Synthesis of the major metabolites of Paroxetine

Article abstract of DOI:10.1016/S0045-2068(03)00040-3

Paroxetine is a well-known antidepressant, used worldwide in therapeutics. In comparison with other selective serotonin reuptake inhibitors, it exhibits the highest activity in serotonin reuptake inhibition. Paroxetine metabolism initially involves its demethylenation to the catechol intermediate, which is then O-methylated at positions C3 or C4. Herein, the chemistry resulting in the syntheses of these metabolites (3S,4R)-4-(4-fluorophenyl)-3-(hydroxymethyl)piperidine and (3S,4R)-4-(4-fluorophenyl)-3-(4-hydroxy-3-methoxyphenoxymethyl)piperidine is described starting from the common intermediate (3S,4R)-4-(4-fluorophenyl)-3-hydroxymethyl-1-methylpiperidine. Additionally, the common intermediate was used to synthesize paroxetine, which had the same structure and stereochemistry as commercial paroxetine, thereby confirming our synthetic route.

Full text of DOI:10.1016/S0045-2068(03)00040-3

BIOORGANIC
CHEMISTRY
Bioorganic Chemistry 31 (2003) 248–258
www.elsevier.com/locate/bioorg
Synthesis of the major metabolites of Paroxetine
Mireia Segura,a,b Lidia Roura,c,1 Rafael de la Torre,a,b
c,
*
ꢀ
and Jesus Joglar
a
Pharmacology Research Unit, IMIM, Barcelona 08003, Spain
b
Universitat Pompeu Fabra, CEXS, Barcelona 08003, Spain
c
Research Unit on Bioactive Molecules (RUBAM), Department of Biological Organic Chemistry,
IIQAB-CSIC, Barcelona 08034, Spain
Received 11 December 2002
Abstract
Paroxetine is a well-known antidepressant, used worldwide in therapeutics. In comparison
with other selective serotonin reuptake inhibitors, it exhibits the highest activity in serotonin
reuptake inhibition. Paroxetine metabolism initially involves its demethylenation to the cate-
chol intermediate, which is then O-methylated at positions C3 or C4. Herein, the chemistry
resulting in the syntheses of these metabolites (3S,4R)-4-(4-fluorophenyl)-3-(hydroxym-
ethyl)piperidine and (3S,4R)-4-(4-fluorophenyl)-3-(4-hydroxy-3-methoxyphenoxymethyl)
piperidine is described starting from the common intermediate (3S,4R)-4-(4-fluorophenyl)-3-
hydroxymethyl-1-methylpiperidine. Additionally, the common intermediate was used to
synthesize paroxetine, which had the same structure and stereochemistry as commercial
paroxetine, thereby confirming our synthetic route.
Ó 2003 Elsevier Science (USA). All rights reserved.
1. Introduction
Paroxetine [(3S,4R)-4-(4-fluorophenyl)-3-(3,4-methylenedioxyphenoxymethyl)-
piperidine] is a well-known selective serotonin reuptake inhibitor (SSRI) antidepres-
sant, used worldwide in therapeutics. In comparison with other SSRIs, it exhibits the
* Corresponding author. Fax: +34-93-204-5904.
E-mail address: joglar@iiqab.csic.es (J. Joglar).
1
~
Present address: Sociedad Espanola de Desarrollos Quımicos, S.A. Marie Curie 33, Barbera del
ꢀ
ꢀ
ꢁ
Valles. Barcelona.
0045-2068/03/$ - see front matter Ó 2003 Elsevier Science (USA). All rights reserved.
doi:10.1016/S0045-2068(03)00040-3

M. Segura et al. / Bioorganic Chemistry 31 (2003) 248–258
249
highest activity in serotonin reuptake inhibition [1]. Paroxetine (1) metabolism (Fig.
1) initially involves its demethylenation to the catechol intermediate (2), followed by
O-methylation at position
3
or 4. The catechol intermediate can be
also metabolized to afford (3S,4R)-4-(4-fluorophenyl)-3-(hydroxymethyl)piperidine
(metabolite III). These metabolites undergo rapid and extensive conjugation to form
glucuronides and sulfate esters [2–5].
Pharmacokinetic data for paroxetine metabolites in humans are limited. Non-lin-
ear pharmacokinetics of paroxetine after repeated doses have been reported [2,3,5,6].
Mechanisms resulting in this non-linearity are not clear. The methylenedioxyphenyl
moiety present in the paroxetine chemical structure, strongly suggests that a carbene
formed in the O-demethylenation reaction may interact with cytochrome P450
(CYP2D6) and lead to the formation of an intermediate enzyme metabolite complex.
This resulting complex may be responsible for a quasi-irreversible inhibition of the
enzymatic activity and thereby explain the non-linear kinetic behavior. Similar mech-
anisms of enzyme metabolite complex formation have been proposed for other com-
pounds bearing the methylendioxyphenyl grouping such as pesticide synergists [7,8].
To verify this hypothesis, it is necessary to determine paroxetine and its metabolites
in biological fluids from patients and healthy volunteers, who had been administered
the drug.
Fig. 1. Metabolic pathway of paroxetine.

