3,4-Dimethoxybenzaldehyde (I) is treated with 1,3-propanedithiol (II) to give 2-(3,4-dimethoxyphenyl)-m-dithiane (III), the oxidation product of which (IV) is reacted with N-[2-(3,4-dimethoxyphenyl)ethyl]-N-methylpropanamine chloride (V) in the presence of a base.