Discovery and structure-activity relationships of novel selective norepinephrine and dual serotonin/norepinephrine reuptake inhibitors

Article abstract of DOI:10.1016/j.bmcl.2004.11.025

Novel arylthiomethyl morpholines are potent selective norepinephrine reuptake inhibitors (NERIs) and dual serotonin/norepinephrine reuptake inhibitors (SRI/NERIs). The target compounds were prepared using a stereochemically versatile synthesis featuring an aldol condensation as the key step. One enantiomer of the 2-methoxy-substituted analogue was found to be a potent and selective norepinephrine reuptake inhibitor, whereas the opposite enantiomer was a potent dual serotonin/norepinephrine reuptake inhibitor.

Full text of DOI:10.1016/j.bmcl.2004.11.025

Bioorganic & Medicinal Chemistry Letters 15 (2005) 699–703
Discovery and structure–activity relationships of novel
selective norepinephrine and dual serotonin/norepinephrine
reuptake inhibitors
John Boot,a Manuel Cases,a Barry P. Clark,a Jeremy Findlay,a Peter T. Gallagher,a
Lorna Hayhurst,a Teresa Man,a Christian Montalbetti,b Richard E. Rathmell,a
a
Helene Rudyk, Magnus W. Walter, Maria Whatton and Virginia Wood
a,
a
a
*
´ `
aEli Lilly & Co. Ltd, Lilly Research Centre, Erl Wood Manor, Windlesham, Surrey GU20 6PH, UK
bEvotec OAI, 151 Milton Park, Abingdon, Oxon OX14 4SD, UK
Received 5 August 2004; revised 18 October 2004; accepted 10 November 2004
Available online 8 December 2004
Abstract—Novel arylthiomethyl morpholines are potent selective norepinephrine reuptake inhibitors (NERIs) and dual serotonin/
norepinephrine reuptake inhibitors (SRI/NERIs). The target compounds were prepared using a stereochemically versatile synthesis
featuring an aldol condensation as the key step. One enantiomer of the 2-methoxy-substituted analogue was found to be a potent
and selective norepinephrine reuptake inhibitor, whereas the opposite enantiomer was a potent dual serotonin/norepinephrine reup-
take inhibitor.
Ó 2004 Elsevier Ltd. All rights reserved.
1. Introduction
O
R
Monoamine reuptake inhibition is an important neuro-
O
O
O
pharmacological strategy for the treatment of major
affective disorders.1 Selective serotonin reuptake inhibi-
tors (SSRIs), such as fluoxetine 1 (ProzacTM), are widely
used in the treatment of depression.
O
S
N
H
N
H
N
H
1: R = 4-CF3 (Fluoxetine)
2: R = 2-Me (Atomoxetine)
3: Duloxetine
4: Reboxetine
Only two selective norepinephrine reuptake inhibitors
(NERIs) are currently in clinical use. Lilly recently
introduced atomoxetine (StratteraTM) 2 for the treatment
of attention-deficit/hyperactivity disorder (ADHD).2
Reboxetine (EdronaxTM) 4, another selective NERI, is
marketed in Europe for the treatment of depression.3
The combination of multiple mechanisms of action in
a single active ingredient is an important concept in psy-
chopharmacology. Combination of serotonin and nor-
epinephrine reuptake inhibition for example in
duloxetine 3 (CymbaltaTM) has been shown to improve
potency and accelerate onset of action of antidepressant
Figure 1. Selective and dual monoamine reuptake inhibitors.
activity.4 Thus, there is continued interest in biogenic
amine reuptake inhibitors with both selective and mixed
activity profiles (Fig. 1).
2. Target design
Atomoxetine 2, fluoxetine 1, and duloxetine 3 contain a
3-aryloxypropylamine scaffold. This privileged struc-
tural motif has the potential for high affinity binding
to biogenic amine transporters. The same motif can also
be found in reboxetine 4 where it is constrained in a
morpholine ring system. The nature of the aromatic
substituent R in fluoxetine and atomoxetine has a
Keywords: Monoamine reuptake inhibitors; Selective norepinephrine
reuptake inhibitor; Dual reuptake inhibitor; SAR; Serotonin;
Norepinephrine.
