PALLADIUM-CATALYSED NEUROLEPTIC SYNTHESIS

a thesis presented by PEYMAN OSTOVAR

in partial fulfilment of the requirem ents for the degree of

DOCTOR OF PHILOSOPHY OF THE

UNIVERSITY OF LONDON

DEPARTMENT OF CHEMISTRY IMPERIAL COLLEGE LONDON SW7 2AY

SEPTEMBER 1989

1

To Masoud, Parvin and Pendar

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Contents

Page

Acknowledgements 6Abbreviations 7Abstract 8Review: Organic synthesis with the short-lived carbon-11 radioisotope 91 .0 In tro d u c tio n 101.1 The 11C Isotope 111.2 P rim ary p recu rso rs 121.2.1 [11C ]C arbon Dioxide 12

1.2 .1.1 Preparation of [^CJcarboxylic acids 12

1.2 .1.2 Preparation of [^Cjalcohols 13

1.2 .1.3 Preparation of [^C]aldehydes 131.2.1.4 Preparation of [^CJaminoacids 13

1.2 .1.5 Reductive [^CJcarboxylation 151.2.2 [11C ]M eth an e 16

1.2 .2.1 Conversion to [^CJdiazomethane 161.2 .2.2 Conversion to [^CJcarbon dioxide 16

1.2.3 M isce llaneous 171.2.3.1 [^C ] Acetylene 171.2 .3.2 [^CJCyanamide 17

1.2 .3.3 [^C ] Guanidine 181.3 Secondary p recu rso rs 181.3.1 [2-11C ]A ce to n e 181.3.2 [c a rb o n y l-11C]AcyI ch lorides 19

1.3.2.1 Preparation 191.3.2.2 [^C]Acetyl chloride 201.3.2 .3 Cyclopropane [^Cjcarbonylchloride 211.3.2.4 [^CJFuranoyl chloride 21

1.3.3 [11C]M ethyl iodide and h igher [11C ]alkyl halides 221.3.3.1 [11C ]M ethyl iodide 22

1.3.3.1.1 Preparation 221.3.3.1.2 Preparation of [^CJaminoacids 221.3.3.1.3 [11C]Methylation of amides 251.3.3.1.4 [^CjMethylation of amines 261.3.3.1.5 [11C]Methylation of oxygen nucleophiles 291.3.3.1.6 Miscellaneous 31

3

1 .3 .3 .2 H igher [11C]aIkyl halides 321.3.3.2.1 Preparation 32

1.3.3.2.2 Preparation of [^CJaminoacids 34

1.3.3.2.3 [^C ] Alkylation of nitrogen nucleophiles 35

1.3.3.2.4 [l lC] Alkylation of oxygen nucleophiles 351 .3 .4 [11C ]C y an id e 36

1.3.4.1 Preparation 361.3.4.2 Conversion to [^CJcarbon dioxide 36

1.3.4.3 Conversion to [11C]methylamine 37

1.3.4.4 Preparation of N -t^C ] amines 37

1.3.4.5 Preparation of [l-^CJam ino acids 38

1.3.4.5.1 Hydrolysis of a - [ l - 11C]aminonitriles 38

1.3.4.5.2 Hydrolysis o f [S-^CJhydantoins 39

1.3.4.6 Preparation of l,4-[4-11C]amino acids 401.3.4.7 Preparation of [carbonyl-1 ̂ co m p o u n d s 40

1.3.4.7.1 [llQjCarboxylic acids 40

1.3.4.7.2 [n C]Aldehydes 41

1.3.4.7.3 [11C]Ketones 42

1.3.4 .8 Preparation of [* ̂ n i t r i le s 42

1.3.4.9 Preparation of [^CJurea 431 .3 .5 [11C ]Form aIdehyde and h igher [11C ]a ld eh y d es 44

1 .3 .5 .1 [11C ]F o rm a ld eh y d e 44

1.3.5.1.1 [1JC]Methylation of amines 44

1 .3 .5 .2 H igher [11C ]a Id eh y d es 451.3.5.2.1 Preparation of [^Cjamino acids 46

1 . 3 .6 [11C ]C arb o n M onoxide 471 .3 .7 [11C ]P h o sg e n e 48

1.3.7.1 Preparation 491.3.7.1.1 Chlorination of [11C]carbon monoxide 491.3 .7 .1.2 Oxidation of [^CJcarbon tetrachloride 491.3.7.2 1 ̂ -Labelling of nitrogen nucleophiles 501.3.7.3 1 ̂ -Labelling of oxygen nucleophiles 51

1 .3 .8 [11C ]M ethy IIith ium 521.3.8.1 Preparation 521.3.8.2 [^CJMethylation of ketones 53

1 .4 C o n c lu sio n 53

References 54

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Results and Discussion 642 .0 In tro d u c tio n 652 .1 G round Rules 662 .2 R e tro sy n th e s is 662 .3 M echanism 672 .4 M odel study 72

2.4.1 Synthesis and carbonylative coupling of 1 -(3-tributylstannylpropyl)piperidine (16) and l-(3-trimethylstannylpropyl)piperidine (17) 72

2.4.2 Synthesis and coupling reactions of l-(3-trimethylstannylprop-2-enyl)piperidine (23) and l-(3-tributylstannylprop-2-enyl)piperidine (24) 75

2.4.3 Cross coupling reactions of 1 -(3-iodoprop-2-enyl)piperidine (30)with 4-fluorophenyl tributylstannane (31) 79

2.4.4 Routes to l-[4-(4-fluorophenyl)-4-oxobut-2-ynyl)piperidine (32) 812.4.4.1 Palladium-catalysed coupling reactions o f 1 -(2-propynyl)piperidine

(26) with 4-fluorophenyl iodide 812.4.4.2 Synthesis and coupling reactions of l-(3-trimethylstannylprop-2-

ynyl)piperidine (34) and l-(3-tributylstannylprop-2-ynyl)piperidine (35) 82

2.4.5 Reduction of the conjugated ketones l-[4-(4-fluorophenyl)-4-oxobut-2-enyl]piperidine (25) and l-[4-(4-fluorophenyl)-4-oxobut-2-ynyl] piperidine (32) 85

2 .5 C onclusions based on the model s tudy 872.6 Synthesis and carbony lative coupling of 3 -m ethyI-l-pheny l

-8 - (3 - tr im e th y Is ta n n y lp ro p y I)- l ,3 ,8 - tr ia z a sp iro [4 .5 ]d e c a -4 -one (38) 88

2 .7 R outes to 8 -[4 -(4 -fluo ropheny l)-4 -oxobu t-2 -yny I]-3 -m ethy l- l-p h e n y l- l ,3 ,8 - tr ia z a sp iro [4 .5 ]d e c a n -4 -o n e (47) 91

2.7.1 Palladium-catalysed coupling reactions of 3-methyl-l-phenyl-8-(prop-2-ynyl)-l,3,8-triazaspiro[4.5]decan-4-one (48) 91

2.7.2 Synthesis and coupling reactions of 3-methyl-1 -phenyl-8-(3-tributylstannylprop-2-ynyl)-l ,3,8-triazaspiro[4.5]decan-4-one (39) 93

2 .8 R ed u c tio n s o f 8 -[4 -(4 -flu o ro p h en y l)-4 -o x o b u t-2 -y n y l]-3 -m e th y l- l-p h e n y l- l ,3 ,8 - tr ia z a sp iro [4 .5 ]d e c a n -4 -o n e (47) 95

2 .9 T andem carbonylative coupling and conjugated ketone reductions for the one-pot synthesis of3-iV-methyl sp iperone 96

2 .1 0 C o n c lu s io n s 972 .1 1 F u tu re developm ents 97

Experimental 99References 130

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Acknowledgements

I wish to warmly thank the following people for their important contributions during the course of this project:

Dr. D. A. W iddowson for his advice, tolerance and irrepressible optimism and encouragement.

Dr. Inderjit Mann, Dr. Charles Lindsay, Dr. Pamela Dickens and Mr. Neil Matthews for their invaluable advice.

The technical staff especially Chris Sausmann and Pete Sulsh.

Ken Jones, Paul Hammerton and John Bilton for the microanalytical, NMR and mass spectroscopy sevices.

Mark Chambers, Neil Matthews and Louis "The King" Diazario for their eagle eyes and critical nature in doing a great job of proof reading this thesis.

My friends who with their friendship and humour have been a constant cheer, especially Paul, Indy, Mac, Pamela, Jo, Micheal, Mark, Louis, Neil, Catherine, Frank, Mehrdad, Mary, Adam, Farhad, James, Richard and Chan.

The Republic of Iran for subsidising my studies during a very difficult period of her history.

Finally and most importantly, a deep gratitude to Masoud, Parvin and Pendar for their constant and unflinching support.

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Abbreviations

Arg ArginineBuLi butyllithium9-BBN 9-Borabicyclo[3.3.1 jnonaneCNS Central nervous systemD-AAO D-Aminoacid oxidaseDABCO 1,4-Diazabicyclo[2.2.2]octanedba DibenzylideneacetoneDCM DichloromethaneDIBAL Diwobutylahiminium hydrideDMS Dimethylsulphidedppf 1, l'-B is(dipheny lphosphino)ferroceneEt20 diethylether

EtOH EthanolGlu Glutamic acidGly GlycineHMPA HexamethylphosphoramideHPLC High performance liquid chromatographyLAH Lithium aluminium hydrideLeu LeucineLys LysineMesylate Methane sulphonateMet MethionineNMR Nuclear magnetic resonance spectroscopy0-tolyl 1,2-DimethylbenzenePh PhenylPhe PhenylalaninePro ProlineR.T. Room temperaturesolv. SolventTf Trifluoromethane sulphonateTHF TetrahydrofuranTLC Thin layer chromatographyTol. TolueneTosylate Toluene sulphonateTyr TyrosineU.V. UltravioletDMF N, N-Dimethylformamidered. Reduction

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ABSTRACT

This thesis is divided into three sections: review, results and discussion and experimental.

