Synthesis and herbicidal activity of N-oxide derivatives

Article abstract of DOI:10.1021/jf9904766

As part of an ongoing program on the chemistry and biological activity of N-oxide-containing molecules, a number of novel 1,2,5-oxadiazole N-oxide, benzo[1,2-c]1,2,5-oxadiazole N-oxide, and quinoxaline N,N'-dioxide derivatives were synthesized and evaluated for their herbicidal activity. Many of these compounds exhibited moderate to good herbicidal pre-emergence activity against Triticum aestivum. Dose - response studies were done on the more representative compounds (12, 20, and 26). The most active compound, butylcarbamoylbenzo[1,2-c]1,2,5-oxadiazole N-oxide, 26, displayed herbicidal activity at concentrations as low as 24 g/ha.

Full text of DOI:10.1021/jf9904766

J. Agric. Food Chem. 2000, 48, 2995−3002
2995
Syn th esis a n d Her bicid a l Activity of N-Oxid e Der iva tives
Hugo Cerecetto,* Eduardo Dias, Rossanna Di Maio, Mercedes Gonza´lez,* Sandra Pacce,
Patricia Saenz, and Gustavo Seoane
Ca´tedra de Qu´ımica Orga´nica, Facultad de Qu´ımica, Universidad de la Repu´blica, CC 1157,
11800 Montevideo, Uruguay
Leopoldo Suescun and Alvaro Mombru´
Laboratorio de Cristalograf´ıa y Qu´ımica del Estado So´lido, Facultad de Qu´ımica,
Universidad de la Repu´blica, Montevideo, Uruguay
Grisel Ferna´ndez, Marisa Lema, and J uana Villalba
Ca´tedra de Cereales y Cultivos Industriales, Facultad de Agronom´ıa, Universidad de la Repu´blica,
Paysandu´, Uruguay
As part of an ongoing program on the chemistry and biological activity of N-oxide-containing
molecules, a number of novel 1,2,5-oxadiazole N-oxide, benzo[1,2-c]1,2,5-oxadiazole N-oxide, and
quinoxaline N,N′-dioxide derivatives were synthesized and evaluated for their herbicidal activity.
Many of these compounds exhibited moderate to good herbicidal pre-emergence activity against
Triticum aestivum. Dose-response studies were done on the more representative compounds (12,
20, and 26). The most active compound, butylcarbamoylbenzo[1,2-c]1,2,5-oxadiazole N-oxide, 26,
displayed herbicidal activity at concentrations as low as 24 g/ha.
Keyw or d s: Herbicide; 1,2,5-oxadiazole N-oxide; benzo[1,2-c]1,2,5-oxadiazole N-oxide; quinoxaline
N,N′-dioxide
INTRODUCTION
occurred to us to examine the herbicidal activity of new
N-oxide-containing compounds developed in our labora-
tory.
The discovery and development of new herbicides is
a long and difficult endeavor. A large number of
herbicides are currently available to assist in controlling
weeds in a variety of crops. Herbicides can be classified
into families based on their chemical structures and
modes of action (Thedoridis et al., 1990, and references
therein). For example, the pyrazole phenyl ether her-
bicides act by inhibiting the protoporphyrinogen oxidase
(Clark, 1996; Nandihalli et al., 1994); the sulfonylureas
and imidazolinones inhibit amino acid biosynthesis
(Kleschick et al., 1990, and references therein; Wepplo,
1990; Hay, 1990); and the bipyridinium herbicides
(Baker and Percival, 1990) act by competing for elec-
trons with the natural substrate ferredoxin.
In the course of our work on N-oxide derivatives
(Monge et al., 1998a; Cerecetto et al., 1999), we devel-
oped a number of compounds that combined in a single
structure several features of the toxophores described
above. Consequently, they contain a quaternary nitro-
gen (as the bipyridinium family) and a heterocyclic
system structurally related to the other families (pyra-
zole phenyl ethers, sulfonylureas, and imidazolinones)
(see Chart 1). On the other hand, several groups have
investigated the mechanism responsible for the biologi-
cal activity of the N-oxide system and have found that
the toxic species is an oxidizing radical (Cahill and
White, 1991; Monge et al., 1995, 1998b). Therefore, it
In this paper we report on the synthesis of N-oxide
compounds derived from 1,2,5-oxadiazole, benzo[1,2-
c]1,2,5-oxadiazole, and quinoxaline heterocycles. The
syntheses and structures of the N-oxide derivatives
described here (1-34) are presented in Schemes 1-3.
1
All new compounds were identified by H NMR and, in
some cases, by 13C NMR, IR, and MS. Their purity was
established by TLC and microanalysis. The herbicidal
activity of these compounds was examined through a
preliminary screening program in both pre- and post-
emergence assays (using Triticum aestivum L. as bio-
logical species). Dose-response studies were performed
with the more representative products of each family
of compounds.
