Design, synthesis, and quantitative structure-activity relationship study of herbicidal analogues of pyrazolo[5,1-d][1,2,3,5]tetrazin-4(3H)ones

Article abstract of DOI:10.1021/jf063271n

A series of pyrazolo[5,1-d][1,2,3,5]tetrazin-4(3H)one derivatives were designed, synthesized, and evaluated for their herbicidal activities where some of these compounds provided <80% control of Brassica campestris at 10 μg/mL. Quantitative structure-activity relationship studies were performed on these compounds using physicochemical parameters (electronic, Verloop, or hydrophobic) as independent parameters and herbicidal activity as a dependent parameter, where herbicidal activity correlated best (r > 0.8) with physicochemical parameters in this set of molecules. The herbicidal activity against B. campestris was mainly affected by the molar refractivity (MR) for R¹, Taft (Es₀) for R² or R⁶, Verloop (L_m) for R³ or R⁵, and electronic parameters (Hammett's constants) for R4. The optimal MR for herbicidal activity is 0.95. The herbicidal activity against Echinochloa crus-galli was mainly related with the substituents' hydrophobic parameter. The optimal π parameters for R ₁ and R⁴ for herbicidal activity are 0.72 and 0.68, respectively. In general, these compounds showed greater herbicidal activity toward B. campestris than E. crus-galli.

Full text of DOI:10.1021/jf063271n

1364 J. Agric. Food Chem. 2007, 55, 1364−1369
Design, Synthesis, and Quantitative Structure−Activity
Relationship Study of Herbicidal Analogues of
Pyrazolo[5,1-d][1,2,3,5]tetrazin-4(3H)ones
YOU-QUAN ZHU, CHAO WU, HUA-BIN LI, XIAO-MAO ZOU,* XUE-KAI SI,
FANG-ZHONG HU, AND HUA-ZHENG YANG*
State Key Laboratory of Elemento-organic Chemistry, Research Institute of Elemento-organic
Chemistry, Nankai University, Tianjin 300071, People’s Republic of China
A series of pyrazolo[5,1-d][1,2,3,5]tetrazin-4(3H)one derivatives were designed, synthesized, and
evaluated for their herbicidal activities where some of these compounds provided >80% control of
Brassica campestris at 10 µg/mL. Quantitative structure-activity relationship studies were performed
on these compounds using physicochemical parameters (electronic, Verloop, or hydrophobic) as
independent parameters and herbicidal activity as a dependent parameter, where herbicidal activity
correlated best (r > 0.8) with physicochemical parameters in this set of molecules. The herbicidal
activity against B. campestris was mainly affected by the molar refractivity (MR) for R1, Taft (Eso) for
R2 or R6, Verloop (Lm) for R3 or R5, and electronic parameters (Hammett’s constants) for R4. The
optimal MR for herbicidal activity is 0.95. The herbicidal activity against Echinochloa crus-galli was
mainly related with the substituents’ hydrophobic parameter. The optimal π parameters for R1 and
R4 for herbicidal activity are 0.72 and 0.68, respectively. In general, these compounds showed greater
herbicidal activity toward B. campestris than E. crus-galli.
KEYWORDS: Pyrazolo[5,1-d][1,2,3,5]tetrazin-4(3H)one; hydrophobic; herbicidal activity; quantitative
structure-activity relationship (QSAR)
INTRODUCTION
other areas (5). The other objective of this study was to further
understand the QSAR for R1-R6 through preparing a series of
C (Figure 1 and Table 1) .
Tetrazinone compounds represented an important class of
heterocyclic compounds. They displayed a wide range of
biological and pharmaceutical activities (1, 2) and have attracted
considerable attention. For example, mitozolomide A and
temozolomide B (Figure 1) exhibited outstanding antineoplastic
activity and drew attention to azolotetrazine systems. For the
sake of finding valuable herbicidal candidates, the imidazotet-
razinone moiety was modified into pyrazolotetrazinone in our
previous report and some of them provided 50-66% control
of Brassica campestris at 200 µg/mL (3). It was also noticed
that many compounds, containing the moiety of polysubstituted
benzene ring, exhibited excellent inhibition activity against
protoporphyrinogen-IX oxidase (4) and that the structure unit
was crucial for their herbicidal activity. To find potent lead
compounds, the compound C (Figure 1) was designed and
synthesized.
MATERIALS AND METHODS
Synthetic Procedures. Proton NMR spectra were obtained at 300
MHz using a Bruker AC-P300 spectrometer in CDCl3 solution with
tetramethylsilane as an internal standard. Chemical shift values (δ) are
given in ppm. Elemental analyses were determined on a Yanaca CHN
Corder MT-3 elemental analyzer. Melting points were taken on a
Thomas-Hoover melting point apparatus and are uncorrected. Solvents
were dried by standard methods and distilled prior to use. Yields were
not optimized.
General Synthetic Procedures for Diazonium Inner Salt (E) and
Aryl Isocyanate (G). Compound E (Scheme 1) was synthesized as
the literature described (3, 6). Compound G was synthesized as the
literature described (7). They were identified by 1H NMR spectroscopy.
General Synthetic Procedure for Compound C (3). The mixture
of 2 mmol of E and 2 mmol of aryl isocyanate G in 5 mL of
dichloromethane was stirred for 40-48 h at room temperature. After
the reaction was completed as monitored by thin-layer chromatography,
the solvent was removed in vacuo. The crude product was purified by
flash column chromatography on silica gel, using ethyl acetate-
petroleum ether (V/V ) 3:1) as the eluant to afford the pure target
product. The melting points, yields, and elemental analyses of
At present, quantitative structure-activity relationships (QSAR),
an important area of chemometrics, are widely utilized to study
the relationship between chemical structures and biological or
other functional activities. QSAR has become increasingly
helpful in understanding many aspects of chemical-biological
interactions in drug and pesticide research as well as in many
1
compounds C1-28 are listed in Tables 1 and 2, and their H NMR
* To whom correspondence should be addressed. Tel: +86-22-23503799.