The] section is concerned with the preparation of a number of simple compounds, containing carbon-11 isotope. Role of these radio-labelling reagents in the synthesis of biologically important tracers, for in vivo studies using the Positron Emission Tomography (PET) technique, is also reviewed.

The results and discussion detail the work carried out in synthesising the dopamine receptor antagonist, 3-TV-methyl spiperone (3-NMSP), in forty minutes or two carbon-11 half lives. The work utilised palladium-catalysed caibonylative coupling of aryl halides with organotin reagents or alkynes under carbon monoxide at atmospheric pressure. A model study was performed to determine the most suitable organotin species as well as the necessary reaction conditions. Trialkylalkynylstannanes were found to be the most efficient coupling partner. Although this implied two additional reduction steps, palladium-catalysed conjugate reduction of alkynylketones using Group 14 hydrides was found to be effective for the task. Tandem coupling and reduction sequences resulted in moderate yields of 3- N-methyl spiperone but, more crucially, within the time constraints set by the sensitivity o f the PET instrument. This synthesis can be adapted to the preparation o f the radio- labelled 3-NMSP using the rarely harnessed, but readily available, radioactive carbon monoxide.

The final section contains the experimental details as well as physical and spectroscopic data, relating to the results and discussion.

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Review

O rganic synthesis with the short-lived carbon-11 radioisotope

9

1 .0 Introduction

The importance of Positron Emission Tomography (PET) as a non-invasive tool for the in vivo study of animals and man has been underlined by the explosion of scientific literature about this technique in recent years1’2. This nascent technology promises utility in a wide range o f exciting investigations such as the study of the brain and the heart; metabolic, regulatory and neurochemical mechanisms, drug distribution and pharmacokinetics3.The basis for PET is the incorporation of short-lived radionuclides in physiologically active molecules which decay in vivo emitting positrons. Positron-emitting radio isotopes are neutron deficient nuclei in which a proton spontaneously decays into a neutron, a positron and a neutrino. Depending on the nucleus, the emitted positron will have a specific maximum kinetic energy (for it is 0.96 MeV) and therefore a specific maximum range (for it is 4.1 mm in H2O) before it interacts with electrons from the tissue2. The

matter-antimatter annihilation that ensues produces 2 y-ray photons of equal energy (511 eV, equivalent to the rest mass of an electron) in opposite directions (Scheme 1).

P+ + e' ------------ 2 hv hv = 511 eV

Scheme 1

It is these penetrating coincident photons that the PET camera, which consists of an array of scintillation crystals, detects.The most widely used radionuclides are HC, ^ F , ^ O , 13n and 76gr These isotopes are prepared by the bombardment of an appropriate target with a beam of accelerated particles. The most common targets and the resulting nuclear reactions are depicted in Table 11)4>5.

T ab le 1.Nuclide t^ m i n ) Nuclear Reaction Target for cvclotron

U C 20.38 MN (p,o)11C N2 + traces of H2 or O2

n B(p,n)n C n B20313n 9.96 160 (p ,a )13N h 2o

12C(d,n)13N c h 4

15o 2.07 14N(d,n)150 N2 + traces o f O218p 109.7 180(p ,n )18F H2o

20N e(d ,a)18F Ne + 0.1% 19F2

The quantity of radioactivity (Curie, Ci) produced is dependent upon the energy of the incident beam (MeV), the beam current (fiA) and the time of bombardment (minutes).For in vivo administration, 7-10 mCi o f the radioactive substrate is usually required

10

therefore the synthesis has to commence with a substantially more radioactive precursor. Since some of the substrates are toxic in large doses, it is important that the syntheses are of high specific activity (Ci/mmol), that is to maximise the ratio of the radioactive to stable isotopes in the equivalent positions. The maximum theoretical specific activity for J1C is 9.218x10^ Ci/mmol1, however this value is never achieved because o f ubiquitous carbon impurities in the atmosphere or in the apparatus. The unintentional dilution o f the radioisotope with the stable one is a "no carrier added" (NCA) synthesis whereas the deliberate dilution is "carrier added" (CA).The efficiency of the synthesis is presented as the non-decay corrected radiochemical yield (RCY) and is the amount of radioactivity available at the end of synthesis (EOS) compared to the quantity at the end of bombardment (EOB). The values quoted however, are usually decay corrected to reflect the chemical potency o f the process. In this review the radiochemical yields mentioned are decay corrected unless otherwise stated.In practice the synthesis, purification, formulation and radiochemical purity checks are completed within 3-4 half-lives, ready for in vivo injection. The PET scanning may be continued up to an hour and a half after administration. Provided sufficient radioactive substrate is available, serial scanning can be performed for pharmacokinetic and metabolic studies.

1.1 The 11C Isotope

The isotope is produced by a variety of methods. These include the proton or deuterium bombardment of 10B or n B boron oxides6’7, and also the irradiation of 12C graphite with bremsstrahlung photons8. Bremsstrahlung photons are high energy yrays, created when a 50 MeV electron beam is directed at an aluminium-tantalum target. These

irradiations are performed in the presence of oxygen gas to form 11C0 2 -

10B(d4i)n C

11B(d,2n)11C11B(p^i)11C12C(Y,n)n C

The most convenient and widely used nuclear reaction however is the proton bombardment o f a nitrogen target9’10.

14N (p,a)n C

Nitrogen gas when co-targeted with traces of oxygen or 5% hydrogen gas generates and 11CH4 respectively.

11

/

From these primary precursors a variety of other structurally simple secondary precursors

such as 1 ̂ - lab e lled aldehydes, alkyl halides, cyanides etc. are made which perform the bulk of the labelling tasks. The conversions are done "on line" and the reagents transported by flow of gas through a series of valves. Computer-controlled automation reduces exposure of the researchers to hazardous radiation and results in greater reproducibility of synthetic yields which otherwise, because of the micromolar quantities being used, give erratic results in human hands.In this review instances where these primary precursors have been used directly in syntheses are cited and then preparation and reactions of the secondary precursors is discussed.

1 .2 Primary Precursors

1 . 2 . 1 [^CJCarbon Dioxide

The reactions o f are predominantly carboxylation o f nitrogen and carbon

nucleophiles; culminating in [^C ] carboxylic acids, alcohols, aldehydes, amino acids, and by reductive carboxylation, in tertiary amines.

1 .2 .1 .1 P rep a ra tio n of [11C ]carboxylic acids

Approximately 65% of the total energy requirement of the heart is met by fatty acid m etabolism 11. Therefore, 1 ̂ - lab e lled fatty acids have been used to differentiate the

infarcted (dead), ishemic (blood flow deficient) or the viable myocardial tissues. [1-11C] Palmitic12’13 ( j) and 3-methyl [1 -^C ] heptadecanoic acid11’13 (2) which are usually used

for the in vivo investigations, were prepared by the long chain Grignard capture of ^C C ^. The radioactive carboxylic acids were released upon treatment with hydrochloric acid solution (Scheme 2).

R1CHCH2MgBr C° 2’ r R1CHCH211COOH I 2) H30 +R2 R2

1- -R *= C i 3 H 2 7 , R2 = H

2— R 1 = C 1 4 H 2 9 , R2 = CH3

Scheme 2

[ l- ^ C ] Acetate14’15, prepared in good radiochemical yield by the interaction of methyl magnesium bromide and ^ C C ^ , is taken up rapidly by the myocardial tissue and has resulted in lucid images and provided metabolic clues about the functioning of the heart.

12

An interesting NCA synthesis of [ l-^ C ] pyruvic acid16 was achieved by the carboxylation of a masked acyl anion, lithium aldimide 4, which was formed when methyllithium was added to the isocyanide 3. The resulting a-iminoacid anion 5 on acid hydrolysis revealed HC-labelled pyruvic acid (6) (Scheme 3). The method can be adopted for other a-keto acids.

MeLiLi I

N ^ C H a11COc

11 COOLi

, b u ^ mA c Hs

c h 3c o 11c o o h

HqO+

6 (15-28%)

Scheme 3

1 .2 .1 .2 P rep a ra tio n o f [11C ]alcohoIs

[l-^CJButanol (7) was prepared as an agent for blood flow measurement by carboxylation of n-propylmagnesium bromide17’18. The magnesium salt o f the resulting n-butanoic acid was then reduced with lithium aluminium hydride. Hydrolysis released the labelled alcohol (Scheme 4). Several substituted benzyl alcohols have also been made en route to labelled aldehydes (Section 1.3.5.2).

i)11co2,2) LAH,3) H20

CJS'O H

(55-74%)

Scheme 4

1 .2 .1 .3 P rep a ra tio n of [11C ]a ld eh y d es

Preparation and uses of radiolabelled aldehydes is discussed separately in section 1.3.5.2.

1 .2 .1 .4 P rep ara tio n of [11C ]am ino acids

A number of [l-^CJ-labelled a-aminoacids have been synthesised by the carboxylation ofa-lithio isocyanides. The use o f these reagents renders the position a to the nitrogen accessible whilst serving as an easily removable protection for the amino group (Scheme

13

5). Racemic mixtures of [l-^ C j-lab e lled alanine1^ 20, phenylalanine21, glycine22, phenylglycine21, methionine25,24, tyrosine25, dopa26>27, ornithine28 and lysine28 have been thus obtained in moderate yields (*10-20%).

RCH2-N = CBase,

-60 °C- RCH—N=C

\Li

1) 11co2,2) H30 +

RCH - NH2 11COOH

R= CH3, Ph, H, PI1CH2, CH2CH2SCH3, 4-hydroxyphenyl, 3,4-dihydroxyphenyl

Scheme 5

[l-^C JL ysine (n= 5) and [l-UC]om ithine (n= 4) which have two amino functionalities were made by m onolithiation o f the required bis-isocyanide 8 followed by the [^CJcarboxylation-hydrolysis protocol in 14-20% yield28(Scheme 6).

