Fungicidal meta-substituted strobilurin analogs

Article abstract of DOI:10.2533/000942903777678614

Strobilurin analogs in which the side chain is in a meta-relationship to the pharmacophore are active fungicides provided an ortho-group is also present. Syntheses, fungicidal screening results and structure-activity relationships for these compounds are described. A new pharmacophore synthesis from aniline precursors is also detailed.

Full text of DOI:10.2533/000942903777678614

CROP PROTECTION RESEARCH
675
CHIMIA 2003, 57, No.11
Chimia 57 (2003) 675–679
© Schweizerische Chemische Gesellschaft
ISSN 0009–4293
Fungicidal meta-Substituted
Strobilurin Analogs
Michael P. Walker*
Abstract: Strobilurin analogs in which the side chain is in a meta-relationship to the pharmacophore are active
fungicides provided an ortho-group is also present. Syntheses, fungicidal screening results and
structure–activity relationships for these compounds are described. A new pharmacophore synthesis from
aniline precursors is also detailed.
Keywords: Crop protection · Cytochrome bc1 complex · Fungicides · Mitochondrial respiration inhibitors ·
Strobilurin analogs
Introduction
also an effective fungicide. Replacement of in Fig. 2. Most important among these side
both double bonds led to a third class of bi- chain modifications are aryl ethers (present
The isolation and structural determination ologically active ring constrained analogs in 5 and 7), aralkyl ethers (in 6 and 9) and
of the fungicidal natural product Strobilurin represented by the naphthalene 4. oximes (exemplified by 8 and 10).Asecond
A (1) from the basidiomycete fungus Stro- From these promising beginnings scien- objective of the optimization work was
bilurus tenacellus was the starting point for tists at BASF and ICI turned their attention to replace the putative pharmacophoric
fruitful research programs at many compa- to optimizing activity mainly on structure 2. β-methoxyacrylate group present in the nat-
nies, which have subsequently led to com- The stilbene unit (or side chain) was still ural product with other groups. ICI, BASF
mercialization of several successful agri- not photostable enough to be part of a com- and Ciba quickly found that an oxime ester
cultural fungicides [1]. The site of action of mercial fungicide so both companies fol- was an effective replacement (in 6 and 8).
this compound was shown to be inhibition lowed an optimization program designed to A variety of other replacements has also
of mitochondrial respiration at the cyto- replace the styrene with other lipophilic been uncovered, notably the carbamate
chrome bc1 complex [2]. The unique groups. After the publication of the first (present in 9) and the oxime amide (i.e. 7).
and simple β-methoxyacrylate containing patents many other companies joined the At Du Pont we have found that certain
structure proved to be readily amenable to search for novel side chains. A wide variety heterocycles such as the triazolinone pres-
modification with retention of activity. The of groups proved capable of standing in for ent in structure 10 serve as cyclic pharma-
natural product was unfortunately very the stilbene as exemplified by the structures cophores [3].
photolabile and therefore not very effective
in testing on whole plants in direct sunlight.
A solution to the photostability problem
was soon arrived at as scientists from both
ICI (now Syngenta) and BASF simply re-
placed the double bond proximal to the β-
methoxyacrylate with an aromatic ring as in
the fungicidally active stilbene 2 (Fig. 1).
Similar exchange of the distal double bond
with an aromatic ring gave the biphenyl 3,
O
O
O
O
O
O
2
1 (Strobilurin A)
*Deceased December 7, 2002
Correspondence: Dr. Thomas M. Stevenson
Du Pont Crop Protection
O
O
O
O
Stine-Haskell Research Center
P.O. Box 30
O
O
Newark, DE 19714, USA
Tel.: +1 302 366 5744
4
3
Fax: +1 302 366 5738
E-Mail: Thomas.M.Stevenson@usa.dupont.com
Fig. 1. Strobilurin A double bond replacement strategy pursued by ICI and BASF

