Prodrug-oriented molecular design of neonicotinoids: Preparation of imidacloprid-related 5,5-dimethoxy-1,3-diazacyclohexane derivatives and their insecticidal activity

Article abstract of DOI:10.1271/bbb.69.705

Prodrug-oriented molecular design was attempted for the potent acyclic neonicotinoid insecticide, clothianidin (1-(2-chloro-5-thiazolylmethyl)-3- methyl-2-nitroguanidine). Molecules bearing a CH₂COCH₂ bridge linking the 1,3-NH ends o

Full text of DOI:10.1271/bbb.69.705

Biosci. Biotechnol. Biochem., 69 (4), 705–713, 2005
Prodrug-Oriented Molecular Design of Neonicotinoids: Preparation of
Imidacloprid-Related 5,5-Dimethoxy-1,3-diazacyclohexane Derivatives
and Their Insecticidal Activity
y
2
Shinzo KAGABU,1; Manabu HIBI,1 and Keiichiro NISHIMURA
1Department of Chemistry, Faculty of Education, Gifu University, Gifu 501-1193, Japan
2Research Institute for Advanced Science and Technology, Osaka Prefecture University, Osaka 599-8570, Japan
Received September 27, 2004; Accepted November 29, 2004
Prodrug-oriented molecular design was attempted
for the potent acyclic neonicotinoid insecticide, clothia-
nidin (1-(2-chloro-5-thiazolylmethyl)-3-methyl-2-nitro-
guanidine). Molecules bearing a CH2COCH2 bridge
linking the 1,3-NH ends of clothianidin or their acetals
would possibly be hydrolyzed, regenerating the mother
compounds. This strategy was used to prepare seven
acetals of clothianidin-based molecules that combined 2-
chloro-5-thiazolylmethyl, 6-chloro-3-pyridylmethyl or
3-tetrahydrofurfuryl with a nitroimine, cyanoimine
or nitromethylene group. The key intermediate, 1,3-
diamino-2,2-dimethoxypropane, was prepared from the
dihydroxyacetone dimer in four steps. A selected acetal
showed a characteristic nerve-impulse pattern for
neonicotinoids on an excised American cockroach
ganglion, although the neuroblocking activity was fairly
low. Some acetals were highly insecticidal against the
pea aphid at 0.8–20 ppm 7 days after a spray treatment,
this being in a contrast to their far weaker activity by
injection into American cockroaches. The biological
results suggest that the intrinsic insecticidal activities of
the acetals are weak, but would exhibit enhanced
activity if hydrolyzed in an external environment.
strategy. In fact, clothianidin can be readily regenerated
from these molecules under hydrolytic conditions
(Fig. 1, eq. 1).2) The oxadiazine derivative, thiameth-
oxam (5), can be viewed as a structure by the same type
of modification and, in principle, this skeleton can be
hydrolyzed, leaving clothianidin.4) Nevertheless, in spite
of the formally similar appearance, thiamethoxam seems
to have some different biological features from those
of the triazine and thiadiazine analogues.5) We have
attributed its distinctive biological performance to the
relatively strong –NCH2–O–CH2N– linkage by compar-
ing the bond energy of C–O with that of C–N or C–S.6)
Thiamethoxam proved practically unchanged under
hydrolytic conditions.7)
These examples indicate that some molecules de-
signed by a prodrug approach can display discrete
biological features that are different not only from the
original molecule, but also from similar structural
variants. We attempted in this study to contrive another
reconstruction of clothianidin. The basic skeleton of
neonicotinoid is constructed from an amidine or a
guanidine conjugated to a powerful electron-withdraw-
ing group like a nitro or cyano one; consequently, the
amidinyl (guanidyl) nitrogen atoms are partially posi-
tive.8–10) By referring to the well-examined hydrolytic
cleavage of 1,3-diacyl and related structures11,12) as
represented by the reaction, RCOCH2COR + H2O !
RCOOH + RCOCH3, we predicted that such a ꢀ-
aminoketone as R1R2NCH2COCH3 could give
R1R2NCH3 in a similar fashion, if the amine part were
positively charged like the nitrogen atoms in the
neonicotinoid skeleton. Thus, 1,3-diazacyclohexa-5-
one derivatives (8) would eventually decay to clothia-
nidin-type acyclic molecules in the applied environment
as illustrated in Fig. 1 (eq. 2). Also, we expected another
biological profile to be shown due to the different
bonding properties from the existing modifications.
Acetal is a well-known protective group of ketone and
is, in principle, deprotected under hydrolytic condi-
tions.13) With this strategy in mind, we prepared a set of
Key words: neonicotinoid insecticide; prodrug; clothia-
nidin; acetal; neuroblocking activity
The molecular design of a so-called prodrug, a
transformation of an original active molecule in order
to show or to enhance its biological activity, is found
time and again in developed pesticides.1) Prodrug-based
design can also be seen in the neonicotinoid field.2)
The potent insecticide, clothianidin (1),3) may be an
accessible target for prodrug molecular design because
of its structure bearing two reactive amine ends. If one
can make up the structure of clothianidin by linking the
amine ends so as to undo the linker before reaching the
site of action, the modified molecule would exhibit
potency due to clothianidin. The construction of triazine
and thiadiazine (6, Y ¼ NMe or S) are examples of this
y
To whom correspondence should be addressed. Fax: +81-58-293-2207; E-mail: kagabus@cc.gifu-u.ac.jp

706
S. KAGABU et al.
X
Me
NH
H
N
Het
Py
Het-CH -
N
NR
2
Het-CH -
2
NNO2
NNO2
Cl
N
2: prototype (Het=Py, X=CH2, R=H)
3: Het=Py, X=CMe2, R=H
4: Het=Py, X=CHOMe, R=H
1: clothianidin (Het=Thy)
S
Thy Cl
Tef
N
5: thiamethoxam (Het=Thy, X=O, R=Me)
X
Me
NH
H
N
O
Het-CH -
2
Het-CH -
2 N
NMe
(1)
Y
Y
6: X= NMe, O, S
7
Y= NNO2, CHNO2, NCN
H2O
O
Me
NR
Me
N
HO2C
Me
NR
Het-CH2
Het-CH -
N
2
Het-CH -
2 N
(2)
NR
Y
Y
Y
10
8
9
Fig. 1. Hydrolysis Paths (eqs. 1 and 2) for Clothianidin and Thiamethoxam and Related Compound.