NH2

** H2N ^ 1(CH2)n 11COOH

n= 4 (ornithine), 5 (lysine)

1) "BuLi (1 eq.),C N (C H 2)nN = C ----- -------------------

2) 11C02,8 3) H30 +

Scheme 6

It is known that the total body decarboxylation rate of L-omithine in patients with proven cancer is much greater than in the normal state. Since L-omithine is a non-essential amino acid and is not incorporated into proteins it can be used to determine the ornithine decarboxylase activity in vivo and therefore monitor the proliferation o f the disease. To this end L-[ 1 -11C]ornithine was prepared by treating the racemic labelled mixture with D-

Amino Acid Oxidase (D-AAO)/Catalase. L-fl-^CJLysine also has potential for studying protein synthesis rates, especially in regional aberrant collagen production of tumors or other diseases. For example, rats with W alker 256 carcinosarcoma have shown a tumor/non-tumor uptake ratio of 4.9 and 4.5 for D L -fl-^C Jo m ith in e and DL-[1-

^Cjlysine respectively at 45 minutes post injection.

D L -tl-^C JP ro line29 (9) is also a non-essential amino.acid. Using PET, it has shown a differential uptake ratio o f 5.9 for tumour to normal tissue. It is prepared by the carboxylation of a-lithiopyrrolidyl-7V-fm-butylformamidine (10) and the subsequent regeneration of the secondary amine functionality (Scheme 7).

14

N!Bu

s- BuLi,

-25*C

Q v Li 1) 11co2,k / 2) HO'

N‘B u

1 0Scheme 7

c o 2hH

9

1 .2 .1 .5 R eductive [11C ]C arb o x y la tio n

This interesting one-pot procedure for labelling secondary amines involves [ ^ C ] caiboxylation of an amido-lithium or an aminotrimethylsilyl bond. The resulting carbamates are then rapidly reduced in situ using LAH or NaAl(OMe>2H 2 to the corresponding[*1 C]methyl amines.

N '- [m e th y l-11C ]Im ipram ine^® (11) (Scheme 8), a potent anti-depressant, and [HCJtamoxifen31, an anti-estrogenic drug used against human breast carcinomas, have been made by this method in good yields (77 and 65% respectively).A m anual NCA synthesis o f N '-(m e th y l-^ C )im ip ram in e starting w ith N '- trimethylsilyldesimipramine (12a) was carried out within 45 minutes from the end ofbombardment (Route A). In the CA synthesis, route B, after [11C]carboxylation of 12b the anion was quenched with methyl chloroformate. The product (13) underwent rearrangement, with the concomitant extrusion o f [^C Jcarbon dioxide. The resulting carbamate (14) was then reduced using LAH, to form the radioactive imipramine 11. N-M ethylations using ^ C C ^ , as opposed to using radioactive methyl iodide or formaldehyde, have the advantage of starting with higher specific activity label since steps whereby isotopic dilution may occur have been eliminated.

R

CH3^ 1CH3 1 1

S)LAH,2) NaOH

RiCH3̂ - 11C02Me

14a) X= Si(CH3)3, b) X= Li

Scheme 8

1. 2.2 [^CJM ethane

[n C]Methane unlike [^CJcarbon dioxide, being relatively unreactive, has to be activated under harsh conditions to produce secondary precursors. A sizable variety of secondary precursors is nevertheless derived which includes compounds such as [ ^ C ] diazomethane3233 ̂cyanide34*36, acetylene37, tetrachloromethane38 and also, circuitously,

carbon dioxide39.[n C] Methane obtained from the target chamber has a higher specific activity (3-5 Ci/mmol after 30 minutes irradiation with a 30 pA beam current) than the radioactive carbon dioxide (2-3 Ci/mmol) produced under similar conditions38.

1 .2 .2 .1 C onversion to [11C ]d ia zo m e th a n e

f1 *C]Diazomethane is made from [* ̂ c h lo ro fo rm 32*33. f1 ̂ C h lo ro fo rm is formed "on line" by the passage of cyclotron produced [^C jm ethane through a heated (380 °C) column of pumice stone impregnated with cupric chloride. [1JC]Chloroform thus obtained, is then converted to [^CJdiazom ethane by treatment with potassium hydroxide in an ethanolic solution of hydrazine (Scheme 9). The radiochemical yield of the reaction is ascertained by the formation of the methyl ester of 4-nitrobenzoic acid33.

11CH4pumice stone/CuCI2

310 °C11CHCI3

NH2NH2, KOH,

EtOH11CH2N2

30% (EOB)

Scheme 9

The temperature o f the pumice stone/cupric chloride column is critical to the product distribution38. At 380 °C [HC]tetrachloromethane is the sole outcome. It is an intermediate in the synthesis of [^Cjphosgene (1.3.7.1.2).

Preparation times are short (10 minutes) and the specific activity of the ^ C I ^ ^ is

excellent (2.5-3.5 Ci/pmol, EOB). Although labelling reactions with [1JC] diazomethane have not yet been reported, this entity promises particular utility for (9 -m ethylation reactions.

1 .2 .2 .2 C onversion to [11C ]carbon dioxide

[^C]M ethane that is released from the target chamber, when carried by a slow stream of nitrogen and oxygen (2%) through a heated (500 °C) column of cobalt (II) and (HI) oxide, is oxidised to [^CJcarbon dioxide39.Significant isotopic dilution occurs that preventative measures have failed to rectify.

16

1 . 2 . 3 M iscellaneous

Other primary precursors can be obtained from the bombardment of different target systems but, although interesting, they have been of little use because of their low specific activity. Some are discussed briefly, below.

1 .2 .3 .1 [11C ]A cety Iene

This precursor is formed when calcium [12C]carbide is bombarded by a beam of protons. The resulting radioactive carbide, on hydrolysis releases [11C]acetylene40’41 (Scheme 10).

GaG '12C(p,d)11C

C a C 11CH ,0

HC 11 CH

Scheme 10

[^CJAcetylide anion, generated by the treatment of the precursor with n-butyllithium, was used to label estrone by forming the alkynyl alcohol 1542. The process suffers from poor specific activity and low efficiency (3-5%, not corrected) (Scheme 11).

1) nBuLi,HC = 11CH ------------------

2) estrone,10-15 min.

1 5Scheme 11

1 .2 .3 .2 [H C JC y an am id e

Proton bombardm ent o f a calcium nitride target has yielded up to 30 mCi of [^CJcyanamide43 (16). Using this labelled compound, [^C jary l or alkylguanidines have been made as potential adrenal imaging agents. As a preliminary study, condensation of [^CJcyanamide with benzylamine was considered and was found to have a radiochemical efficiency of 20%, forming benzyl guanidine (17) within 60 minutes (EOB) (Scheme 12).

„ 1) PhCH2NH2,HoN — C = N ------------------------- ■

2 )A1 6

NH11C

- HIsT v NH: CH2Ph

1 7Scheme 12

17

1 .2 .3 .3 [11C]Guanidine

Proton irradiation of a mixture of liquid ammonia and nitrous oxide (200 : 69) was found to generate a spectrum o f 1 ̂ -lab e lled products, among which [HCjguanidine (18) was the m ajor com ponent (m ax.42% )44. I11C] Methane, methylamine, cyanamide (16), formaldehyde, formic acid, urea etc. were also observed. Nitrous oxide is believed to scavenge solvated electrons, increasing the concentration of NH2 radicals which are vital to

the creation of [HC] guanidine.This precursor has been used in cyclocondensation reactions to form radioactive pyrimidines. 2,4,6-Triamino-[2-11C]pyrimidine (19) was formed in good radiochemical yields on condensation of malononitrile with 1844 (Scheme 13).

11H2N '

NHnn h 2

1 8

NC CNNaOMe

EtOH, 140 °C

Scheme 13

1 .3 Secondary Precursors

1 . 3 . 1 [H C ]A cetone

[11C] Acetone is used for introducing labelled isopropyl groups to molecules. It is obtained from the reaction of two equivalents of methyllithium with cyclotron-produced

(Scheme 14). From approximately 1 Ci of ^C C ^ , 600mCi of [^C]acetone is obtained 5

minutes after the release of radioactive carbon dioxide from the target chamber45.

11C 02 + MeLiO

H , C ^ O L i

MeLi

OLi11A

OLi

HoO

11Me

; c ,Me

Scheme 14

18

The stoichiometric equivalence of methyllithium is crucial as excess reagent reacts with [2- ^C ] acetone to produce undesired P-HQpbutanol.[2-^ C ] Acetone has been used in preparing the radiolabelled p-adrenergic receptor ligands practolol46, pindolol47 and propanolol48. The syntheses involved initial condensation of the [HCJacetone with a primary amine followed by the reduction of the resulting imine functionality with sodium cyanoborohydride. Pindolol (20), which is believed to be superior to both practolol and propanolol, was obtained rapidly (30 min.) and with high specific activity (0.6-1.0 Ci/mmol) when imine 21 was reduced (Scheme 15).

OHC T ^ ^ N H . OH

C r ^ ^ N +H "C L

Scheme 15

1 . 3 . 2 [carbonyI-HC]AcyI chlorides

1 .3 .2 .1 P rep a ra tio n

Production of these electrophilic precursors has depended on the efficient capture of n C0 2

by appropriate Grignard reagents. The resulting magnesium [11C]carboxylates on treatment with phthaloyl chloride in the presence of 2,6-di-ferf-butyl pyridine (2,6-DTBP), have furnished radioactive acyl chlorides in high yield («80%) (Scheme 16). Acyl chlorides synthesised have been limited in number but include species such as [HC] acetyl4^'52, cyclopropyl carbonyl53*54 and fiiranoyl chlorides55.