Fax: +86-22-23503627. E-mail: nk_yanghz@126.com.
are listed in Table 3.
10.1021/jf063271n CCC: $37.00
© 2007 American Chemical Society
Published on Web 01/20/2007

Analogues of Pyrazolo[5,1-d][1,2,3,5]tetrazin-4(3H)ones
J. Agric. Food Chem., Vol. 55, No. 4, 2007 1365
Figure 1. Chemical structures of A, B, and C.
Table 1. Melting Points and Yields of Compounds C1
−28
Table 2. Elemental Analysis of Compounds C1−28
yield
R6 appearance (%)
mp
C)
elemental analysis (%, calcd)
H
compd
R1
R2 R3 R4
R5
(
°
compd
C
N
C1
C2
C3
C4
C5
C6
C7
C8
CH3S
CF3
CH3
CH3S
CF3
CH3
CH3S
CF3
CH3S
CH3S
CF3
F
F
F
F
F
F
F
F
H
H
H
H
H
H
H
H
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
H
propynyloxy
propynyloxy
allyloxy
allyloxy
allyloxy
n-propoxy
n-propoxy
n-propoxy
i-propoxy
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
yellow
white
yellow
yellow
white
white
yellow
white
yellow
yellow
white
white
yellow
yellow
liquid
yellow
yellow
yellow
white
white
white
white
white
34.3 132
33.7 117
77.2 122
60.8 129
47.6 105
39.7 103
60.5 130
32.3 110
62.3 126
85.1 145
62.9 124
76.2 107
77.6 144
58.3
−
134
119
123
130
106
105
131
111
128
146
125
108
145
C1
C2
C3
C4
C5
C6
C7
C8
46.44 (46.63)
44.37 (44.41)
49.95 (50.07)
46.30 (46.42)
44.11 (44.22)
50.05 (49.82)
46.16 (46.21)
44.09 (44.03)
46.27 (46.21)
44.90 (45.11)
42.60 (42.77)
49.12 (49.05)
44.95 (45.11)
42.54 (42.77)
49.02 (49.05)
45.34 (45.11)
42.50 (42.77)
45.01 (44.85)
39.28 (39.53)
44.74 (44.85)
41.68 (41.53)
39.40 (39.53)
44.72 (44.85)
41.48 (41.53)
39.59 (39.53)
41.38 (41.30)
36.90 (36.76)
39.59 (39.53)
3.10 (2.99)
2.28 (2.19)
3.87 (3.71)
3.46 (3.44)
2.75 (2.62)
4.14 (4.18)
3.72 (3.88)
3.08 (3.04)
3.74 (3.88)
3.11 (3.03)
2.33 (2.15)
3.21 (3.29)
2.90 (3.03)
2.38 (2.15)
3.40 (3.29)
3.13 (3.03)
2.19 (2.15)
2.72 (2.76)
1.87 (1.77)
2.73 (2.76)
2.54 (2.56)
1.64 (1.77)
2.83 (2.76)
2.50 (2.56)
1.65 (1.77)
2.46 (2.31)
1.51 (1.44)
1.65 (1.77)
15.90 (16.00)
15.21 (15.23)
17.15 (17.17)
15.80 (15.92)
14.96 (15.17)
16.98 (17.09)
16.03 (15.85)
15.26 (15.10)
15.78 (15.85)
17.56 (17.54)
16.40 (16.63)
19.18 (19.07)
17.44 (17.54)
16.37 (16.63)
19.22 (19.07)
17.43 (17.54)
16.68 (16.63)
17.60 (17.43)
15.54 (15.37)
17.56 (17.43)
16.18 (16.14)
15.49 (15.37)
17.56 (17.45)
16.34 (16.14)
15.38 (15.37)
16.09 (16.06)
14.03 (14.29)
15.48 (15.37)
−
−
−
−
−
−
−
−
−
−
−
−
C9
F
H
C9
C10
C11
C12
C13
C14
H
H
CF3
CF3
CF3
H
H
H
C10
C11
C12
C13
C14
C15
C16
C17
C18
C19
C20
C21
C22
C23
C24
C25
C26
C27
C28
H
H
H
H
H
H
H
H
CH3
CH3S CF3
CF3
CF3
C15
C16
C17
C18
C19
C20
C21
C22
C23
C24
C25
C26
C27
C28
CH3
CH3S
CF3
CH3
CF3
CH3
CH3S Cl
CF3
CH3
CH3S
CH3
CH3S Cl
CF3
CF3
H
H
H
CF3
CF3
Cl
H
H
H
H
H
H
H
H
CF3
CF3
CF3
Cl
Cl
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Cl
Cl
Cl
H
73.5 134
70.1 150
66.5 133
84.6 149
32.9 110
79.7 127
78.4 134
48.3 116
84.6 130
94.5 139
32.1 90
49.1 88
32.6 78
−
−
−
−
−
−
−
−
−
−
−
−
−
−
136
151
134
150
111
129
135
118
132
140
92
H
CF3
CF3
CF3
H
H
H
H
H
H
H
H
Cl
H
H
CF3 Cl
CF3 Cl
H
H
H
white
yellow
yellow
yellow
yellow
yellow
Cl
CF3
CF3
CF3
90
80
109
Cl
H
CF3 Cl
44
108
h at 28 ((1) °C. The lengths of 10 rape roots selected from each plate
were measured, and the means were calculated. The percentage
inhibition was used to describe the control efficiency of the compounds.
