New iodoreboxetine analogues for SPECT imaging of the noradrenaline transporter

Article abstract of DOI:10.1016/j.bmcl.2008.08.041

A new route for the stereoselective synthesis of iodinated reboxetine analogues has been developed for the generation of SPECT imaging agents for the noradrenaline transporter (NAT). (2S,3S)- and (2R,3R)-iodoreboxetine were prepared and biological testing against various mono-amine transporters showed these compounds to be potent and selective for NAT.

Full text of DOI:10.1016/j.bmcl.2008.08.041

Bioorganic & Medicinal Chemistry Letters 18 (2008) 4940–4943
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
New iodoreboxetine analogues for SPECT imaging
of the noradrenaline transporter
Nicola K. Jobson a, Andrew R. Crawford b, Deborah Dewar b, Sally L. Pimlott c, Andrew Sutherland a,
*
a WestChem, Department of Chemistry, The Joseph Black Building, University of Glasgow, Glasgow G12 8QQ, UK
b Division of Clinical Neuroscience, Faculty of Medicine, University of Glasgow G12 8QQ, UK
c West of Scotland Radionuclide Dispensary, University of Glasgow and North Glasgow University Hospital NHS Trust, Glasgow G11 6NT, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
A new route for the stereoselective synthesis of iodinated reboxetine analogues has been developed for
the generation of SPECT imaging agents for the noradrenaline transporter (NAT). (2S,3S)- and (2R,3R)-
iodoreboxetine were prepared and biological testing against various mono-amine transporters showed
these compounds to be potent and selective for NAT.
Received 25 July 2008
Revised 12 August 2008
Accepted 14 August 2008
Available online 19 August 2008
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Reboxetine
Stereoselective synthesis
SPECT
Imaging
Noradrenaline
Transporter
The noradrenaline transporter (NAT), which is located at the
pre-synaptic terminal of noradrenergic neurons, plays a key role
in noradrenergic neurotransmission by regulating the concentra-
tion of noradrenaline in the synaptic cleft via a re-uptake mecha-
nism.1,2 Alterations in synaptic levels of noradrenaline have been
implicated in a number of neuropsychiatric and neurodegenerative
disorders such as depression, attention-deficit/hyperactivity disor-
der, anxiety and Alzheimer’s disease. In addition, NAT is a major
target for drugs targeted at a range of psychiatric conditions.3–6
Non-invasive imaging techniques such as single photon emis-
sion computed tomography (SPECT) or positron emission tomogra-
phy (PET) could provide useful tools for determining the status of
NAT in patients with neuropsychiatric or neurodegenerative disor-
ders and the assessment of drug occupancy of the transporter
in vivo. Most of the studies involved in achieving such a goal have
focused on the development of radioiodinated analogues of
reboxetine 1, the well-known noradrenaline re-uptake inhibitor,7
for in vivo SPECT imaging.8–10 As most of the general NAT studies
of reboxetine analogues have focused on the (2S,3S)-1 and the
(2R,3R)-stereoisomers,8,9,11 we became interested in investigating
iodoanalogues of the (2S,3R)- and the (2R,3S)-stereoisomers as pos-
sible SPECT imaging agents for NAT. This led to the development of
a stereoselective synthesis of (2S,3R)-iodoreboxetine 2 and (2R,3S)-
iodoreboxetine 3 and in vitro testing of these compounds with
homogenised rat brain identified 3 as the most potent for NAT with
a Ki value of 58.2 nM.10
O
O
OEt
N
H
1
I
I
O
O
O
O
OEt
OEt
N
H
N
H
3
2
In an effort to gain further insight into how the stereochemistry of
reboxetine affects binding affinity with NAT and to produce a more
potent iodoanalogue for SPECT imaging, the preparation of the two
other stereoisomers of 2 and 3, (2S,3S)-iodoreboxetine 4 and
(2R,3R)-iodoreboxetine 5 was proposed. In this Letter, we now re-
port the stereoselective synthesis of iodoanalogues 4 and 5 and
the screening of these compounds for affinity with NAT, the seroto-
nin transporter (SERT) and the dopamine transporter (DAT).
* Corresponding author. Tel.: +44 141 330 5936; fax: +44 141 330 4888.
E-mail address: andrews@chem.gla.ac.uk (A. Sutherland).
0960-894X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2008.08.041