19

2,6-DTBP11co2R— MgBr ----------- 11i a OMgBr Cl

R = CH3f

O

ClCl

o

Scheme 16

1 .3 .2 .2 [1lC ]A cety l ch loride

This precursor has been used in labelling primary52 and secondary50 amines, and was applied to the synthesis of radioactive melatonin49a (22), 6-fluoromelatonin49b (23) and

[carbonyl-1 *C] acetazol amide51 (24). Melatonin is a neurohormone secreted by the pineal gland and has possible roles in the biological rhythms o f life. It also plays a part in depressive illnesses and is administered to alleviate the effects o f "jet-lag". 6 - [^ClFluoromelatonin is currently used in preference to [^Cjm elatonin as it has a longer biological half life. The two compounds have been prepared from the appropriate 5- methoxy tryptamines (25) (Scheme 17).

N-NA k

H a N S O ^ S ^ S O o N H 11dOMe

24 (15%)

M eO .C r \

n h 2 c h 311c o c i MeO

X ^ 35 min. XH

25

NH11COCH,

Scheme 17

H

22 (X= H, 13%)23 (X= F, 35%)

1 .3 .2 .3 Cyclopropane [11C]carbonyl chloride

N-Methylcyclopropane is a common substructure in many opiate receptor ligands. It enhances the antagonistic property of morphine and its analogues. HC-Labelling of this region has been achieved by reacting cyclopropaneC1 JC]carbonyl chloride with an appropriate secondary amine and reducing the resulting amide using LAH.Diprenorphine54 and cyclophran53 (26) have thus been made in radioactive form (Scheme18).

Scheme 18

1 .3 .2 .4 [ l1C ]F uranoyI ch lo ride

This precursor was developed in conjunction with the synthesis of [^CJprazosin55 (27), a powerful and selective a-adreno-receptor antagonist and a vasodilator.Acylation of the secondary amine 28 proceeded efficiently (30-40%) and provided [^CJprazosin with good specific activity (700-1000 mCi/pmol) (Scheme 19).

28 27

Scheme 19

21

*

1 . 3 . 3 [*] C]MethyI iodide and higher [H CJalkyl halides

[HCJMethyl iodide1 . 3 . 3 . 1

[^C ] Methyl iodide is the most used precursor2, involved in the methylation of a variety of functionalities. A selection of interesting examples is set out below.

1 .3 .3 .1 .1 P rep a ra tio n

[^CjM ethyl iodide has been prepared as a primary precursor by proton bombardment of a nitrogen gas-hydrogen iodide target56. The major product however is [HCjmethane (45%,

c.f. 25% ^C H ^I) and the purification is both arduous and time consuming.

The routine production of 11CH3l is as a secondary precursor, and proceeds first by the

reduction of the cyclotron-produced **CC>2 with lithium aluminium hydride. Hydrolysis of

the resulting salt gives [HCJmethanol which is then converted to ^ C H jI by either heating

at reflux with hydrogen iodide57-61, or treating it with the solid reagent, diphosphorous tetraiodide62 (Scheme 20). The final step can also be accomplished by treating ^ C ^ O H

with ethylphosphonium iodide65.

4 11C 02 + 3 LiAIH4 ------------- LiAI(011CH3)4 + 2 LiAI02

HI.

11CH3I

4 H20

4 11CH3OH + LiOH + AI(OH)3

Scheme 20

1 .3 .3 .1 .2 P rep a ra tio n of [11C ]am in o ac id s

The synthesis and PET applications of L- and D- [methyl-11 C]methionine (29) has recently received wide attention64-69. Preparation is fully automated and is performed in two stages. First, the S-benzyl protected homocysteine 30 is deprotected in a liquid ammonia/sodium metal environment. In the second step, after ammonia has been driven off by a stream of nitrogen, the remaining sulphide anion is reacted with H CH 3I (Scheme 21). Depending on

the chirality of the initial protected homocysteine, stereochemically pure [^Cjmethionine can be obtained.

22

BzS C02H 1) Na/ NH3(|) 11CH3S ŝ ^ ^ C 0 2H

NH2

3 0

i T W n h 2

29 (40-90%)Scheme 21

L- [11 C]Methionine has been used to investigate Alzheimer’s disease as well as protein synthesis rates in brain and lung tumors66.Neuropeptides, Substance P70 (31) and Met-enkephalin71»72 (32) were similarly labelled on their methionine substituent. Substance P which is an undecapeptamide is produced in the central nervous system by the cells of the dorsal root ganglia. It is a transmitter of the pain stimulus chiefly in the sensoiy neurons and is concentrated mainly in the midbrain and the spinal cord. Enkephalin has similar properties and distribution.

Arg-Pro-Lys-Pro-Glu-Glu-Phe-Phe-Gly-Leu-[11 C]Met-NH2

31 uTyr-Gly-Gly-Phe-[11C]Met

32

Two routes have been developed for the labelling o f other amino acids, they both make use of a lithiation-alkylation protocol. The first approach involves formation of the anion from an a-isocyanoacetate, quenching with a [^CJalkyl halide and hydrolysis reveals the amino acid. Racemic [3-1 ̂ a la n in e (33) was prepared in this manner73, starting with fm-butyl-2-isocyanoacetate (34), en route to L-P-^CJlactic acid (Scheme 22).

q CO^ Bu

34

1) nBuLi, -78 °C

2) 11CH3I, R.T., *■3) H30 +

11CH3

c o 2h

33

c o 2h

Scheme 22

The above methodology has been superceded in favour of anions derived from imines such as N-(diphenylmethylene)glycine-terf-butylester74' 77 (35).The racemic [3-1 *01 alanine obtained using imine 35, was transformed into a variety of other [^ C ] labelled amino acids. By manipulating appropriate enzyme systems it was possib le to convert 33 into L -tyrosine, L -dopa, L -tryptophan and L-5- hydroxytryptophan76.An inherent failing of both these methylation routes is the squandering of up to 50% of the radioactivity as it exists in the wrong chiral form. In an attempt to rectify this drawback

23

asymmetric syntheses based on chiral auxiliaries have now appeared. Although chiral inductions have not been absolute, they have resulted in significant enantiomeric enrichments. Amino Acid Oxidase/catalase enzymes are still required for purifying the desired stereoisomer but in terms of retention of radioactivity, a major improvement has

taken place. For example L -P-^C Jalanine has been formed in 89% enantiomeric excess, starting with the chiral imine 3678,7^ (Scheme 23).

1) Base,

2 ) 11CH3I, r3) H2NOH,

EtOH

h 3o + 11c h 3

*H 2N ^ C 02H

33 (12-28%)

Scheme 23

The chiral handle can,altematively, be part of the ester functionality as was demonstrated by the use of (-)-8-phenylmenthan-3-yl N-(diphenylmethylene)-glycinate78’80’81 (37). Under phase transfer catalysis with tetrabutylammonium hydroxide and accompanied with sonication, [UCJmethylation proceeded in moderate yield (40%) and formed L-[3- 11C]alanine (33) in 52% enantiomeric excess8®’81.

1) Bu4N+OH’',2) 11CH3I, 11CH3

3) NH2OH, * H2N ^ COzH4) “OH

33

Scheme 24

24

1 .3 .3 .1 .3 [11C]MethyIation of amides

A group of biologically important tracers have been thus produced, among them is 3-N- [llQm ethylspiperone (3-[HC]NMSP) (38) which is derived from spiperone82 (39). The methylations were performed using tetrabutylammonium hydroxide in DMF solvent83 and sometimes accompanied with sonication at elevated temperatures84’85 (Scheme 25). Spiperone and its 3-N-methyl derivative are potent dopamine receptor antagonists and have been used as neuroleptic drugs in the treatment of schizophrenia. Relative concentrations o f dopaminergic receptors in various regions o f the brain have been investigated with [UCJspiperone and [3-11C]NMSP. It has been observed that in man at 70-130 minutes post-injection a caudate/cerebellum uptake ratio of 4 exists and that the region of highest concentration is in the basal ganglia82.

38 (21%)

Scheme 25

Benzodiazepines are also receptor ligands and a number of thi s type of compounds have been used clinically for treating anxiety, insomnia and epilepsy. One potent ligand, Ro 15-

1788 (40) and its metabolites have been labelled, using ^ C H jI (Scheme 26), in an effort

to locate the appropriate receptors86"88.

11c h 3i , a

NaOH, 5 min.

Scheme 26

Cl

Whilst preparing [^CH'-chlorodiazepam8^ (41) an interesting practical ploy was used to minimise reaction and handling times^0. 1-DesmethyM-chlorodiazepam in acetone was absorbed on an acrylic yam and the reaction with [HC]methyl iodide was carried out in the injection loop of a liquid chromatography apparatus. Reaction time was 15 minutes, and the product was obtained with high specific activity (500-1000Ci/mmol).

1 .3 .3 .1 .4 [11C ]M ethylation o f am ines

Amine [^CJmethylations are numerous and the chemistry involved is simple and standard, therefore only the ^C-labelling o f those compounds whose physiological properties are interesting has been highlighted. One such compound is l-m ethyl-4-phenyl-l,2,5,6- tetrahydropyridine91»92 (MPTP, 42). This compound was formed inadvertently as an impurity in several synthetic illicit drugs. It induces irreversible Parkinsonism and devastates the brain by selective degradation of the nerve cells of the Substantia nigra.

Labelled MPTP was made inefficiently (5-7%) by the action of PTP (43) with ^ C H jI in a

total preparation time of 33-37 minutes (EO B)^ (Scheme 27).

Scheme 27

Administration of [^CJMPTP to Rhesus monkeys has indicated high uptake in the striatum and midbrain, including the Substantia nigra. M onitoring for 2 hours showed no elimination from these regions unlike the other areas. Pre-treatment with spiperone did not affect the level of uptake, indicating lack of any great affinity for post synaptic dopamine receptors^1.

26

Monoamine oxidase (MAO) is a vital enzyme in the brain, oxidatively deaminating endogenous neurotransmitters and administered drugs. It exists in two forms: MAO-A and MAO-B. Suicide substrates have been developed that specifically inhibit either form and, when labelled with HC, pinpoint the areas of MAO activity. Clorgyline94-96 (44) [s m irreversible inhibitor o f MAO-A. It was labelled by [H Q m ethy la tion o f the 2- propargylamine 45 (Scheme 28).

