Metofluthrin: A potent new synthetic pyrethroid with high vapor activity against mosquitoes

Article abstract of DOI:10.1271/bbb.68.170

(1R)-trans-Norchrysanthemic acid fluorobenzyl esters are synthesized and their structure-activity relationships are discussed. These esters show outstanding insecticidal activity against mosquitoes. In particular, the 2,3,5,6-tetrafluoro-4-methoxymethylbe

Full text of DOI:10.1271/bbb.68.170

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Metofluthrin: A Potent New Synthetic Pyrethroid
with High Vapor Activity against Mosquitoes
Kazuya UJIHARAa, Tatsuya MORIa, Tomonori IWASAKIa, Masayo SUGANOa, Yoshinori
SHONOa & Noritada MATSUOa
a Agricultural Chemicals Research Laboratory, Sumitomo Chemical Co., Ltd.
Published online: 22 May 2014.
To cite this article: Kazuya UJIHARA, Tatsuya MORI, Tomonori IWASAKI, Masayo SUGANO, Yoshinori SHONO & Noritada
MATSUO (2004) Metofluthrin: A Potent New Synthetic Pyrethroid with High Vapor Activity against Mosquitoes, Bioscience,
Biotechnology, and Biochemistry, 68:1, 170-174, DOI: 10.1271/bbb.68.170
To link to this article: http://dx.doi.org/10.1271/bbb.68.170
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Biosci. Biotechnol. Biochem., 68 (1), 170–174, 2004
Metofluthrin: A Potent New Synthetic Pyrethroid with High Vapor
Activity against Mosquitoes
Kazuya UJIHARA,y Tatsuya MORI, Tomonori IWASAKI, Masayo SUGANO,
Yoshinori SHONO, and Noritada MATSUO
Agricultural Chemicals Research Laboratory, Sumitomo Chemical Co., Ltd.,
4-2-1 Takatsukasa, Takarazuka, Hyogo 665-8555, Japan
Received July 31, 2003; Accepted September 22, 2003
(1R)-trans-Norchrysanthemic acid fluorobenzyl esters
are synthesized and their structure-activity relation-
ships are discussed. These esters show outstanding
insecticidal activity against mosquitoes. In particular,
the 2,3,5,6-tetrafluoro-4-methoxymethylbenzyl analog
(metofluthrin) exhibits the highest potency, being ap-
proximately forty times as potent as d-allethrin in a
mosquito coil formulation when tested against southern
house mosquitoes (Culex quinquefasciatus). Metoflu-
thrin also exhibits a significant vapor action at room
temperature.
closed spaces to control fabric insect pests in closets.
Empenthrin does not, however, show satisfactory in-
secticidal activity against mosquitoes. Much recent
attention has been directed at the development of
devices to control mosquitoes by using products in
ambient-temperature devices because of their increased
safety and ease of use, especially during outdoor
activities. This development activity has resulted in a
variety of fan-powered mosquito vaporizers and asso-
ciated formulations which are now being marketed.
These devices, however, have performance limitations
that are imposed by the insecticidal activity of the active
ingredient used. In order to overcome some of these
limitations, we undertook extensive research to find a
new pyrethroid with a higher vapor action that was
highly active against mosquitoes.
As a result of the research, we found that (1R)-trans-
norchrysanthemic acid fluorobenzyl esters showed vapor
activity against mosquitoes. In particular, the 2,3,5,6-
tetrafluoro-4-methoxymethylbenzyl analog (1, metoflu-
thrin) exhibited the highest potency, being approximate
forty times as potent as d-allethrin (3) in a mosquito coil
formulation when tested against southern house mos-
quitoes (Culex quinquefasciatus). This paper describes
syntheses of the esters and presents their structure-
activity relationships.
Key words: metofluthrin; norchrysanthemic acid; vapor
action; mosquito coil; fan vaporizer
Studies of the structural modification of natural
pyrethrins span more than half a century. As a result,
a number of synthetic pyrethroids with diverse charac-
teristics have been invented not only for the control of
household insect pests, but also for agricultural use.
