Dihydropiperazine neonicotinoid compounds. Synthesis and insecticidal activity

Article abstract of DOI:10.1021/jf021185r

Syntheses of various isomeric dihydropiperazines can be approached successfully by taking advantage of the regioselective monothionation of their respective diones. Preparation of the precursor unsymmetrical N-substituted piperazinediones from readily available diamines is key to this selectivity. The dihydropiperazine ring system, as exemplified in 1-[(6-chloropyridin-3-yl)methyl]-4-methyl-3-oxopiperazin-2-ylidenecyanamide (4) and 1-[(2-chloro-1,3-thiazol-5-yl)methyl]-4-methyl-3-oxopiperazin-2- ylidenecyanamide (25), has been shown to be a suitable bioisosteric replacement for the imidazolidine ring system contained in neonicotinoid compounds. However, placement of the cyanoimino electron-withdrawing group further removed from the pyridine ring, as in 4-[(6-chloropyridin-3-yl)methyl]-3-oxopiperazin-2-ylidenecyanamide (3a), or relocation of the carbonyl group, as in 1-[(6-chloropyridin-3-yl)methyl]-4-methyl-5-oxopiperazin-2-ylidenecyanamide (5), results in significantly decreased bioisosterism. The dihydropiperazine ring system of 4 and 25 also lends a degree of rigidity to the molecule that is not offered by the inactive acyclic counterpart 2-[(6-chloropyridin-3-yl)-methyl-(methyl)amino]-2-(cyanoimino)-N, N-dimethylacetamide (6). A pharmacophore model is proposed that qualitatively explains the results on the basis of good overlap of the key pharmacophore elements of 4 and imidacloprid (1); the less active regioisomers of 4 (3a, 5, and 6) feature a smaller degree of overlap.

Full text of DOI:10.1021/jf021185r

J. Agric. Food Chem. 2003, 51, 3035−3042 3035
Dihydropiperazine Neonicotinoid Compounds. Synthesis and
Insecticidal Activity
JACK G. SAMARITONI,* DAVID A. DEMETER, JAMES M. GIFFORD,
GERALD B. WATSON, MARGARET S. KEMPE, AND TIMOTHY J. BRUCE
Discovery Research, Dow AgroSciences, 9330 Zionsville Road, Indianapolis Indiana 46268-1054
Syntheses of various isomeric dihydropiperazines can be approached successfully by taking advantage
of the regioselective monothionation of their respective diones. Preparation of the precursor
unsymmetrical N-substituted piperazinediones from readily available diamines is key to this selectivity.
The dihydropiperazine ring system, as exemplified in 1-[(6-chloropyridin-3-yl)methyl]-4-methyl-3-
oxopiperazin-2-ylidenecyanamide (4) and 1-[(2-chloro-1,3-thiazol-5-yl)methyl]-4-methyl-3-oxopiperazin-
2-ylidenecyanamide (25), has been shown to be a suitable bioisosteric replacement for the
imidazolidine ring system contained in neonicotinoid compounds. However, placement of the
cyanoimino electron-withdrawing group further removed from the pyridine ring, as in 4-[(6-chloropyridin-
3-yl)methyl]-3-oxopiperazin-2-ylidenecyanamide (3a), or relocation of the carbonyl group, as in 1-[(6-
chloropyridin-3-yl)methyl]-4-methyl-5-oxopiperazin-2-ylidenecyanamide (5), results in significantly
decreased bioisosterism. The dihydropiperazine ring system of 4 and 25 also lends a degree of rigidity
to the molecule that is not offered by the inactive acyclic counterpart 2-[(6-chloropyridin-3-yl)-methyl-
(methyl)amino]-2-(cyanoimino)-N,N-dimethylacetamide (6). A pharmacophore model is proposed that
qualitatively explains the results on the basis of good overlap of the key pharmacophore elements of
4 and imidacloprid (1); the less active regioisomers of 4 (3a, 5, and 6) feature a smaller degree of
overlap.
KEYWORDS: Dihydropiperazine; neonicotinoid; synthesis; molecular modeling; pharmacophore; nicotinic
acetylcholine receptor; bioisosterism; cotton aphid
INTRODUCTION
(3, 4) as well as acyclic examples (18). We report herein that
the dihydropiperazine ring is also bioisosteric (19) with these
examples. Compounds 3a and 4 feature dihydropiperazine rings
in which a carbonyl group has been inserted into the imidazo-
lidine ring of 2 at points a and b, respectively (Figure 1). These
compounds, which have not been previously reported, were
found to bind to the nAChR of housefly head membrane (Musca
domestica) and afforded control of sap-feeding insects such as
cotton aphid (CA, Aphis gossypii). This paper describes the
syntheses and biological activities of a number of dihydropip-
erazine derivatives targeted to assess the potential of this ring
system for use in neonicotinoid compounds.
Since the advent of imidacloprid (1) 15 years ago (1-3), the
search for new neonicotinoid insecticides has been an intense
and competitive effort on the part of several research groups
within the agrochemical industry (4-10). Neonicotinoid com-
pounds, a term proposed by Yamamoto to describe this class
of chemistry (11), are potent insecticides that primarily control
sap-feeding pests and are relatively safe toward mammals,
making this class of chemistry particularly attractive. This
selectivity arises, in part, from a lower affinity for the mam-
malian nicotinic acetylcholine receptor (nAChR) than for the
insect receptor (12, 13). This particular mode of action is not
well represented in the arsenal of insect control agents available
to the grower and thus is quite suitable for use in integrated
pest management practices (14).
EXPERIMENTAL PROCEDURES
Chemistry. Melting points are uncorrected. All reagents purchased
were used without further purification. Solvents were dried using 3 Å
molecular sieves. Chromatography was performed using 230-400 mesh
ASTM silica gel 60 from EM Science, Darmstadt, Germany. Proton
NMR spectra were obtained on a Varian Gemini 300 spectrometer using
deuteriochloroform as solvent unless otherwise indicated and are
reported in parts per million (δ) downfield from tetramethylsilane as
internal reference. Infrared spectra were obtained on a Bio-Rad
spectrophotometer using potassium bromide pellets or as neat oils and
are reported as wavenumbers (cm-1). Mass spectra were obtained on a
A major emphasis in this search has involved the modification
of the substructure containing the electron-withdrawing group
C ) X. In the case of 1 and 2 this substructure is the
imidazolidine ring. A variety of replacements for this ring have
been found to be suitable, including five- and six-membered
saturated and unsaturated systems (2, 15-17) and fused rings
* Author to whom correspondence should be addressed [telephone (317)
337-3157; fax (317) 337-3215; e-mail jgsamaritoni@dow.com].
10.1021/jf021185r CCC: $25.00 © 2003 American Chemical Society
Published on Web 04/10/2003

3036 J. Agric. Food Chem., Vol. 51, No. 10, 2003
Samaritoni et al.
1621; MS (EI-DIP), m/z 265 ([M + 2]+, 19), 263 (M+, 55), 238 (35),
220 (35), 126 (100).
Anal. Calcd for C11H10ClN5O: C, 50.10; H, 3.82; N, 26.56. Found:
C, 50.16; H, 3.73; N, 26.38.