250
M. Segura et al. / Bioorganic Chemistry 31 (2003) 248–258
We are particularly interested in the two main metabolites that appear after the O-
demethylenation of the 3,4-methylenedioxybenzene ring, namely the catechol (2) and
its methoxy derivative (3) (Fig. 1). Such compounds have been described in very pre-
liminary drug development reports, but to our knowledge the synthesis and pharma-
cokinetics in humans have not been described in the general literature. In order to
conduct detailed pharmokinetics, it is necessary to synthesize 2 and 3 because these
two metabolites are not commercially available. In the present manuscript, the syn-
thetic chemistry leading to these standards is described starting from the common
intermediate (3S,4R)-4-(4-fluorophenyl)-3-hydroxymethyl-1-methylpiperidine (4).
The synthetic route was validated by the synthesis of paroxetine, whose stucture
and stereochemistry were identical to that of commercially available paroxetine.
2. Material and methods
2.1. General methods
(3S,4R)-4-(4-fluorophenyl)-3-hydroxymethyl-1-methylpiperidine (4) was a gift of
a pharmaceutical company. All other chemicals and reagents were purchased from
ꢀ
Sigma-Aldrich Quımica, S.A. (Alcobendas, Spain). Solvents were obtained from
Panreac Quımica, S.A. (Barcelona, Spain). Reactions sensitive to moisture were car-
ꢀ
ried out under argon atmosphere. Commercial grade reagents were used directly
without further purification (unless otherwise indicated). Solvents were dried by
standard methods and distilled before use. Purification of products by column chro-
matography was performed on Merck silica gel 60. TLC was carried out on pre-
coated silica gel Merck 60 F254 (0.25 mm) sheets. FT-IR spectra were recorded on
1
a Michelson Bomem MB-120 and are reported in cmꢀ1. H and 13C NMR spectra
were obtained in CDCl3 solutions (unless otherwise indicated) on a Varian Gemini
1
XL200 or a Varian Unity 300 spectrometer, operating at 200 and 300 MHz for H
and 50 and 75 MHz for 13C, respectively. Chemical shifts are reported in delta (d)
units, parts per million (ppm) downfield from ðCH3Þ Si, or in ppm relative to the sin-
4
1
glet at 7.26 ppm of CDC3 for H, in ppm relative to the central line of a triplet at
77.0 ppm of CDCl3 for 13C, and in ppm relative to the singlet at 0 ppm of the Cl3CF
for 19F. 13C NMR multiplicities were determined by DEPT (Distortionless Enhance-
ment by Polarization Transfer) experiments using standard pulse sequences. A
Perkin–Elmer polarimeter was used for optical rotatory measures. Gas chromatog-
raphy-mass spectrometry (GC-MS) was performed on a Fisons gas chromatograph
(8000 series) coupled to a Fisons MD-800 quadrupole mass selective detector. The
system was equipped with a non-polar DB-5 (J&W) capillary column (25 m Â
0.25 lm  0.22 mm I.D.) using helium as carrier gas at a flow rate of 1 mL/min.
The oven was heated starting at 80 °C at a rate of 5 °C/min up to 200 °C, then it
was maintained for 1 min and then heated again at 10 °C/min up to 280 °C. Samples
were injected in the splitless mode although the split valve was closed for 48 s. Injec-
tor and interface were set at 250 °C. The mass spectrometer was operated by electron
ionization (EI, 70 eV) and in the scan mode (working range 40–600 amu).

M. Segura et al. / Bioorganic Chemistry 31 (2003) 248–258
251
2.2. Synthesis of paroxetine metabolites 2 and 3
2.2.1. p-Toluenesulfonate derivative of 4 [9]
In a 25 mL round bottom flask were placed 2 g (8.9 mmol) of (3S,4R)-4-(4-fluor-
ophenyl)-3-hydroxymethyl-1-methylpiperidine (4) and 855 mg (8.9 mmol) of trimeth-
ylamine hydrochloride in 30 mL of anhydrous methylene chloride under an argon
atmosphere. The flask was placed in an ice bath and 2.5 g (13.5 mmol) of p-toluene-
sulfonyl chloride were added and the solution was stirred for 4 h. Then the mixture
was adjusted to pH 7.0 with 1 N HCl aqueous solution, and water (100 mL) was
added to the mixture, which was extracted with ethyl acetate (3 Â 50 mL). The com-
bined organic layer was washed with brine (3 Â 25 mL), dried over anhydrous
MgSO4 and filtered. The solvent was removed under vacuum and the crude product
obtained was purified by silica-gel column chromatography using ethyl acetate/meth-
anol/ammonia (20:2:1) as the eluent yielding 2.98 g (7.90 mmol, 89%) of compound 5
as the free amine which was converted to its hydrochloride salt with an ethanolic so-
lution of hydrogen chloride. m.p.: 121–122 °C. IR (neat, mmax): 2939, 2897, 2791,
1
1598, 1509, 1362, 840, 820, 670. H-NMR (200 MHz): 7.63–7.57 (m, 2H), 7.28–
7.20 (m, 2H), 7.05–6.78 (m, 4H), 3.71 (dd, J ¼ 2:5 and 10.0 Hz, 1H), 3.54 (dd,
J ¼ 6:0 and 10.0 Hz, 1H), 3.02–2.81 (m, 2H), 2.42 (s, 3H), 2.29 (s, 3H), 2.15–1.69
(m, 6H). 13C-NMR (75 MHz): 163.5, 160.3, 145.5, 135.4, 131.6, 130.0, 129.1,
128.9, 127.8, 115.9, 115.7, 68.3, 56.2, 54.7, 43.8, 40.2, 38.6, 30.1, 21.6. 19F-NMR
(282 MHz): )116.7 to )116.6 (m, 1F). GC-MS (m=z): 58, 91,192, 206, 222, 377. Anal.
