Strategy for 14C-labeling of a series of bis(heteroaryl) piperazines

Article abstract of DOI:10.1002/jlcr.1880

Four bis(heteroaryl)piperazines labeled with carbon-14 in the 2-position of imidazole moiety were prepared as part of a 4-step (or 5-step) sequence from 5-hydroxymethyl-2-mercapto-1-benzylimida-zole-[2-¹⁴C] as a key synthetic intermediate which

Full text of DOI:10.1002/jlcr.1880

Research Article
Received 19 July 2010,
Revised 15 January 2011,
Accepted 19 January 2011
Published online 30 May 2011 in Wiley Online Library
(wileyonlinelibrary.com) DOI: 10.1002/jlcr.1880
14
Strategy for C-labeling of a series of
bis(heteroaryl)piperazines
Ã
Omid Khalili Arjomandi, Nader Saemian, Gholamhossein Shirvani,
Mohsen Javaheri, and Kameh Esmailli
Four bis(heteroaryl)piperazines labeled with carbon-14 in the 2-position of imidazole moiety were prepared as part of a 4-
step (or 5-step) sequence from 5-hydroxymethyl-2-mercapto-1-benzylimida-zole-[2-14C] as a key synthetic intermediate
which has been synthesized from potassium [14C]-thiocyanate.
Keywords: Carbon-14; Bis(heteroaryl) piperazines; Imidazoles
was CH3CN:H2O:triethylamine (600:400:0.75) with flow rate:
1 ml/min. Radioactivity was determined using a Beckman
Introduction
Bis(heteroaryl)piperazines (BHAPs) have been a rich source of
biologically active molecular entities and are related to a wide
family of compounds with well-known pharmacological proper-
ties.1–3 A variety of analogues of BHAPs (such as: U-80493E) were
synthesized and evaluated for their inhibition of human
immunodeficiency virus type1 reverse transcriptase.4 Sometimes
replacement of the substituted aryl moiety with various
aromatic systems provided bis (heteroaryl) piperazines that
were10–100 fold more potent than U-80493E (for instance:
Atevirdine).5–8 According to previous structure and activity
relationship studies on Atevirdine, the compound 3a was
introduced by Hadizadeh and Mehrparvar, in which 1-(3-
alkylamino-2-pyridyl) piperazine part of the molecule was
unchanged and 5-alkoxy-2-indolylcarbonyl part of molecule
was replaced by benzyl-2-alkylthio-imidazolylcarbonyl moiety.9
To further elucidate the mechanism of action and to support
the on-going metabolism studies, the need arose to synthesize
the corresponding carbon-14 compounds, with the label
situated in a biologically stable site.10 In this paper the synthesis
of 5-hydroxymethyl-2-mercapto-1-benzylimidazole-[2-14C] 1 and
5-(hydroxymethyl)-1-benzylimidazole-[2-14C] 2 and the conver-
sion of these compounds to the four bis(heteroaryl)piperazines-
[2-14C](3a-d) is described (Figure 1).
LS6500 liquid scintillation spectrometer. Mass spectra were
obtained on a Finnigan TSQ-70 instrument. Infrared spectra
were recorded on a Nicolet IR 550 spectrometer.
Potassium [14C]-thiocyanate 5
A mixture of potassium [14C]-cyanide 4 (357.5 mg, 508.75 MBq),
sulfur (193 mg) and acetone (8.3 ml) was refluxed for 45 min. The
solvent was evaporated and the residue mixed with sulfur
(96 mg) and acetone (7.2 ml), and mixture refluxed for 45 min.
The combined acetone solutions were evaporated with a stream
of nitrogen. The product 5 was obtained with a quantitative
radiochemical yield.
5-(Hydroxymethyl)-2-mercapto-1-benzylimidazole-2-[14C] 1
A mixture of benzylamine hydrochloride (574 mg, 4 mmol) 1,3-
dihydroxyacetone dimer (321 mg) and Potassium [14C]-thiocya-
nate 5 (514 mg, 490 MBq) in 1-butanol(2.5 ml) and glacial acetic
acid (0.4 ml) was stirred vigorously at room temperature for 48 h.