CROP PROTECTION RESEARCH
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CHIMIA 2003, 57, No.11
Twist
N
N
MeO
Side-Chain
O
O
O
O
Phenyl Ring
O
O
O
CN
N
CO2Me
O
O
Pharmacophore
6 (kresoxim-methyl)
5 (azoxystrobin)
Fig. 3. Side-chain enforces a twist in strobil-
urin analogs
F
F
O
N
F
O
O
O
O
O
N
N
HN
O
7 (metominostrobin)
8 (trifloxystrobin)
Side Chain Orientation
F
In our attempts to optimize the activity
of strobilurin analogs derived from our tri-
azolinone pharmacophore, we studied the
effects of side chain position with respect to
the pharmacophore. Previous work at ICI
had established a clear preference for the
point of attachment of the styrene side
chain to be ortho to the β-methoxyacrylate
for good activity [4]. Compounds with
styrene groups meta or para to the pharma-
cophore were essentially inactive. Despite
this precedent we were convinced that at
least part of this preference for ortho-sub-
stitution could be attributed to the contribu-
tion of the side chain in twisting the phar-
macophore out of plane (Fig. 3). From
X-ray crystallographic measurements the
twist between the pharmacophore and the
aromatic ring was reported by ICI to be 86°
[4]. We believed that if we synthesized stro-
bilurin analogs with a methyl group ortho
to the triazolinone (thereby imparting a
twist to the pharmacophore) that the side
chain might also be successfully attached to
other positions on the ring. The myriad of
different side chains that had been patented
in strobilurins was also compelling evi-
dence that a large lipophilic binding site
was present in the cytochrome bc1 complex
that might accommodate substitution at
other positions.
F
O
N
O
N
F
Cl
N
N
O
N
O
O
O
O
N
N
9 (pyraclostrobin)
10 (DPX KZ165)
Fig. 2. Examples of various strobilurin pharmacophores and side chains
O
O
1) ClCOCl
O
ClCOCl
2) NH2NMe2
N
Cl
O
85-90%
H
N
35-55%
HN
NH2
N
N
N
O
11 a-d
12 a-d
13 a-d
F
F
F
4
1) AlCl3
N
S
2) NaOMe
3
5
N
O
Side chain
O
3) ThiadazolylCl
K2CO3
N
F
O
F
F
20-50%
N
N
Side Chain
14 a-d
Scheme 1. Synthesis of four side chain positional isomers
Table 1. Comparative activities of the four positional isomers
Triazolinone Pharmacophores
Side Chain
Position
2
3
4
(14c)
—
5
(14d)
—
We chose to begin our investigation of
side chain position with heterocyclic ethers
derived from 3-aryl-1,2,4-thiadiazoles.
Compounds bearing this side chain have
extremely high levels of fungicidal activity
and starting materials to make all four of the
potential targets are readily available [5].
As shown in Scheme 1 synthesis began
with o-toluidines 11a–d with a methoxy
group in each of the four possible positions.
(14a)
++++
(14b)
++
Fungicidal
Activitya
Mitochondrial
Inhibition ppbb
11
55
> 5000
> 5000
a Fungicidal activity was measured on the following pathogens: Erysiphe graminis (wheat powdery mildew), Puccinia recon-
dita (wheat leaf rust) and Pseudocercosporella herpotrichoides (wheat foot rot). The following rating system is used in all of
the tables: + denotes activity at 200 ppm, ++ denotes broad spectrum activity at 200 ppm, +++ denotes broad spectrum
activity at 40 ppm and ++++ denotes broad spectrum activity at 10 ppm or below. See [6] for details of the assays.
b Measured with beef heart mitochondria by Mr. Rand Schwartz and represents the concentration in ppb required to inhibit
50% of the mitochondrial function.

