Synthesis and structure-activity studies of benzyl ester meperidine and normeperidine derivatives as selective serotonin transporter ligands

Article abstract of DOI:10.1016/j.bmc.2010.09.060

A series of benzyl esters of meperidine and normeperidine were synthesized and evaluated for binding affinity at serotonin, dopamine and norepinephrine transporters. The 4-methoxybenzyl ester 8b and 4-nitrobenzyl ester 8c in the meperidine series and 4-me

Full text of DOI:10.1016/j.bmc.2010.09.060

Bioorganic & Medicinal Chemistry 18 (2010) 8356–8364
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry
journal homepage: www.elsevier.com/locate/bmc
Synthesis and structure–activity studies of benzyl ester meperidine
and normeperidine derivatives as selective serotonin transporter ligands
Xiaobo Gu a, Sari Izenwasser b, Dean Wade b, Amy Housman b, Gerard Gulasey b, Jill B. Rhoden a,
Christopher D. Savoie a, David L. Mobley a, Stacey A. Lomenzo a, Mark L. Trudell a,
⇑
a Department of Chemistry, University of New Orleans, New Orleans, LA 70148, USA
b Department of Psychiatry and Behavioral Sciences, University of Miami Miller School of Medicine, Miami, FL 33136, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of benzyl esters of meperidine and normeperidine were synthesized and evaluated for binding
affinity at serotonin, dopamine and norepinephrine transporters. The 4-methoxybenzyl ester 8b and
4-nitrobenzyl ester 8c in the meperidine series and 4-methoxybenzyl ester 14a in the normeperidine
series exhibited low nanomolar binding affinities at the SERT (Ki values <2 nM) and high SERT selectivity
(DAT/SERT >1500 and NET/SERT >1500).
Received 12 August 2010
Revised 21 September 2010
Accepted 23 September 2010
Available online 29 September 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Meperidine
Normeperidine
Serotonin transporter
Dopamine transporters
Norepinephrine transporters
Depression
1. Introduction
F
O
CF3
O
The serotonin transporter has been a therapeutic target for
depression and anxiety for several decades.1–3 Selective serotonin
reuptake inhibitors (SSRIs, e.g., citalopram, fluoxetine and paroxe-
tine) have widely been prescribed for patients suffering from this
common psychiatric disorder (Fig. 1). However, despite the success
of these drugs they suffer from significant side effects and thus
there remains a need for new medications.4 Within the past dec-
ade, the rationale for the development of new pharmacotherapies
has expanded to target molecules that exhibit potent serontonin
reuptake inhibition as well as to affect the function of other mono-
amine transporters or serotonin receptors. This approach has led to
the development of dual serotonin/norepinephrine reuptake inhib-
itors (SNRIs, e.g., venlafaxine and duloxetine).5,6 These dual inhib-
itors have been accepted as more efficacious than SSRIs for the
treatment of depression with reduced side effects.7,8
NC
O
O
O
N
N
N
H
F
H
Citalopram
Fluoxetine
Paroxetine
S
O
OH
NH
OCH3
N
H
More recently, pharmacological evidence suggests that triple
monoamine uptake inhibitors (TUIs), targeting dopamine trans-
porters (DAT) as well as serotonin transporters (SERT) and norepi-
nephrine transporters (NET) may be even more efficacious and
exhibit improved safety profiles as antidepressants.9 The prototyp-
ical TUI, DOV 216,303 was found to be both safe and effective in
Venlafaxine
NH
Duloxetine
O
H
N
N
O
WAY-262398
F
⇑
Corresponding author. Tel.: +1 504 280 7337; fax: +1 504 280 6860.
E-mail addresses: mtrudell@uno.edu, MLTCM@jazz.ucc.uno.edu (M.L. Trudell).
Figure 1. SSRIs, SNRIs TUIs and SSRI/5-HT1A antagonsits.
0968-0896/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmc.2010.09.060

X. Gu et al. / Bioorg. Med. Chem. 18 (2010) 8356–8364
8357
Phase II clinical studies on depression, however, recently has been
dropped from further development due to concerns about the pat-
ent lifetime.10,11 While a balanced profile of inhibition at the three
monoamine transporters has been suggested as desirable for the
development of new antideressant therapeutics, compounds that
inhibit serotonin reuptake as well as block 5-HT1A autoreceptors,12
which inhibit serotonergic neuronal firing in the presence of sero-
tonin (5-HT1A antagonism) have also been the focus of numerous
studies.3,13–17 It is believed that dual SSRI/5-HT1A antagonists
(e.g., WAY-262398)18 may lead to medications with improved
therapeutic utility. Despite these advances in depression/anxiety
therapy, SSRIs remain among the most widely prescribed drugs
for the treatment of depression and anxiety related disorders.
Moreover, the modulation of the serotonin transporter remains
an important focus in the development of new medications target-
ing depression.19
Our efforts to develop novel molecular scaffolds targeting
monoamine transporter systems have led to the development of
several classes of selective DAT ligands and selective SERT li-
gands.20–23 Previously, we have reported on a series of derivatives
of meperidine (1) that exhibited modest potency and selectivity for
the SERT (Fig. 2).21,22 However, unlike the prototypical SSRIs fluox-
etine and paroxetine, the meperidine analogues 2 lack a secondary
amine and two aromatic moieties common to the SERT pharmaco-
phore.24,25 In light of these deficiencies it was of interest to exam-
ine this class of meperidine-based SERT ligands to determine if
SERT potency could be improved and the monoamine transporter
selectivity influenced by the addition of a second strategically
placed aryl ring system. To this end, we envisaged a series of ben-
zyl ester normeperidine derivatives 3 that would be similar to the
SERT pharmacophore. As illustrated in Figure 3, the alignment of
predicted favorable solvated conformers of benzyl normeperidine
3, fluoxetine and paroxetine suggested that appropriately substi-
tuted normeperidine benzyl esters should exhibit high affinity for
the SERT.26 A normeperidine benzyl ester 3 that exhibited potent
SERT affinity could then be evaluated at other monoamine trans-
porters or serotonin receptors to determine the potential focus
for medication development (e.g., SSRI, TUI, duel SSRI/5-HT1A
antagonists). Herein we report the synthesis and monoamine
transporter affinity of novel benzyl ester meperidine and norme-
peridine derivatives.
2. Chemistry
Based upon previous structure–activity studies of meperidine
analogues,21 we focused on four 4-aryl (Ar1) piperidine scaffolds
(Ar1 = 3,4-dichlorophenyl, 4-Ph-Ph, 4-I-Ph and 2-naphthyl). Previ-
ous studies had shown these aryl groups to contribute to the high
potency and selectivity at SERT observed for the meperidine ethyl
ester derivatives 2a–d (Table 1) as well as a variety of piperidine
and tropane derivatives.27–30 As illustrated in Scheme 1, an initial
series of normeperidine derivatives 4 were prepared from the
corresponding ethyl esters 2 using 1-chloroethyl chloroformate
(ACE-Cl). This two-step one-pot process typically afforded the
normeperidine derivatives 4 in 59–73% yield. The benzyl ester
derivatives were prepared from the corresponding nitriles 5
(Scheme 1) that were readily available using synthetic methods
previously reported from our laboratory.21 Hydrolysis of the nitrile
moiety of 5 was achieved in a methanolic solution of sodium
hydroxide (25% wt) at reflux, followed by an acidic work-up (2 N
HCl) to afford the carboxylic acids 6 as the hydrochloride salts in
>90% yield. Without purification, the carboxylic acids 6 were con-
verted into the acid chlorides 7 with thionyl chloride and then
treated with the appropriately substituted benzyl alcohol under
phase-transfer conditions. This furnished the benzyl esters 8–11
in 25–62% yield for the two-step process. The normeperidine ana-
logues 12–15 were then prepared in 70–85% yield using ACE-Cl
procedure described above.
