Synthesis of novel nithiazine analogues containing the 1,4-dihydropyridine structure, and their bioactivity as insecticides

Article abstract of DOI:10.1007/s11164-013-1020-8

A series of novel nithiazine analogues bearing the 1,4-dihydropyridine structure have been designed and synthesized by reaction of nithiazine, malononitrile or ethyl cyanoacetate, and benzaldehyde. The synthesized compounds were identified by ¹

Full text of DOI:10.1007/s11164-013-1020-8

Res Chem Intermed
DOI 10.1007/s11164-013-1020-8
Synthesis of novel nithiazine analogues containing
the 1,4-dihydropyridine structure, and their bioactivity
as insecticides
•
Zhongzhen Tian Shuxia Cui Zhiping Xu
•
Received: 6 December 2012 / Accepted: 3 January 2013
Ó Springer Science+Business Media Dordrecht 2013
Abstract A series of novel nithiazine analogues bearing the 1,4-dihydropyridine
structure have been designed and synthesized by reaction of nithiazine, malono-
nitrile or ethyl cyanoacetate, and benzaldehyde. The synthesized compounds were
1
identified by H NMR and IR spectroscopy, and elemental analysis. Preliminary
bioassays indicated that most of the compounds had moderate insecticidal activity
against Aphis craccivora. The relationship between molecular structure and bio-
logical activity is discussed.
Keywords Nithiazine Á Neonicotinoid insecticides Á 1,4-Dihydropyridine Á
Biological activity
Introduction
Since the introduction of imidacloprid in 1990s [1], the neonicotinoid insecticides
have rapidly become an important chemical class of insecticides. Similarly to the
naturally occurring alkaloid nicotine, all neonicotinoids act selectively on the insect
central nervous system (CNS) as agonists of post-synaptic nicotinic acetylcholine
receptors (nAChRs) [2–5]. Neonicotinoid insecticides are characterized by persis-
tent effects, broad range, good systemic properties, and low toxicity to mammals
Z. Tian (&) Á S. Cui
Shandong Provincial Key Laboratory of Fluorine Chemistry and Chemical Materials,
School of Chemistry and Chemical Engineering, University of Jinan, 106 Jiwei Road,
Jinan 250022, People’s Republic of China
e-mail: chm_tianzz@ujn.edu.cn
Z. Xu
Shanghai Key Laboratory of Chemical Biology, School of Pharmacy, East China University
of Science and Technology, Shanghai 200237, People’s Republic of China
123

Z. Tian et al.
and aquatic life [6, 7]. Hence, the development of neonicotinoid insecticides has
been a promising innovation in pesticides.
Nithiazine (tetrahydro-2-(nitromethylene)-2H-1,3-thiazine) was discovered by
Shell in 1978. Electrophysiological radioligand binding assays showed that
nithiazine and related nitromethylenes targeted insect nAChRs [8]. Compared with
imidacloprid, nithiazine had greater activity against housefly adults and corn
earworm larvae, and lower mammalian toxicity. However, owing to its rapid
degradation under both hydrolytic and photolytic conditions (induced by the
unstable 2-nitromethylene chromophore; electronic absorption kmax = 343 nm) in
field trials, nithiazine was never commercialized for broad agricultural use [9, 10].
It has, however, been reported that the 2-nitromethylene chromophore may
effectively improve photostability [11–13]. Herein, our interest is in the design of a
novel nithiazine family by introducing the 1,4-dihydropyridine structure into the
lead compound to overcome its photoinstability, and synthesis of a series of new
nithiazine analogues. The 1,4-dihydropyridine structure is frequently used as a
subunit of antiatherosclerotic, anticancer, and antidiabetic drugs [14–17]. It can be
prepared conveniently by multicomponent reactions (MCRs) using nithiazine, aryl
carbonyl compounds, and malononitrile as starting materials.
1
Compound structures were characterized by IR and H NMR spectroscopy and
elemental analysis. Preliminary bioassays indicated that most of the new compounds
had moderate insecticidal activity against Aphis craccivora (Fig. 1).
Results and discussion
Synthesis
Nithiazine analogues bearing the 1,4-dihydropyridine structure (3, 4) were
synthesized as shown in Scheme 1. The search for one-pot condensation reaction
conditions was started by reaction of nithiazine, malononitrile or ethyl cyanoacetate
(1), and benzaldehyde (2) (molar ratio 1:1.3:1.3) for 15–20 h in boiling ethanolic
piperidine. The purity of the compounds was checked by TLC and elemental
analysis. Spectral data (IR, 1H NMR) of the newly synthesized compounds 3, 4 were
in full agreement with their proposed structures. The IR spectra of compounds 3 and
4 contained characteristic peaks at approximately 1,674 and 2,192 cm-1, because of
the presence of ester (–COOEt) and nitrile (–C:N) groups, and a peak ascribed
1
to –N–H stretch appeared in the region 3,222–3,440 cm-1. In H NMR spectra of
compounds 3 and 4, the protons of –NH2 of the dihydropyridine ring resonate at
approximately d 6.2 ppm.
Fig. 1 Structures of
imidacloprid and nithiazine
123