1-[(6-Chloropyridin-3-yl)methyl]piperazine-2,3-dione 3-O-Meth-
yloxime (3b). To a mixture of 0.080 g (0.96 mmol) of methoxylamine
hydrochloride in 1.0 mL of absolute ethanol was added in one portion
a solution of 106 mg (0.393 mmol) of the thioimidate 10a in 2 mL of
absolute ethanol. The solution was stirred at room temperature for 4 h
and then added to 25 mL of ice water. The pH was adjusted from 3 to
7 with solid sodium bicarbonate, and then the solution was extracted
once with dichloromethane. The extract was dried (MgSO4) and
concentrated to give 85 mg of an oil, which was chromatographed on
silica gel using the following gradient: dichloromethane and progressing
to 95:5 dichloromethane/methanol. Combining like fractions afforded
85 mg (72%) of 3b as a glass containing 11% w/w of dichloromethane,
which could not be removed under high vacuum: 1H NMR δ 8.31 (d,
1H, J ) 2.2 Hz), 7.72 (dd, 1H, J ) 8.0 Hz and J ) 2.5 Hz), 7.32 (d,
1H, J ) 8.1 Hz), 5.37 (br s, 1H), 4.68 (s, 2H), 3.97 (s, 3H), 3.45-3.36
(m, 4H); IR (neat) cm-1 2942, 1680, 1616; MS (EI-DIP), m/z 270 ([M
+ 2]+, 5), 268 (M+, 13), 237 (30), 173 (100), 126 (100). Additional
drying at high temperature and high vacuum resulted in the following
analytical data.
Figure 1.
Hewlett-Packard model 5989A mass spectrometer using the electron
impact-direct insertion probe (EI-DIP), chemical ionization (CI), fast
atom bombardment (FAB), chemical desorption (CD), or electrospray
(ES) technique and are reported as m/z. Microanalyses were performed
by Midwest Microlab of Indianapolis, IN.
1-[(6-Chloropyridin-3-yl)methyl]piperazine-2,3-dione (8). A solu-
tion of 925 mg (4.98 mmol) of the diamine 7 and 730 mg (4.99 mmol)
of diethyloxalate in 30 mL of ethanol was heated under reflux for 20
h, then cooled to room temperature, and concentrated to a residue, which
was recrystallized from ethanol to afford 448 mg (38%) of 8: mp
1
170.5-173.5 °C; H NMR δ 8.33 (d, 1H, J ) 2.4 Hz), 7.70 (dd, 1H,
J ) 8.3 Hz and J ) 2.4 Hz), 7.51 (br s, 1H), 7.35 (d, 1H, J ) 8.4 Hz),
4.69 (s, 2H), 3.52 (br s, 4H); IR (KBr) cm-1 3538, 3205, 1696, 1671;
MS (EI-DIP), m/z 241 ([M + 2]+, 11), 239 (M+, 29), 126 (100), 70
(83).
Anal. Calcd for C11H13ClN4O2: C, 49.17; H, 4.87; N, 20.85.
Found: C, 48.62; H, 4.85; N, 20.10.
1-[(6-Chloropyridin-3-yl)methyl]piperazine-2,3-dione 3-Oxime
Hydrochloride (3c). To a mixture of 147 mg (2.11 mmol) of
hydroxylamine hydrochloride in 2 mL of ethanol was added 142 mg
(0.526 mmol) of the thioimidate 10a followed by 2 mL of ethanol.
Within minutes a solution existed, and within 1 h a precipitate was
present. After stirring overnight at room temperature, the precipitate
was collected and air-dried to afford 83.5 mg (54%) of 3c as an off-
Anal. Calcd for C10H10ClN3O2: C, 50.11; H, 4.20; N, 17.53.
Found: C, 50.22; H, 4.27; N, 17.35.
1-[(6-Chloropyridin-3-yl)methyl]-3-thioxopiperazin-2-one (9). A
mixture of 2.65 g (11.1 mmol) of the dione 8 and 1.23 g (5.53 mmol)
of phosphorus pentasulfide in 30 mL of dry pyridine was heated at 80
°C for 3 h, then cooled, and added to 350 mL of ice water. The solution
was then extracted twice with dichloromethane, and the combined
extracts were dried (MgSO4). Concentration gave 2.47 g (87%) of 9 as
a pale yellow powder: 1H NMR δ 8.68 (br s, 1H), 8.35 (d, 1H, J )
2.8 Hz), 7.75 (dd, 1H, J ) 8.0 Hz and J ) 2.5 Hz), 7.35 (d, 1H, J )
8.4 Hz), 4.69 (s, 2H), 3.51-3.62 (m, 4H); IR (KBr) cm-1 3145, 1670;
MS (EI-DIP), m/z 257 ([M + 2]+, 28), 255 (M+, 68), 227 (17), 194
(42), 126 (100), 86 (64), 70 (61). A portion was recrystallized from
ethanol, mp 161.5-162.5 °C, to give the following analytical data.
Anal. Calcd for C10H10ClN3OS: C, 46.96; H, 3.94; N, 16.43; S,
12.54. Found: C, 47.01; H, 4.01; N, 16.33; S, 12.41.
1-[(6-Chloropyridin-3-yl)methyl]-3-(methylthio)-5,6-dihydropy-
razin-2(1H)-one (10a). To a mixture of 100 mg (2.50 mmol) of 60%
sodium hydride/mineral oil dispersion in 3 mL of dry dimethylforma-
mide was added dropwise at 0 °C a solution of 570 mg (2.23 mmol)
of the thione 9 in 6 mL of dimethylformamide. The mixture was allowed
to warm to room temperature over 1 h, then cooled in an ice bath, and
treated with 0.14 mL (319 mg, 2.25 mmol) of methyl iodide in one
portion. The contents were stirred at room temperature for 2 h and
were added to 90 mL of ice water. The aqueous mixture was extracted
twice with dichloromethane, and the combined extracts were dried
(MgSO4). Concentration gave 680 mg of an oil, which was chromato-
graphed on silica gel using gradient elution beginning with dichlo-
romethane and proceeding to 4:1 dichloromethane/ethyl acetate,
affording 369 mg (62%) of 10a as a light yellow solid: mp 83-84
°C; 1H NMR δ 8.33 (d, 1H, J ) 2.6 Hz), 7.68 (dd, 1H, J ) 8.2 Hz and
J ) 2.4 Hz), 7.33 (d, 1H, J ) 8.4 Hz), 4.61 (s, 2H), 3.79 (t, 2H, J )
6.4 Hz), 3.38 (t, 2H, J ) 6.4 Hz), 2.28 (s, 3H); MS (ESI), m/z 271 ([M
+ H + 2]+, 37), 269 ([M + H]+, 100).
1
white powder: mp 198-199 °C; H NMR (DMSO-d6) δ 11.38 (br s,
1H), 9.24 (br s, 1H), 8.42 (d, 1H, J ) 2.5 Hz), 7.85 (dd, 1H, J ) 8.0
Hz and J ) 2.6 Hz), 7.54 (d, 1H, J ) 8.0 Hz), 4.67 (s, 2H), 3.59 (m,
2H), 3.47 (m, 2H); IR (KBr) cm-1 2949, 2762, 1667; MS (EI-DIP),
m/z 256 ([M + 2 - HCl]+, 1), 254 ([M - HCl]+, 4), 238 (54), 155
(45), 126 (100).
Anal. Calcd for C10H12Cl2N4O2: C, 41.20; H, 4.18; N, 19.08.
Found: C, 41.19; H, 4.25; N, 19.10.