*
Corresponding author. Tel.: +44 1276 484786; fax: +44 1276
483305; e-mail: magnus.walter@lilly.com
0960-894X/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2004.11.025

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J. Boot et al. / Bioorg. Med. Chem. Lett. 15 (2005) 699–703
Aryloxy Domain
R
OH
Arylthio Domain
OH
Linker
H
H
O
N
Domain
O
N
Ph
O
i
Ph
R
+
O
O
S
O
N
O
R' = H
O
3JHH = 3 Hz
Ph
3JHH = 8 Hz
Ph
Ph
N
H
N
H
6: threo (R,S/S,R)
7: erythro (S,S/R,R)
5
major product
minor product
Arylthiomethyl Morpholines
ii, iii
ii, iii
Figure 2. Arylthiomethyl morpholine targets.
OH
OH
H
O
H
O
significant influence on the activity and selectivity of
compounds as inhibitors of the serotonin transporter
(SERT) and/or the norepinephrine transporter (NET).
Ph
Ph
N
N
Ph
9: erythro (R,S/S,R)
Ph
The objective of this study was to investigate biogenic
amine reuptake inhibition (activity against NET, SERT
and the dopamine transporter, DAT) of compounds
with a sulfur atom in the linker domain (Fig. 2).
8: threo (S,S/R,R)
Scheme 1. (i) LDA (1.5equiv), THF, À78°C, 30min; PhCHO (1equiv)
À78°C, 1h (inverse addition: anion to aldehyde) 56%; (ii) BH3, THF,
60°C, 2h followed by MeOH; (iii) HCl (aq), 60°C, 1h, 68% (major ds
8), 60% (minor ds 9).
3. Chemistry
C-2 of the morpholine ring and the benzylic hydrogen.
The initial assignment of the relative stereochemistry
was later confirmed by X-ray crystallographic analysis.8
Diborane-mediated reduction of the lactam gave mor-
pholine benzyl alcohols 8 and 9 in good yield.
The target design outlined above required the develop-
ment of a flexible and stereoselective synthetic approach
to arylthiomethyl morpholines. Existing routes to
reboxetine 4 and analogues thereof introduce the aryl-
oxy domain early with closure of the morpholine ring
occurring at a late stage in the synthetic scheme.5,6
The stereochemical assignment of aminoalcohol 8 de-
rived from the major aldol product was confirmed by
conversion to 2-ethoxyaryloxy analogue 4 which was
compared to an authentic sample of reboxetine (Scheme
2). Reboxetine is marketed as a mixture of the (S,S) and
(R,R) form. Aminoalcohol 8 was converted to the corre-
sponding benzyl bromide using polymer-supported tri-
phenylphosphine and carbon tetrabromide with
inversion of configuration at the benzylic position. Bro-
mide displacement with 2-ethoxy phenol gave N-pro-
tected arylether 10. Due to the presence of the
hydrogenolytically labile benzyl group a two-step depro-
tection procedure was adopted.9 Treatment of 10 with a-
chloroethyl chloroformate followed by methanolysis of
the intermediate a-chloroethyl carbamate removed the
protecting group. After conversion into the hydrochlo-
ride salt the sample obtained via this route from threo-
alcohol 8 was found to be identical to an authentic sam-
ple of commercial (S,S/R,R) reboxetine 4. Attempts to
introduce the aryloxy group before reduction of the
We envisaged the approach shown in Figure 3 in which
a suitable intermediate, such as an activated arylmethyl
morpholine A, would allow variation of the arylthio do-
main through introduction of a variety of thiophenols at
a late stage in the synthetic scheme. The morpholine ring
would be derived from readily available N-benzyl pro-
tected morpholinone 5.7
Reaction between benzaldehyde and the enolate derived
from readily available N-benzyl-2-morpholinone 5 gave
an acceptable yield of the two diastereomeric benzylal-
cohols 6 and 7 in a ratio of 2.2/1 (Scheme 1).7 The aldol
products could be separated by chromatography on sil-
ica gel. The assignment of relative stereochemistry was
based on the coupling constants between the proton at
R
X
S
O
N
O
+
R
HS
N
H
EtO
A
OH
EtO
O
Ph
H
Arylthiomethyl Morpholines
O
N
X = Leaving Group
O
Ph
H
i, ii
iii, iv
O
N
H
Ph
O
Ph
Ph
O
N
N
H
O
+
Ph
8: threo (S,S/R,R)
10
4: Reboxetine (S,S/R,R)
H
O
Ph
Scheme 2. (i) PS–PPh3, CBr4, CHCl3, reflux, 62%; (ii) 2-EtO–
C6H4OH, KOtBu, t-BuOH, 80°C, 80%; (iii) a-chloroethyl chlorofor-
mate, Et3N, DCM, 40°C; (vi) MeOH, reflux, 43% over two steps.