The mixture was suction filtered and the solid was washed with
water (3 ꢀ 3 ml) followed by ether(3 ꢀ 3 ml). The title product 1
precipitated upon cooling were purified by washing with ether,
crystallization from methanol/ether and drying under high
vacuum to give the title compound 1 (722 mzg, 303 MBq)
(yield: 61.83%) (Rt = 2.4 min), 1H-NMR (DMSO-d6, TMS): d12.23
(s, 1H, absent in D2O Shake); d7.18–7.49 (m, 6H, aromatic & H–C4
imidazole); d5.48 (s, 2H); d5.25 (t, 1H, J = 5 Hz, absent in
D2O Shake); d4.56 (d, 2H, J = 5 Hz; s in D2O); MS (70 eV):
m/z = 222 (M1).
Experimental
Barium [14C] carbonate was converted to potassium [14C]
cyanide according to the standard procedure.11 IR spectra were
recorded on a Bruker FT-IR, Vector 22 instrument and the1H-
NMR spectra were recorded on a Varian unity plus 400
spectrometer (400 MHz). HPLC: equipment consisted of: a waters
Radioisotope Section/Nuclear Science Research School, Nuclear Science
&
1525 Binary HPLC pump, a waters 2487 Dual l absorbance Technology Research Institute, P.O. Box: 11365-3486, Tehran, Iran
detector, a waters Breeze Data processing system, Column:
m Bondapak18 (150 ꢀ 4.6 mm) and 5 mm diameters and uv
detection at l = 254 nm. The isocratic elution was used for
*Correspondence to: Nader Saemian, Radioisotope Section/Nuclear Science
Research School, Nuclear Science & Technology Research Institute, P.O. Box:
11365-3486, Tehran, Iran.
chromatographic separation in room temperature. The eluent E-mail: nsaemian@aeoi.org.ir
J. Label Compd. Radiopharm 2011, 54 363–366
Copyright r 2011 John Wiley & Sons, Ltd.

O. K. Arjomandi et al.
manganese dioxide (13 mmol) in chloroform (10 ml) under
reflux condition, the title compound 10 was obtained in
90.5% yield (Rt = 1.28 min), 1H-NMR (CDCl3, TMS): d9.58 (s, 1H,
CHO); d8.25 (s, 1H, H–C2 imidazole); d7.72 (s, 1H, H–C4
imidazole); d7.25–7.12 (m, 5H, aromatic); d5.47 (s, 2H, –CH2N–);
IR: 1657 cmꢁ1 (C = O). MS (70 eV): m/z = 188 (M1).
N
14C
IsoPr-HN
N N
N
14C
HS
CH2OH
N
Y
X
N
1
Bn
N
Bn
3
N
14C
1-Benzyl-2-methylthioimidazole-5-carboxylic acid-2-[14C] 8
X
Y
H
CH2OH
3a CH3S-
3b CH3S-
C=O
CH2
C=O
CH2
N
The compound 7 (1.82 mmol, 423 mg, 167.85 MBq), silver nitrate
(2.5 mmol), sodium hydroxide (3 mmoles), and distilled water
(14 ml) were combined and stirred overnight. The mixture was
filtered and its pH was adjusted between 3 and 4 by adding HCl
(2N). A precipitate was formed, which was isolated by filtration
to give the title compound 8 (1.36 mmol, 125.42 MBq, 338 mg) in
74.7% yield (Rt = 2.1 min), 1H-NMR (CDCl3, TMS): d7.81 (s, 1H,
H–C4 imidazole); d7.33–7.01 (m, 5H, aromatic); d5.47 (s, 2H,
–CH2N–); d2.67 (s, 3H, CH3–S–); IR: 1700 cmꢁ1. MS (70 eV): m/
z = 250 (M1).
3c
H
H
2
Bn
3d
Figure 1. The key imidazole-[2-14C] 1 & 2 for labelling in the 2-position of imidazole
moiety of bis (heteroaryl) piperazines 3a-d.
