Figure 1: Illustrations of earliest and lastest examplars

raw material. However, when a challenging substitution pattern is required, ab-initio synthetic routes are often used.

• Derivatives of pyrazole-4- carboxamide, often with fluorinated side-chains, have become very popular, with the first appearing in 1997, when Sumitomo

Chemical launched the rice fungicide, furametpyr. Nine AIs carrying this side-chain are profiled in Volume 39. In Table 2, a list of recent agrochemical actives that are still in some stage of development is presented. It demonstrates very well the wide range of chemistries being used to create new compounds with new or improved modes of action. A few interesting examples are worth pointing out: • Several benzoxaboroles have recently reached early testing stage. Boragen appears to have transferred the germicidal activity that similar compounds have exhibited into the agrochemical sector.

• Many compounds under development in the US, Europe and Japan make use of pyrazoles, very often substituted pyrazole-4-carboxamides (see Figure 2, which illustrates how Sumitomo Chemical’s furametpyr subsequently inspired new lead compounds in the R&D laboratories of several European agrochemical companies). This very popular substructural feature, most often found in insecticides, is usually produced from a suitably substituted hydrazide and a 1,3-diketo compound.

• A recent herbicide candidate, introduced by Nissan Chemical, contains an azetidinone. This is far more familiar in beta-lactam antibiotics, where its reactivity leads to an irreversible reaction with the receptor site.

• In Table 1, thiazoles, together with their reduced forms, are listed as a common chemical feature. There are eight under active development at the time of writing, emphasizing their continuing importance in new agrochemical actives.

Given the three dimensional

receptor sites in which these molecules must interact with the receptor site, it might be thought that homochiral compounds would predominate. However, although there was a strong trend towards such compounds between 2000- 2015, the additional hurdles involved with achieving regulatory approval proved to be a deterrent. Ideally, new active ingredients should lack chiral centres. This point leads nicely to the final important factor that must always be borne in mind when developing an agrochemical; cost of production and thus the final price of the finished formulation. Successful agrochemical products usually contain active ingredients that are priced at no more than $150/kg and very often less than $80/kg. This usually constrains the complexity and length of manufacturing processes but, of course, there are exceptions, where the activity of the product (in grams/hectare) is high. However, these tend to be exceptional. Many good products have failed to reach their full potential by being too

costly and this lesson has been well learned within the industry.

In conclusion

This short review illustrates the importance of the ever-improving palette of new chemistries being adopted by both bioscience industries, pharmaceuticals and agrochemicals. Keeping up with these latest developments has been a very enjoyable pursuit for the editors of the Ag Chem New Compound Review. Being able to alert those involved in applying agrochemistry around the world has proved to be a worthwhile activity and should be maintained as long as crop pests continue to resist the world’s farmers’ best efforts to keep us fed.

Further information Dr Rob Bryant Agranova 34 The Drive, Orpington Kent BR6 9AP United Kingdom P: +44 1689 600 501 E: W:

Table 2: Chemical features chosen from a selection of recent agrochemical actives (2015- 2021) -ordered by development stage-

Issue 2 • March/April 2021 15

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