No abstract

The biochemical and physiological effects of 10 isoxazoles were investigated. The amount of protoporphyrin IX caused to accumulate by the compounds correlated well with their herbicidal activity. Protoporphyrinogen oxidase (Protox) was inhibited competitively in the proximity of the catalytic site. However, the Protox I 50 values of the methyl esters and acid chloride derivatives were lower than expected on the basis of their in vivo herbicidal activity. The results suggest that some tolerance mechanism, other than differential absorption and translocation, may protect the plants against these compounds. The molecular properties of 9 isoxazoles and 2 other well-known inhibitors of different herbicide groups were compared to those of protoporphyrinogen (Protogen). The most active compounds have similar bulk, electronic, and energy properties that approximate half of the Protogen molecule. Furthermore, these compounds have atoms/groups on the ring that generate distinct negative electrostatic potential fields that may mimic the reactive part of the Protogen molecule.

The synthesis and SAR of herbicidal N-substituted 2,6-bis(polyfluoromethyl)-l,2-dihydropyridine-3,5-dicarboxylates are described. N-Substituted dihydropyridines were readily prepared by treatment of the corresponding dihydropyridyl anions with electrophiles. Herbicidal activity of the N-substituted dihydropyridines is correlated with the rate of hydrolytic cleavage of the N-substituent to afford 2 or 3. Dihydropyridine 2 is shown to undergo oxidation in the soil to afford herbicidal 1, thereby relating the activity of N-substituted dihydropyridines to the corresponding aromatic herbicides.

Certain heretofore unknown tertiary α‐isocyanacetanilides 1 are reported for the first time, prepared by methods necessarily different from those employed to prepare the previously cited aliphatic α‐isocyano‐acetamides 2. Like 2, 1 has been shown to readily undergo dialkylation with alkyl halides. Moreover, 1 also gives rise to oxazole and oxazoline derivatives upon reaction with acyl halides and aldehydes, respectively, resembling the similarly activated α‐isocyano esters and sulfones. Further, in a demonstrable ring‐chain tautomeric equilibration, apparently restricted to tertiary amides, 1 readily cyclizes, providing the only known synthesis of 5‐amino‐2,4‐unsubstituted oxazoles 3. In other examples 4‐chloro and 4‐alkyl‐5‐amino‐oxazoles are produced. The materials and reactions are characterized, and compared with previous related studies.