SYNTHESIS AND STRUCTURE-ACTIVITY-RELATIONSHIPS OF MITICIDAL 4,5-DIHYDROPYRAZOLE-5-THIONES

A series of novel 4,5-dihydropyrazole-5-thiones (DHPs) was synthesised by treating the corresponding dihydropyrazolones with Lawesson's reag ent and evaluated for miticidal activity against two-spotted spider mi tes (Tetranychus urticae Koch). Of these, 3-(4-chlorophenyl)-4,4-dimet hyl-1-phenyl-4, 5-dihydropyrazole-5-thione, yl-4-methyl-1-phenyl-4,5-d ihydropyrazole-5-thione, l-4,5-dihydropyrazole-5-thione-4-spirocyclope ntane and ifluoromethyl-phenyl)-4,5-dihydropyrazole-5-thione were high ly active (pEC(50) > 4 . 0) and were more effective than the miticide dicofol (pEC(50) = 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 phe nyl and thioxo groups on the 1-, 3- and 5-positions of DHPs respective ly were required for activity. Quantitative SAR studies using physicoc hemical parameters of substituents and the capacity factor k' as a hyd rophobicity index suggested that: (a) the activities of all types of D HPs examined were mainly dominated by hydrophobicity, (b) the bulkines s 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 P-ow value of c. 5 . 0.