred.

The radiochemical reaction yield is preferably at least 10%, more preferably at least 20%, e.g. at least 25%, especially at least 30 %. Yields of greater than 50%, e.g. greater than 60% can also be achieved. The reactions described herein are preferably effected with no carrier added.

¹ 'C-labelled starting materials can be prepared by conventional techniques typically from cyclotron produced "C-labelled carbon dioxide. For example, ¹¹C- labelled methylene iodide can be prepared from cyclotron produced ^πC-labelled CO₂ by a catalytic iodination reaction via ' C-labelled methane as is well known in the art. Conversion to the triflate is achieved readily, e.g. as described in the examples. Alternative procedure is the "wet phase" iodination, reduction of CO2 to methanol with LAH followed by HI treatment (see fx. Stone-Elander et al. J Med Chem 1985 Sep;28(9): 1325-8.

¹⁸F labelled starting materials can be prepared using published techniques such as described previously by Henriksen et al. (J Labeld Compds Radiopharm. 2005; 48: 771-779) and Iwata et al. (Appl Radiat Isot. 2002;57:347-352), for [¹⁸F]fluoroethyl tosylate and [¹⁸F]fluromethyl bromide, respectively.

The amine on which this reaction is carried out can vary greatly but it needs to be a primary or secondary amine. Ideally, the amine will be free of other functional groups which would interfere with carbamate formation, e.g. other highly nucleophilic functionalities unless protecting groups are introduced on such functionalities. The amine can however comprise all manner of organic structures such as alkyl, alkenyl, alkynyl, aryl, heterocycles, and groups such as esters, epoxides, ethers, halides etc.

In one embodiment, the primary or secondary amine can be deprotonated prior to reaction with carbon dioxide and the compound RX/R'X so as to form an anion of the primary of secondary amine. Deprotonation could be effected using any suitable base such as an alkyl lithium compound, e.g. BuLi. Carbamate formation does however take place readily with the non deprotonated amine so deprotonation is preferably not effected as it adds a potentially unnecessary step to the process. The person skilled in the art will appreciate that this technique is broadly applicable and the nature of the amine on which the reaction is carried out is not critical as long as the reactive amine group is primary or secondary.

In a highly preferred embodiment, the amine on which the reaction is carried out is the normethylcarbamoyl of (+) GR 89696, (+)GR 89696 or (-) GR89696 (also known as GR103545) (i.e. GR89696 without the COOCH₃ group).

Thus, viewed from another aspect the invention provides a process for the conversion of a compound of formula (II)

to a ^uC-labelled or ¹⁸F labelled carbamate of formula (iii) or (iv)

(* represents a chiral centre which may be (+), (-) or racemic) comprising reacting said compound of formula (II) with a compound RX or R¹X as hereinbefore defined and carbon dioxide, preferably in the presence of a metal carbonate and an ammonium salt. When R is methyl, the compound formed is GR89696.

Preferably R represents ¹¹C labelled methyl group. Preferably the metal carbonate is caesium carbonate. Preferably the ammonium salt is NBu₄TfOr NBu₄I. Preferably X is triflate or a halide.

This process for the manufacture of GR89696, in particular its (-) enantiomer GR103545, yields the compounds in higher labelling yields and higher specific activities than previously have been obtainable.

This technique is, of course, applicable to other similar amino compounds. Viewed from another aspect therefore, the process of the invention may be carried out on an optionally substituted amine of formula (V)

(V)

The amine of formula (V) may carry a wide variety of substituents although again, these should not interfere with the reaction claimed. Any nucleophilic substituents should therefore be protected. The skilled man will appreciate that all manner of substitutions could be made to this backbone.