3. Biology
O
Binding affinities for the serotonin, dopamine and norepineph-
rine transporters were determined by the ability of the drug to dis-
place the transporter selective radiolabeled ligands [3H]citalopram,
[3H]WIN 35,428 and [3H]nisoxetine, respectively, from the
monoamine transporters in rat brain tissue. The binding affinities
of all compounds listed in Table 1 initially were determined for the
SERT and DAT. Typically selected compounds that exhibited either
SERT binding affinities with Ki values <10 nM or that exhibited
DAT/SERT selectivity >100 were then evaluated at norepinephrine
transporters to determine a monoamine transporter selectivity
profile. Although meperidine exhibits sub-micromolar affinity for
O
O
Ar1
Ar1
O
Ar2
O
O
N
H
N
N
CH3
CH3
1
2
3
Figure 2. Meperidine derivatives
l-opioid receptors (Ki = 0.92 lM), previous structure–activity stud-
ies with substituted aryl analogues revealed that the 3,4-dichloro-
phenyl, 4-iodophenyl and 2-naphthyl meperidines exhibited
reduced affinity at
-opioid receptor affinity was deemed unnecessary at this time.
l
-opioid receptors.21 Therefore evaluation of
l
4. Results and discussion
Our initial expectation of improved SERT potency by the intro-
duction of a secondary amine group into the meperidine skeleton
was readily validated upon evaluation of the normeperidines 4 at
the SERT. In general, a six-to-ten-fold improvement in SERT affinity
was observed for the normeperidines 4 (e.g., 4c, Ki = 2.5 nM) over
the corresponding meperidine derivatives 2 (e.g., 2c, Ki = 21.1 nM).
Furthermore, the benzyl ester 8a (Ki = 3.9 nM) exhibited signifi-
cantly improved binding affinity for the SERT relative to meperi-
dine (1, Ki = 414 nM) and the corresponding ethyl ester 2a
(Ki = 18.7 nM). The nearly six to ten-fold increase in SERT affinity
Figure 3. Superimposed predicted favorable solvated conformers of
fluoxetine (red) and paroxetine (yellow).
3 (blue),

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X. Gu et al. / Bioorg. Med. Chem. 18 (2010) 8356–8364
Table 1
Monoamine transporter affinity and selectivity
Compounda
Ar1
Ar2
SERT (Ki, nM)b
DAT (Ki, nM)b
NET (Ki, nM)b
DAT/SERT
NET/DAT
NET/SERT
Fluoxetine
Paroxetine
1
2a
2b
2c
2d
4a
4b
4c
4d
8a
8b
8c
8d
8e
8f
8g
8h
9
12a
12bf
12cf
12df
12e
13
2.0 0.1c
0.05 0.0003c
413 44e
18.7 2.6e
47.7 4.1e
21.1 2.4e
7.2 0.10e
21.6 1.6
3.3 1.0
2.5 0.8
14.1 2.1
3.9 0.5
1.7 0.2
1.0 0.1
784 39d
98 1.7d
473 11c
59 0.7c
NT
392
2000
43
0.60
0.59
240
1200
Ph
17,800 2700e
125 15e
3,4-Cl2-Ph
4-Ph
4-I-Ph
2-Naphthyl
3,4-Cl2-Ph
4-Ph-Ph
74,500 5100e
4824 633e
519,000 51,000e
71,000 9700e
NT
6.7
600
0.71
160
62
4000
110
25,000
9900
6801 3060e
3250 200e
1140 380e
321 58
3209 388
3500 321
710 138
2970 297
2792 626
1530 334
5226 152
2563 48
3471 238
5629 508
4913 525
2312 334
919 138
1264 142
1295 263
2892 1539
2244 200
1109 84
2925 715
1777 265
4342 1090
889 463
160
150
160
159
980
1400
51
760
1600
1500
1200
370
260
520
500
47
NT
NT
NT
4-I-Ph
2-Naphthyl
3,4-Cl2-Ph
3,4-Cl2-Ph
3,4-Cl2-Ph
3,4-Cl2-Ph
3,4-Cl2-Ph
3,4-Cl2-Ph
3,4-Cl2-Ph
3,4-Cl2-Ph
4-Ph-Ph
3,4-Cl2-Ph
3,4-Cl2-Ph
3,4-Cl2-Ph
3,4-Cl2-Ph
3,4-Cl2-Ph
4-Ph-Ph
Ph
901 122
300 29
333 54
NT
580 52
NT
NT
2549 77
NT
3.3
0.11
0.22
230
180
330
4-CH3O-Ph
4-NO2-Ph
3,4-Cl2-Ph
4-Br-Ph
4-I-Ph
4-Ph-Ph
2-Naphthyl
Ph
4.3 0.5
7.0 0.9
0.23
0.52
83
13.2 1.8
10.9 2.1
9.9 3.7
49.5 7.4
9.2 3.1
10.2 2.7
6.4 1.9
30.2 4.9
46.7 17
2.0 0.3
0.6 0.2
5.9 2.0
5.9 2.7
2.9 0.2
2.0 0.5
26.5 2.4
260
Ph
NT
100
110
227
96
4-CH3O-Ph
4-NO2-Ph
3,4-Cl2-Ph
4-CF3-Ph
Ph
4-CH3O-Ph
4-NO2-Ph
3,4-Cl2-Ph
4-CH3O-Ph
4-NO2-Ph
3,4-Cl2-Ph
3133 578
1444 415
11,930 1060
7902 869
NT
2731 698
2069 322
6059 1598
2349 127
2.5
1.1
4.1
3.5
307
226
400
170
48
560
4900
300
740
310
370
120
14af
14bf
14cf
15af
15bf
15cf
4-I-Ph
4-I-Ph
4-I-Ph
2-Naphthyl
2-Naphthyl
2-Naphthyl
0.93
1.2
1.4
2.6
1.3
5.2
4600
350
1000
810
470
630
732 132
3212 311
937
7
16,557 4034
a
b
c
d
e
f
All compounds were tested as the hydrochloride salt, unless noted otherwise. NT: Not tested.
All values are the mean SEM of three or four experiments preformed in triplicate.
Value taken from Ref. 31.
Value taken from Ref. 32.
Value taken from Ref. 21 and based upon inhibition of [3H]paroxetine.
Tested as the oxalate salt.
was accompanied by a significant decrease in DAT affinity. Thus
the benzyl ester 8a was not only more potent than the correspond-
ing ethyl ester 2a at the SERT, it was more SERT selective as well.
A series of substituted benzyl ester 4-(3,4-dichlorophenyl)-
meperidine derivatives 8 were evaluated at the SERT and the
DAT, since the 4-(3,4-dichlorophenyl)-meperidine scaffold was
the most readily available of the meperidine derivatives from
previous studies.21 In general, all of the benzyl esters 8a–8h
exhibited high affinity for the SERT (Ki values ꢀ10 nM) and good
DAT/SERT selectivity (DAT/SERT >100). It was noteworthy, that
the 4-methoxybenzyl ester 8b (Ki = 1.7 nM) and the 4-nitrobenzyl
ester 8c (Ki = 1.0 nM) were the most potent compounds at the SERT
of the meperidine series and were equally DAT/SERT selective. The
similarity in potency and selectivity of these two compounds was
consistent with the SERT pharmacophore, which favors a H-bond
acceptor group on the benzyl group (Ar2) of 8.24,25 The NET binding
affinity also was determined for selected derivatives of 8 and was
found to increase relative to the corresponding ethyl esters 2.
However, the NET affinity of the benzyl esters (e.g., 8b Ki = 300 nM)
was significantly lower than that exhibited for the SERT and
resulted in NET/SERT selectivity >100, (e.g., 8b, NET/SERT = 180)
albeit less selective than the corresponding ethyl ester (e.g., 2a,
NET/SERT = 4000). Alternatively, the 4-phenyl meperidine deriva-
tive 9 exhibited potency at the SERT (Ki = 49.5 nM) and the DAT
(Ki = 2312 nM) that was not significantly different than that of
the ethyl ester 2c. It was interesting to note that the 4-phenyl
congenor 9 was the only compound of the benzyl ester series that
did not exhibit significantly improved SERT affinity over the
corresponding ethyl esters. Overall, the results of the normeperi-
dine ethyl esters 4 and the meperidine benzyl esters 8 were very
satisfying and supported our rationale for the ligand design.
The affinity and selectivity of the benzyl esters 8b and 8c was
very similar to fluoxetine and suggested that the meperidine deriv-
atives were binding to the SERT in a similar motif. Since fluoxetine
is a secondary amine it was of interest to explore a potential syn-
ergistic effect of the N–H group with the benzyl ester moiety on
SERT affinity. A series of normeperidine benzyl ester analogues
12–15 were evaluated at monoamine transporters. The normeper-
idine congenors 12 of the 4-(3,4-dichlorophenyl) analogues were
slightly less potent than the corresponding N-methyl derivatives.
