Yield:-

Reaction Conditions:

Stage #1: methyl 5-methoxylbenzo[b]thiophene-2-carboxylatewith boron tribromide in dichloromethane at 0; for 2 h;
Stage #2: with water in dichloromethane at 0;

Steps:

6.2 5-Hydroxy-benzo[b]thiophene-2-carboxylic acid methyl ester

Step 2 5-Hydroxy-benzo[b]thiophene-2-carboxylic acid methyl ester The above prepared 5-methoxy-benzo[b]thiophene-2-carboxylic acid methyl ester (0.199 g, 0.895 mmol) was dissolved in 6 mL of CH₂Cl₂, cooled to 0° C., and treated with BBr₃ (1.79 mL of 1M solution in CH₂Cl₂, 2 eq.). After 2 additional h at 0° C., the reaction mixture was carefully poured onto crashed ice, twofold extracted with AcOEt, washed with water, dried over sodium sulfate, and evaporated to dryness. Flash chromatography (SiO₂, hexane/AcOEt=7/3) yielded 0.145 g of the title compound as off-white solid.

References:

US2009/29976,2009,A1 Location in patent:Page/Page column 19-20