It is preferred however, if the phenyl ring carries at least one halide substituent, e.g. two halides. Preferably, these are chlorides, most preferably meta and para to the acetyl linker. The piperazine ring may be substituted by a group (CH₂) NR₃R₄ wherein R₃ OrR₄ independently represents a hydrocarbyl group (e.g. a Ci_-6 hydrocarbyl group such as a Ci_-6 alkyl group) or both R₃ and R₄ taken together form an optionally substituted non aromatic hydrocarbyl ring, e.g. a five or 6 membered ring, which may be saturated or unsaturated. This ring may itself have substituents such as halides, hydroxyl, oxo, alkyl, alkenyl, alkoxy, alkyl ester, or =CCO₂C_1-6alkyl.

The invention also encompasses compounds having the backbone of formula (V) which have undergone the process claimed to form a ¹⁸ F or ' ¹C labelled carbamate. Viewed from another aspect therefore, the invention provides an optionally substituted, ¹¹C or ¹⁸F labelled compound of formula (VI) or (VII)

(VI) (VII)

where R/R¹ are as hereinbefore defined with the proviso that R is not ' ¹CH₃ or a salt thereof.

The compound GR89696 or an enantiomer thereof ¹¹C labelled on the ester carbonyl is of particular interest and also forms an aspect of the invention. It will be appreciated that the carbamates of the invention can be subsequently converted into salts if desired.

Since the reaction described here is carried out in one pot, it lends itself to ready automation using conventional techniques. The reaction claimed can therefore be readily carried out in hot-cells thus minimising exposure of workers to hazardous radiation.

I R 1 1

F and C-labelled carbamates formed by the process of the invention can be formed into medicaments and used as radioligands for imaging, in particular for imaging the κ-opioid receptors. Imaging of this system may allow advances in the treatment and/or diagnosis of various conditions such as drug abuse and drug addiction, pathological pain, Tourette's syndrome, epilepsy, psychosis, hallucinations, or neurodegenerative diseases such as Parkinson's and Alzheimer's disease and prion-mediated disease. The opioid system is also critical in addiction and psychosis and the assessment of this system in individuals with these ailments may allow new diagnoses to be made. Individuals addicted to alcohol, cocaine, amphetamines and opioid based drugs such as heroin may benefit from treatments or diagnoses made using the carbamates of the invention.

The carbamates of the invention can be used immediately after formation in the techniques described above. Since time is so critical in the use of these compounds, they will typically be administered by injection in a saline solution. The skilled worker in this field is aware of the necessary injection protocols and dosages which need to be used.

The carbamates are of particularly utility in PET and the invention further comprises therefore the use of the carbamates of the invention in the manufacture of a medicament /radiopharmaceutical and formulation thereof for use in a method of diagnosis using positron emission tomography.

The invention will now be described with reference to the following non- limiting examples. Example 1.

Carbamate 2 was found in moderate yield (range 18-36 %) via the three component coupling of amine 1, CO₂, and [ ⁿC]methyl iodide. Full results are presented in Table 1. Here, 2 mg of 1 was dissolved in 300 μl of DMF containing 3 molar equivalents Of Cs₂CO₃ and 3 molar equivalents of tetrabutyl ammonium iodide. CO₂ (15 ml/min) was bubbled through the solution for one hour. Cyclotron produced [¹¹C]CO₂ was converted to [¹¹C]CH₃I by the catalytic gas-phase iodination reaction via [¹¹C]CH₄. [¹¹C]CH₃I, swept with a He-flow at 50 ml/min into the reaction vial. The reaction mixture was allowed to stir at different temperatures for different time intervals up to 5 min. Aliquots drawn from the reaction mixture were analyzed by means of UV/radio-HPLC. Product identity was confirmed by coelution of the radioactive product with authentic, non radioactive, 2.