1-[(6-Chloropyridin-3-yl)methyl]piperazine-2,3-dione 3-Thiosemi-
carbazone (3d). A mixture of 504 mg (1.87 mmol) of the thioimidate
10a and 170 mg (1.87 mmol) of thiosemicarbazide in 10 mL of dry
ethanol was heated at 50-60 °C for 4 h. Upon cooling, the precipitate
was collected to afford 513 mg (88%) of 3d as a light yellow solid:
1
mp >270 °C; H NMR (DMSO-d6) δ 9.83 (s, 1H), 8.37 (d, 1H, J )
2.4 Hz), 8.12 (br s, 1H), 7.85 (dd, 1H, J ) 8.3 Hz and J ) 2.4 Hz),
7.51 (d, 1H, J ) 8.4 Hz), 7.48 (s, 1H), 7.31 (br s, 1H), 4.63 (s, 2H),
3.50 (m, 2H), 3.33 (m, 2H); IR (KBr) cm-1 3416, 3274, 1614; MS
(EI-DIP), m/z 240 ([M + 2 - NCSNH2]+, 23), 238 ([M - NCSNH2]+,
74), 155 (100).
Anal. Calcd for C11H13ClN6OS: C, 42.24; H, 4.19; N, 26.87.
Found: C, 42.27; H, 4.14; N, 26.60.
N-{4-[(6-Chloropyridin-3-yl)methyl]-3-oxopiperazin-2-ylidene}-
1,1,1-trifluoromethanesulfonamide (3e). A solution of 204 mg (0.756
mmol) of the thioimidate 10a and 510 mg (3.42 mmol) of trifluo-
romethanesulfonamide in 2 mL of tetrahydrofuran was heated at 50-
55 °C for 10 h and then allowed to cool. The solution was concentrated
to a residue, which was chromatographed on silica gel and eluted with
dichloromethane followed by dichloromethane/ethyl acetate mixtures
to afford 50 mg (18%) of 3e: mp 142-152 °C; 1H NMR δ 9.98 (br s,
1H), 8.34 (d, 1H, J ) 3.1 Hz), 7.74 (dd, 1H, J ) 8.2 Hz and J ) 2.6
Hz), 7.37 (d, 1H, J ) 8.4 Hz), 4.69 (s, 2H), 3.63-3.70 (m, 4H); IR
(neat) cm-1 3288, 1688, 1635; MS (EI-DIP), m/z 372 ([M + 2]+, 0.6),
370 (M+, 2), 269 (18), 237 (100).
Anal. Calcd for C11H10ClF3N4O3: C, 35.63; H, 2.72; N, 15.11.
Found: C, 35.84; H, 2.77; N, 14.93.
1-[(6-Chloropyridin-3-yl)methyl]-3-ethoxy-5,6-dihydropyrazin-
2(1H)-one (10b). A solution of 177 mg (0.656 mmol) of the thioimidate
10a and 97 mg (0.65 mmol) of trifluoromethanesulfonamide in 20 mL
of absolute ethanol was stirred at room temperature for 2 h and then
Anal. Calcd for C11H12ClN3OS: C, 48.97; H, 4.48; N, 15.58.
Found: C, 48.88; H, 4.54; N, 15.35.
4-[(6-Chloropyridin-3-yl)methyl]-3-oxopiperazin-2-ylidenecyana-
mide (3a). A solution of 96.0 mg (0.356 mmol) of the thioimidate 10a
and 166 mg (3.95 mmol) of cyanamide in 2.5 mL of ethanol was heated
at 50-60 °C for 3 h and then allowed to cool and stand overnight.
The precipitate was collected and washed with cold ethanol to afford
75 mg (80%) of 3a as white crystals: mp 185-187 °C; 1H NMR
(DMSO-d6) δ 9.86 (br s, 1H), 8.41 (d, 1H, J ) 2.4 Hz), 7.84 (dd, 1H,
J ) 8.3 Hz and J ) 2.4 Hz), 7.52 (d, 1H, J ) 8.3 Hz), 4.63 (s, 2H),
3.57 (m, 2H), 3.42 (m, 2H); IR (KBr) cm-1 3215, 2182, 1676, 1645,

Insecticidal Activity of Dihydropiperazine Neonicotinoid Compounds
J. Agric. Food Chem., Vol. 51, No. 10, 2003 3037
under reflux for 10 h. Upon cooling, the solution was concentrated in
vacuo to give a residue, which was chromatographed on silica gel
eluting with dichloromethane and then 9:1 dichloromethane/methanol
to afford 135 mg (77%) of 10b: mp 100-107 °C; 1H NMR δ 8.33 (d,
1H, J ) 2.1 Hz), 7.70 (dd, 1H, J ) 8.3 Hz and J ) 2.4 Hz), 7.34 (d,
1H, J ) 8.4 Hz), 4.63 (s, 2H), 4.18 (q, 2H, J ) 7.1 Hz), 3.63 (m, 2H),
3.39 (m, 2H), 1.38 (t, 3H, J ) 7.1 Hz); MS (EI), m/z 269 ([M + 2]+,
8), 267 (M+, 26), 239 (17), 126 (100). GC-MS analysis revealed an
impurity at greater retention time accounting for 6% of the total signal
from the flame ionization detector and resulted in an erroneous
elemental analysis. The impurity was not identified.
1-Methylpiperazine-2,3-dione (11). A solution of 1.46 g (10.0
mmol) of diethyloxalate and 0.741 g (10.0 mmol) of N-methylethyl-
enediamine in 6 mL of absolute ethanol was heated under reflux for
20 h and then concentrated to a residue, which was recrystallized from
4 mL of ethanol to afford 0.686 g (54%) of 11: mp 153.5-156.5 °C
[lit. 158 °C (i-PrOH) (20)]; 1H NMR δ 8.03 (br s, 1H), 3.47 (br s, 4H),
3.11 (s, 3H); IR (KBr) cm-1 3231, 1697, 1669; MS (EI-DIP), m/z 128
(M+, 28), 100 (100).
mmol) of the piperazinone 14. The mixture was warmed to 41 °C,
whereupon hydrogen gas evolution began to be observable. After
heating at 41-48 °C until deprotonation was complete, the mixture
was cooled to 20 °C and treated dropwise with 1.21 g (7.47 mmol) of
2-chloro-5-chloromethylpyridine in 3 mL of tetrahydrofuran. This was
immediately followed by dropwise addition of 6 mL of dry dimethyl-
formamide, and then the contents were heated at 45-50 °C for 7 h.
Upon cooling, the mixture was concentrated in vacuo to remove the
volatiles. The residue was diluted with heptane and concentrated again.
This was repeated once more with heptane. The residue was then
triturated under 100 mL of dichloromethane. The solid was filtered
and the filtrate concentrated to give 2.63 g of a solid, which was
recrystallized from 55 mL of methanol to afford 856 mg (43%) of 4 as
1
white needles: mp 189-190.5 °C; H NMR (DMSO-d6) δ 8.41 (d,
1H, J ) 2.4 Hz), 7.85 (dd, 1H, J ) 2.6 Hz and J ) 8.2 Hz), 7.53 (d,
1H, J ) 8.2 Hz), 4.74 (s, 2H), 3.67 (m, 2H), 3.56 (m, 2H), 2.96 (s,
3H); IR (KBr) cm-1 2185, 1680; MS (EI-DIP), m/z 277 (M+, 14), 126
(100).
Anal. Calcd for C12H12ClN5O: C, 51.89; H, 4.36; N, 25.22. Found:
C, 52.01; H, 4.37; N, 25.17.
Anal. Calcd for C5H8N2O2: C, 46.87; H, 6.29; N, 21.87. Found: C,
46.81; H, 6.24; N, 21.94.
