PROCESS SIMPLIFICATION FOR PRECURSOR COMPOUND
Technical Field of the Invention
The present invention relates to a method to obtain radiopharmaceutical precursors, and
particular to protected amino acid derivatives which are used as precursors for
production f radiolabeled amino acids for use in in vivo imaging procedures such as
positron emission tomography (PET). The invention further includes a method to
obtain said radiolabelled amino acids.
Description of Related Art
In recent years, a series of radioactive halogen-labelled amino acid compounds
including [1 F]l-amino-3-fluorocyclobutanecarboxylic acid ([ F]-FACBC) have been
designed as novel radiopharmaceuticals. [ F]-FACBC is considered to be effective as a
diagnostic agent for highly proliferative tumours, because it has a property of being
take up specifically by amino acid transporters. EP1 97801 5(A1) provides precursors
for the [, F]-FACBC compound and methods to obtain said precursors.
EP197801 5(A1) specifically discloses a method to obtain the precursor l -(N-(tbutoxycarbonyl)
amino)-3-[((trifluoromethyl)sulfonyl)oxy]-cyclobutane-l-carboxylic
acid ethyl ester wherein said method comprises the following steps:
EP1 97801 5(A 1) describes that step 1 of the above reaction scheme comprises
hydrolysis f s>w-5-(3-benzyloxycyclobutane)hydantoin 1 by addition of barium
hydroxide Ba(OH)2 to the solution and refluxing the mixture at 14°C for 24 hours or
longer. In the ethyl esterification step 2, syn- -amino-3-benzyloxycyclobutane- 1-
carboxylic acid 2 is dissolved in ethanol (EtOH) and reacted with thionyl chloride
(SOCl2) to yield syn- -annno-3-benzyloxycyclobutane- 1-carboxylic acid ethyl ester 3.
Step 3 comprises addition of / -buto.xycarbonyl (Boc) to the a i e function by
reaction of 3 with tert-butyl dicarbonate (Boc)20 , and the resultant material is purified
by chromatography to obtain - l -(N-(/-butoxycarbonyl)amino)-3-benzyloxycyclobutane-
1-carboxylic acid ethyl ester 4. The benzyl-protected intermediate 4 is then
deprotected i step 4 by dissolving compound 4 in ethanol (EtOH), adding palladium on
activated carbon (Pd/C) and applying a small positive ¾-pressure over the reaction
mixture. The resultant material is purified by chromatography to yield syn-\ -(N-(tbutoxycarbonyl)
amino)-3-benzyloxy-cyclobutane-l -carboxylic acid ethyl ester 5 for use
in step 5, which comprises reaction of 5 with trifluoromethanesulfonic anhydride
(Tf20), followed by chromatographic purification with subsequent re-crystallization of
the material order to obtain syn- 1-(A-(/-butoxycarbonyl)amino )-3-
[((trifiuoromethy])sulfonyl)oxy]-cyclobutane-l -carboxylic acid ethyl ester 6. Similar
methods are described in EP2230229 and US20 0016626. In the case of all of these
prior art teachings, the methods are suitable for small scale preparation for research
purposes.
It would be desirable to reduce the complexity of the above-described multistep
chemical reaction in order to reduce process time, use of equipment and chemicals, and
to facilitate scale-up.
Summary of the Invention
The present invention is a method for the preparation of precursor compounds for [ F]-
FACBC and similar compounds that is simplified with respect to known methods. The
method of the invention leaves out one of e purification steps taught by the prior art
and as such permits the resultant precursor compounds o be obtained in a more costand
time-efficient manner.

