Preemergent activity

Figure 2d. Three-dimensional perspective plot of the Hansch equation describing preemergence activity of 3-substituted TFMS herbicides on Foxtail grass (0.1% Tween 80 present)...

Although the synthesis program did not result in any commercial herbicides, two types of pyridazines were discovered which produced unexpected results, both in the level and the type of activity. The first series, which includes the 3-methoxy- and the 3-dimethylaminopyridazines, resulted in a high level of postemergence activity not observed in the lead or in the other 3-sub-stituted-pyridazines. Secondly, based on a comparison with the N-oxides of other pyridazines, the N-oxide of the 3-hydropyridazine resulted in unexpectedly high preemergence activity, yet without phytotoxicity to cotton. 

Alkylation of the nitrogen atom renders the molecule inactive. N-(Acetoxymethylation) retains preemergent activity while increasing postemergent broadleaf weed activity over that of the parent. Being derivatives of formaldehyde, these hemiaminals hydrolyze readily in soil and plant to the active parent NH-compound. 

Herbicidal Activity. The subject furanones are primarily active as preemergence and preplant incorporated (PRE/PPI) materials and are moderately active when applied postemergence. The structure-activity relationships discussed below pertain to the preemergence activity. 

Fluazifop-butyl has a preemergence activity too, lasting about 3-6 weeks according to soil type. This activity is only 25-50% of the herbicidal effect of postemergence application. Its selectivity is believed to be due to rapid degradation followed by conjugate formation in broad-leaved plants (Plowman et ai, 1980). 

A CH or Cl substitution on the benzene ring similarly reduces preemergence activity. This, however, does not hold true for 2,6-substituted compounds. 

In the case of analogous dialkyl derivatives, the postemergence activity of the 4-methyl derivative is stronger than that of the 4-trifluoromethyl derivative, while the opposite is true of their preemergence activity. 

As a starting point for structural optimization of the second lead, we divided the core structure into three subsections, the phenyl (X), the imino group (Y), and the thiazoline nucleus (R and R ) (Figure 5 ). First, we synthesized the 2-(N-ethoxycarbonylimino)-l,3-thiazolines that possessed a variety of substituents at each position of the phenyl ring to examine the effect on herbicidal activity. Preemergence activities are summarized in Figure 6. Substitution on the phenyl moiety was preferred in the meta position and we observed a progressive... 

Figure 4. Effect of Heterocyclic Ring on Relative Preemergence Activity...

In addition to the 8 Ib/acre tests, several compounds with prominent preemergence activity were retested at either 0.12, 0.25, 0.5, or 1.0 Ib/acre on the following plant species, as well as the five species listed above barnyard grass (Echinochloa crusgalli), velvetleaf (AbutiIon theophrasti), wild oat (Ayena fatua), zinnia... 

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