1) k2c o 3,

2) 11CH3I

45 44

Scheme 28

L-Deprenyl (46) is MAO-B's nem esis94"96. It too contains a 2-propynyl amine functionality and, exhibits a much diminished activity as the D-stereoisomer. Methylation of propargylamine 47 is performed in the presence of potassium carbonate95 (Scheme 29).

err 1) k2c o 3,2) 11CH3I Q T T46 (25-40%)

Scheme 29

The preliminary results o f the PET studies have indicated high MAO activity in the corpus striatum, thalamus and the brain stem. MAO-B is believed to be affected by MPTP-induced parkinsonism and therefore trials are underway where L-deprenyl is used to control the symptoms of Parkinson's disease97.The causes o f depression have been attributed to a functional lack o f noradrenalin or serotonin (48) in the limbic system. To enhance the concentration of this neurotransmitter at the central synapses it is possible to inhibit either their re-uptake or their enzymatic degradation. To achieve the first, tricyclic anti-depressants such as imipramine or sertraline have been used and for the second, the use of MAO inhibitors has been fruitful.

27

[^CJIm ipram ine98’99 (11) and [^C ]sertraline100 (49) have been prepared but as yet the outcome of their PET studies has not been not available.Double methylation was a problem encountered in the labelling o f sertraline, it arises because of the greater nucleophilicity of the secondary compared with the primary amine. An interesting strategy used in the synthesis of iV -f^Q m ethyl chlorophentermine101 (50), circumvents this problem via methylation of the sodium salt o f chlorophentermine trifluoroacetamide (51). The drawback in this synthesis however is the necessity of purification by HPLC prior to the hydrolysis step (Scheme 30).

50

Scheme 30

Other biologically active amines that have been labelled include, morphine analogues which were used to study opioid receptors102"104, benzazepine [11C]SCH 23390105"107 (52) that was developed as a tracer ligand for dopamine type-1 receptors (spiperone (39) and rac lo p rid e (6 3 ) are dopam ine type-2 recep to r an tagonists). N -[m ethyl- ^CJNom isfensine10 ̂(53) was used in evaluating monoamine re-uptake sites at the pre- synaptic dopaminergic terminals whilst [11C]iodoantipyrine109 (54) acted as a brain blood flow marker. [^CJM eptazinol110*111 (55, a central analgesic) and [^CJsuriclone112 (56), a member of the cyclopyiTolone family with anxiolytic and hypnotic properties, have also been made and used for PET tracing.1 ^ -L ab elled choline113’114, piperidinocholine and pyrrolidinocholine115 have provided

28

clues about age-related memory disturbances such as senile dementia and Alzheimer's disease where cholinergic dysfunction arises from significant reductions in the levels of acetyltransferase enzyme in the cerebral cortex and hippocampal region.

1 .3 .3 .1 .5 [ l 1C ]M ethy!ation o f oxygen nucleophiles

A normal brain uses D-glucose for its energy requirements, therefore an analogue of it, 3- [methyl-HQI-D-glucose (P -^ C jM G , 57), was prepared with the intention of studying regional glucose metabolism116,117 A preliminary study has shown high uptake of this tracer in the brain. [3-u C]MG was made by treating the potassium salt o f 1,2,5,6- diisopropylidene-D-glucose (58) with followed by stripping of the protectinggroups in acidic medium (Scheme 31).

Scheme 31

29

Excellent differentiation between a secondary and a tertiary alcohol was observed whilst

synthesising two opioid receptor ligands [^ C Jb u p re n o rp h in e 118 (BPN, 59) and [11C]diprenorphine119 (DPN, 60). Although the efficiency of the methylation was poor, tracers were made with high specific activity (1425 and 1740 Ci/mmol respectively) (Scheme 32).

59-(R=OH, R '^ B u ), 60-(R=CH3, R'=OH)

Scheme 32

[llCJB PN andfUCJDPN are PET tracers for opiate receptors and are used for in vivo pharmacokinetic studies; BPN is a partial agonist whereas DPN is a potent opiate antagonist. Carfentanyl120 (61) is also an opiate receptor antagonist and is approximately 7000 times more potent than morphine. It was labelled by the [HCJmethylation of the sodium salt of the carboxylic acid 62 (Scheme 33).

Scheme 33

[HC]Methyl esters of prostaglandins D2, E2121 and 9J3-PGD2122 were made similarly, but underwent rapid hydrolysis in vivo and were of limited use.In the study of dopamine D-2 receptors, the use of spiperone and its 3-N-methyl derivative has been superceded by a more specific ligand, the substituted benzamide Raclopride123 (63). Raclopride has been labelled at two sites, at the amine functionality with [^CJethyl

30

iodide124 or at the phenolic hydroxyls with [^C Jm ethyl iodide125 (Scheme 34). O- Methylation was faster than iV-ethylation and gave better yields of [^C jraclopride (by *10% )124. In monkeys, PET imaging has revealed a caudate to cerebellum uptake ratio of 10 while in humans caudate or putamen binding is 4-5 times greater than that of the cerebellum126.

Scheme 34

1 .3 .3 .1 .6 M isce llan eo u s

[^ C ] Alkyl halides have the potential of forming phosphorus ylids and hence the exciting prospect of constructing labelled olefinic bonds127. The [HQmethylphosphonium salt 64, obtained from the reaction of ^ C H jI and triphenylphosphine, formed the phosphorane 65 on treatment with n-butyllithium. Subsequent condensation with benzaldehyde produced radioactive styrene (Scheme 35). Although no more examples are available, the labelled Wittig reaction may have wide application.

11CH3I, "BuLiPPh3 --------------- ► Ph3P+11CH3 l' ------------------► Ph3P = 11CH,

toluene 3 2

Scheme 35

Primary and secondary aliphatic nitro compounds can be converted to aldehydes and ketones by the N ef reaction128*129. Combined with the acidity of the a-proton, this poses interesting possibilities. The first preparation o f a nC-labelled nitroalkane (66) involved

exposure o f to silver nitrite at room temperature to form [HQInitromethane130-132.Sodium nitrite can efficiently replace silver nitrite, however the yields of the labelled precursor 66 ranges between 35 to 80% (Scheme 36).

31

AgN02/NaN02

R.T.11c h 3i 11ch3no2

66

Scheme 36

An interesting masked acyl anion equivalent was used in the synthesis o f 21- t1 ̂ p ro g es te ro n e133 (67). Anion formation adjacent to the a-tosylisocyanide 68 followed by [^Cjm ethylation and subsequent acid hydrolysis revealed the [^C jm ethyl ketone 67 (Scheme 37).

Scheme 37

1 . 3 . 3 . 2 Higher [H cja lk y l halides

1 .3 .3 .2 .1 P rep a ra tio n

One-pot protocols have been developed for the synthesis o f 1 ̂ -lab e lled alkyl and benzyl iodides.

Under optimum conditions 11C0 2 is absorbed by solutions of Grignard reagents. After

LAH reduction o f the resulting f1 carboxyl ate salts, the lithium aluminium alkoxide complexes are heated with hydrogen iodide to yield the labelled alkyl or benzyl iodides (Scheme 38).

32

RMgBr + 1 1 COz R1 1 C„OII

OMgBr

1) LAH,2) HI, A

r 1 1 c h 2i

R= M e134'135, Et, Pr, ^Pr133,4-anisyl, 4-cWorophenyl, veratryl133

Scheme 38

In the alkyl series, preparation times increased gradually with increasing molecular weight and branching135.Formation of alkyl or benzyl halides, other than the iodides is possible. For example, whilst searching of a biologically inert tracer for the in vivo study of cerebral blood flow, f1 d im e th y l fluoride (69) was made by treating ^ C H jI with tetraethylammonium fluoride

at elevated temperature137 (Scheme 39).

1 1 ch3i + E y v T FMeCN, 110 °C

1 0 min.1 1 c h 3f

69

Scheme 39

[HCJBenzyl chloride was prepared en route to synthesising D L43-1 ̂ jp h e n y l alanine. Preparation resembled the benzyl iodide synthesis except that [^CJbenzyl alcohol was liberated and isolated after LAH reduction of the [^CJcarboxylate salt. Conversion to the chloride was accomplished efficiently by a tris(n-octyl)phosphine-tetrachloromethane system13** (Scheme 40).

MgBr 1) 1 1 C02,

2) LAH,3) H30 +

a

C^ O H P (rvo c ty l)3’ ,CCL

n<

a

Cl

Scheme 40

A number of opioid receptor ligands such as buprenorphine (59), diprenoiphine (60) andpethidine have a nitrogen-bound methylcyclopropyl group. The route outlined in Scheme38 is not appropriate for the preparation of the required [l-^CJiodomethylcyclopropane

33

precurso r13 1 4 0 (70). The carbocation formed, under acidic medium, from the [1- ^CJcyclopropylmethanol undergoes undesired rearrangements. Instead of liberating the alcohol therefore, [HC]bromomethylcyclopropane (71) was made by treating the lithium aluminium alkoxide complex with tosyl bromide at high temperature. Conversion to the iodide was achieved by the Finkelstein reaction (Scheme 41). Thermal instability of p- toluenesulphonyl iodide (tosyl iodide) prevented it being used with the complex139.

[> — MgBr1) 11co2,2) LAH

. „ ̂ Tos-Br,!1CH20 ’ M + --------------

A, 6 min.[>— 1 1 CH2Br

71

M= lithium-aluminium complex

Nal, A, acetone, 1 min.

0 — 11c h 2i

70

Scheme 41

1 .3 .3 .2 .2 P rep a ra tio n [11C ]am ino acids

Anions formed from N -(diphenylm ethylene)glycine-ferr-butylester (35) have been quenched with a variety of [^ C ] alkyl iodides to provide the radiolabelled amino acids phenylalanine74*77, 2-aminobutyric acid74, valine74*141, norvaline74, leucine74 and norleucine74 in operable yields (Scheme 42).

