Herbicide groups, structure-activity relationships

The first chapter in the herbicide section is devoted to synthetic efforts related to the herbicide Command, currently being developed by FMC Corporation. Here we see detailed the various synthetic and structure-activity relationships of this important group of compounds. These compounds exert their phytotoxic effect by their bleaching action on a wide variety of economic weeds. An important observation was that soybeans were not affected at normal use rates. These compounds act upon the carotene and chlorophyll biosynthesis of the plant. Here are a group of synthetic pathways that are peculiar to plants and a few microorganisms and are susceptible to chemical attack. 

In order to obtain more powerful PDHc El inhibitors with effective herbicidal activity, all substituents R, R, R, R, and Yn in structure I were systemically modified. Through structural modification or optimization, more than thirty novel series of structure I including alkylphosphonates, monosalts of alkylphosphonic acids, alkylphosphinates, monosalts of alkylphosphinic acid, cycUc phosphate, and caged bicyclic phosphates were designed and synthesized (Scheme 1.20). Their herbicidal activities were evaluated. Structure-activity relationship was then established and analyzed. Enzyme inhibition, toxicities and usage of representative compounds in this group were also stodied later. 

Endothall is a structural analogue of cantharidin (Fig. 20). Its mode of action is apparently inhibition of protein phosphatase(s). A struc-ture/activity relationship study demonstrated that the presence of the oxygen bridge and the location of the two carboxylic groups play important roles in the activity of the molecules. The unusual 7-oxabicyclo[2.2.1]heptane ring of endothal is similar to that of cinmethylin, another natural product-like herbicide for which the mode of action has also eluded scientists for many years. 

Diary] and triaryl or naphthyl carbamates exhibit low herbicidal activity. The substitution of the aryl radical for a heterocyclic radical gives heterocyclic alkyl and dialkyl ureas, of which many examples have been prepared in recent years. The herbicidal activity of urea derivatives containing a heterocyclic radical, such as benzthiazole, thiazole, thiadiazole, oxadiazole and pyridine, is favourable if one or two methyl groups are substituted at the N -nitrogen. The carrier of total or selective action in these derivatives is presumably the heterocyclic part of the molecule. More recently several new groups of compounds have become known, mainly in the patent literature, for which the structure-activity on relationships are still to be elucidated. 

Swithenbank et al. (1971) synthesised forty related benzamide derivatives to study the relationship between chemical structure and action. The herbicidal activity of N-alkyl-3,S-dichlorobenzamides was enhanced by, y-unsaturation, and activity was further improved by a,a-dimethyl groups. In the case of dimethylpro-pynyl nzamides, derivatives with halogen substituents on the aromatic ring are the most active, substitution in positions 3 and 5 being optimal. 

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