This was not surprising since a similar result was observed in the
ethyl ester series where the N-methyl derivative 2b was more po-
tent at the SERT than 4a. However, the 4-phenylnormeperidine
benzyl ester 13 did exhibit a substantial increase in SERT affinity
and a modest increase in DAT affinity. This supported our rationale
for incorporation of both the N–H and benzyl ester moieties into
the meperidine scaffold. Based upon the improved SERT affinity
of 13, we were encouraged to further investigate the structure–
activity of the highly potent and SERT selective 4-(4-iodophenyl)
(14) and 4-(2-naphthyl) (15) normeperidine scaffolds with the
most potent substituted benzyl esters of the meperidine series
(Ar2 = 4-CH3OPh, 4-NO2Ph and 3,4-Cl2Ph). As envisaged the iodo
and naphthyl analogues, 14 and 15, exhibited improved potency
and selectivity for the SERT relative to that of the corresponding
dichloro derivatives. The 4-(4-iodophenyl) normeperidine
4-methoxybenzyl ester 14a was found possess subnanomolar
potency at the SERT and was the most potent ligand of the study
for the SERT (Ki = 0.6 nM). In addition, 14a was also the most selec-

X. Gu et al. / Bioorg. Med. Chem. 18 (2010) 8356–8364
8359
Standard Grade). Petroleum ether refers to pentanes with a boiling
point range of 30–60 °C. NMR spectra were recorded on a Varian-
Gemini400 MHz spectrometer. Chemicalshifts are reported as d val-
ues with tetramethylsilane (TMS), employed as the internal stan-
dard. The compounds were characterized by NMR as the free base
then converted into the corresponding salt. Melting points were
determined for the salts and are uncorrected. Elemental analyses
were obtained from Atlantic Microlabs, Inc., Norcross, GA.
O
O
Ar1
Ar1
O
O
i
N
H
N
CH3
4
2
O
O
Ar1
Ar1
Ar1 CN
6.1. General procedure for the preparation of hydrochloride
salts
Cl
OH
iii
ii
N
•HCl
N
•HCl
N
The base (50–100 mg) was dissolved in a minimum amount of
diethyl ether (1–2 mL) and added to a saturated ethereal solution
(10 mL) of anhydrous hydrogen chloride. The hydrochloride salts
crystallized and were washed with Et2O (3 Â 2 mL) and purified by
trituration with Et2O and ethyl acetate. Fractional moles of water
could not be prevented, despite vigorous drying (110 °C, 1 h) under
vacuum (0.01 mm Hg). All compounds were homogeneous on
thin-layer chromatography (CHCl3/CH3OH/NH4OH, 90:9:1).
CH3
CH3
CH3
6
5
7
O
O
Ar1
Ar1
Ar2
O
Ar2
O
i
iv
N
H
N
CH3
6.2. General procedure for the preparation of oxalate salts
8 Ar1 = 3,4-Cl2-Ph
9 Ar1 = 4-Ph-Ph
10 Ar1 = 4-I-Ph
12 Ar1 = 3,4-Cl2-Ph
13 Ar1 = 4-Ph-Ph
14 Ar1 = 4-I-Ph
The base (50–100 mg) was dissolved in a minimum amount of
anhydrous THF (1–2 mL) and added to a THF solution (10 mL) of
anhydrous oxalic acid (1 equiv). The salts crystallized and were
washed with anhydrous THF (1 Â 2 mL) and then washed with
Et2O (3 Â 2 mL) and purified by trituration with Et2O and ethyl
acetate. Fractional moles of water could not be prevented, despite
vigorous drying (110 °C, 1 h) under vacuum (0.01 mm Hg). All
compounds were homogeneous by thin-layer chromatography
(CHCl3/CH3OH/NH4OH, 90:9:1).
11 Ar1 = 2-naphthyl
15 Ar1 = 2-naphthyl
Ar2 = Ph, 4-BrPh, 4-I-Ph, 4-CH3OPh, 4-NO2Ph,
4-CF3Ph, 4-Ph-Ph, 3,4-Cl2Ph, 2-naphthyl
Scheme 1. General synthetic procedures. Reagents and conditions: (i) ACE-Cl,
NaHCO3, ClCH2CH2Cl, reflux; then CH3OH, reflux, 3 h; (ii) NaOH, CH3OH, reflux;
then 2 N HCl; (iii) SOCl2, reflux; (iv) Ar2CH2OH, Bu4NHSO4, 5% NaOH (aq), CH2Cl2.
6.3. General procedure for the preparation of normeperidine
derivatives 4
tive compound for SERT (DAT/SERT = 4900; NET/SERT = 4600) and
exhibited SERT selectivity that exceeded that of fluoxetine and par-
oxetine (Table 1). The 2-naphthyl derivatives 15a and 15b also
exhibited high affinity and high selectivity for the SERT relative
to the DAT and NET but were greater than 10-fold less selective
for the SERT than 14a.
A solution of 2 (6.1 mmol), sodium bicarbonate (9.1 mmol), and
1-chloro-ethylchloroformate (52 mmol) in 1,2-dichloroethane
(27 mL) was heated to reflux under and atmosphere of nitrogen
for 48 h. The product was filtered to remove the sodium bicarbon-
ate and the solvent was removed under reduced pressure. Metha-
nol (155 mL) was added and the mixture was heated to reflux for
3 h. The solvent was removed under reduced pressure. Chloroform
was added and washed with 1.8 N NaOH (30 mL) and water
(30 mL) then dried (Na2SO4). The crude product was purified by
column chromatography (SiO2, CHCl3/CH3OH, 12:1) to afford the
normeperidine analogues 4 as waxy solids in 58–73% yield.
5. Conclusion
In conclusion, we have synthesized a series of benzyl ester
derivatives of meperidine and normeperidine as SERT selective
compounds based upon the SERT pharmacophore. In general, the
benzyl esters were significantly more potent at monoamine trans-
porters than the corresponding ethyl esters and exhibited low
nanomolar binding affinity for the SERT. The 4-methoxybenzyl es-
ters 8b, 14a and 4-nitrobenzyl ester 8c were the most SERT selec-
tive ligands of the study. Based upon these results, both the
meperidine benzyl ester scaffold and the normeperidine benzyl es-
ter scaffold seem to be well suited for the development of new
compounds that display high potency and selectivity for the SERT.
Studies to further characterize these compounds at serotonin
receptors and in vivo are under way and will be reported in due
course.
6.3.1. Ethyl 4-(3,4-dichlorophenyl)-piperidine-4-carboxylate (4a)
Tan solid (830 mg, 65% yield) 1H NMR (400 MHz, CDCl3) d 7.45
(d, J = 2.0 Hz, 1H), 7.41 (d, J = 8.8 Hz, 1H), 7.21 (dd, J = 2.8,8.8 Hz,
1H), 5.76 (s, 1H), 4.16 (q, J = 7.2 Hz, 2H), 3.25 (d, J = 12.8 Hz, 2H),
2.90 (t, J = 12.0 Hz, 2H), 2.59 (d, J = 13.2 Hz, 2H), 2.02 (m, 2H),
1.20 (t, J = 6.8 Hz, 3H). 13C NMR: (101 MHz, CDCl3) d 172.8, 142.1,
132.8, 131.6, 130.5, 127.9, 125.1, 61.6, 48.4, 42.9(2), 32.7(2), 13.9.
Mp 203–205 °C (HCl salt). Anal. Calcd for C14H17NO2Cl2ÁHCl: C,
49.65; H, 5.36; N, 4.14. Found: C, 49.77; H, 5.42; N, 4.08.
6. Experimental section
6.3.2. Ethyl 4-biphenyl-4-yl-piperidine-4-carboxylate (4b)
Tan solid (1.4 g, 73% yield). 1H NMR (400 MHz, CDCl3) d 7.58–7.31
(m, 9H), 4.17 (q, J = 6.8 Hz, 2H), 3.36 (d, J = 12.8 Hz, 2H), 3.01 (t,
J = 11.2 Hz, 2H), 2.69 (d, J = 14.0 Hz, 2H), 2.25 (m, 2H), 1.20 (t,
J = 6.8 Hz, 3H). 13C NMR (101 MHz, CDCl3) d 173.3, 140.3, 140.2,
140.1, 128.7(2), 127.4(3), 126.9(2), 125.9(2), 61.4, 48.3, 42.5(2),
31.8(2), 13.9. Mp 148–151 °C (HCl salt). Anal. Calcd for C20H23NO2Á
HCIÁH2O: C, 66.01; H, 7.20; N, 3.85. Found: C, 66.35; H, 7.04; N, 3.75.