MATERIALS AND METHODS
Ch em ist r y. Syn t h et ic Met h od s. All starting materials
were commercially available research-grade chemicals and
used without further purification. Compounds 1-18, 26, and
33 and intermediates I-IX and XI-XIV were prepared
according to procedures found in the literature (Gasco et al.,
1991; Monge et al., 1995, 1998a; Fruttero et al., 1989; Smith
and Boyer, 1963; Edwards and Bambury, 1975; Cerecetto et
al., 1998, 1999). All solvents were dried and distilled prior to
use (Perrin and Armarego, 1988). All reactions were carried
out in a nitrogen atmosphere. The typical workup included
washing with brine and drying the organic layer with Na2-
SO4. Melting points were determined using a Leitz microscope
heating stage model 350 apparatus and are uncorrected.
Elemental analyses were obtained from vacuum-dried samples
* Authors to whom correspondence should be addressed
[(H.C.) fax (598) 2 924 19 06, e-mail hcerecet@bilbo.edu.uy;
(M.G.) fax (598) 2 924 19 06, e-mail megonzal@bilbo.edu.uy].
10.1021/jf9904766 CCC: $19.00 © 2000 American Chemical Society
Published on Web 06/13/2000

2996 J. Agric. Food Chem., Vol. 48, No. 7, 2000
Cerecetto et al.
Ch a r t 1
Sch em e 1
(over phosphorus pentaoxide at 3-4 mmHg, 24 h at room
temperature) and performed on a Fisons EA 1108 CHNS-O
analyzer and were within (0.4% of theoretical values. Infrared
spectra were recorded on a Perkin-Elmer 1310 instrument,
using potassium bromide tablets for solid and oil products; the
P r ep a r a tion of Sem ica r ba zon es 19-23. General Proce-
dure. A mixture of 17 (1 equiv), semicarbazide (hydrochloride
semicarbazide, methyl carbazate, VII, VIII, or IX) (1 equiv),
p-TsOH (catalytic amounts), and toluene as solvent was stirred
at 40-60 °C until the carbonyl compound was not present
(followed by TLC on SiO2, 1% MeOH in CH2Cl2). After the
workup process, the residue was purified as indicated.
frequencies are expressed in cm-1 1H NMR and 13C NMR
.
spectra were recorded on a Varian XL-100 (at 100 MHz)
instrument or on a DPX-Bruker 400 (at 400 and 100 MHz)
instrument, with tetramethylsilane as the internal reference
and in the indicated solvent; the chemical shifts are reported
in parts per million. Mass spectra were recorded on a Shi-
madzu GC-MS QP 1100 EX instrument at 70 eV.
1-[(Benzo[1,2-c]1,2,5-oxadiazol-5(6)-yl N1-oxide)methylidene]-
semicarbazide, (19), was purified by column chromatography
[SiO2, EtOAc/MeOH (0-10%)] and then crystallized from
MeOH, yellow needles (30%): mp 215.0-218.0 °C; IR νmax
1
3410, 3250, 1650, 1570, 1050 cm-1; H NMR (DMSO-d6, 100

Synthesis and Herbicidal Activity of N-Oxide Derivatives
J. Agric. Food Chem., Vol. 48, No. 7, 2000 2997
Sch em e 2
224.0 °C; IR νmax 3400, 3080, 2930, 1670, 1610, 1390 cm-1; 1H
NMR (DMSO-d6, 400 MHz) δ 3.27 (s, 3H), 3.32 (q, J ) 6.0 Hz,
2H), 3.43 (t, J ) 6.3 Hz, 2H), 7.33 (t, J ) 5.8 Hz, 1H), 7.50-
7.85 (m, 2H), 7.90 (s, 1H), 8.00-8.30 (m, 1H), 10.79 (s, 1H);
13C NMR (DMSO-d6, 100 MHz) δ 38.57, 57.79, 70.97, 110.40,
112.40, 114.40, 115.60, 117.61, 127.65, 131.20, 136.40, 136.78,
152.80, 155.08; MS, m/z (abundance) 279 (M+•, 2.0%), 178
(7.5%). Anal. Calcd for C11H13N5O4: C, 47.31;H, 4.66;N, 25.09.
Found: C, 47.39; H, 4.64; N, 24.79.
1-[(Benzo[1,2-c]1,2,5-oxadiazol-5(6)-yl N1-oxide)methylidene]-
4-(2-phenylethyl)semicarbazide, (22), was purified by column
chromatography [SiO2, petroleum ether/EtOAc (30-90%)] and
then crystallized from petroleum ether/EtOAc, yellow needles
(55%): mp 201.0-203.0 °C; IR νmax 3395, 3185, 3070, 2930,
1
1675, 1595, 1360 cm-1; H NMR (DMSO-d6, 100 MHz) δ 2.82
(t, J ) 7.0 Hz, 2H), 3.42 (q, J ) 6.0 Hz, 2H), 7.25 (m, 6H),
7.60-8.50 (m, 4H), 10.80 (bs, 1H); MS, m/z (abundance) 325
(M+•, 1.0%), 309 (1.0%). Anal. Calcd for C16H15N5O3: C, 59.08;
H, 4.62; N, 21.54. Found: C, 59.10; H, 4.53; N, 21.50.
