Opportunities for isotopic labelling via phase-tagged synthesis with organogermanium linkers

Article abstract of DOI:10.1002/jlcr.1302

Strategies for the use of germanium-based linkers for the phase-tagged synthesis of isotopically labelled intermediates are described. Protocols of potential use in the devolatilization of volatile intermediates prepared from [¹⁴C]-CO₂

Full text of DOI:10.1002/jlcr.1302

Journal of Labelled Compounds and Radiopharmaceuticals
J Label Compd Radiopharm 2007; 50: 281–285.
Published online in Wiley InterScience
JLCR
(www.interscience.wiley.com). DOI: 10.1002/jlcr.1302
Short Research Article
Opportunities for isotopic labelling via phase-tagged
synthesis with organogermanium linkersy
ALAN C. SPIVEY1,*, LAETITIA J. MARTIN1, CATHERINE NOBAN1, TEYRNON C. JONES2, GEORGE J. ELLAMES3 and
ANDREW D. KOHLER3
1 Department of Chemistry, South Kensington Campus, Imperial College, London SW7 2AZ, UK
2 Department of Chemistry, Sheffield University, Sheffield S3 2HF, UK
3 Department of Isotope Chemistry and Metabolite Synthesis, sanofi-aventis, Willowburn Ave., Alnwick, Northumberland NE66 2JH, UK
Received 1 August 2006; Revised 29 January 2007; Accepted 4 February 2007
Abstract: Strategies for the use of germanium-based linkers for the phase-tagged synthesis of isotopically labelled
intermediates are described. Protocols of potential use in the devolatilization of volatile intermediates prepared from
[
14C]-CO2 and [14C]-PhBr to aid ADME studies on drug substances and their metabolites are discussed in relation to
SR47035 (an advanced intermediate en-route to the 5-HT2 receptor antagonist SR46349B) and 4-amino-4-
phenylpiperidine (a key intermediate in the synthesis of NK3 receptor antagonist SR142801, osanetant).
Additionally, protocols with potential for the preparation of [123I] and [18F]-labelled SPECT/PET imaging/diagnostic
agents are discussed in relation to an [123I]-labelled analogue of SR141716 (rimonabant, a CB1 receptor antagonist)
and 6-[18F]-(S)-DOPA (for diagnosis of Parkinson’s disease). Copyright # 2007 John Wiley & Sons, Ltd.
Keywords: phase-tagged synthesis; solid-phase synthesis (SPS); isotope labelling; organogermanium; devolatilization; SPECT;
PET
O
Me
NH
N
Ac
NMe
O
OH
F
OH
N
N
N
HO
14
C
NH
3
14
123
C
I
N
CO
O
2
18
Cl
F
Cl
Cl
Cl
14C]-SR47035
[
14C]-SR142801
[
123I]-SR141716 analogue 6-[18F]-(S)-DOPA
[
Introduction
attached to a ‘phase-tag’ which allows the molecule to
be readily separated from compounds that are not
tagged.1,2 The best known example is solid-phase
synthesis (SPS) wherein an insoluble polymer support
is used as the phase-tag and separation is by simple
filtration; which can be automated. In an isotopic
labelling context, SPS offers two additional benefits:
firstly, the process of phase-tagging renders the sub-
strate involatile and secondly, if an appropriately low
loading of the substrate onto the tag is achieved, then
conveniently handled quantities of, e.g. derivatized
polymer can be used for the preparation of the
extremely small quantities of labelled product required
for many applications. Herein, we describe the devel-
opment of potentially generic strategies for the pre-
paration of isotopically labelled compounds of
Phase-tagged synthesis encompasses methods of
synthesis in which either the reagents/catalysts or
substrates used in a synthetic sequence are covalently
*Correspondence to: Alan C. Spivey, Department of Chemistry, South
Kensington Campus, Imperial College, London SW7 2AZ, UK.
E-mail: a.c.spivey@imperial.ac.uk
Contract/grant sponsor: EPSRC
Contract/grant sponsor: BBSRC
Contract/grant sponsor: ORS
Contract/grant sponsor: sanofi-aventis
Contract/grant sponsor: GSK
Contract/grant sponsor: GE Healthcare
yProceedings of the Ninth International Symposium on the Synthesis
and Applications of Isotopically Labelled Compounds, Edinburgh,
16–20 July 2006.
Copyright # 2007 John Wiley & Sons, Ltd.

