METHOD OF SYNTHESIS
Technical Field of the Invention
The present invention resides in the field of chemical synthesis. More
specifically, the present invention relates to novel methods useful in the
5 synthesis of a positron emission tomography (PET) tracer, and novel
intermediates useful in said method.
Description of Related Art
WO 94127591 describes certain substituted guanidines and their use for treatment
andlor prophylaxis of neurological conditions such as epilepsy, neurodegenerative
10 conditions andlor nerve cell death resulting from e.g. hypoxia, hypoglycemia, brain
or spinal chord ischemia, brain or spinal chord trauma. WO 94127591 describes
that the substituted guanidines can be prepared by the reaction of an amine,
typically an amine salt such as an amine hydrochloride, with a preformed alkyl or
aryl cyanamide (Safer et a1 1948 J Org Chem; 13: 924) or the corresponding N-
15 substituted alkyl or aryl cyanamide.
WO 20041007440 and WO 20061136846 describe radiolabelled guanidine
derivatives and their use for imaging central nervous system (CNS) receptors and
teach synthesis of these radiolabelled derivatives from precursor compounds. For
example, WO 20061136846 teaches a compound of Formula (A):
or a salt or solvate thereof, wherein:
R' is hydrogen or C1.4alkyl;
R2 and R4 are each independently selected from alkyl, [ " ~ ] - ~ l - 4 a l ka~nld,
['8~]-~1f.l4uo roalkyl provided that at least one of R2 and R4 is ['1~]-~1-4alkoyr l
[ ' 8 ~ ] - ~fl~uo.r4oa lkyl; and,
R3 is halo.
WO 20061136846 teaches that the above compound of Formula (A) is
synthesised by reaction of a suitable source of "C or ' 8 w~it h a precursor
5 compound of Formula (B):
wherein one of R2 or R4 is hydrogen, and the other is hydrogen, C1-4 alkyl, or a
thiol protecting group such as benzyl; R' is hydrogen or Cl-4alkyl, and R3 is
halo.
10 WO 20061136846 also teaches that the method to obtain the precursor
compound of Formula (B) above wherein R2 is hydrogen is based on that
disclosed by Hu et a1 (J. Med. Chem. 1997; 40(26): 4281-9), wherein a
compound of Formula (C).
15 or a salt or solvate thereof, wherein R3 is halo and P' is a thiol protecting group;
is reacted with a compound of Formula (D):
wherein R' is hydrogen or Cq-4 alkyl and R4 is as defined for the desired
compound of Formula (B).
This method has also been recently reported by Robins et a1 (2010 Bioorg Med
Chem Lett; 20: 1749-51) as a successful way to obtain the following 1 8 ~ -
labelled S-fluoroalkyl diarylguanidines:
5 However, the above-described method of preparing the radiolabelling precursor
of Formula (B) suffers from a number of problems. First of all, tin chloride is
used in a reduction step used in the preparation of the compound of Formula
(C) from sulfonylchloride starting material. Residual tin complicates the workup
by gel forming tin hydroxides at pH >2. Furthermore, following this reduction
10 step, a benzyl protective group is introduced at the thiol and this group has to
be removed at the end of the synthesis, requiring use of AICI3 followed by flash
chromatography. There is therefore a need for a method to prepare this
radiolabelling precursor that overcomes these problems.
In addition, and not described in the prior art, the present inventors have found
15 that the compound of Formula (B) wherein R* is hydrogen decomposes to form
a disulfide impurity, even under what would be considered "inert" conditions,
which complicates subsequent radiolabelling to obtain the corresponding
compound of Formula (A). There is therefore an additional need for alternative
strategies to obtain said compound of Formula (A) that do not suffer from this
20 disadvantage.
Summarv of the Invention
The present invention provides a method to prepare a radiolabelled guanidine
derivative. A novel intermediate and it's method of synthesis are also provided.
In the method of the invention, use of a step comprising iodine oxidation has
25 the advantage that the iodine oxidises any tin present as a consequence of the
initial tin chloride reduction step. No protecting groups are required in the
method of the invention, thereby eliminating the need for a deprotection step.
In addition, the method of the invention overcomes the problem observed by
the present inventors with formation of a disulphide impurity. The method of
5 the invention therefore overcomes a number of problems associated with
known methods for the preparation of radiolabelled guanidine derivatives.
