Reaction of aryl-2-hydroxypropenoic derivatives with boron tribromide

(Z)-Mono, di or trimethoxyphenyl-2-hydroxypropenoic acids 1a–d gave mixtures of (E) and (Z) mono, di or trihydroxyphenyl-2-hydroxypropenoic acids 2a–d when treated with boron tribromide. The isomerisation proceeds during the work-up and depends on the duration of the hydrolysis and the number of oxygens on the aromatic ring. When the aromatic ring was substituted with a methoxy group at the ortho position, a cyclisation occurs, and 3-hydroxycoumarins 3 and benzofuran-2-carboxylic acids 4 can be obtained. 3-Hydroxycoumarin 3a can also be obtained almost quantitatively from the reaction of methyl 3-(2-methoxyphenyl)-2,3-epoxypropanoate with boron tribromide.