Radiosynthesis of tritium-labeled novel nitromethylene neonicotinoids compounds with NaB3H4

Article abstract of DOI:10.1002/jlcr.1862

Paichongding and Cycloxaprid are two novel neonicotinoids with good industrialization prospects for their high activity against imidacloprid- resistant pest. In this paper, the radiosynthesis of [³H ₂]-Paichongding and [³H₂]-Cycloxaprid was achieved using a NaB³H₄ reduction. The title compounds were obtained with chemical purities of 98.7 and 98.4%, and radiochemical purities of 97.3 and 98.6%, respectively. The labeled compounds could be used as radiotracers for further study of metabolism and toxicology. Copyright

Full text of DOI:10.1002/jlcr.1862

Research Article
Received 5 September 2010,
Revised 4 November 2010,
Accepted 15 November 2010
Published online 17 February 2011 in Wiley Online Library
(
wileyonlinelibrary.com) DOI: 10.1002/jlcr.1862
Radiosynthesis of tritium-labeled novel
nitromethylene neonicotinoids compounds
3
with NaB H
4
a
Chao Li, Xiao-Yong Xu, Ju-Ying Li, Qing-Fu Ye, and Zhong Li
a
a
by
a
Ã
Paichongding and Cycloxaprid are two novel neonicotinoids with good industrialization prospects for their high activity
against imidacloprid-resistant pest. In this paper, the radiosynthesis of [ H ]-Paichongding and [ H ]-Cycloxaprid was
3
3
2
2
3
achieved using a NaB H reduction. The title compounds were obtained with chemical purities of 98.7 and 98.4%, and
4
radiochemical purities of 97.3 and 98.6%, respectively. The labeled compounds could be used as radiotracers for further
study of metabolism and toxicology.
3
Keywords: radiosynthesis; tritium-labeled; neonicotinoids; NaB H4
Introduction
Experimental
Dicyclic compound Paichongding and oxabridged compound General
Cycloxaprid (Figure 1) are two novel neonicotinoids with cis-
Radiochemical sodium borohydride (80 mCi with a specific
configuration discovered by Qian et al. Both of them can
effectively control sucking and biting insects of broad spectrum,
such as cowpea aphids (Aphis craccivora), brown planthopper
activity of 15 Ci/mmol) was purchased from American Radi-
olabeled Chemicals Inc. (USA). 6-Chloronicotinoyl chloride was
obtained from Alfa Aesar. All other chemicals were obtained
from commercial sources as chemically pure or higher grade and
were used without further purification. Methanol for HPLC was
of chromatographic grade from SK Chemicals (Korea) and
(Nilaparvata lugens), Fabricius (Nephotettix bipunctatus), and
have low mammalian toxicity and favorable environmental
profiles. Moreover, they exhibit significant activity against
1
,2
imidacloprid-resistant brown planthopper.
ꢁ1
ultrapure water (18.2 MO ꢀ cm , 251C) was prepared on Milli-Q
It was found that Paichongding interacted at similar sites to
imidacloprid and clearly discriminated between the high and
low binding sites, potently interacting at the high-affinity
imidacloprid binding site but only weakly at the low-affinity
academic instrument (Millipore, France). The other reagents
were of analytical or higher grade and were used without
purification. Melting points (mp) were recorded on B u¨ chi B540
apparatus (Buchi Labortechnik AG, Flawil, Switzerland) and are
¨
3
imidacloprid binding site. The configuration of Paichongding is
cis, while three proposals for modes of action (MOA) of
1
13
uncorrected. H NMR and C NMR spectra were recorded on a
BrukerAM-400 (400 MHz) spectrometer with CDCl or DMSO-d
4
–6
3
6
neonicotinoids are based on trans-configuration.