The herbicidal activity was summarized in Table 4.
Scheme 1
Inhibition of the Seedling Growth of Barnyard Grass [E. crus-galli
(L.) BeauV]. Ten E. crus-galli seeds were placed into a 50 mL cup
covered with a layer of glass beads and a piece of filter paper at the
bottom, to which 5 mL of inhibitor solution had been added in advance.
The cup was placed in a bright room, and the seeds were allowed to
germinate for 65 h at 28 ((1) °C. The heights of the above-ground
parts of the seedlings in each cup were measured, and the means were
calculated. The percentage inhibition was used to describe the control
efficiency of the compounds. The herbicidal activity is summarized in
Table 4.
RESULTS AND DISCUSSION
Preparations. The syntheses of E, G, and C have been
reported (3, 6, 7) (Scheme 1). The diazotization of compound
D using nitrous acid at 0-5 °C followed by neutralization with
saturated aqueous sodium carbonate gave 5-diazo-4-ethoxycar-
bonyl-1H-pyrazole E (3, 6). G was synthesized as the literature
described (7) by reaction of triphosgene with aromatic amine.
The target molecules C, described in Table 1, were obtained
by the cycloaddition of E with aryl isocyanate G in anhydrous
CH2Cl2 at room temperature.
QSAR Analysis. Different substituents were introduced into
the pyrazole and benzene rings of target products C. Their
herbicidal activities were given in Table 4.
By replacing the methylsulfonyl group (R1) of related
compounds of ref 3 by methyl, methylthio, or trifluoromethyl
Bioassays. The herbicidal activities of the target compounds (C1-
28) were determined with B. campestris L. and Echinochloa crus-galli
L. Beauv as samples of annual dicotyledonous and monocotyledonous
plants, respectively, using a previously reported procedure (8). For all
of the bioassay tests, each treatment was repeated three times.
Treatment. The emulsions of purified compounds were prepared by
dissolving them in 100 µL of N,N-dimethylformamide with the addition
of 2 µL of Tween 20. The mixture of the same amount of water, N,N-
dimethylformamide, and Tween 20 was used as control.
Inhibition of the Root-Growth of Rape (B. campestris L.). Rape seeds
were soaked in distilled water for 4 h before being placed on a filter
paper in a 6 cm Petri plate, to which 2 mL of inhibitor solution had
been added in advance. Usually, 15 seeds were used on each plate.
The plate was placed in a dark room and allowed to germinate for 65

1366 J. Agric. Food Chem., Vol. 55, No. 4, 2007
Zhu et al.
Table 3. 1H NMR of Compounds C1
−28
compd
1H NMR (CDCl3),
CH2CH3), 2.59 (s, 1H, H), 2.71 (s, 3H,
CH2C 6.0 Hz, Ar
H), 7.29 (d, 1H, 3JHH H), 7.43 (d, 1H, 3JHH
CH2CH3), 2.60 (s, 1H, 7.2 Hz, CH2CH3),
H), 4.53 (q, 2H, 3JHH
H), 7.30 (d, 1H, 3JHH H), 7.46 (d, 1H, 3JHH
6.3 Hz, Ar 9.0 Hz, Ar H)
7.2 Hz, CH2CH3), 2.76 (s, 3H, CH2CH3),
CH3), 4.50 (q, 2H, 3JHH
CH2CH CH2), 5.37 (m, 2H, CH2CH CH2),
6.3 Hz, Ar
H), 7.41 (d, 1H, 3JHH
δ
C1
1.46 (t, 3H, 3JHH
CH2CH3), 4.81 (s, 2H,
1.46 (t, 3H, 3JHH
7.2 Hz,
CH2C
)
7.2 Hz,
−
−
−
−
C
t
−
SCH3), 4.51 (q, 2H, 3JHH
8.7 Hz, Ar−
)
7.2 Hz,
H)
−
t
)
−
)
C2
C3
)
−
−C
t
)
−
4.82 (s, 2H,
t
)
−
)
7.2 Hz,
−
1.46 (t, 3H, 3JHH
)
−
−
)
−
4.63 (m, 2H,
−
)
−
)
)
CH2), 6.03 (m, 1H,
−CH2CH)