Synthesis and bioactivity of novel nithiazine analogues
The mechanism proposed for the reaction is shown in Scheme 2. Assembly of 3
and 4 can be explained via initial Michael addition of 5 to the ylidenic bond in
nithiazine, leading to the formation of an acyclic intermediate, 6, which cyclizes to
form the intermediate 7 via nucleophilic attack of the NH group on the cyano
carbon; this is followed by tautomerization to the final products (3, 4). Formation of
5 is via Knoevenagel condensation of 1 with an appropriate aromatic aldehyde 2.
Insecticidal activity
The compounds were screened for insecticidal activity against A. craccivora. Most
had moderate insecticidal activity at 500 mg L-1. The results are listed in Table 1.
Compounds 3a, 3b, 3c, 3d, and 4b had 50–90 % insecticidal activity (i.e. mortality)
at 500 mg L-1, and compounds 3e and 4c had 30–50 % activity at 500 mg L-1
.
Among these products, 3b had the highest insecticidal activity—53.1 % bioactivity
at 125 mg L-1. Comparison of the two groups of compounds revealed compounds 3
had greater insecticidal activity; a possible reason is that the nitrile group had
stronger electron-withdrawing activity and smaller volume.
Experimental
Physical measurements
All melting points (mp) were obtained with a Bu¨chi melting point B540 and are
uncorrected. 1H NMR spectra were recorded on a Bruker AV-400 (400 MHz)
spectrometer with TMS as internal reference. Chemical shifts are reported in d
(parts per million) values. Combustion analysis for determination of elemental
composition was performed with an Elementar vario EL III. Infrared spectra were
measured on a Nicolet FT-IR-20SX instrument as potassium bromide (KBr) disks.
All chemicals and reagents were purchased from standard commercial suppliers.
General procedure for preparation of compounds 3a–e and 4a–e
A solution of malononitrile or ethyl cyanoacetate (10 mmol) in anhydrous alcohol
(15 mL) was added dropwise to a solution of aryl aldehyde (10 mmol) in anhydrous
Scheme 1 Synthetic route to compounds 3 and 4
123

Z. Tian et al.
Scheme 2 Mechanism proposed for formation of 3 and 4
Table 1 Insecticidal activity of new compounds 3a–e and 4a–e against Aphis craccivora
Entry
Y
R
Concentration (mg L-1
)
Mortality (%)
3a
3b
CN
CN
H
500
500
125
500
500
500
500
500
500
500
500
50.2
90.2
53.1
61.7
15.0
33.4
69.8
35.3
39.2
10.7
59.4
CH3
3c
3d
3e
4a
4b
4c
4d
4e
CN
OCH3
NO2
Cl
CN
CN
COOEt
COOEt
COOEt
COOEt
COOEt
H
CH3
OCH3
NO2
Cl
alcohol (15 mL) at room temperature. After stirring for 5 min, piperidine
(0.1 mmol), used as catalyst, was added dropwise. The resulting mixture was
stirred for another 2 h. Nithiazine (7.5 mmol) was then added and the reaction
mixture was heated under reflux for 15–20 h, then cooled to room temperature. The
solid crystalline products were isolated by filtration and washed with CH2Cl2.
The crude products were purified by recrystallization from ethanol (95 %) to give
the pure compounds.
123