282 A. C. SPIVEY ET AL.
Devolatilization of potentially 14C-labelled pharma-
ceutical intermediates for ADME studies
pharmaceutical/pharmacological interest exploiting
the above features of phase-tagged synthesis. Addi-
tionally, we exploit the unique properties of an
arylgermanium bond between the substrate and the
phase tag.3–6 The precise properties of the aryl
germanium linkage can be tuned by varying the two
‘spectator groups’ on the germanium atom,6 but in
general this linkage is extremely resistant to cleavage
by basic and nucleophilic conditions but is readily
cleaved in a ‘traceless’ fashion by, e.g. TFA or with
concomitant introduction of functionality at the posi-
tion of previous attachment by, e.g. Iþ, Brþ, Clþ and Fþ.
This allows ‘traceless’ devolatilization on the one hand
and late-stage introduction of potentially isotopically
labelled I and F atoms for SPECT/PET imaging on the
other. All the work described has been performed using
non-isotopically enriched materials as proof-of-concept
for the approaches – clearly, optimization will be
required prior to ‘hot’ application.
Radioisotopically labelled intermediates are generally
toxic at high concentrations and require rigorous
containment. Issues of containment are invariably
most acute during the first steps in a synthesis starting
from a volatile-labelled starting material and proceed-
ing via low-molecular weight volatile synthetic inter-
mediates. Devolatilization by direct immobilization of
the labelled precursor onto a polymeric support would
clearly solve these problems by rendering the labelled
material completely involatile during subsequent
transformations. That only a limited set of isotopically
labelled ‘building blocks’ are employed routinely by
radiochemists is advantageous for developing generic
devolatilization protocols – only a limited number of key
immobilization techniques and subsequent functiona-
lization protocols need to be developed in order for
these techniques to be widely applicable. It was there-
fore our objective to develop modes of attachment that
would prove as robust and flexible as possible.
Results and discussion
Our initial objective was to provide proof-of-concept
for the devolatilization of [14C]-labelled intermediates
formed following carboxylation of an aryl lithium
species with CO2 generated from [14C]-BaCO3. Specifi-
Preparation of the germanium-based linkers
The efficient synthesis of germanium-based linkers for
SPS from commercial GeCl4 has been described pre-
viously (Scheme 1).3,4,6
cally, we decided to develop
a SPS approach to
SR47035, which is an advanced intermediate en-route
to the sanofi-aventis 5-hydroxytryptamine (5-HT2)
receptor antagonist, SR46349B. During preliminary
studies within sanofi-aventis towards the synthesis of a
Immobilization of an aromatic substrate to a polymer
support in this fashion provides a robust platform for
further chemical manipulation while benefiting from
the above-mentioned synthetically appealing attributes
of the germanium-aryl bond.
[
14C]-labelled analogue it had been noted that the
2-fluoroacetophenone intermediate was appreciably
volatile (Scheme 2).
Cl
R
Ge
R
R
Ge
R
Cl Cl
Ge
1) RMgX (Xs.)
Ge
Cl
Cl
Ar
Cl
2)
OMe
3 steps
[~69%]
1) HCl in Et2O
2) ArLi
O
OMe
O
HO
£
Br
Cs2CO3, NBu4I
MeCN, 90˚C
[~70-90%]
R = Me, CF3, Et, i-Pr, p-tol, Ph
p-FC6H4, 3,5-di(CF3)C6H3
'off-the-shelf' stable resin
Scheme 1 Scheme available in colour online at www.interscience.wiley.com
1) MeLi
2) TMSCl
3) NH4Cl
1) n-BuLi
2) 14CO2
OH
F
OH
F
F
F
OH
1) HCl, i-PrOH
F
OHC
H
Me
N
14C
O
14CO2H
14C
Me
14C
Me
Me
Br
N
Me
N
Me
O
0.5 fumarate
O
volatile
N
Me
O
SR47035
SR46349B
5-HT2 antagonist
2) fumaric acid
Scheme 2 Scheme available in colour online at www.interscience.wiley.com
Copyright # 2007 John Wiley & Sons, Ltd.
J Label Compd Radiopharm 2007; 50: 281–285
DOI: 10.1002.jlcr