The MOA
as the solvent and TMS as the internal standard. Chemical shifts
are reported in d (parts per million) values. High-resolution mass
spectra were recorded under electron impact (70 eV) condition
using a MicroMass GCT CA 055 instrument. Analytical thin-layer
chromatography (TLC) was carried out on precoated plates
and binding sites of cis-configuration nitromethylene neonico-
tinoids are still not clear. Isotope-substituted Paichongding and
Cycloxaprid make it convenient to identify the MOA of these cis-
configuration neonicotinoids. At the same time, the synthesis of
labeled compounds is required in order to investigate their
metabolism, degradation and environmental behavior in detail
for future development. The use of radiolabeled compounds
presents a promising approach for the detection and accurate
analysis of compounds or products. Tritium, the general
radionuclide, provides an opportunity for achieving the minimal
a
Shanghai Key Laboratory of Chemical Biology, School of Pharmacy, East China
University of Science and Technology, Shanghai 200237, People’s Republic of China
b
Institute of Nuclear-Agricultural Science, Zhejiang University, Hangzhou
10029, Zhejiang Province, People’s Republic of China
3
disruption of the molecular structure, low raw material price
7
and convenient synthesis. Therefore, this paper describes the *Correspondence to: Zhong Li, Shanghai Key Laboratory of Chemical Biology,
3
synthesis of tritium-labeled Paichongding ([ H
]-Paichongding, School of Pharmacy, East China University of Science and Technology, 130
2
3
Meilong Road, Shanghai 200237, People’s Republic of China.
E-mail: lizhong@ecust.edu.cn
2
Scheme 1) and tritium-labeled Cycloxaprid ([ H ]-Cycloxaprid,
Scheme 2), which could be used as radiotracers for the study
of the MOA, metabolism, degradation and environmental
behavior.
y
Correspondence to: Qing-Fu Ye, Institute of Nuclear-Agricultural Science,
Zhejiang University, 268 Kaixuan Road, Hangzhou 310029, Zhejiang Province,
People’s Republic of China. E-mail: qfye@zju.edu.cn
J. Label Compd. Radiopharm 2011, 54 256–259
Copyright r 2011 John Wiley & Sons, Ltd.

C. Li et al.
2
-Chloro-5-hydroxymethylpyridine (2)
O N
2
dicyclic
3
To the solution of freshly prepared NaB H
0 mCi/mmol, 8.0 mmol, NaOH/MeOH, 0.1 M) in anhydrous
methanol (250 mL), 6-chloronicotinoyl chloride (1) (5 g,
8.6 mmol) was added in batches at room temperature (251C),
4
(52 mL, 80 mCi,
N
N
O
1
Cl
N
O N
2
2
Paichongding
N
NH
the reaction was kept for 4 h. After completion, the solution was
concentrated in vacuum to 60 mL and treated with aqueous
NaH PO (1.0 M, 100 mL), followed by extraction with dichloro-
2
methane (5 ꢂ 100 mL). The organic layers were combined,
dried over anhydrous Na SO overnight. After the solvent was
evaporated under reduced pressure, the residue was subjected
to flash chromatograph (ethyl acetate/hexane 1:4, v/v) to afford
white solid 2 (602 mg), yield 53%.
Cl
N
O N
2
oxabridged
4
O
N
N
6-Cl-PMNI
2
4
Cl
N
Cycloxaprid
Figure 1. Structures of 6-Cl-PMNI, Paichongding and Cycloxaprid.
2
-Chloro-5-chloromethylpyridine (3)
Thionyl chloride (15 mL) was added to the solution of 2 (602 mg)
in CHCl (30 mL), and the mixture was refluxing for 3 h and then
cooled to room temperature. The solvent and thionyl chloride
were removed under reduced pressure to give crude 3, which
was used in the next step without further purification.
O2N
O N
2
3
O
*
*
N
NH
N
N
OH
Cl
N
Cl
N
[3
H2]-6-Cl-PMNI
6
N-(2-Chloro-5-pyridylmethyl)ethylenediamine (4)
To a solution of ethylenediamine (25 mL) in acetonitrile (5 mL)
cooled in an salt ice bath (-15 to ꢁ101C) was added 3 in
acetonitrile (25 mL) dropwise by using a pressure equalizer
funnel under argon within 45 min. The mixture was allowed to
warm to room temperature and the reaction was continued
overnight. To the resulting solution was added water (20 mL)
followed by extraction with dichloromethane (6ꢂ 40mL). The
O2N
OH
*
N
N
O
Cl
N
[3
H2]-Paichongding
2 4
combined organic layers were dried over anhydrous Na SO , and
evaporated under reduced pressure to afford crude 4, which
could be used for the next step without further purification.