Calcd. for C20H24FNO3S: C: 63.64; H: 6.41; F: 5.03; N: 3.71; O: 12.72; S: 8.49.
20
D
Found: C: 63.52; H: 6.47; N: 3.70. ½aꢁ ¼ ꢀ42:9 (c ¼ 0:01, methanol).
2.2.2. (3S,4R)-3-(3,4-dibenzyloxyphenoxymethyl)-4-(4-fluorophenyl)-1-methylpiper-
idine (6)
0.48 g (1.6 mmol) of 3,4-dibenzyloxyphenol [10] dissolved in 10 mL of anhydrous
DMF were slowly added to a stirred suspension of NaH (2.12 mmol) in 10 mL of
DMF. The mixture was refluxed for 3 h, left to reach room temperature and then
the flask was placed in an ice bath. To this solution, were added 0.5 g (1.32 mmol)
of tosylate 5 dissolved in 10 mL of DMF and then heated to reflux for 4 h. Once
it was at room temperature, water was slowly added in order to eliminate the remain-
ing NaH and the mixture was adjusted to pH 7 with the addition of 1N HCl solution.
Water (250 mL) was added to the mixture which was extracted with ethyl acetate
(10 Â 50 mL). The organic layer was washed with water (4 Â 25 mL), brine
(2 Â 25 mL), dried over anhydrous MgSO4, and filtered. The solvent was evaporated
under vacuum and the crude product was purified by flash chromatography (hexane/
EtOAc 3:1; then EtOAc/MeOH/NH3 20:3:1) to give 0.54 g (1.06 mmol, 80%) of the
free amine 6 as a reddish solid. IR (neat, mmax): 3033, 2936, 2785, 1604, 1510,
1
1466, 1221, 1173, 1026, 832, 755, 697. H-NMR (300 MHz): 7.48–7.24 (m, 10H),
7.20–7.11 (m, 2H), 7.10–6.93 (m, 2H), 6.80 (d, J ¼ 9:0 Hz, 1H), 6.47 (d, J ¼ 3:0 Hz,
1H), 6.20 (dd, J ¼ 3:0 and 8.5 Hz, 1H), 5.10 (s, 2H), 5.06 (s, 2H), 3.58 (dd, J ¼ 3:0
and 9.0 Hz, 1H), 3.46 (dd, J ¼ 7:0 and 9.5 Hz, 1H), 3.23 (dd, J ¼ 2:5 and 11.0 Hz,
1H), 3.01 (d, J ¼ 11:0 Hz, 1H), 2.46 (dt, J ¼ 4:5 and 11.5 Hz, 1H), 2.40 (s, 3H),

252
M. Segura et al. / Bioorganic Chemistry 31 (2003) 248–258
2.35–2.20 (m, 1H), 2.15–1.80 (m, 4H). 13C-NMR (75 MHz): 161.4 (JC–F ¼ 244 Hz),
154.0, 150.0, 142.9, 139.4, 137.5, 136.9, 128.8, 128.7, 128.4, 128.3, 127.7, 127.6,
127.4, 127.2, 116.9, 115.5, 115.2, 105.1, 103.2, 72.5, 71.0, 68.8, 59.3, 56.0, 46.3,
43.3, 41.9, 34.1. Anal. Calcd. for C33H34FNO3: C: 77.47; H: 6.70; F: 3.71; N: 2.74;
20
D
O: 9.38. Found: C: 77.32; H: 6.79; N: 2.77. ½aꢁ ¼ ꢀ38:2 (c ¼ 0:01, methanol).
2.2.3. (3S,4R)-4-(4-fluorophenyl)-3-(3-methoxy-4-benzyloxyphenoxymethyl)-1-
methylpiperidine (7)
By following the above procedure, treatment of 0.55 g (2.41 mmol) of 3-methoxy-
4-benzyloxyphenol (synthesized according to [10]) with NaH (2.41 mmol) and 0.76 g
(2.01 mmol) of tosylate 5 yielded 7 as the free amine, after column chromatography
purification (hexane/AcOEt 3:1) (0.47 g, 1.07 mmol, 53%). IR (neat, mmax): 2937,
2785, 1510, 1222, 1199. 1H-NMR (300 MHz): 7.41–7.19 (m, 5H), 7.18–7.10 (m,
2H), 6.99–6.90 (m, 2H), 6.67 (d, J ¼ 8:5 Hz, 1H), 6.37 (d, J ¼ 2:5 Hz, 1H), 6.08
(dd, J ¼ 2:5 and 8.5 Hz, 1H), 5.02 (s, 2H), 3.57 (dd, J ¼ 3:0 and 9.5 Hz, 1H), 3.45
(dd, J ¼ 7:0, and 9.5 Hz, 1H), 3.20 (m, 1H), 3.08 (s, 3H), 2.98 (d, J ¼ 10:8 Hz,
1H), 2.44 (dt, J ¼ 4:5 and 11.5 Hz, 1H), 2.36 (s, 3H), 2.30–2.18 (m, 1H), 2.13–1.76
(m, 4H). 13C-NMR (75 MHz): 161.5 (JC–F ¼ 244 Hz), 154.0, 150.7, 142.3, 139.5,
137.4, 128.8, 128.7, 128.4, 127.7, 127.4, 115.5, 115.4, 115.2, 103.7, 100.9, 72.0,
68.9, 59.5, 56.1, 55.8, 46.4, 43.4, 42.0, 34.2. 19F-NMR (282 MHz): )117.1 to
)116.9 (m, 1F). GC/MS (m=z): 109, 140, 153, 218, 262, 358, 401. Anal. Calcd. for
C27H30FNO3: C: 74.46; H: 6.94; F: 4.36; N: 3.22; O: 11.02. Found: C: 74.34; H:
20
D
6.97; N: 3.20. ½aꢁ ¼ ꢀ59:5 (c ¼ 0:01, methanol).