The mother liquor of 4 was concentrated to a residue, which was
dissolved in dichloromethane, filtered, and chromatographed on silica
gel using ethyl acetate as eluant to give 139 mg (7%) of 15 as a solid:
1-Methyl-3-thioxopiperazin-2-one (12). To a mixture of 4.90 g
(11.8 mmol) of 97% Lawesson’s reagent in 140 mL of dry toluene
was added at 0 °C 3.01 g (23.5 mmol) of the dione 11. The contents
were mechanically stirred at 45-50 °C for 1.5 h and then at room
temperature for 3 h. The mixture was filtered to afford 5.7 g of a solid,
which was dissolved in 200 mL of dichloromethane. The solution was
treated with silica gel, and the solvent was removed in vacuo to afford
a dry flowable solid, which was placed at the top of a column of silica
gel. Elution with dichloromethane followed by gradually increasing
the polarity to 95:5 dichloromethane/methanol gave 2.8 g (83%) of 12
as a yellow solid containing a trace amount of Lawesson’s reagent
byproducts as evidenced by proton NMR. Recrystallization of a small
amount (320 mg) from methanol afforded 202 mg of 12 as bright yellow
1
mp 113-116 °C; H NMR δ 8.45 (d, 1H, J ) 2.4 Hz), 7.79 (dd, 1H,
J ) 2.4 Hz and J ) 8.4 Hz), 7.35 (d, 1H, J ) 8.7 Hz), 4.80 (s, 2H),
3.74 (m, 2H), 3.46 (m, 2H), 3.09 (s, 3H); IR (KBr) cm-1 2229, 1679,
1634; MS (EI-DIP), m/z 279 ([M + 2]+, 7), 277 (M+, 21), 236 (27),
126 (100).
Anal. Calcd for C12H12ClN5O: C, 51.89; H, 4.36; N, 25.22. Found:
C, 52.01; H, 4.42; N, 24.97.
1-Methylpiperazin-2,5-dione (16). A solution of 9.74 g (66.6 mmol)
of glycylsarcosine in 60 mL of ethylene glycol was heated at reflux
for 40 min. The ethylene glycol was removed in vacuo to afford a
solid that was recrystallized from 2-propanol to afford 7.02 g (82%)
1
1
crystals: mp 215-216 °C (dec); H NMR (DMSO-d6) δ 11.06 (br s,
of 17: mp 135.5-139 °C [lit. 141-143 °C (i-PrOH) (21)]; H NMR
1H), 3.55 (m, 2H), 3.52 (m, 2H), 2.95 (s, 3H); IR (KBr) cm-1 3172,
1662, 1535; MS (EI-DIP), m/z 144 (M+, 100), 69 (63).
δ 6.76 (br s, 1H), 4.03 (s, 2H), 3.98 (s, 2H), 3.00 (s, 3H); IR (KBr)
cm- 3235, 1687, 1658; MS (EI-DIP), m/z 129 ([M + H]+, 100).
1
Anal. Calcd for C5H8N2OS: C, 41.64; H, 5.59; N, 19.43; S, 22.24.
Found: C, 41.87; H, 5.72; N, 19.71; S, 21.87.
Anal. Calcd for C5H8N2O2: C, 46.87; H, 6.29; N, 21.87. Found: C,
47.04; H, 6.29; N, 21.80.
5,6-Dihydro-1-methyl-3-methylthiopiperazin-2-one (13). To a
mixture of 1.96 g (13.6 mmol) of the thione 12 in 25 mL of dry
acetonitrile was added at 0 °C 0.888 mL (2.02 g, 14.3 mmol) of methyl
iodide followed by 1.88 g (13.6 mmol) of anhydrous potassium
carbonate. The contents were allowed to warm to room temperature
and stir overnight. Methyl iodide (0.089 mL) was again added, and
after 6 h, the mixture was filtered and the filtrate concentrated to a
residue, which was triturated under 50 mL of dichloromethane and
filtered again. The filtrate was diluted with dichloromethane and dried
over MgSO4. Concentration gave 1.8 g of a yellow oil, which was
chromatographed on silica gel eluting with dichloromethane/ethyl
acetate mixtures to give 1.38 g (64%) of 13 as a solid: mp 38.5-40.5
°C; 1H NMR δ 3.81 (t, 2H, J ) 6.5 Hz), 3.42 (t, 2H, J ) 6.5 Hz), 3.04
(s, 3H), 2.25 (s, 3H); IR (KBr) cm-1 1666, 1597; MS (EI-DIP), m/z
158 (M+, 90), 143 (65), 115 (30), 87 (50), 72 (100).
1-Methyl-5-thioxopiperazin-2-one (17) and 1-Methylpiperazin-
2,5-dithione (18). A mixture of 460 mg (3.58 mmol) of 16 and 748
mg (1.79 mmol) of 97% Lawesson’s reagent in 22 mL of dry toluene
was heated at 55-60 °C for 1 h, cooled to room temperature, and
filtered to afford 434 mg of a white solid, which was an equimolar
mixture of 17 and 18 as evidenced by proton NMR and GC-MS
analysis. The mixture was dissolved in 250 mL of dichloromethane
and treated with silica gel. After removal of the solvent, the dry powder
was placed at the head of a column of silica gel and eluted with a
gradient beginning with dichloromethane and proceeding to 95:5
dichloromethane/methanol. First to be eluted was 150 mg (26%) of
the dithione 18: mp 214-218 °C (dec); 1
H NMR (DMSO-d6) δ 10.81
(br s, 1H), 4.56 (s, 2H), 4.31 (s, 2H), 3.33 (s, 3H); IR (KBr) cm-1
1598, 1556; MS (EI-DIP), m/z 160 (M+, 100), 58 (72).
Anal. Calcd for C5H8N2S2: C, 37.47; H, 5.03; N, 17.48; S, 40.02.
Found: C, 37.52; H, 5.01; N, 17.38; S, 40.00.
Anal. Calcd for C6H10N2OS: C, 45.54; H, 6.37; N, 17.71. Found:
C, 45.32; H, 6.49; N, 17.57.
The monothione 17 (160 mg) was eluted shortly thereafter. It was
found to be contaminated with 18. Recrystallization from methanol
gave 70 mg (14%): mp 153.5-155.5 °C; 1H NMR (DMSO-d6) δ 10.61
(br s, 1H), 4.35 (s, 2H), 3.84 (s, 2H), 2.83 (s, 3H); IR (KBr) cm-1
1685, 1650; MS (EI-DIP), m/z 144 (M+, 100), 58 (31).
5,6-Dihydro-1-methyl-3-cyanoiminopiperazin-2-one (14). A mix-
ture of 1.27 g (8.03 mmol) of the thioimidate 13 and 1.32 g (31.4 mmol)
of cyanamide in 10 mL of absolute ethanol was heated at 58-60 °C
for 4 h and then allowed to cool. The precipitate was collected and
recrystallized from methanol to afford 570 mg (47%) of 14 as white
crystals: mp 202-4 °C (dec); 1H NMR (DMSO-d6) δ 9.79 (br s, 1H),
3.55 (m, 2H), 3.42 (m, 2H), 2.96 (s, 3H); IR (KBr) cm-1 3198, 2194,
2163, 1624; MS (EI-DIP), m/z 152 (M+, 100).
Anal. Calcd for C5H8N2OS: C, 41.64; H, 5.59; N, 19.43; S, 22.24.
Found: C, 41.35; H, 5.59; N, 19.07; S, 21.84.