Retailed Description of the Invention
In one aspect, the present invention provides a method to obtain a positron
emission tomography (PET) tracer of Formula I:
~ r r h n r n i n .
V V I IGI G I 1 I .
X' is an X group selected from C1-4 alkyl or halo;
Y' is a Y group selected from hydrogen or C1-4 alkyl;
Z' is a Z group which is CI-4 alkyl; and,
Q is ["CICA-4 alkyl- or ['8~]-~l-4fluoroalkyl-;
wherein said method comprises:
(a) providing a compound of Formula II:
wherein:
XI' and XI2 are the same and are both an X group as defined for x';
Y" and YI2 are the same and are both a Y group as defined for Y', and,
Z" and 2'' are the same and are both a Z group as defined for z',
(b) reducing said compound of Formula II with a reducing agent to obtain a
compound of Formula ill:
x'' is an X group as defined for XI;
\ 121 r is a Y group as defined for Y'; and,
2'' is a Z group as defined for z',
(c) reacting the compound of Formula Ill as obtained in step (b), with either
[' ' C]CI-4 alkyl-LG' or ['8~]-~f1lm~4o r o a l k ~wl -h~er~ein~ L.G ' and LG* are
independently halo, or the group -0-SO~w-Rhe'r ein R' represents an
optionally-substituted C6-10a ryl, an optionally-substituted C1-4a lkyl, or
CI-4 fluoroalkyl, wherein said reacting is carried out in a suitable solvent.
The term "PET tracer" refers to a chemical compound that comprises a
radionuclide that undergoes positron emission decay (also known as positive beta
5 decay), and is therefore detectable using PET imaging. The most commonly used
radionuclides for PET are I8F and "c.
The term "alkyl", alone or in combination, means a straight-chain or branchedchain
alkyl radical having the general formula CnHZn+'. Examples of such radicals
include methyl, ethyl, and isopropyl.
10 The term "halogen" or "winl the context of the present invention means a
substituent selected from fluorine, chlorine, bromine or iodine.
The term "reducing agent" (also commonly referred to as a "reductant" or
"reducer") is the element in an oxidation-reduction reaction that donates an
electron to another species. For the present invention, non-limiting examples
15 of suitable reducing agents for use in step (b) in the above-defined method
inciude: sodium borohyariae jNaBH4j, zinc in nydrocnioric acid, zinc in acetic
acid, magnesium in hydrochloric acid, sodium hydrogentelluride (NaTeH) in
ethanol, lithium aluminium hydride (LiAIH4) in tetrahydrofuran, indium in
ammonium chloride, and sodium hydride (NaH). A preferred reducing agent is
20 NaBH4. In a preferred embodiment, the reducing agent is bound to a solid
phase such as a resin in the form of particles such as beads.
The term "fluoroalkvl" refers to an alkyl as defined above that comprises a
fluorine atom in place of a hydrogen. Specifically, the term fluoroalkyl as used
herein is taken to mean ['8~]fluoroalkyla, nd as such the fluorine atom
25 comprised therein is radioactive I8F Preferably, said fluoroalkyl includes a
single I8F atom, most preferably at the terminal end of the chemical group.
The term "leaving group" refers to a moiety suitable for nucleophilic substitution
and is a molecular fragment that departs with a pair of electrons in heterolytic
bond cleavage.
The term "aJJ" refers to a monovalent aromatic hydrocarbon having a single
ring (i.e. phenyl) or fused rings (i.e. naphthalene).
A chemical group defined herein as "optionallv substituted" may either have no
substituents or may include one or more substituents. Preferred substituents
5 include CIm4 alkyl, C1-4 haloalkyl, halo, and nitro, wherein alkyl is as defined
above, "haloalkvl" is an alkyl as defined above that comprises a halo, halo is as
defined above, and "mrefyers to the group -NO2. Accordingly, examples of
preferred -0-SO~-Rg' roups for the present invention include: toluenesulfonic
acid, nitrobenzenesulfonic acid, benzenesulfonic acid, trifluoromethanesulfonic
10 acid, fluorosulfonic acid, and perfluoroalkylsulfonic acid.
The reaction step (c) with ["c]c~-~al kyl-LG' or [ ' 8 ~ ] - ~ 1f l-u4~ r o a l k ~isl -a~n ~ ~
alkylation reaction carried out in a suitable solvent. A "suitable solvent" is one in
which the reactants are readily soluble and readily react to result in the desired
product. Such a suitable solvent may be selected from the group comprising N,N-
15 dimethylformamide (DMF), acetone, dichloromethane (DCM), chloroform,
dimethylsulphoxide (DMS), methanol, ethanol, propanol, isopropanol,
tetrahydrofuran (THF), or acetonitrile.