5
Figure 3. Retrosynthetic analysis of arylthiomethyl morpholines.

J. Boot et al. / Bioorg. Med. Chem. Lett. 15 (2005) 699–703
701
Double Inversion Strategy
OH
R
H
O
Double Inversion
Strategy
OH
Br
S
R
Ph
H
O
H
H
O
N
O
S
Ph
Ph
i
ii,iii
iv
Ph
H
N
O
Ph
(S,S/R,R)
N
N
H
Ph
HCl
(S,S/R,R)
N
H
Ph
Ph
8: threo (S,S/R,R)
8
11
13: R = 2-MeO
Single Inversion
Strategy
Scheme 3. (i) PPh3, CBr4, CHCl3, reflux, 62%; (ii) arylthiol, Cs2CO3,
DMF, 95°C, 74% (R = 2-MeO); (iii) a-chloroethyl chloroformate,
Et3N, DCM, 40°C; (iv) MeOH, reflux, 75% over two steps.
R
OH
S
H
H
O
O
Ph
Ph
N
N
lactam moiety in 6 and 7 led to formation of significant
amounts of eliminated product and were abandoned.
This double inversion strategy was successfully em-
ployed in the synthesis of the target arylthiomethyl mor-
pholines (Scheme 3).
Double Inversion
Strategy
H HCl
Ph
9: erythro (R,S/S,R)
(S,R/R,S)
Figure 4. Stereochemical strategies to (S,S/R,R) arylthiomethyl
morpholines.
To complement this efficient route to the desired threo
(S,S/R,R) products from the major aldol product 6 we
also investigated a single inversion approach (Scheme
4). This would then allow us to use the minor diastereo-
mer 9 in the preparation of the target molecules.
aldol condensation could be used as starting material
for the synthesis of the target molecules.
4. Stereochemistry and SAR
Formation of an oxy-phosphonium intermediate under
Mitsunobu-type conditions employing 1,1-(azodicar-
bonyl)dipiperidine (ADDP)10 followed by displacement
with 2-ethoxyphenol gave the expected (S,S/R,R)
diastereomer mixture of reboxetine after standard
deprotection with a-chloroethyl chloroformate. In an
alternative approach, mesylate 12 could be obtained
under standard conditions, but displacement with 2-eth-
oxyphenol did not lead to the desired product. Displace-
ment of mesylate 12 with arylthiols using caesium
carbonate as base in DMF was successful and gave
access to the desired arylthiomethyl morpholine targets
in similar yields to the double inversion strategy.
All target compounds were tested as S,S/R,R racemates
for reuptake inhibition at the norepinephrine (NET),
serotonin (SERT) and dopamine transporter (DAT).8
Table 1 shows the inhibitory activity of a limited set of
2-substituted-thioaryl morpholines compared with
(R,R/S,S) reboxetine (4).
Replacement of oxygen in reboxetine by sulfur in aryl-
thiomethyl morpholines 13 to 16 maintained potent
levels of norepinephrine reuptake inhibition. Whilst
thioarylmethyl morpholines 13 to 17 did not inhibit
dopamine reuptake, compounds 13 to 16 were potent
serotonin reuptake inhibitors. The methoxy analogue
of reboxetine (corresponding to 14) had been reported
to give poor serotonin reuptake inhibition.5,10 NET
and SERT inhibition of methyl substituted morpholines
15 to 17 appeared to decrease in the order ortho > meta
> para. To investigate the interesting possibility of
dual activity against both the serotonin and norepineph-
rine transporter in single compounds racemic 2-methoxy
analogue 14 was separated into its enantiomers 14a and
14b using chiral HPLC (Table 2). The faster eluting iso-
mer 1 was found to be a potent selective norepinephrine
reuptake inhibitor whereas the slower eluting isomer
showed potent inhibition (NET and SERT Ki 6 20nM)
of both norepinephrine and serotonin reuptake.