7.08 (d, 1H, 3JHH
)
−
9.0 Hz, Ar H)
−
C4
C5
C6
1.46 (t, 3H, 3JHH
4.63 (m, 2H,
)
−
7.2 Hz,
CH2CH
6.0 Hz, Ar
6.9 Hz,
CH2CH3), 4.53 (q, 2H, 3JHH
CH2CH CH2), 6.05 (m, 1H, CH2CH
9.0 Hz, Ar H)
7.2 Hz,
CH2CH2CH3), 1.45 (t, 3H, 3JHH
CH2CH2CH3), 2.75 (s, 3H,
CH3), 3.98 (t, 2H, 3JHH
7.2 Hz, 7.2 Hz, Ar
CO2CH2CH3), 7.04 (d, 1H, 3JHH
H)
−
CH2CH3), 2.71 (s, 3H,
−
SCH3), 4.50 (q, 2H, 3JHH
)
7.2 Hz,
−
CH2),
CH2CH3),
)
CH2), 5.41 (m, 2H,
−
CH2CH
)
)
)
CH2), 6.04 (m, 1H,
−CH2CH
)
7.08 (d, 1H, 3JHH
)
−
H), 7.42 (d, 1H, 3JHH
9.0 Hz, Ar
6.9 Hz, CH2CH3), 4.64 (m, 2H,
6.3 Hz, Ar−
−H)
1.46 (t, 3H, 3JHH
5.43 (m, 2H,
)
−
−
)
−
−CH2CH
dCH2),
−
)
CH2), 7.07 (d, 1H, 3JHH
)
H),
7.44 (d, 1H, 3JHH
)
−
1.07 (t, 3H, 3JHH
3JHH
7.2 Hz,
4.49 (q, 2H, 3JHH
3JHH
9.2 Hz, Ar
1.07 (t, 3H, 3JHH
3JHH
6.8 Hz,
4.50 (q, 2H, 3JHH
3JHH
8.8 Hz, Ar
)
−
−
)
7.2 Hz,
−
CO2CH2CH3), 1.87 (m, 2H,
6.8 Hz, CH2CH2CH3),
H), 7.39 (d, 1H,
)
−
)
−
)
−
−
)
−
)
C7
)
−
7.2 Hz,
−
CH2CH2CH3), 1.45 (t, 3H, 3JHH
SCH3), 3.98 (t, 2H, 3JHH
CO2CH2CH3), 7.04 (d, 1H, 3JHH
6.0 Hz, Ar−
)
7.2 Hz,
−
CO2CH2CH3), 1.88 (m, 2H,
6.8 Hz, CH2CH2CH3),
H), 7.40 (d, 1H,
)
CH2CH2CH3), 2.70 (s, 3H,
7.2 Hz,
H)
−
)
−
)
−
)
)
−
C8
1.08 (t, 3H, 3JHH
)
7.2 Hz,
−
CH2CH2CH3), 1.45 (t, 3H, 3JHH
6.4 Hz,
6.4 Hz, Ar
CH(CH3)2), 1.45 (t, 3H, 3JHH
7.2 Hz,
7.2 Hz, CO2CH2CH3), 4.53 (m, 1H,
OCH(CH3)2), 7.08 (d, 1H, 3JHH
8.8 Hz, Ar H)
CH2CH3), 2.72 (s, 3H,
)
6.8 Hz,
CH2CH2CH3), 4.52 (q, 2H,
H), 7.42 (d, 1H, 3JHH
CO2CH2CH3), 2.70 (s, 3H,
6.4 Hz,
−CO2CH2CH3), 1.88 (m, 2H,
3JHH
3JHH
)
)
6.8 Hz,
−
CH2CH2CH3), 3.99 (t, 2H, 3JHH
)
)
−
7.2 Hz,
−
CO2CH2CH3), 7.04 (d, 1H, 3JHH
7.2 Hz,
)
−
)
8.8 Hz, Ar
−H)
C9
1.40 (d, 6H, 3JHH
)
−
)
−
−SCH3),
4.50 (q, 2H, 3JHH
Ar
1.46 (t, 3H, 3JHH
−
−
)
−
H), 7.39 (d, 1H, 3JHH
)
−
−
C10
)
7.2 Hz,
−
SCH3), 4.51 (q, 2H, 3JHH
)
7.2 Hz,
−CH2CH3,),
7.84 (m, 4H, Ar
−
H)
C11
C12
1.46 (t, 3H, 3JHH
)
7.2 Hz,
7.2 Hz,
−
−
CH2CH3), 4.53 (q, 2H, 3JHH
)
7.2 Hz,
−
CH2CH3), 7.85 (m, 4H, Ar
−H)
1.46 (t, 3H, 3JHH
)
CH2CH3), 2.76(s, 3H,
−
CH3), 4.50 (q, 2H, 3JHH
)
7.2 Hz, CH2CH3),
−
7.75 (m, 4H, Ar
1.46 (t, 3H, 3JHH
−
H)
C13
)
7.2 Hz,
−
CH2CH3), 2.71 (s, 3H, − ) 7.2 Hz, −CH2CH3),
SCH3), 4.50 (q, 2H, 3JHH
7.75 (m, 4H, Ar
−
H)
C14
C15
1.45 (t, 3H, 3JHH
)
7.2 Hz,
7.2 Hz,
−
−
CH2CH3), 4.51 (q, 2H, 3JHH
)
7.2 Hz,
−
CH2CH3), 7.77 (m, 4H, Ar
−H)
1.46 (t, 3H, 3JHH
)
CH2CH3), 2.76 (s, 3H,
−
CH3), 4.50 (q, 2H, 3JHH
)
7.2 Hz, CH2CH3),
−
7.85 (m, 4H, Ar
1.46 (t, 3H, 3JHH
−
H)
C16
)
7.2 Hz,
−CH2CH3), 2.71 (s, 3H,
−
SCH3), 4.50 (q, 2H, 3JHH
)
7.2 Hz,
−CH2CH3),
7.85 (m, 4H, Ar
−
H)
C17
C18
1.46 (t, 3H, 3JHH
)
7.2 Hz,
7.2 Hz,
−
−
CH2CH3), 4.53 (q, 2H, 3JHH
CH2CH3), 2.75 (s, 3H,
CH3), 4.50 (q, 2H, 3JHH
)
7.2 Hz,
−
CH2CH3), 7.87 (m, 4H, Ar
−H)
1.45 (t, 3H, 3JHH
)
−
)
7.2 Hz, CH2CH3),
−
7.70 (m, 3H, Ar
−
H)
C19
C20
1.45 (t, 3H, 3JHH
)
7.2 Hz,
6.8 Hz,
−
−
CH2CH3), 4.52 (q, 2H, 3JHH
)
7.2 Hz,
−
CH2CH3), 7.72 (m, 3H, Ar
−H)
1.46 (t, 3H, 3JHH
)
CH2CH3), 2.76 (s, 3H,
−
CH3), 4.50 (q, 2H, 3JHH
)
6.8 Hz, CH2CH3),
−
7.84 (m, 3H, Ar
1.46 (t, 3H, 3JHH
−
H)
C21
)
7.2 Hz,
−CH2CH3), 2.71 (s, 3H,
−
SCH3), 4.50 (q, 2H, 3JHH
)
7.2 Hz,
−CH2CH3),
7.82 (m, 3H, Ar
−
H)
C22
C23
1.46 (t, 3H, 3JHH
)
)
7.2 Hz,
7.2 Hz,
−
−
CH2CH3), 4.53 (q, 2H, 3JHH
)
7.2 Hz,
−
CH2CH3), 7.83 (m, 3H, Ar
−H)
1.46 (t, 3H, 3JHH