Synthesis and bioactivity of novel nithiazine analogues
6-Amino-9-nitro-8-phenyl-2,3,4,8-tetrahydropyrido[2,1-b][1, 3]thiazine-7-carboni-
1
trile (3a) Yield: 79 %, m.p. 214.6–215.3 °C; H NMR (400 MHz, DMSO-d6):
d (ppm) 1.99–2.07 (m, 1H), 2.46–2.49 (m, 1H), 2.88–2.95 (m, 1H), 3.01–3.06 (m,
1H), 3.49–3.55 (m, 1H), 4.03–4.10 (m, 1H), 4.87 (s, 1H, PhCH), 6.56 (s, 2H, NH2),
7.12 (d, J = 8.4 Hz, 2H, PhH), 7.24 (t, J = 7.2 Hz, 1H, PhH), 7.32 (t, J = 7.2 Hz,
2H, PhH); IR (KBr/cm-1) m: 3,394, 3,207, 3,027, 2,943, 2,912, 2,188, 1,660, 1,610,
1,500, 1,432, 767, 710; Anal. Calcd for C15H14N4O2S: C, 57.31; H, 4.49; N, 17.82,
Found: C, 57.21; H, 4.50; N, 17.87.
6-Amino-9-nitro-8-(p-tolyl)-2,3,4,8-tetrahydropyrido[2,1-b][1, 3]thiazine-7-carbo-
1
nitrile (3b) Yield: 71 %; m.p. 212.7–213.4 °C; H NMR (400 MHz, DMSO-d6):
d (ppm) 1.98–2.06 (m, 1H), 2.25 (s, 3H, CH3), 2.87–2.94 (m, 1H), 3.00–3.05 (m,
1H), 3.46–3.53 (m, 1H), 4.03–4.09 (m, 1H), 4.83 (s, 1H, PhCH), 6.54 (s, 2H, NH2),
7.00 (d, J = 8.0 Hz, 2H, PhH), 7.12 (d, J = 8.0 Hz, 2H, PhH); IR (KBr/cm-1
)
m: 3,404, 3,325, 3,034, 2,966, 2,921, 2,192, 1,662, 1,608, 1,490, 1,421, 854; Anal.
Calcd for C16H16N4O2S: C, 58.52; H, 4.91; N, 17.06, Found: C, 58.57; H, 4.96; N,
17.01.
6-Amino-8-(4-methoxyphenyl)-9-nitro-2,3,4,8-tetrahydropyrido[2,1-b][1,3]thia-
1
zine-7-carbonitrile (3c) Yield: 60 %, m.p. 208.8–209.8 °C; H NMR (400 MHz,
DMSO-d6): d (ppm) 1.98–2.06 (m, 1H), 2.46–2.48 (m, 1H), 2.87–2.94 (m, 1H),
2.99–3.05 (m, 1H), 3.47–3.54 (m, 1H), 3.71 (s, 3H, OCH3), 4.03–4.10 (m, 1H), 4.82
(s, 1H, PhCH), 6.53 (s, 2H, NH2), 6.88 (d, J = 8.8 Hz, 2H, PhH), 7.03 (t,
J = 8.8 Hz, 2H, PhH); IR (KBr/cm-1) m: 3,400, 3,026, 2,968, 2,925, 2,194, 1,660,
1,608, 1,490, 1,421, 848; Anal. Calcd for C16H16N4O3S: C, 55.80; H, 4.68; N,
16.27, Found: C, 55.86; H, 4.70; N, 16.26.
6-Amino-9-nitro-8-(4-nitrophenyl)-2,3,4,8-tetrahydropyrido[2,1-b][1,3]thiazine-7-
carbonitrile (3d) Yield: 87 %; m.p. 229.8–230.5 °C; 1H NMR (400 MHz, DMSO-
d6): d (ppm) 2.03–2.10 (m, 1H), 2.46–2.47 (m, 1H), 2.91–2.98(m, 1H), 3.03–3.09
(m, 1H), 3.55–3.62 (m, 1H), 4.01–4.07 (m, 1H), 5.06 (s, 1H, PhCH), 6.70 (s, 2H,
NH2), 7.42 (d, J = 8.8 Hz, 2H, PhH), 8.20 (d, J = 8.4 Hz, 2H, PhH); IR (KBr/
cm-1) m: 3,431, 3,338, 3,242, 3,080, 2,975, 2,933, 2,190, 1,660, 1,620, 1,513, 1,432,
1,346, 813; Anal. Calcd for C15H13N5O4S: C, 50.13; H, 3.65; N, 19.49, Found: C,
50.20; H, 3.60; N, 19.51.
6-Amino-8-(4-chlorophenyl)-9-nitro-2,3,4,8-tetrahydropyrido[2,1-b][1,3]thiazine-
7-carbonitrile (3e) Yield: 83 %; m.p. 216.4–217.4 °C; 1H NMR (400 MHz,
DMSO-d6): d (ppm) 1.98–2.08 (m, 1H), 2.44–2.48 (m, 1H), 2.88–2.95 (m, 1H),
3.01–3.06 (m, 1H), 3.50–3.57 (m, 1H), 4.01–4.08 (m, 1H), 4.60 (s, 1H, PhCH), 6.62
(s, 2H, NH2), 7.15 (d, J = 8.4 Hz, 2H, PhH), 7.38 (d, J = 8.4 Hz, 2H, PhH); IR
(KBr/cm-1) m: 3,431, 3,334, 3,236, 3,035, 2,935, 2,190, 1,658, 1,616, 1,504, 1,425,
831; Anal. Calcd for C15H13ClN4O2S: C, 51.65; H, 3.76; N, 16.06, Found: C, 51.68;
H, 3.81; N, 16.11.
Ethyl 6-amino-9-nitro-8-phenyl-2,3,4,8-tetrahydropyrido[2,1-b][1,3]thiazine-7-
carboxylate (4a) Yield: 74 %; m.p. 147.9–148.5 °C; 1H NMR (400 MHz,
CDCl3): d (ppm) 1.26 (t, J = 7.0 Hz, 3H, CH3), 2.17–2.25 (m, 1H), 2.40–2.47
123

