1,2-Dimethyl-5-hydroxyindole-3-acetic acid ethyl ester (I) is acetylated with acetic anhydride affording the O-acyl derivative (II), which is brominated to the corresponding dibromide compound (III). The reaction of (III) with thiophenol in KOH yields (IV), which is then submitted to a conventional Mannich condensation with formaldehyde and dimethylamine in acetic acid, giving the free base of arbidol (V), which is treated with aqueous hydrochloric acid.