CH2CH3), 2.75 (s, 3H,
CH2CH3), 2.71 (s, 3H,
CH2CH3), 2.77 (s, 3H,
CH2CH3), 2.71 (s, 3H,
−
−
−
−
CH3), 4.49 (q, 2H, 3JHH
)
7.2 Hz, CH2CH3),
−
7.88 (m, 3H, Ar
1.46 (t, 3H, 3JHH
7.90 (m, 3H, Ar
1.47 (t, 3H, 3JHH
−H)
C24
C25
C26
)
7.2 Hz,
−
−
−
−
SCH3), 4.50 (q, 2H, 3JHH
) 7.2 Hz, −CH2CH3),
−H)
)
7.2 Hz,
H)
6.8 Hz,
H)
CH3), 4.51 (q, 2H, 3JHH
CH3), 4.38 (q, 2H, 3JHH
)
)
7.2 Hz,
7.2 Hz,
−
CH2CH3),
CH2CH3),
7.84 (s, 2H, Ar
−
1.38 (t, 3H, 3JHH
7.84 (s, 2H, Ar
)
−
−
C27
C28
1.46 (t, 3H, 3JHH
)
7.2 Hz,
6.8 Hz,
CH2CH3), 4.42 (q, 2H, 3JHH
7.2 Hz,
)
6.8 Hz,
−CH2CH3), 7.87 (s, 2H, Ar
−H)
1.45 (t, 3H, J
)
−
CH2CH3), 4.53 (q, 2H, J
)
−CH2CH3), 7.90 (m, 3H, Ar−
H)
groups, the herbicidal activity was generally improved. When
a trifluoromethyl group existed at the meta-position in the
benzene ring, most of them (C) showed similar inhibition at
100 and 10 µg/mL (e.g., C10, C20, C23, C24, and C28) for B.
capestris. When the substituent at the para-position in benzene
ring was chloro- or trifluoromethyl groups, most of the
corresponding compounds showed lower herbicidal activity
(except C15, C18, and C23). The comparison between C18 and
C23 implied that the presence of trifluoromethyl group may
have some influence on their metabolism process. It was also
observed that introduction of a polysubstituted benzene ring also
improved the bioactivity. For example, when R1, R2, R3, R4,
R5, and R6 were methyl, hydrogen, trifluoromethyl, chloro,
hydrogen, and hydrogen, respectively, the corresponding com-
pound C23 provided 89% control of B. campestris at 10 µg/
mL. In order to gain insight into structure-activity relationships
of compounds C, the data were analyzed by a physicochemical-
based QSAR (Hansch) approach using physicochemical param-
eters as independent and herbicidal activity data (% I) at 100
µg/mL being converted to log {I/[(100 - I) × MW]} listed in

Analogues of Pyrazolo[5,1-d][1,2,3,5]tetrazin-4(3H)ones
J. Agric. Food Chem., Vol. 55, No. 4, 2007 1367
Table 4. Herbicidal Activity of Compounds (% I; Concentration,
µg/mL)
B. campestris root test
E. crus-galli cup test
100 10
compd
100
10
C1
C2
C3
C4
C5
C6
C7
C8
0
0
0
25
2
0
2
5
13
37
14
86
23
9
90
18
2
0
0
0
0
0
0
0
0
0
32
±
±
±
±
±
±
±
±
±
1
2
2
1
2
1
1
1
1
0
28
24
22
16
18
12
20
20
0
0
0
0
0
0
0
0
0
0
0
0
0
0
±
±
2
1
±
±
±
±
±
±
±
±
±
±
±
±
1
1
2
2
2
2
2
1
2
1
1
2
C9
C10
C11
C12
C13
C14
C15
C16
C17
C18
C19
C20
C21
C22
C23
C24
C25
C26
C27
C28
34
±
2
1
0
1
6
Figure 2. Relationship between observed and calculated activity by eq 1.
Table 6. Correlation Matrix of the Parameters Used in QSAR Study
0
±
±
1
2
5
±
0
0
7
82
4
±
±
±
1
2
1
9
10
22
18
14
21
0
11
2
±
±
±
±
±
±
2
1
1
2
1
2
(MR
−
0.95)2
Eso
σp
Lm
2
9
6
0
0
±
±
±
1
1
1
0
0
0
(MR
−
0.95)2
1.000
0.242
0.072
0.325
63
0
Eso
σp
1.000
0.359
0.412
−
1.000
0.295
65
13
10
89
24
25
9
±
±
±
±
±
±
±
±
±
2
2
1
2
1
2
1
2
1
61
±
2
Lm
−
1.000
0
0
±
±
±
±
±
±
±
2
1
1
2
1
1
1
2
±
1
2
0
0
0
0
3
0
0
89
22
4
3
2
±
±
±
±
±
±
1
1
1
1
1
2
4
were not involved in the following QSAR analysis. Preliminary
QSAR analysis showed the molar refractivity [(MR - 0.95)2]
(r ) 0.822) for R1, and the Verloop parameter (Lm) parameter
for R5 or R3 (r ) 0.381), to a certain extent, showed correlation.