phenyl) o fused rings (i.e. naphthalene). Unless otherwise defined, such aryl groups
typically contain from 6 to 10 carbon ring atoms.
The term "debenzylation " refers to the cleavage of a benzyl substituent from a compound.
The term "benzyl" refers to a group with chemical structure .H CH -. Debenzylation is a
method well-known in the art and i generally carried out by "catalytic hydrogenation",
which is a reaction whereby a carbon-carbon bond s cleaved or undergoes "lysis" by
hydrogen. Hydrogenolysis usually carried out catalytically, e.g. using palladium on
carbon (Pd/C) as a catalyst. When a catalyst such as Pd/C s used in the debenzylation step,
the catalyst s removed from the reaction mixture by filtration prior to the next step. The
term "filtration" refers to the mechanical separation o solids from fluids. Non-limiting
examples of suitable filtration means for use in the present invention include glass sinter
funnel or glass fibre filer in addition to a filter funnel, although other more specialised filter
methods are also suitable. Generally, following the debenzylation step (a) and prior o the
conversion step (b), the reaction solvent is removed by drying. Drying may be carried out
by methods well-known to the person skilled i the art e.g. by evaporation under nitrogen
flow and/or vacuum drying.
The term "obtained directly" refers to the fact that following step (a) and prior to step (b),
no purification steps are carried out o the reaction mixture. n particular, the reaction
mixture obtained by carrying out step (a) is not further purified by chromatography prior to
carrying out step (b). Alternatively stated, step (a) is carried out with the proviso that the
reaction mixture obtained from step (a) is not purified prior to carrying out step (b), and
particular with the proviso that the reaction mixture of step (a) is not purified by
chromatography following step (a) and prior to step (b). The term ''chromatography'' is
well-known in the art and refers to a laboratory technique for the separation of chemical
substances from each other i a mixture of different substances. Chromatographic
separation involves passing the mixture dissolved in a mobile phase through a stationary
phase, which separates the molecule of interest from other molecules i the mixture based
on differential partitioning between the mobile and stationary phases.
The term "a suitable form ofX" means X as defined herein in a form that can displace the
hydroxy! function in a substitution reaction.

Hydantoin 1 was treated with 3N aqueous sodium hydroxide at 180°C followed by ditertbutyl
dicarbonate to provide the N-Boc acid 5 . Methyl ester 6 was obtained in high
yield by reacting 5 with trimethylsilyl diazomethane.
It is within the ordinary skill in the art to adapt the above-described prior art methods to
obtain other compounds of Formula a that fall within the definition of the present
invention. Suitably, the starting hydantoin compound includes a mixture f the syn- and
anti-enantiomers. There is no need for actively separating enantiomers, at any stage of the
process.
In a preferred embodiment of the invention. X the group -0-S0 2-R . Most preferably
when X is -0-S0 2-R3 it is selected from the group consisting of tolucnesulfonic acid,
nitrobenzenesulfonic acid, bcnzenesulfonic acid, trifluoromethanesulfonic acid,
f!uorosulfonic acid, and perfluoroalkylsulfonic acid. In an especially preferred embodiment
-O-SO 3 3 2-R is trifluoromethanesulfonic acid. The group -O-SO2-R can be added in step (b)
of the method o the invention by reaction of the compound of Formula lb with a
electrophilic derivative of the desired -O-SO2-R group, which an example of a "suitable
form of X" . For example, where it s desired to add tri fiuoromethanesul fon ic acid, the
compound of Formula b can be reacted with trifluoromethanesulfonic anhydride.
In an alternative preferred embodiment, X is halogen. When X is halogen it i most
preferably bromo or chloro. Step (b) of the method of the invention wherein X is a halogen
may be carried out by methods well known to those skilled in the art. For example, a
compound of Formula I wherein X is chloro can be obtained by reaction of the compound
o Formula b with a chloride-containing reagent such as thionyl chloride, phosphorous
pentachloride (PC15), phosphorous trichloride (PC13), each of which are examples of a
"suitable form of X". A compound of Formula I wherein X is bromo can be obtained by
reaction of a compound of Formula lb with a brom ine-con ai ng reagent such as
hydrobromic acid (HBr) or phosphorous tribromide (PBn), again, each of which are
examples o a "suitable form of X".
Preferably, R1 is methyl or ethyl and is most preferably ethyl. This preferred definition of
R1 equally applies to R ' and R2 1.