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The triazolinone intermediates 13a–d were be only moderately selective (less than 2:1 Side Chain Optimization
built up by isocyanate formation, followed in favor of the desired product). However,
by reaction with 1,1-dimethylhydrazine separation was not difficult and the isolated
The highest levels of activity were seen
and ring formation with triphosgene [3]. benzylic bromides 19a–c could be convert- for the oxime amide and oxime ester phar-
The phenols were liberated by reaction with ed to the corresponding oxime final prod- macophores and we wanted a more robust
aluminum chloride. Displacement of the ucts without difficulty. Similar chemistry method for synthesis of intermediates to
chlorides with sodium methoxide gave the produced the methoxy acrylate 21d, but the pursue an analog program to explore struc-
four isomeric precursors to the target struc- bromination and oxime formation proceed- ture–activity relationships on the oxime
tures. Reaction of each isomer with com- ed in much lower overall yield. The tria- sidechain. Since selective functionalization
mercially available 5-chloro-3-(3,5-bis- zolinone precursor 18 was made from 2,5- of the 5-methyl group was our main syn-
trifluoromethylphenyl)-1,2,4-thiadiazole dimethylaniline by the route shown in thetic obstacle, we searched for potential
(Maybridge) in the presence of potassium Scheme 1. As expected, good levels of ac- starting materials that already were appro-
carbonate gave the desired strobilurin tivity were observed for the products as priately functionalized at the 5-position.
analogs 14a–d.
seen in Table 2, confirming the ability of the Commercially available 3-amino-4 methyl-
The results, from fungicidal and mito- methyl group to impart a twist to the other benzyl alcohol (22) was an appealing start-
chondrial respiration testing for the four strobilurin pharmacophores.
isomers, are shown in Table 1. As expected
ing material, but its use required the devel-
and in line with previously known structure
activity relationships, the highest activity
was observed with the side chain in the
1) Mg
2-position. Compounds with side chains in
the 4- and 5-positions were virtually inac-
tive in both fungicidal and mitochondrial
testing. Gratifyingly, when the side chain
was placed in the 3-position high activity
was also observed. This showed that the
twist imparted by the methyl group was
indeed sufficient to allow binding of the
3-substituted product to the cytochrome
bc1 complex. As a control we also synthe-
sized and tested the analogous 3-substitut-
ed compound lacking the ortho-methyl
group and found it to be essentially inactive
in the assays. [6]
2) ClCOCOCl
NH2OMe
MeOH
X = Br
45%
81%
O
O
O
N
X
O
O
O
15
17
16
NBS
AIBN
X = NH2
30 %
1) Ph3P=CHOMe
2) NBS, AIBN
MeNH2
86 %
As in
< 10 %
Scheme 1
40 %
Br
NBS, AIBN
40%
NBS, AIBN
Traditional Pharmacophores
N
O
O
O
O
N
We next set out to discover if the posi-
tive effect of an ortho-methyl group would
extend to other known strobilurin pharma-
cophores. Our synthetic approach was
based on an α-ketoester intermediate that
would allow us to make oxime ester, oxime
amide and methoxy acrylate pharma-
cophores from a common synthetic inter-
mediate (Scheme 2). We investigated sev-
eral reagents for the introduction of this
function and settled on the use of oxalyl
chloride. We formed the Grignard reagent
derived from 2-bromo-p-xylene and added
it to a –70 °C solution of oxalyl chloride in
THF. Quenching with methanol gave the
desired phenyl glyoxylate 16 in moderate
yield (the main byproduct was the corre-
sponding benzil). This could be converted,
for example, to the oxime ester 17 by reac-
tion with methoxylamine and on to the
oxime amide 20 with methylamine. This
approach afforded the desired intermedi-
ates from an inexpensive starting material,
but differentiation of the two methyl groups
turned out to be more problematic than ex-
pected as the radical bromination proved to
N
N
45%
Pharm
19
HN
20
18
NaH
25-70%
N
HO
CF3
N
O
CF3
Pharm
21 a-d
Scheme 2. Synthesis of various pharmacophores with the oxime side chain at position 3
Table 2. Comparative activity of various pharmacophores with the oxime side chain
N
O
O
O
O
O
O
Strobilurin
Pharmacophore
O
O
N
N
N
N
O
O
HN
(21d)
(21b)
+++
(21c)
(21a)
++++
Fungicidal
Activity
+++
++

CROP PROTECTION RESEARCH
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CHIMIA 2003, 57, No.11
opment of an effective method for convert-
ing the amino group to the oxime ester or
amide (Scheme 3).Aldoximes are known to
react with diazonium salts in the presence
of copper salts to form aromatic ketoximes
[7]. If the oxime of methylglyoxylate could
participate in such a coupling, a direct syn-
thesis of the oxime ester pharmacophore
might be possible. Indeed, when the diazo-
nium coupling reaction was carried out in
the presence of copper sulfate and sodium
sulphite the desired ester oxime was isolat-
ed in good yield [8]. Methylation of the
oxime with methyl iodide or dimethylsul-
fate in the presence of potassium carbonate
produced the desired (E)-isomer of the
oxime ester 23 in high selectivity and good
yield. Conversion of the alcohol to the chlo-
ride 24 could be carried out by treatment
with methanesulfonyl chloride. Reaction of
the benzylic chloride with various oximes
gave the target molecules, which could be
converted with methylamine to the amide
oximes 25a–c. It was also possible to make
ethers 26a–g of various chain lengths from
24, which allowed us to gauge the effect of
chain length on activity (n = 0, 1, and 2).
The most active materials were those with
n = 0 (Scheme 3).
1) NaNO2, CuSO4
HCl, Na2SO3
MeSO2Cl
TEA
OH
OH
HO
O
89%
O
O
N
N
NH2
22
O
O
2) K2CO3, MeI
23
39%
X
N
1) Oxime
NaH
Cl
O
2) MeNH2
30-50%
H
N
O
O
O
N
N
O
O
25a-g
24
1) NaH, HO(CH2)nPh
30-70%
n
O
O
O
N
O
26 a-c
Scheme 3. Novel synthesis of traditional pharmacophores via diazotization
Table 3. Relative activities of various oxime substituents
Subst.
3,5-DiCl 3-SiMe3 3-OCF3
(25d)
3-Cl
(25e)
3-Me
(25f)
4-Me
(25g)
3,5-CF3
(25a)
(25b)
(25c)
Alternative ortho-Groups
Fungicidal
Activity
++++
+++
+++
++++
+++
++
++
It was also apparent that methyl groups
were not the only groups that might be able
to cause a twist in strobilurin pharma-
cophores. We found it was straightforward
to synthesize compounds with a variety of
halogens as well as a methoxy group ortho
to the pharmacophore with the oxime side
chain (Scheme 4). Beginning with the req-
uisite anilines 27a–f with a 3-methyl and
the various ortho-substituents, we were
able to use the diazotization route to install
the pharmacophore in moderate yield. Rad-
ical bromination of the methyl group gave
the requisite benzylic bromides. Reaction
with the 3-trifluoromethylacetophenone
oxime, and amidation completed the se-
quence to give 29a–f. As shown in Table 4,
the methyl substituent had the highest lev-
els of fungicidal activity.
1) NBS
1) NaNO2 CuSO4
HCl, Na2SO3
2) NaH, Oxime
3) MeNH2
R
HO
O
R
20-45%
O
O
N
N
NH2
O
O
2) K2CO3/MeI
30-40%
27 a-f
28 a-f
N
F
O
F
F
R
H
N
O
N
O
29 a-f
Scheme 4. Synthesis of various ortho-substituents
Table 4. Relative fungicidal activities of various ortho-groups
R
Me
(21a)
++++
F
(29a)
++
Cl
(29b)
++
Br
(29c)
+
I
OMe
(29e)
+
H
(29f)
-
(29d)
++
Fung.
Activity

