Boron Trichloride → 5-(2-Formyl-3-hydroxyphenoxy)pentanoic acid

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5-(2-Formyl-3-methoxyphenoxy)pentanoic acid (Example 1) (25 g, 0.099 M) was dissolved in dry dichloromethane (600 ml) with stirring at -70° C. Boron trichloride (50 g) in dry dichloromethane (100 ml) was then added from a pressure-equalising dropping funnel at such a rate that the temperature of the reaction mixture did not rise above -60° C. (~1/4 hr). The mixture was stirred at -70° C. (1/4 hr) and then warmed to room temperature by immersion in lukewarm water (~1/4 hr). After stirring at room temperature (1 hr) the mixture was carefully poured into 10% sodium acetate solution (500 ml). The resulting mixture was filtered and the layers separated. The aqueous phase was extracted once with dichloromethane and the combined organic solutions evaporated. The residue was dissolved in ethyl acetate/ether (1:1) and extracted with 5% sodium bicarbonate solution (4×250 ml). The combined extracts were acidified with concentrated hydrochloric acid and extracted with ethyl acetate. The combined extracts were washed with water, dried (sodium sulphate) and evaporated. The residue was dissolved in acetone (100 ml) and treated, with swirling, with 40/60 petrol (400 ml). The pale-yellow supernatant solution was decanted from the red tarry residue and filtered. Evaporation gave 5-(2-formyl-3-hydroxyphenoxy)pentanoic acid, m.p. 96°-98° C. from benzene/petrol.

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