The Collaborative International Dictionary of English v.0.48:

Protocatechuic \Pro`to*cat`e*chu"ic\, a. (Chem.)
   Pertaining to, derived from, or designating, an organic acid
   which is obtained as a white crystalline substance from
   catechin, asafetida, oil of cloves, etc., and by distillation
   itself yields pyrocatechin.
   [1913 Webster]