All chemicals were purchased from Aldrich Chemical Co., Mil-
waukee, WI. Unless otherwise noted. Anhydrous THF, CH2Cl2 and
CH3OH were purchased from Mallinkrodt Baker. Toluene and Et2O
(DrisolvÒ, EMD Chemicals) were purchased from VWR International.
Chromatographyreferstoflashcolumnchromatographyonsilicagel
(Sorbent Technologies Silica Gel 60 Å, 230–400 mesh, 32–63 lm

8360
X. Gu et al. / Bioorg. Med. Chem. 18 (2010) 8356–8364
6.3.3. Ethyl 4-(4-iodophenyl)-piperidine-4-carboxylate (4c)
Yellow solid (430 mg, 40% yield). 1H NMR (400 MHz, CDCl3): d
7.67 (d, J = 8.4 Hz, 2H), 7.11 (d, J = 8.8 Hz, 2H), 4.14 (q, J = 7.2 Hz,
2H), 3.31 (d, J = 13.2 Hz, 2H), 2.88 (t, J = 11.2 Hz, 2H), 2.58 (d,
J = 13.6 Hz, 2H), 1.99 (m, 2H), 1.18 (t, J = 11.6 Hz, 2H). 13C NMR
(101 MHz, CDCl3): d 173.4, 143.5, 142.0, 132.9, 137.7, 127.7, 93.0,
61.4, 48.9, 43.3, 33.2, 14.0. Mp 220–222 °C (HCl salt). Anal. Calcd
for C14H18NO2IÁHCIÁ1/4H2O: C, 42.02; H, 4.91; N, 3.50. Found: C,
42.01; H, 4.09; N, 3.50.
6.5.2. 4-Methoxylbenzyl 4-(3,4-dichlorophenyl)-1-methylpiperi-
dine-4-carboxylate (8b)
White solid (385 mg, 51% yield). 1H NMR (400 MHz, CDCl3) d
7.40 (d, J = 2.3, 1H), 7.34 (dd, J = 8.4, 4.1 Hz, 1H), 7.20 (m, 1H),
7.13 (m, 2H), 6.91–6.77 (m, 2H), 5.04 (s, 2H), 3.80 (s, 3H), 2.74
(d, J = 9.2 Hz, 2H), 2.53 (d, J = 12.6 Hz, 2H), 2.23 (s, 3H), 2.12–2.10
(m, 2H), 1.92 (d, J = 10.8 Hz, 2H). 13C NMR (101 MHz, CDCl3) d
173.6, 159.6, 132.9, 131.5, 130.6, 130.2(2), 128.5(2), 127.8, 125.6,
118.7, 114.1, 67.1, 55.9, 55.5, 53.4, 48.6, 46.3, 33.9(2). Mp
162–165 °C (HCl salt). Anal. Calcd for C21H23Cl2NO3ÁHClÁH2O: C,
54.50; H, 5.66; N, 3.03. Found: C, 52.37; H, 5.60; N, 3.03.
6.3.4. Ethyl 4-naphthalen-2-yl-piperidine-4-carboxylate (4d)
White solid (136 mg, 58% yield). 1H NMR (400 MHz, CDCl3): d
7.98–7.46 (m, 7H), 4.19 (q, J = 7.6 Hz, 2H), 3.49 (d, J = 13.2 Hz,
2H), 3.12 (t, J = 11.2 Hz, 2H), 2.80 (d, J = 14.4 Hz, 2H), 2.45 (m,
2H), 1.16 (t, J = 6.8 Hz, 3H). 13C NMR (101 MHz, CDCl3): d 8 172.8,
137.7, 133.1, 132.4, 128.6, 128.0(2), 127.3, 126.3(2), 124.5, 123.2,
61.6, 48.2 42.0(2), 30.7(2), 13.9. Mp 223–225 °C (HCl salt). Anal.
Calcd for Cl8H21NO2ÁHCl: C, 67.60; H, 6.93; N, 4.38. Found: C,
67.25; H, 6.89; N, 4.45.
6.5.3. 4-Nitrobenzyl 4-(3,4-dichlorophenyl)-1-methylpiperi-
dine-4-carboxylate (8c)
Yellow solid (370 mg, 47% yield) 1H NMR (free base): 1H NMR
(400 MHz, CDCl3) d 8.16 (d, J = 8.8 Hz, 2H), 7.40 (dd, J = 2.4,
8.4 Hz, 2H), 7.29 (d, J = 8.8 Hz, 2H), 7.13 (dd, J = 2.4, 2.4 Hz, 1H),
5.17 (s, 2H), 2.74 (s, 2H), 2.56–2.53 (d, J = 12.4 Hz, 2H), 2.24 (s,
3H), 2.15–2.10 (dt, J = 10.8 Hz, 2H), 2.00–1.98 (d, J = 11.2 Hz, 2H).
13C NMR (101 MHz, CDCl3) d 173.4, 147.9, 142.8, 135.5, 133.1,
131.9(2), 130.8(2), 128.6(2), 125.6, 124.0, 65.7, 53.3(2), 48.6,
46.3, 33.7(2). Mp 187–189 °C (HCl salt). Anal. Calcd for
6.4. General procedure for the preparation of carboxylic acids 6
C
20H20N2O4Cl2ÁHClÁ1/4H2O: C, 51.74; H, 4.67; N, 6.03. Found: C,
A solution of the nitrile (5, 5.2 mmol) and 25% (wt) NaOH
(34 mL) in methanol (100 mL) was stirred at reflux overnight, then
cooled down to room temperature. The mixture was reduced to
half the volume under reduced pressure and extracted with Et2O
(3 Â 30 mL). The aqueous layer was cooled to 0 °C, then acidified
to pH 2 with 1 M HCl solution and then extracted with ethyl ace-
tate (3 Â 75 mL). A white suspension was formed and the white so-
lid was filtered by vacuum filtration. The solid was recrystallized
from H2O/MeOH (4:1, v/v) to afford the carboxylic acid hydrochlo-
ride salts 6 as white crystals. The salts were then carried on to the
next step without characterization or further purification.
51.74; H, 4.75; N, 5.83.
6.5.4. 3,4-Dichlorobenzyl 4-(3,4-dichlorophenyl)-1-methyl-
piperidine-4-carboxylate (8d)
Tan solid (438 mg, 50% yield) 1H NMR (free base): 1H NMR
(400 MHz, CDCl3) d 7.40 (dd, J = 2.4, 8.4 Hz, 2H), 7.36 (s, 1H), 7.18
(dd, J = 2.0, 2.1 Hz, 2H), 7.00 (dd, J = 2.0, 2.0 Hz, 1H), 5.04 (s, 2H),
2.83 (s, 2H), 2.57 (d, J = 12.8 Hz, 2H), 2.30 (s, 3H), 2.19 (dt,
J = 8.0 Hz, 2H), 2.05 (d, J = 10.4 Hz, 2H). 13C NMR (101 MHz, CDCl3)
d 173.4, 141.3, 135.9, 133.1, 132.7, 132.6, 131.8, 130.8, 130.0,
128.5(2), 127.3, 125.6, 65.5, 53.3(2), 48.6, 46.3, 33.7(2). Mp
182–183 °C (HCl salt). Anal. Calcd for C20H19NO2Cl4ÁHCl: C, 49.67;
H, 4.17; N, 2.90. Found: C, 49.44; H, 4.27; N, 2.84.
6.5. General procedure for the preparation of benzyl esters 8a–i,
9, 10a–c and 11a–c
6.5.5. 4-Bromobenzyl 4-(3,4-dichlorophenyl)-1-methylpiperi-
dine-4-carboxylate (8e)
Thionyl chloride (20 mL) was transferred to a 100 mL round-
bottomflask fitted with a condenserand chargedwith the carboxylic
acid 6 (1.85 mmol). The mixture was heated to reflux overnight with
stirring under N2. Excess thionyl chloride was removed by distilla-
tion to afford the crude acid chloride 7. A solution of alcohol
(1.85 mmol) and Bu4NHSO4 (0.36 mmol) in CH2Cl2 (20 mL) was
transferred to the residue. The mixtures were cooled to À5 °C then
5% NaOH (3 mL) was added. Stirring was continued at À5 °C for 1 h
and then the mixture was allowed to warm room temperature. The
reaction mixture was monitored by TLC every 30 min until the start-
ing material was consumed. The organic layer was separated,
washed with water, dried (Na2SO4), filtered and concentrated under
reduced pressure. Purification by column chromatography (MeOH/
CHCl3, 2:98) gave the benzyl esters 8–11 as waxy solids or oils in
25–62% yield.