The table on the right shows the abbreviations of the heterocyclic structural elements.
diazacyclohexanone acetals bearing various functional
groups and examined their biological activity toward
insects.
was poured into a solution composed of 15 ml of water
and 30 ml of conc. HCl with ca. 50 g of crushed ice, the
solution being acidified to pH 3 by further adding a 10%
HCl solution. This mixture was extracted with chloro-
form (4 ꢀ 30 ml), the chloroform layer being washed
with a 10% HCl solution (2 ꢀ 10 ml), and then with
water and finally dried over magnesium sulfate. Evap-
oration of the solvent and subsequent recrystallization of
the residual solid from ether gave 19.7 g (67.1%) of the
product as colorless crystals, mp 102–103 ꢁC. IR ꢂmax
(KBr): 1350, 1330, 1180 cmꢂ1. NMR ꢁH (CDCl3): 3.10
(6H, s), 3.33 (6H, s), 4.23 (4H, s); ꢁC: 37.7, 48.8, 63.6,
98.0. MS m=z (%): 292 (Mþ, 5), 261 (70), 166 (100),
103 (20). Anal. Found: C, 28.29; H, 5.50; S, 21.60%.
Calcd. for C7H16O8S2: C, 28.26; H, 5.52; S, 21.93%.
1,3-Diamino-2,2-dimethoxy-1,3-propanediol bis-p-
toluenesulfonate (14). This compound was prepared
according to the above-mentioned procedure, but using
p-toluenesulfonyl chloride instead of methanesulfonyl
chloride. Yield: 76.5%. Mp 113–115 ꢁC. IR ꢂmax (KBr):
1350, 1190, 1175 cmꢂ1. NMR ꢁH (CDCl3): 2.46 (6H, s),
3.10 (6H, s), 3.92 (4H, s), 7.35 (2H, d, J ¼ 8:2), 7.75
(2H, d, J ¼ 8:2); ꢁC: 21.7, 48.3, 63.8, 97.9, 128.0, 130.0,
132.1, 145.3. MS m=z (%): 259 (Mþ, 100), 103 (18).
Anal. Found: C, 51.50; H, 5.60; S, 14.90%. Calcd. for
C19H24O8S2: C, 51.34; H, 5.44; S, 14.43%.
Materials and Methods
Preparation of the compounds.
All melting point (mp) data are uncorrected. IR
spectra were measured with a Perkin Elmer FTIR 1600
spectrometer, and NMR spectra were obtained by a
Varian Gemini 2000 C/H (400 MHz) instrument. The
chemical shifts were recorded in ꢁ (ppm) and the
coupling constants, J, in Hz. Mass spectra were recorded
by a Jeol JMS-700 instrument (EI, 70 eV).
2,2-Dimethoxypropanediol (12). To a solution of
dihydroxyacetone dimer (15.0 g, 0.05 mmol) in dried
methanol (200 ml) was added trimethyl orthoformate
(17.5 g, 0.16 mol) and p-toluenesulfonic acid (0.06 g).
The mixture was stirred for 24 h at ambient temperature,
before anhydrous sodium carbonate (180 mg) was added
and stirring continued for a further 24 h. Most of the
methanol was evaporated, and the residue was chroma-
tographed on silica gel with chloroform as the eluent,
giving 7.60 g (50% yield) of a product as a colorless
liquid. The spectral data were identified by the reported
data for 12.14)
2,2-Dimethoxy-1,3-propanediol bismethanesulfonate
(13). A solution of methanesulfonyl chloride (12.8 g,
0.28 mol) in pyridine (150 ml) was added dropwise to
2,2-dimethoxypropanediol (13.6 g, 0.1 mol) while ice
cooling. After 4 hour of stirring, the reaction mixture
1,3-Diazido-2,2-dimethoxypropane (15). Sodium
azide (6.86 g, 112 mmol) was added to a solution of
bismethanesulfonate (13; 7.69 g, 26.3 mmol) in hexam-
ethylphosphoramide (120 ml). The mixture was stirred
for 60 h at 120 ꢁC, before being cooled to room

Prodrug Design of Clothianidin Insecticide
707
temperature, diluted with 200 ml of water, and extracted
with isopropyl ether (IPE; 3 ꢀ 30 ml). The combined
IPE layers were dried over magnesium sulfate. After
evaporating IPE at below 40 ꢁC, chromatography of the
residue on silica gel with a mixture of hexane and ethyl
acetate (9:1, v/v) gave a colorless liquid. This product
was used in the next step without further purification.
Crude yield: 2.90 g (59.9%). IR (NaCl, neat): 2110,
1140 cmꢂ1. NMR ꢁH (CDCl3): 3.29 (6H, s), 3.42 (4H, s);
ꢁC: 48.8, 50.25, 100.8.
The yield of 15 was 10% by using tosylate 14 instead
of mesylate 13 after 120 h of stirring at 120 ꢁC in DMF.
