"protocatechualdehyde ... can be made via different routs; known are e.g.
the method by REIMER-TIEMANN (from pyrocatechol [10]) ...
also the demethylation of vanillin [13] resp. veratrumaldehyd [3]
leads to 3,4-dihydroxybenzaldehyde. by the method of LANGE (1962, [5])
vanillin can be demethylated with high yield (87%) to crystalline
protocatechualdehyd of good purity ..."
[3] DREYFUS, ger. pat. Patent DE193958 (http://l2.espacenet.com/dips/viewer?PN=DE193958&CY=gb&LG=en&DB=EPD)
[Frdl. 9, 161 (1908-10)]
[5] LANGE R.G., Monsanto Chem. Co. (1962),
cited in FIESER/FIESER, p 1010-... (see lit. [3])
[10] REIMER und TIEMANN, Ber. 9, 1268 (1876);
TIEMANN und KOPPE, Ber. 14, 2015 (1881)
[13] TIEMANN und HAARMANN, Ber. 7, 620 (1874)