MHz) δ 6.60 (bs, 2H), 7.65 (d, J ) 9.0 Hz, 0.4H), 7.70 (d, J )
8.0 Hz, 0.6H), 7.80 (d, J ) 1.0 Hz, 0.4H), 7.85 (s, 0.6H), 7.89
(s, 0.4H), 8.00 (dd, J 1 ) 1.0 Hz, J 2 ) 8.0 Hz, 0.6H), 8.20 (dd,
J 1 ) 1.0 Hz, J 2 ) 9.0 Hz, 0.4H), 8.49 (d, J ) 1.0 Hz, 0.6H),
10.50 (bs, 0.4H), 10.68 (bs, 0.6H); MS, m/z (abundance) 221
(M+•, 28.1%), 205 (3.6%). Anal. Calcd for C8H7N5O3‚1/3H2O: C,
42.29; H, 3.38; N, 30.84. Found: C, 42.59; H, 3.00; N, 30.74.
2′-[(Benzo[1,2-c]1,2,5-oxadiazol-5(6)-yl N1-oxide)methylidene]-
1-methoxymethanehydrazide, (20), was purified by crystalliza-
tion from toluene, yellow amorphous solid (73%): mp 208.0-
210.0 °C; IR νmax 3220, 3100, 2960, 1705, 1165 cm-1; 1H NMR
(DMSO-d6, 100 MHz) δ 3.82 (s, 3H), 7.80 (d, J ) 6.0 Hz, 1H),
7.85 (d, J ) 2.0 Hz, 1H), 7.96 (dd, J 1 ) 2.0 Hz, J 2 ) 6.0 Hz,
2′-[(Benzo[1,2-c]1,2,5-oxadiazol-5(6)-yl N1-oxide)methylidene]-
1-(morpholine-4-yl)methane hydrazide, (23), was purified by
column chromatography [SiO2, EtOAc/MeOH (0-30%)] and
then crystallized from MeOH/EtOH, bright red plates (48%):
mp 234.0-237.0 °C; IR νmax 3200, 3050, 2850, 1640, 1610, 1380,
1240, 1100 cm-1 1H NMR (DMSO-d6, 400 MHz) δ 3.44 (m,
;
4H), 3.61 (m, 4H), 7.50-8.00 (m, 3H), 8.20 (s, 1H), 10.85 (s,
1H); 13C NMR (DMSO-d6, 100 MHz) δ 44.13, 65.82, 111.03,
113.10, 114.48, 115.86, 118.28, 127.24, 130.00, 139.31, 139.91,
152.76, 153.78; MS, m/z (abundance) 291 (M+., 0.3%), 275
(0.2%). Anal. Calcd for C12H13N5O4‚1/2H2O: C, 48.00; H, 4.67;
N, 23.33. Found: C, 48.39; H, 4.67; N, 22.93.
1H), 8.10 (s, 1H), 10.50 (bs, 1H); MS, m/z (abundance) 236 (M+•
,
40.8%), 220 (1.2%). Anal. Calcd for C9H8N4O4‚1/2H2O: C, 44.08;
H, 3.67; N, 22.86. Found: C, 44.23; H, 3.34; N, 22.56.
1-[(Benzo[1,2-c]1,2,5-oxadiazol-5(6)-yl N1-oxide)methylidene]-
4-(2-methoxyethyl)semicarbazide, (21), was purified by crystal-
lization from EtOH/MeOH, yellow needles (52%): mp 223.0-
P r ep a r a tion of Am id es 24, 25, a n d 27. General Proce-
dure. A mixture of 18 (1 equiv), SOCl2 (10 equiv), and DMF
(catalytic amounts) was heated at reflux for 3 h. The excess of
SOCl2 was eliminated by distillation at reduced pressure, and
the residue was cooled to 0 °C. At this moment a mixture of
Sch em e 3

2998 J. Agric. Food Chem., Vol. 48, No. 7, 2000
Cerecetto et al.
amine (2-methoxyethylamine, morpholine, or 4-chloroaniline)
(1 equiv), Et3N (2 equiv), and dry CH2Cl2 was added, and the
reaction was stirred at room temperature for 12 h. After the
workup process, the residue was purified as indicated.
5(6)-(2-Methoxyethyl)ca rba moylbenzo[1,2-c]1,2,5-oxa di-
azole N1-oxide, (24), was purified by column chromatography
[Al2O3, petroleum ether/EtOAc (0-30%)] and then crystallized
from petroleum ether/EtOAc, yellow needles (30%): mp 115.5-
116.0 °C; IR νmax 3300, 3075, 2930, 2830, 1635, 1580, 1305
F igu r e 1. Isomeric forms of compounds 18-27 at room
temperature (through NMR analysis).
1
cm-1; H NMR (DMSO-d6, 400 MHz) δ 3.28 (s, 3H), 3.45 (t, J
) 5.2 Hz, 2H), 3.47 (q, J ) 4.8 Hz, 2H), 7.50-8.50 (m, 3H),
8.93 (bt, 1H); 13C NMR (DMSO-d6, 100 MHz) δ 39.87, 57.80,
70.05, 112.71, 115.49, 116.20, 117.66, 128.20, 129.02, 131.32,
132.03, 137.74, 148.71, 148.95, 163.75, 164.41; MS, m/z
(abundance) 237 (M+•, 10.1%), 205 (6.3%), 189 (31.2%). Anal.