Example 2

CO₂, ["CJmethyl triflate

Cs₂CO₃, TBATf, DMF

Carbamate 2 was found in high yields (range 54-72 %) through the three component coupling of amine 1, CO₂, and [ ⁿC]methyl triflate. Full results are presented in Table 1. Here, 2 mg of 1 was dissolved in 300 μl of DMF containing 3 molar equivalents of Cs₂CO₃ and 3 molar equivalents of tetrabutyl ammonium trifluoromethane sulfonate. CO₂ (15 ml/min) was bubbled through the solution for one hour. Cyclotron produced [¹¹C]CO₂ was converted to [¹¹C]CH₃I by the catalytic gas-phase iodination reaction via [¹¹C]CH₄. [¹¹C]CH₃I, swept with a He-flow at 50 ml/min through a column of 1.6mm internal diameter, length 50 mm column filled with silver triflate and thereafter into the reaction vial. The reaction mixture was allowed to stir at different temperatures for different time intervals up to 5 min. Aliquots drawn from the reaction mixture were analyzed by means of UV/radio- HPLC. Product identity was confirmed by coelution of the radioactive product with authentic non radioactive 2.

Table 1. Reaction yield¹ of N-benzyl methyl-carbamate (2) under various conditions

Tetrabutyl Methylation Reaction Reaction Yield of ammonium reagent temperature time carbamate compound (⁰C) interval product (min) (%)

TBAI [¹¹C]CH₃I 25 5 36

TBAI [¹¹C]CH₃I 50 5 21

TBAI [¹¹C]CH₃I 70 5 18

TBATf [¹¹C]CH₃I 50 5 25

TBATf [¹¹C]CH₃I 70 5 19

TBATf [¹¹C]CH₃OTf 25 2 69

TBATf [¹¹C]CH₃OTf 25 5 72

TBATf [¹¹C]CH₃OTf 35 2 62

Analytical yield as measured by radio-HPLC. TBAI = tetrabutyl ammonium ioidide, TBATf= tetrabutyl ammonium trifiuoromethane sulfonate.

Example 3.

The fumarate acid salt of 3 (GR89696 ) 75 mg (0.14 mmol) was dissolved in THF (4 mL) and heated and stirred at reflux. Bu₄NF (1.8 mL, 1 M in THF) was added and the reaction mixture was refluxed for 5 h. After cooling, a solution OfK₂CO₃ satd. (10 mL) was added and the aqueous phase was extracted with EtOAc (3 x10 mL). The combined organic layers were dried over MgSO₄, filtered and the solvent was removed under reduced pressure. The resulting oil was purified by chromatography using kieselgel 60 (0.040-0.063 mm) n-hexane and ethyl acetate as eluents (50:50, volume/volume). The fractions containing the products were combined and solvent was removed under reduced pressure yielding 4 (41 mg, 82 %).

Example 4

In preparation for ^uC-labeling reactions, 2 mg of the 4 was dissolved in 300 μl of DMF containing 3 molar equivalents OfCs₂CO₃ and 3 molar equivalents of tetrabutyl ammonium iodide. CO₂ (15 ml/min) was bubbled through the solution for one hour. Cyclotron produced [¹¹C]CO₂ was converted to [^{l 1}C]CH₃I by the catalytic gas-phase iodination reaction via [¹¹C]CH₄. [¹¹C]CH₃I, swept with a He-flow at 50 ml/min into the reaction vial. The reaction mixture was allowed to stir at different temperatures for different time intervals up to 5 min. Aliquots were drawn from the reaction mixture and analyzed by means of UV/radio-HPLC. Product identity was confirmed by coelution of the radioactive product with authentic 3. Example 5

2 mg of 4 was dissolved in 300 μl of DMF containing 3 molar equivalents of Cs₂CO₃ and 3 molar equivalents of tetrabutyl ammonium trifluoromethane sulfonate. CO₂ (15 ml/min) was bubbled through the solution for one hour.

Cyclotron produced [¹¹C]CO₂ was converted to [¹¹C]CH₃I by the catalytic gas-phase iodination reaction via [¹¹C]CH₄. In some experiments [¹¹C]CH₃I, swept with a He- flow at 50 ml/min through a column of 1.6mm internal diameter, length 50 mm column filled with silver triflate and thereafter into the reaction vial. The reaction mixture was allowed to stir at different temperatures for different time intervals up to 5 min. Aliquots were drawn from the reaction mixture and analyzed by means of UV/radio-HPLC. Product identity was confirmed by coelution of the radioactive product with authentic 3. Results are presented in Table 2.