1,3-Diamino-2,2-dimethoxypropane (16). A solution
of diazide 15 (2.03 g, ca. 11.0 mmol) in 50 ml of
methanol was hydrogenated with 135 mg of 10%
palladium/carbon at atmospheric pressure. After 15
hour of shaking, the organic layer was filtered and the
filtrate was distilled, quantitatively giving a diamine
(1.47 g) with a boiling point of 100–104 ꢁC at 20 mmHg.
IR (NaCl, neat): 3280, 1600, 1460 cmꢂ1. NMR ꢁH
(CDCl3): 1.40 (4H, bs), 2.85 (4H, s), 3.23 (6H, s); ꢁC:
41.3, 48.1, 102.8. MS m=z (%): 122 (Mþ, 2), 104 (100),
74 (88), 72 (98). HRMS (EI) M/z (Mþ): calcd. for
C4H14N2O2, 122.1055; found, 122.1056.
2-Nitroimino-5,5-dimethoxy-1,4,5,6-tetrahydropyrimi-
dine (17). To an ice-cooled solution of 1,3-diamino-2,2-
dimethoxypropane (16) (0.33 g, 2.46 mmol) was added
S,S0-dimethyldithionitroiminocarbonate (0.41 g, 2.46
mmol) in small portions. The mixture was stirred at
room temperature for 3 h, the precipitated crystals were
filtered and washed thoroughly with hexane. Yield:
280 mg (55.8%). Mp: 197 ꢁC (dec). IR (KBr): 1620,
1610, 1380, 1300, 1100 cmꢂ1. NMR ꢁH (DMSO-d6):
3.20 (6H, s), 3.36 (4H, s), 8.85 (1H, bs); ꢁC: 43.9, 48.6,
156.4. MS m=z (%): 205 (Mþ, 32), 158 (33), 129 (100).
Anal. Found: C, 35.29; H, 5.90; N, 27.60%. Calcd. for
C6H12N4O4: C, 35.29; H, 5.93; N, 27.44%.
2-(6-Chloro-3-pyridylmethyl)-5,5-dimethoxy-1,4,5,6-
tetrahydro-[2H]-pyrimidine (22). A solution of 1,3-
diamino-2,2-dimethoxypropane (1.47 g, 11.1 mmol) and
6-chloronicotinaldehyde (1.25 g, 8.88 mmol) in 30 ml of
benzene was refluxed for 3 h using a Dean-Stark trap.
The residual liquid, after evaporating the benzene, was
used in the next step without further purification. IR
(NaCl, neat): 1455, 1105, 1049, 747, 734 cmꢂ1. NMR ꢁH
(CDCl3): 2.32 (2H, d, J ¼ 13:7), 2.87 (2H, d, J ¼ 13:7),
3.25 (3H, s), 3.27 (3H, s), 4.63 (1H, s), 7.30 (1H, d,
J ¼ 8:6), 7.89 (1H, dd, J ¼ 8:6=2:3), 8.62 (1H, d,
J ¼ 2:3); ꢁC: 48.0, 49.4, 69.5, 92.8, 123.7, 135.6, 137.3,
148.5, 150.1. MS m=z (%): 256 (Mþ, 0.7), 199 (6), 126
(28), 104 (85), 58 (100). HRMS (EI) M/z (Mþ): calcd.
for C11H16ClN3O2, 257.0932; found, 257.0941.
m), 2.67 (2H, m), 3.21 (6H, s), 3.24 (2H, m), 3.65 (1H,
1H, m), 3.75 (1H, m), 3.89 (2H, m); ꢁC: 29.7, 44.8, 47.9,
49.7, 49.8, 68.3, 71.0, 73.8, 93.1. MS m=z (%): 216 (Mþ,
2), 186 (14), 145 (77), 88 (100). HRMS (EI) M/z (Mþ):
calcd. for C10H20N2O3, 216.1473; found, 216.1479.
N-(6-Chloro-3-pyridylmethyl)-2,2-dimethoxy-1,3-di-
aminopropane (24). A solution of compound 22
(ꢃ2:5 g) in a mixture of ethanol and water (5:1, v/v)
was treated with 1.35 g of sodium borohydride in
portions. The mixture was stirred for 120 h at room
temperature. The inorganic solids were filtered off, and
the resulting filtrate was concentrated. The residual
semisolid was extracted with acetonitrile, and the
acetonitrile was evaporated. This operation was repeat-
ed, and the combined residue was used for the next step
without further purification. A small portion of the
residue was distilled in a micro-distillation apparatus
(100 ꢁC, 0.1 mmHg) for analysis. IR (NaCl, neat): 1568,
1455, 1046 cmꢂ1. NMR ꢁH (CDCl3): 2.74 (2H, s), 2.86
(2H, s), 3.20 (6H, s), 3.80 (2H, s), 7.29 (1H, d, J ¼ 8:6),
7.68 (1H, dd, J ¼ 8:6=2:3), 8.33 (1H, d, J ¼ 2:3); ꢁC:
42.9, 48.2, 48.9, 50.5 102.1, 124.0, 134.6, 138.6, 149.3,
150.1. MS m=z (%): 259 (Mþ, 1), 227 (38), 126 (13),
103 (23), 88 (100). HRMS (EI) m=z (Mþ): calcd. for
C11H18ClN3O2, 259.1089; found, 259.1091.
