Oxidative coupling of cinnamic acid derivatives and their radical-scavenging activities

4-Hydroxycinnamic acid derivatives, including ferulic acid (FA) (1), sinapic acid (SA) (2) and caffeic acid (CA) (3), are widely distributed in the plant kingdom, and undergo oxidative cross-coupling leading to the corresponding dehydrodimers, trimers and even higher oligomers in plants. In order to evaluate the antioxidative ability of these 4-hydroxycinnamic acid derivatives and their oligomers, we synthesized 8-8′-bis-lactone-dimers (8-8′-DiFA (4), 8-8′-DiSA (5) and 8-8′-DiCA (6)), as well as a new trimer (7), by the reaction of the corresponding monomers (1–3) with Ag2O in methanol, and assayed their free radical-scavenging activity by the reaction with 2,2-diphenyl-1-picrylhydrazyl radical (DPPH•). It was found that CA (3) and 8-8′-DiCA (6) bearing o-dihydroxyl groups exhibited significantly higher radical-scavenging activity than those bearing no such groups, and oxidative coupling of CA (3) resulted in remarkable enhancement in the activity.