3,6-Dihydro-1-methyl-5-methylthiopiperazin-2-one (19). A mix-
ture of 1.9 g (10.2 mmol) of a 50 wt % mixture of the monothione/
dithione (17/18), 1.82 g (13.2 mmol) of potassium carbonate, and 1.24
mL (2.83 g, 19.9 mmol) of methyl iodide in 25 mL of dry acetonitrile
was stirred at room temperature overnight. The mixture was filtered
and the filtrate concentrated to dryness. The residue was chromato-
graphed on silica gel using dichloromethane as eluant and progressing
to 94:6 dichloromethane/methanol. A yield of 1.5 g (65%) of 19 was
obtained as a liquid: 1H NMR δ 4.28 (t, 2H, J ) 2.9 Hz), 4.02 (t, 2H,
Anal. Calcd for C6H8N4O: C, 47.36; H, 5.30; N, 36.82. Found: C,
47.11; H, 5.30; N, 36.42.
1-[(6-Chloropyridin-3-yl)methyl]-4-methyl-3-oxopiperazin-2-
ylidenecyanamide (4) and (6-Chloropyridin-3-yl)methyl(4-methyl-
3-oxo-3,4,5,6-tetrahydropyrazin-2-yl)cyanamide (15). To a mixture
of 347 mg (8.67 mmol) of 60% sodium hydride/mineral oil dispersion
in 9 mL of dry tetrahydrofuran was added in one portion 1.08 g (7.10

3038 J. Agric. Food Chem., Vol. 51, No. 10, 2003
Samaritoni et al.
J ) 2.9 Hz), 2.99 (s, 3H), 2.36 (s, 3H); MS (EI-DIP), m/z 158 (M+,
100), 143 (7), 125 (43), 72 (31), 57 (43).
graphed on silica gel using 98:2 dichloromethane/methanol as eluant
and proceeding to 93:7 over a 1 h period to afford 740 mg (48%) of
23. A small amount was triturated under ethyl ether to give an analytical
3,6-Dihydro-1-methyl-5-cyanoiminopiperazin-2-one (20). A solu-
tion of 870 mg (5.5 mmol) of the thioimidate 19 and 870 mg (21 mmol)
of cyanamide in 6 mL of ethanol was heated at 60-70 °C for 3 h.
Removal of volatiles in vacuo at 40-45 °C gave a residue that was
chromatographed on silica gel eluting with 95:5 dichloromethane/
methanol to afford 640 mg of 20 contaminated with cyanamide. This
material was recrystallized from methanol (9 mL) to give 434 mg (52%)
of pure 20: mp 188-192 °C (dec); 1H NMR (DMSO-d6) δ 9.62 (br s,
1H), 4.33 (br s, 2H), 3.84 (s, 2H), 2.87 (s, 3H); MS (EI-DIP), m/z 152
(M+, 100).
1
sample of a white solid: mp 87-88 °C; H NMR δ 7.39 (br s, 1H),
3.12 (s, 3H), 3.04 (s, 3H), 2.99 (d, 3H, J ) 4.5 Hz); IR (KBr) cm-1
3313, 2195, 1667, 1614; MS (EI-DIP), m/z 155 ([M + H]+, 5), 154
(M+, 4), 126 (19), 72 (100).
Anal. Calcd for C6H10N4O: C, 46.74; H, 6.54; N, 36.34. Found: C,
46.85; H, 6.47; N, 36.02.
2-[(6-Chloropyridin-3-yl)methylmethylamino-2-cyanoimino]-N,N-
dimethylacetamide (6). To a suspension of 230 mg (5.76 mmol) of
60% sodium hydride/mineral oil in 3 mL of dry dimethylformamide
cooled in an ice bath was added dropwise via syringe a solution of
740 mg (4.80 mmol) of the amidine 23 in 5 mL of dry tetrahydrofuran.
After 15 min, a solution of 855 mg (5.28 mmol) of 2-chloro-5-
chloromethylpyridine in 5 mL of tetrahydrofuran was added dropwise
via syringe. This was followed by the addition of 3 mL of dimethyl-
formamide. The contents were stirred overnight at room temperature
and then concentrated in vacuo to remove most of the solvent. The
residue was triturated under 100 mL of dichloromethane and then
filtered, and the filtrate was dried over sodium sulfate. Concentration
gave 1.75 g of an oil, which was chromatographed on silica gel eluting
with 98:2 dichloromethane/methanol for 1 h and proceeding to 95:5
over a 0.5 h period to afford 990 mg (74%) of 7: mp 141-144 °C; 1H
NMR (DMSO-d6) δ 8.41 (d, 1H, J ) 1.8 Hz), 7.82 (dd, 1H, J )
8.2 Hz and J ) 2.6 Hz), 7.58 (d, 1H, J ) 8.3 Hz), 4.73 (m, 2H), 3.05
(s, 3H), 2.97 (s, 3H), 2.94 (s, 3H); IR (KBr) cm-1 2183, 1657; MS
(EI-DIP), m/z 280 ([M + H]+, 9), 279 (M+, 6), 208 (38), 126 (48), 72
(100).
Anal. Calcd for C6H8N4O: C, 47.36; H, 5.30; N, 36.82. Found: C,
47.37; H, 5.31; N, 36.59.
1-(2-Chloropyridin-5-yl)methyl-3,6-dihydro-4-methyl-2-cyanoim-
inopiperazin-5-one (5). To a solution of 2 mL of dry tetrahydrofuran
and 2 mL of dry dimethylformamide cooled in an ice bath was added
in one portion 100 mg (2.50 mmol) of 60% sodium hydride/mineral
oil dispersion. This was followed by portionwise addition of 383 mg
(2.50 mmol) of the amidine 21. The mixture was heated to 60 °C, then
allowed to cool to room temperature, and treated dropwise via syringe
with a solution of 405 mg (2.50 mmol) of 2-chloro-5-chloromethylpy-
ridine in 2 mL of dry tetrahydrofuran. After stirring at room temperature
for 1 h, the contents were heated at 40 °C for 8 h. The mixture was
cooled to room temperature, diluted with heptane, and concentrated in
vacuo to remove as much of the dimethylformamide as possible. The
resulting oil was chromatographed on silica gel eluting with 96:4
dichloromethane/methanol to afford 590 mg (85%) of 5: mp 123-
125 °C; 1H NMR δ 8.38 (d, 1H, J ) 2.5 Hz), 7.71 (dd, 1H, J ) 2.6 Hz
and J ) 8.1 Hz), 7.38 (d, 1H, J ) 8.4 Hz), 4.72 (s, 2H), 4.55 (s, 2H),
4.00 (s, 2H), 3.05 (s, 3H); IR (KBr) cm-1 2181, 1670; MS (EI-DIP),
m/z 279 ([M + 2]+, 8), 277 (M+, 23), 126 (100).
Anal. Calcd for C12H14ClN5O: C, 51.52; H, 5.04; N, 25.04. Found:
C, 51.37; H, 5.00; N, 24.84.
N-[(Tetrahydrofuran-3-yl)methyl]-5,6-dihydro-4-methyl-2-cy-
anoiminopiperazin-3-one (24). To a suspension of 120 mg (3.0 mmol)
of 60% sodium hydride/mineral oil dispersion in 3 mL of tetrahydro-
furan was added in one portion 387 mg (2.54 mmol) of 14. The mixture
was heated at 40-45 °C for 1 h and was then cooled to room
temperature. It was then treated dropwise via syringe with a solution
of 458 mg (2.54 mmol) of tetrahydrofuran-3-ylmethyl sulfonate (23)
in 3 mL of tetrahydrofuran followed by 3 mL of dimethylformamide.