N-(Tetrahydrofuran-3-ylmethyl)-2,2-dimethoxy-1,3-di-
aminopropane (25). This compound was similarly
prepared from 23. IR (NaCl, neat): 1460, 1105, 1050
cmꢂ1. NMR ꢁH (CDCl3): 1.12 (1H, m), 1.59 (1H, m),
2.02 (1H, m), 2.37 (1H, m), 2.63 (2H, m), 2.77 (1H, m),
2.82 (2H, m), 3.22 (6H, s), 3.50 (1H, m), 3.82 (1H, m),
3.86 (2H, m); ꢁC: 30.5, 39.6, 43.0, 48.1, 49.9, 53.5, 67.8,
71.9, 102.3. MS m=z (%): 220 (Mþ + 2, 44), 219
(Mþ + 1, 55), 186 (33), 158 (43), 104 (100). HRMS
(EI) m=z (Mþ): calcd. for C10H22N2O3, 218.1630;
found, 218.1632.
1-(6-Chloro-3-pyridylmethyl)-2-nitroimino-5,5-dime-
thoxy-1,4,5,6-tetrahydropyrimidine (18).
Method 1.
A solution of diamine 24 (1.68 g,
6.49 mmol) and S,S0-dimethyldithionitroiminocarbonate
(1.10 g, 6.62 mmol) in 10 ml of chloroform was stirred
for 12 h at room temperature. The chloroform was
distilled off and the residual liquid was subjected to
preparative thin-layer chromatography on silica gel
using a mixture of chloroform and ethanol (20:1, v/v)
as the eluent, giving 140 mg (40% yield) of colorless
crystals with mp 169 ꢁC. IR ꢂmax (KBr): 3274, 1596,
1558, 1448, 1333, 1248, 1130, 1093 cmꢂ1. NMR ꢁH
(CDCl3): 3.21 (6H, s), 3.35 (2H, m), 3.50 (2H, m), 4.71
(2H, s), 7.34 (1H, d, J ¼ 8:1), 7.75 (1H, dd, J ¼
8:1=2:6), 8.31 (1H, d, J ¼ 2:6), 9.87 (1H, bs); ꢁC: 43.9,
48.9, 49.4, 50.7, 92.9, 124.7, 130.2, 139.0, 149.0, 151.4,
155.8. MS m=z (%): 330 (Mþ, 2), 283 (79), 126 (54), 45
(100). Anal. Found: C, 43.59; H, 5.03; N, 21.60%.
Calcd. for C12H16ClN5O4: C, 43.71; H, 4.89; N, 21.24%.
Method 2. To an ice-cooled solution of 2-nitroimino-
5,5-dimethoxy-1,4,5,6-tetrahydropyrimidine (17; 0.30 g,
1.47 mmol) in DMF was added sodium hydride (60%,
2-(Tetrahydrofuran-3-ylmethyl)-2,2-dimethoxy-1,4,5,
6-[2H]-tetrahy-dropyrimidine (23). This compound was
prepared according to the procedure used for 22 with
3-tetrahydrofurancarbaldehyde. IR (NaCl, neat): 1455,
1105, 1047, 897, 753 cmꢂ1. NMR ꢁH (CDCl3): 1.54 (2H,
bs), 1.80 (1H, m), 2.02 (1H, m), 2.15 (1H, m), 2.30 (1H,

708
S. KAGABU et al.
25 mg, 0.625 mmol) and the mixture was stirred for
30 min at room temperature. A solution of 6-chloro-3-
pyridylmethyl chloride (240 mg, 1.47 mmol) in 4 ml of
DMF was next added dropwise. The mixture was stirred
overnight, before DMF was distilled off under reduced
pressure. Preparative thin-layer chromatography on
silica gel of the residue gave the product in a 43.5%
yield (130 mg).
yellow crystals in 17.5% yield (250 mg). Mp: 192–
193 ꢁC. IR ꢂmax (KBr): 3135, 2169, 1610, 1559, 1464,
1433, 1313, 1091 cmꢂ1. NMR ꢁH (CDCl3): 3.18 (6H, s),
3.25 (2H, m), 3.36 (2H, m), 4.58 (2H, s), 6.54 (1H, s),
7.28 (1H, d, J ¼ 8:4), 7.71 (1H, dd, J ¼ 8:4=2:6), 8.30
(1H, d, J ¼ 2:6); ꢁC: 44.0, 48.8, 49.4, 50.9, 93.8, 118.8,
124.5, 130.8, 138.7, 148.9, 151.1, 157.8. MS m=z (%):
309 (Mþ, 14), 278 (84), 264 (77), 126 (84), 43 (100).
Anal. Found: C, 47.59; H, 5.00; N, 17.21%. Calcd. for
C13H16ClN5O4: C,47.64; H, 4.92; N, 17.10%.
1-(6-Chloro-3-pyridylmethyl)-2-nitromethylene-5,5-
dimethoxy-1,4,5,6-tetrahydropyrimidine (27). A solution
of diamine 24 (2.21 g, 8.59 mmol) and 1,1-bismethyl-
thio-2-nitroethene (1.47 g, 8.88 mmol) in 30 ml of eth-
anol was refluxed for 16 h. The precipitated crystals
were collected and washed with hexane. Yield: 175 mg
(6.3%). Mp: 208 ꢁC. IR ꢂmax (KBr): 3130, 1583, 1458,
1429, 1273, 1091 cmꢂ1. NMR ꢁH (CDCl3): 3.27 (6H, s),
3.42 (2H, m), 3.52 (2H, m), 4.44 (2H, s), 6.54 (1H, s),
7.38 (1H, d, J ¼ 8:4), 7.56 (1H, dd, J ¼ 8:4=2:6), 8.31
(1H, d, J ¼ 2:6), 10.84 (1H, bs); ꢁC: 42.8, 48.9, 51.0,
53.3, 93.4, 98.3, 124.8, 128.9, 136.9, 147.9, 151.6,
154.3. MS m=z (%): 329 (Mþ, 38), 282 (93), 126 (100).
Anal. Found: C, 47.29; H, 5.30; N, 17.30%. Calcd. for
C13H17ClN4O4: C, 47.49; H, 5.21; N, 17.04%.
