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New twenty 2-(n-octyl)-3-(n-propyl)isothiourea derivatives(S) were synthesized which is modified from the insedicidal Buprofezine (Applaud) in the selective insecticidal activities in-vitro against Diamond-Back moth (Plutella Xylostella Linnaeus). The structure activity relationships(SAR) between the insecticidal activity(pI_(50)) and a various physicochemical parameters of the substituent(Z) of S were analyzed by the multiple regression technique. The activities would depend largely on the MR, π and L₁ parameters. The SAR was rationalized by parabolic function of MR, π, and L₁ constant, where the optimal values of the constants were L₁= 5.22 Å, MR=15.70 C㎥/㏖ and π=1.60, respectively. The steric effects play an important role in determining insecticidal activity. The SAR suggest that the S derivatives having a substituents with a small breadth and an appropriate length as Z group showed potent activity. From the results, the iso-propyl group(Z) substituent (5) with three carbon atom was the most effective compound.
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