DuPont sulfonylurea herbicides

Extensive use of two more recendy developed classes of herbicides will further dramatically reduce the amount of appHed to control weeds. The sulfonylurea herbicides are extremely active compounds first discovered in the mid-1970s at DuPont they have been discussed extensively (78). Sulfonjiurea herbicides have experienced a rapid and widespread success since thek commercial introduction in 1982 with chlorsulfuron (Table 5). The sulfonylureas are appHed at rates of 2—75 g/ha. The chemistry of the sulfonylurea molecule permits the synthesis of a very large number of useful analogues, consequentiy many new herbicides are anticipated for crop production. As of this writing (1996), over 350 patents have been issued to about 27 agricultural companies covering tens of millions of stmctures known or expected to be herbicidaHy active. 

Research work in the US potentially offers herbicide-resistant sunflower varieties that have been developed by non-biotechnological techniques. Express , being developed by DuPont and Pioneer, shows resistance to sulfonylurea herbicide. Another variety, Clearfield , is resistant to imazamox. The key decision on these products depends on the decision under Section 18 of the Environmental Protection Agency. Should Section 18 label be approved, Micogen CO. already has enough Clearfield seeds to plant 15000 acres next year. Other companies will have Clearfield seeds ready in 2003. Express is expected to be commercially available in a few years (The Sunflower, December, 2001). 

In 1982, DuPont introduced the sulfonylurea herbicides which are effective at remarkably low dose rates an important example is chlorsulfuron 22 used for selective control of broad-leaved weeds in cereals at rates of 10-40 gha this herbicide can be prepared from chlorobenzene 21 (Scheme 5) (see also Chapter 6, ref 33), The first stage of the synthesis, namely the reaction of chlorobenzene with chlorosulfonic acid yields a mixture of the o- and / -sulfonyl chlorides, which may be separated by fractional distillation, since the o-isomer has the lower bp (Chapter 6, ref 33). 

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