Table 2. Reaction yield¹ of GR89696 (3) under various conditions

Tetrabutyl Methylation Reaction Reaction Yield of ammonium reagent temperature time interval product (%) compound²

TBATf [¹¹C]CH₃I 25 5 34

TBATf [¹¹C]CH₃OTf 25 2 65

TBATf [¹¹C]CH₃OTf 25 5 69

¹ Analytical yield as measured by radio-HPLC. ² TBATf= tetrabutyl ammonium trifluoromethane sulfonate.

Example 6

2 mg of the 4 was dissolved in 300 μl of DMF containing 3 molar equivalents of Cs₂CO₃ and 3 molar equivalents of tetrabutyl ammonium trifluoromethane sulfonate. CO₂ (15 ml/min) was bubbled through the solution for one hour.

Cyclotron produced [¹¹C]CO₂ was converted to [¹¹C]CH₃I by the catalytic gas-phase iodination reaction via [¹¹C]CH₄. [¹¹C]CH₃I, swept with a He-flow at 50 ml/min through a column of 1.6mm internal diameter, length 50 mm column filled with silver triflate and thereafter into the reaction vial. The reaction mixture was stirred at ambient temperature for 2 min. The mixture was added 1 ml of a mixture consisting of acetonitrile 0.1 M ammonium formiate ( 25:75, v/v) and purified by preparative HPLC on a μ-Bondapak 10 mm internal diameter, length 250 mm, 5 μm particles, eluted with acetonitrile: 0.1 M ammonium formate ( 25:75, v/v) at a flow rate of 8 ml/min The product eluting at 12.3 min were collected. Product identity of [ⁿC]3 was confirmed by coelution with authentic 3 as analyzed by UV/radio-HPLC. [ⁿC]3 was isolated in a radiochemical yield of 58±4 %, a specific activity in the range of 1247-1562mCi/ μmol in a syntheses time of 32 min after end-of- bombardment, and with a radiochemical purity of > 97 %.

Example 7

The ¹⁸F-fluoroalkylating agents 2-[¹⁸F]fluoroethyltosylate and [^l8F]fluoromethyl bromide were prepared as described previously by Henriksen et al. (J. Labeld. Compds. Radiopharm. 2005; 48: 771-779) and Iwata et al. (Appl Radiat Isot. 2002;57:347-352), respectively.

2 mg of 1 was dissolved in 300 μl of DMF containing 3 molar equivalents of Cs₂CO₃ and 3 molar equivalents of tetrabutyl ammonium trifiuoromethane sulfonate. CO₂ (15 ml/min) was bubbled through the solution for one hour. After addition of the F-fluoroalkylating agent in question, the reaction mixture was allowed to stir at different temperatures for different time intervals. Aliquots drawn from the reaction mixture were analyzed by means of UV/radio-HPLC.

Carbamic acid derivatives 5 and 6 was found in yields within the range of 2-23 % via the three component coupling of amine 1, CO₂, and ¹⁸F-fluoroalkylation agents. Full results are presented in Table 3.

Table 3. Reaction yield¹ of N-benzyl-¹⁸F-fluoroalkyl-carbamic acid derivatives under various conditions

Tetrabutyl Fluoro-alkylating Reaction Reaction Yield of ammonium reagent temperature time carbamate compound² (⁰C) interval product (min) (%)

TBAOTf [¹⁸F]CH₂Br 70 10 7

TBAOTf [¹⁸F]CH₂Br 50 10 11

TBAOTf [¹⁸F]CH₂Br 35 5 18

TBAOTf [¹⁸F]CH₂Br 35 10 23

TBAOTf [¹⁸F]CH₂CH₂OTs 70 10 < 3

TBAOTf [¹⁸F]CH₂CH₂OTs 50 5 6

TBAOTf [¹⁸F]CH₂CH₂OTs 50 1 10

TBAOTf [¹⁸F]CH₂CH₂OTs 50 15 12

¹ Analytical yield as measured by radio-HPLC. TBAOTf= tetrabutyl ammonium trifluoromethane sulfonate.