2.2.4. (3S,4R)-3-(3,4-dibenzyloxyphenoxymethyl)-4-(4-fluorophenyl)-1-vinyloxycar-
bonylpiperidine (8)
To a stirred solution of 6 (553 mg, 1.08 mmol) and anhydrous potassium carbon-
ate (2.1 g, 15.3 mmol) in anhydrous 1,2-dichloroethane (15 mL) was added vinyl
chloroformate (1.1 mL, 11.5 mmol). The reaction mixture was refluxed for 4 h, the
solvent was evaporated and the residue partitioned between ethyl acetate and brine
(200 mL, 1:1). The organic layer was decanted and the aqueous layer extracted with
ethyl acetate (5 Â 25 mL). The combined organic layer was washed with brine
(2 Â 10 mL), dried over anhydrous MgSO4 and filtered. The solvent was evaporated
under vacuum to give a crude brown oil which was purified by column chromatog-
raphy (hexane and then hexane/EtOAc 3:1) yielding 0.32 g (0.56 mmol, 52%) of 8 as
1
colorless oil. IR (neat, mmax): 3033, 2921, 2865, 1716, 1509, 1435, 1220, 1149. H-
NMR (200 MHz): 7.52–7.20 (m, 11H), 7.15–9.90 (m, 4H), 6.79 (d, J ¼ 8:5 Hz,
1H), 6.48 (d, J ¼ 3:0 Hz, 1H), 6.19 (dd, J ¼ 2:5 and 6.5 Hz, 1H), 5.11 (s, 2H), 5.06
(s, 2H), 4.82 (dd, J ¼ 1:5 and 13.5 Hz, 1H), 4.54 (d, J ¼ 13:5 Hz, 1H), 4.50 (dd,
J ¼ 1:5 and 6.5 Hz, 1H), 4.35 (d, J ¼ 13:5 Hz, 1H), 3.61 (dd, J ¼ 3:0 and 9.5 Hz,
1H), 3.45 (dd, J ¼ 6:0 and 9.5 Hz, 1H), 2.95 (m, 2H), 2.72 (dt, J ¼ 4:0 and
11.5 Hz, 1H), 2.15–1.94 (m, 1H), 1.90–1.61 (m, 2H). 13C-NMR (75 MHz): 161.9
(JC–F ¼ 244 Hz), 153.4, 150.1, 143.5, 139.7, 137.4, 136.9, 128.9, 128.5, 128.4, 127.9,
127.7, 127.5, 127.3, 115.9, 115.8, 115.7, 105.2, 103.4, 72.4, 71.1, 66.9, 41.6, 39.4,

M. Segura et al. / Bioorganic Chemistry 31 (2003) 248–258
253
30.9, 30.0. Anal. Calcd. for C35H34FNO5: C: 74.06; H: 6.04; F: 3.35; N: 2.47; O:
20
D
14.09. Found: C: 74.22; H: 6.21; N: 2.44. ½aꢁ ¼ ꢀ38:5 (c ¼ 0:01, methanol).
2.2.5. (3S,4R)-4-(4-fluorophenyl)-3-(3-methoxy-4-benzyloxyphenoxymethyl)-1-
vinyloxycarbonylpiperidine (9)
Following the above procedure, treatment of a solution 7 (426 mg, 0.98 mmol)
and anhydrous potassium carbonate (1.27 g, 9.2 mmol) in 1,2-dichloroethane
(12 mL) with vinyl chloroformate (0.62 mL, 6.92 mmol) gave 0.27 g (0.55 mmol,
56%) of 9, after purification by flash silica-gel column chromatography. IR (neat,
m
max): 2919, 1717, 1647, 1509, 1450, 1437, 1221, 1197, 1150, 844. 1H-NMR
(300 MHz): 7.42–7.21 (m, 6H), 7.16–7.09 (m, 2H), 7.01–6.91 (m, 2H), 6.70 (d,
J ¼ 8:5 Hz, 1H), 6.42 (d, J ¼ 3:0 Hz, 1H), 6.12 (bs, 1H), 5.04 (s, 2H), 4.78 (dd,
J ¼ 3:0 and 14.0 Hz, 1H), 4.55 (br, 1H), 4.48 (dd, J ¼ 1:5 and 6.5 Hz, 1H), 4.34
(br, 1H), 3.82 (s, 3H), 3.63 (dd, J ¼ 2:5 and 9.5 Hz, 1H), 3.49 (t, 1H), 3.04–2.80
(m, 2H), 2.73 (dt, J ¼ 4:0 and 11.5 Hz, 1H), 2.15–1.96 (m, 1H), 1.90–1.66 (m, 2H).
13C-NMR (75 MHz): 161.6 (JC–F ¼ 245 Hz), 153.7, 152.4, 150.7, 142.5, 138.7,
138.6, 137.4, 128.8, 128.7, 128.4, 127.7, 127.3, 115.7, 115.4, 115.3, 103.6, 101.0,
95.4, 71.9, 68.0, 55.9, 47.3, 44.5, 43.8, 42.0, 41.7, 33.9, 33.5, 30.9. 19F-NMR
(282 MHz): )116.4 to )116.3 (m, 1F). Anal. Calcd. for C29H30FNO5: C: 70.86; H:
20
D
6.15; F: 3.86; N: 2.85; O: 16.27. Found: C: 70.75; H: 6.17; N: 2.83. ½aꢁ ¼ ꢀ16:8
(c ¼ 0:01, methanol).