Ph

Ph) f= N C H 2C 02*Bu

3 5

1) Base,

2) [11C]RI,3) H30 +

c o 2h

Scheme 42

L-t1 ̂ P heny la lan ine74 and L-t^C]valine141 were isolated after racemic reaction mixtures were exposed to D-AAO/catalase. Enantiomeric purities in the order of 98-99% were obtained as the unwanted stereoisomer was converted to the a-keto acid.Enantiomeric enrichment using the chiral auxiliaries 36 and 37 gave [^C lnorvaline78 andt1 ̂ pheny la lan ine80*81 respectively. The former was made in 87% while the latter was formed in 55% enantiomeric excess.

34

1 .3 .3 .2 .3 [HC]Alkylation of nitrogen nucleophiles

Muscarinic cholinergic receptors are an important entity in the CNS and their function is seriously impaired in Huntington's chorea, Alzheimer's disease and senile dementia. [J1C] Dextim ide142 (72) is a ligand of these receptors and is labelled simply by alkylating a piperidyl nitrogen with radioactive benzyl iodide (Scheme 43).

P h

72 (20%)

Scheme 43

Similarly [^ C ] ethyl and butyl iodide were used to label amine functionalities in the preparation of [1JC] lidocaine and bupivacaine135.

1 .3 .3 .2 .4 [f lC jA lk y la tio n o f oxygen nucleophiles

The [^C Jethylester of benzodiazepine Ro 15-1788 (40) was formed by treating the carboxylic acid 73 with labelled ethyl iodide in presence o f 2,2,6,6-tetramethylpiperidine88 (Scheme 44).

73Scheme 44

40 (15-20%)

1 .3 .4 [UC]Cyanide

1 .3 .4 .1 P rep a ra tio n

Several methods exist for the production of [^C]cyanide anion or hydrogen cyanide34" 36,143. For example proton irradiation of a N2 -H2 target mixture generates UCH4 and ammonia. This mixture when passed slowly through a column of platinum wire at 550 °C is transformed into H ^ C N l44 (Scheme 45). The chemical conversion is virtually quantitative.

1 4 N(p, a)11CHz (5%)

11C K NH<Pt, 550 °C

H11CN

Scheme 45

Alternatively , may be formed separately by the hydrogen reduction of over

a heated nickel catalyst and ammonia added laterl45.Another scheme involved the addition of dry hydrogen and ammonia to cyclotron-produced ^ 0 0 2 and passage over platinum wire maintained at 950 °C 14^ (Scheme 46). The

emerging H 1 *CN was then absorbed on sodium hydroxide-impregnated quartz wool. This synthesis required lower proton energies (9.4 MeV c.f. 18 MeV for the route)

and lower beam currents (IOjiA c.f. 30|iA), however equal irradiation times resulted in less radioactivity.

14N(p , a ) 11C *• 11co2h 2 / n h 3

Pt, 950 °C*- H11CN

Scheme 46

1 .3 .4 .2 C onversion to [11C ]carbon dioxide

[n C]Cyanide is converted to n C0 2 by passing it through a column packed with a mixture

of cobalt (II) and (HI) oxides and an inert ceramic material maintained at 670 °C36»147 (Scheme 47).

H11CNCoO/ C0 2 O3 ,

670 °C11co2(20-60%)

Scheme 47

36

The fall in specific activity that accompanies this preparation is believed to arise from the contaminants in the oxidants or 12C(>2 adsorbed on the ceramic material.

1 .3 .4 .3 C onversion to [11C ]m eth y Iam in e

[1lC]Methylamine is a potentially useful tool in the preparation of labelled methylamino species as well as in Mannich reactions14**. An efficient synthesis (50-70%, 20 min.) of

this precursor proceeds by hydrogenation of [^CJcyanide with Adam's catalyst (PtC^) in sulphuric acid solution, producing radioactive methylamine with good specific activity

(1.6-2.5 Ci/pmol)14^ (Scheme 48). Success of the reaction however depends critically on the complete removal of excess ammonia used in the synthesis of the radioactive cyanide.

„ 1) NaOH,H11CN ■ ■ ------»

2) H2, Pt02,h3o +,

3) NaOH, AScheme 48

1 1 c h 3 n h 2

1 .3 .4 .4 P rep ara tio n of A -[H C ]a m in e s

Amines and polyamines are good biochemical markers. For example, since adult brain tissue does not normally divide, the uptake and m etabolism of putrescine (1,4- diam inobutane, 74) is a marker for cell growth and proliferation of brain tumors. [^C jPutrescine150’151 is obtainable from the borane reduction of labelled succinonitrile (75). *0]Succinonitrile is formed by Michael addition of potassium [^C jcyanide toacrylonitrile (Scheme 49). Separation of the different compounds was performed on a cellulose phosphate ion exchange column which separates polyamines o f varying basicity by buffer solutions of increasing pH and ionic strength151.

1% KOH,K11CN + --------------

THFN11 C CN

75

BH3 .DMS

11CH2N '

74 (35-40%)Scheme 49

37

Radioactive cyanide can be introduced into molecules by other means. These include enzymatic incorporation, as in the synthesis o f [H C ]phenylethanolam ine using mandelonitrile lyasel52, or by Sj^2 displacement of leaving groups such as bromide153,154,

chloride154 or quaternary ammonium salts155. The latter was used in the preparation of the neurotransmitter and vasoconstrictor drug, [UQserotonin (48) (Scheme 50). Reduction of

[^C jn itrile s to [^C Jam ines has been routinely performed with LAH155 , sodium borohydride or borane complexes152.

Scheme 50

1) LAH,2) H2/ Pd

48 («5%)

1 .3 .4 .5 P rep a ra tio n of [ l - ^ C la m in o acids

Using [^C Jcyan ide as a precursor, two pathways are available for making [1- ^C jam inoacids: a) by hydrolysing a -fl-^C Jam in o n itriles , b) by hydrolysing [5- ^CJhydantoins.

1 .3 .4 .5 .1 H ydrolysis of a - [ l - 11C ]a m in o n itr i ! e s

The radioactive cyanide group was introduced into the system by an efficient Sj^2

displacement of a sulphite moiety from an a-am inobisulphite species, 77. Using an appropriate carbonyl compound, a-am ino bisulphites 77 are obtained by a two step process. First, an a-hydroxysulphinate salt 76 is formed when the carbonyl compound is reacted with sodium bisulphite. Treatment with 25% ammonia solution then efficiently converts 76 to the amino bisulphite 77. Hydrolysis of the resulting labelled 1-aminonitrile, 78, at elevated temperature in the presence of concentrated acid furnished the amino acids

38

79145,156 (Scheme 51).

pi NaHS03RS r ' R1HO SO,Na

NH4OH, 25%- V R1NaQ3S n NH2

76 77

1 1 CN", A-----------------

6 N HCI,

160 °C

78

R1c o 2h

79

R*= Ph, C H jPh, Bz, iso- c j h ? , iso-C4H 9 , 11-C4H 9 , cyclohexane145, 3,4-dimethoxybenzyl156R2= H

Scheme 51

The procedure is, however, prone to steric hindrance at the nucleophilic displacement step, R2 groups other than hydrogen have resulted in poor yields. The hydrolysis of the a - [ l- ^CJaminonitrile is also sometimes slow and the starting adducts have been recovered from the reaction mixture145.

1 .3 .4 .5 .2 H ydrolysis of [5 -H C lh y d a n to in s

[5-1 ̂ JH ydantoins are formed by the modified Bucherer-Strecker technique. An aldehyde or ketone is treated with a source of ammonium ions, normally ammonium carbonate or ammonium chloride, together with [^C jcyanide. Heating of the mixture results in the formation of the labelled hydantoin 80. Base hydrolysis o f 80 at elevated temperature rapidly causes ring opening and release of the [ l -11C]amino acid 79 (Scheme 52).

K1 1 CN, A,

NH4 (C03 )2_. V -N H 1) NaOH, A R1 ' --------

< V ^ O 2 )H 30 +

R*

p / H

- R1VHOO11C NH2

80 79

Scheme 52

Formation of the hydantoin may be facilitated by using an appropriate a-hydroxysulphinate salt 76 instead of a carbonyl compound157*15**. The racemic mixtures that are obtained can

39

be resolved enzymatically, though this is ultimately an unsatisfactory means of purification.

Various radiolabelled amino acids have been thus prepared, includingfl-^C] L-alanine158, L -phenylalan ine15^, DL-tryptophan157*160*161, L-leucine162, DL-valine163 and 1- aminocyclopentane carboxylic acid164*165.Hydantoins are themselves biologically active compounds166-167 and in particular P -^ C ]- 5,5-diphenylhydantoin168, which is used in the treatment of epileptic seizures has been harnessed in PET imaging experiments.

1 .3 .4 .6 P rep a ra tio n o f l,4 -[4 -11C ]am ino acids

y-Aminobutyric acid (GABA, 81) is the only member of this group to have been labelled in this manner. The synthesis was based on the Michael addition of [^CJcyanide ion to ethyl acrylate. The radioactive nitrile 82 was then selectively reduced and the amino ester hydrolysed to give [^ C ] GABA16^ (Scheme 53). Experimental details, however, are lacking.

C02EtNa11CN

N11C C02 Et.1 ) red.

822) H30 +

81

Scheme 53

1 .3 .4 .7 P rep a ra tio n of [carbony l-11C] com pounds

1 .3 .4 .7 .1 [H C ]C arboxy lic acids

Radioactive carboxyl groups are derived, as seen already in previous sections, from the hydrolysis o f the corresponding [^CJnitriles. In this way, long chain alkylnitriles153 and lactonitrile (an a-hydroxynitrile) have yielded fatty acids170 and lactic acid171 respectively. An interesting application of this conversion was in the synthesis of (ethylenediamine)(l-

malonate)platinum(II)172 (^C -P tenm al, 83) which is an analogue of cisplatin {cis- dichlorodiammineplatinum(II)}, a highly active antitumor drug. Cisplatin, however, has severe toxic side effects; locating and determining the mode and the centres of these toxic actions may help to improve the drag.[l-^ C JM alo n a te was made by Sjq2 substitution of [H C]cyanide for bromine in

bromoacetate, followed by base hydrolysis of the resulting nitrile. Complexation with (diaquo)(ethylenediamine)platinum(II) gave 83 in operable yields (17-40%) (Scheme 54).