Among these pyrethroids, empenthrin (2) (Fig. 1) is
noteworthy for having a high vapor action at room
temperature, making it suitable for use as a fumigant in
Materials and Methods
General methods. NMR spectra were recorded with a
JEOL JNM-AL400 NMR spectrophotometer, with tet-
ramethylsilane used as an internal standard. The starting
materials were prepared by known methods.1,2)
Syntheses of the new compounds. Syntheses of the
new norchrysanthemates are summarized in Scheme 1.
(2,3,5,6-Tetrafluorophenyl)methyl (1R,3R)-2,2-dimeth-
yl-3-((1Z)-1-propenyl)cyclopropanecarboxylate (5). Di-
isopropyl azodicarboxylate (a 40% solution in toluene,
Fig. 1.
y
To whom correspondence should be addressed. Fax: +81-797-74-2129; E-mail: ujihara@sc.sumitomo-chem.co.jp

Metofluthrin with High Vapor Action against Mosquitoes
171
Scheme 1. Synthetic Routes to the Norchrysanthemic Acid Fluorobenzyl Esters.
Reagents and conditions: a) pyridine, THF; b) i-PrOCON=NCOOi-Pr, PPh3, THF; c) O3, MeOH, EtOAc, ꢀ78ꢁC; Me2S; d) CH3CH2PPh3Br,
t-BuOK, THF.
2.0 ml, 4.0 mmol) was added to a mixed solution of
(1R,3R)-2,2-dimethyl-3-((1Z)-1-propenyl)cyclopropane-
carboxylic acid (0.42 g, 3.0 mmol), (2,3,5,6-tetrafluoro-
phenyl)methanol (0.49 g, 2.7 mmol), triphenylphosphine
(0.93 g, 3.5 mmol) and tetrahydrofuran (20 ml). After
16 hours, the reaction solution was concentrated under
reduced pressure, and the resulting residue was sub-
jected to silica gel column chromatography with hexane-
ethyl acetate (20:1 by volume) as the eluant to give 5
(0.80 g, 93% yield) as a colorless oil. NMR ꢀH (CDCl3Þ:
1.15 (3H, s), 1.29 (3H, s), 1.47 (1H, d, J ¼ 5:3 Hz), 1.70
(3H, dd, J ¼ 6:9 Hz, 1.6 Hz), 2.19 (1H, br dd,
J ¼ 8:1 Hz, 5.3 Hz), 5.12 (1H, d, J ¼ 10:6 Hz, 8.1 Hz,
1.6 Hz), 5.24 (1H, t, J ¼ 1:6 Hz), 5.25 (1H, t, J ¼
1:6 Hz), 5.60 (1H, dqd, J ¼ 10:6 Hz, 6.9 Hz, 1.1 Hz),
7.10 (1H, tt, J ¼ 9:7 Hz, 7.4 Hz).