In a preferred embodiment of the method of the invention said compound of
Formula II is a compound of Formula Ila:
wherein:
X" and xI2 are the same and are both an X group as suitably and
preferably defined herein,
Y" and Y" are the same and are both a Y group as suitably and
preferably defined herein; and,
5 Z" and zI2 are the same and are both a Z group as suitably and
preferably defined herein.
Preferably for the method of the invention said X group is halo, most preferably
chloro.
Preferably for the method of the invention said Y group is C1-4 alkyl, most
10 preferably methyl.
Preferably for the method of the invention said Z group is methyl.
In a preferred embodiment for the intermediates and the product of the method of
the invention, said X group is chloro, said Y group is methyl and said Z group is
metnyi.
15 In summary, the compound of Formula II provided in step (a) of the method of the
invention is obtained by reacting a compound of Formula IV:
wherein x3' and x~~ are the same and are both an X group as suitably and
preferably defined herein;
20 with a compound of Formula V:
wherein y4' is a Y group as suitably and preferably defined herein, and z4' is a Z
group as suitably and preferably defined herein.
The coupling of the compound of Formula IV with the cyanamide of Formula V
5 may be performed without solvent, or in the presence of a high boiling nonprotic
solvent such as chlorobenzene, toluene, or xylene. This reaction may be
effected at elevated temperature, for example 50 to 200°C, preferably at
around 130°C.
The above-defined compound of Formula IV is obtained by iodine oxidation of a
10 compound of Formula VI
wherein x5' is an X group as suitably and preferably defined herein.
Table 1 below illustrates a known method to obtain a compound of Formula Ill
alongside the methods used in the present invention:
15
( Prior Art
x5Fo1rmula VI0 NH2
1 add thiol protecting group
+
y4I
I
N '- / N 0 - s z 4 ~ Formula V
/
Present Invention
~"F9ormula VI "~
SH
Formula IV
I
Formula V
1
H yI 2I
x 2\ 1 ~ NNH ~ N/ ~ " o l
spl
Formula I I I (protected)
In Table 1:
j?&mosphere
H Y"
S.s
z 1 2 s r~b +X12 N N y12 H Formula I1
1 deprotection
H r1
XZQI I N N~HI N,+OZZI
SH
Formula Ill
each X, Y and Z group is as suitably and preferably defined herein for X, Y and
Z, respectively; and,
I reduction
H T2I
SH
Formula Ill
5 P' is a thiol protecting group.
Protecting groups are well known to those skilled in the art. For thiol groups
suitable protecting groups are benzyl, trityl, and 4-methoxybenzyl. The use of
further protecting groups are described in 'Protective Groups in Organic
Synthesis', Theorodora W. Greene and Peter G. M. Wuts, (Fourth Edition,
10 John Wiley & Sons, 2007).
The cyanamide starting material of Formula V in Table 1 can be prepared
according to the method described by Hu ef a1 (1997 J Med Chem; 40: 4281-
4289), by reaction of cyanogen bromide with the primary amine in diethyl ether,
or by alkylation of an arylcyanamide with sodium hydride or alkyl halide in
15 tetrahydrofuran. The nitrobenzenesulfonyl chloride starting material illustrated
in Table 1 is commercially available. The first step in Table 1, common to the
prior art method and the method of the present invention, is reduction of the
nitrobenzenesulfonyl chloride starting material to form the aminobenzenethiol
intermediate of Formula VI. In the second step of the prior art method, a
5 protecting group is placed onto the thiol group, which is removed at the end of
the synthesis using known methods. For example, where the thiol protecting
group is benzene, AIC13 and flash chromatography may be used for its removal.
The disulfide of Formula IV is obtained in the method of the invention by iodine
oxidation of the aminobenzenethiol intermediate of Formula VI. This step has
10 the advantage that the iodine additionally oxidises any tin present. In the prior
art method, residual tin complicates the workup because the tin hydroxides
form gels at pH >2. The method used in the present invention overcomes this
problem as oxidised tin does not form gels, thereby facilitating extractive work
up of the product. The method is furthermore advantageous because use of
15 the disulfide intermediate of Formula IV circumvents the need to protect the
thiol group. The method used in the present invention to obtain a compound of
Formula Ill illustrated in Table I above may be regarded as another aspect of
the present invention.