ISOTOPIC LABELLING VIA PHASE-TAGGED SYNTHESIS 283
We developed a solution-phase surrogate for SPS
using an ethoxyethanol functionalized derivative of a
dimethylgermyl linker as a mimetic for the same linker
attached to a commercial polyethyleneglycol (PEG)-
grafted polystyrene (PS) resin (Scheme 3).
using a solution-phase surrogate for SPS, has estab-
lished an efficient protocol for immobilization of the
Grignard reagent derived from bromobenzene and for
its conversion to a mixture of meta- and para-sub-
stituted pinnacolatoboronic esters (Scheme 4).
For this solution phase model, the key carboxylation
step proceeded smoothly using an excess of solid CO2
(78% yield) and traceless cleavage from the linker was
accomplished using 50% trifluoromethansulphonic
acid in dichloromethane. As yet, however, yields for
the carboxylation step when using strictly one equiva-
lent of gaseous CO2 in conjunction with a hypogel1
immobilized precursor have been poor. Further optimi-
zation of this process is ongoing.
Suzuki coupling then allows access to a functiona-
lized styrene derivative that we hope to convert via e.g.
hydroamination and then ipso-protodegermylative tra-
celess cleavage into the requisite 4-amino-4-phenylpi-
peridine en route to the target structure.
Isotopically labelled halo-degermylation ! SPECT/
PET imaging agents
We also wanted to address some of the challenges
presented by the handling and use of [14C]-bromoben-
zene. This compound is among the most commonly
employed labelled starting materials in radiosynthesis
after [14C]-CO2 and can be difficult to contain. If
immobilization is to be the very first step, and the
bromine ‘handle’ of bromobenzene is used to attach the
molecule to the resin then the immobilized labelled
benzene needs to be activated to allow further functio-
nalization. We considered that iridium catalyzed C–H
activation/borylation might provide a powerful method
by which to achieve this as the resulting aryl boronic
ester would constitute a versatile precursor to a wide
number of synthetically diverse functional groups.
Specifically, we decided to explore this concept by
developing a SPS approach to SR142801 (osanetant),
which is a sanofi-aventis neurokinin (NK3) receptor
antagonist. This molecule contains the 4-amino-4-
phenylpiperidine pharmacophore that is common to
several neuroactive pharmaceuticals. Our initial work,
Single photon emission computed tomography (SPECT)
and positron emission tomography (PET) are emerging
medical imaging methods which allow the concentra-
tion and movement of short-lived emitters in living
tissues to be imaged in real time. Consequently, there
is a growing demand for efficient methods for the
preparation of molecules containing appropriate nuclei
{e.g. [123I] (t1/2 13.2 h) for SPECT and [18F] (t1/2
110 min) and [11C] (t1/2 20 min) for PET}. The prepara-
tion of molecules labelled in this manner presents some
unique synthetic challenges, primarily due to their
short half-lives. As a consequence, a synthesis must be
completed from the appropriate precursor and the
product purified to a high level very rapidly (within
2–3 half-lives).
Our objective to develop methods for the introduction
of isotopically labelled halogens into aromatic precur-
sors concomitant with cleavage from a phase tag via
electrophilic ipso-halodegermylation. The analogous
process of electrophilic ipso-halodestannylation of
1) MeLi
2) TMSCl
3) NH4Cl
Me Me
Me Me
Me
Me
Ge
1) BuLi
2) CO2
Ge
R'
F
OH
F
10% MSA
MeOH
Ge
F
OEt
O
F
R
R
SR46439B
5-HT2 antagonist
O
O
C
Me
C
O
C
OH
Br
[91%]
[78%]
involatile
OH
OHC
R
50%TFMSA
PhSMe, CH2Cl2
R' - GeMe2R
R' = H, SR47035
[54%]
[95%]
Scheme 3 Scheme available in colour online at www.interscience.wiley.com
B2pin2
[IrCl(COD)]2
dtbpy
Br
NCbz
Me
Me
Ge
O
1)
Ac
NMe
TfO
Me
Me
Ge
R
OEt
O
C
Cl
TlOH, Pd(PPh3)4
LiBr, THF, RT
N
(1eq)
octane, ∆
Me Me
Ge
Me Me
C
N
Ge
R
R
[62%]
2) Mg(m)
O
B
O
C
[90%]
C
C
Cl
14C]-SR142801
neurokinin antagonist
[84%]
N
R
[
Cbz
Cl
(m/p = ~2:1)
Scheme 4 Scheme available in colour online at www.interscience.wiley.com
Copyright # 2007 John Wiley & Sons, Ltd.
J Label Compd Radiopharm 2007; 50: 281–285
DOI: 10.1002.jlcr