N. K. Jobson et al. / Bioorg. Med. Chem. Lett. 18 (2008) 4940–4943
4941
OH
OH
I
I
a
NH
NH2
OH
OH
I
O
I
Cl
13
6
O
O
O
O
b
OEt
OEt
N
H
N
H
I
I
5
4
The first stage of this study required the development of a stereose-
lective synthesis of (2S,3S)-iodoreboxetine 4. The first key interme-
diate in this route, amino alcohol 6 was prepared in six steps as
shown in Scheme 1. 4-Iodobenzyl alcohol 7 was used as the starting
material and converted in a one-pot Swern oxidation/Horner–
Wadsworth–Emmons reaction under Masumune–Roush conditions
c
O
O
HO
HO
N
H
O
N
to E-a
,b-unsaturated ester 8 in 99% yield.12 Reduction of 8 using
Boc
14
15
DIBAL-H gave allylic alcohol 9 in 99% yield. We planned to create
the stereogenic centres of 6 using a Sharpless asymmetric dihydr-
oxylation.13 Sharpless and co-workers have shown that allylic chlo-
rides are excellent substrates for the asymmetric dihydroxylation
reaction.14 Thus, allylic alcohol 9 was converted to the correspond-
ing allylic chloride 10 using triphenylphosphine and N-chlorosuc-
cinimide and this was subjected to an asymmetric
OEt
F
d
Cr(CO)3
16
I
I
dihydroxylation using AD-mix-
a which gave diol 11 in 84% yield
and in an excellent 98% e.e.15
Treatment of 11 with sodium hydroxide led to efficient forma-
tion of epoxide 12 and this underwent a regioselective ring-open-
ing reaction with aqueous ammonia solution to give the desired
amino alcohol 6 in 90% yield.
The next stage of the synthesis of (2S,3S)-iodoreboxetine 4 re-
quired the formation of the morpholine ring and a similar ap-
proach as previously described for our synthesis of 2 and 3 was
utilised (Scheme 2).10 Thus, reaction of amino alcohol 6 with chlo-
roacetyl chloride gave amide 13 and ring closure was then effected
e
O
N
O
O
O
OEt
OEt
N
H
Boc
4
17
Scheme 2. Reagents and conditions: (a) chloroacetyl chloride, NEt3, MeCN, À10 °C
to RT, 57%; (b) sodium tert-butoxide, t-BuOH, 40 °C, 65%; (c) i—BH3.Me2S, THF, 0 °C
to RT; ii—Boc2O, Et3N, DMAP (cat.), CH2Cl2, 40% over two steps; (d) NaH, DMF then
I2, 63%; (e) TFA, CH2Cl2, 60%.
CO2Et
a
OH
using sodium tert-butoxide which gave morpholinone 14 in 51%
yield over the two steps.16 Reduction of 14 was then carried out
using borane-dimethylsulfide and the resulting amine was Boc-
protected to give 15 in 40% yield over the two steps.
I
I
8
7
b
Our initial strategy for introducing the 2-ethoxyphenyl group of
4 involved bromination of the benzyl alcohol component of 15, fol-
lowed by nucleophilic displacement with 2-ethoxyphenol. While
bromination of 15 using carbon tetrabromide and polymer sup-
ported triphenylphosphine proceeded smoothly, all attempts at
displacement of the bromide using 2-ethoxyphenol gave only com-
plex mixtures of products.11b Instead, a nucleophilic aromatic sub-
stitution reaction previously described by Tamagnan and co-
workers involving chromium tricarbonyl activated 2-ethoxyfluor-
ophenol 16 was utilised and this gave 17 in 63% yield.17 Finally, re-
moval of the Boc-protecting group under standard conditions gave
(2S,3S)-iodoreboxetine 4 in 60% yield.
This approach was also used for the synthesis of (2R,3R)-iodore-
boxetine 5 (Scheme 3). In this case, allylic chloride 10 was sub-
jected to an asymmetric dihydroxylation using AD-mix-b, which
gave the corresponding diol 18 in 62% yield and in an excellent
98% e.e. Diol 18 was then converted to (2R,3R)-iodoreboxetine 5
in 8-steps as described for compound 4.
c
Cl
OH
I
I
10
9
d
OH
OH
e
Cl
O
OH
I
I
12
11
f
OH
NH2
OH
I
6
The binding of compounds 4 and 5 at noradrenaline, serotonin
and dopamine transporters was evaluated in vitro using homoge-
nates of rat brain (Table 1).18 (2S,3S)-Analogues of reboxetine gen-
erally have high affinity for NAT and can be up to 10- to 100-fold
more selective than the corresponding (2R,3R)-stereoisomer.8,11d
Scheme 1. Reagents and conditions: (a) i—(COCl)2, DMSO, NEt3, CH2Cl2, À78 °C; ii—
triethyl phosphonoacetate, LiCl, DBU, MeCN, 99% over two steps; (b) DIBAL-H (2.2
equiv), Et2O, À78 °C to RT, 99%; (c) N-chlorosuccinimide, PPh3, CH2Cl2, 0 °C, 69%; (d)
AD-Mix-
a, MeSO2NH2, NaHCO3, t-BuOH, H2O, 0 °C, 84%; (e) NaOH, THF, 0 °C, 98%;
(f) 25% NH3 (aq), MeCN, 90%.

