Pyroxasulfone: A highly efficient herbicide

Pyroxasulfone was developed as a pre-emergence herbicide to control grass and small-seeded broadleaf weeds. A dose of 100-250 gai/ha of pyroxasulfone was sufficient to control these weeds.In fields of genetically modified crops, pyroxasulfone controlled weeds that were resistant to non-selective herbicides.^[1]

1. Physicochemical properties

Pyroxasulfone has unique physicochemical properties that are optimal for its use as an herbicide particularly its relatively low log P value and solubility in water compared with those of chloroacetanilide herbicides such as alachlor, acetochlor and metolachlor. Pyroxasulfone is also hydrolytically stable at all pH values at 25°C, and is therefore less susceptible to decompo-sition.^[2]

Fig.1. pyroxasulfone as an herbicide

2. The advantages of pyroxasulfone

These are three favorable features of pyroxasulfone: (1) Pyroxasulfone can control a large number of annual weeds, particularly grasses; this also includes herbicide-resistant weeds such as Lolium multiflorum(Italian ryegrass) and Alopecurus aequalis(water foxtail)(grasses) and Amaranthus spp.(broadleaf weeds). (2) Pyroxasulfone has high pre-emergence activity and longer residual activity than similar products, with residual activity still high 2 months after application. (3)Pyroxasulfone shows good selectivity for corn, soybeans, wheat, turf, cotton, potato and onion, and we intend to extend its use to other crops in future.

3. The Action mechanism of pyroxasulfone^[3]

(1) Inhibition of shoot elongation and proliferation of cultured cells of plants: Pyroxasulfone potently inhibited the shoot growth of susceptible plants. This herbicide do not inhibit seed germination itself but potently inhibit the shoot elongation of germinated seeds. Herbicidal symptoms of plants treated with herbicides were the interference with new growth during germination and early emergence; these symptoms were very similar to those of plants treated with VLCFAE inhibiting herbicides, such as chloroacetoamide-type herbicides.

(2) Inhibitory on the biosynthesis of VLCFAs in susceptible-plant cultured cells: The contents of fatty acid in plant cultured cells were quantied by gas chromatography after derivatization to their methyl esters. As a result, reduction of VLCFAs with more than 20-carbon-chain and accumulation of VLCFA precursors with less than 18-carbon-chain were observed in rice cultured cells treated with pyroxasulfone, as observed in barnyard millet cultured cells treated with fenoxasulfone. Specifically, the accumulation of pentadecanoic acid was remarkable.

(3) Inhibition of the VLCFAE activity from etiolated plant seedlings: The inhibitory effects of pyroxasulfone on the VLCFAE activities of plants were evaluated by the inhibition of incorporation of malonyl-CoA into acyl-CoA precursors. Pyroxasulfone potently inhibited the VLCFAE activities, which catalyze the five successive elongation steps examined in susceptible plants, such as Italian ryegrass and rice. Compared with the inhibitory potency to VLCFAE activity by pyroxasulfone among plants, the inhibitions of VLCFAEs in crops were moderately weaker than those in susceptible plants. Such a difference in the inhibition of VLCFAE activity was indicated to be one of the factors involved in the selectivity of pyroxasulfone between weeds and crops.

(4) Inhibition of recombinant VLCFAEs of Arabidopsis and rice: Pyroxasulfone potently inhibited the VLCFAE activity of recombinant Arabidopsis FAE1 in a time-dependent manner. On the other hand, pyroxasulfone potently inhibited the VLCFAE activity of recombinant rice Q6F365 in a time-independent manner, as indicated for the inhibition manner of VLCFAEs from etiolated plant seedlings by isoxazoline-type herbicides. Thus, a critical difference was found in the inhibition manner by pyroxasulfone between FAE1 and Q6F365. This difference is presumed to be due to the enzymatic properties between FAE1 and Q6F365, which was also inferred by the phylogenetic analysis of plant VLCFAEs.

4. the synthesis process of pyroxasulfone

3-chloro-5,5-dimethyl-4,5-dihydroisoxazole was prepared by cyclization and chlorination using ethyl 3,3-dimethacrylate as a raw material, and then reacted with thiourea to obtain 5,5-dimethyl(4,5-dihydroisoxazol-3-yl)thiocarboxamidine hydrochloride. 4-(chloromethyl)-5-(difluoromethoxy)-1-methyl-3-(trifluoromethyl)-1H-pyrazole was prepared by using ethy 4,4,4-trifluoroacetoacetate as a raw material through cyclization, methylolation, difluoromethylation, and substitution chlorination. Finally, pyroxasulfone was obtained through substitution and oxidation reactions with the content greater than 99%, and the total yield 35%.^[4]

References

[1]Yoshihiro Yamaji, Weed control efficacy of a novel herbicide, pyroxasulfone, [J]Journal Of Pesticide Science, 39(3),165-169(2014).

[2]Kumiai Chemical Industry: Pyroxasulfone Global Technical Bulletin(2011).

[3]Yoshitaka Tanetani, Action mechanism of isoxazoline-type herbicides, [J]Journal Of Pesticide Science, 37(3)261-262.(2012).

[4]Yunhe Shen, Study on the synthesis process of pyroxasulfone, [J]Nongyao Shijie, 42(9) 40-43.