5-(Hydroxymethyl)-1-benzylimidazole-2-[14C] 2
To 17.4 ml of 10% HNO3 was added 5-(Hydroxymethyl)-2-
mercapto-1-(phenylmethyl)imidazole 1 (722 mg, 303 MBq) with
vigorous strirring. The mixture was briefly contained in an ice
bath (5 min) to moderate the mild exotherm and evolution of
reddish gas, the mixture was stirred further for 2 h at 251C and
suction filtered through a course fritted filter funnel (to remove
solids), and the filtrate was basified with 4 N NaOH to precipitate
the product. This mixture was cooled in an ice bath and the solid
was collected by suction filtration, washed with water, and dried
in vacuo to yield (481 mg, 236 MBq) (77.9%) of a 2 light brown
solid. (Rt = 2.1 min), 1H-NMR ( DMSO-d6, TMS): d7.75(s, 1H);
d7.18–7.49 (m, 6H, aromatic & H–C4 imidazole); d5.48 (s, 2H);
d5.25 (t, 1H, J = 5 Hz, absent in D2O Shake); d4.56 (d, 2H, J = 5 Hz;
s in D2O). MS (70 eV): m/z = 190 (M1).
1-Benzyl-imidazole-5-carboxylic acid-2-[14C] 11
11 was prepared according to the above-described procedure
for 8 by stirring a mixture of 10 (1.82 mmol, 338.5 mg,
167.85 MBq), silver nitrate (2.5 mmoles), and hydroxide
(3 mmoles) in distilled water (14 ml) at room temperature. The
title compound 11 was obtained (1.37 mmol, 277 mg,
125.89 MBq) in 75 % yield (Rt = 1.9 min), 1H-NMR (CDCl3, TMS):
d8.31 (s, 1H, H–C2 imidazole); d7.76 (s, 1H, H–C4 imidazole);
d7.33–7.01 (m, 5H, aromatic); d5.47 (s, 2H, –CH2N–); IR:
1705 cmꢁ1. MS (70 eV): m/z = 204 (M1).
1-Benzyl-5-hydroxymethyl-2-methylthioimidazole-2-[14C] 6
4-[3-(isopropylamino)-2-pyridyl]piperazine 9
To a strirring suspension of the compound 1 (3 mmol, 660 mg,
277 MBq) in methanol (45 ml) was added dropwise sodium
hydroxide (1 N, 3.1 ml)at room temperature. The clear pale
yellow suspension was stirred for 10 min. Iodomethane (0.26 ml)
was added dropwise and stirring was continued overnight. After
evaporation of the methanol the residue was suspended in
water and extracted with chloroform (3 ꢀ 10 ml). The solvent
was evaporated and the residue was crystallized from ethyl
acetate to give the title compound 6 (216 MBq, 548 mg,
2.34 mmol) (yield: 77.9%) (Rt = 1.8 min),1H-NMR (CD3OD, TMS):
d7.48–7.16 (m, 6H, aromatic & H-C4 imidazole); d5.48 (s, 2H,
–CH2–N); d4.65 (s, 2H, –CH2O–); d2.55 (s, 3H, CH3–S–). MS (70 eV):
m/z = 236 (M1).
The title compound 9 was synthesized according to the
standard procedure.7
1-(1-benzyl-2-methylthio-2-[14C]-5-imidazolyl)-4-[3-(1-isopropyl
amino)-2-pridyl] piperazine 3a
1-Benzyl-2-methylthioimidazole-5-carboxylic acid 8 (1.36 mmol,
125.42 MBq, 338 mg) was added to a solution of 1, 10-
carbonyldiimidazole (CDI, 1.36 mmol, 223 mg) in dry tetrahy-
drofuran (2.55 ml) at room temperature. After 1 h of stirring the
reaction was cooled to 01C and 4-[3-(isopropylamino)-2-pyr-
idyl]piperazine 9 (1.27 mmol, 280 mg) dissolved in dry tetrahy-
drofuran (4 ml) was added via cannula. Then the reaction was
slowly warmed to room temperature and stirred for further 20 h.
