Norflurazon structure

A great diversity in molecular structure is observed among herbicides which inhibit carotene biosynthesis as is exemplified by the structures of norflurazon, fluridone and difunone (shown below). Nonetheless, many of these compounds, which comprise a subset of the larger group known as bleaching herbicides, appear to inhibit the same step in the biosynthetic pathway to the carotenoids (1 ). The inhibited step is the desaturation of 15-cis phytoene to 15- cis phytofluene (Figure 1) and the build-up of phytoene in plants and in cell-free systems which have been treated with these herbicides is well documented (2-4). 

If our hypothesis is correct, this hypothetical binding site should also accommodate fluridone, norflurazon and difunone and some possible binding orientations of these molecules are compared with furanone 13 in Figures 7-9. Note that we have attempted to depict the molecules in such a way that key structural features, e.g., the CF3 phenyl and vinylogous amide subunits, occupy the same positions as nearly as possible. Finally, it should be emphasized that considerable further work is required to demonstrate that the furanones actually inhibit phytoene desaturase and to further probe the possibility of a common binding site for the proven inhibitors including those such as fluorochloridone (10) and the m-phenoxybenzamides (4), which do not incorporate the vinylogous amide substructure. 

Owing to the similarity of desaturation reactions catalyzed by PDS or ZDS, differentiation in the plant is not easy to detect. Most of the herbicidal inhibitors probably inhibit both, although to a different extent [6j. If strong inhibition of PDS has taken place with accumulation of phytoene, then the compound s ability to inhibit ZDS cannot be seen. Figure 4.1.2 shows that the commercial products primarily inhibit PDS [6, 8, 27-29]. Cell-free studies exemplified by norflurazon and fluridone have shown them to act as reversible noncompetitive inhibitors of PDS [27]. Other PDS active structures are shown below in Table 4.1.2 and in Section 4.1.4.10. 

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