QSAR of Fungicidal Δ³‐1,2,4‐Thiadiazolines. Reactivity‐Activity Correlation of SH‐Inhibitors

Abstract

Abstract Structure‐activity relationships of fungicidal Δ 3 −1,2,4‐thiadiazolines were investigated by using molecular orbital (MO) calculations. Semiempirical MO calculations by MNDO‐PM3 method suggested that the reactivity of HOMO at the sulfur atom in the thiadiazoline ring is important to inhibit SH‐enzymes. In order to quantify the reactivity of the thiadiazolines, we defined an index, R( 1 ), which is the electron density of HOMO at the sulfur divided by the orbital energy of HOMO. The fungicidal activity against cucumber downy mildew highly correlated with the R( 1 ) index and hydrophobicity of molecule. The significance of R( 1 ) and its square term in the QSAR equations suggested that a suitable range of reactivity is required to control the plant disease through the inhibition of SH‐enzymes.