White solid (411 mg, 50% yield). 1H NMR (400 MHz, CDCl3) d
7.45–7.41 (m, 3H), 7.37 (d, J = 8.0 Hz, 1H), 7.18–7.15 (m, 2H),
7.07 (d, J = 8.0 Hz, 1H), 5.04 (s, 2H), 2.76 (d, J = 8.4 Hz, 2H), 2.53
(d, J = 12.0 Hz, 2H), 2.24 (s, 3H), 2.13 (t, J = 12.0 Hz, 2H), 1.94 (t,
J = 10.0 Hz, 2H). 13C NMR (101 MHz, CDCl3) d 173.6, 138.6, 134.9,
131.8(2), 131.5, 130.6, 130.2, 129.5, 128.5(2), 127.8, 122.6, 67.1,
55.5, 53.4, 48.5, 46.5, 32.1(2). Mp 140–142 °C (HCl salt). Anal. Calcd
for C20H20BrCl2NO2ÁHClÁ1/2H2O: C, 47.78; H, 4.41; N, 2.79. Found:
C, 47.70; H, 4.47; N, 2.73.
6.5.6. 4-Iodobenzyl 4-(3,4-dichlorophenyl)-1-methylpiperidine-
4-carboxylate (8f)
White solid (466 mg, 50% yield). 1H NMR (400 MHz, CDCl3) d
7.64 (m, 2H), 7.38 (m, 2H), 7.18 (m, 1H), 6.93 (d, J = 8.0 Hz, 2H),
5.03 (s, 2H), 2.75 (d, J = 8.8, 2H), 2.53 (d, J = 12.0 Hz, 2H), 2.24 (s,
3H), 2.11 (t, J = 10.4 Hz, 2H), 1.94 (t, J = 10.8 Hz, 2H). 13C NMR
(101 MHz, CDCl3) d 172.6, 138.6, 137.6(2), 135.0, 131.5, 130.6,
130.2, 130.1, 129.5(2), 127.8, 93.8, 67.1, 55.5, 53.4, 48.5, 46.5,
32.1(2). Mp 150–151 °C (HCl salt). Anal. Calcd for C20H20Cl2I-
NO2ÁHClÁH2O: C, 43.00; H, 4.15; N, 2.51. Found: C, 43.01; H, 4.02;
N, 2.39.
6.5.1. Benzyl 4-(3,4-dichlorophenyl)-1-methylpiperidine-4-
carboxylate (8a)
White solid (356 mg, 51% yield). 1H NMR (400 MHz, CDCl3) d
7.43 (d, J = 2.4 Hz, 1H), 7.36 (d, J = 8.4 Hz, 1H), 7.30 (m, 3H), 7.19
(m, 2H), 7.18 (d, J = 8.4, 2.4 Hz, 1H), 5.10 (s, 2H), 2.76 (d,
J = 8.8 Hz, 2H), 2.58 (d, J = 12.4 Hz, 2H), 2.24 (s, 3H), 2.12 (m, 2H),
1.92 (m, 2H). 13C NMR (101 MHz, CDCl3) d 173.3, 135.4, 132.7,
131.3, 130.4, 129.5, 128.5, 128.3(2), 127.6(2), 127.1, 125.4, 67.0,
53.1(2), 48.4, 46.1, 33.6(2). Mp 186–188 °C (HCl salt). Anal. Calcd
for C20H21Cl2NO2ÁHClÁ1/2H2O: C, 56.68; H, 5.47; N, 3.31. Found:
C, 56.54; H, 5.53; N, 3.29.
6.5.7. Biphenylmethyl 4-(3,4-dichlorophenyl)-1-methylpiperi-
dine-4-carboxylate (8g)
White solid (428 mg, 51% yield). 1H NMR (free base):
d
7.59– 7.53 (m, 4H), 7.44 (dd, J = 6.8, 2.0 Hz, 3H), 7.37 (d,
J = 8.8 Hz, 2H), 7.26 (d, J = 8.0 Hz, 2H), 7.20 (dd, J = 2.4, 6.4 Hz,

X. Gu et al. / Bioorg. Med. Chem. 18 (2010) 8356–8364
8361
1H), 5.15 (s, 2H), 2.77 (d, J = 8.0 Hz, 2H), 2.57 (d, J = 12.8 Hz, 2H),
2.25 (s, 3H), 2.15 (t, J = 10.8 Hz, 2H), 1.96 (t, J = 10.8 Hz, 2H). 13C
NMR (101 MHz, CDCl3) d 173.5, 140.5, 139.7, 138.4, 135.0, 132.1,
130.9, 130.7, 129.4(3), 128.2(2), 127.9(2), 127.7(2), 127.6, 127.5,
67.1, 53.2, 53.0, 47.0, 43.6, 32.7, 32.4. Mp 169–170 °C (HCl salt).
Anal. Calcd for C26H25Cl2NO2ÁHClÁH2O: C, 61.32; H, 5.55; N, 2.75.
Found: C, 61.33; H, 5.48; N, 2.84.
7.11–7.08 (m, 3H), 6.96 (dd, J = 4.8, 1.6 Hz, 1H), 5.03 (s, 2H), 2.74
(br s, 2H), 2.55 (d, J = 12.9 Hz, 2H), 2.25 (s, 3H), 2.09 (m, 2H), 2.0
(m, 2H). 13C NMR (101 MHz, CDCl3)
d 173.9, 155.4, 142.3,
138.0(2), 136.1, 130.7, 129.7, 128.0(2), 127.1(2), 93.4, 65.4,
53.4(2), 49.5, 44.4, 35.5(2). Anal. Calcd for C20H20Cl2INO2: C,
47.64; H, 4.00; N, 4.06. Found: C, 47.30; H, 4.29; N, 3.87.
6.5.14. 4-Methoxylbenzyl 4-(naphthalen-2-yl)-1-methylpiperi-
dine-4-carboxylate (11a)
6.5.8. 2-Naphthylmethyl-4-(3,4-dichlorophenyl)-1-methyl-
piperidine-4-carboxylate (8h)
Colorless oil (209 mg, 29% yield). 1H NMR (400 MHz, CDCl3) d
7.80–7.74 (m, 4H), 7.50–7.45 (m, 3H), 7.12 (d, J = 8.0 Hz, 2H),
6.77 (d, J = 8.0 Hz, 2H), 5.06 (s, 2H), 3.75 (s, 3H), 2.80 (br s, 2H),
2.71 (d, J = 12.0 Hz, 2H), 2.26 (s, 3H), 2.18–2.12 (m, 4H). 13C NMR
(101 MHz, CDCl3) d 174.2, 159.8, 133.5, 132.6, 130.2(2), 128.5(2),
128.4, 128.0, 127.6, 126.4, 126.3, 125.1, 124.1, 114.0(2), 66.9,
55.5, 53.3(2), 48.9, 45.8, 33.4(2). Anal. Calcd for C25H27NO3: C,
77.09; H, 6.99; N, 3.60. Found: C, 76.89; H, 7.03; N, 3.44.
White solid (403 mg, 51% yield). 1H NMR (free base):
d
7.84–7.74 (m, 2H), 7.40–7.36 (m, 2H), 7.60 (s, 1H), 7.51–7.45 (m,
2H), 7.34 (d, J = 8.0 Hz,1H) 7.27 (s, 1H) 7.18 (m,1H), 5.27 (s, 2H),
2.75 (d, J = 5.6 Hz, 2H), 2.57 (d, J = 12.0 Hz, 2H), 2.23 (s, 3H), 2.14
(t, J = 10.8 Hz, 2H), 1.95 (t, J = 10.8 Hz, 2H). Mp 200–204 °C (HCl
salt). Anal. Calcd for C24H23Cl2NO2ÁHClÁ1.5H2O: C, 58.61; H, 5.53;
N, 2.85. Found: C, 58.37; H, 5.50; N, 2.83.
