An efficient route towards R-2-phenoxypropionic acid synthesis for biotransformative production of R-2-(4-hydroxyphenoxy)propionic acid

R -2-(4-hydroxyphenoxy)propionic acid ( R -HPPA) is a key intermediate for the synthesis of classic herbicides with high selectivity against grassy weed. The main route for R -HPPA biosynthesis is to hydroxylate the substrate R -2-phenoxypropionic acid ( R -PPA) at C-4 position with microbes. In order to provide sufficient R -PPA for the industrial production of R -HPPA, an effective R -PPA synthesis method was established and optimized in this work. The synthesis process mainly consisted of two steps: (1) synthesis of S -2-chloropropionic acid from L-alanine via diazotization and chlorination reactions; and (2) synthesis of R -PPA from S -2-chloropropionic acid and phenol via etherification reaction. The optimal reaction conditions were as follows: HCl: NaNO 2 : KI: L-Ala = 2.0: 1.2: 0.7: 1.0 (in molar), 125 °C reflux for 1.5 h, with KI as catalyst, and KI: S -2-chloropropionic acid: phenol = 0.075: 1.2: 1.0 (in molar). Under these conditions, an improved molar conversion rate (74.9%, calculated in phenol) was achieved. After extraction using anionic exchange resin Amberlite IRA-400 (CI), R -PPA product with a purity of 95.08% was obtained. The purified R -PPA was identified and evaluated in the application of the biotransformative production of R -HPPA. The results indicated that the synthesized R -PPA supported the R -HPPA biosynthesis with a comparable yield as that of the standard R -PPA. The R -PPA synthesis method provided herein exhibited the advantages of low price and easy availability of raw materials, less toxicity of reagents, simple manipulations, and low equipment/instrument requirements.