The present inventors have observed that the compound of Formula Ill
20 decomposes to form the disulfide of Formula 11, even under what would be
considered "inert" conditions. This problem is effectively overcome by storage of
the disulfide compound of Formula II instead of the compound of Formula Ill. In
the method to obtain a PET tracer of Formula I the compound of Formula II is
reduced by step (b) immediately before the radiolabelling step (c). In order to
25 facilitate this, steps (b) and (c) are preferably carried out in the same vessel. A
further advantage of this strategy over the prior art methods is that it is not
necessavJ l tc! inc!ude a base in the reacticr! step (c). !r!c!usicn cf a bare in the
reaction is required by the methods taught by the prior art to deprotonate the thiol
in order to allow reaction with the radiolabelled synthon. Suitable bases taught in
30 the prior art methods include inorganic bases such as potassium carbonate,
potassium hydroxide, or sodium hydride, or organic bases such as a trialkylamine,
for example triethylamine, diisopropylethylamine, or dimethylaminopyridine. In the
method of the present invention, once the compound of Formula II is reduced in
step (b) it can be reacted directly in step (c) with ["c]c~.~a lkyl-LG' or [ ' 8 ~ ] - ~ 1 - 4
fluoroalkyl-LG2 to obtain the PET tracer of Formula I without any requirement to
add a base.
5 ["c]cI-4 alkyl-LG' provided in step (c) of the method can be prepared using
methods well-known in the art of radiochemistry. For example, methyl iodide
can be prepared by reduction of [''C]carbon dioxide with lithium aluminium hydride
followed by reaction with hydroiodic acid. m carbon dioxide is usually produced
by the 1 4 ~ ( p l a )r1e'a~c tion from nitrogen gas containing trace amounts of oxygen.
10 methyl triflate can be prepared from met methyl iodide, or by gas phase
reaction of methyl bromide preprared from ["C]methane. All these methods
are described in more detail in "Aspects on the Synthesis of "c-~abelled
Compounds", Chapter 3 of Handbook of Radiopharmaceuticals (2003 Welch &
Redvanly eds. pp 141-194). A preferred ["c]cT-~ alkyl-LG' is selected from
15 ["~]meth~l-LGo'r ethyl-LG', and LG' is preferably iodo.
['8~]-~1f.lu4o roalkyl-LG2 provided in step (c) of the method can be prepared by
radiolabelling alkyldihalides or sulfonates using ['8~]fluoride. ['8~]~luoridise
typically obtained as an aqueous solution which is a product of the irradiation of an
[180]-water target. It has been widespread practice to carry out various steps in
20 order to convert ['8~]~luoridineto a reactive nucleophilic reagent, such that it is
suitable for use in nucleophilic radiolabelling reactions. These steps include the
elimination of water from ['8~]-fluoride ion and the provision of a suitable
counterion (Handbook of Radiopharmaceuticals 2003 Welch & Redvanly eds. ch.
6 pp 195-227). Suitable counterions include large but soft metal ions such as
25 rubidium or caesium, potassium complexed with a cryptand such as ~ r y ~ t o f ioxr ~ ~ ,
tetraalkylammonium salts. A preferred ['8~]-~1fl.u4o roalkyl-1G2i s ['8~]-fluoroethyl-
LG' wherein LG' is preferably a suifonate, most preferably tosylate.
Preferably for the method to obtain said PET tracer of Formula I, the reducing
step (b) and the reacting step (c) are carried out in immediate sequence. The
30 term "in immediate sequence" should be understood to mean that the reacting
step (c) is carried out as soon as possible, i.e. directly, after the reducing step
(b), such that there is as little time as practically possible between the two steps
and ideally no time between the two steps. In this way, any decomposition of
the compound of Formula Ill to form the disulfide is minimised, thereby
facilitating the radiolabelling reaction.
5 In a particularly preferred embodiment, the above-described method to obtain said
PET tracer of Formula I is automated. PET tracers, and [18~]-tracerisn particular,
are now often conveniently prepared on an automated radiosynthesis apparatus.