3
3
]-Paichongding with [ H
Scheme 1. Preparation of [ H
dehyde (Ã the label site).
2
2
]-6-Cl-PMNI and crotonal-
3
2
-Chloro-5-((2-(nitromethylene)imidazolidin-1-yl)½ H2ꢃ-
3
methyl)pyridine (½ H2ꢃ-6-Cl-PMNI)
O
O
O
A
6
mixture of l,l-bis(methylthio)-2-nitroethylene (1000 mg,
.1 mmol) and N-(2-Chloro-5-pyridylmethyl)ethylenediamine
1M HCl aq
in anhydrous ethanol (2 mL) was refluxed for 8 h affording a
clear yellow solution. The solvent was removed in vacuo after
cooling to room temperature. The crude product was purified
by silica gel chromatography using dichloromethane:acetone
(4:1, v/v) as an eluent to give white power with 820 mg,
yield 40%.
O N
2
O
O
O N
2
*
O
*
N
NH
N
N
Cl
N
Cl
N
3
1
-((6-Chaloropyridin-3-yl)½ H2ꢃ-methyl)-7-methyl-8-nitro-
1,2,3,5,6,7-hexahydroimidazo[1,2-a]pyridine-5-ol (6)
[3
H2]-Cycloxaprid
[3
H2]-6-Cl-PMNI
3
A mixture of [ H ]-6-Cl-PMNI (254 mg, 1 mmol), anhydrous
2
acetonitrile (3 mL), crotonaldehyde (0.14 mL, 1.7 mmol) and
acetic acid (0.09 mL) was stirred at 40–451C for 26 h and the
solvent was evaporated. The crude was purified by silica gel
chromatography using dichloromethane: acetone (1:2, v/v)
3
3
2 2
Scheme 2. Preparation of [ H ]-Cycloxaprid with [ H ]-6-Cl-PMNI and butanedial
(Ã the label site).
giving 131 mg of the pure material in 40% chemical yield,
1
(
silica gel 60 F254), and spots were visualized with ultraviolet mp = 171.1–175.61C; HNMR (400 Mz, DMSO-d ): d 8.34 (d,
6
light. X-ray diffraction was performed with a Bruker Smart 1000. J = 2.1 Hz, 1H), 7.80 (dd, J1 = 2.4 Hz, J2 = 8.3 Hz, 1H), 7.49
TLC was performed with GF254 plates and radioactive TLC plates (d, J = 8.2 Hz, 1H), 6.29 (br d, J = 6.2 Hz, 1H), 4.82 (m, 1H), 4.73
were scanned on Fujifilm BAS-1800II laser-based fluorescence (d, J = 15.5 Hz, 1H), 4.59 (d, J = 15.5 Hz, 1H), 3.58–3.74 (m, 4H),
and radioisotope imaging system (Fuji Co., Japan). Radioactivity 3.11–3.16 (m, 1H), 1.88–1.95 (m, 1H), 1.69–1.75 (m, 1H), 1.03 (d,
1
3
was measured on WinSpectral-1414 liquid scintillation spectro- J = 6.6 Hz, 1H) ppm; CNMR (400 Mz, DMSO-d
meter (Wallac, Finland).
6
):d158.5, 149.6,
139.7, 132.6, 74.4, 109.3, 76.0, 51.9, 49.8, 45.2, 38.6, 28.3,
J. Label Compd. Radiopharm 2011, 54 256–259
Copyright r 2011 John Wiley & Sons, Ltd.
www.jlcr.org

C. Li et al.
1
0.1 ppm; HRMS(EI ) calcd for C H N O Cl (M ), 324.0989; monitored by TLC. Then the solution was neutralized with a
35
1
2
14 17 4 3
3
7
1
17 4 3 3
found, 324.1014. calcd for C14H N O Cl (M ), 3 26.0960; found, saturated solution of NaHCO and extracted with dichloro-
3
1
26.0964; Anal. Calcd for C14
H
7.25, Found: C, 51.91; H, 5.15; N, 16.95.
