Biological Activity of Two Stereoisomers of theN-Thienyl Chloroacetamide Herbicide Dimethenamid

Abstract

Due to the presence of an asymmetrically substituted C atom, dimethenamid [2‐chloro‐N‐(2,4‐dimethyl‐3‐thienyl)‐N‐(2‐methoxy‐1‐methylethyl)acetamide], a recently introduced N‐thienyl chloroacetamide herbicide, exists as two stereoisomers (S and R) having differing herbicidal activities as demonstrated with a selection of weeds and Lemna minor. The activity of the two isomers was investigated in greater detail with the green alga Scenedesmus acutus and compared to that of alachlor [2‐chloro‐N‐(2,6‐diethylphenyl)‐N‐(methoxymethyl)acetamide]. As with alachlor, the S isomer (5 μM) strongly inhibited algal growth and fatty acid desaturation while the R isomer had no effect. In short‐term experiments (up to 5·5 h), the S isomer and alachlor (100 μM) inhibited [14C]acetate uptake and its incorporation into fatty acids in the same manner, while the R isomer did not. Incorporation of [14C]acetate into a non‐lipid fraction of the algae was strongly inhibited by alachlor and the S isomer (100 μM) and only slightly by the R isomer. A 50% inhibition of incorporation of [14C]oleic acid into the same non‐lipid fraction was attained with less than 10‐7 M of the S isomer while 10‐5 M of the R form of dimethenamid achieved only a 40% inhibition. The same stereospecificity of the compound on growth, fatty acid desaturation, acetate uptake and oleic acid incorporation provides strong evidence that dimethenamid may act upon a primary, specific target in lipid metabolism. Furthermore, the comparable biological activities of dimethenamid and alachlor indicate that this target is common to both N‐phenyl and N‐thienyl chloroacetamide herbicides. © 1997 SCI.

Chloroacetamide Mode of Action, II: Inhibition of Very Long Chain Fatty Acid Synthesis in Higher Plants

“…Similar find ings with the enantiomers of metolachlor and dimethenamid in S. acutus (Couderchet et al, 1997) give evidence of an enzyme target for chloroacet amide attack. As shown in Table II, representa tives of herbicide classes different from chloroand oxyacetamides also affected VLCFA synthe sis.…”

Finding the Target Site of Chioroacetamides: A Thorny Subject

“…24,25). A 50% inhibition was obtained with 8 X 10-8 M metazachlor,26>9X10-gMSdimethenamid,25or with 2 X 10-' M S-metolachlor (see Fig.…”

The Significance of Organosulfur Compounds in Crop Protection: Current Examples from Fungicide Research

“…15 The exchange of the o,o'-dialkylated phenyl ring of the chloroacetamide herbicide metolachlor (11) by a 2,4-dimethylthiophene as well as the introduction of a thiophene ring into the methoxypropylamine sidechain both led to commercial herbicides dimethenamid (10) and thenylchlor (12). 16 But also heterocyclic rings can be successfully replaced by sulfur-containing rings. Both aromatic and aliphatic heterocycles of the neonicotinoid insecticide imidacloprid (14) are successfully mimicked by sulfur-containing rings, the exchange of the pyridine by a thiazole led to imidaclothiz (13), the transposition of the imidazoline by a thiazoline to thiacloprid (15).…”