There are several commercially-available examples of such apparatus, including
racer lab^^ and ~astlab'(~b oth from GE Healthcare Ltd). Such apparatus
10 commonly comprises a "cassette", often disposable, in which the radiochemistry is
performed, which is fitted to the apparatus in order to perform a radiosynthesis.
The cassette normally includes fluid pathways, a reaction vessel, and ports for
receiving reagent vials as well as any solid-phase extraction cartridges used in
post-radiosynthetic clean up steps. The present invention therefore provides in
15 another aspect a cassette for carrying out these steps wherein said cassette
comprises:
(i) a vessel containing the compound of Formula II as suitably and
preferably defined herein;
(ii) means for reacting said compound of Formula ll with a reducing agent
to form a compound of Formula Ill, wherein said reducing agent and
said compound of Formula Ill are as suitably and preferably defined
herein; and,
(iii) means for reacting said compound of Formula Ill with either ["c]cI..~
alkyl-LG' or ['8~]-~1f-~4 u o r o a l k ~tol -o~b~ta~in a PET tracer of
Formula I, wherein LG', LG2 and said PET tracer of Formula I are as
suitably and preferably defined herein.
The means for reacting said compound of Formula II with said reducing agent may
be a vessel containing the reducing agent in solution (or in a soluble) form,
wherein the reducing agent is passed through the vessel containing the compound
30 of Formula II in order to effect the reduction. Alternatively, said means may be a
vessel in which the reducing agent is bound to a solid phase, wherein the
compound of Formula II is passed through the vessel containing the reducing
agent in order to effect the reduction. The suitable and preferred embodiments
described herein for the reduction step (b) and the reaction step (c) also apply to
5 the method as carried out on the cassette of the invention.
The cassette may additionally comprise:
(i) an ion-exchange cartridge for removal of excess ["c]c~-a~lk yl-
LG' or ['8~]-~1f.lu4~ roalk~l-LG*.
The PET tracer of Formula I obtained by the method of the invention is useful as a
10 radioligand for the NMDA receptor and can be used in an in vivo diagnostic or
imaging method such as positron emission tomography (PET). A PET tracer of
Formula I as defined above, or a salt or solvate thereof, may be used to image the
NMDA receptor in healthy human volunteers. Because the PET tracer of Formula
I is useful for in vivo imaging of NMDA receptors it thus also has utility in the
15 diagnosis of NMDA-mediated disorders, such as stroke, brain or spinal chord
trauma, epilepsy, Alzheimer's disease, or H~mtington'sd isease.
Brief Description of the Examples
Example 1 describes the synthesis of 1,l I-(5,5'-disulfanediylbis(2-chloro-5,lphenylene))
bis(3-methyl-3-(3-(methylthio)phenyl)guanidine).
20 Example 2 describes the synthesis of 3-(2-chloro-5-mercaptopheny1)-I-methyl-
1 -(3-(methy1thio)phenyl)guanidine.
Example 3 describes the synthesis of 3-(2-chloro-5-((2-fluoroethyl)thio)phenyl)-
I -methyl-I -(3-(methy1thio)phenyl)guanidine.
Example 4 describes the synthesis of 3-(2-chloro-5-((2-fluoroethyl)thio)phenyl)-
25 I -methyl-I -(3-(methy1thio)phenyl)guanidine using resinbound borohydride.
List of Abbreviations used in the Examples
DCM dichloromethane
EtOH ethanol
HPLC high performance liquid chromatography
M molar
5 MeOH methanol
mmol millimole(s)
NMR nuclear magnetic resonance
10 RT room temperature
Examples
Unless otherwise specified, the intermediates and reagents used in the
examples were purchased from Sigma Aldrich, Merck, or Alfa Aesar.
Example 1: Synthesis of I, 1 '-(5,5'-disulfanediylbis(2-chloro-5,l-
1 5 phenylene))bis(3-methyl-3-(3-(methylthio)phenyl)_cruanidine)
l(a) Synthesis of 55'-disulfanediylbis(2-chlorobenzenaminium chloride
C' CICI-
c1
Tin(ll) chloride (33.32g, 175.74mmol) was dissolved in 30% hydrochloric acid
(99.7mL) and 4-chloro-3-nitrobenzene-I-sulfonyl chloride (5.00g, 19.553mmol)
was added before submerging the flask in a 125°C preheated oil bath. After 3
hours all solid material had dissolved and the reaction mixture was allowed to
5 cool to RT, which caused spontaneous crystallization.