The contents were then heated to 90-100 °C while a stream of nitrogen
removed most of the tetrahydrofuran. After 10h the mixture was allowed
to cool and was concentrated to a residue which was triturated under
dichloromethane to give 320 mg of a yellow solid. The filtrate was
concentrated to a residue which was chromatographed on silica gel
(230-400 mesh) using 95/5 dichloromethane/methanol as eluant to give
200 mg (33%) of 30, mp 170-6 °C; 1H NMR δ 3.45-3.96 (m, 10H),
3.13 (s, 3H), 2.74 (m, 1H), 2.07 (m, 1H), 1.66 (m, 1H); IR (KBr) cm-1
2166, 1676; MS (ES+) m/z 237 ([M+1]+, 11).
Anal. Calcd for C12H12ClN5O: C, 51.89; H, 4.36; N, 25.22. Found:
C, 51.66; H, 4.35; N, 24.94.
N-Methyl-2-methylamino-2-thioxoacetamide (21). To a mixture
cooled in ice of 4.95 g (37.2 mmol) of ethyl thiooxamate in 5 mL of
dry tetrahydrofuran was added over a 1-2 min period 41 mL (82 mmol)
of a 2.0 M solution of methylamine in tetrahydrofuran. The contents
were then heated at 40 °C for 1 h and then stirred at room temperature
for 2 h. The solution was concentrated to dryness to afford 4.8 g (98%)
of 21 as a solid. Recrystallization of 167 mg of 21 from methanol
1
afforded 100 mg: mp 129-131 °C [lit. 122-4 °C (22)]; H NMR δ
9.61 (br s, 1H), 8.21 (br s, 1H), 3.25 (d, 3H, J ) 5.2 Hz), 2.94 (d, 3H,
J ) 4.8 Hz); IR (KBr) cm-1 3303, 3247, 1678; MS (EI-DIP), m/z 133
([M + H]+,1), 65 (100).
Anal. Calcd for C4H8N2OS: C, 36.34; H, 6.10; N, 21.20. Found:
C, 36.53; H, 6.13; N, 21.03.
2-Methylimino-2-methylthio-N,N-dimethylacetamide (22). To a
suspension of 2.4 g (0.060 mol) of 60% sodium hydride/mineral oil
dispersion in 20 mL of dry tetrahydrofuran was added at 15-20 °C
3.56 g (0.0269 mol) of the thioacetamide 21 through a solid addition
funnel over a 20 min period. The contents were allowed to stir at room
temperature for 1 h, then cooled to 15 °C, and treated with 4.3 mL
(0.069 mol) of methyl iodide in 4 mL of tetrahydrofuran dropwise via
syringe over a 10-15 min period. The contents were stirred at room
temperature for 3 h and then concentrated to a solid residue, which
was triturated under 250 mL of ethyl ether. The mixture was filtered,
and the filtrate was dried over sodium sulfate. Concentration gave 4.67
g of a liquid, which was chromatographed on silica gel using ethyl
acetate as eluant to afford 2.66 g (62%) of 22 as a mixture of syn and
anti isomers: 1H NMR (data for predominant isomer given) δ 3.26
(s, 3H), 3.03 (s, 6H), 2.40 (s, 3H); IR (KBr) cm-1 1660, 1624; MS
(EI-DIP), m/z 160 (M+, 6), 145 (10), 113 (42), 88 (90), 72 (100).
Anal. Calcd for C6H12N2OS: C, 44.97; H, 7.55; N, 17.48. Found:
C, 44.62; H, 7.46; N, 17.40.
Anal. Calcd. for C11H16N4O2: C, 55.91; H, 6.83; N, 23.71. Found:
C, 55.75; H, 6.77; N, 23.60.
N-(2-Chlorothiazol-5-yl)methyl-5,6-dihydro-4-methyl-2-cyanoimi-
nopiperazin-3-one (25). To a suspension of 145 mg (3.62 mmol) of
60% sodium hydride/mineral oil dispersion in 4 mL of dry tetrahy-
drofuran was added 460 mg (3.02 mmol) of 14 in one portion. The
mixture was heated at 40 °C for 0.5 h, cooled in ice, and then treated
with a solution of 592 mg (3.24 mmol) of 92% 2-chloro-5-chloro-
methylthiazole (24) in 3 mL of tetrahydrofuran followed by 3 mL of
dry dimethylformamide. The contents were heated at 40-45 °C for 18
h, cooled to room temperature, then diluted with heptane, and
concentrated to a solid residue, which was triturated under dichlo-
romethane. The mixture was filtered, and the filtrate was dried over
sodium sulfate. Concentration gave a residue that was chromatographed
on silica gel (230-400 mesh) using dichloromethane as eluant and
progressing in a gradient manner to 97:3 dichloromethane/methanol to
give 400 mg (44%) of 31: mp 201-204 °C; 1H NMR δ 7.53 (s, 1H),
4.82 (s, 2H), 3.69 (m, 2H), 3.59 (m, 2H), 3.11 (s, 3H); IR (KBr) cm-1
2176, 1679; MS (EI-DIP), m/z 285 ([M + 2]+, 11), 283 (M+, 27), 248
(24), 190 (29), 132 (100).
2-Cyanoimino-2-methylamino-N,N-dimethylacetamide (23). A
solution of 1.61 g (10.0 mmol) of 22 and 670 mg (16 mmol) of
cyanamide in 4 mL of absolute ethanol was heated at 90 °C for 10 h.
The ethanol was removed in vacuo, the residue was triturated under
dichloromethane and filtered, and the filtrate was dried over sodium
sulfate. Concentration gave 1.4 g of a residue, which was chromato-
Anal. Calcd for C10H10ClN5OS: C, 42.33; H, 3.55; N, 24.68.
Found: C, 42.25; H, 3.39; N, 24.36.

Insecticidal Activity of Dihydropiperazine Neonicotinoid Compounds
J. Agric. Food Chem., Vol. 51, No. 10, 2003 3039
Binding to Nicotinic Acetylcholine Receptor. Adult houseflies
(Musca domestica) were first frozen with liquid nitrogen and then
shaken to separate the heads from the bodies. Heads were recovered
by sieving. Housefly head homogenate was prepared by first placing
the heads in ice-cold (4 °C) buffer containing 0.2 M sucrose, 1 mM
ethylenediaminetetraacetic acid, 10 mM Trizma-HCl, and 0.1 mM
phenylmethanesulfonyl fluoride and then homogenizing in a blender.
The resulting homogenate was then filtered through cheesecloth, and
the effluent was centrifuged at 1000g (4 °C) for 15 min. The supernatant
was then centrifuged again at 35000g (4 °C) for 20 min. The supernatant
from the first high-speed spin was discarded, and the remaining pellet
was resuspended in the aforementioned buffer and was then centrifuged
a second time at 35000g for 20 min. The supernatant was again
discarded, and the remaining pellet was resuspended in 50 mM Tris-
HCl at pH 7.4. Aliquots of head membrane homogenate were stored
at -80 °C.
When needed, aliquots of protein were thawed and resuspended in
binding buffer containing 120 mM NaCl and 50 mM Trizma-HCl, pH
7.4. Experiments were performed in 96-well polypropylene microtiter
plates (Beckman Coulter, Fullerton, CA). [3H]Imidacloprid was syn-
thesized by Amersham Pharmacia Biotech (Piscataway, NJ; 30.1
mCi/mmol) and was stored at -80 °C in ethanol. All other chemicals
were obtained from Sigma Chemical Co. (St. Louis, MO).