3.7 mmol), methanol (20 ml) and ethyl acetate (20 ml) at
ꢀ78ꢁC until the color of the solution had changed to
blue. Nitrogen gas was then blown into the solution to
remove the excess ozone, 5 ml of dimethylsulfide was
added, and the solution was allowed to stand for 12
hours at room temperature. The reaction solution was
concentrated under reduced pressure. To the resulting
residue were added 20 ml of acetone, 2 ml of water and
0.2 g of p-toluenesulfonic acid monohydrate and the
mixture was allowed to stand for 2 hours at room
temperature. The reaction solution was poured into
water and extracted with diethyl ether. The organic layer
was dried over anhydrous sodium sulfate and concen-
trated under reduced pressure. The residue was sub-
jected to silica gel column chromatography with hexane-
ethyl acetate (10:1 by volume) as the eluant to give
(2,3,5,6-tetrafluoro-4-methylphenyl)methyl (1R,3R)-3-
formyl-2,2-dimethylcyclopropanecarboxylate (0.98 g,
82% yield). Potassium tert-butoxide (0.23 g, 2.0 mmol)
was added to a stirred mixture of ethyltriphenylphos-
phonium bromide (1.1 g, 3.0 mmol) in tetrahydrofuran
(30 ml) under ice-cooling. After 15 minutes, a tetrahy-
drofuran (5 ml) solution containing (2,3,5,6-tetrafluoro-
4-methylphenyl)methyl (1R,3R)-3-formyl-2,2-dimethyl-
cyclopropanecarboxylate (0.32 g, 1.0 mmol) was added
to the mixture. After 30 minutes more, the reaction
mixture was filtered through a Celite pad, and the filtrate
was concentrated under reduced pressure. The residue
was subjected to silica gel column chromatography with
hexane-ethyl acetate (20:1 by volume) as eluant to give
7 (0.22 g, 67% yield) as a colorless oil. NMR ꢀH
(CDCl3Þ: 1.14 (3H, s), 1.28 (3H, s), 1.45 (1H, d,
J ¼ 5:3 Hz), 1.70 (3H, dd, J ¼ 7:0 Hz, 1.7 Hz), 2.17
(1H, br dd, J ¼ 8:4 Hz, 5.3 Hz), 2.28 (2H, t, J ¼ 2:1 Hz),
5.11 (1H, ddq, J ¼ 10:7 Hz, 8.4 Hz, 1.7 Hz), 5.20 (1H, t,
J ¼ 1:5 Hz), 5.21 (1H, t, J ¼ 1:5 Hz), 5.59 (1H, dqd,
J ¼ 10:7 Hz, 7.0 Hz, 1.3 Hz).
(2,3,5,6-Tetrafluoro-4-methylphenyl)methyl (1R,3R)-
2,2-dimethyl-3-((1Z)-1-propenyl)cyclopropanecarboxy-
late (7). (1R,3R)-2,2-Dimethyl-3-(2-methyl-1-prope-
nyl)cyclopropanecarbonyl chloride (2.06 g, 11.1 mmol)
was added to a mixed solution of (2,3,5,6-tetrafluoro-4-
methylphenyl)methanol (1.78 g, 9.2 mmol) and pyridine
(0.87 g, 11 mmol) in tetrahydrofuran (20 ml) under ice-
cooling, and the mixture was stirred for 8 hours at room
temperature. The reaction mixture was poured into
100 ml of ice-cooled water and extracted twice with
100 ml each of ethyl acetate. The combined ethyl acetate
extracts were washed with saturated brine, dried over
anhydrous sodium sulfate and concentrated under
reduced pressure to give a crude product, which was
subjected to silica gel column chromatography with
hexane-ethyl acetate (20:1 by volume) as eluant to give
(2,3,5,6-tetrafluoro-4-methylphenyl)methyl (1R,3R)-2,2-
dimethyl-3-(2-methyl-1-propenyl)cyclopropanecarboxy-
late (2.75 g, 87% yield). Oxygen containing ozone was
blown into a mixed solution of this compound (1.27 g,

172
K. UJIHARA et al.
(2,3,5,6-Tetrafluoro-4-methoxymethylphenyl)methyl
s), 1.24 (3H, s), 1.48 (3H, d, J ¼ 5:4 Hz), 1.68 (3H, dd,
J ¼ 6:6 Hz, 1.4 Hz), 2.03 (1H, br dd, J ¼ 8:2 Hz,
5.4 Hz), 3.48 (2H, dt, J ¼ 6:3 Hz, 1.3 Hz), 5.07–5.24
(5H, m), 5.62 (1H, dq, J ¼ 15:1 Hz, 6.5 Hz), 5.89 (1H,
ddt, 16.7 Hz, 10.3 Hz, 6.3 Hz).