The crystals (contaminated with Tin) were filtered off, dissolved in water
(250mL) and portions of iodine solution (5OmgImL) were added until HPLC
analysis confirmed that all the 2-chloro-5-mercaptobenzenaminium chloride
had been converted to 5,5'-disulfanediylbis(2-chlorobenzenaminium) chloride.
10 The solution was filtered, and water (400mL) was added to the filtrate, followed
by stirring and neutralization using NaOH solution (-lmL, 10%). The solution
was extracted with diethyl ether (4 x 150mL), dried with magnesium sulphate
(anhydrous) and filtered. To the ether solution was added HCI (dry, 1 M in
diethyl ether, IOmL), the solution was filtered and the filtrate was dried under
15 vacuum to give 5,5'-disulfanediylbis(2-chlorobenzenaminium) chloride as an
off-white powder (21,47g, 63%).
1 H NMR (400 MHz, CDCI3): 6 7 154 (dl J = 8.3 Hz, 2H), 6 6.865 (dl J = 2.2 Hz,
2H), 6 6.783 (dd, J1 = 2.2 Hz, J2=8.3 Hz, 2H), 6 4.080 (broad s, 4H)
l(b) Synthesis of I , 1'-(5,5'-disulfanediylbis(2-chloro-5, I-phenylene))bis(3-
A mixture of 5,5'-disulfanediylbis(2-chlorobenzenaminium) chloride (1 .0g,
2.6mmol) and N-methyl-N-(3-(methyIthio)phenyl)cyanamide (1.83g, 10.3 mmol)
was heated to 130°C. This thick slow-stirring melt was left for 17h (HPLC yield
after 1 hour ~ 8 0 %t)h~en allowed to cool to RT. The solid was dissolved in
DCM (25mL), extracted with water (3 x 200mL) and the combined aqueous
phases back-extracted with DCM (50mL). The aqueous phase was neutralised
with NaHC03 and extracted with diethyl ether (3 x l50mL). The combined
5 organic phases were dried with magnesium sulphate (-5g), filtered and
concentrated to dryness under reduced pressure to give 1 ,IT-(5,5'-
disulfanediylbis(2-chloro-5,l -phenylene))bis(3-methyl-3-(3-
(methy1thio)phenyl)guanidine) (1.16g, I .7mmol, 67%, HPLC purity 94.8%) as
off-white powder.
1 10 H NMR (400 MHz, CDCI3): 6 7.300 (t, J = 7.9 Hz, 2H), 6 7.298 (d, J = 8.3 Hz,
2H), 6 7.175 (t, J = 1.9 Hz, 2H), 6 7.122 (ddd, Jl = 1.0, J2= 1.8 HZ, J 3 ~ 7 . 9H Z,
2H), 6 7.108 (d, J = 2.3 Hz, 2H), 6 7.054 (ddd, J1 = 1.0, J2 = 2.2 HZ, J3 = 7 9
Hz, 2H), 6 7.049 (dd, Jj = 2.3, J2 = 8.3 Hz, 2H), 6 3.893 (broad s, 4H), 6 3.338
(s, 6H), 6 2.494 (s, 6H).
1 5 Example 2: Synthesis of 3-(2-chloro-5-mercaptopheny1)-I-methyl-143-
~methyithiojphenyij_quanidhe
1 NaBH,
S\s HCI -- QN.l:O., CI H I
\s
CI
1,l '-(disuIfanediylbis(2-chloro-5,l -phenylene))bis(3-methyl-3-(3-
(methy1thio)phenyl)guanidine) (3.lg, 4.6mmol), obtained as described in
20 Example I , was suspended in EtOH (62mL) and then sodium borohydride
(0.5g, 13.8mmol) was added in portions. The reaction solution was left to stir
overnight under an inert atmosphere. The reaction was cooled to 10 degrees
before quenching with hydrochloric acid in ether (2M) and concentrated to
dryness under reduced pressure to give a creamy solid. The solid was purified
25 by column chromatography (CHCI3->7% MeOH in CHCI3) to give 3-(2-chloro-5-
-1 8-
mercaptcs]nhenyl)-3- metl?yi-1-(3-(~rre$hylthirs~~pheny!)giianid(i2r.'51ge : 6:7mmol,
73.6%) as a white foam.
