Sulcotrion

The triketones (Fig. 20), the most recent class of herbicides, were discovered in 1986 and introduced commercially (e.g. sulcotrione) in 1991. While several papers report that the triketones herbicides were derived from the plant secondary metabolite leptospermone (Fig. 20) that was isolated from the bottlebrush plant [Callistemon spp.) [Ref. 101], there is a report that suggests that the discovery may have been more serendipitous. ... 

The enzyme p-hydroxyphenylpyruvate dioxygenase is involved in the conversion of p-hydroxyphenylpyruvate into homogentisate, a key step in plastoquinone biosynthesis. Inhibition of this enzyme has an indirect effect on carotenoid biosynthesis as plastoquinone is a co-factor of the enzyme phytoene desaturase. The new maize herbicide isoxaflutole and the triketone herbicides such as sulcotrione (Figure 2.7), inhibit p-hydroxyphenylpyruvate dioxygenase and this leads to the onset of bleaching in susceptible weeds and ultimately plant death.4... 

The pKu of the herbicide sulcotrion is 3.13 (Tomlin, 1997). Would you have expected that this compound is such a strong acid Write down the structure of the conjugate base of sulcotrion and try to explain the rather strong acidity of this herbicide. 

Structure illustrating triketone moiety of A. (-)-usnic, B. (+)-usnic, and C. sulcotrione. 

Inhibitors of carotenoid synthesis also lead to chlorophyll destruction by destabilizing the photosynthetic apparatus. Total carotenoid content decreased with increased (-)-usnic concentration (Fig. 1.4). Carotenoid biosynthesis can be interrupted by inhibiting the enzyme phytoene desaturase that converts phytoene to carotenes or by inhibiting the enzyme HPPD responsible for plastoquinone (required for phytoene desaturase activity) synthesis.14 Usnic acid possesses some of the structural features of the triketone HPPD inhibitors, such as sulcotrione (Fig. 1.1C).8 (-)-Usnic acid had a strong inhibitory activity on HPPD, with an apparent IC50 of 70 nM, surpassing the activity obtained with the commercial herbicide sulcotrione (Fig. 1.5). 

Leptospermone (72) is an allelochemical produced by the bottlebrush plant (Calispermon spp.).46 This compound is herbicidal, causing bleaching symptoms, and is a potent inhibitor of HPPD. The triketone-type HPPD-inhibiting herbicides such as sulcotrione (73) were developed based on the structure of... 

Most commercial HPPD inhibitors e.g., sulcotrione and isoxaflutole) are competitive time-dependent (tight-binding) inhibitors. As such, these herbicides bind to the enzyme very tightly with T A of dissociation ranging from a few hoins to several days, as opposed to milliseconds for traditional reversible inhibitors. Sorgoleone does not behave as these herbicides and appears to be a reversible inhibitor of HPPD. This quinone is structurally more planar than the traditional HPPD inhibitors, so it may not form a stable reaction intermediate. Instead, its backbone may resemble the conformation of one of the later intermediate step in the reaction mechanism of HPPD. 

Figure 3. Effect of the p-triketone (-)-usnic acid (circles), the benzoquinone sorgoleone (triangles) and the commercial herbicide sulcotrione (squares) on the activity of p-hydroxyphenylpyruvate dioxygenase. The dotted line represents 50% inhibition of enzyme activity.

L.G. Freitas, C.W. Gotz, M. Ruff, H.P. Singer, S.R. Muller, Quantification of the new triketone herbicides, sulcotrione and mesotrione, and other important herbicides and metabolites, at the ng/l level in surface waters using LC-MS-MS, J. Chromatogr. A, 1028(2004)277. 

Selected physical chemical, toxicological, and environmental properties of sulcotrione are listed in Table 4.3.1, column 1. 

With the introduction of isoxafiutole (IFT) (17, Fig. 4.4.7) new crops, com and sugarcane, came in the focus of HPPD inhibitors of type heterocycles. IFT is not the first HPPD compound for com, this was sulcotrione in 1990, but it was the first for pre-emergence application. Reported 1995 by Luscombe et al. [20] the compound had been initially patented 1991 [21] by Rhone-Poulenc Agriculture Limited (now Bayer CropScience). 

Topramezone is aimed at the post-emergence control of major grass and broad-leaf weeds in corn crops worldwide. This means that this new corn compound differentiates itself from sulcotrione and mesotrione in that it shows real cross spectrum activity like isoxaflutole and it is not limited to mainly broadleaf weed control. 

Fig. 2 Inhibition of p-hydroxyphenylpymvate dioxygenase by manuka oil ( ), the j8-triketone-rich fraction ( ), and its individual jS-triketone components leptospermone ( ), flavesone (A), and grandiflorone (T). The synthetic herbicide sulcotrione (o) was added as positive control. Each data point represents the mean of two independent experiment 1 SD (N = 6) (Figure from [44]. With permission)...

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