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.

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 Ag20 in methanol, and assayed their free radical-scavenging activ- ity by the reaction with 2,2-diphenyl-l-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.