Calcd for C10H11N3O4: C, 50.63; H, 4.65; N, 17.72. Found: C,
50.72; H, 4.63; N, 17.60.
5(6)-(Morpholine-4-yl)ca rba moylbenzo[1,2-c]1,2,5-oxa di-
azole N1-oxide, (25), was purified by column chromatography
[Al2O3, petroleum ether/EtOAc (0-20%)] and then crystallized
from petroleum ether/EtOAc, pale yellow needles (49%): mp
160.5-162.0 °C; IR νmax 3080, 2910, 2850, 1615, 1585, 1525,
1250, 1105 cm-1; 1H NMR (CDCl3, 100 MHz) δ 3.50-3.90 (m,
8H), 7.35-8.00 (m, 3H); MS, m/z (abundance) 249 (M+., 59.7%),
233 (6.0%). Anal. Calcd for C11H11N3O4: C, 53.01; H, 4.42; N,
16.87. Found: C, 53.41; H, 4.57; N, 16.80.
5(6)-(4-Chlorophenyl)ca rba moylbenzo[1,2-c]1,2,5-oxa di-
azole N1-oxide, (27), was purified by column chromatography
[Al2O3, CH2Cl2/MeOH (0-30%)] and then crystallized from
DMF/H2O, brownish yellow needles (34%): mp 231.5-233.5 °C;
IR νmax 3400, 3090, 3060, 1670, 1575, 1310, 835, 815 cm-1; 1H
NMR (DMSO-d6, 100 MHz) δ 7.43 (d, J ) 9.0 Hz, 2H), 7.80
(m+d, 4H), 8.32 (s, 1H), 10.62 (bs, 1H); MS, m/z (abundance)
289 (M+•, 40.5%), 273 (9.1%). Anal. Calcd for C13H8ClN3O3: C,
53.89; H, 2.76; N, 14.51. Found: C, 53.68; H, 2.80; N, 14.53.
P r ep a r a tion of Qu in oxa lin e N,N′-Dioxid es 28 a n d 29.
General Procedure. A stirred mixture of XIII (1 equiv),
semicarbazide (methyl carbazate or VIII) (1 equiv), p-TsOH
(catalytic amounts), and toluene as solvent was heated at 40-
60 °C until the carbonyl compound was no longer present
(followed by TLC SiO2, 1% MeOH in CH2Cl2). After the workup
process, the residue was purified as indicated.
NMR (DMSO-d6/D2O, 100 MHz) δ 3.75 (s, 3H), 7.87 (s, 0.4H),
8.10 (m, 1H), 8.16 (s, 0.6H), 8.20 (s, 0.4H), 8.25 (s, 0.6H), 8.32
(d, J ) 11.0 Hz, 0.6H), 8.39 (d, J ) 9.0 Hz, 0.4H); MS, m/z
(abundance) 302 (M+•, 10.1%), 286 (25.5%), 213 (6.3%). Anal.
Calcd for
Found: C, 45.37; H, 3.87; N, 25.90.
C12H10N6O4‚H2O: C, 45.00; H, 3.75; N, 26.25.
1-[(2(3)-Amino-3(2)-cyanoquinoxalin-6-yl N1,N4-dioxide)-
methylidene]-4-(2-methoxyethyl) semicarbazide, (31), was ob-
tained as a red-brown solid (77%): mp (the mixture) > 300.0
°C; IR νmax 3390, 3310, 3270, 2920, 2200, 1670, 1630, 1530,
1340, 1120 cm-1; 1H NMR (DMSO-d6, 400 MHz) δ 3.29 (s, 3H),
3.33 (q, J ) 5.5 Hz, 2H), 3.43 (t, J ) 6.1 Hz, 2H), 7.19 (t, J )
5.7 Hz, 0.5H), 7.23 (t, J ) 5.5 Hz, 0.5H), 7.96 (s, 0.5H), 8.02
(s, 0.5H), 8.04 (d, J ) 11.0 Hz, 0.5H), 8.08 (bs, 1H), 8.12 (bs,
1H), 8.23 (s, 1H), 8.26 (d, J ) 9.0 Hz, 0.5H), 8.33 (d, J ) 11.0
Hz, 0.5H), 8.47 (d, J ) 9.1 Hz, 0.5H), 10.63 (s, 0.5H), 10.77 (s,
0.5H); MS, m/z (abundance) 345 (M+., 0.5%), 329 (2.7%), 313
(2.6%). Anal. Calcd for C14H15N7O4‚1/2H2O: C, 47.46; H, 4.52;
N, 27.68. Found: C, 47.50; H, 4.50; N, 27.89.