2-(Tetrahydrofuran-3-ylmethyl)-2-nitromethylene-5,5-
dimethoxy-1,4,5,6-tetrahydropyrimidine (28). A solution
of diamine 25 (1.08 g, 4.97 mmol) and 1,1-bismethyl-
thio-2-nitroethene (0.85 g, 5.20 mmol) in 20 ml of eth-
anol was refluxed for 20 h. The solvent was distilled off
and subsequent column chromatography (silica gel) of
the residual semisolid with CHCl3/ethanol (20:1 and
then 10:1, v/v) afforded 570 mg (40% yield) of a
product as colorless crystals with mp 132 ꢁC. IR ꢂmax
(KBr): 3159, 2944, 1576, 1429, 1306, 1202, 1133, 1118,
1086, 1049, 759 cmꢂ1. NMR ꢁH (CDCl3): 1.60 (1H, m),
2.10 (1H, m), 2.64 (1H, m), 3.15 (1H, m), 3.27 (1H, m),
3.29 (6H, s), 3.40 (2H, m), 3.48 (2H, m), 3.58 (1H, m),
3.77 (2H, m), 3.93 (1H, m), 6.66 (1H, s), 10.86 (1H, bs);
ꢁC: 29.6, 38.2, 43.1, 48.7, 67.3, 70.4, 93.2, 98.4, 154.0.
MS m=z (%): 289 (Mþ, 45), 242 (68), 88 (74), 43 (100).
Anal. Found: C, 50.29; H, 7.30; N, 14.30%. Calcd. for
C12H21N3O5: C, 50.16; H, 7.37; N, 14.63%.
1-(2-Chloro-5-thiazolylmethyl)-2-nitroimino-5,5-dime-
thoxy-1,4,5,6-tetrahydropyrimidine (19). This com-
pound was prepared according to the foregoing method
2, using 2-chloro-5-thiazolylmethyl chloride instead of
6-chloro-3-pyridylmethyl chloride. Yield: 12%. Mp:
177 ꢁC. IR ꢂmax (KBr): 3282, 1594, 1555, 1463, 1423,
1331, 1235, 1122, 1094, 1072, 1046 cmꢂ1. NMR ꢁH
(CDCl3): 3.22 (6H, s), 3.42 (2H, m), 3.48 (2H, m), 4.70
(2H, s), 7.48 (1H, s), 9.72 (1H, bs); ꢁC: 44.1, 46.0, 48.8,
50.6, 93.1, 134.6, 140.6, 154.2, 155.2. MS m=z (%): 336
(Mþ, 3), 289 (51), 253 (100), 132 (85). Anal. Found: C,
35.64; H, 4.13; N, 21.00; S, 9.89%. Calcd. for
C10H14ClN5O4S: C, 35.77; H, 4.20; N, 20.86; S, 9.55%.
1-(6-Chloro-3-pyridylmethyl)-3-methyl-2-nitroimino-
5,5-dimethoxy-1,4,5,6-tetrahydropyrimidine (20). To an
ice-cooled solution of 18 (80 mg, 0.309 mmol) in 2.5 ml
of DMF was added sodium hydride (60%, 25 mg,
0.625 mmol). After 30 min of stirring at room temper-
ature, methyl iodide (140 mg, 1.00 mmol) in 2.5 ml of
DMF was added dropwise. The mixture was stirred for
5 h at 60 ꢁC, and subsequently DMF was distilled off
under reduced pressure. Preparative thin-layer chroma-
tography on silica gel of the residue gave the product in
a 37.7% yield (40 mg). Mp: 136 ꢁC. IR ꢂmax (KBr):
1605, 1397, 1278 cmꢂ1. NMR ꢁH (CDCl3): 3.17 (3H, s),
3.18 (6H, s), 3.33 (2H, m), 3.51 (2H, m), 4.70 (2H, s),
7.36 (1H, d, J ¼ 8:2), 7.78 (1H, dd, J ¼ 8:2=2:5), 8.31
(1H, d, J ¼ 2:5); ꢁC: 39.0, 49.1, 50.5, 51.7, 53.5, 94.0,
124.9, 129.3, 139.4, 149.2, 151.9, 158.2. MS m=z (%):
344 (Mþ, 2), 297 (98), 169 (50), 126 (100). Anal. Found:
C, 45.29; H, 5.30; N, 20.30%. Calcd. for C12H16ClN5O4:
C, 45.42; H, 5.28; N, 20.37%.
1-(2-Chloro-5-thiazolylmethyl)-3-methyl-2-nitroimino-
5,5-dimethoxy-1,4,5,6-tetrahydropyrimidine (21). This
compound was similarly prepared from 19. Yield:
23.5%. Mp: 157 ꢁC. IR ꢂmax (KBr): 1598, 1528, 1415,
1383, 1333, 1296, 1090, 1048 cmꢂ1. NMR ꢁH (CDCl3):
3.13 (3H, s), 3.22 (6H, s), 3.40 (2H, m), 3.48 (2H, m),
4.75 (2H, s), 7.19 (1H, s); ꢁC: 39.2, 47.5, 49.1, 50.4,
53.6, 94.5, 134.1, 141.2, 154.5, 157.7. MS m=z (%): 350
(Mþ, 3), 303 (33), 267 (100), 132 (48). Anal. Found: C,
37.29; H, 4.30; N, 20.31; S, 9.21%. Calcd. for
C11H16ClN5O4S: C, 37.77; H, 4.61; N, 20.02; S, 9.17%.