1H NMW (580 MHz, CDC13): 6 9.686 (s: IH). 5 8.531 js,2H), 6 7.332 (t. S = 7.8
I-fz, IM), 6 7.032 bd, JZ8.4 Hz: 11-4). 67.022 gd, JETi,8 Hz, IH), 6 7.0.14 (s,
5 ~Hg,B6922(s.1H),6G.892jcf,J=8.4Hz1, H),bB855(d:J='T.BHz,IH).6
3.740 (s, qH), 6 3.656 (s, 3H), 5 2.460 (s, 3H).
10 I,1' -(d8suffatnediylbi~f2~~chl~r-poh-5e,n1y Iene))bis(3-ep~et~~yt-3-E3-
jrriietEsyithio)phe~~yI)guanidine(3) .0g, 4,$5mmoI), obtair-sed as described in
Example 4, was a dissolved in ethanol j120mb) and 2-fluaroethyl 4-
wrsthylbenmenesulf~nate( 2%"9 .8rnrnof) was added. The clear sol hi ti or^ $.as
heated to 60°C under inert atmosphere and sodium borohydride (0.8g.
15 22.3mmof) was added in portions. The reaction was heated for 45 minutes
before being concentrated 'so dryness under reduced pressure. The crude was
pi~rifiedb y column chrawratagraphy (CHC13-=.2tihM eOH in CHC13) to give 3-(2-
chforo-5-((2-fl~!oroet~~yi)thio)phe-rfr~~ye-l~f ~y-f(-34-( me$hyi'shio)phei?yI)gu~~~idi~e
(2.29, 5.6 ~ne~fo6i2, .996) as a &ear siowly crystalizir~go il.
28 1 H NNMW (400MHz, CDCIA: 67.28-7.35 (m, 2H3, 61.05-7.25 (m,4 H3, 66.90-8.95
(dd. .j1-3.'/ 8Hz, 4,-8.33Hz 3 H). 54.6 (t. J=6.T5Hz, 4 H), 54.5 (t, Ji=G.-J5)-iiz.3, knf).
63.9 (broad S, 1 H), 63.4 gs, 3H), 53.2(t, J-6.75Hr. 1 H), 63.1 5 {t, J=6,75!4z,
3 H). 82.52 (S, 3H)61 .65 (broad S, -1 H-ii)
WO 20111141568 PCTlEP20111057757
Example 4: Synthesis of 3-(2-chloro-5-((2-fluoroethy1)thio)phenvl~-Imethyl-
I-(3-(methy1thio)phenyl)guanidine using resinbound Borohvdride
I , I '-(disulfanediylbis(2-chloro-5,l -phenylene))bis(3-methyl-3-(3-
5 (methy1thio)phenyl)guanidine) (0.15 g, 0.22 mmol), obtained as described in
Example 1, and 2-fluoroethyl4-methylbenzenesulfonate (0.10 g, 0.45 mmol)
was dissolved in ethanol (96%, 10 mL), and solid phase supported borohydride
(0.45g, -1 I lmmol) was added The resulting reaction suspension was heated
under inert atmosphere to 60°C for 17 hours. The suspension was filtered and
10 the solid phase material was washed with ethanol (96%, 2*5mL). The
combined organic phases were concentrated under reduced pressure to give a
moist crude that was redissolved in dichloromethane (1 OmL), dried with
magnesium sulfate (-0.3 g, 2.492 mmol), filtered and concentrated to dryness
to give 3-(2-chloro-5-((2-f1uoroethyl)thio)ph eny-I- methyl-I -(3-
15 (methy1thio)phenyl)guanidine (0.13 g, 0.32 mmol, 70.7 % yield) as a clear oil.