1-[(2(3)-Amino-3(2)-cyanoquinoxalin-6-yl N1,N4-dioxide)-
methylidene]-4-(2-phenylethyl) semicarbazide, (32), was ob-
tained as a red solid (78%); mp (the mixture) > 300.0 °C; IR
ν
max 3405, 3310, 3270, 2930, 2860, 2240, 1675, 1600, 1530, 1340
cm-1 1H NMR (DMSO-d6, 400 MHz) δ 2.82 (t, J ) 7.7 Hz,
;
2H), 3.40 (q, J ) 6.4 Hz, 2H), 7.22-7.34 (m, 6H), 7.96 (s, 0.6H),
8.01 (s, 0.4H), 8.04 (d, J ) 11.0 Hz, 0.6H), 8.09 (bs, 0.8H), 8.13
(bs, 1.2H), 8.22 (d, J ) 9.1 Hz, 0.4H), 8.25 (s, 0.6H), 8.27 (s,
0.4H), 8.33 (d, J ) 11.0 Hz, 0.6H), 8.44 (d, J ) 9.3 Hz, 0.4H),
10.63 (s, 0.6H), 10.77 (s, 0.4H); MS, m/z (abundance) 391 (M+.,
0.2%), 375 (0.6%), 359 (1.6%). Anal. Calcd for C19H17N7O3: C,
58.31; H, 4.35; N, 25.06. Found: C, 58.52; H, 4.03; N, 25.30.
2′-[(Benzo[1,2-c]1,2,5-oxadiazol-5-yl)methylidene]-1-meth-
oxymethanehydrazide, (34). A mixture of 20 (1 equiv), Ph3P
(1.1 equiv), and EtOH as solvent was heated at reflux for 1.5
h (Boyer and Ellzey, 1961). The EtOH was eliminated by
distillation at reduced pressure. After the workup process, the
residue was purified by column chromatography [SiO2, CH2-
Cl2/MeOH (0-1%)] and then crystallized from MeOH, yellow
needles (46%): mp 199.0-201.0 °C; IR νmax 3250, 3200, 1746,
1694, 1238 cm-1; 1H NMR (DMSO-d6, 400 MHz) δ 3.74 (s, 3H),
8.04 (dd, J 1 ) 1.2 Hz, J 2 ) 9.5 Hz, 1H), 8.07 (d, J ) 9.9 Hz,
2′-[(2,3-Dimethylquinoxalin-6-yl N1,N4-dioxide)methylidene]-
1-methoxymethanehydrazide, (28), was purified by crystalliza-
tion from toluene, yellow solid (83%): mp >300.0 °C; IR νmax
3570, 3490, 3190, 3050, 1720, 1565, 1310 cm-1 1H NMR
;
(DMSO-d6, 100 MHz) δ 2.76 (s, 3H), 2.78 (s, 3H), 3.74 (s, 3H),
8.12 (s, 1H), 8.30 (dd, J 1 ) 2.0 Hz, J 2 ) 8.0 Hz, 1H), 8.40 (d,
J ) 2.0 Hz, 1H), 8.78 (d, J ) 8.0 Hz, 1H), 10.30 (bs, 1H); MS,
m/z (abundance) 290 (M+•, 86.5%), 274 (47.2%), 257 (17.9%).
Anal. Calcd for C13H14N4O4: C, 53.79; H, 4.83; N, 19.31.
Found: C, 53.50; H, 4.53; N, 19.49.
1H), 8.17 (d, J ) 1.0 Hz, 1H), 8.18 (s, 1H), 11.50 (bs, 1H); 13
C
1-[(2,3-Dimethylquinoxalin-6-yl N1,N4-dioxide)methylidene]-
4-(2-phenylethyl)semicarbazide, (29), was purified by crystal-
lization from toluene, yellowish green solid (68%): mp 257.0-
259.0 °C; IR νmax 3400, 3330, 3220, 3080, 2920, 1670, 1525,
1315 cm-1; 1H NMR (DMSO-d6, 400 MHz) δ 2.61 (s, 3H), 2.64
(s, 3H), 2.84 (t, J ) 8.0 Hz, 2H), 3.39 (q, J ) 7.8 Hz, 2H), 7.28
(m, 6H), 8.08 (s, 1H), 8.41 (d, J ) 8.0 Hz, 1H), 8.44 (d, J ) 8.0
Hz, 1H), 8.51 (s, 1H), 10.69 (s, 1H); MS, m/z (abundance) 379
NMR (DMSO-d6, 100 MHz) δ 53.10, 116.32, 117.38, 130.33,
139.42, 142.00, 149.90, 150.12, 154.75; MS, m/z (abundance)
220 (M+•, 1.6%), 190 (0.1%), 189 (0.2%). Anal. Calcd for
C9H8N4O3: C, 49.09;H, 3.64;N, 25.45. Found: C, 49.38; H, 3.64;
N, 25.27.
Biology. Herbicide Evaluation. The compounds described
above were evaluated in pre-emergence herbicide assays. Some
of them (4, 5, 15, and 18) were also examined for their
postemergence herbicidal activity. The preliminary herbicidal
efficacies are based upon evaluations on T. aestivum L. Dose-
response studies were done over compounds 12, 20, and 26.
All data were statistically analyzed (ANOVA), and the experi-
ments were run in duplicate.