Basic workup (CH2Cl2,NaHCO3, Na2SO4), concentration in vacuo,
and purification by flash column chromatography (CHCl3)
afforded the title compound 3a (1.1 mmol, 101.5 MBq, 495 mg)
in 81% yield (Rt = 3.15 min), 1H-NMR (CDCl3, TMS): d7.85 (s, 1H, H-
C4 imidazole); d7.65 (dd, 1H, H-pyridine); d7.15 (m, 5H, aromatic);
d6.85 (m, 2H, Hpyridine); d5.37 (s, 2H, –CH2N–); d4.20 (br, 1H,
NH); d3.80–3.40 (m, 5H, CH-iso-propyl & CH2-piperazine); d2.88
(m, 4H, CH2-piperazine); d2.64 (s, 3H, CH3–S–); d1.22 (d, 6H,
2CH3); IR: 1622 cmꢁ1 (C = O). MS (70 eV): m/z = 452 (M1).
1-Benzyl-2-methylthio-5-formylimidazole2-[14C] 7
A stirring of the compound 6 (2 mmol, 468 mg, 184.46MBq) and
manganese dioxide (13 mmol) in chloroform (10 ml) was refluxed
overnight. The mixture was cooled to room temperature and
filtered. The chloroform was evaporated and the residue was
crystallized from ether to give the title compound 7 (1.82 mmol,
423 mg, 167.85 MBq) in 91% yield (Rt = 1.35 min), 1H-NMR (CDCl3,
TMS): d9.60 (s, 1H, CHO); d7.77 (s, 1H, H-C4 imidazole); d7.25–7.12
(m, 5H, aromatic); d5.47 (s, 2H, –CH2N–); d2.67 (s, 3H, CH3–S–); IR:
1655 cmꢁ1 (C = O). MS (70 eV): m/z = 234 (M1).
1-(1-benzyl-2-[14C]-5-imidazolyl)-4-[3-(1-isopropyl amino)-2-
pridyl] piperazine 3c
1-Benzyl-5-formylimidazole-2-[14C] 10
The 3c was prepared according to the above-described procedure
for 3a, by stirring 11(1.36 mmol, 124.97 MBq, 275 mg), (CDI,
1.36 mmol, 223 mg), and 4-[3-(isopropylamino)-2-pyridyl]piperazine
10 was prepared according to the above-described procedure
for
7 by heating 2 (2 mmol, 184.48 MBq, 376 mg) and
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J. Label Compd. Radiopharm 2011, 54 363–366

O. K. Arjomandi et al.
9 (1.27 mmol, 280 mg) in dry tetrahydrofuran. The title compound 1H-NMR (CDCl3, TMS): d8.3 (s, 1H, H–C2 imidazole); d7.80 (s, 1H,
3c (1.1 mmol, 101.13 MBq, 445 mg) was obtained in 81% yield H–C4 imidazole); d7.65 (dd, 1H, H-pyridine); d7.15 (m, 5H,
(Rt = 3.01 min), 1H-NMR (CDCl3, TMS): d8.3 (s, 1H, H–C2 imida- aromatic); d6.85 (m, 2H, Hpyridine); d5.37 (s, 2H, –CH2N–); d4.13
zole); d7.80 (s, 1H, H–C4 imidazole); d7.65 (dd, 1H, H-pyridine); (br, 1H, NH); d3.53 (m, 1H); d3.47 (s, 2H); d3.09 (m, 4H); d2.59 (m,
d7.15 (m, 5H, aromatic); d6.85 (m, 2H, Hpyridine); d5.37 (s, 2H, 4H); d1.23 (d, 6H, 2CH3). MS (70eV): m/z = 392 (M1).
–CH2N–); d4.20 (br, 1H, NH); d3.80ꢁ3.40 (m, 5H, CH-iso-propyl &
CH2-piperazine); d2.88 (m, 4H, CH2-piperazine); d2.64 (s, 3H,
Results and discussion
CH3–S–); d1.22 (d, 6H, 2CH3); IR: 1624 cmꢁ1 (C = O). MS (70 eV):
m/z = 406 (M1).