Figure 4 gives a summary of our stereochemical strate-
gies to the target arylthiomethyl morpholines. Using
either a double or a single inversion strategy both the
threo product 6 and the erythro product 7 of the key
Single Inversion Strategy
EtO
OH
H
O
Ph
O
i, ii, iii
H
O
N
Ph
4: (S,S/R,R)
Ph
N
H
Reboxetine
iv
9
OMs
H
R
O
N
Ph
v
S
H
5. Summary
O
Ph
Ph
12
We have developed an efficient and highly versatile route
to arylthiomethyl morpholine analogues of reboxetine.
Selective norepinephrine reuptake inhibition and dual
norepinephrine/serotonin reuptake inhibition was found
to reside in different enantiomers of a member of this
series. More detailed SAR studies of both series and
N
H
Scheme 4. (i) ADDP, 2-EtO–C6H4OH, PBu3, THF 40°C, 56%; (ii) a-
chloroethyl chloroformate, Et3N, DCM, 40°C; (iii) MeOH, reflux,
45% over two steps; (iv) MsCl, pyridine, DCM, room temperature; (v)
arylthiol, Cs2CO3, DMF, 95°C.

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J. Boot et al. / Bioorg. Med. Chem. Lett. 15 (2005) 699–703
Table 1. Inhibition of monoamine reuptake for norepinephrine, serotonin and dopamine
R
X
H
O
Ph
(S,S/R,R)
N
H
Compound
X
R
Ki (nM)a
NET
SERT
DAT
13
14
S
S
S
S
S
O
H
17.6 2.4
10.7 2.2
8.3 1.0
36 0.3
1.2 1.1
522.1 48.7
>200 (9.7 3.0%)c
226.9 34.9
>200 (51 0.2%)c
>200 (8 1.1%)c
>200 (2 1.7%)c
2-OMe
2-Me
3-Me
4-Me
2-OEt
15
16
0.2 0.1
3.2 1.2
>100 (19.3 2.3)b
>100 (25 18%)b
108.6 7.4
364.3 19.3
1.9 0.2
17
Reboxetine 4
a Values are means of at least three experiments.
b %-displacement at 100nM.
c %-displacement at 1000nM. For experimental details of assay conditions see Ref. 8. Minimum significant ratio (MSR): NET:2.5; SERT:3.0;
DAT:1.6. NET, SERT and DAT binding data for fluoxetine 1, atomoxetine 2, and duloxetine 3 has recently been published.11
Table 2. Inhibition of monoamine reuptake for norepinephrine, serotonin and dopamine
MeO
S
H
O
Ph
N
H
Compound
Stereochemistry
Ki (nM)a
SERT
NET
DAT
14
(SS/RR)
Isomer 1
Isomer 2
10.7 2.2
1.7 0.4
24.6 2.3
1.2 1.1
66.2 3.0
1.5 0.2
>200 (9.7 3.1%)b
>200 (2.8 1.8%)b
>200 (19.0 3.4%)b
14a
14b
a Values are means of at least three experiments.
b %-displacement at 1000nM.
assignment of absolute stereochemistry will be commu-
nicated in due course.
(1H, d, 8Hz), 4.10 (1H, d, 11Hz), 3.85–4.00 (4H, m),
3.74 (1H, dt, 1Hz, 11Hz), 2.82–3.02 (2H, m), 2.66–
3.02 (3H, m); m/z 316 [M + H]+. Compound 14a was ob-
tained after separation by chiral HPLC on a Chiralcel-
OJ(3624) semi-preparative column using a gradient of
heptane/isopropyl alcohol as eluent (ee >95%, sample
concentration 0.2mg/ml 1.0ml/min, sample solvent:
50% heptane, 50% ethanol, 0.2 dimethylethyl amine),
tR = 9.32min. Compound 14b was obtained after sepa-
ration by chiral HPLC using the same conditions;
tR = 11.45min (ee >95%).
