Drotaverine hydrochloride is used as antispasmodic drug arising from papaverine class of biologically active compounds. It is marketed also under the name of NO-SPA. Drug Master File of drotaverine documents a list of potential impurities that may arise during both, the chemical synthesis and partial degradation. One of those regulated compounds is 6,7-diethoxy-1-(4-ethoxy-3-hydroxybenzyl)-3,4-dihydroisoquinolinium chloride. The latter is an imine tautomer of drotaverine and additionally differs from the parent structure with free 3-hydroxy group in the benzylic substituent. We report here for the first time the full synthesis of the above mentioned standard impurity. The developed synthetic route allows to obtain several grams of the required product. The synthesis consists of 11 steps with the key-process being Bischler-Napieralski cyclisation of isoquinoline cycle. In turn, the linear precursor was obtained from commercially available 2-(3,4-diethoxyphenyl)ethylamine and (3-benzyloxy-4-ethoxyphenyl)acetic acid. The latter was obtained in 6 steps from commercial 3,4-dihydroxybenzaldehyde. Synthetic sequence towards this carboxylic acid started with an orthogonal protection of both hydroxyl groups at C(3) and C(4) and was followed by reduction of aldehyde. Then the transformation of the resulting benzyl alcohol led to benzyl chloride and its nucleophilic substitution with KCN provided benzyl cyanide,hydrolysis of which gave the above mentioned carboxylic acid required for the amide coupling.