6.5.9. 4-Trifluoromethyl-4-(3,4-dichlorophenyl)-1-methyl-
piperidine-4-carboxylate (8i)
6.5.15. 4-Nitrobenzyl 4-(naphthalen-2-yl)-1-methylpiperidine-
4-carboxylate (11b)
Colorless oil (400 mg, 48% yield). 1H NMR (free base): (400 MHz,
CDCl3) d 7.57 (d, J = 8.0 Hz, 2H), 7.39 (m, 2H), 7.29 (d, J = 8.4 Hz,
2H), 7.18 (dd, J = 2.0 Hz 1H), 5.15 (s, 2H), 2.80 (s, 2H), 2.56 (d,
J = 12.8 Hz, 2H), 2.28 (s, 3H), 2.17 (t, J = 10.8 Hz 2H), 2.01 (t,
J = 10.8 Hz, 2H). 13C NMR (101 MHz, CDCl3) d 173.4, 139.5, 135.4,
133.1, 130.8, 128.5, 128.3(2), 125.8(2), 125.7(2), 125.6, 122.8,
66.3, 53.2(2), 48.5, 46.2, 33.6(2). Anal. Calcd for C21H20Cl2F3NO2:
C, 56.52; H, 4.52; N, 3.14. Found: C, 56.33; H, 4.71; N, 3.08.
Yellow oil (276 mg, 37% yield). 1H NMR (400 MHz, CDCl3) d 7.97
(d, J = 8.8 Hz, 2H), 7.83–7.75 (m, 4H), 7.50–7.46 (m, 3H), 7.18 (d,
J = 8.6 Hz, 2H), 5.16 (m, 2H), 2.82 (br s, 2H), 2.73 (m, 2H), 2.27 (s,
3H), 2.26–2.17 (m, 4H). 13C NMR (101 MHz, CDCl3) d 174.2,
147.7, 143.2, 133.5, 132.6, 128.6(2), 128.2(2), 127.7(2), 126.6(2),
125.2, 124.1, 123.8(2), 65.3, 53.6(2), 49.2, 46.5, 33.9(2). Anal. Calcd
for C24H24N2O4: C, 71.27; H, 5.98; N, 6.93. Found: C, 71.00; H, 6.01;
N, 6.87.
6.5.10. Benzyl 4-(4-phenyl-phenyl)-1-methylpiperidine-4-
carboxylate (9)
6.5.16. 3,4-Dichlorobenzyl 4-(naphthalen-2-yl)-1-methyl-
piperidine-4-carboxylate (11c)
Yellow solid (256 mg, 36%). 1H NMR (400 MHz, CDCl3) d 7.55
(m, 4H), 7.42 (m, 4H), 7.33–7.17 (m, 6H), 5.13 (s, 2H), 2.83 (br s,
2H), 2.65 (d, J = 12.8 Hz, 2H), 2.28 (s, 3H), 2.19 (t, J = 10.8 Hz 2H),
2.09 (t, J = 10.8 Hz, 2H). 13C NMR (101 MHz, CDCl3) d 174.1,
140.5, 139.8, 135.8, 128.7(2), 128.3(2), 128.0(2), 127.8(2), 127.2,
127.1, 126.9(2), 126.3(2), 66.5, 53.3(2), 48.4, 46.1, 33.6(2). Mp
201–202 °C (HCl salt). Anal. Calcd for C26H27NO2ÁHClÁH2O: C,
70.98; H, 6.87; N, 3.18. Found: C, 70.71; H, 7.00; N, 3.01.
Yellow oil (316 mg, 40% yield). 1H NMR (400 MHz, CDCl3) d
7.83–7.76 (m, 4H), 7.55–7.43 (m, 2H), 7.29–7.19 (m, 2H), 7.14 (d,
J = 1.9 Hz, 1H), 7.00–6.86 (m, 1H), 5.03 (s, 2H), 2.89 (br s, 2H),
2.72 (m, 2H), 2.32 (s, 3H), 2.18 (m, 4H). 13C NMR (101 MHz, CDCl3)
d 172.0, 140.2, 135.1, 133.5, 133.0, 132.7, 131.1(2), 130.9(2),
130.0(2), 128.0(2), 127.5(2), 125.2, 66.2, 56.0(2), 48.9, 42.1, 30.4,
29.7. Anal. Calcd for C24H23Cl2NO2: C, 67.29; H, 5.41; N, 3.27.
Found: C, 67.19; H, 5.43; N, 3.10.
6.5.11. 4-Methoxylbenzyl 4-(4-iodophenyl)-1-methylpiperidine-
4-carboxylate (10a)
6.6. General procedure for the preparation of normeperidine
analogues 12a–e, 13, 14a–c and 15
Colorless oil (215 mg, 25% yield). 1H NMR (400 MHz, CDCl3) d
7.60 (d, J = 8.8 Hz, 2H), 7.13–7.06 (m, 4H), 6.82 (d, J = 8.8 Hz, 2H),
5.08 (m 2H), 3.80 (s, 3H), 2.73 (br s, 2H), 2.54 (d, J = 12.8 Hz, 2H),
2.20 (s, 3H), 2.12 (m, 2H), 1.94 (m, 2H). 13C NMR (101 MHz, CDCl3)
d 173.2, 159.9, 138.0, 137.8(2), 130.1(2), 128.2, 127.9, 127.5,
114.0(2), 93.0, 66.9, 55.5, 53.4(2), 48.7, 46.3, 33.7(2). Anal. Calcd
for C21H24INO3: C, 54.20; H, 5.20; N, 3.01. Found: C, 54.00; H,
5.30; N, 2.92.
A solution of the meperidine benzyl ester (0.60 mmol), sodium
bicarbonate (9.1 mmol) and 1-chloro-ethyl chloroformate
(5.0 mmol) in 1,2-dichloroethane (20 mL) was heated to reflux un-
der an atmosphere of nitrogen for 48 h. The mixture was filtered to
remove any solids and the solvent was removed under reduced
pressure. Methanol (50 mL) was added and the mixture was
heated to reflux for 3 h. The solvent was removed under reduced
pressure. Chloroform was added, washed with 1.8 N NaOH
(30 mL) and water (30 mL), and then dried (Na2SO4). The crude
product was purified by column chromatography (SiO2, CHCl3/
CH3OH, 12:1) to afford the normeperidine analogues 12–15 as
waxy solids.
6.5.12. 4-Nitrobenzyl 4-(4-iodophenyl)-1-methylpiperidine-4-
carboxylate (10b)
Yellow oil (300 mg, 34% yield). 1H NMR (400 MHz, CDCl3) d 8.2
(d, J = 8.8 Hz, 2H), 7.66 (d, J = 8.4 Hz, 2H), 7.33 (d, J = 8.8 Hz, 2H),
7.17 (d, J = 8.8 Hz, 2H), 5.16 (s, 2H), 2.75 (s, 2H), 2.58 (m, 2H),
2.30 (s, 3H), 2.19–2.06 (m, 2H), 2.01 (m, 2H). 13C NMR (101 MHz,
CDCl3) d 173.8, 155.4, 143.1, 138.0(2), 134.8, 128.3(2), 123.9(2),
123.3(2), 93.2, 65.4, 53.4(2), 48.8, 46.4, 36.5(2). Anal. Calcd for
6.6.1. Benzyl 4-(3,4-dichlorophenyl) piperidine-4-carboxylate
(12a)
White solid (125 mg, 57% yield). 1H NMR (400 MHz, CDCl3) d
7.42 (d, J = 2.4 Hz, 1H), 7.36 (d, J = 8.0 Hz, 1H), 7.32–7.29 (m, 3H),
7.20–7.17 (m, 3H), 5.11 (s, 2H), 3.07 (d, J = 12.0 Hz, 2H), 2.77 (t,
J = 10.8 Hz, 2H), 2.55 (d, J = 12.8 Hz, 2H), 2.40 (br s, 1H), 1.87 (m,
2H). 13C NMR (101 MHz, CDCl3) d 173.6, 143.2, 135.7, 133.0,
131.6, 130.7(2), 128.6, 128.8, 128.6, 128.4, 128.3, 125.6, 67.3,
49.5, 44.0(2), 34.7(2). Mp 177–179 °C (HCl salt). Anal. Calcd for
C
20H21IN2O4: C, 50.01; H, 4.41; N, 5.83. Found: C, 49.88; H, 4.51;
N, 5.72.
6.5.13. 3,4-Dichlorobenzyl 4-(4-iodophenyl)-1-methylpiperi-
dine-4-carboxylate (10c)
Yellow oil (447 mg, 48% yield). 1H NMR (free base): 1H NMR
(400 MHz, CDCl3) d 7.66 (d, J = 8.8 Hz, 2H), 7.35 (d, J = 8.0 Hz, 1H),

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X. Gu et al. / Bioorg. Med. Chem. 18 (2010) 8356–8364
C19H19Cl2NO2ÁHCl: C, 56.95; H, 5.03; N, 3.50. Found: C, 56.60; H,
2H), 6.82 (d, J = 8.7 Hz, 2H), 5.03 (s, 2H), 3.80 (s, 3H), 3.21 (m,
2H), 2.80 (m, 2H), 2.58 (m, 3H), 2.07 (m, 2H). 13C NMR (101 MHz,
CDCl3) d 173.3, 159.9, 138.0(2), 137.7, 130.2(2), 128.0, 127.9,
127.5, 114.2(2), 93.4, 67.3, 55.5, 49.8, 44.1(2), 33.7(2). Mp 205–
206 °C (oxalate salt). Anal. Calcd for C20H22INO3ÁC2H2O4: C, 48.81;
H, 4.47; N, 2.59. Found: C, 49.50; H, 4.71; N, 2.52.