2.2.6. (3S,4R)-3-(3,4-dibenzyloxyphenoxymethyl)-4-(4-fluorophenyl)-piperidine (10)
Anhydrous hydrogen chloride was bubbled through a stirred solution of 8 (0.21 g,
0.37 mmol) in anhydrous 1,2-dichloroethane (5 mL) for 45 min. After the solvent was
evaporated, 2 mL of 1,2-dichloroethane were added, and the solvent was removed by
evaporation. The resulting orange residue was dried under vacuum for 3 h, then it
was taken up in absolute ethanol (10 mL) and refluxed for 1.5 h. Ethanol was evap-
orated to yield 10 (0.21 g, 0.44 mmol, 73%) as a pink solid. IR (neat, mmax): 2929,
1
2356, 1607, 1515, 1461, 1226, 1177. H-NMR (200 MHz): 9.82 (brs, 1H), 7.41–7.20
(m, 10H), 7.14 (t, 2H), 6.94 (t, 2H), 6.73 (d, J ¼ 9:0 Hz, 1H), 6.42 (d, J ¼ 2:0 Hz,
1H), 6.12 (dd, J ¼ 2:0, and 9.0 Hz, 1H), 5.07 (s, 2H), 5.01 (s, 2H), 3.73–3.50 (m,
3H), 3.49–3.42 (m, 1H), 3.25–2.70 (m, 3H), 2.70–2.50 (m, 1H), 2.50–2.23 (m, 1H),
2.10–1.90 (m, 1H). 13C-NMR (75 MHz): 162.0 (JC–F ¼ 246 Hz), 153.4, 150.1, 143.5,
139.7, 137.4, 136.9, 128.9, 128.5, 128.4, 127.8, 127.7, 127.4, 127.3, 116.8, 115.9,
115.6, 111.8, 105.2, 103.4, 72.4, 71.1, 66.9, 46.8, 44.5, 41.6, 39.4, 30.9, 30.0. 19F-
NMR (282 MHz): )115.5 to )115.3 (m, 1F). Anal. Calcd. for C32H32FNO3: C:
77.24; H: 6.48; F: 3.82; N: 2.81; O: 9.65. Found: C: 77.12; H: 6.44; N: 2.85.
20
D
½aꢁ ¼ ꢀ57 (c ¼ 0:01, methanol).
2.2.7. (3S,4R)-4-(4-fluorophenyl)-3-(3-methoxy-4-benzyloxyphenoxymethyl)piperi-
dine (11)
Following the above procedure, treatment of a solution of 9 (220 mg, 0.45 mmol) in
1,2-dichloroethane (5 mL) with anhydrous hydrogen chloride, evaporation of solvent
and subsequent heating of an ethanolic solution under reflux provided a yellow oil

254
M. Segura et al. / Bioorganic Chemistry 31 (2003) 248–258
which was purified by column chromatography (ethyl acetate/methanol/ammonia
20:3:1) to give 170 mg (87%) of the free amine 11 as colorless solid. m.p.: 66–67 °C.
1
IR (neat, mmax): 2932, 2796, 1598, 1511, 1222, 1199. H-NMR (300 MHz): 7.42–7.22
(m, 5H), 7.20–7.09 (m, 2H), 7.00–6.90 (m, 2H), 6.66 (d, J ¼ 9:0 Hz, 1H), 6.36 (d,
J ¼ 3:0 Hz, 1H), 6.07 (dd, J ¼ 3:0 and 9.0 Hz, 1H), 5.02 (s, 2H), 3.80 (s, 3H), 3.56
(dd, J ¼ 3:0 and 9.5 Hz, 1H), 3.45 (dd, J ¼ 7:0 and 16.0 Hz, 1H), 3.44 (Br, 1H), 3.18
(d, J ¼ 11:0 Hz, 1H), 2.80–2.50 (m, 2H), 2.40–2.20 (m, 1H), 2.14–2.00 (m, 1H), 1.90–
1.70 (m, 2H). 13C-NMR (75 MHz): 161.4 (JC–F ¼ 243 Hz), 154.0, 150.7, 142.3, 139.8,
137.4, 128.7, 128.4, 127.7, 127.4, 127.3, 115.5, 115.4, 115.2, 103.7, 100.9, 93.8, 88.3,
72.0, 68.9, 55.8, 44.4. 19F-NMR (282 MHz): )115.5 to )115.3 (m, 1F). Anal. Calcd.
for C26H28FNO3: C: 74.09; H: 6.70; F: 4.51; N: 3.32; O: 11.39. Found: C: 73.96; H:
20
D
6.72; N: 3.35. ½aꢁ ¼ ꢀ45:9 (c ¼ 0:01, methanol).
