Studies on the Application of Ketene Dithioacetals to Pesticides

Abstract

During the studies on the application of organic sulfur compounds to agricultural fungicides, some of ketene dithioacetals (III) were found to be effective for rice blast control. In order to prepare III, the reaction of carbon disulfide with active methylene compounds was investigated and a large number of related compounds were synthesized.In those, one of the most potent compounds, isoprothiolane (5b, diisopropyl 1, 3-dithiolan-2-ylidenemalonate) is now used practically as a systemic fungicide to control rice blast disease.The structure-activity relationships of isoprothiolane analogs against rice blast were studied and summarized as follows.Of the compounds with ring structure (4), only 1, 3-dithiolanes showed high effect and systemic property. Those without ring structure (3) were lacked in systemic and persistent properties, though some of them showed strong fungicidal activity. Double bond and two sulfur atoms were necessary. That was to say, ketene dithioacetal structure in isoprothiolane was essential for rice blast control. The optimum size of alkyl groups in 5 seemed to be C2-C5 alkyl with 4-8 total carbon number. Thioester or amide analogs (20, 21) were less active than the corresponding ester compounds.An interesting property of isoprothiolane against planthopper was observed from the field tests on rice blast. That was the remarkable suppression of population density of planthoppers in the field. This discovery implies the importance of close observation to evaluate biological activities.As one of another biological activity of III, strong fungicidal property against rice sheath blight was found in some compounds having 4-chloromethyl-1, 3-dithiolane moiety.