(1R,3R)-2,2-dimethyl-3-((1Z)-1-propenyl)cyclopropane-
carboxylate (1, metofluthrin). (1R,3R)-2,2-dimethyl-3-
((1Z)-1-propenyl)cyclopropanecarbonyl chloride (1.82
g, 1.05 mmol) was added to a solution of (2,3,5,6-
tetrafluoro-4-methoxymethylphenyl)methanol (2.24 g,
10 mmol) and pyridine (0.87 g, 10 mmol) in tetrahydro-
furan (20 ml) under ice-cooling, and the mixture was
stirred for 8 hours at room temperature. The reaction
mixture was poured into 100 ml of ice-cooled water and
extracted twice with 100 ml each of ethyl acetate. The
combined ethyl acetate extracts were washed with
saturated brine, dried over anhydrous sodium sulfate
and concentrated under reduced pressure to give a crude
product, which was subjected to silica gel column
chromatography with hexane-ethyl acetate (20:1 by
volume) as the eluant to give 2 (3.17 g, 88% yield) as a
colorless oil. NMR ꢀH (CDCl3Þ: 1.15 (3H, s), 1.28 (3H,
s), 1.46 (1H, d), 1.70 (3H, dd), 2.18 (1H, dd), 3.41 (3H,
s), 4.59 (2H, s), 5.08–5.12 (1H, m), 5.24 (2H, s), 5.58–
5.62(1H, m).
(2,3,5,6-Tetrafluoro-4-methoxyphenyl)methyl (1R,3R)-
2,2-dimethyl-3-((1Z)-1-propenyl)cyclopropanecarboxy-
late (11). This compound was prepared according to the
procedure described for 7. NMR ꢀH (CDCl3Þ: 1.14 (3H,
s), 1.28 (3H, s), 1.45 (1H, d, J ¼ 5:4 Hz), 1.70 (3H, dd,
J ¼ 6:9 Hz, 1.7 Hz), 2.18 (1H, br dd, J ¼ 8:4 Hz,
5.4 Hz), 4.10 (3H, t, J ¼ 1:4 Hz), 5.11 (1H, ddq,
J ¼ 10:5 Hz, 8.4 Hz, 1.7 Hz), 5.18 (1H, t, J ¼ 1:6 Hz),
5.19 (1H, t, J ¼ 1:6 Hz), 5.60 (1H, dqd, J ¼ 10:5 Hz,
7.1 Hz, 1.4 Hz).
Evaluation of the insecticidal effectiveness of the test
compounds. Topical application for evaluating the
insecticidal efficacy against common house mosquitoes
(Culex pipiens pallens) complied with the method
described by Yamaguchi et al.3)
(2,3,4,5,6-Pentafluorophenyl)methyl (1R,3R)-2,2-di-
methyl-3-((1Z)-1-propenyl)cyclopropanecarboxylate (6).
This compound was prepared according to the procedure
described for 5. NMR ꢀH (CDCl3Þ: 1.15 (3H, s), 1.28
(3H, s), 1.45 (3H, d, J ¼ 5:4 Hz), 1.70 (3H, dd, J ¼
6:8 Hz, 1.7 Hz), 2.18 (1H, br dd, J ¼ 8:4 Hz, 5.4 Hz),
5.11 (1H, ddq, J ¼ 10:6 Hz, 8.4 Hz, 1.7 Hz), 5.21 (1H, br
s), 5.60 (1H, dqd, J ¼ 10:6 Hz, 7.0 Hz, 1.2 Hz).
Evaluation of the vapor action of a non-heated
formulation of each test compound at room temperature
against common house mosquitoes (Culex pipiens
pallens). A test compound (100 mg) was dissolved in
20 ml of acetone and applied onto a sheet of filter paper
(20 cm ꢂ 50 cm), the acetone then being removed by air-
drying. In the center of a 28 m3 test chamber (4.3 m ꢂ
2.65 m ꢂ 2.45 m height), the filter paper was hung from
the ceiling with the upper end of the filter paper 1.7 m in
height from the floor. Four nylon-net cages (cylindrical,
30 cm in diameter and 20 cm in height) each containing
20 female common house mosquitoes (Culex pipiens
pallens) were hung from the ceiling with the base of
each cage 60 cm from the floor. One cage was placed in
each corner of the room, 60 cm horizontally from the
filter paper. The number of knocked down mosquitoes
was counted at designated intervals for 60 minutes. In
order to circulate air in the chamber, a fan was
positioned under the treated filter paper, and a board
was placed between the fan and the filter paper to
prevent direct air flow between the two (see Fig. 2).