In the case of di-ortho or di-meta substituents, only the
substituent with higher Eso or Lm values was under consider-
ation. Stepwise regression analysis of different combinations
of these parameters were studied, which led to the derivation
of eq 1, with the best correlation (r ) 0.880).
26
22
28
15
±
22
14
13
0
Table 5. Topological and Physicochemical Parameters of Compound
C
compd
(MR−
0.95)2
Eso
σp
Lm
X1obs
X1calcd
X1 ) log{I/[(100 - I) × MW]}
where MW is molecular weight and
C1
C2
C3
C4
C5
C6
C7
C8
0.19
0.20
0.15
0.19
0.20
0.15
0.19
0.20
0.19
0.19
0.20
0.15
0.19
0.20
0.15
0.19
0.20
0.15
0.20
0.15
0.19
0.20
0.15
0.19
0.15
0.19
0.20
0.20
−
0.46
0.46
0.46
0.46
0.46
0.46
0.46
0.46
0.46
0
0.23
0.23
0.23
0.23
0.23
0.23
0.23
0.23
0.23
0
0
0
0
0
0.54
0.54
0.54
0.23
0.23
0
6.58
6.58
6.22
6.22
6.22
6.05
6.05
6.05
4.80
3.30
3.30
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
3.30
3.30
3.30
3.30
3.30
1.00
1.00
1.00
3.30
−3.84
−4.15
−2.55
−3.79
−4.10
−2.52
−3.76
−4.07
−3.59
−2.95
−3.26
−2.04
−3.28
−3.59
−2.09
−3.33
−3.65
−2.34
−3.89
−1.97
−3.21
−3.52
−2.01
−3.25
−2.36
−3.60
−3.91
−3.56
−
−
−
−
−
−
−
−
−
−
3.12
4.35
X1 ) 3.419 ((00.893) - 31.053 ((4.580) (MR - 0.95)2 +
0.274 ((0.137) Eso - 1.320 ((0.519) σp - 0.141 ((
0.060) Lm (1)
−4.34
−3.95
−3.47
−2.83
−3.41
−1.78
−3.13
−3.63
−1.61
−3.26
−4.31
−2.37
C9
C10
C11
C12
C13
C14
C15
C16
C17
C18
C19
C20
C21
C22
C23
C24
C25
C26
C27
C28
where n ) 23, r ) 0.881, s ) 0.4135, and F ) 15.615.
The analysis indicates that (i) the coefficients of the (MR -
0.95)2 terms meant that the MR for R1 had an important impact
on the herbicidal activity and the optimum value of MR for
herbicidal activity was about 0.95 (such as ethynyl group), the
compound that showed the highest herbicidal activity; (ii) the
presence of bulkier substituents with positive Eso values at the
ortho-position of the benzene ring positively contributed for the
activity; (iii) the electronic-donating groups for the para-position
in the benzene ring helped to elevate their herbicidal activity;
and (iv) for the di-meta substituents (R3 and R5), herbicidal
activity was mainly affected by the substituent with the higher
Verloop parameter (Lm) value. Because the sums of the values
at the substituent pairs performed less well than the maximum,
this suggested that a steric clash with the tetrazinone carbonyl
was important.
0
−2.40
−2.40
−2.40
0
0
0
−2.40
−2.40
−0.97
−0.97
−0.97
0
−
−
−
−
−
−
−
−
−
2.34
3.46
3.61
1.66
3.1
3.12
3.68
3.24
3.45
0
0
0.23
0.23
0.54
0.54
0.54
0.23
0
−0.97
−0.97
−0.97
0
The intercorrelation of variable was given in Table 6, and
the calculated activities for them by eq 1 were in good agreement
with the observed activity (Table 6 and Figure 2).
Variations in the herbicidal activities against E. crus-galli
were also analyzed (Table 7). Because the herbicidal activity
for C10, C13, C20, and C28 was zero, the four target molecules
were also not involved in the following QSAR analysis.
Preliminary QSAR analysis showed the hydrophobic parameter
Table 5 as dependent parameters. The values of the physico-
chemical parameters in the study were taken from the literature
(9), and multiparameter linear regression analysis was carried
out using SYBYL 6.9 (Tripos Inc.).
Variations in the herbicidal activities against B. campestris
(Table 5) were first analyzed. Because the herbicidal activity
for C1-3, C6, and C19 was zero, the five target molecules

1368 J. Agric. Food Chem., Vol. 55, No. 4, 2007
Zhu et al.