4.95; N, 3.33.
6.6.2. 4-Methoxylbenzyl 4-(3,4-dichlorophenyl) piperidine-4-
carboxylate (12b)
White solid (167 mg, 71% yield). 1H NMR (400 MHz, CDCl3) d
7.38–7.29 (m, 2H), 7.16–7.13 (m, 3H), 6.83 (d, J = 8.5 Hz, 2H),
5.05 (s, 2H), 3.80 (s, 3H), 3.06 (m, 2H), 2.77 (t, J = 11.2 Hz, 2H),
2.72 (br s, 1H), 2.50 (d, J = 12.0 Hz, 2H), 1.81 (m, 2H). 13C NMR
(101 MHz, CDCl3) d 173.6, 159.9, 143.2, 132.9, 131.5, 130.6,
130.2, 128.4(2), 127.7, 125.5(2), 114.0, 67.1, 55.5, 49.4, 44.0(2),
34.6(2). Mp 182–183 °C (oxalate salt). Anal. Calcd for
6.6.8. 4-Nitrobenzyl 4-(4-iodophenyl) piperidine-4-carboxylate
(14b)
Yellow solid (201 mg, 72% yield). 1H NMR (400 MHz, CDCl3) d
8.14 (d, J = 8.6 Hz, 2H), 7.66 (t, J = 8.0 Hz, 2H), 7.26 (d, J = 8.0 Hz,
2H), 7.11 (d, J = 8.6 Hz, 2H), 5.17 (s, 2H), 3.06 (d, J = 12.5 Hz, 2H),
2.77 (t, J = 11.6 Hz, 2H), 2.54 (d, J = 12.6 Hz, 2H), 1.91–1.84 (m,
3H). 13C NMR (101 MHz, CDCl3) d 173.9, 155.3, 143.1, 138.0(2),
133.7, 128.4(2), 128.1(2), 123.9(2), 93.2, 65.4, 49.7, 44.1(2),
34.6(2). Mp 207–209 °C (oxalate salt). Anal. Calcd for C19H19I-
N2O4ÁC2H2O4: C, 45.34; H, 3.80; N, 5.04. Found: C, 46.44; H, 4.15;
N, 4.69.
C
20H21Cl2NO3ÁC2H2O4Á0.1H2O: C, 54.36; H, 4.81; N, 2.88. Found:
C, 54.70; H, 4.91; N, 2.87.
6.6.3. 4-Nitrobenzyl 4-(3,4-dichlorophenyl) piperidine-4-
carboxylate (12c)
Yellow solid (169 mg, 69% yield). 1H NMR (400 MHz, CDCl3) d
8.15 (d, J = 8.5 Hz, 2H), 7.45–7.36 (m, 3H), 7.32 (m, 2H), 7.19 (d,
J = 8.0 Hz, 1H), 5.19 (s, 2H), 3.07 (d, J = 12.7 Hz, 2H), 2.77 (t,
J = 11.5 Hz, 2H), 2.54 (d, J = 13.1 Hz, 2H), 2.39 (br s, 1H), 1.87 (m,
2H). 13C NMR (101 MHz, CDCl3) d 173.4, 147.9, 142.8(2), 133.1,
131.8, 130.8(2), 128.6, 128.4, 125.5, 123.9(2), 65.7, 49.5, 43.9(2),
34.6(2). Mp 175–178 °C (oxalate salt). Anal. Calcd for
6.6.9. 3,4-Dichlorobenzyl 4-(4-iodophenyl) piperidine-4-
carboxylate (14c)
White solid (206 mg, 70% yield). 1H NMR (400 MHz, CDCl3) d 7.65
(d, J = 8.5 Hz, 2H), 7.35 (d, J = 8.2 Hz, 1H), 7.13 (d, J = 1.8 Hz, 1H), 7.09
(d, J = 8.5 Hz, 2H), 6.96 (dd, J = 8.2, 1.8 Hz, 1H), 5.02 (s, 2H), 3.07 (m,
2H), 2.79 (m, 2H), 2.53 (m, 3H), 1.87 (m, 2H). 13C NMR (101 MHz,
CDCl3) d 173.8, 142.3, 138.0(2), 136.1, 132.9, 132.5, 131.1, 130.7,
129.8, 128.1, 127.2, 93.4, 65.3, 49.6, 44.0(2), 30.6(2). Mp 198–
200 °C (oxalate salt). Anal. Calcd for C19H18Cl2INO2ÁC2H2O4: C,
43.47; H, 3.47; N, 2.41. Found: C, 43.92; H, 3.59; N, 2.39.
C
19H18Cl2N2O4ÁC2H2O4: C, 50.52; H, 4.04; N, 5.61. Found: C,
50.60; H, 4.15; N, 5.33.
6.6.4. 3,4-Dichlorobenzyl 4-(3,4-dichlorophenyl) piperidine-4-
carboxylate (12d)
Tan solid (182 mg, 70% yield). 1H NMR (400 MHz, CDCl3) d 7.40
(d, J = 8.3, 2H), 7.32 (d, J = 1.9 Hz, 1H), 7.17–7.06 (m, 2H), 6.98 (dd,
J = 8.2, 1.6 Hz, 1H), 5.06 (s, 2H), 3.44 (d, J = 12.8 Hz, 2H), 3.04 (t,
J = 11.7 Hz, 2H), 2.68 (d, J = 14.4 Hz, 2H), 2.42–2.21 (m, 2H). 13C
NMR (101 MHz, CDCl3) d 171.5, 159.9, 139.9, 143.2, 132.9(2),
131.4, 130.7, 130.2, 128.0(2), 127.5, 125.1, 67.1, 49.0, 44.0(2),
34.6(2). Mp 142–143 °C (oxalate salt). Anal. Calcd for
6.6.10. 4-Methoxylbenzyl 4-(naphthalen-2-yl) piperidine-4-
carboxylate (15a)
White solid (162 mg, 72% yield). 1H NMR (400 MHz, CDCl3) d
7.88–7.66 (m, 4H), 7.54–7.40 (m, 3H), 7.09 (d, J = 8.8 Hz, 2H),
6.76 (d, J = 8.8 Hz, 2H), 5.05 (s, 2H), 3.75 (s, 3H), 3.15 (m, 3H),
2.84 (m, 2H), 2.67 (m, 2H), 2.03 (t, J = 10.4 Hz, 2H). 13C NMR
(101 MHz, CDCl3) d 174.5, 159.8, 140.3, 133.5, 132.6, 130.1,
128.5, 128.4(2), 128.1, 127.6, 126.4, 126.2, 124.9, 124.2, 114.0(2),
66.8, 55.4, 49.9, 44.2(2), 34.8(2). Mp 197–198 °C (oxalate salt).
Anal. Calcd for C24H25NO3ÁC2H2O4Á1/3H2O: C, 66.32; H, 5.91; N,
2.97. Found: C, 66.34; H, 5.95; N, 2.93.
C
19H17Cl4NO2ÁC2H2O4Á1/2H2O: C, 47.39; H, 3.79; N, 2.63. Found:
C, 47.60; H, 3.83; N, 2.74.
6.6.5. 4-Trifluoromethylbenzyl 4-(3,4-dichlorophenyl) piperi-
dine-4-carboxylate (12e)
White solid (188 mg, 72% yield). 1H NMR (400 MHz, CDCl3) d
7.56 (d, J = 8.0 Hz, 2H), 7.42 (d, J = 2.4 Hz, 1H), 7.27 (d, J = 8.4 Hz,
1H), 7.28 (d, J = 8.0 Hz, 2H), 7.18 (dd, J = 8.4, 2.4 Hz, 1H), 5.14 (s,
2H), 3.06 (m, 2H), 2.78 (m, 2H), 2.53 (d, J = 13.2 Hz, 2H), 2.23 (br
s, 1H), 1.85 (m, 2H). 13C NMR (101 MHz, CDCl3) d 173.5, 142.9,
139.6, 133.0, 131.7, 130.9, 128.4, 128.3, 125.8(2), 125.7(2), 125.5,
122.8, 66.2, 49.5, 44.1(2), 34.7(2). Mp 112–113 °C (HCl salt). Anal.
