MOLECULAR SIMILARITY OF PEROXIDIZING HERBICIDES - BIOISOSTERISM IN DELTA(2)-1,2,4-THIADIAZOLINES AND RELATED HETEROCYCLIC-COMPOUNDS

A series of fused DELTA2-1,2,4-thiadiazoline derivatives (I) showed similar herbicidal activity to that of tetrahydrophthalimides (II) and 1,2,4-triazolin-3-ones (III). In order to understand the bioisosterism of I, II and III, steric and electrostatic properties of them were studied by means of molecular orbital calculations and computational techniques. The molecular electrostatic potentials of their model molecules were similar to each other in terms of the patterns of potential contours. The conformational analysis for the rotation of benzene ring in I-III indicated that they are flexible within some range of the rotation, and that they are commonly stable when the torsional angle is ca. 240-degrees. The Carbo's similarity index was employed to quantify the electrostatic similarity of the molecules, and the values of the index were in the range of 0.83-0.93 when two of the molecules were superimposed by the least square fitting. The SIMPLEX optimization procedure was also examined to find the best superimposition to maximize the electrostatic similarity, and the obtained superimposed models were almost same as the ones by the least square fitting. Based upon such aspects, a hypothetical model of active structure was drawn to account for the bioisosterism of herbicides having dissimilar backbone structures.