In this approach, according to the synthetic pathway shown in
Scheme 1, potassium [14C]-thio-cyanate
5 was obtained
quantitatively via the reaction between potassium [14C] cyanide
4 and sulfur in acetone.12,13
2-(4-((1-benzyl-2-(methylthio)-2-[14C]-imidazol-5-yl)methyl)-
piperazin-1-yl)-N-isopropylpyridin-3-amine 3b
The key imidazole-[2-14C] 1 was produced in 61.8% yield
during 48 h by stirring a mixture of potassium [14C] thiocyanate
5, 1,3-dihydroxyacetone dimer and benzylamine hydrochloride
in n-butanol and glacial acetic acid.14 The compound 6 was
achieved from addition of methyl iodide to 1 in the presence of
sodium hydroxide in methanol.15 Oxidation of 6 to 7 was then
carried out with good yield (91%) by using manganese dioxide
in chloroform.16 In the next step, 7 was further oxidized to 8 by
treatment with an alkaline solution of silver nitrate.17 Pyridylpi-
perazine part of the molecule (3a–d) has been synthesized as
part of a 4-step sequence from piperazine and 2-chloro-3-nitro
pyridine. In the final step the coupling of 2-methylthio-1-
benzylimidazole-5-carboxylic acid-[2-14C] 8 with pyridylpipera-
zine 9 was carried out in the presence of 1,10-carbonyl
diimidazole in THF, and 3a was produced in 81% yield.18 The
product 3a could then be converted to 3b ( in 86.5% yield) by
treatment with DIBAL-H in THF.19 Our approach to the synthesis
of 3c and d is shown in Scheme 2. In this route, the key
To a refluxing solution of 3a (1.1 mmol, 101.5 MBq, 495 mg) in
9 ml of THF was added 4.5 ml of 1M DIBAL-H solution in
n-hexane (4.5 mmol), and mixture was refluxed for 4 h with
stirring. Then 87 ml of water was added to the reaction mixture
and extracted with dichloromethane (120 ml). After the extract
was dried over anhydrous MgSO4, the solvent was removed and
the residue was purified by a column chromatography on silica
gel (ethylacetate:hexane, 1:5) to give the title compound 1b
(415 mg, 0.95 mmol, 87.79 MBq) (yield: 86.5%). (Rt = 3.5 min),
1H-NMR (CDCl3, TMS) : d7.65 (dd, 1H, H-pyridine); d7.15 (m, 5H,
aromatic); d7.12 (s, 1H, H–C4 imidazole); d6.85 (m, 2H, Hpyridine);
d5.37 (s, 2H, –CH2N–); d4.13 (br, 1H, NH); d3.53(m, 1H); d3.47
(s, 2H); d3.09 (m, 4H); d2.59( m, 4H); d2.64 (s, 3H, CH3–S–); d1.23
(d, 6H, 2CH3). MS (70 eV): m/z = 438 (M1).
2-(4-((1-benzyl-2-[14C]-imidazol-5-yl)methyl)piperazin-1-yl)-
N-isopropylpyridin-3-amine 3d
The 3d was prepared according to the above-described imidazole-[2-14C] 1 was converted to the compound 2 by
procedure for 1b, by heating 3c (1.1mmol, 101.13MBq, treatment with nitric acid (10%).20 Then the desired products 3c
445 mg) and DIBAL-H (4.5ml of 1 M) in THF at reflux condition. and d were synthesized as part of a three and four stages
The title compound 1d was achieved in 87% yield. (Rt = 3.35min), sequence, respectively, from 221,22 according to the Scheme 2.
Scheme 1.
J. Label Compd. Radiopharm 2011, 54 363–366
Copyright r 2011 John Wiley & Sons, Ltd.
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O. K. Arjomandi et al.
Scheme 2.
These three compounds (3b–d) were introduced for the first
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time and their biological studies of these compounds are still
under investigation. Also all the products 3a–b labeled with
carbon-14 have been synthesized for the first time.
Conclusion
In this paper, we have presented a convenient synthetic
pathway for the labeling of a series of BHAPs with carbon-14
in the 2-position of imidazole moiety, via multistage sequence
from a key imidazole-[2-14C].
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Acknowledgements
We gratefuly acknowledge the help of Dr R. Dowlatabadi
(Tehran University of Medical Science, Faculty of Pharmacy) and
Mr N. Ali-Reza Zadeh (AEOI) for 1H-NMR spectroscopy and
radioactivity determination of synthesized samples, respectively.
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