1-(6-Chloro-3-pyridylmethyl)-2-cyanoimino-5,5-dime-
thoxy-1,4,5,6-tetrahydropyrimidine (26). A solution of
diamine 24 (1.08 g, 4.97 mmol) and S,S0-dimethyl-N-
cyanodithioiminocarbonate (0.70 g, 4.82 mmol) in 20 ml
of ethanol was refluxed for 20 h. After evaporating the
solvent, chromatography on silica gel using CHCl3/
ethanol (15:1, v/v) as the eluent gave the product as pale
Biological tests.
Chemicals. Compounds 2,15) 3/4,16) and 31/3217)
were prepared according to the reported procedures.
Reagent-grade piperonyl butoxide (PB) purchased from
Tokyo Kasei Kogyo Co. (Tokyo, Japan) was used as an
inhibitor of oxidative metabolism. NIA 16388 (prop-
argyl propyl benzenephosphonate; NIA) was the same as
the sample used in our previous studies.18–22) NIA
originally is an inhibitor of the hydrolytic metabolism of
pyrethroids.23)
Insecticidal tests against the green peach aphid
(Myzus persicae). Paper towels were placed on the base
of a cage and watered. On them, wet rice seeds were
sown, and the cage stored for germination in the dark at

Prodrug Design of Clothianidin Insecticide
709
Table 1. Biological Activities of the Tested Compounds
Compound
no.
Spray insecticidal rating (%)a,b
Injection insecticidal activity MLD (mmmol)b
Neuroblocking potencyb
20
4
0.8
0.16
Alone
+(PB + NIA)
BC (mM)
2
18
19
20
21
26
27
28
3
100
100
100
90
100
100
100
90
100
90
90
60
0.015
ꢄ0:38
ꢄ0:34
ꢄ0:36
ꢄ0:36
ꢄ0:40
ꢄ0:44
ꢄ0:36
NT
9:6 ꢀ 10ꢂ5
0.19
0.24
0.0045
ꢃ0:56
ꢄ0:56
ꢄ0:56
ꢄ0:54
ꢃ0:62
ꢃ1:0
60
60
50
30
ꢃ0:36
ꢃ0:36
ꢄ0:40
0.35
100
100
90
100
60
60
60
40
20
60
0
0
NT
NT
NT
NT
NT
100
60
60
NT
NT
NT
NT
100
ꢄ0:36
NT
NT
>0:40d
0.0051d
ꢄ0:54
NT
NT
0
4
40
0c
0
NT
>0:40d
0.0051d
31
32
NT
100
>0:65d
0.0085d
100
aThe percentage killing rate at three specified doses in ppm for Myzus persicae 7 d after the treatment. See the text for the details.
bMLD, minimum lethal dose; BC, neuroblocking concentration. See the text for the details. NT, not tested.
c60% at 100 ppm.
dRef. 36.
30 ꢁC for 4 d. After storage, the etiolated small rice
plants about 2 cm tall were sprayed from above with an
aqueous solution of a test compound. The concentration
of the compound was adjusted in four stages from 20
to 0.16 ppm. After spraying, ten nymphs were released
on the plants, and the mortality was determined in %
after 7 d.
silver chloride. The number of spontaneous discharges
that were larger than approximately 15 mV was consec-
utively counted with a pulse counter (MET-1100, Nihon
Kohden, Tokyo, Japan) for every 30-sec period. The
frequency was usually quite high in a few minutes after
setting, and then normally subsided. When the frequency
had decreased to a range of 30–400 counts per 30 sec in
about 2 min, the saline solution was exchanged for
another saline solution containing a test compound
dissolved in methanol solution containing DMSO. The
final concentration of the organic solvent was lower
than 1% (v/v), which did not affect the nerve activity.
Measurements were conducted at 22–25 ꢁC. The eval-
uation of the neuroblocking activity was based on the
previously reported procedure.21,22) The time when the
number of firing first decreased to fewer than 10 counts
per 30 s, which is defined as t in min, was determined.
The effect on the frequency and the time to subside
depended on the concentration of the compound
(Fig. 3). Similar measurements were conducted at two
concentrations for each compound, more than three
nerve preparations being used for each concentration of
each compound. A concentration-response relationship
for each compound (data not shown) enabled the
concentration required to reach 1 in terms of log t was
determined and defined as BC (in M). The BC values are
listed in Table 1.
Insecticidal test against the American cockroach. The
insecticidal assay against the male adult American
cockroach, Periplaneta americana L., was conducted
as previously described.18–22) Various volumes (1–10 ml)
of each compound dissolved in methanol containing
dimethyl sulfoxide (DMSO) were injected into the
abdomen of a cockroach. Organic solvents alone in this
range did not have any toxic effect. Details of the dosage
were fundamentally the same as described previous-
ly.18,22) The dose was varied in stages of 1.25 times in
moles. In some experiments, a methanol solution (1 ml)
containing PB (50 mg) and NIA (50 mg) was injected 1 h
before injection of the test compound. The metabolic
inhibitors in these amounts did not have any toxic effect.
Three insects were used to test each dose of each
compound and were kept at 22–25 ꢁC for 24 h after the
injection. The minimum dose at which two of three
insects were killed was taken as the minimum lethal
dose (MLD) in mol. Paralyzed insects were counted as
dead. The MLD values for the test compounds are listed
in Table 1. Each value is the mean of at least two
experiments with a deviation of 0.64–1.6 times.