Pre-emergence Herbicide Tests. Sterilized sand was placed
in 24 × 40 × 7 cm plastic flats with plastic inserts containing
8 × 8 × 6 cm cells (pH of the potting medium is near
neutrality). On top of the sand in each cell was placed a
predetermined number of seeds, which were then covered with
1 cm of additional sand. A known amount of test compound
was dissolved or suspended and applied directly to the soil
surface, using a laboratory belt sprayer calibrated to deliver
∼200 L/ha. One of the following carriers was employed as
(M+•, 7.8%), 362 (3.0%), 346 (3.1%). Anal. Calcd for C20
21N5O3: C, 63.32; H, 5.54; N, 18.47. Found: C, 63.00; H, 5.53;
N, 18.50.
-
H
P r ep a r a tion of Qu in oxa lin e N,N′-Dioxid es 30-32. Gen-
eral Procedure. A mixture of benzo[1,2-c]1,2,5-oxadiazole N-
oxide (20, 21, or 22) (1 equiv) and malononitrile (1.1 equiv)
was stirred for 10 min at 0 °C. At this temperature a solution
of Et3N (1 drop) in DMF was added, and the mixture was
allowed to stand at room temperature over 24 h and then
filtered off. The solid product (30-32) was washed with Et2O,
boiling MeOH, boiling DMF, and boiling H2O.
2′-[(2(3)-Amino-3(2)-cyanoquinoxalin-6-yl N1,N4-dioxide)-
methylidene]-1-methoxymethane hydrazide, (30), was obtained
as a red-orange solid (73%): mp (the mixture) > 300.0 °C; IR
1
νmax 3290, 3070, 2230, 1730, 1625, 1530, 1330,1230 cm-1; H

Synthesis and Herbicidal Activity of N-Oxide Derivatives
J. Agric. Food Chem., Vol. 48, No. 7, 2000 2999
F igu r e 2. (A) ORTEP diagram of compound 9; (B) packing diagram of compound 9 with unit cell. Both diagrams were generated
with the XZORW package (Zsolnai and Pritzkow, 1995).
Ta ble 1. P r e-em er gen ce Her bicid a l Activity of N-Oxid e
vehicle for each tested compound: acetone/H2O, 50:50 (v/v) (A);
Der iva tives a ga in st T. a estivu m L.
DMSO/H2O, 60:40 (v/v) (B); acetone/DMSO, 50:50 (v/v) (C);
acetone/DMSO/H2O/Tween 80, 15:35:15:35 (v/v) (D). The
activityb,c
dosagea
% of germinated
seeds
samples were tested between 6.4 and 19.6 kg/ha, due to the
different molecular weights of the assayed compounds; in all
the cases the employed amount of the product was 50 mol/ha.
Five replications were done for each compound; a replication
was represented by 40 seeds per cell. After treatment, the flats
were placed in a greenhouse (20 °C), where they received
regular watering (overhead). Evaluations were conducted until
two to three true leaves were present after application (∼2
weeks after treatment). Results of the greenhouse herbicide
evaluation were expressed on a 0-4 rating scale (Eussen et
al., 1990; J oshi et al., 1990; Karp et al., 1997). The scale is
based on the determination of the decreasing of the following
parameters: length of first leaf (lfl), aerial weight (aw), and
radicular weight (rw) compared to those of an untreated
control (with only carrier). In this scale, 1 represents minimum
injury and 4 represents good activity (4 ) 100 to 80% of
reduction of the evaluated parameters with respect to the
control, 3 ) 79 to 50% of reduction of the evaluated parameters
with respect to the control, 2 ) 49 to 20% of reduction of the
evaluated parameters with respect to the control, and 1 ) 19
to 1% of reduction of the evaluated parameters with respect
to the control); 0 represents increment of the parameters with
respect to untreated control. Also, the percentage of germi-
nated seeds was calculated. Atrazine was included in all of
the assays to have a positive control (2.5 kg/ha, 12 mol/ha).
Post-emergence Herbicide Tests. Tests were prepared in an
identical manner to the pre-emergence tests. After the weeds
had three to four true leaves, the plants were treated with
the test compound dissolved in carrier B. The spray chamber
settings and application rates were identical to those of the
pre-emergence treatments. After spraying, the plants were
returned to the greenhouse and not watered overhead for 48
h. During these 2 days water was applied directly to the sand
suface. The same evaluations were conducted 10 days after
application.