Z. Tian et al.
(m, 1H), 2.85–2.99 (m, 2H), 3.67–3.74 (m, 1H), 3.98–4.04 (m, 1H), 4.15 (q,
J = 7.1 Hz, 2H, CH2), 5.57 (s, 1H, PhCH), 6.21 (s, 2H, NH2), 7.15–7.26 (m, 5H,
PhH); IR (KBr/cm-1) m: 3,440, 3,222, 3,072, 3,026, 2,983, 2,933, 1,678, 1,629,
1,550, 1,490, 1,431, 1,245, 1,122, 754, 700; Anal. Calcd for C17H19N3O4S: C,
56.50; H, 5.30; N, 11.63, Found: C, 56.42; H, 5.22; N, 11.70.
Ethyl 6-amino-9-nitro-8-(p-tolyl)-2,3,4,8-tetrahydropyrido[2,1-b][1,3]thiazine-7-
carboxylate (4b) Yield: 65 %; m.p. 140.8–141.6 °C; 1H NMR (400 MHz,
CDCl3): d (ppm) 1.26 (t, J = 3.6 Hz, 3H, CH3), 2.19–2.31 (m, 1H), 2.25 (s, 3H),
2.41–2.46 (m, 1H), 2.86–2.97 (m, 2H), 3.67–3.75 (m, 1HS), 3.98–4.05 (m, 1H), 4.14
(q, J = 7.5 Hz, 2H, CH2), 6.54 (s, 1H, PhCH), 6.17 (s, 2H, NH2), 7.04 (d,
J = 8.4 Hz, 2H, PhH), 7.08 (d, J = 8.0 Hz, 2H, PhH); IR (KBr/cm-1) m: 3,415,
3,230, 3,026, 2,977, 2,925, 2,871, 1,674, 1,552, 1,508, 1,425, 1,251, 1,124, 842;
Anal. Calcd for C18H21N3O4S: C, 57.58; H, 5.64; N, 11.19, Found: C, 57.47; H,
5.72; N, 11.09.
Ethyl 6-amino-8-(4-methoxyphenyl)-9-nitro-2,3,4,8-tetrahydropyrido[2,1-b]
[1,3]thiazine-7-carboxylate (4c) Yield: 61 %; m.p. 173.9–175.4 °C; 1H NMR
(400 MHz, CDCl3): d(ppm) 1.26 (t, J = 7.0 Hz, 3H, CH3), 2.17–2.26 (m, 1H),
2.39–2.48 (m, 1H), 2.84–2.98 (m, 2H), 3.67–3.74 (m, 1H), 3.75 (s, 3H, OCH3),
3.98–4.04 (m, 1H), 4.14 (q, J = 7.2 Hz, 2H, CH2), 5.51 (s, 1H, PhCH), 6.20 (s, 2H,
NH2), 6.77 (d, J = 8.8 Hz, 2H, PhH), 7.11 (d, J = 8.8 Hz, 2H, PhH); IR (KBr/
cm-1) m: 3,438, 3,290, 3,039, 2,983, 2,933, 2,898, 2,835, 1,672, 1,639, 1,504, 1,432,
1,245, 1,108, 829; Anal. Calcd for C18H21N3O5S: C, 55.23; H, 5.41; N, 10.73,
Found: C, 55.27; H, 5.42; N, 10.77.
Ethyl 6-amino-9-nitro-8-(4-nitrophenyl)-2,3,4,8-tetrahydropyrido[2,1-b][1,3]thia-
1
zine-7-carboxylate (4d) Yield: 72 %; m.p. 206.4–207.2 °C; H NMR (400 MHz,
CDCl3): d (ppm) 1.25 (t, J = 3.6 Hz, 3H, CH3), 2.23–2.30 (m, 1H), 2.46–2.53 (m,
1H), 2.9–3.04 (m, 2H), 3.74–3.81 (m, 1H), 3.99–4.05 (m, 1H), 4.15 (q, J = 7.2 Hz,
2H, CH2), 5.66 (s, 1H, PhCH), 6.31 (s, 2H, NH2), 7.37 (d, J = 8.4 Hz, 2H, PhH),
8.11 (d, J = 8.8 Hz, 2H, PhH); IR (KBr/cm-1) m: 3,415, 3,297, 3,234, 3,107, 3,076,
2,972, 2,935, 2,904, 1,674, 1,631, 1,548, 1,510, 1,425, 1,348, 1,247, 1,130, 827;
Anal. Calcd for C17H18N4O6S: C, 50.24; H, 4.46; N, 13.79, Found: C, 50.17; H,
4.37; N, 13.89.
Ethyl 6-amino-8-(4-chlorophenyl)-9-nitro-2,3,4,8-tetrahydropyrido[2,1-b][1,3] thi-
azine-7-carboxylate (4e) Yield: 80 %; m.p. 183.1–183.9 °C; 1H NMR (400 MHz,
CDCl3): d (ppm) 1.25 (t, J = 3.4 Hz, 3H, CH3), 2.18–2.28 (m, 1H), 2.41–2.49 (m,
1H), 2.887–3.003(m, 2H, NCH2CH2CH2S), 3.684–3.750(m, 1H, 3.98–4.04 (m, 1H),
4.14 (q, J = 7.1 Hz, 2H, CH2), 5.54 (s, 1H, PhCH), 6.23 (s, 2H, NH2), 7.13 (d,
J = 8.4 Hz, 2H, PhH), 7.20 (d, J = 8.4 Hz, 2H, PhH); IR (KBr/cm-1) m: 3,369,
3,226, 3,041, 2,979, 2,937, 2,902, 1,672, 1,641, 1,575, 1,513, 1,486, 1,441, 1,242,
1,118, 835; Anal. Calcd for C17H18ClN3O4S: C, 51.58; H, 4.58; N, 10.61, Found: C,
51.63; H, 4.52; N, 10.57.
123

