Convenient Preparation and Quantification of 5,5'-Diferulic Acid.

5,5'-Diferulic acid (5,5'-DFA), which is one of the cross-linking residues in plant cell walls, was prepared by using a facile procedure. The phenol oxidation of vanillin with FeCl3 gave divanillin, which was further devoted to a Perkin reaction to give the desired product. It was found on (13)C-NMR that the chemical shift of C-5 of ferulic acid (FA) clearly shifted downfield, when this carbon is quaternarized by the oxidative dimerizaton to 5,5'-DFA, while those of other carbons of 5,5'-DFA are fundamentally same as those of FA. Also prepared was [9,9'-(13)C2]-5,5'-DFA, which was proved to be a good internal standard on GC-MS quantification of endogenous 5,5'-DFA from plant tissues.