Table 7. Topological and Physicochemical Parameters of Compound
C
∑
(
π
R3
5 + πR6) X2obs
+
2
compd
π
R1
(
π
R1)2
πo
πp
(πp)
πR
X2calcd
C1
C2
C3
C4
C5
C6
C7
C8
0.61
0.88
0.56
0.61
0.88
0.56
0.61
0.88
0.61
0.61
0.88
0.56
0.61
0.88
0.56
0.61
0.88
0.56
0.88
0.56
0.61
0.88
0.56
0.61
0.56
0.61
0.88
0.88
0.37
0.77
0.31
0.37
0.77
0.31
0.37
0.77
0.37
0.37
0.77
0.31
0.37
0.77
0.31
0.37
0.77
0.31
0.77
0.31
0.37
0.77
0.31
0.37
0.31
0.37
0.77
0.77
0.14 0.71
0.14 0.71
0.14 0.71
0.14 0.71
0.14 0.71
0.14 0.71
0.14 0.71
0.14 0.71
0.14 0.71
0.50
0.50
0.50
0.50
0.50
0.50
0.50
0.50
0.50
0
0
0
0
0
0.77
0.77
0.77
0.50
0.50
0
0.10
0.10
0.60
0.60
0.60
1.05
1.05
1.05
0.36
0.88
0.88
0
0
0
0
0
0
0
0
0.88
0.88
0.88
0.88
0.88
0.71
0.71
0.71
0.88
−
−
−
−
−
−
−
−
−
2.97
3.07
3.11
3.19
3.38
3.27
3.51
3.27
3.25
−3.03
−3.30
−3.43
−3.14
−3.42
−3.54
−3.25
−3.52
−3.09
−4.03
−4.30
−3.86
−3.82
−3.84
−3.40
−3.11
−3.39
−3.08
−3.07
−4.31
−3.82
−4.09
−3.54
−3.25
−3.36
−3.07
−3.34
−3.52
Figure 3. Relationship between observed and calculated activity by eq 2.
Table 8. Correlation Matrix of the Parameters Used in QSAR Study
C9
C10
C11
C12
C13
C14
C15
C16
C17
C18
C19
C20
C21
C22
C23
C24
C25
C26
C27
C28
0
0
0
0
−
−
4.62
3.76
0.88
0.88
0.88
0
0
0
0
0
0
0.88
0.88
0.88
∑
(
π
R3
πR
5 + πR6)
+
−3.63
−3.52
−3.15
−3.28
−3.39
−3.23
2
π
R1
(
π
R1)2
πo
πp
(
πp)
π
R1
π
1.000
1.000
0.094
0.224
0.205
0.048
(
R1)2
1.000
0.098
0.225
0.205
0.051
πo
πp
(
1.000
0.383
0.324
0.192
0.88 0.71
0.88 0.71
−
−
−
−
−
−
−
−
−
1.000
0.977
0.079
2
πp
)
1.000
0.120
0.71
0.71
0.71
0
0
0
0
0.71
0.71
∑
π
(
R
π
R3
+
−
−
1.000
0
0
−3.55
−4.35
−3.95
−3.06
−3.19
−3.08
−3.44
5 + πR6)
0.50
0.50
0.77
0.77
0.77
0.5
0
0.71 0.88
0.71 0.88
0.71 0.88
Table 9. Topological and Physicochemical Parameters of Compound
C
0
0.71
2
X1obs
πp
(
πp)
πm
X2obs
X2calcd
Σ[πR1 + (πR1)2] (r ) 0.391) for R1, Σ(πP + (πP)2 (r ) 0.721)
for R4, and Σ(πR3 + πR5 + πR6) for R3, R5, and R6 (r ) 0.253),
to a certain extent, showed correlation. Stepwise regression
analysis of different combinations of these parameters was
studied, which led to the derivation of eq 2, with a best
correlation (r ) 0.844).
C4
C5
C7
C8
−3.12
−4.35
−4.34
−3.95
−3.47
−3.41
−1.78
−3.63
−1.61
−3.26
−4.31
−2.37
−2.34
−3.46
−3.61
−1.66
−3.10
−3.12
−3.68
−3.24
0.71
0.71
0.71
0.71
0.71
0
0.50
0.50
0.50
0.50
0.50
0
0.60
0.60
1.05
1.05
0.36
0.88
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.88
0.88
0.88
0.88
0.00
0.00
0.00
−3.19
−3.38
−3.51
−3.27
−3.25
−4.62
−3.76
−3.63
−3.52
−3.15
−3.28
−3.39
−3.23
−3.55
−4.35
−3.95
−3.06
−3.19
−3.08
−3.44
−3.37
−3.21
−3.40
−3.45
−3.22
−4.10
−3.93
−3.69
−3.48
−3.27
−3.13
−3.20
−3.21
−4.10
−4.08
−3.68
−3.49
−3.29
−3.21
−3.27
C9
C11
C12
C14
C15
C16
C17
C18
C19
C21
C22
C23
C24
C25
C26
C27
0
0
0
0
X2 ) log{I/[(100 - I) × MW]}
where MW is molecular weight and
0.88
0.88
0.88
0.71
0.71
0
0.77
0.77
0.77
0.50
0.50
0
X2 ) 34.520 ((15.107) πR1 - 23.985 ((10.389) (πR1)2 +
0.289 ((0.171) πo + 2.264 ((0.809) πP - 1.663 ((0.958)
0
0
0.71
0.71
0.88
0.88
0.88
0.50
0.50
0.77
0.77
0.77
(π )2 - 0.230 ((0.135) (πR3 + πR5 + πR6) - 16.006
∑
P
((5.317) (2)
where n ) 24, r ) 0.844, s ) 0.2524, and F ) 6.994.
Equation 2 indicated that the substituent R2 positively
contributed for the activity while R3, R5, and R6 at the benzene
ring negatively contributed. The coefficients of the (πR1)2 and
πR1 terms meant that the hydrophobic parameter (πR1) for R1
had an optimum value for C and was about 0.72 (such as chloro
atom, 0.71). The coefficients of the (πP)2 and πP terms also
meant that the hydrophobic parameter (πP) for R4 had an
optimum value for C and was about 0.68 [such as CH2CH2(1-
pyrrolidyl), 0.69]. This result showed that when the other
conditions were the same and πR1 and πP were 0.72 and 0.68,
respectively, the corresponding compounds had the highest
herbicidal activity. The intercorrelation of variable was given
in Table 8, and the calculated activities for them by eq 2 were
in good agreement with the observed activity (Table 8 and
Figure 3).