Dose-Response Studies. This study was performed under
conditions identical to those used for the pre-emergence
herbicidal tests. The concentrations of compounds used were
12.0, 25.0, 50.0, and 75.0 mol/ha for derivatives 12 (2.400,
5.000, 10.000, and 15.000 kg/ha) and 20 (2.920, 6.150, 12.300,
and 18.450 kg/ha). Because compound 26 showed the best
herbicidal activity, we used lower concentrations, 0.1, 2.0 and
50.0 mol/ha (0.024, 0.472, and 11.800 kg/ha), in the dose-
response studies.
compd (kg/ha) carrier
lfl
aw rw
1
2
3
4
5
6
7
8
9
9.5
13.1
13.1
10.5
16.9
14.9
15.1
17.3
16.1
6.4
B
B
C
B
B
C
C
A
B
B
B
B
B
B
B
B
B
B
B
D
B
D
B
B
B
B
D
D
B
D
D
B
B
1
0
0
0
1
I
1
2
3
1
I
100
100
100
100
100
100
100
100
100
100
89
2
2
1
Id
1
1
2
0
1
1
1
1
0
0
4
3
4
4
I
1
1
1
1
1
3
0
3
4
I
10
11
12
13
14
15
16
17
18
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
9.3
10.0
13.5
10.1
6.8
7.5
8.2
3
3
15
95
92
I
100
100
100
100
75
NS
100
NS
100
100
46
3
2
1
1
3
1
1
1
3
0
1
3
9.0
12.3
14.0
16.3
15.0
11.9
12.5
11.8
14.5
14.5
19.0
16.0
17.7
19.6
18.1
11.0
NSe NS NS
2
NS
2
3
3
3
0
0
2
2
NS NS
1
3
3
3
0
0
4
0
4
3
0
1
3
3
4
0
1
3
4
4
3
0
96
100
100
75
90
84
2
3
90
100
0
a
b
The employed amount of the product was 50 mol/ha. Key to
the parameters: length of first leaf (lfl), aerial weight (aw),
radicular weight (rw). c Reduction of the parameter compared to
an untreated control; 1 represents minimum injury, 4 represents
maximum activity, and 0 represents increment of the parameters
d
(see Biology under Materials and Methods). I denotes that
compound caused no injury to the target weed. e NS denotes that
the results are not statistically significant, the solubility of the
products is very low, and the biological activities are erratic.
Cr ysta llogr a p h y. Suitable needle-shaped single crystals
of 9 were obtained by slow evaporation from EtOAc in the

3000 J. Agric. Food Chem., Vol. 48, No. 7, 2000
Cerecetto et al.
F igu r e 3. (A) Percentage of reduction of the evaluated parameters [aerial weight (shaded bars) and radicular weight (white
bars)] with respect to the control at different doses for compounds 12, 20, and 26. (B) Dose-response curves, log dose (g/ha)
versus percentage of germinated seeds, for compounds 12, 20, and 26.
monoclinic system. Space group is P21/n, with four molecules
per unit cell, and cell constants of a ) 5.197(2) Å, b ) 18.883(3)
Å, c ) 15.1147(17) Å, â ) 97.308(19)o, V ) 1471.1(6) Å3.
Intensities were collected in a Rigaku AFC7S diffractometer
at room temperature with monocromated Mo KR radiation (λ
) 0.7107 Å). Of the 3333 collected reflections (range ) 2.16°
< θ < 27.48°), 2970 were independent and 1510 were observed
(I > 2σI). Structure was solved by direct methods, and all
atoms were freely refined. Some conformational restraints
were applied to the disordered phenyl ring to avoid unrealistic
The 1,2,5-oxadiazole derivatives were obtained as
shown in Scheme 1. The benzo[1,2-c]1,2,5-oxadiazole
system was prepared using the appropriate nitrophenyl
azides (Scheme 2). Cyclocondensation of these azides in
boiling toluene yielded the desired heterocycles. The
semicarbazones 19-23 were obtained in variable yields
(30-73%). The amides 24-27 were obtained in moder-
ate yields.
The compounds 18-27 exist as a mixture of isomers
at room temperature (XV and XV′, Figure 1). This
phenomenon was observed through the corresponding
NMR spectra (proton and carbon), which showed com-
plex groups of signals in the aromatic zone (7.30-8.50
and 110-155 ppm, respectively) at room temperature.
The spectra simplified at higher temperature, where one
of these isomers predominates. Careful examination of
coupling constants and chemical shifts indicated that
the major isomer at high temperature is XV, which is
the 1,5-disubstituted heterocycle (Harris et al., 1963;
Boulton et al., 1967; Boulton and Middleton, 1974;
parameters (Sheldrick, 1990). The final residuals were R1
)
0.0788 and wR2 ) 0.2363 for the observed reflections. Solutions
and refinement of the structure were performed with the
SHELX 97 package (Sheldrick, 1997).
RESULTS AND DISCUSSION
Syn th esis. Our general strategy for the design of the
structures was based on the conjunction of N-oxide
systems and different functionalities (aldehyde, acid,
semicarbazone, amine, and nitrile), to determine the
optimum requirement for herbicidal activity.

Synthesis and Herbicidal Activity of N-Oxide Derivatives
J. Agric. Food Chem., Vol. 48, No. 7, 2000 3001
Cerecetto et al., 1999). On the other hand, compounds
1-14 were obtained as a single product (13C NMR and
crystallographic studies), and the N-oxide location was
established by X-ray diffraction analysis. In this regard,
the single-crystal X-ray structure of compound 9, de-
picted in Figure 2, was used to confirm that the N-oxide
moiety was not isomerized under the reaction conditions
(Gasco et al., 1972; Calvino et al., 1993; Monge et al.,
1998a).
