[CH3:1][C:2]1[C:3]2[CH:10]=[CH:9][C:8]([C:11]([OH:13])=[O:12])=[CH:7][C:4]=2[S:5][CH:6]=1.Cl.[CH3:15]O>>[CH3:15][O:12][C:11]([C:8]1[CH:9]=[CH:10][C:3]2[C:2]([CH3:1])=[CH:6][S:5][C:4]=2[CH:7]=1)=[O:13]A solution of 3-methylbenzo[b]thiophene-6-carboxylic acid (3.65 g) in methanol (100 ml) was saturated with hydrogen chloride gas and the solution was heated under reflux for 4 hours, and then evaporated. The residue was dissolved in ether and the solution was washed with water, sodium bicarbonate solution and dried (Na2SO4). Evaporation of the solvent gave 3-methylbenzo[b]thiophene-6-carboxylic acid methyl ester (3.70 g) pure enough for further reaction. A sample crystallized from petrol (b.p. 40°-60°) had m.p. 46°-47° C.
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