2.2.8. (3S,4R)-3-(3,4-dihydroxyphenoxymethyl)-4-(4-fluorophenyl)-piperidine (2)
In a pressure resistant glass reactor was dissolved 66 mg (0.13 mmol) of 10 in 15 mL
of previously degassed methanol. To this solution was added a catalytic amount of Pd/
C. Hydrogenation was performed at 3 bar pressure with continuous stirring at room
temperature for 4 h. After filtration of the residue through Celite and concentration,
2 was obtained as a pink oil (38 mg, 0.12 mmol, 92%). IR (neat, mmax): 3191, 1511,
1
1226, 1165. H-NMR (300 MHz, CD3OD): 7.34–7.22 (m, 2H), 7.12–7.02 (m, 2H),
6.59 (d, J ¼ 9:0 Hz, 1H), 6.27 (d, J ¼ 3:0 Hz, 1H), 6.08 (dd, J ¼ 3:0 and 9.0 Hz, 1H),
3.72–3.42 (m, 4H), 3.22–3.07 (m, 2H), 3.02–2.82 (m, 1H), 2.37 (br, 1H), 2.03 (br,
2H). 13C-NMR (75 MHz, CD3OD): 163.3 (JC–F ¼ 243 Hz), 153.6, 147.0, 140.7,
139.2, 130.3, 130.2, 116.7, 116.5, 116.4, 105.9, 104.2, 68.6, 45.5, 42.7, 40.8, 31.6.
20
D
19F-NMR (282 MHz): )115.8 to )115.7 (m, 1F). ½aꢁ ¼ ꢀ69 (c ¼ 0:01, methanol).
2.2.9. (3S,4R)-4-(4-fluorophenyl)-3-(3-methoxy-4-hydroxyphenoxymethyl)piperidine
(3)
Following the above procedure, treatment of 19 mg (0.045 mmol) of 11 provided
1
13 mg (0.040 mmol, 89%) of 3 as a pink oil. H-NMR (300 MHz, CD3 OD): 7.28–
7.22 (m, 2H), 7.06–6.96 (m, 2H), 6.61 (d, J ¼ 9:0 Hz, 1H), 6.37 (d, J ¼ 3:0 Hz,
1H), 6.13 (dd, J ¼ 3:0 and 9.0 Hz, 1H), 3.77 (s, 3H), 3.62–3.34 (m, 4H), 3.22–3.10
(m, 1H), 2.80–2.60 (m, 3H), 2.20 (br, 2H). 13C-NMR (75 MHz, CD3OD): 162.2
(JC–F ¼ 243 Hz), 153.9, 149.4, 141.4, 141.2, 130.1, 130.0, 116.3, 116.0, 106.3, 101.5,
82.9, 70.1, 47.2, 45.1, 43.3, 35.3. 19F-NMR (282 MHz): )115.9 to )115.6 (m, 1F).
20
D
½aꢁ ¼ ꢀ60:5 (c ¼ 0:005, methanol).
2.3. Synthesis of paroxetine
2.3.1. (3S,4R)-4-(4-fluorophenyl)-3-(3,4-methylenedioxyphenoxymethyl)-1-methyl-
piperidine (12)
Following the same procedure as described for the preparation of 6, treatment of
224 mg (1.59 mmol) of 3,4-methylenedioxyphenol with NaH and subsequent reaction
with 500 mg (1.32 mmol) of tosylate 5 provided, after column chromatography puri-
fication, 370 mg (1.08 mmol, 80%) of 12 as free amine. IR (neat, mmax): 2936, 2787,

M. Segura et al. / Bioorganic Chemistry 31 (2003) 248–258
255
1
1510, 1489, 1185, 1038, 832. H-NMR (300 MHz): 7.18–7.10 (m, 2H), 7.01–6.90 (m,
2H), 6.60 (d, J ¼ 8:5 Hz, 1H), 6.32 (d, J ¼ 2:5 Hz, 1H), 6.11 (dd, J ¼ 2:5 and 8.5 Hz,
1H), 5.85 (s, 2H), 3.55 (dd, J ¼ 3:0 and 9.5 Hz, 1H), 3.42 (dd, J ¼ 7:0 and 9.5 Hz,
1H), 3.20–3.15 (m, 1H), 3.00–2.90 (m, 1H), 2.42 (dt, J ¼ 5:5 and 11.5 Hz, 1H),
2.34 (s, 3H), 2.26–2.11 (m, 1H), 2.10–1.75 (m, 4H). 13C-NMR (75 MHz): 161.5
(JC–F ¼ 244 Hz), 154.3, 148.1, 141.5, 139.6, 128.8, 128.7, 115.5, 115.2, 107.8, 105.5,
101.0, 97.9, 69.5, 59.5, 56.1, 46.5, 43.5, 42.1, 34.3. 19F-NMR (282 MHz): )117.2 to
)117.0 (m, 1F). GC/MS (m=z): 58, 83, 109, 191, 206, 343. Anal. Calcd. for
C20H22FNO3: C: 69.95; H: 6.46; F: 5.53; N: 4.08; O: 13.98. Found: C: 70.04; H:
20
D
6.39; N: 4.12. ½aꢁ ¼ ꢀ75:9 (c ¼ 0:01, methanol).
