4- Phenoxybutanoic acid

Astles PC, Brealey C, Brown TJ, Facchini V, Handscombe C, Harris NV, McCarthy C, McLay IM, Porter B, Roach AG, Sargent C, Smith C, Walsh RJA. Selective endothelin A receptor antagonists. 3. Discovery and structure-activity relationships of a series of 4-phenoxybutanoic acid derivatives. J Med Ghent 1998 41 2732-44. 

Subsequent carbonation followed by hydrolysis produced 4-phenoxybutanoic acid in over 70% yield. 

There is a close relationship between the effect of various substances as auxins and their herbicidal effects, with some easily explained exceptions. 4-Phenoxybutanoic acids (e.g., MCPB) have no auxin effect, yet they are efficient herbicides in many plants, providing they are metabolically transformed to the corresponding acetic acid. Although MCPA is selective with toxicity mainly against broadleaves, MCPB has an even more narrow selectivity because broadleaved plants, which do not metabolize it to MCPA, are not killed. 

R)-3-Phenoxybutanoic acid and the corresponding butyl (S)-ester were obtained by Burkholderia cepacia lipase-catalyzed enantioselective esterification of the racemic acid with 1-butanol in hexane containing anhydrous sodium sulfate to remove the water produced during the reaction (Figure 6.17) [64]. 

Of the alkanoic acids with a phenoxy group in the 2 position, D-enan-tiomers of propanoic and butanoic acid have higher activities than phenoxy acetic acids. If the phenoxy group is further away, there must be a metabolic activation, as shown for 4-(4-chloro-2-methyl-phenoxybutanoic acid (MCPB). 

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