4942
N. K. Jobson et al. / Bioorg. Med. Chem. Lett. 18 (2008) 4940–4943
OH
As expected, (2S,3S)-iodoreboxetine 4 has high affinity for NAT
with a Ki value of 53.8 nM. Surprisingly, (2R,3R)-iodoreboxetine 5
has also significant potency for NAT with a similar Ki value of
64.0 nM. Our previous work with stereoisomers, 2 and 3 and our
result for (2S,3S)-analogue 4 shows that the 3S-stereogenic centre
is required for high affinity with NAT.10 The fact that (2R,3R)-ste-
reoisomer also binds to NAT with high affinity, strongly suggests
that this compound must bind in a different manner to the other
stereoisomers. The addition of a large iodine atom to the phenyl
ring of 5 compared to the parent (2R,3R)-reboxetine compound,
means that the two aromatic rings are now more similar in size
and thus, this may allow these moieties to alternate binding pock-
ets, producing a similar 3-D shape at the C3-position as with ster-
eoisomers 3 and 4, resulting in similar levels of potency.
Cl
a
Cl
I
OH
I
10
18
8 Steps
I
O
O
OEt
Compounds 4 and 5 are significantly more selective for NAT
than the other mono-amine transporters. In particular, (2S,3S)-
iodoreboxetine 4 is approximately 50-fold and 25-fold more
selective for NAT than SERT and DAT respectively and thus, future
analogues of 4 may have both the potency and selectivity required
for developing an effective imaging agent for NAT.
N
H
5
Scheme 3. Reagents and conditions: (a) AD-Mix-b, MeSO2NH2, NaHCO3, t-BuOH,
H2O, 0 °C, 62%.
In conclusion, we have developed a new stereoselective route
for the synthesis of (2S,3S)-iodoreboxetine 4 and (2R,3R)-iodore-
boxetine 5 and have shown these compounds to have high affinity
and selectivity for NAT. The preparation and testing of all four ster-
eoisomers of this iodoreboxetine analogue has generated valuable
insight into how the different stereoisomers of reboxetine bind to
NAT and our work has revealed that the (2R,3S)- and (2S,3S)-ster-
eoisomers in particular, have the highest affinity for NAT. Work is
currently underway to further confirm this hypothesis and to gen-
erate more potent and selective analogues around a (2R,3S)- and
(2S,3S)-morpholine backbone for the development of an effective
SPECT imaging agent for NAT.
Table 1
Binding affinity of reboxetine analogues 4 and 5 with NAT, SERT and DAT (NAT data
for 2 and 3 included for comparison)10
Compound
NAT Ki (nM)a
SERT Ki (nM)b
DAT Ki (nM)b
I
320.8 9.0
—
—
O
O
OEt
N
H
Acknowledgements
210
Financial support from EPSRC (DTA award to N.K.J.), BBSRC
(Industrial Studentship with GSK to A.R.C.) and Medical Research
Scotland is gratefully acknowledged.
I
58.2 9.4
53.8 2.7
64.0 2.4
—
—
Supplementary data
O
O
Experimental procedures and spectroscopic data for all com-
pounds synthesised as well as details for competition binding as-
says. Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.bmcl.2008.08.041.
OEt
N
H
310
I
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O
O
OEt
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5
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b

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