Claims

Claims

1. A process for the preparation of a ¹¹C of ¹⁸F-labelled carbamate comprising reacting a primary or secondary amine or an anion thereof with carbon dioxide and a compound of formula RX or R¹X, wherein X represents a leaving group; R represents an alkyl or alkylene containing group;

R¹ represents an alkyl or alkylene containing group which group comprises a ¹⁸F-label; and said group R and/or the carbon dixoide has a ' 'C-labelled carbon atom.

2. A process as claimed in claim 1 for the preparation of a ^uC-labelled carbamate comprising reacting a primary or secondary amine or an anion thereof with carbon dioxide and a compound of formula RX, wherein X represents a leaving group; and

R represents an alkyl or alkylene containing group; said group R and/or the carbon dioxide having a ⁿC-labelled carbon atom.

3. A process as claimed in claim 1 for the preparation of a ¹⁸F-labelled carbamate comprising reacting a primary or secondary amine or an anion thereof with carbon dioxide and a compound of formula R¹X, wherein R¹ represents an alkyl or alkylene containing group which group comprises a ¹⁸F-label; and

X represents a leaving group.

4. A process as claimed in any preceding claim wherein X is a halide or sulphonate ester.

5. A process as claimed in any preceding claim wherein X is iodide or triflate.

6. A process as claimed in any preceding claim wherein R is a ' 'C-labelled Ci_-7 alkyl group.

7. A process as claimed in any preceding claim wherein R is ' ¹C labelled methyl.

8. A process as claimed in any preceding claim wherein the carbon dioxide is non radioactive.

9. A process as claimed in any preceding claim wherein R¹ is Cj_-7 alkyl group carrying an ¹⁸F label.

10. A process as claimed in any preceding claim wherein the reaction is effected in the presence of a metal carbonate.

11. A process as claimed in any preceding claim wherein said carbonate is caesium¹ carbonate.

12. A process as claimed in any preceding claim wherein the reaction is effected in the presence of an ammonium salt.

13. A process as claimed in any preceding claim wherein the ammonium salt is N(Ci_-6 alkyl)₄ ⁺ halide or sulphonate ester.

14. A process as claimed in any preceding claim wherein the ammonium salt is NBu₄TfOr NBu₄I.

15. A process as claimed in any preceding claim wherein said amine is an optionally substituted amine of formula (V)

16. A process as claimed in any preceding claim wherein the amine is the normethylcarbamoyl analogue of GR 89696 or an enatiomer thereof.

17. A ¹⁸F or ⁿC-labelled carbamate prepared by a process as hereinbefore described or a salt thereof.

18. A pharmaceutical composition containing such a carbamate or salt thereof, e.g. a saline solution of said carbamate.

19. The use of a ¹⁸F or ⁿC-labelled carbamate as hereinbefore described in the manufacture of a medicament for use in a method of diagnosis or treatment of a disease or condition, such as drug addiction, Tourette's syndrome, epilepsy, psychosis or Alzheimer's disease.

20. A method of diagnosis or treatment of a condition as described above comprising administering to a patient a carbamate of the invention.

21. An optionally substituted ¹¹C or ¹⁸F labelled compound of formula (VI) or (VII)

(VI) (VII)

where R/R¹ are as hereinbefore defined with the proviso that R is not ¹¹CH₃ or a salt thereof.

22. The compound GR89696 or an enatiomer thereof ¹¹C labelled on the ester carbonyl or a salt thereof..