17ClN
4
O
3
: C, 51.78; H, 5.28; N, methane, the combined extracts were dried over anhydrous
Na SO , filtered and concentrated in vacuo to give an oily residue.
2 4
1
Purification by silica gel chromatography using dichloromethane:
acetone (4:1, v/v) as an eluent giving 165 mg of a canary yellow
solid, yield 51%. mp= 149.0–150.01C; HNMR (400Mz, DMSO-
3
1
methyl-8-nitro-5-propoxy-1,2,3,5,6,7-hexahydroimida-
-((6-Chaloropyridin-3-yl)½ H2ꢃ-methyl)-5-propyloxy-7-
1
3
zo[1,2-a]pyridine (½ H2ꢃ-Paichongding)
d
7
4
6 1 2
):d 8.35 (d, J = 2.4 Hz, 1H), 7.81 (dd, J = 2.4 Hz, J = 8.4 Hz, 1H),
.51 (d, J = 8.4 Hz, 1H), 5.36–5.39 (s, 2H), 5.00 (d, J = 15.6Hz, 1H),
.68 (d, J = 15.6Hz, 1H), 3.57–3.73 (m, 4H), 1.94–2.04 (m, 4H) ppm;
To the above compound 6 (131 mg, 0.4 mmol) in dichloro-
methane was added propan-1-ol (0.84 mL, 11.25 mmol) and
hydrochloric acid (0.08 mL), the mixture was refluxed and stirred
for 36 h, then the solvent was removed and the residue was
purified by column chromatography (dichloromethane:ace-
tone = 4:1, v/v) affording a pure yellow solid in 55% yield.
1
mp = 130.2-131.91C; HNMR (400 Mz, CDCl
1
3
CNMR (400Mz, DMSO-d ):d 155.6, 149.7, 149.6, 139.7, 132.6,
6
1
24.5, 109.6, 87.0, 75.1, 51.2, 50.3, 46.6, 31.9,31.7 ppm; HRMS
35
1
ESI ) calcd for
1
Cl(M1H) , 323.0911; found,
(
C H N O
14 16 4 3
3
3
7
1
23.0912. calcd for C H N O Cl (M1H) , 325.0811; found,
3
325.0895. calcd for
1
4
16
4
3
5
1
ClNa(M1Na) , 345.0730;
C
14
H
16
N
4
O
N
3
3
):d 8.31–8.33 (m, 1H),
3
7
1
ClNa(M1Na) , 347.0701;
found, 345.0722. cacld for C14
found, 347.0692.
H
16
4
O
3
7.86 (dd, J = 8.4 Hz, 0.5H), 7.82 (dd, J = 2.4 Hz,
J = 8.2 Hz, 0.5H), 7.33 (dd, J = 0.8 Hz, J = 1.2 Hz, 0.5H), 7.31
1
= 2.4 Hz, J
2
1
2
2
1
2
(
(
1 2
dd, J = 0.8 Hz, J = 1.2 Hz, 0.5H), 4.88 (d, J = 15.2 Hz, 0.5H), 4.82
3
d, J = 15.2 Hz, 0.5H), 4.61–4.67 (m, 1H), 4.55 (dd, J
1
= 3.6 Hz, The chemical and radiochemical purities of ½ H2ꢃ-Paichongd-
3
J2 = 6.0 Hz, 0.5H), 4.50 (t, J = 3.6 Hz, 0.5H), 3.95–4.02 (m, 0.5H), ing and ½ H2ꢃ-Cycloxaprid
3
(
2
.81–3.86 (m, 0.5H), 3.64–3.72 (m, 1H), 3.35–3.59 (5H), 2.14–2.21
m, 0.5H), 1.99–2.01 (m, 1H), 1.75-1.82 (m, 0.5H), 1.54–1.65 (m,
H), 1.22–1.27 (d, J = 6.8 Hz, 0.5H), 1.23 (d, J = 6.8 Hz, 0.5H), 0.95
The chemical purity was determined by HPLC. HPLC condition:
Diamonsil C18 (2) 5 mm, 250 ꢂ 4.6 mm, solvent: acetonitrile/
3
water = (65:35, v/v) for [ H
1
2
]-Paichongding and (75:25, v/v)
(
t, J = 6.2 Hz, 0.5H), 0.92 (t, J = 6.2 Hz, 0.5H) ppm; HRMS(EI ) calcd
1
3
for [ H ]-Cycloxaprid, flow: 1 mL/min. The retention time of
3
3
5
1
Cl (M1) , 366.1459; found, 366.1487.