40

1) 1 1 CN'*■ "00 1 1 C — 'COO'

H2O s ^ N H q Hz0 ' ^ N H 2J

83

Scheme 54

1 .3 .4 .7 .2 [11C ]A ld eh y d es

The transformation of [H Q nitrile to aldehyde has, to date, been used only in the synthesis

of 2-deoxy-D -[l-11C]glucose (2D[11C]G, 84), a tracer for regional metabolic rates. Radioactivity was introduced by the nucleophilic displacement of a triflate moiety, 85, with [^ C lc y a n id e 173-175. The resulting nitrile 86 was reduced first to an imine and then hydrolysed in acidic medium to give the labelled aldehyde (Scheme 55).The triflate leaving group was found to be superior to iodide173 or tosylate174 even though the alkyl triflate 85 is very labile and needs to be freshly made prior to the reaction. Raney alloy in formic acid was also determined to be more effective than the more common metallohydride reductions175*176.

(CF3so2)2o Na11CNROH r o s o 2 c f 3

85

^ R11CN

86

H2 C— Ni / Al,h c o 2h

H

*OH

84Scheme 55

41

1.3.4.7.3 [u C ]K e to n es

An interesting synthesis of [P -^C Jhep tadecan-P -one177 (87) was based upon the trifluoroacetic acid mediated rearrangement of the "ate" complex 88. The complex was formed when di-n-octylthexylborane (89) was reacted with labelled cyanide precursor. After alkaline peroxide workup, the radioactive ketone was obtained in good radiochemical yield (50-70%) (Scheme 56).

H15C7

H15C7

K11CN

H1 7 C8̂ ' Thexyl

8 9 88 1) (CF3C0)20,2) NaOH, H20 2

11h 17c17^8 CoH8n 17

87Scheme 56

In a synthesis of [carbonyl-1 *C]spiperone178 (39), the benzoyl ketone was made in a labelled form by a two step process. First, phenyl Grignard addition to the 1C] nitrile provided an iminium salt which on hydrolysis in protic medium furnished [^Q spiperone in good yield (20-30%) (Scheme 57).

1) Na11CN

2) 4-F-PhMgBr3) NH4 CI

39

Scheme 57

1 .3 .4 .8 P rep ara tio n o f [IfC Jn itr ile s

A variety of ways o f incorporating [11C]cyanide ions into organic molecules have already been outlined. A number of radioactive nitriles have been made, not as intermediates but as an end in themselves. Among these is an interesting NCA preparation of [HC] labelled benzonitrile using tricarbonylfluorobenzenechrom ium (O ) com plex17^ 180 (90). Nucleophilic displacement of the fluoride ion is facilitated by the chromium tricarbonyl unit. Decomplexation occurred simultaneously with the substitution reaction (Scheme 58).

42

(CO)3Cr

90

Na1 1 CN, A

DMSO

Scheme 58

11 CN

(50%)

Mirroring cyanohydrin formation, 4-iodoanilinophenylaceto[*1 C]nitrile181 (91) was made, by addition of radioactive cyanide to the inline 92 in acidic medium at elevated temperature, in very good yield (77%) (Scheme 59). Nitrile 91 is a tracer for regional brain perfusion.

Na11CN +AcOH, H20,

MeOH, APh H

92 91

Scheme 59

1 .3 .4 .9 P rep ara tio n of [11C ]u rea

[^C JU rea (93) is used increasingly as a biological marker and as a precursor to other labelled compounds16̂ ,182-185 An optimised NCA synthesis of this species depends upon the permanganate oxidation of [^CJcyanide ion186-188. The reaction which was performed in strongly alkaline solutions (pH 13.5) and at elevated temperature (75 °C) produced [^CJcyanate. After conversion to ammoniumcyanate 94, thermal transformation at 180 °C yielded radioactive urea with excellent radiochemical efficiency (95%) in a total preparation time of 16 minutes187 (Scheme 60).

H0‘3 K11CN + 2 KMn04 + H20 --------------► 3 K 0 11CN + 2 MnQ2 + 2KOH

2 K011CN + (NH4)2S04

n h 4 o 11c n

v11,2 NH40 ' 'CN + K2 S04

94

O^ 11C

h2n ^ n h 2

93Scheme 60

43

[^CJThioureas have been similarly prepared, except that radiolabelled thiocyanate was obtained by heating a solution of [1JC]cyanide with elemental sulphur189.

1 .3 .5 [11C]FormaIdehyde and higher [HCJaldehydes

1 .3 .5 .1 [HClFormaldehyde

[ 11 C]Formaldehyde is routinely synthesised by the oxidation of [^C jm ethanol that is

obtained by the LAH reduction of cyclotron-produced ^ C O j190. Oxidations have been carried out by metal mediated or enzyme catalysis.Metal mediated oxidations are performed in columns packed with silver needles191"193 or an iron-molybdenum mixture194,195, at elevated temperatures (350-390°C). High specific activities (1500-3000 Ci/mmol) o f labelled formaldehyde were obtained within 8-13 minutes from the end of irradiation196.Enzymatic oxidations o f ^ C H jO H are achieved using immobilised alcohol oxidase

(E.C.1.1.3.13.) in an oxygen environment197,198. Preparation is efficient (80-95%) and provides [^ C ] form aldehyde with moderate specific activity (400-450 Ci/mmol, EOB) (Scheme 61).

Ag/

1 4 N(p,a)11C 11COc1) LAH

2) H30 +11

y f Fe-Mo, A

CH3OH H11CHO

\ /EC.1.1.3.13

Scheme 61

1 .3 .5 .1 .1 [llC JM eth y la tio n of am ines

Reductive N-formylation is the route of choice when the use of ^ C H jI is not feasible, or

is unsatisfactory. This procedure involves the initial condensation o f radioactive formaldehyde with a primary or secondary amine, followed by reduction of the resulting [HC] inline with either sodium cyanoborohydride192,195,199,200 or a neutral solution of potassium phosphite196.N-fm ethyl-1 ̂ JS co p o lam in e195’196 (95), a muscarinic receptor antagonist is routinelysynthesised this way. The use of was not possible since, under the alkaline

conditions required for the methylation reaction, the epoxide ring was destroyed. Reductiveformylation is performed efficiently (20-43%) under neutral conditions within 45 minutes

196from the end of bombardment (Scheme 62).

44

HO

Ph

1 ) H1 1 CHO, pH6.5

2) KHP03i AO

95 (20-43%)

Scheme 62

In addition to scopolamine a number of other amino compounds have been labelled using

[^C Jform aldehyde. These include nor-etorphine20® (an opiate receptor ligand), imipraminel92»201 ( l l ) , nicotine192, chlorpromazine192 and polyamines like putrescine199. In the synthesis of N-[m ethyl-^C ]erythrom ycin A202 (96) it was observed that the sodium cyanoborohydride reduction of the imine derivative was not adequately selective therefore, in a variation from the routine, on treatment with H ^CH O , the carbinolamine 97 was isolated. Hydrogenation of this compound using palladium on carbon (Scheme 63) accomplished the [11C]methylation in good yield (50%), within 10 minutes EOB.

H H11CHO

Me IR97

H2, Pd/C, A

50%M e'N ^ 1CH3

R

96

M e SN^ 11CH3

R

M e ^ 11CH3

1 . 3 . 5 . 2 Higher [HCJaldehydes

[^CJBenzaldehyde (98) and [^Q veratraldehyde (99) were obtained from the oxidation of the corresponding [l-^C jbenzy l alcohols20^ 204. The labelled alcohols are made by the route already discussed, employed in the preparation of [l-^C Jbenzyl halides (section1.3.3.2). Tetrabutylammonium hydrogenchromate is the oxidant commonly used (Scheme

45

64), though an interesting variation utilises titanocene dichloride and iso-butylmagnesium bromide for the sequential reduction of the magnesium [^CJcarboxylate and oxidation to radioactive aldehyde205.

R2

MgBr1) 11co22) LAH

3) HzO4) Bu4 NHCr03

11CHO

98 (R1= R2= H)99 (R1= R2 = OMe)

Scheme 64

1 .3 .5 .2 .1 P rep ara tio n of [11C ]am ino acids

Racemic [3- 1 ̂ p h e n y la la n in e (100) and [S-^C Jdopa (101) are accessible by the condensation of ^jbenzaldehydes 98 and 99 with 2-phenyl-5-oxazolone in the presence of DABCO or triethylamine204. Acid hydrolysis of the resulting radioactive benzylidene oxazolones 102 and 103 with hydrogen iodide and red phosphorus exposed the a-amino acid functionality, and in the case of the veratryl compound 103 the methoxy substituents were demethylated also (Scheme 65).

100 (R1 =R2=H, 20-30%)101 (R1 =R2=OH, 8-15%)

Scheme 65

46

In a variation of the above theme, base mediated ring opening of 102 revealed [3- ^C jphenylpyruvic acid (104). Enzymatic treatment o f this compound with glutamic- oxaloacetic acid transaminase (E.C.2.6.1.1.) yields [3-1 ̂ p h eny la lan ine206 (Scheme 66).

glutamic oxaloacetic 1 0 0acid acid

Scheme 66

For amino acids labelled in the 2- position, a modified Bucherer-Strecker reaction may be applied. But only phenylglycine (105) has been prepared in this manner so far207 (Scheme 67). Greater availability of [^C]aldehydes could render this route an attractive alternative to the more established syntheses.