284 A. C. SPIVEY ET AL.
aryltrialkylstannane precursors is used widely for this
purpose.7 However, the tin-containing precursors and
the tin halide by-products are extremely toxic making
precursor synthesis and product purification hazar-
dous. We considered that it would be highly advanta-
geous if the labelling precursor contained non-toxic
germanium (cf. tin) and was bound to the solid phase –
allowing release of pure labelled product into solution.
This would obviate the need for purification thereby
saving valuable time and improving activity (and hence
imaging resolution).
currently being screened to optimize the key cleavage
step.
We were also interested in introducing fluorine into
aromatic compounds in this fashion. 6-[18F]-Fluoro-
(S)-DOPA is a clinically important tracer molecule
which is routinely employed for the diagnosis of
Parkinson’s disease by PET. Using the electrophilic
fluorinating agent Selectfluor1, we found that a sui-
tably protected (S)-DOPA derivative, immobilized via
germanium at the 6-position onto a solution phase
surrogate for SPS, could be cleaved to give a clean
mixture of ipso-fluorodegermylated and ipso-protode-
germylated products (Scheme 6).
Our initial objective was to provide proof-of-concept
for ipso-halodegermylative cleavage of an aryl group
from a solid-supported arylgermane precursor under
conditions appropriate for introduction of isotopically
labelled iodide {e.g. [123I]-NaI}. Thus, treatment of
40methoxybiphenyl immobilized on argogel1 resin3
with dichloramine-T (DCT) as the oxidant in conjunc-
tion with unlabelled NaI afforded the analytically pure
biaryliodide in 45% yield (unoptimized). This concept
was adopted for the preparation of an iodine analogue
of CB1 receptor antagonist SR141716 (rimonabant)
(Scheme 5).
It is expected that optimization of the cleavage
conditions, using [18F]-F2 or [18F]-AcOF should allow
for an improved ratio of fluorinated to protonated
products and that the application of microfluidic and
microwave technology in combination with a flow-
through cell containing the immobilized precursor will
enable much shorter reaction and deprotection times.
Conclusion
Iodo-rimonabant was prepared by a route adapted
from those of Seltzman8 and Lan9 and immobilized
onto hypogel1 resin following conversion to the corre-
sponding Grignard reagent. Oxidative iodination using
DCT when applied to this substrate resulted in some
oxidative degradation of the hydrazide moiety of
rimonabant and so alternative, milder oxidants are
We have provided proof-of-concept for phase-tagged
synthesis of isotopically labelled compounds exploiting
the unique reactivity profile of an aryl germanium bond
between the phase tag and the substrate. The methods
hold promise for the devolatilization of early synthetic
intermediates derived from [14C]-CO2 and [14C]-bromo-
benzene for ADME studies and for the preparation of
NH2.HCl
O
HN
O
O
Me
NH
N
Me
NH
N
O
Me
NH
N
1) LHMDS
Cl
2) (CO2Et)2
Cl
N
Et
1) i-PrMgCl
I
N
N
N
Ar
N
H
N
2) KOH, MeOH [96%]
X
2)
Me
Me
I
3) SOCl2, DMF
Ge
Ge
Me
hypogel®
NaI, CuI
MeNHCH2CH2NHMe
dioxane [98%]
NH2
[91%]
4)
Cl
N
X = Br
X = I
H
Cl
Me
Cl
O
Cl
Cl
Cl
Cl
iodinated SR141716
for SPECT imaging
0.3 mmol/g
Scheme 5 Scheme available in colour online at www.interscience.wiley.com
OMe
2BF4
CH2Cl
OMe
OH
N
N
MeO
NBoc2
MeO
HO
NBoc2
NH3
1) aq.HBr 48%, ∆
2) DOWEX-50
F
N
Selectfluor®
O
CO2
6
OEt
O
GeMe2
O
F
N
F
MeCN, ∆
[14%]
[98%]
6-F-(S)-DOPA
>98% ee
Scheme 6 Scheme available in colour online at www.interscience.wiley.com
Copyright # 2007 John Wiley & Sons, Ltd.
J Label Compd Radiopharm 2007; 50: 281–285
DOI: 10.1002.jlcr

ISOTOPIC LABELLING VIA PHASE-TAGGED SYNTHESIS 285
SPECT/PET tracers containing [123I]-aryliodide and
18F]-arylfluoride moieties.
3. Spivey AC, Diaper CM, Rudge AJ. J Chem Soc Chem
Commun 1999; 835–836.
[
4. Spivey AC, Diaper CM, Rudge AJ. J Org Chem 2000;
65: 5253–5263.
5. Spivey AC, Srikaran R, Diaper CM, Turner DJ. Org
Biomol Chem 2003; 1: 1638–1640.
Acknowledgements
We are grateful to the EPSRC, the BBSRC, the ORS,
sanofi-aventis, GSK and GE Healthcare for funding of
our germanium/isotope-labelling program.
6. Spivey AC, Turner DJ, Turner ML, Yeates S. Synlett
2004; 111–115.
7. Glaser M, Luthra SK, Brady F. Int J Oncol 2003; 22:
253–267.
8. Seltzman HH, Carroll FI, Burgess JP, Wyrick CD,
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Copyright # 2007 John Wiley & Sons, Ltd.
J Label Compd Radiopharm 2007; 50: 281–285
DOI: 10.1002.jlcr