Table 1. Binding Affinity to the nAChR of Housefly Head Membrane
and Activity on Cotton Aphid
a
compound
K (nM) ± SEM
50 ppm
5 ppm
0.5 ppm
i
2
17b
+++
+++
+++
++
+++
+
++
+
−
+++
nt
nt
−
+
−
nt
3a (R ) CN)
441 ± 106
1530 ± 159
3340 ± 886
3919 ± 754
593 ± 243
6 ± 3.3
3b (R ) OMe)
3c (R ) OH‚HCl)
3d (R ) NHCSNH )
−
2
3e (R ) SO CF3)
+++
+++
++
−
2
4
5
+++c
nt
>10000
−
6
8
9
15
24
25
3833 ± 1623
400 ± 101
1777 ± 226
>10000
+
−
+++
+
−
−
nt
+
+
nt
+++
+++
−
−
+++
517 ± 223
4 ± 1.4
nt
nt
++c
+++ ) 90−100% mortality; ++ ) 70−89% mortality; + ) 50−69% mortality;
− ) <50% mortality. a Standard error of measurement with n g 3, b n ) 1, c at
0.78 ppm; nt ) not tested.
For all experiments, [3H]imidacloprid was diluted to a final test
concentration of 2 nM. Test compounds were first dissolved at high
concentrations in DMSO and then diluted to the appropriate test
concentrations. The highest concentration of DMSO, 0.25%, had no
demonstrable effect on the binding of [3H]imidacloprid. Tissue, test
compounds, and radioactivity were combined and incubated at room
temperature (22 °C) for 1 h. The assay was terminated by filtra-
tion through a GF/C glass fiber filter, rinsing with ice-cold binding
buffer, and employing a TomTec automated harvester (TomTec,
Hamden, CT). Bound radioactivity was counted on a Wallac 1205
Betaplate liquid scintillation counter (Wallac, Inc., Gaithersburg, MD).
Nonspecific binding was defined by binding in the presence of 10-4
M nicotine. Each compound was run three times in triplicate, except
where noted.
Figure 2.
Biology. Cotton Aphid (CA; Aphis gossypii). Summer crookneck
squash seedlings (Cucurbita pepo) at the expanded cotyledon stage were
pruned to a single cotyledon. All stages of cotton aphid were transferred
to the seedlings by placing infested leaf sections on seedlings 16-24
h prior to the application of the test material. As the infested sections
dry out, the aphids move to the succulent plant material. The dried out
leaf sections were removed, and plants were examined for good
infestation prior to application of experimental compounds
RESULTS AND DISCUSSION
Chemistry. The key methodology used in the syntheses of
compounds 3-6 involved discrimination between carbonyl
groups by selectively converting one of them to a thiocarbonyl.
In the route used to synthesize 3a-e (Table 1), the carbonyl
groups of the dihydro-2,3-piperazinedione system were dif-
ferentiated by preparing the N-alkylated derivative 8 from the
diamine 7 (30) and diethyloxalate (Figure 2). The C-3 carbonyl
was then selectively converted to the thiocarbonyl using
phosphorus pentasulfide to give 9. The thioimidate 10a was
obtained under strongly basic conditions using sodium hydride
and was found to be stable toward silica gel chromatography,
although the crude thioimidate was used with success in the
final reaction, preparation of the amidines 3. Amidines 3a-d
were prepared in ethanol; however, solvolysis of 10a occurred
in the presence of trifluoromethanesulfonamide to give the ethyl
imidate 10b. This is not a surprising result considering the weak
nucleophilicity of trifluoromethanesulfonamide and its greater
acidity compared to the other amines used. The desired amidine
3e was obtained in low yield when the reaction was carried out
in tetrahydrofuran.
The positions occupied by cyanoimino and carbonyl in 3a
are switched in the structure of 4. N-Methyldihydropiperazine-
2,3-dione (11) was prepared by the condensation of N-meth-
ylethylenediamine and diethyloxalate (Figure 3). Conversion
to the 3-thione 12 was found to be more convenient using
Lawesson’s reagent, and then sulfur was methylated under
mildly basic conditions. Alkylation of 14 with 5-chloromethyl-
2-chloropyridine (31) proceeded to give both regioisomers 4
and 15 in a 10:1 ratio with the desired isomer 4 predominating.
The experimental compounds were dissolved at 1 mg/mL in 90:10
acetone/alcohol and then diluted in water containing 0.05% Tween 20.
Serial dilutions were made to yield solutions of 50, 5.0, and 0.50 ppm.
Application was made with a hand-held Devilbiss, air-brush sprayer.
The squash cotyledons were sprayed on both the upper and lower sides
of the cotyledon until runoff, and then all plants within the treatment
were sprayed evenly until the remaining spray solution was completely
used. Each rate had four replications (plants). Controls consisted of
eight plants treated with diluent prepared with a blank stock solution
only.
Tests were held in a holding room for 72 h at approximately 74 °F,
40% relative humidity, and 24 h photoperiod prior to grading. Tests
were graded by assessing the live aphid count (all nonwinged stages)
on the underside of each cotyledon using a dissecting binocular
microscope. Live count results were used to calculate percent control
using Abbott’s correction formula for solvent blank controls.
Molecular Modeling. Overlays of molecular models were con-
structed in Sybyl 6.6 (Tripos, St. Louis, MO) using the active analogue
method (25, 26). The molecules were minimized using the Tripos force
field (27) and Ga¨steiger-Hu¨ckel charges (28, 29). The minimization
parameters used were as follows: method ) powell, termination criteria
) energy, minimum energy change ) 0.00001, maximum iterations
) 1000000, dielectric function ) distance, dielectric constant ) 2.0.
Default values were used for all other variable parameters. FlexS 1.5
(34) was used for rigid overlays of minimized structures.

3040 J. Agric. Food Chem., Vol. 51, No. 10, 2003
Samaritoni et al.
Figure 3.
Figure 5.
Figure 4.
Figure 6.
The amidine carbons in 3a and 4 (C-3 and C-2, respectively)
are both flanked by carbonyl groups, which may impart
undesired reactivity to the amidine moiety such as susceptibility
to hydrolysis. The dihydropiperazine 5 was targeted with this
in mind as it features the carbonyl group now at C-5 of the
ring. Its synthesis, outlined in Figure 4, begins with the
cyclodehydration of glycylsarcosine under refluxing ethylene
glycol to give the required dihydropiperazinedione 16 in good
yield (21, 32). Treatment with half of an equivalent of
Lawesson’s reagent resulted in unavoidable formation of the
dithione 18 along with an equal amount of the desired 17.
Careful monitoring of this reaction, which was carried out in
toluene, revealed that no reaction occurred at 40 °C, but at 50
°C both products were observed to have been formed. Chro-
matographic separation of 17 and 18 was problematic, so it was
decided to treat the mixture to the methylation conditions. This
afforded the desired S-methylated product 19, which was easily
purified by silica gel chromatography. Conversion to the amidine
20 with cyanamide was then followed by alkylation, which gave
5 as the only detectable isomer. Its structure was established
through selective INEPT carbon-13 NMR experiments.
