Synthesis and Chemistry of Agrochemicals III - American Chemical

Chapter 29 N-Benzoyl-N-alkyl-2-aminothiazole Proinsecticides 1

Marty C. Wilkes , Paul B. Lavrik, and John Greenplate

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Monsanto Agricultural Products Company, 700 Chesterfield Village Parkway, St. Louis, MO 63198

When a series of seventeen N-benzoyl-2-amino-5-chloro4-trifluoromethylthiazole amides were prepared and screened as potential Heliothis virescens (tobacco budworm) (TBW)) insecticides, an N-methyl group at the amide nitrogen eliminated phytotoxicity and reduced acute rat toxicity. Subsequently, the effect of deuterium incorporation in the N-methyl group on TBW mortality was investigated. The NCD derivative was inactive while the NCH derivative exhibited a diet incorporation LC of 109 ppm (+/- 20 ppm) against TBW suggesting a deuterium isotope effect in abstraction of the methyl hydrogen atoms by cytochrome P-450 enzymes. 3

3

50

Two market forces drive industrial pesticide research progress supply and demand. On the demand side, the need for new classes of environmentally acceptable pesticides is increasing. On the supply side, existing pesticides are being removed from the market for selectivity and resistance reasons. In searching for new classes of agrochemicals such as insecticides, nonselective toxins are relatively easily discovered (7). The general problem is whether there is any systematic way to improve these toxins' selectivity patterns and restrict their lethality to the target pest by taking advantage of metabolic pathways in the target (2). While this approach of selectivity "clean up" remains largely a dream, limited steps can be taken in this direction. In the course of screening for classes of chemical insecticides we discovered a class of amides 1 shown below in Figure 1. When R=H 1

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In Synthesis and Chemistry of Agrochemicals III; Baker, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

328

SYNTHESIS A N D CHEMISTRY O F AGROCHEMICALS

compound l a exhibited a tobacco budworm (TBW) L C of 1-10 ppm via diet incorporation. Unfortunately these compounds were phytotoxic and exhibited rat toxicity effects. Our approach to this problem was to vary the substituent R to create a series of potential propesticides (5). We introduced various substituents at the R group that would be readily removed by the insect and create a pesticide in situ. We hoped to take advantage of the relatively rapid metabolism rate of insects relative to other organisms in the process. Secondly, we developed a new in vivo method of actually demonstrating that these propesticides were in fact converted to the pesticide in the insect and we chose to use deuterium isotope effects for this demonstration. Downloaded by STANFORD UNIV GREEN LIBR on June 22, 2012 | http://pubs.acs.org Publication Date: September 22, 1992 | doi: 10.1021/bk-1992-0504.ch029

50

Synthesis of N-Benzoyl-N-Alkyl-2-Aminothiazoles Figure 2 below displays our most generally applicable route to the desired thiazole propesticides (4). Beginning with the traditional Hantzch thiazole synthesis (5) we condensed l,l,l-trifluoro-3-bromo-2-propanone with an alkylthiourea in refluxing water to obtain the hydrobromide salt of the aminothiazole 3 and the resulting mixture was extracted with sodium hydroxide to give thefreeamine in moderate to excellent yield depending on the R substituent. Aminothiazoles 3 in turn were selectively chlorinated in the 5-position with N-cWorosuccinimide in refluxing acetonitrile to obtain the 5-chlorothiazole 2 in moderate to excellent yield. Finally 2 was condensed with an acid chloride to obtain the amides 1 in moderate to excellent yields depending on the substituents present. This synthesis worked quite well for simple R groups such as hydrogen, ethyl, isopropyl, methoxy, benzyl, and phenyl type substituents. However, in cases where the byproduct hydrochloric acid formed in the third step caused amide hydrolysis or other side reactions with the R group, then an alternative synthesis was used as shown below in Figure 3. The sodium salt of the amine, or amide, was prepared and condensed with the acid chloride to form the respective amide or imide. To prepare other substituted alkylaminothiazoles,freeradical chemistry of the methyl group was invoked as shown below in Figure 4. Utilizing benzoyl peroxide in carbon tetrachloride on l b afforded a 24% yield of the desired chloromethyl derivative lk. However a more efficient route was discovered, on changing to the reagent sulfuryl chloride. Irradiating the reaction mixure with 366 nm light for 48 hours, a 98% yield of the chloromethyl derivative was obtained. The idea was to introduce various substituents which could later be readily removed by the insect by reacting the chloromethyl compound with a series of nucleophiles as shown below in Figure 5. For example, reacting thiophenol with triethylamine gave the triethylammonium salt that readily displaced the chloride ion. Potassium cyanide in acetonitrile also worked quite well with


III

29.

WILKES E T A L .