2
for C17
H
23
N
4
O
3
[
[
H ]-Paichongding was 6.87 min, 12.16 min for its isomers, and
2
3
3
]-Cycloxaprid was 6.44 min. The chemical purity of [ H
H
2
2
]-
]-Cycloxaprid was 98.7 and 98.4%,
3
5
-((6-Chloro-pyridin-3-yl)½ H2ꢃ-methyl)-7-nitro-11-oxa-2,5-
3
2
,6
3
Paichongding and [ H
respectively.
2
diaza-tricyclo[6.2.1.0 ]undec-6-ene (½ H2ꢃ-Cycloxaprid)
To a mixture of tetrahydro-2,5-dimethoxy-furan (0.48 mL,
.7 mmol) and [ H
The radiochemical purity was measured with HPLC-LSC
2
]-6-Cl-PMNI (259mg, 1.02mmol) in acetonitrile method, which is operated as follows: all the components were
3
3
(
3mL) was added hydrochloric acid (1.8mL, 1 mol/L), the resulting collected by flashing bottle after separation of labeled
0
solution was stirred at 351C for 4 h and the reaction was compounds by HPLC, the radioactivity of all the components(A )
O
3
NaB H
4
O
Cl
N
*
*
SOCl2
OH
Cl
Cl
N
Cl
N
O
3
NaB H
4
Cl
2
3
Cl
N
1
O N
S
S
O2N
2
*
NH2
*
N
H
N
NH
Cl
N
Cl
N
NH2
4
H N
2
[3
H2]-6-Cl-PMNI
N
Cl
H
*
N
N
H
*
Cl
N
5
3
Scheme 3. Preparation of [ H
3
]-6-Cl-PMNI with 6-chloronicotinoyl chloride (1) and NaB H
2
4
(Ã the label site).
www.jlcr.org
Copyright r 2011 John Wiley & Sons, Ltd.
J. Label Compd. Radiopharm 2011, 54 256–259

C. Li et al.
and the target component(A ) was measured with LSC. The ratio prepared by 2,5-dimethoxytetrahydrofuran. The conversion of
e
8
of A
purity of [ H
8.6%, respectively.
e
and A
0
indicates the radiochemical purity. Radiochemical Cycloxaprid can also be achieved by the reaction of 6-Cl-PMNI
3
3
2
]-Paichongding and [ H
2
]-Cycloxaprid was 97.3 and and 2,5-dimethoxytetrahydrofuran in one pot. After the reaction
was completed, the crude product was purified by silica gel
column chromatography twice. The overall chemical and
radiochemical yield was 20.7 and 29.7%, respectively. The
chemical purity was 98.4%, while the radiochemical purity was
9
Results and discussion
The radiosynthesis of tritium-labeled 2-chloro-5-((2-(nitromethy- 98.6%. The specific activity was 14.4 mCi/mmol.
lene) imidazolidin -1-yl)methyl)pyridine (6-Cl-PMNI) (Figure 1) has
9
been reported by Latli et al., which involves high specific activity Acknowledgements
3
NaB H and a multi-step microsynthesis. It required microreactor
4
and difficult operation to achieve the final resolution. In this This work was financial supported by National Basic Research
3
paper, a high-specific radioactivity methanolic NaB H
4
solution Program of China (973 Program, 2010CB126104), Ministry of
was first diluting by non-labeled NaBH , and this was taken to Agriculture of China (200803034), National High Technology
4
synthesize the target compounds offering the characteristics of a Research and Development Program of China (863 Program
simple operation, high utilization ratio and providing sufficient 2010AA10A204) and National Science Foundation China Pro-
quantities for further study. Various starting materials have been gram Grant (20872034). This work was also partly supported by
reported in introducing the tritium atoms into the flexible Science Foundation of Chinese University, Shanghai Foundation
9
–11
linkage.