Because it is known that acyclic analogues of 1 and 2 retain
efficacy associated with the imidazolidines, it seemed prudent
to prepare the acyclic example 6 to determine the effect on
biological activity of unhindered rotation about the C2-C3
bond. Figure 5 outlines the synthesis of 6 starting from
commercially available ethyl thiooxamate. When treated with
aqueous methylamine at room temperature, the thiooxamate
yielded two products in addition to the desired 21 identified by
GC-MS as N,N′-dimethyloxamide and the N-methylamidine of
the starting thiooxamate. It was found that these byproducts
could be completely suppressed by using methylamine in
tetrahydrofuran under anhydrous conditions. Compound 21 was
then N,S-dimethylated to give the thioimidate 22, which was
considerably less reactive toward cyanamide than the thioimi-
dates 10a, 13, and 19. Alkylation of 23 under the standard
conditions gave 6, the proton NMR spectrum of which in
deuteriochloroform exhibits restricted rotation within the mol-
ecule.
In addition to the 2-chloropyridinyl moiety of 4, 3-tetrahy-
drofuranylmethyl and a thiazolylmethyl were also introduced
at N1 of the dihydropiperazine 14 to give 24 and 25 (Figure 6)
using the same methodology as described above for the synthesis
of 4.
Biology. Compounds were first evaluated for their ability to
bind to the nAChR of housefly head membrane (M. domestica)
followed by an in vivo assay in which percent mortality of cotton
aphid (CA; A. gossypii) on squash was observed at 50, 5, and
0.5 parts per million (ppm). The results of these tests are
compiled in Table 1. Compounds 4 and 25 exhibit the highest
affinity for the nAChR, that is, the lowest Ki values, and were
found to be the most active in vivo on CA. Compound 4
demonstrated broad-spectrum activity affording control of sweet
potato whitefly (WF), brown planthopper (BPH), and green
leafhopper (GLH) (data not shown). Compounds with higher
Ki values were invariably less active on CA than 4 and 25;
however, the Ki value was not always a good predictor of in
vivo activity within this set. For example, 24 performs more
poorly in vivo at 50 ppm than anticipated on the basis of its in
vitro result similar to those for 3a, 3e, and 8, which gave
excellent control of CA. Neither metabolism nor distribution
studies were conducted to shed light on anomalies such as this,
although there was no evidence to suggest that any compound
within Table 1 was chemically unstable under the in vitro
experimental conditions.
Compounds 3a and 5, piperazinone regioisomers of 4, are
seen to possess 2-4 orders of magnitude less affinity, respec-
tively, for the receptor, and this is reflected in lower efficacy
on CA. Compound 6, which mimics the piperazinone substitu-
tion pattern of 4 but is acyclic, was found to perform poorly in
both in vitro and in vivo assays.

Insecticidal Activity of Dihydropiperazine Neonicotinoid Compounds
J. Agric. Food Chem., Vol. 51, No. 10, 2003 3041
Figure 7. Four compounds used to define the pharmacophore model (1,
2, 4, and 26).
Figure 9. Overlays of 2, 3a, 5, and 6 with 4 (orange).
degree of variation in the overlap of their five- and six-
membered rings, respectively. Both of these molecules dem-
onstrate tight binding to the housefly head nicotinic receptor as
well as high efficacy on CA. Note that the NH of 2 is equivalent
in position to the carbonyl group of 4 rather than the N-methyl
group. This suggests that the NH/carbonyl position does not
provide a significant degree of binding to the receptor because
opposite charges are represented by these two functionalities.
Compounds 3a, 5, and 6, which exhibit decreased binding to
the receptor with consequential loss of mortality observed
against CA, each possess features or changes in geometries that
reduce overlap of the pharmacophore elements with those of 4
(Figure 9). Only the terminal nitrogen atom of the cyanoimino
group of 3a overlaps with the cyanoimino group of 4, and
overlap of their chloropyridinyl groups has also been reduced.
Although the chloropyridinyl and cyanoimino moieties of 5 and
4 superimpose well, the carbonyl oxygen of 5 occupies a region
of space not encroached upon by 4 and may well be responsible
for the observed lack of activity. The acyclic analogue 6 features
a carbamoyl group which cannot adopt coplanarity with its
cyanoimino group, an arrangement that would require 20 kcal/
mol. This results in very poor overlap of the oxygen atoms of
the carbamoyl groups of 4 and 6 and places the N,N-
dimethylamino group of the carbamoyl functionality of 6 into
a region of molecular space not impinged upon by 4 or the three
reference compounds.
Figure 8. Overlay of 1 (orange), 2, 4, and 26.
Molecular Modeling. In an effort to understand the results
of the in vitro and in vivo testing in a qualitative manner,
compound 4 and the reference compounds 1, 2, and 26 (10,
Figure 6), all potent binders in the imidacloprid housefly head
binding assay (1, ∼0.1 nM; 2, 17 nM; 4, 6 nM; and 26, 0.4
nM), were first modeled and their minimized conformations
overlaid to define our pharmacophore model. The procedure
involved building 2 in a conformation similar to its reported
crystal structure (33), minimizing it, and then doing a rigid
overlay of 1 and 4 onto 2 using FlexS 1.5 (34). The minimized
structure of 26 was also overlaid on 1 using FlexS. Figure 7
shows these minimized structures of the four compounds. All
possess two hydrogen bond acceptors (labeled A and B) and a
chlorine atom on the pyridine ring (labeled C), which we
consider key pharmacophore elements, and overlap well as
illustrated in Figure 8. This pharmacophore model differs from
the previous one published by Okazawa and co-workers (35,
36). In their model, the chloropyridinyl ring is in an extended
conformation pointing away from the nitroimino or cyanoimino
hydrogen bond accepting groups. In our model, the chloropy-
ridinyl ring is in a folded conformation pointing toward the
nitroimino or cyanoimino hydrogen bond accepting groups.
Compound 26 fits well in the folded shape but cannot adopt
the extended conformation needed to fit the Okazawa model.
Therefore, we feel the folded shape explains our current binding
data better than the extended model. The two active analogues,
2 and 4 (see Figure 9, 4 in orange), show a high degree of
overlap of their NCN and chloropyridinyl groups and a small
Conclusions. Syntheses of various isomeric dihydropipera-
zines can be approached successfully by taking advantage of
the regioselective monothionation of their respective diones.
Preparation of the precursor unsymmetrical N-substituted pip-
erazinediones from readily available diamines is key to this
selectivity.
The dihydropiperazine ring system, as exemplified in 4 and
25, has been shown to be a suitable bioisosteric replacement
for the imidazolidine ring system contained in neonicotinoid
compounds such as 2. However, placement of the cyanoimino
electron-withdrawing group further removed from the pyridine
ring, as in 3a, or relocation of the carbonyl group, as in 5, results
in significantly deceased bioisosterism. The dihydropiperazine
ring system of 4 and 25 also lends a degree of rigidity to the
molecule that is not offered by the inactive acyclic counterpart
6. The pharmacophore model proposed here qualitatively

3042 J. Agric. Food Chem., Vol. 51, No. 10, 2003
Samaritoni et al.
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ABBREVIATIONS USED
1, 2-chloro-5-({2-[oxido(oxo)hydrazono]imidazolidin-1-yl}-
methyl)pyridine; 2, 1-[(6-chloropyridin-3-yl)methyl]imidazoli-
din-2-ylidenecyanamide; 26, N-[(2E)-1-[(6-chloro-3-pyridinyl)-
methyl]-2(1H)-pyridinylidene]-2-pyrazinecarboxamide.
ACKNOWLEDGMENT
We thank Scott Thornburgh for the proton and carbon-13 NMR
spectral analyses for compounds 4, 5, and 15.
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Received for review December 5, 2002. Revised manuscript received
February 28, 2003. Accepted February 28, 2003.
JF021185R