2.3.2. (3S,4R)-4-(4-fluorophenyl)-3-(3,4-methylenedioxyphenoxymethyl)-1-vinyloxy-
carbonylpiperidine (13)
Following the same procedure as described for the preparation of 8, treatment of
a solution of 12 (300 mg, 0.87 mmol), and anhydrous potassium carbonate (1.23 g,
8.21 mmol) in 1,2-dichloroethane (5 mL) with vinyl chloroformate (0.55 mL,
6.18 mmol) gave, after purification by flash silica-gel column chromatography,
0.26 g (0.65 mmol, 75%) of (3S,4R)-4-(4-fluorophenyl)-3-(3,4-methylenedioxyphen-
oxymethyl)-1- vinyloxycarbonylpiperidine. IR (neat, mmax): 2924, 2870, 1716, 1488,
1
1469, 1436, 1221, 1186, 1151. H-NMR (300 MHz): 7.28–7.20 (m, 1H), 7.16–7.07
(m, 2H), 7.00–6.91 (m, 2H), 6.60 (d, J ¼ 8:5 Hz, 1H), 6.34 (d, J ¼ 2:5 Hz, 1H),
6.26 (dd, J ¼ 2:5 and 8.5 Hz, 1H), 5.86 (s, 2H), 4.79 (dd J ¼ 1:5 and 14.0 Hz, 1H),
4.50 (br, 1H), 4.46 (dd, J ¼ 1.5 and 6.5 Hz, 1H), 4.33 (d, J ¼ 12:5 Hz, 1H), 3.60
(dd, J ¼ 2:5 and 9.5 Hz, 1H), 3.50–3.41 (m, 1H), 3.02–2.81 (m, 2H), 2.80–2.65 (m,
1H), 2.10–1.96 (m, 1H), 1.88–1.65 (m, 2H). 13C-NMR (75 MHz): 161.6
(JC–F ¼ 245 Hz), 154.1, 152.4, 148.1, 142.5, 141.7, 138.6, 128.8, 128.7, 115.7, 115.4,
107.8, 105.5, 101.1, 98.0, 95.3, 68.5, 47.3, 44.5, 43.7, 41.8, 33.4. 19F-NMR (282 MHz):
)116.4 (s, 1F). GC/MS (m=z): 109, 151, 262, 356, 399. Anal. Calcd. for C22H22FNO5:
C: 66.16; H: 5.55; F: 4.76; N: 3.51; O: 20.03. Found: C: 66.04; H: 5.47; N: 3.62.
20
D
½aꢁ ¼ ꢀ24:0 (c ¼ 0:01, methanol).
2.3.3. (3S,4R)-4-(4-fluorophenyl)-3-(3,4-methylenedioxyphenoxymethyl)piperidine,
Paroxetine (1)
Following the same procedure as described for the preparation of 10, treatment of
a solution of 13 (76 mg, 0.19 mmol) in 1,2-dichloroethane (3.5 mL) with anhydrous
hydrogen chloride, evaporation of solvent and subsequent heating of an ethanolic
solution under reflux provided an orange–yellow oil that was purified by column
chromatography (ethyl acetate/methanol/ammonia 20:3:1) to give 60 mg (96%) of
the free amine 1 as white solid. IR (neat, mmax): 3403, 2921, 1509, 1488, 1470,
1
1186. H-NMR (300 MHz, CD3OD): 7.28–7.20 (m, 2H), 7.06–6.96 (m, 2H), 6.61
(d, J ¼ 9:0 Hz, 1H), 6.33 (d, J ¼ 2:5 Hz, 1H), 6.13 (dd, J ¼ 2:5 and 9.0 Hz, 1H),
5.84 (s, 2H), 3.70–3.34 (m, 4H), 3.24–3.14 (m, 1H), 2.86–2.66 (m, 3H), 2.22–2.06
(m, 1H), 1.86–1.76 (m, 2H). 13C-NMR (75 MHz, CD3OD): 163.0 (JC–F ¼ 242 Hz),
155.7, 149.6, 143.1, 141.0, 140.9, 130.2, 130.1, 116.4, 116.1, 108.8, 106.5, 102.4,
20
D
98.7, 70.1, 46.9, 44.8, 43.0, 34.8. ½aꢁ ¼ ꢀ66 (c ¼ 0:01, methanol).

256
M. Segura et al. / Bioorganic Chemistry 31 (2003) 248–258
3. Results and discussion
The preparation of both metabolites was planned following the synthesis de-
scribed for paroxetine. Thus, commercially available 3,4-dibenzyloxybenzaldehyde
and 4-benzyloxy-3-methoxybenzaldehyde were converted to the corresponding phe-
nols [10]. (3S,4R)-4-(4-fluorophenyl)-3-hydroxymethyl-1-methylpiperidine, 4 [11]
was converted to its tosylate derivative 5 [9] (Fig. 2) which by treatment with the so-
dium salt of the corresponding phenol was converted to the intermediates 6 and 7.
N-demethylation of these compounds by reaction with vinyl chloroformate and
subsequent heating with hydrogen chloride [12] in absolute ethanol provided both
3,4-disubstituted phenoxypiperidines 10 and 11 as their hydrochloride salts, which
by hydrogenation in the presence of a catalytic amount of palladium on charcoal
yielded the corresponding (3S,4R)-4-(4-fluorophenyl)-3-(3,4-dihydroxyphenoxym-
ethyl)piperidine 2, and (3S,4R)-4-(4-fluorophenyl)-3-(4-hydroxy-3-methoxyphen-
oxymethyl)piperidine 3, respectively.
As it is known that the configuration of the C3 stereocenter could be inverted
during the displacement of the corresponding sulfonate ester of trans 4-(4-fluorophe-
nyl)-3-hydroxymethyl-1-methylpiperidine with the sodium salt of 3,4-methylenedi-
oxyphenol [13], the trans configuration of synthetic piperidines 2 and 3 was
confirmed by NMR analysis of compound 7 (1H-, 13C-NMR, NOE, and heterocor-
relation experiments). After assingment of all the signals, one may conclude that
Fig. 2. Synthesis of paroxetine main metabolites 2 and 3. Reagents and conditions. (a) TsCl, Et3N,
Me3NꢂHCl, CH2Cl2, 0 °C, 1.5 h, 90%; (b) NaH, DMF, add a solution of corresponding phenol in
DMF, reflux, 3 h, cool to RT and add a solution of 5 in DMF, reflux, 4 h, then cool to RT; (c)
CH2@CHOCOCl, K2CO3, ClCH2CH2Cl, reflux, 4 h, evaporation to dryness; (d) HCl(g) in absolute
EtOH, reflux, 1.5 h; (e) H2, Pd–C, MeOH, 4 h.

