The quantitative relationship between the structure of optically active 4‐substituted 2‐methoxy‐1,3,2‐oxazaphospholidine 2‐sulfides (4‐RMOS) and their insecticidal activity on the house fly, Musca domestica L., was analysed using reported physicochemical parameters and regression analysis. The configuration at the C4 atom was more important for insecticidal activity than that at the phosphorus atom, since the coefficient of the dummy parameter for the C4‐ configuration was larger than that for the configuration of the phosphorus atom: (S)c(R)p diastereoisomers showed the highest insecticidal activity, and the activity decreased in the order of (S)c(R)p > (S)c(S)p > (R)c(R)p > (R)c(S)p. The electronic nature of the substituent at the C4 position was the most important, followed by the steric and hydrophobic effects: the more electron‐donating, the less bulky and the more hydrophobic the 4‐substituent, the higher the insecticidal activity. Thus the highest activity was obtained for the isopropyl derivatives, and the activity decreased in the order of isopropyl > isobutyl > ethyl > sec‐butyl > benzyl > phenyl > methyl > tert‐butyl.
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