1 H NMR (400MHz, CDCI3): 67.28-7.35 (m, 2H), 67.05-7.25 (m, 4H), 66.90-6.95
(dd, J1=3. 1 8Hz, J2=8.33Hz I H), 64.6 (t, J=6.75Hz1 1 H), 64.5 (t, J=6.75Hz1 1 H),
63.9 (broad S, I H), 63.4 (s, 3H), 63.2(t, J=6.75Hz, I H), 63.1 5 (t, J=6.75Hz1
1 H), 62.52 (S, 3H)61.65 (broad S, 1 H)

Claims
1) A method to obtain a positron emission tomography (PET) tracer of
Formula I:
5 wherein:
X' is an X group selected from C1., alkyl or halo;
Y' is a Y group selected from hydrogen or Clq alkyl;
Z' is a Z group which is Cl alkyl; and,
Q is ["CICI-4 alkyl- or ['8~]-~l-4fluoroalkyl-;
10 wherein said method comprises:
(a) providing a compound of Formula 11:
wherein:
X" and xI2 are the same and are both an X group as defined for XI;
Y" and YI2 are the same and are both a Y group as defined for Y'; and,
Z" and z ' a~re the same and are both a Z group as defined for z':
5 (b) reducing said compound of Formula II with a reducing agent to obtain a
compound of Formula Ill:
x2' is an X group as defined for XI;
Z2' is a Z group as defined for z',
(c) reacting the compound of Formula Ill as obtained in step (b), with either
["CICI-4 alkyl-LG' or [I8~]-cl4f l u o r ~ a l k ~wl -h~er~ei~n ,L G' and L Ga~re
independently halo, or the group -0-~02-Rw' herein R' represents an
optionally-substituted C6-'() aryl, an optionally-substituted C1-4 alkyl, or
15 CIm4 fluoroalkyl, wherein said reacting is carried out in a suitable solvent.
2) The method as defined in Claim 1 wherein said compound of Formula II is
a compound of Formula Ila:
wherein:
X" and xI2 are the same and are both an X group as defined in Claim
1,
Y" and Y ' a~re the same and are both a Y group as defined in Claim 1;
and,
Z" and zf2 are the same and are both a Z group as defined in Claim 1
3) The method as defined in either Claim 1 or Claim 2 wherein said X group is
halo.
10 4) The method as defined in any one of Claims 1-3 wherein said Y group is
C1-4 alkyl.
5) The method as defined in any one of Claims 1-4 wherein said Z group is
methyl.
""\J The methcd ss defined i:: myc ze cf CIaims ?=5w herei:: ssid X grcup is
15 chloro, said Y group is methyl and said Z group is methyl.
7) The method as defined in any one of Claims 1-6 wherein said reducing
agent used in step (b) is selected from sodium borohydride (NaBH4), zinc in
hydrochloric acid, zinc in acetic acid, magnesium in hydrochloric acid,
sodium hydrogentelluride (NaTeH) in ethanol, lithium aluminium hydride
-23-
(LiAIH4) in tetrahydrofuran, indium in ammonium chloride, and sodium
hydride (NaH).
8) The method as defined in any one of Claims 1-7 wherein in said reacting
step (c) the compound of Formula Ill is reacted with [18~]-~1f.lu4o roalkyl-
5 LG*.
9) The method as defined in any one of Claims 1-8 wherein said reducing
step (b) and said reacting step (c) are carried out in immediate sequence.
10) The method as defined in any one of Claims 1-9 wherein said reducing
step (b) and said reacting step (c) are carried out in the same vessel.
10 1 1 The method as defined in any one of Claims 1-10 wherein said method is
automated.
12) A method for the preparation of the compound of Formula Ill as defined in
any one of Claims 1 and 3-6, said method comprising reacting a compound
of Formula IV:
wherein x3' and x3' are the same and are both an X group as defined in
any one of Claims 1, 3 and 6;
with a compound of Formula V:
wherein y4' is a Y group as defined in any one of Claims 1, 4 and 6, and
z4' is a Z group as defined in any one of Claims 1, 5 and 6;
to obtain a compound of Formula II as defined in any one of Claims 1-6;
5 and,
reducing said compound of Formula II using a reducing agent as defined
in either Claim 1 or Claim 7 to obtain said compound of Formula Ill.
13) A cassette for carrying out the method as defined in Claim 11, wherein said
cassette comprises:
10 (i) a vessel containing the compound of Formula I1 as defined in any one
of Claims 1-6;
(ii) means for reacting said compound of Formula II with a reducing agent
to form a compound of Formula Ill as defined in any one of Claims 1
and 3-6; and,
15 (iii) means for reacting said compound of Formula Ill with either ["c]cI.~
alkyl-LG' or [ ' 8 ~ ] - ~ 1fl-u4or oalkyl-LG2 to obtain a PET tracer of
Formula I, wherein LG', LG2, and said PET tracer of Formula I are
as defined in Claim 1
14) The cassette as defined in Claim 12 which further comprises:
20 (iv) an ion-exchange cartridge for removal of excess ["c]cI-4 alkyl-LG'
or [ 1 8 ~ ] - ~fl~uo.r4o alkyl-LG2.