15 with other derivatives, 16-18, 20, 22, 24-27). When
the substituent was an amide, highest activities were
found (24-27). The effect of amide-N substitution on
herbicidal activity shows that lipophilicity may be also
responsible for increased activity, and thus, compounds
26 and 27 were more active than compounds 24 and
25. The expansion of the heterocycle to 2,3-dimethyl-
quinoxaline di-N-oxides decreases activity (compare 28
to 20 and 29 to 22). However, when the expansion was
to 2-amino-3-cyanoquinoxaline di-N-oxides, the activity
increased significantly (compare 30 to 20 and 32 to 22).
Finally, the absence of the N-oxide moiety produced a
sharp loss of activity (see compounds 20 and 34). Dose-
response studies indicated that compounds 12 and 20
were active, even at a concentration 4 times lower than
the initial (10.000 and 12.300 kg/ha, respectively). In
addition, compound 26 was still active at a concentra-
tion 500 times lower than the initial concentration
(11.800 kg/ha).
In another effort to obtain new N-oxide derivatives,
we expanded the oxadiazole system to obtain the quin-
oxaline 1,4-dioxide nucleus (Scheme 3). When compound
20 or 23 reacted with butanone in morpholine (Edwards
and Bambury, 1975), the desired dimethylquinoxaline
was not obtained. In both cases a main product was
1
generated (intermediate X, characterized by H NMR
and MS), which could not be transformed into the
desired product by treatment with Et3N/MeOH at
reflux. Therefore, we employed an alternative route to
obtain the desired heterocycles, in which the aldehyde
XIII was the carbonylic reactant. Beirut reaction of
benzo[1,2-c]1,2,5-oxadiazole N-oxide with malononitrile
in the presence of Et3N at low temperature gave
compounds 30-33 in good yields (73-80%) as a mixture
of 6- and 7-substituted isomers (Ley and Seng, 1975;
Albin and Pietra, 1991; Cheeseman and Cookson, 1979;
Tennant and Mason, 1971).
Studies determining cytotoxicity to mammalian cells
for these groups of compounds showed that they have
low toxicity (Monge et al., 1995, 1998a; Cerecetto et al.,
1999).
In summary, a series of 1,2,5-oxadiazole N-oxide,
benzo[1,2-c]1,2,5-oxadiazole N-oxide, and quinoxaline di-
N-oxide derivatives were synthesized and evaluated for
their herbicidal activities. Preliminary biological studies
found that some of them exhibit significant herbicidal
activity. The structural requirements indicated that the
N-oxide moiety could be the phytophore, and electronic
effects and the lipophilic-hydrophilic balance of the
compounds may explain some differences in biological
activity. Further studies, such as electrochemical mea-
surements and EPR spectroscopy to determine the
ability to form radicals, lipophilicity studies, and theo-
retical calculations, are currently in progress to deter-
mine mechanisms of action for these compounds.
The deoxygenated derivative 34 was prepared by
reaction of the corresponding N-oxide (20) with Ph3P
in boiling EtOH (Scheme 3) (Howard and Olszewski,
1959).
Her bicid a l Activity. The comparative herbicidal
activity of the target compounds was measured at the
whole plant level via a greenhouse assay (Table 1). Some
of the N-oxide derivatives investigated showed activity
in the pre-emergence tests. Compounds 21 and 23 were
insoluble in all of the vehicles assayed; this fact yields
to results that are not statistically significant (in some
replications moderate activities were observed). The
compounds evaluated in postemergence assays (4, 5, 15,
and 18) were not phytotoxic. Dose-response studies
were performed for some of the more active compounds
(12, 20, and 26) (Figure 3).
Herbicidal activity was very sensitive to the type of
N-oxide supporting heterocycle. 4-Phenyloxadiazole N-
oxide derivatives (compounds 1-9) possess moderate to
low pre-emergence activity. Compounds containing
mesomeric electron-withdrawing groups at position 3
(e.g., compounds 2 and 3) showed some level of activity.
On the other hand, compounds containing inductive
electron-withdrawing groups in this position (com-
pounds 4-8) were less active or inactive. 3-Methyloxa-
diazole N-oxide derivatives (10-14) were more active
than the 4-phenyl analogues (comparing compounds
with the same side chain, 12 to 2 and 14 to 3). This
family of compounds had high pre-emergence activity.
The change of the phenyl group to the methyl in the
1,2,5-oxadiazole system could be playing an important
role in the biological activity. Obviously, the different
electronic effects of these moieties could be responsible
for their dissimilar activities, but the lipophilic-hydro-
philic contribution of these groups may also be impor-
tant, affecting their transport through membranes. In
general, the benzo[1,2-c]1,2,5-oxadiazole N-oxide deriva-
tives (compounds 15-27) were found to possess good
pre-emergence activity. Herbicidal activity was sensitive
to substitution at the benzo position (compare compound
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Received for review May 7, 1999. Revised manuscript received
February 28, 2000. Accepted April 26, 2000. This work was
supported by CONICYT-Uruguay (Grant 347/95).
J F9904766