Phenoxyacetic acids Discovery

While each of these milestones brought forth a renewed interest in chemical weed control and led to more research in the field, the use of chemicals for selective weed control in crops was very limited and not very successful until the discovery and development of 2,4-D, MCPA, and other phenoxyacetic acid herbicides in the 1940s. These compounds were the first truly selective herbicides that could reliably kill broadleaf weeds in cereal crops, including com, and they quickly developed widespread popularity and use after World War II. With this major milestone, new application technology emerged, including the low-volume sprayer, and new herbicide formulations were developed. 

Among the numerous discoveries that we owe to botanists and pharmacognosts are the development of tryptophan metabolites, and especially indolylacetic acid. This compound acts as growth hormone in plants. Para-chlorinated phenoxyacetic acids (MCPA or methoxone 2,4-D or chloroxone) are mimics of indolylacetic acid (bioisostery) and show similar phytohormonal properties at high doses they serve as weeders. Ring-chlorinated phenoxyacetic acids were later introduced in molecules as varied as meclofenoxate (cerebral metabohsm), clofibrate (Upid metabolism) and ethacrynic acid (diuretic). 

In 1939, the highly efficient contact insecticide DDT (an abbreviation of the technically incorrect name dichlorodiphenyl-trichloroethane) was introduced to the market. Its use, as well as the use of several other organochlorine compounds, spread worldwide in the following years. Important active substances that appeared on the market in the period after World War II include carbamate insecticides and herbicides. Also significant was the discovery of herbicidal phenoxyacetic acids, which represent the first group of the so-caUed hormonally active pesticides. Around the middle of the last century a number of other biologically active substances were discovered, many of which, such as herbicides based on substituted urea, s-triazines (1,3,5-triazines), quaternary ammonium salts or insecticidal synthetic pyrethroids and many others, are still used in many countries around the world. 

Of these later discoveries, picloram (6.47) ( Tordon ) 3,5,6-trichloro-4-aminopyridine-2-carboxylic acid, resembles the phenoxyacetic acids in mode of action, but is much more potent, and highly persistent. It is being widely used (Kefford, 1966). 

Among the numerous discoveries that we owe to the botanists and the pharmacognosts, the precocious interest for tryptophan metabolites has to be evoked, especially the interest for indolylacetic acid. This compound acts as growth hormone in plants. Para-chlorinated phenoxyacetic... 

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