THIS is what SWIM was looking for...
Demethylation of Vanillin to form Protocatechuic Aldehyde
The demethylation is effected by heating the vanillin, in a suitable solvent or medium, with an ether-cleaving agent such as the hydrogen halides (hydrogen chloride, hydrogen bromide, hydrogen iodide), the organic primary ammonium halides (such as aniline hydrochloride, ethanolamine hydrochloride), the organic secondary ammonium halides (such as dimethylamine hydrobromide, N-ethylaniline hydrochloride), the organic tertiary ammonium halides (such as pyridine hydrochloride, the hydrochlorides of the picoline-lutidine fraction of coal tar distillate) and the halides of aluminum, zinc, iron, tin, antimony and boron (such as anhydrous AlCl3, AlBr3, ZnCl2, SnCl4, SbCl5, BF3).
Best results are obtained by the use of demethylating agents of hydrogen chloride, hydrogen bromide, aluminum chloride, aluminum bromide, aniline hydrochloride and pyridine hydrochloride.
The demethylation is effected by heating vanillin in a suitable medium or solvent with at least a reacting proportion of the ether-splitting reagent, at a temperature between 50@C. and 250@C., whereupon it is converted to protocatechuic aldehyde and the methyl halide, which distills off during the reaction. A wide group of media or solvents may be employed for effecting this demethylation of the vanillin, including water, acetic acid, benzene, nitrobenzene, methanol, ethanol, n-propanol and isopropanol. It is also feasible to effect this demethylation by fusing the vanillin, in the absence of solvent, with the ether-splitting reagent whenever said reagent is not volatile at the temperature of the demethylation (e.g. 50@-250@). Thus vanillin may be demethylated to protocatechuic aldehyde by heating in the absence of a solvent with aniline hydrochloride, pyridine hydrochloride, aluminum chloride, zinc chloride, at 50@-250@ C., until methyl halide evolution is complete.
The reaction product of demethylation step consists predominantly of protocatechuic aldehyde (85%-95%) with minor amounts of unreacted vanillin. The reaction product is freed of concomitant ether-cleaving reagent (e.g. by slurrying in water in which such ether-cleaving reagents are soluble, and filtering off the protocatechuic aldehyde) and is then employed in the second step of the process.
The second step of the process is for bourbonal (ethylvanillin). This is like Rhodiums page, but gives a better reaction conditions and choices. Pyridine is a very disgusting reagent, and SWIM will not work with it.