Conversely, by heating protocatechuic acid with potash and methylene iodide, piperonylic acid was regained.
Hydrolysis by alkaline solutions gives a sugar and caffeic acid; whilst fusion with potassium hydroxide gives protocatechuic acid.
These results show that piperonylic acid is the methylene ether of protocatechuic acid.
Moringa-tannin or maclurin, C1,H1006 H20, found in Morus tinctoria, hydrolyses on fusion with caustic potash to phloroglucin and protocatechuic acid.
The prism formula also received support from the following data: protocatechuic acid when oxidized by nitrous acid gives carboxytartronic acid, which, on account of its ready decomposition into carbon dioxide and tartronic acid, was considered to be HO C(COOH) 3.
Alkali fusion of eugenol gives protocatechuic acid.