Synthesis and bioactivity of novel nithiazine analogues
Biology assay
All compounds were dissolved in acetone and diluted with water containing Triton
X-100 (0.1 mL L-1) to obtain a series concentrations of 500.0 mg L-1 and others
for bioassays.
Pea aphids (A. craccivora) were dipped in accordance with a slightly modified
FAO dip test. Tender shoots of soybean with 40–60 healthy apterous adults were
dipped for 5 s into dilute solutions of the chemicals containing Triton X-100
(0.1 mL L-1), the superfluous fluid was removed, and the shoots were placed in a
conditioned room (23 1 °C, RH 50 %). Water containing Triton X-100
(0.1 mL L-1) was used as control. Mortality was assessed after 24 h, and the data
were corrected and subjected to probit analysis.
Conclusion
Synthesis of a variety of nithiazine derivatives was successfully achieved by use of
MCRs. The synthesized compounds were identified by 1H NMR and IR
spectroscopy and elemental analysis. Preliminary bioassays indicated that most of
the compounds had moderate insecticidal activity against A. craccivora at
500 mg L-1. Study of structure–activity relationships showed the nitrile group
was more important than the ester group for high biological activity.
Acknowledgments We thank the National Science Foundation China Program (grant 20902037), the
Promotive Research Fund for Excellent Young and Middle-aged Scientists of Shandong Province (grant
BS2010NY001), and the Opening Fund of Shanghai Key Laboratory of Chemical Biology (grant
SKLCB-2008-08).
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