Results and Discussion
Neurophysiological assay. A neurophysiological test
of the compounds was conducted as previously describ-
ed.19–22) In brief, a nerve preparation containing the
abdominal fifth and sixth ganglia of a male adult
American cockroach was excised and placed in a saline
solution. One of two bundles of the nerve cord was taken
up from the thoracic side with saline into a glass tube, in
which a silver wire was set as an electrode. As the
reference electrode, another wire was set outside the cut
end of the tube. The silver wires were thinly coated with
The synthetic scheme is depicted in Fig. 2. The key
intermediate, 1,3-diaminoacetone dimethyl acetal (16),
was prepared from dihydroxyacetone dimer 11 in four
steps. The SN2 type of substitution of the bis-tosylate
(14) to azides (15) was extremely sluggish in this
process. The conventional procedure in refluxing ace-
tone, ethanol, or acetonitrile with or without crown ether
resulted in recovery of the starting tosylate, and even
such harsh conditions as 120 h of heating at 120 ꢁC in

710
HO
S. KAGABU et al.
O OH
CH2OH
HC(OMe)3
1. NaN3 / HMPA
2. H2 / Pd-C
MsCl or TosCl
Pyridine
(MeO)2C(CH2OR)2
(MeO)2C(CH2X2)2
15: X=N3
(MeO)2C(CH2OH)2
HOCH2 O
PTS / MeOH
13: R=Ms
14: R=Tosyl
11
12
16: X=NH2
MeO OMe
MeO OMe
MeO OMe
MeI
NaH / DMF
(MeS)2C=NNO2
HetCH2Cl
MeN
N-CH2Het
NNO2
HN
N-CH2Het
NNO2
16
HN
NH
NNO2
17
NaH / DMF
18: Het=Py
20: Het=Py
19: Het=Thy
21: Het=Thy
MeO OMe
MeO OMe
(MeS)2C=NNO2
or
MeO
NaBH4
Het-CHO
H2NCH2-C-CH2NHCH2Het
MeO
16
HN
NH
Het
HN
N-CH2Het
(MeS)2C=NCN
or
X
(MeS)2C=CHNO2
22: Het=Py
23: Het=Tef
24: Het=Py
25: Het=Tef
18: Het=Py, Y=NNO2
26: Het=Py, X=NCN
27: Het=Py, X=CHNO2
28: Het=Tef, X=CHNO2
Fig. 2. Preparation Scheme for 1,3-Diazacyclohexane Acetals.
See Fig. 1 for the structural abbreviations of Py, Thy and Tef.
ized 1,3-diaminopropane will provide practical access to
a precursor for therapeutically interesting imidazoben-
zodiazepines,24) 1,3-diazacyclohexanes25) and antitumor
cisplatine-related metal ligands.26) The final hetaryl-
methyl 1,3-diazacyclohexanone acetals were prepared
by two routes. The first route started with 2-nitroimino-
tetrahydropyrimidine acetal (17) that was in turn
prepared by the condensation of N-nitroimidodithio-
carbonate27) with diamine 16. The free NH of 17 was
readily substituted with 6-chloronicotinyl chloride and
chlorothiazolylmethyl chloride using sodium hydride as
the base, respectively giving 18 and 19, and further
another NH was methylated without difficulty. The
second route made use of 1-(6-chloronicotinyl)- or
1-(tetrahydrofuranylmethyl)-1,3-diaminopropane acetal
24 or 25 as the precursor. The protocol starting with
dehydrative condensation of hetarylaldehyde with 1,3-
diaminopropane in refluxing benzene and followed by
reduction of the formed C=N bond has been applied as
a routine method for N-hetarylmethyl-substituted 1,3-
diamine.15,28) However, with 1,3-diamino-2,2-dime-
thoxypropane as the substrate, 2-hetaryl-hexahydropyr-
imidines (22/23) were formed instead of the expected
imine. Variation of the edduct ratio or replacement with
other solvents did not recede the step to the intra-
molecular cyclization. Fortunately, however, we found
that hexahydropyrimidines (22/23) could also be re-
duced to 1,3-diaminopropane derivatives (24/25) with
sodium borohydride, although very slowly. The final
reactions for compound(s) 18 (or 26–28) proceeded
without difficulty.
8000
6000
4000
2000
0
t
0
5
10
15
20
Time (min)
Fig. 3. Time Course Characteristics for the Effect of Compounds 2
and 18 on the Spontaneous Discharge in Excised Central Nerve
Cords of American Cockroaches.
After counting for 5 min, the nerve preparations were applied with
compounds 2 ( , 1:3 ꢀ 10ꢂ6 M; , 5:0 ꢀ 10ꢂ6 M) and 18 ( , 5:7 ꢀ
10ꢂ4 M) at zero min.
DMF only gave a 10% yield of the diazide. We
optimized the process by using bis-mesylate 13 as the
substrate and HMPA as the solvent, and we could obtain
desired product 15 in a 60% yield after 60 h at 120 ꢁC.
Hydrogenolysis of this diazide to diamine 16 proceeded
smoothly. This straightforward approach to functional-

Prodrug Design of Clothianidin Insecticide
711
Neonicotinoid is especially effective against chewing
and sucking insects.29) We chose in this study the green
peach aphid (Myzus persicae), a target insect for this
insecticide class, to evaluate the insecticidal activity of
the present compounds. Spray treatment with the tested
acetals was highly insecticidal against aphids at 20 ppm
after seven days, except for the tetrahydrofuranyl
derivative (28) which had modest activity. The nitro-
imine derivatives (18–21) showed the most appreciable
insecticidal effect at as low a concentration as 4–
0.8 ppm, although they did not match the effect of
compound 2 at 0.16 ppm. This was in contrast to the far
lower activity of the 5,5-dimethyl (3) and 5-methoxy
derivatives (4). The higher activity of the dimethyl
acetals than compound 3 bearing a likely steric incre-
ment or compound 4 carrying a similar hydrophilic
element suggests that a certain property of the carbonyl-
protecting group would have been involved in the
exhibition.