(4-Ethyl-2,3,5,6-tetrafluorophenyl)methyl (1R,3R)-2,
2-dimethyl-3-((1Z)-1-propenyl)cyclopropanecarboxylate
(8). This compound was prepared according to the
procedure described for 1. NMR ꢀH (CDCl3Þ: 1.14 (3H,
s), 1.23 (3H, t, J ¼ 7:6 Hz), 1.28 (3H, s), 1.46 (1H, d,
J ¼ 5:4 Hz), 1.70 (3H, dd, J ¼ 6:8 Hz, 1.7 Hz), 2.18
(1H, dd, J ¼ 8:4 Hz, 5.4 Hz), 2.77 (2H, q, J ¼ 7:6 Hz),
5.10 (1H, ddq, J ¼ 10:7 Hz, 8.4 Hz, 1.7 Hz), 5.21 (1H, t,
J ¼ 1:3 Hz), 5.22 (1H, t, J ¼ 1:3 Hz), 5.59 (1H, dqd,
J ¼ 10:7 Hz, 6.8 Hz, 1.3 Hz).
(2,3,5,6-Tetrafluoro-4-propylphenyl)methyl (1R,3R)-
2,2-dimethyl-3-((1Z)-1-propenyl)cyclopropanecarboxy-
late (9). This compound was prepared according to the
procedure described for 1. NMR ꢀH (CDCl3Þ: 0.97 (3H,
t, J ¼ 7:5 Hz), 1.13 (3H, s), 1.28 (3H, s), 1.46 (1H, d,
J ¼ 5:4 Hz), 1.64 (2H, sext, J ¼ 7:5 Hz), 1.70 (3H, dd,
J ¼ 6:8 Hz, 1.7 Hz), 2.18 (1H, dd, J ¼ 8:4 Hz, 5.4 Hz),
2.72 (2H, t, J ¼ 7:5 Hz), 5.11 (1H, ddq, J ¼ 10:7 Hz,
8.4 Hz, 1.7 Hz), 5.21 (1H, t, J ¼ 1:3 Hz), 5.22 (1H, t,
J ¼ 1:3 Hz), 5.59 (1H, dqd, J ¼ 10:7 Hz, 6.8 Hz,
1.3 Hz).
Evaluation of biological efficacy in a mosquito coil
formulation. The preparation of test mosquito coils
complied with the method described by Yamaguchi et
al.3) The test coil was fitted on a coil holder and placed
at the center of the chamber (4.3 m ꢂ 2.65 m ꢂ 2.45 m
height). The coil was ignited and then 100 adult female
mosquitoes were released into the chamber. The number
of knocked down mosquitoes was counted at designated
intervals for 75 minutes.
(4-Allyl-2,3,5,6-tetrafluorophenyl)methyl (1R,3R)-2,
2-dimethyl-3-((1Z)-1-propenyl)cyclopropanecarboxylate
(10). This compound was prepared according to the
procedure described for 7. NMR ꢀH (CDCl3Þ: 1.13 (3H,
LD50 and KT50. These values were calculated by the
probit method.4)

Metofluthrin with High Vapor Action against Mosquitoes
173
Fig. 4. Effectiveness of 2,3,5,6-Tetrafluorobenzyl Chrysanthemate
and Its Norchrysanthemic Analog in an Ambient-temperature
Formulation against Culex pipiens pallens.
Table 1. Insecticidal Effectiveness of Metofluthrin and Its Analogs
against Culex pipiens pallens
Fig. 2. Method for Evaluating the Vapor Activity of a Formulation at
Ambient-temperature.