The reduction of 1 by sodium borohydride in of Science and Technology (09391911800, 09XD1401300) and
anhydrous methanol achieved higher radiochemical yield. As Shanghai Leading Academic Discipline Project, Project Number:
illustrated in the Scheme 3, compound 1 was reduced in B507. 111 Project (No. B07023).
(80 mCi, specific activity 10mCi/mmol)
3
methanol using NaB H
4
to give 2 after 3 h at room temperature. Refluxing with excess
thionyl dichloride in chloroform, compound 2 was converted to 3
smoothly. When compound 3 was added dropwise into
ethylenediamine in acetonitrile to obtain 4, slow addition and
low temperature were required to avoid the by-product 5 being
easily formed. After the completion of the reaction, excess
References
[
[
1] X. S. Shao, W. W. Zhang, Y. Q. Peng, Z. Li, Z. Z. Tian, X. H. Qian,
Bioorg. Med. Chem. Lett. 2008, 18, 6513–6516.
2] X. S. Shao, H. Fu, X. Y. Xu, X. L. Xu, Z. W. Liu, Z. Li, X. H. Qian,
J. Agric. Food. Chem. 2010, 58, 2696–2702.
ethylenediamine was removed under reduced pressure to provide
3
the crude compound 4. Finally, the desired product [ H ]-6-Cl-
[3] X. Xu, H. Bao, X. S. Shao, Y. Zhang, X. Yao, Z. Liu, Z. Li, Insect. Mol.
Biol. 2010, 19, 1–8.
2
PMNI was afforded by the condensation of
bis(methylthio)-2-nitroethylene in ethanol under reflux.
The synthesis of Paichongding and Cycloxaprid has been
4 with l,l-
[
[
[
[
4] M. Tomizawa, I. Yamamoto, Nihon. Noyaku. Gakkaishi. 1993, 18,
91–98.
5] S. Kagabu, H. Matsuno, J. Agric. Food. Chem. 1997, 45,
276–281.
6] M. Tomizawa, D. L. Lee, J. E. Casida, J. Agric. Food. Chem. 2000, 48,
1
,2
reported in the earlier literature. Reaction of half of the fresh
3
2
synthesized [ H ]-6-Cl-PMNI with crotonaldehyde in acetonitrile
6016–6024.
afforded 6, which was treated with propan-1-ol in acetonitrile,
purified by silica gel column chromatography twice to prepare
7] J. A. Kozocas, J. E. Bupp, M. J. Tanga, J. T. Pluhar, I. W. Wainer,
J. Label. Compd. Radiopharm. 2010, 53, 68–72.
3
the product [ H
2
]-Paichongding. The overall chemical and
[8] Z. M. Yang, Q. F. Ye, Q. H. Tang, L. Lu, Z. Y. Chen, Acta. Chim. Sin.
2005, 63, 1999–2003.
radiochemical yield was 8.8 and 10.5%, respectively. The
chemical purity was 98.7% and the radiochemical purity was
[9] B. Latli, C. Than, H. Morimoto, P. G. Williams, J. E. Casida, J. Label.
Compd. Radiopharm. 1996, 38, 971–978.
9
7.3%. The specific activity was 11.9 mCi/mmol.
3
The other half of [ H
[
[
10] C. L. K. Lee, T. P. Loh, Org. Lett. 2005, 7, 2965–2967.
11] B. latli, J. E. Casida, J. Label. Compd. Radiopharm. 1992, 31,
609–613.
2
]-6-Cl-PMNI reacted with butanedial to
3
obtain [ H ]-Cycloxaprid, and the butanedial needs to be freshly
2
J. Label Compd. Radiopharm 2011, 54 256–259
Copyright r 2011 John Wiley & Sons, Ltd.
www.jlcr.org