11? (NH4)2c o 3,

' H KCN, A

N H ,I dc^cocf

105 (20%)

Scheme 67

1 .3 .6 [HCJCarbon monoxide

[llCJCarbon monoxide is obtained "on-line'1 by the reduction o f cyclotron-produced U C0 2 over zinc powder maintained at 380 °C208. It is also the main product of the proton

irradiation of a nitrogen-oxygen (2%) target mixture20^[HC]Carbon monoxide is routinely produced as an intermediate in the synthesis of radioactive phosgene210*211 (Section 1.3.7.1.1). It has also been used in the preparation of severa l1 ̂ -lab e lled amides212. These preparations involved the carbonylation of lithium dialkylamides at low temperatures to form highly reactive acyl anion species. Carbonylating

47

lithium piperidide for example, formed the carbamoyl lithium 106. Quenching with water or alkyl iodides formed the [^C Jform am ide 107 and [^C Jam ides 108 and 109 respectively (Scheme 68). Although trapping of carbon monoxide in this instance was poor (10-20%), the methodology has some attractive prospects such as addition of the reactive carbamoyl lithiums to carbonyl compounds to form a-hydroxycarboxamides. The resulting [^C]amides can also be reduced to the corresponding amines.

O

108 (R=* Me, 12%)109 (R= Et, 15%)

Scheme 68

Carbonylation of organoboranes was used in the synthesis of [l-^CJbutanol213. Using the trialkylborane 110 (derived from 9-BBN and propene), [^CJcarbonylation followed by sequential LAH reduction and alkaline hydrogen peroxide workup, furnished radiolabelled butanol 7 in 33-71% yield over 60 minutes (Scheme 69). This route is not as efficient as the carboxylation of Grignard-reduction protocol described in Scheme 4.

1- KBHfO-Prk,2- 11CO,

3- LAH,4- NaOH, H20 2

11 c^ OH

7

Scheme 69

1 .3 . 7 [H C ]Phosgene

[^CjPhosgene is an effective reagent for forming labelled carbonyl compounds. Nitrogen and oxygen nucleophiles have been used in partnership with labelled phosgene (vide infra)

48

but, perhaps because of practical difficulties, carbon nucleophiles like Grignards have not. Nevertheless this precursor is now routinely produced in many research centres.

1 .3 .7 .1 P rep a ra tio n

Synthesis of H C O C lj is based upon two very different approaches, chlorination o f

[^C]carbon monoxide or oxidation of [^CJcarbon tetrachloride.

1 .3 .7 .1 .1 C hlorination of [H C ]carb o n m onoxide

Preparation of [11C]carbon monoxide was discussed in the previous section.

Chlorination o f 11 CO is performed either by the U.V. irradiation o f a chlorine mixture using a mercury lamp210 or by slowly passing the ^ C O through a column of platinum tetrachloride at 430 °C211 (Scheme 70).

Cl2 , U.V.

11Zn, 380 °C

COc 11CO11

PtCI4 ,4 3 0 °C

COCI2

Scheme 70

The first chlorination route is faster (10 min. c.f. 20 min.) and higher yielding (80% c.f. 30-50%) but, more critically, it has a lower specific activity (=100 mCi/pmol) than the second method (400-500 mCi/pmol). The cause of this isotopic dilution is believed to be the carrier carbon dioxide produced during the irradiation and also impure chlorine gas or platinum tetrachloride.

1 .3 .7 .1 .2 O xidation of [H C ]carb o n te tra ch lo rid e

This approach which has recently been developed, is a fast way (10 min. EOB) of producing substantial amounts of radioactive phosgene (375-500 mCi) of high specific activity (1400-1600 mCi/jimol)214’215.

[^C]Carbon tetrachloride is obtained when cyclotron-produced is passed through

pumice stone impregnated with cupric chloride at 380-400 °C. Addition of oxygen (2%) to the helium gas flow and transportation through a second oven containing iron filings heated to 300 °C, efficiently converts [11C]carbon tetrachloride to labelled phosgene (Scheme 71).

49

1 4N(p, a ) u CH2 ,5% pumice stone,

CuCI2, 380 °C11 c h 4

Scheme 71

- 1 1 CCI4

Fe, 0 2 2% 300 °C

1 1 COCI2

Since 1 *CH4 is produced with a specific activity (3-5 Ci/flmol) greater than (2-3

Ci/|imol), the oxidative route is superior in this respect to the chlorination protocol.

1 .3 .7 .2 11C-LabeIIing o f n itrogen nucleophiles

Amines are the nucleophiles most often used with [^CJphosgene, producing radioactive ureas. One such interesting urea is [H C JS arC N U 216 (111), an analogue o f the chemotheraputic agent BCNU (l,3-bis(2-chloroethyl)nitrosourea). SarCNU is taken up by brain tumors and is therefore used in pharmacokinetics and metabolic studies. In its

synthesis the amine hydrochloride 112 on heating with ^ C O C ^ formed the isocyanate113. Condensation of sarcosinamide (114) with the isocyanate followed by nitrosation of the resulting urea (115), with N2O3, furnished radioactive SaiCNU (Scheme 72).

Intramolecular capture of [11C]phosgene has been another strategy favoured by workers in this field. Diamino compounds thus provide labelled cyclic ureas and, aminoamide species give rise to [^CJhydantoins. Therefore substrates such as the new p-adrenergic receptor ligand, [1lC]CGP12177217 (116), and [2-1 lC]-5,5-diphenyl hydantoin218.219 (117) have become available.

111 (22%) 115Scheme 72

H C r ^ Y ^ N f H / B u ' O H•“CO

H116

2

o117

1 .3 .7 .3 11C -LabeIIing o f oxygen nucleophiles

[^C]Dimethyl and diethyl carbonate have been prepared as relays in the synthesis of other

interesting tracers220’221. As expected, reaction of ^ C O C lj with methanol and ethanol and their sodium salts is rapid and quantitative. Mono-alkoxidation is also feasible, absorption o f 11COCl2 in ethanol at room temperature produced [HC]ethyl chloroformate (118). This

precursor was converted to diethyl carbonate when heated at reflux in ethanol for 2-5 minutes or into labelled ethyl caibamate (119) when ammonia gas was bubbled through the reaction mixture222 (Scheme 73).Radiolabelled dimethyl and diethyl carbonates have been used to synthesise [2-

^Cjphenobarbital220 (120) and [2-**C]DMO221*22 ̂ (5,5-dimethyl-2,4-oxazolidinedione, 121). DMO is a biologically inert compound and is used for in vivo pH measurements and has potential as a tracer for assessing regional cerebral pH. It was obtained in good yield when 2-hydroxy-wo-butyramide (HIBA, 122) and the [HCjdialkyl carbonates were heated in basic environment (Scheme 74).

o EtOH, O EtOH, A

^ C l R.T.—► 11A

Cl OB 2-5 min.

1 1 8

O11C

E t O ^ O E t

( 100% )

NH3

O11c

H2 N " n OEt

1 1 9 (4 0 % )

Scheme 73

51

o> ^ nh2

OH

O+

RO" ^OR

oRO'Na+,ROH, « a

----------------------- ► O' NH

1500 'H o122 121

R= OMe — — 20-56%OEt — — 60%

Scheme 74

O11 nH N ^ 'N H

o ^ S A oP h '^ E t

120

The use of dialkyl carbonates is in fact not necessary and [2-11C]DMO has been made directly and efficiently (40-60%) from [^CJphosgene and HIBA in the presence of freshly powdered potassium hydroxide224.

1 .3 .8 [H C lM e th y llith iu m

Radioactive methyllithium, as a nucleophilic methylating agent, has attracted some interest and has been used in conjunction with ketones225"22**. Success with this precursor has however been qualified and examples of its use are scant.

1 .3 .8 .1 Preparation

[^C JM ethyl lithium was generated by the metal-halogen exchange reaction between

11C H 3l and n-butyllithium at -80 °C226. [^C JM ethyl- and butyllithium exist in an equilibrium which lies to the right (Scheme 75). Equilibrium was established within 2.5 minutes. The alternative preparation, by the reaction of labelled methyl iodide and lithium beads, was not as effective owing to poor reproducibility and inferior yields227.

-80 °C11CH3I + C4H9Li . 11CH3Li + C4H9I

Scheme 75

52

1 .3 .8 .2 [11C]Methylation of ketones

Several 17 keto-steroids have been methylated using this precursor225-228. One such compound was 17 a-finethyl-^C ) testosterone227, a tracer for estrogen receptors present

in human tumors that respond to hormonal therapy. It was prepared using Li, in

specific activities sufficient for PET imaging (*1-2 Ci/pmol). [^C jM ethyllithium was made in situ by the addition of n-butyllithium to a solution of ^ C H jI and the 17-keto- steroid. The generated radioactive methyllithium was therefore consumed as soon as it was formed and before it condensed into aggregates. The yield of the reactions were however poor (15%), not least because of the competing enolisation pathway.Lack of selectivity and difficulty in handling this precursor has resulted in waning of interest in recent years.

1 .4 C o n c lu s io n s

In this review the ^C-precursors generated by the cyclotron bombardment of various targets and their transformation to more versatile secondary precursors was discussed. Preparation o f several biologically active tracers, using these precursors, and their physiological significance was also briefly mentioned.The chemistry used in the labelling experiments is, for the most part, simple and well established. Amine and amide [^CJmethylation can be carried out efficiently and labelled aminoacids can be made with a degree of flexibility as to the position of the label. Simple [^ C ] alcohols, aldehydes and carboxylic acids can also be made, tolerance for other functionalities however, is low as reaction conditions are severe.The challenge, lies in producing new labelling agents from the limited range of primary precursors, or using existing ones in new reactions. [^ClNitroalkanes for example can act as convenient a-methylamino anion synthons, with particular relevance to f11C] aminoacid syntheses. Radiolabelled alkyl and benzyl halides may be used more widely in preparation of radioactive Wittig reagents. Acid chlorides, that hitherto have been only seldom used together with soft carbon nucleophiles such as organozinc reagents, may be used to form unsymmetrical ketones. [ilQ C aibon monoxide is another precursor whose utility has not been adequately explored. The ensuing study was a step to demonstrate its potential in this area.

53

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