N-Benzoyl-N-alkyl-2-aminothiazole Proinsecticides 329


Figure 1. Structures of the thiazole amide series. .CF s

Ρ

3

Step A

CP >—Br

water, reflux

R—NH

66-99% yield O

R=H-, Et-, iPr-, CH OCH CH -, benzyl-, plienyl3

2

2

StepB CH CN, reflux

N-CI

3

F,C CF

^ ^ n - J \ FoC

«

α

/

StcpC

RNH

J > — Cl

R'COCI, toluene reflux 17-99% yield

29-92% yield Figure 2. Synthesis of thiazole amides.

\

NaH, ether 2)3,5-(CF ;) P h c o c i 3

2

1) NaH, ether \

2) 3,5-(CF ) PhCOCl 3 2

_/ΓΧ *

C l

^

s

// Λ X

3

S

Figure 3. Synthesis of amides via sodium salts.


330

SYNTHESIS AND CHEMISTRY O F A G R O C H E M I C A L S III

18-crown-6. Potassium thiocyanide succeeded in acetone while potassium acetate in acetonitrile reacted well in the presence of 18-crown-6. Potassium iodide was successful if the reaction was repeated two times. Each S 2 transformation required a different solvent or additive. To prepare the d3-methyl thiazole lj the synthesis shown in Figure 6 below was used. The acetamide protecting group allowed selective alkylation at the amide position provided the acetamide methyl group did not contain electron withdrawing groups. No thiazole N-alkylation or amidate formation occurred with acetamides. We selectively hydrolyzed off the acetamide portion with sodium hydroxide in methanol to give the d3 amide which then was treated with an acid chloride as before to give the d3 methyl thiazole lj. In the C-13 NMR of lj the d3 methyl appeared as a heptet at 34 ppm.


N

Biological Data: The above target series of amides 1 was tested in our biological assays (4,6). A diet incorporation assay was used where the amides were dissolved in acetone and incorporated thoroughly into the diet of the insect and mortality was measured six days after application of the neonatant tobacco budworms. The center of Table I shows the substituent R and the right side shows the range of diet incorporation values. On repeating this assay a year later these values were still representative on our strain of TBW. The best examples were the lead compound la, N-acetyl, acetoxymethyl, and iodomethyl derivatives which exhibited TBW insecticidal activity. In die starred cases we observed phytotoxicity on one or more crop or weed species. There were, however, two selective cases; the methyl and benzyl derivatives. Methomyl was used as the standard positive control which exhibited an L C of 2-3 ppm in our assays. Our strain of TBW was resistant to several insecticides including chlordimeform. On comparing the N-methyl case l b with l a in a preliminary mammalian toxicity study, as shown below in Figure 7, l a exhibited a rat acute oral toxicity of 69 mg/kg whereas l b exhibited a value of 501 mg/kg. Thus, l b was roughly seven times safer to mammals and nonherbicidal as well. 50

Cytochrome P-450 N-Methyl Oxidations: It was postulated that the N-methyl was hydroxylated by insect cytochrome P-450 enzymes and this intermediate would then lose formaldehyde to give the insecticidal compound l a (7,8). We attempted to synthesize the hydroxymethyl intermediate but it tended to disproportionate to form dimers, trimers, and tetramers. The apparently stable acetoxymethyl analog did, however exhibit insecticidal activity. In the literature for drugs there are two mechanisms postulated for cytochrome P-450 oxidations (9) shown below in Figure 8. The first involves a loss of a hydrogen atom to give a methyl radical which is then hydroxylated.


29.

N-Benzoyl-N-alkyl-2-aminothiazole Proinsecticides

WILKES E T A L

F Q

F C

3

3

.0

F c'

CH

3

s

b)S0 CI ,366nmln)

3

2

F

2

/

fa

s'

c

lb

lk


a) 24% yield b) 98% yield

Figure 4. Synthesis of N-chloromethyl thiazoles.

Cl

Cl

°Λ^_Ν

Nucleopliile (Nu)

y

F C

-CF

N

•

2

^

3

O*.

CH CI

CH Nu 2

F C^ ^ ^ C F

3

3

3

lk

Figure 5. Synthesis of N-alkyl substituted thiazoles.

3 C

\_

\\

F s C

v

C

—

CDATEBA

• (Γ\-/™

\\

3

~ Ν τ ~

\

CH Cl ,K C0 ,NaOH 2

H

2

2

l

3

\

>L^ / ~ N

S

2f

X

CD

5 10% NaOH, MeOH

F Q^ 3

.CF

FC

3

3

3,5-(CF ) PliCOCl toluene, reflux

4

3 2

s

^

^

s

Figure 6. Synthesis of d3 amide lj.




Table I. Biological Data

NHCH

Methomyl

Number

R-

1a

H-

1b

Me-

T B W LC-50 diet i n c o r p . 1-10 p p m * ( p h y t o t o x i c ) 70-125 p p m

1c

Et-

inactive

1d

Allyl-

inactive

1e

iPr-

inactive

1f

Acetyl-

1-10 p p m *

19 1h

MeOEt-

inactive

Benzyl-

1i

5-10 p p m

Ph-

inactive

1) 1k

d3-Me-

inactive

Cl-Me-

inactive *

11

PhS-Me-

inactive

1m

NC-Me-

1n

NCS-Me-

10-50 p p m *

10

AcO-Me-

1-10 p p m *

1p

3,5-CF3Bz-Me-

1q MethomylI

l-Me-

50-100 p p m *

inactive 1-10 p p m * 2-3 p p m


3


29.