M. Segura et al. / Bioorganic Chemistry 31 (2003) 248–258
257
Fig. 3. Chemical synthesis of paroxetine 1. Reagents and conditions. (a) NaH, DMF, then add a solution
of 3,4-methylenedioxyphenol in DMF, reflux, 3 h, cool to Rt and add a solution of 5 in DMF, reflux, 4 h,
then cool to RT; (b) CH2@CHOCOCl, K2CO3, ClCH2CH2Cl, reflux, 4 h, evaporation to dryness; (c)
HCl(g) in absolute EtOH, reflux, 1.5 h.
hydrogen attached to C4 displays three vicinal coupling constants (11.5, 11.0, and
4.5 Hz). The first two correspond typically to 1,2-trans-diaxial system and the third
one to a vicinal axial-equatorial which would only be possible if substituents at C3
and C4 are in a 1,2-trans-diaxial disposition. This confirms that no inversion at C3
had occurred during the displacement reaction.
This stereochemistry was also confirmed by comparison of synthetic and commer-
cial samples of paroxetine. Accordingly, treatment of tosylate derivative 5 (Fig. 3)
with the sodium salt of 3,4-methylenedioxyphenol resulted in the N-methylpiperidine
intermediate 12, which by demethylation following the same procedure as described
above, yielded a product whose structure and stereochemistry was confirmed as par-
oxetine 1 by comparison of its analytical data with those of an authentic sample.
4. Conclusion
This is the first report describing the synthesis of paroxetine main metabolites
(3S,4R)-4-(4-fluorophenyl)-3-(3,4-dihydroxyphenoxymethyl)piperidine 2 and (3S,
4R)-4-(4-fluorophenyl)-3-(4-hydroxy-3-methoxyphenoxymethyl)piperidine 3, that
are crucial for our future in vivo studies on the metabolism of paroxetine.
Acknowledgments
This investigation was supported by: FIS 98/0181, CIRIT 99-SGR-00187, and
PNSD(Spain).
References
€
[1] S. Hartter, B. Hermes, A. Szegedi, C. Hiemke, Ther. Drug Monit. 16 (1994) 400–406.
[2] I.F. Tulloch, A.M. Johnson, J. Clin. Psychiatry 53 (1992) 7–12.

258
M. Segura et al. / Bioorganic Chemistry 31 (2003) 248–258
[3] C.M. Kaye, R.E. Haddock, P.F. Langley, G. Mellows, T.C. Tasker, B.D. Zussman, W.H. Greb, Acta
Psychiatr. Scand., Suppl. 350 (1989) 60–75.
[4] R.E. Haddock, A.M. Johnson, P.F. Langley, D.R. Nelson, J.A. Pope, D.R. Thomas, F.R. Woods,
Acta Psychiatr. Scand., Suppl. 350 (1989) 24–26.
[5] N.S. Gunasekara, S. Noble, P. Benfield, Drugs 55 (1998) 85–120.
[6] K.L. Dechant, S.P. Clissold, Drugs 41 (1991) 225–253.
[7] M. Murray, Curr. Drug Metab. 1 (2000) 67–84.
[8] P.R. Ortiz de Montellano, N.O. Reich, in: P.R. Ortiz de Montellano (Ed.), Cytochrome P-450,
Plenum Press, New York, 1986, pp. 280–283.
[9] Y. Yoshida, Y. Sakakura, N. Aso, S. Okada, Y. Tanabe, Tetrahedron 55 (1999) 2183–2192.
[10] A.F. Barrero, E.J. Alvarez-Manzaneda, R. Chahboun, Tetrahedron 54 (1998) 5635–5650.
[11] (a) Christensen, J.A., Squires, R.F., 1973. UK Patent 1,422,263;
(b) Christensen, J.A., Squires, R.F., 1977. US Patent 4,007,196 .
[12] (a) R.A. Olofson, R.C. Schnur, L. Bunes, J.P. Pepe, Tetrahedron Lett. (1977) 1567–1570;
(b) K. Willcocks, R.D. Barnes, D.C. Rusttidge, D.J.D. Tidy, J. Labelled Compd. and Radiopharm.
33 (1993) 783–794.
[13] Christensen et al. reported the conversion of the (ꢃ)-trans isomer of the methane sulfonic ester
derivative of 4 into a mixture of (ꢃ)-trans and mainly (ꢃ)-cis of compound 10 by reaction with the
sodium salt of 3,4-methylenedioxyphenol (see reference [11b]). In this regard, the same authors [see
€
Christensen, J.A.; Engelstoft, M.; Schaumburg, K.; Schou, H.; Watjen, F. (1983) Tetrahedron Lett.,
24, 5151–5152.] published work on the preparation of a similarly substituted piperidine (Femoxetine)
stating that a mixture of cis and trans (85:15) benzenesulphonyl ester was converted, by treatment
with the sodium salt of p-methoxyphenol, into a cis/trans (15/85) of Femoxetine, but if pure (ꢃ)-trans
sulphoester is used as the starting material pure (ꢃ)-trans Femoxetine is the product, indicating in this
case a clean SN 2 reaction. This contradiction led us to investigate the relative stereochemical outcome
of our reaction.