The insecticidal activity by injecting into the insect
body can be evaluated after regulating factors involving
decomposition in the environment and the ease of
cuticular penetration. The activity determined by this
injection method with the cockroach for acetals 18–21
and 26–28 without metabolic inhibitors was much more
than 20 times lower than that of the prototype (2). It is
not clear whether the lower activity exhibited by
injection was related to low hydrolysis rates of the
acetals in the insect during the testing period of 24 h.
Insecticides are metabolized by various enzyme
systems. A recent study on neonicotinoids using re-
combinant cytochrome P450 (CYP450) isozymes
showed that reduction at the nitroimine substituent is
the main metabolic process for imidacloprid, and the
oxidation at the imidazolidine moiety is a side path.30,31)
On the other hand, there are some metabolic inhibitors
that prevent or minimize the deactivation of an
insecticide by enzymes and generally enhance the
activity exhibited when neat. NIA and PB are known
as respective inhibitors of the hydrolytic and oxidative
metabolism of pyrethroids,23) and their synergistic effect
is also apparent in neonicotinoids.19–22) We thought that
these synergists might also inhibit the hydrolysis of
these acetals to ketones, thus retaining the low activity.
However, the addition of PB and NIA brought about an
activity increase as was obvious in compounds 18, 19
and 27, and recognizably so in compounds 20 and 21.
These results suggest that the synergists mainly inter-
fered with the metabolic attack occurring at the nitro-
imino and analogous electron-withdrawing group of
these compounds, rather than with any hydrolysis of the
acetal.
Neonicotinoid compounds give a characteristic nerve
impulse pattern from the action at the insect nicotinic
acetylcholine receptor, i.e. enhancement of spontaneous
firing followed by the blockage of neural activity.19–22)
Figure 3 relates the frequency of spontaneous discharges
in the excised central nerve cords of American cock-
roaches over time resulting from the treatment with
compounds 2 and 18. The two-phase characteristics of
the initially increasing and then decreasing were clearly
apparent. With compound 2, the frequency decreased to
zero around 13 min after the start of treatment at 1:3 ꢀ
10ꢂ6 M. With compound 18, however, some discharges
were still counted even 20 min after the start of
treatment at 5:7 ꢀ 10ꢂ4 M, this being the highest
concentration tested. By estimating in a similar manner
to that for the previous electrophysiological experi-
ments,21,22,32–34) the blocking concentration for com-
pounds 18, 26 and 27 was almost the same in a range of
0.6–1.0 mM. The much larger values than for prototype 2
of 4.5 mM are due to the evidently weak neuroblocking
activity of these acetals. The other acetals were such
weak blockers that we could not identify any definite
nerve-blocking phenomenon at as high a concentration
as 0.5 mM in this 30-min measurement period.
We have already observed the almost parallel rela-
tionship between the activities against sucking insects
and the estimates using American cockroaches.20,29) The
present acetal derivatives were generally weak in
insecticidal activity by injection as well as in neuro-
blocking activity. However, it is notable that a few of
them were fairly high in this insecticidal estimates
toward aphids 7 days after spraying. These biological
results suggest that the intrinsic insecticidal activity of
the acetals is weak and some substances enhanced this
activity during the testing period. As depicted in eq. 2 in
Fig. 1, acetal hydrolysis and the subsequent 1,3-diacyl
type of cleavage leading to active compound, e.g. 32,
will be a plausible transformation under such green-
house conditions. Another possible decomposition proc-
ess is based on the well-documented path for the
dealkylation metabolism of alkylamines through oxida-
tion (hydroxylation) at the ꢃ-methylene,35) as depicted in
Fig. 4. However, the obviously lower aphidicidal activ-
ity of compound 31 than of cyclic acetal 1836) would
O
MeO OMe
O-H
H
P450 et al.
CH -
2 N
CH -
2 N
NR
NR
CH -
2 N
NHR
NNO2
or air (O2)
NNO2
NNO2
Cl
Cl
Cl
N
N
N
29
30
18 (R=H)
19 (R=Me)
31 (R=H)
32 (R=Me)
Fig. 4. Alternative Possible Cleavage Path of Acetals 18/19 to 31/32.

712
S. KAGABU et al.
14) Ferroni, E. L., DiTella, V., Ghanayem, N., Jeske, R.,
Jodlowski, C., O’Connell, M., Styrsky, J., Svoboda, R.,
Venkataraman, A., and Winkler, B. M., A three-step
preparation of dihydroxyacetone phosphate dimethyl
acetal. J. Org. Chem., 64, 4943–4945 (1999).
15) Shiokawa, K., Tsuboi, S., Kagabu, S., and Moriya, K.,
Heterocyclic compounds. Eur. Pat. Appl., EP 0192060
A1 (1986).
16) Kagabu, S., Nishiwaki, H., Sato, K., Hibi, M., Yamaoka,
N., and Nakagawa, Y., Nicotinic acetylcholine receptor
binding of imidacloprid-related diaza compounds with
various ring sizes and their insecticidal activity against
Musca domestica. Pest Manag. Sci., 58, 483–490 (2002).
17) Maienfisch, P., Huerlimann, H., and Haettenschwiler, J.,
A novel method for the preparation of N,N-disubstituted-
N0-nitroguanidines, including a practical synthesis of the
neonicotinoid insecticide clothianidin. Tetrahedron Lett.,
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suggest either no such cleavage or no contribution to the
activity. There would also be a question of whether the
ketone itself was responsible for the biological activity.
Trials to deprotect the acetal group to the ketone in
various acidic solutions at various temperatures resulted
in recovery of the acetal or in an intractable residual
reaction mixture. Deprotection under neutral or milder
conditions according to the published protocols6) also
failed. Alternatively, oxidation of the known 5-hydroxy-
1,3-diazacyclohexane16) has not succeeded either so far.
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