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CHIMIA 2003, 57, No.11
Conclusions
[1] For a comprehensive review of the evolu-
tion of strobilurins see: H. Sauter, W.
Steglich, T. Anke, Angew. Chem. Int. Ed.
1999, 38, 1328.
Strobilurin analogs with meta-side-
chains are quite active against a variety of
commercially important plant pathogens.
Highest levels of activity were found for
diseases of cereal crops such as wheat foot
rot, wheat leaf rust, wheat glume blotch and
especially, wheat powdery mildew. Effica-
cy for the optimal compounds such as 21a
was displayed at 2 ppm and below for
various powdery mildews. Good activity
was also displayed on other important
pathogens such as apple scab, rice blast and
grape downy mildew.
We were able to show that meta-substi-
tuted strobilurins can have interesting lev-
els of fungicidal activity provided that a
substituent ortho to the pharmacophore was
also present. This work contradicted previ-
ous findings that the sidechain needed to be
in the ortho-position relative to the phar-
macophore. We surmise that the ortho-
group enforces a twist on the pharma-
cophore leading to a conformation suitable
for binding to the cytochrome bc1 complex,
similar to the effect of the traditional ortho-
side chain.
[2] W.F. Becker, G. von Jagow, T. Anke, W.
Steglich, FEBS Lett. 1981, 132, 329.
[3] R.J. Brown, B. Ashworth, J.E. Drumm,
D.A. Frasier, M.A. Hanagan, C. Happer-
sett, G.E. Koether, D.J. Robinson, K-M.
Sun, P. Wojtkowski, in ‘Synthesis and
Chemistry of Agrochemicals VI’, Eds.
D.R. Baker, J.G. Feynes, G.P. Lahm, T.P.
Selby, T.M. Stevenson, American Chemi-
cal Society, Washington D.C., pp.
327–341.
[4] K. Beautement, J.M. Clough, P.J. de
Fraine, C.R.A. Godfrey, Pestic. Sci. 1991,
31, 499.
[5] R.J. Brown, D.M.T. Chan, M.H. Howard,
D.J. Daniel, D.A. Clark, T.P. Selby (Du
Pont), World Patent Application WO
97/00612, 1997.
[6] Experimental details, physical properties
and fungicidal activity for compounds in
this paper are described in: M.P. Walker
(Du Pont), World Patent Application WO
99/28305, 1999.
[7] W.F. Beech, J. Chem. Soc. 1954, 1297.
[8] A similar transformation was subsequent-
ly reported by Novartis: R. Waditschatka
(Novartis), World Patent Application WO
00/63162, 2000.
Acknowledgements
We would like to thank all the members of
the plant disease control group at Du Pont for
providing the biological evaluations presented
here. Mitochondrial respiration assays were car-
ried out by Mr. Rand Schwartz and Dr. Douglas
Jordan. Analytical measurements were obtained
by Mr. James Doughty, Mr. John Groce and Ms.
Gina Blankenship. We also acknowledge helpful
discussions with Dr. David A. Clark and Dr.
Richard J. Brown. We would also like to thank
Dr. Darius J. Robinson and Ms. Berlin Stokes
(who synthesized the original thiadiazolyl tria-
zolinone (14a), which served as the comparison
point for our original investigation). We would
also like to thank Dr. James V. Hay and Dr.
William K. Moberg for their enthusiastic sup-
port of this project.
Received: September 15, 2003