When the herbicidal activity against whether E. crus-galli or
B. campestris was zero, the corresponding compound was not
included. In the case of di-meta substituents, only the substituent
with the larger hydrophobic parameter (πm) was under consid-
eration.
X2 ) -4.166 ((0.272) - 0.130 ((0.082) X1 + 2.499 ((
1.098) πp - 2.241 ((1.378) (πp)2 - 0.433 ((0.202) πm (3)
where n ) 20, r ) 0.798, s ) 0.2793, and F ) 6.561.
The coefficients of X1 term meant that, for the same
compound, its herbicidal activity against B. campestris was
about 1.3 times that against E. crus-galli. The coefficients of
the (πp)2 and πp terms also meant that when the hydrophobic
parameter (πp) for R4 was about 0.56, the selectivity for
inhibiting the seedling growth of E. crus-galli was the highest.
The intercorrelation of variable was given in Table 10, and the
In addition, the selectivity between E. crus-galli and B.
campestris for this kind compounds was studied and led to the
derivation of eq 3, with best correlation (r ) 0.798) (Table 9).

Analogues of Pyrazolo[5,1-d][1,2,3,5]tetrazin-4(3H)ones
J. Agric. Food Chem., Vol. 55, No. 4, 2007 1369
showed greater herbicidal activity toward B. campestris than
E. crus-galli.
LITERATURE CITED
(1) Barraja, P.; Diana, P.; Lauria, A.; Montalbano, A.; Almerico,
A. M.; Dattolo, G.; Cirrincione, G. Synthesis and antiproliferative
activity of [1,2,3,5]tetrazino[5,4-a]indoles, a new class of azolo-
tetrazinones. Biol. Med. Chem. 2004, 13 (2), 295-300.
(2) Diana, P.; Barraja, P.; Lauria, A.; Almerico, A. M.; Dattolo, G.;
Cirrincione, G. Pyrrolo[2,1-d][1,2,3,5]tetrazines, a new class of
azolotetrazines related to the antitumor drug temozolomide.
Synthesis 1999, 12, 2082-2086.
(3) Wu, Y.-C.; Zou, X.-M.; Hu, F.-Z.; Yang, H.-Z. Design and
synthesis of novel sulfone- containing pyrazolo[1,5-a]pyrimidines
and pyrazolo[5,1-d][1,2,3,5]tetrazine- 4(3H)-ones. J. Heterocycl.
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(4) Hirai, K.; Uchida, A.; Ohno, R. Herbicide Classes in DeVelop-
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Verlag: Berlin, Heidelberg, 2002; pp 255-274.
(5) van de Waterbeemd, H. Introduction. In Chemometric Methods
in Molecular Design; van de Waterbeemd, H., Ed.; VCH
Publishers: New York, 1995; pp 1-13.
Figure 4. Relationship between observed and calculated activity by eq 3.
Table 10. Correlation Matrix of the Parameters Used in QSAR Study
2
X1obs
1.000
πp
(
πp)
πm
X1obs
πp
−
−
−
0.010
0.011
0.302
1.000
0.980
0.289
2
(
πp
)
1.000
0.392
πm
−
−
1.000
(6) Zou, X.-M.; Cheng, Y.-H.; Ren, X.-L.; Hu, F.-Z.; Yang, H.-Z.
Synthesis of some new hydrazone compounds containg pyrazolyl
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calculated activities for them by eq 3 were in good agreement
with the observed activity (Table 10 and Figure 4).
(7) Du, X.-H.; Xu, X.-S.; Xu, Z.-Y. A safe and effective method
for preparation of aromatic isocyanates using bis(trichlorometh-
yl)carbonate. Chin. J. Pestic. Sci. 2002, 4, 83-85 (in Chinese).
(8) Zhu, Y.-Q.; Zou, X.-M.; Hu, F.-Z.; Yao, C.-S.; Liu, B.; Li, Y.-
H.; Yang, H.-Z. Synthesis and herbicidal evaluation of novel
3-(R-hydroxy-substitutedbenzylidine) pyrrolidine-2,4-diones. J.
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(9) Hansch, C.; Leo, A.; Hoekman, D. Exploring QSAR; ACS
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Washington, DC, 1995.
In summary, we further extended the study of SAR of
pyrazolo[5,1-d][1,2,3,5]tetrazin-4(3H)ones and gained insight
about the pharmacophore through physicochemical-based QSAR
study. The herbicidal activity in the rape root test for compound
C was mainly affected by the MR for R1, Taft (Eso) for R2 or
R6, Verloop’s sterimol (Lm) for R3 or R5, and electronic
parameters (Hammett’s constants, σp) for R4. When MR was
about 0.95, the compound showed the highest herbicidal activity
and for the di-meta substituents (R3 and R5) at the benzene ring,
their herbicidal activity was mainly affected by the substituent
with higher Verloop’s sterimol (Lm) parameter value.
The herbicidal activity in the barnyard grass cup test for
compound C was mainly related with the substituents’ hydro-
phobic parameter. When the other conditions were the same
and the hydrophobic parameters (π) for R1 and R4 were 0.72
and 0.68, respectively (eq 2), the corresponding compounds had
the highest herbicidal activity. In general, these compounds
Received for review November 12, 2006. Revised manuscript received
December 14, 2006. Accepted December 15, 2006. We acknowledge the
support of this work by the National Key Project for Basic Research
(2003CB114400), the National Natural Science Foundation of China
(20372040), and the Research Foundation for the Doctoral Program of
Higher Education (20050055042).
JF063271N