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N-Benzoyl-N-alkyl-2-aminothiazole Proinsecticides

333

Rat acute oral LD 69mg/kg

Rat acute oral LD 501 mg/kg

Rabbit dermal LD >200mg/kg

Rabbit dermal LD >200mg/kg

Severe rabbit eye irritation

Slight rabbit eye irritation

50

50

50

50

Figure 7. Mammalian toxicity of N-thiazoyl amides.

k /k - 3 H

D

Figure 8. Cytochrome P-450 oxidation.




334

This mechanism exhibits a very strong kinetic deuterium isotope effect of, on average, about eight and can rangefromabout six to eleven. An example of an enzyme which operates in this range is horseradish peroxidase which has a kH/kD of 8.72. A C-D bond which is stronger than a C-H bond is being broken givingriseto the large deuterium isotope effect. The second mechanism involves the loss of an electron followed by a loss of a proton to give the same radical. In this case the bond is broken later in the transition state exhibiting a reduced kinetic deuterium isotope effect. If either mechanism was operative in the insect, we should observe a reduction in insect mortality for the d3 methyl case. A special assay was run comparing side by side l a and lj. Compound l b repeatedly exhibited a L C of 109 ppm (+/- 20 ppm). lj exhibited essentially no activity up to 500 ppm. These results shown in Figure 9 below suggest that the deuterium isotope effect is operational resulting in a greater than five fold difference in hydroxymethylation, and hence, activation rate. Breaking the methyl C-H bond lies on the pathway to creating an insecticide. This effect is due to the sum of different enzymes which are present in the insect (10). We assume that the deuterium atom has a negligible effect on transport within the insect. 50

lb TBW L C so ~ 90 ppm

lj TBW L C

5 0

>500 ppm

Figure 9. Deuterium isotope effect in TBW mortality.

Conclusion: The N-methyl and N-benzyl groups are the best substituents of seventeen tested for the amide nitrogen in conferring selective insecticidal activity. N-Alkyl thiazoles with unsubstituted alkyl groups consistently exhibit reduced herbicidal effects. The remarkable difference in TBW insecticidal activity between l b and lj indicates that l b is acting as a proinsecticide. It remains to be shown in situ that l b is converted to la. When several metabolites are simultaneously present within the insect it becomes impossible to ascribe the resulting mortality to any one of them. Thus this type of deuterium incorporation experiment will only work if it is operating on the first step in the transformation. Deuterium incorporation studies may be a valuable mechanistic tool for investigation of other whole organism processes. This type of experiment has not been commonly applied in agriculture to date.


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N'Benzoyl-N-alkyl-2-aminothiazole Proinsecticides

Acknowledgments: We thank Steve Sims and Paul Gahr who performed several insect assays as well as Gabriel Srouji and Diane Broccolino who generously provided us several thiazole samples and Peter Beak who contributed expert advice on this project.


Literature Cited: 1. Brooks, G . T. In Progress in Drug Metabolism; Bridges, J. W.; Chasseaud, L . F., Eds.; Taylor and Francis: 1984; Vol. 8, 101-188. 2. Fukuto, T. R. In Pesticide Synthesis Through Rational Approaches; Magee, P. S.; Kohn, G. K.; Menn, J. J., Eds.; American Chemical Society: Washington, D. C., 1984; Vol. 255; 87-101. 3. Drabek, J.; Neuman, R. In Progress in Pesticide Biochemistry and Toxicology; Hutson, D. H.; Roberts, T. R.;,Eds.; Wiley and Sons: New York, New York, 1985, Vol. 5; 35-87. 4. Wilkes, M .C.;Lavrik, P. B.; Greenplate, J. J. Agric. Food Chem. 1991, 39, 1652-1657. 5. Kaye, P. T.; Meakins, G . D.; Willbe, C.; Williams, P. R. J. Chem. Soc., Perkin Trans. 1, 1981, 2335-2339. 6. Marrone, P. G.; Ferri, F. D.; Mosley, T. R.; Memke, L . J. J. Econ. Entomol., 1985, 78, 290-293. 7. Metcalf, R. L.; Fukuto, T. R.; Collins, C.; Borck, K.; El-Aziz, S. Α.; Munoz, R.; Cassil, C. C. J. Agric. Food Chem., 1968, 16, 300-311. 8. Guengerich, F. P.; Macdonald, T. L . Acc. Chem Res. 1984, 17, 9-16. 9. Miwa, G . T.; Walsh, J. S.; Kedderis, G . L.; Hollenberg, P. F. J. Biol., 1983, 258, 14445-14449. 10. Levine, W. G . Drug Metabolism Reviews, 1991, 23, 253-309. RECEIVED February 10, 1992


Synthesis and Chemistry of Agrochemicals III - American Chemical

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