Calcd for C20H18Cl2F3NO2ÁHClÁ1/3H2O: C, 50.60; H, 4.18; N, 2.95.
Found: C, 50.65; H, 4.27; N, 2.92.
6.6.11. 4-Nitrobenzyl 4-(naphthalen-2-yl) piperidine-4-
carboxylate (15b)
White solid (170 mg, 73% yield). 1H NMR (400 MHz, CDCl3) d
7.97 (d, J = 8.0 Hz, 2H), 7.85–7.77 (m, 4H), 7.48 (m, 2H), 7.17 (d,
J = 8.4 Hz, 2H), 5.16 (s, 2H), 3.14 (m, 2H), 2.86 (m, 2H), 2.80 (m
2H), 2.67 (br s, 1H), 2.06 (t, J = 10.9 Hz, 2H). 13C NMR (101 MHz,
CDCl3) d 172.7, 147.9, 142.3, 133.4, 132.8, 129.2, 128.6, 128.4,
128.3(2), 127.7, 127.1(2), 127.0, 125.2, 123.9, 123.2, 65.9, 48.6,
42.0(2), 30.5(2). Mp 182–185 °C (oxalate salt). Anal. Calcd for
6.6.6. Benzyl 4-(4-phenyl-phenyl) piperidine-4-carboxylate (13)
Tan solid (100 mg, 45%). 1H NMR (400 MHz, CDCl3) d 7.55 (m,
4H), 7.42 (m, 4H), 7.33 (t, J = 7.5 Hz, 1H), 7.25 (m, 3H), 7.17 (m,
2H), 5.12 (s, 2H), 3.08 (dt, J = 12.9, 3.3 Hz, 2H), 2.80 (m, 2H), 2.69
(s, 1H), 2.62 (d, J = 13.2 Hz, 2H), 1.94 (m, 2H). 13C NMR (101 MHz,
CDCl3) d 174.1, 141.7, 140.5, 139.9, 135.8, 128.7(2), 128.4(2),
128.0(2), 127.8(2), 127.3, 127.2, 126.9(2), 126.2(2), 66.6, 49.4,
43.9(2), 34.5(2). Mp 201–202 °C (HCl salt). Anal. Calcd for
C
23H22N2O4ÁC2H2O4Á1/2H2O: C, 61.34; H, 5.15; N, 5.72. Found: C,
61.85; H, 5.04; N, 5.68.
6.6.12. 3,4-Dichlorobenzyl 4-(naphthalen-2-yl) piperidine-4-
carboxylate (15c)
White solid (177 mg, 71% yield). 1H NMR (400 MHz, CDCl3) d
7.83–7.75 (m, 4H), 7.54–7.47 (m, 3H), 7.24 (d, J = 8.0 Hz, 1H),
7.20 (d, J = 2.0 Hz,1H), 7.89 (dd, J = 8.0, 2.0 Hz, 1H), 5.02 (s, 2H),
3.12 (m, 2H), 2.93 (s, 1H), 2.87 (t J = 11.2 Hz, 2H), 2.68 (d,
J = 11.2 Hz, 2H), 2.05 (m, 2H). 13C NMR (101 MHz, CDCl3) d 174.2,
139.7, 136.2, 133.5, 132.8, 132.7, 132.4, 130.6, 129.8, 128.7,
128.4, 127.7, 127.2, 126.6, 126.5, 125.1, 123.9, 65.2, 49.9, 44.1(2),
34.6(2). Mp 207–208 °C (oxalate salt). Anal. Calcd for
C
25H25NO2ÁHClÁ1/4H2O: C, 72.81; H, 6.48; N, 3.40. Found: C,
72.77; H, 6.43; N, 3.48.
6.6.7. 4-Methoxylbenzyl 4-(4-iodophenyl) piperidine-4-
carboxylate (14a)
White solid (198 mg, 73% yield). 1H NMR (400 MHz, CDCl3) d
7.62 (d, J = 8.6 Hz, 2H), 7.11 (d, J = 8.6 Hz, 2H), 7.03 (d, J = 8.6 Hz,
C
23H21Cl2NO2ÁC2H2O4: C, 59.53; H, 4.60; N, 2.78. Found: C, 59.53;
H, 5.02; N, 2.60.

X. Gu et al. / Bioorg. Med. Chem. 18 (2010) 8356–8364
8363
6.7. [3H]Citalopram binding assay
fuging (50,000g for 10 min at 4 °C). The resulting pellet was then
washed and centrifuged two more times. The final pellet was
resuspended to a concentration of 80 mg/ml (original wet weight).
Assays were conducted in the above Tris buffer with volume to-
taled to 0.5 mL and a tissue concentration of 8 mg/tube. [3H]Nisox-
etine (specific activity 80 Ci/mmol; final concn 0.5 nM, Perkin
Elmer, MA) was added and the incubation continued for 1 h on
ice. Incubations were terminated by rapid filtration through What-
man GF/B filters, presoaked in 0.05% polyethylenimine (PEI). Non-
Brains from male Sprague–Dawley rats weighing 200–225 g
(Taconic Labs) were removed, midbrain dissected and rapidly fro-
zen. Membranes were prepared by homogenizing tissues in 25 vol-
umes (w/v) of 50 mM Tris containing 120 mM NaCl and 5 mM KCl,
(pH 7.4 at 25 °C), using a Brinkman Polytron (setting 6 for 20 s) and
centrifuged at 20,000g for 10 min at 4 °C. The resulting pellet was
resuspended in buffer, recentrifuged and resuspended in buffer to
a concentration of 7.5 mg/mL. Ligand binding experiments were
conducted in assay tubes containing 0.5 mL buffer for 60 min at
room temperature. Each tube contained 1.4 nM [3H]citalopram
(Perkin Elmer, MA) and 1.5 mg midbrain tissue (original wet
specific binding was defined using 1
assays, an initial screen was conducted to assess displacement of
nisoxetine at a concentration of 1 and 10 M of the unknown com-
lM desipramine. For these
l
pound. If there was greater than 50% displacement of nisoxetine, a
Ki value was determined in subsequent studies. Data were ana-
lyzed with GraphPad Prism software (San Diego, California).
weight). Nonspecific binding was determined using 10 lM fluoxe-
tine. Incubations were terminated by rapid filtration through
Whatman GF/B filters, presoaked in 0.3% polyethylenimine, using
a Brandel R48 filtering manifold (Brandel Instruments Gaithers-
burg, Maryland). The filters were washed twice with 5 ml cold buf-
fer and transferred to scintillation vials. Beckman Ready Safe
(3.0 mL) was added and the vials were counted the next day using
a Beckman liquid scintillation counter (Beckman Coulter Instru-
ments, Fullerton, California). Data were analyzed by using Graph-
Pad Prism software (San Diego, California).
Acknowledgments
This research was supported by the National Institute on Drug
Abuse (DA11528, MLT) and the Louisiana Optical Network Initia-
tive Institute (DLM), supported by the Louisiana Board of Regents
Post-Katrina Support Fund Initiative grant LEQSF (2007-12)-ENH-
PKSFI-PRS-01.
6.8. [3H]WIN 35,428 binding assay
References and notes
Male Sprague–Dawley rats (200–250 g, Taconic, Germantown,
NY) were decapitated and their brains removed to an ice-cooled
dish for dissection of the caudate-putamen. The tissue was homog-
enized in 30 volumes ice-cold modified Krebs-HEPES buffer
(15 mM HEPES, 127 mM NaCl, 5 mM KCl, 1.2 mM MgSO4, 2.5 mM
CaCl2, 1.3 mM NaH2PO4, 10 mM glucose, pH adjusted to 7.4) using
a Teflon/glass homogenizer and centrifuged at 20,000g for 10 min
at 4 °C. The resulting pellet was then washed two more times by
resuspension in ice-cold buffer and centrifugation at 20,000g for
10 min at 4 °C. Fresh homogenates were used in all experiments.
Binding assays were conducted in modified Krebs-HEPES buffer
on ice. The total volume in each tube was 0.5 mL and the final con-
centration of membrane after all additions was approximately 0.3%
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Whatman GF/B glass fiber filter paper (presoaked in 0.1% BSA in
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6.9. [3H]Nisoxetine binding assay
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8364
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