6. Selected experimental data
Compound 6: MW 297.36; C18H19NO3: 1H NMR
(CDCl3): d 7.36–7.41 (2H, m), 7.16–7.31 (6H, m),
6.86–6.91 (2H, m), 5.14 (1H, d, 3Hz), 4.71 (1H, d,
14Hz), 4.48 (1H, d, 3Hz), 4.25 (1H, d, 14Hz), 4.20
(1H, br, s), 3.89 (1H, ddd, 11Hz, 3Hz, 2Hz), 3.67
(1H, dt, 11Hz, 3Hz), 3.16 (1H, dt, 12Hz and 4Hz),
2.86 (1H, br, d, 12Hz); LCMS: m/z 298 [M + H]+.
Acknowledgements
Compound 7: MW 297.36; C18H19NO3: 1H NMR
(CDCl3): d 7.55–7.61 (2H, m), 7.36–7.50 (6H, m),
7.25–7.31 (2H, m), 5.21 (1H, d, 2Hz), 5.09 (1H, d,
8Hz and 2Hz), 4.73 (2H, s), 4.37 (1H, d, 8Hz), 4.01
(1H, ddd, 12Hz, 3Hz, 2Hz), 3.77 (1H, dt, 11Hz,
4Hz), 3.50 (1H, dt, 12Hz, 4Hz), 3.16 (1H, br, d,
12Hz); LCMS: m/z 298 [M + H]+.
We would like to thank the analytical group at Erl
Wood for excellent support, and Dr. Fionna Martin
and Dr. Jeremy Gilmore for helpful discussions.
References and notes
Compound 14: MW 315.44; C18H21NO2S: 1H NMR
(CDCl3): d 7.14–7.34 (7H, m), 6.74–6.84 (2H, m), 4.50
1. Bymaster, F. P.; McNamara, R. K.; Tran, P. V. Exp.
Opin. Inv. Drugs 2003, 12, 531–543.

J. Boot et al. / Bioorg. Med. Chem. Lett. 15 (2005) 699–703
703
2. Bymaster, F. P.; Katner, J. S.; Nelson, D. L.; Hemrick-
Luecke, S. K.; Threlkeld, P. G.; Heiligenstein, J. H.;
Morin, S. M.; Gehlert, D. R.; Perry, K. W. Neuropsycho-
pharmacology 2002, 27, 699–711.
3. Scates, A. C.; Doraiswamy, P. M. Ann. Pharmacotherapy
2000, 34, 1302–1312.
4. Schatzberg, A. F. J. Clin. Psychiat. 2003, 64, 30–37.
5. Melloni, P.; Carniel, G.; Della Torre, A.; Bonsignori, A.;
Buonamici, M.; Pozzi, O.; Ricciardi, S.; Rossi, A. C. Eur.
J. Med. Chem. 1984, 19, 235–242.
8. For details see: Walter, M. W.; Clark, B. P.; Gallagher, P.
T.; Haughton, H. L.; Rudyk, H. C. E. WO
2004017977NET assay: Gobel, J.; Saussy, D. L.; Goetz,
A. J. Pharmacol. Toxicol. Methods 1999, 42, 237–
244; SERT/DAT assay: Ramamoorthy, S.; Giovanetti,
E.; Qian, Y.; Blakely, R. J. Biol. Chem. 1998, 273, 2458.
9. Olofson, R. A.; Abbott, D. E. J. Org. Chem. 1984, 49,
2795–2799.
10. Tsunoda, T.; Yamamiya, Y.; Ito, S. Tetrahedron Lett.
1993, 34, 1639–1642.
6. Melloni, P.; Della Torre, A.; De Munari, S.; Meroni,
M.; Tonani, R. Gazz. Chim. Ital. 1985, 115, 159–
163.
7. Dobrev, A.; Nechev, L.; Ivanov, K. Liebigs Ann. Chem.
1989, 815–818.
11. Boot, J. R.; Brace, G.; Delatour, C. L.; Dezutter, N.;
Fairhurst, J.; Findlay, J.; Gallagher, P. T.; Hoes, I.;
Mahadevan, S.; Mitchell, S. N.; Rathmell, R. E.; Rich-
ards, S. J.; Simmonds, R. G.; Wallace, L.; Whatton, M. A.
Bioorg. Med. Chem. Lett. 2004, 14, 5395–5399.