A series of novel 4,5-dihydropyrazole-5-thiones (DHPs) was synthesised by treating the corresponding dihydropyrazolones with Lawesson's reagent and evaluated for miticidal activity against two-spotted spider mites (Tetranychus urticae Koch). Of these, 3-(4-chlorophenyl)-4,4-dimethyl-1-phenyl-4, 5-dihydropyrazole-5-thione, 3-(4-chlorophenyl)-4-ethyl-4-methyl-l-phenyl-4,5-dihydropyrazole-5-thione, 3-(4-chlorophenyl)-1-phenyl-4,5-dihydropyrazole-5-thione-4-spirocyclopentane and 4,4-dimethyl-l-phenyl-3-(4-trifluoromethylphenyl)-4,5-dihydropyrazole-5-thione were highly active (pEC,, > 4-0) and were more effective than the miticide dicofol (pEC,, = 3.879), which has traditionally been used for the control of phytophagous mites. Structure-activity relationship (SAR) studies were performed on each position of the pyrazole ring of DHPs. The results indicated that the unsubstituted phenyl, 4-substituted phenyl and thioxo groups on the 1-, 3-and 5-positions of DHPs respectively were required for activity. Quantitative SAR studies using physicochemical parameters of substituents and the capacity factor k' as a hydrophobicity index suggested that: (a) the activities of all types of DHPs examined were mainly dominated by hydrophobicity, (b) the bulkiness of 4-substituents of the 3-phenyl ring favoured the activity and (c) the log k' optimum for all DHPs was 1.675, equivalent to a log Po, value of c. 5.0.

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