Compound
R
R.T.a)
5
H
F
30
100
200
490
250
500
360
2500
10
6
7
Me
Et
8
9
Pr
Fig. 3.
10
11
allyl
OMe
CH2OMe
metofluthrin (1)
empenthrin (2)
d-allethrin (3, standard)
100
Results and Discussion
a)
Relative toxicity (R.T.) against Culex pipiens pallens based on LD50 by
the topical application method.
Ohno5) and Elliott6) independently reported insectici-
dal norchrysanthemic acid esters in the 1970s (Fig. 3).
According to their reports, these norchrysanthemic acid
esters showed comparable insecticidal activity to that of
the corresponding chrysanthemates.7) However, further
studies were discontinued at that time because they
could not find any justification to develop these
norchrysanthemic acid esters due to the difficulty of
synthesizing norchrysanthemic acid. Despite this, we
directed our attention to the norchrysanthemic acid
esters because they had a lower molecular weight and
showed comparable insecticidal activity to that of the
corresponding chrysanthemates. We synthesized various
insecticidal norchrysanthemic acid esters and screened
them for vapor activity.
Table 2. Effectiveness of Metofluthrin in Various Formulations
against Mosquito Species
KT50 [min]a)
Conc. [%] metofluthrin d-allethrin
Formulation
Species
non-heated C. pipiens pallens
27
>60
0.013
0.02
0.04
0.2
49
35
22
mosquito coil C. pipiens pallens
54
58
0.005
0.2
42
mosquito coil C. quinquefasciatus
a)
KT50: time for 50% knocked-down calculated by the probit method.
As a result of the screening, we found 2,3,5,6-
tetrafluorobenzyl norchrysanthemate (5) had a faster
knockdown activity (KT50, time for 50% knocked-down
calculated by the probit method) than the chrysanthe-
mate (4)8) against mosquitoes as shown in Fig. 4. We
then synthesized the derivatives with substituents at the
4-position on the phenyl ring of compound 5.
All analogs had much higher activity against mosqui-
toes than empenthrin and compound 5 by the standard
topical application method as shown in Table 1. The
relative toxicity reached the maximum with between
two and three carbon atoms at the 4-position (5–9).
Unsaturation (10) and incorporation of an oxygen atom
(11) also showed substantial activity, inter alia, the 4-
*
methoxymethyl derivative (1, metofluthrin ) exhibiting
the highest lethal potency, being over twenty five times
as active as d-allethrin. The results presented in Table 2
clearly demonstrate that metofluthrin exhibited a sig-
nificant level of vapor action against mosquitoes at room
temperature. Metofluthrin also exhibited a high levels of
knockdown activity in coil formulations when tested
against mosquito species as shown in Table 2. In
particular, metofluthrin exhibited similar activity to that
of d-allethrin against southern house mosquitoes (Culex
*
ISO 1750 provisionally approved.

174
K. UJIHARA et al.
pyrethroid. Jap. J. Sanit. Zool., 32, 59–66 (1981).
4) Bliss, C. I., The determination of the dosage mortality
curve from small numbers. Quart. J. Pharm. Pharmacol.,
11, 192–216 (1938).
5) Okuno, Y., Itaya, N., Mizutani, T., Ohno, N., Matsuo, T.,
and Kitamura, S., Japan Kokai Tokkyo Koho, 47043333
(Feb. 19, 1972).
6) Elliott, M., Farnham, A. W., Janes, N. F., Needham, P. H.,
and Pulman, D. A., Potent pyrethroid insecticides from
modified cyclopropane acids. Nature, 244, 456–457
(1973).
quinquefasciatus) at only a fortieth of the active ingre-
dient level.
Metofluthrin will be marketed for environmental
health use, having high knockdown activity against
mosquitoes and an excellent mammalian safety profile.
Metofluthrin is suitable for use in various existing source
devices like mosquito coils, and also in the more novel
devices such as a fan vaporizers and treated paper strip.
References
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Sci., 7, 499–502 (1976).
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