COOEt
NHBoc Compound 2a
wherein Et is ethyl and Boc is t rt-Butyloxycarbonyl.
In a preferred embodiment, steps (c) and (d) are carried out on an automated synthesiser.
[ F]-radiotracers are now often conveniently prepared on an automated radiosynthesis
apparatus. There are several commercially-available examples of such apparatus, including
Traccrlab™ and Fastlab™ (both from GE Healthcare Ltd). Such apparatus 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 post-radiosynthetic clean up steps.
A typical cassette for automated synthesis of a compound of Formula 11 includes:
(i) a vessel containing a compound of Formula I as defined herein; and
(ii) means for eluting the vessel with a suitable source of [ F]-fluoride as
defined herein.
(iii) an ion-exchange cartridge for removal of excess [ F]-f!uoridc; and,
(iv) a cartridge for deprotection of the compound of Formula a to form the
compound of Formula II.
The invention will now be described by means of the following experimental examples:
Brief Description of the Examples
Example 1 describes the synthesis of 5-(3-benzyloxycyclobutane)hydantoin.
Example 2 describes the synthesis of l-amino-3-(benzyloxy)cyclobutanecarboxylic
acid.
Example 3 describes the synthesis of l-Arnino-3-benzyloxy-cyclobutanecarboxylic acid

for 16 hours 10 minutes.
Solid material from vacuum drying (257.2g) was dissolved in water-isopropanol (15.0 1,
1:1). The solution was stirred at ambient temperature for 70 minutes and un-dissolved
particles were removed by filtration through a glass sinter filter (pore size 3). Filtrate
evaporated in vacuo at 45-49"C to approximately 1/3 of the starting volume. The
resulting slurry was cooled to 7.6°C, filtered through a glass sinter filter funnel (pore
size 3) and washed with cold water (2.0 1, < 7°C). The filter cake was transferred to a
Schott Duran glass bottle and in vacuo at 36°C for 1 h 40 min. Yield: 229.6 g (71 %).
NMR (500 MHz, DMS ,) d (ppm): 10.63 (s, 111. Ni ), 8.24 (s, 1H, NH), 7.38-
7.27 (m, 5H, Bz), 4.32 (s, 1H, C¾-Bz), 4.06-3.98 (m, 1H, CH-ring), 2.68-2.61 (m, 2H,

dichloromethane (77.5 ml) and pyridine (2.95 ml. 36.6 mmol), and the resulting
solution cooled to <5°C. Triflic anhydride (3.01 ml, 17.9 mmol) was added to the
mixture portions over 23 minutes with the reaction temperature kept at <5°C during
the addition. The resulting mixture was stirred on a ice bath for 31 minutes (reaction
progress monitored by TLC) and upon complete reaction water (70 ml) was added to
quench the reaction.
The reaction mixture was extracted with Et20 ( 150 ml), the water phase discarded the
organic phase washed with HC1 (75 ml, M), brine (75 sat.aq.) and dried over
Na2S04. The mixture was filtered through a glass sinter filter and the filtrate evaporated
in vacuo at <30°C to afford crude Compound 1. The crude product was re-dissolved in
dichloromethane (40 ml) and adsorbed onto Si0 2 (9.5 g).
The crude product was purified by flash chromatographic purification using Si0 2
column, isocratic elution with pentanc: diethyl ether (3:1). The product fractions were
combined and evaporated in vacuo at <30°C to afford Compound 1 (3.22g).
Compound was further puri lied by re-crystallization by dissolving the material in
diethyl ether (7.7 ml) and slowly stirring the mixture at <35°C until all solids had
dissolved. The reaction mixture was slowly cooled to 25°C over 45 minutes and further
stirred at this temperature for 1hour 25 minutes. The solution was further cooled to
<5°C and gently stirred at this temperature for 54 minutes, then further cooled to <-20°C
and stirred at this temperature for 1 hour 4 minutes, ice-cold heptane (25 ml) was added
and the solution gently stirred for 1 hour 20 minutes. Formed crystals were collected by
filtration using a pre-cooled glass sinter filter and washed with ice cold heptane (25 ml,
<-5°C). The reaction afforded Compound 1 2.86 g (61 %).